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CELL  BIOLOGY

1. Define cell theory

Cell theory is a theory that asserts that the cell is the constituent unit of the living beings.

Before the discovery of the cell, it was not recognized that the living beings were made of building blocks like cells.

The cell theory is one of the basic theories of Biology.

2. Explain why viruses are not considered as living cells

The virus is considered the only alive beings that do not have cells. Virus are constituted by genetic material (DNA or RNA) wrapped by a protein capsule. They do not have membrane and cell organelles neither self-metabolism.

3. State and explain Two major classification of cells

Cells can be classified as eukaryotic or prokaryotic.

Prokaryotic cell is that without a delimited nucleus.

Eukaryotic cells are those with nucleus delimited by membrane.

4. Explain the  constituents and  functions of Plasma Membrane

The plasma membrane is the outer membrane of the cell it delimits the cell itself and a cell interior with specific conditions for the cellular function. Since it is selectively permeable the plasma membrane has an important role for the passage of substances inwards or outwards.

The main constituents of the plasma membrane are phospholipids, proteins and carbohydrates. The phospholipids, amphipathic molecules, are regularly organized in the membrane according to their polarity: two layers of phospholipids form the lipid bilayer with the polar part of the phospholipids pointing to the exterior of the layer and the non polar phospholipid chains in the interior. Proteins can be found embedded in the lipid bilayer and there are also some carbohydrates bound to proteins and to phospholipids in the outer face of the membrane.

5. Distinguish between cytoplasm and nucleoplasm

 

6. Distinguish between plasma membrane and cell wall

Plasma membrane and cell wall is not the same thing. Plasma membrane, also called cell membrane, is the outer membrane common to all living cells and it is made of a phospholipid bilayer, embedded proteins and some appended carbohydrates.

Because cell membranes are fragile, in some types of cells there are even outer structures that support and protect the membrane, like the cellulose wall of plant cells and the chitin wall of some fungi cells. Most bacteria also present an outer cell wall made of peptidoglycans and other organic substances.

7. State and explain the major cell constituents of cell walls in bacteria, protists, fungi and plants?

In bacteria cell wall is made of peptidoglycans; among protists algae have cell wall made of cellulose; in fungi, the cell wall is made of chitin (the same substance that makes the exoskeleton of arthropods); in plants, the cell wall is made of cellulose too.

8. Identify the outer  Outer Wrapping Of Cells

Lipid membranes do not form only the outer cover of cells. Cell organelles, such as the Golgi complex, mitochondria, chloroplasts, lysosomes, the endoplasmic reticula and the nucleus, are delimited by membranes too.

9. Explain based on The Presence Of Nucleus The Difference Between Animal And Bacterial Cells.

Animal cells (cells of living beings of the kingdom Animalia) have an interior membrane that delimits a cell nucleus and thus they are eukaryotic cells; in these cells the genetic material is located within the nucleus. Bacterial cells (cells of living beings of the kingdom Monera) do not have organized cellular nucleus and so they are prokaryotic cells and their genetic material is found dispersed in the cytosol

10. Identify  three main parts of a eukaryotic cell

The eukaryotic cell can be divided into two main portions: the cell membrane that separates the intracellular space from the outer space phisically delimiting the cell; the cytoplasm, the interior portion filled with cytosol (the aqueous fluid inside the cell); and the nucleus, the membrane-delimited internal region that contains the genetic material.

11. Distinguish between prokaryotes and eukaryotes  in terms of

• Flagella
• Respiration

12. What Are The Main Structures Within The Cell Nucleus?

Within the cell nucleus the main structures are: the nucleolus, an optically dense region, spherical shaped, where there are concentrated ribosomal RNA (rRNA) associated to proteins (there may be more than one nucleolus in a nucleus); the chromatin, made of DNA molecules dispersed in the nuclear matrix during the cell interphase; the karyotecha, or nuclear membrane, the membrane that delimits the nucleus.

13. Differentiate  between chromatin and chromosome

The chromatin, dispersed in the nucleus, is a set of filamentous DNA molecules associated to nuclear proteins called histones. Each DNA filament is a double helix of DNA and thus a chromosome.

14. List the major cytoplasmic structures present in animal cells

The main cytoplasmic structures of the cell are the centrioles, the cytoskeleton, lysosomes, mitochondria, peroxisomes, the Golgi apparatus, the endoplasmic reticula and ribosomes.

15. State and explain  Cytoplasmic Inclusions

Cytoplasmic inclusions are cytoplasmic molecular aggregates, such as pigments, organic polymers and crystals. They are not considered cell organelles.
Fat drops and glycogen granules are examples of cytoplasmic inclusions.

16. Explain the  biological function of ribosomes

Ribosomes can be found free in the cytoplasm, adhered to the outer side of the nuclear membrane or associated to the endoplasmic reticulum membrane defining the rough endoplasmic reticulum. Ribosomes are the structures where protein synthesis takes place.

17. Distinguish between  smooth and rough endoplasmic reticulum

The endoplasmic reticulum is a delicate membranous structure contiguous to the nuclear membrane and present in the cytoplasm. It forms an extense net of channels throughout the cell and it is divided in rough and smooth types.

The rough endoplasmic reticulum has great amount of ribosomes adhered to the external side of its membrane. The smooth endoplasmic reticulum does not have ribosomes attached to its membrane. Ribosomes are the site of protein synthesis. Therefore RER are more dominant in cells which are actively involved in synthesis and export of proteins   e.g. in pancreatic acinar cells and plasma cells.

The main functions of the rough endoplasmic reticulum are synthesis and storage of proteins made in the ribosomes. The smooth endoplasmic reticulum plays a role in the lipid synthesis and, in muscle cells it is importante in the conduction of the contraction stimulus. SER   is found in those cells which are specialized in lipid metabolism and which secretes steroids such as cells of adrenal cortex, the testes and ovary. Also SER is also present in liver cells where it helps in detoxification of drugs and poisons.

18. Highlight four functions of  golgi bodies

The Golgi apparatus, also known as Golgi bodies or Golgi complex, is a cellular organelle with various functions related to the processing, modification, and sorting of proteins and lipids. Here are four key functions of Golgi bodies:

• Protein Modification and Sorting: One of the primary functions of Golgi bodies is the modification and sorting of proteins. Proteins synthesized in the endoplasmic reticulum (ER) are transported to the Golgi apparatus, where they undergo post-translational modifications. These modifications can include glycosylation (addition of carbohydrate groups), phosphorylation (addition of phosphate groups), and cleavage of specific protein regions. Additionally, Golgi bodies sort proteins into different vesicles, which are then transported to specific cellular destinations, such as lysosomes, plasma membrane, or secretory vesicles.

 

• Lipid Metabolism and Processing: Golgi bodies also play a role in lipid metabolism and processing. They receive lipids from the ER and modify them through enzymatic reactions. These modifications can include the addition of fatty acids, removal of phosphate groups, or formation of complex lipids. Golgi bodies are involved in the synthesis of various lipids, such as phospholipids and sphingolipids, which are important components of cellular membranes.
• Formation of Lysosomes: Golgi bodies are involved in the formation of lysosomes, which are cellular organelles responsible for intracellular digestion. Golgi vesicles containing enzymes fuse together to form mature lysosomes. These lysosomes then carry out the degradation of macromolecules, recycling cellular components, and participating in cellular defense mechanisms.
• Secretion of Mucus and Secretory Proteins: Golgi bodies are essential for the secretion of various substances, including mucus and secretory proteins. Specialized Golgi regions, called secretory vesicles, package proteins and other substances for export outside the cell. These vesicles fuse with the plasma membrane, releasing their contents to the extracellular space. The secretion of proteins and other substances is vital for various physiological processes, such as cell communication, immune response, and maintenance of tissue homeostasis.

These are just a few of the functions of Golgi bodies, which highlight their importance in cellular processes related to protein and lipid metabolism, vesicular trafficking, and secretion.

 

19. State four functions of cell plasma membrane

• A Physical Barrier

The plasma membrane surrounds all cells and physically separates the cytoplasm, which is the material that makes up the cell, from the extracellular fluid outside the cell. This protects all the components of the cell from the outside environment and allows separate activities to occur inside and outside the cell.

• Selective Permeability

Plasma membranes are selectively permeable (or semi-permeable), meaning that only certain molecules can pass through them. Water, oxygen, and carbon dioxide can easily travel through the membrane. Generally, ions (e.g. sodium, potassium) and polar molecules cannot pass through the membrane; they must go through specific channels or pores in the membrane instead of freely diffusing through. This way, the membrane can control the rate at which certain molecules can enter and exit the cell.

• Endocytosis and Exocytosis

Endocytosis is when a cell ingests relatively larger contents than the single ions or molecules that pass through channels. Through endocytosis, a cell can take in large quantities of molecules or even whole bacteria from the extracellular fluid. Exocytosis is when the cell releases these materials. The cell membrane plays an important role in both of these processes. The shape of the membrane itself changes to allow molecules to enter or exit the cell. It also forms vacuoles, small bubbles of membrane that can transport many molecules at once, in order to transport materials to different places in the cell.

• Cell Signaling

Another important function of the membrane is to facilitate communication and signaling between cells. It does so through the use of various proteins and carbohydrates in the membrane. Proteins on the cell “mark” that cell so that other cells can identify it. The membrane also has receptors that allow it to carry out certain tasks when molecules such as hormones bind to those receptors.

20. State and explain the function of  lysosomes

Lysosomes are hydrolase-containing vesicles that detach from the Golgi apparatus. Lysosomes have digestive enzymes (hydrolases) that are made in the rough endoplasmic reticulum and stored in the Golgi apparatus .

Lysosomes make autophagic and heterophagic digestion: autophagic digestion by digesting residual substances from the cellular metabolism; heterophagic digestion by digesting substances that enter the cell. Lysosomes enfold the substances to be degraded forming digestive vacuoles, or residual vacuoles, that later migrate toward the plasma membrane fusing with it and liberating (exocytosis) the digested material to the exterior.

21. Identify  Cell Organelles That Participate In The Cell Division and In The Formation Of Cillia And Flagella Eukaryotic Cells

The organelles that participate in the cell division and in the formation of cilia and flagella of some eukaryotic cells are the centrioles. Some cells have cillia (paramecium, the bronchial ciliated epithelium, etc.) or flagella (flagellate protists, sperm cells, etc.); these cell structures are composed by microtubules originated from the centrioles. Centrioles also make the aster microtubules that are very important for cell division.

 

22. Explain the Morphological, Chemical And Functional Similarities And Differences Between Lysosomes And Peroxisomes

Similarities: lysosomes and peroxisomes are small membranous vesicles that contain enzymes and enclose residual substances from internal or external origin degrading them. Differences: lysosomes have digestive enzymes (hydrolases) that break substances to be digested into small molecules; peroxisomes contain enzymes that degrade mainly long-chained fatty acids and amino acids and that inactivate toxic agents including ethanol; within peroxisomes there is the enzyme catalase, responsible for the oxidation of organic compounds by hydrogen peroxide

(H2O2) and, when this substance is in excess, by the degradation of the peroxide into water and molecular oxygen.

23. Explain the Endosymbiotic Origin Of Mitochondria.

It is presumed that mitochondria were primitive aerobic prokaryotes that were engulfed in mutualism by primitive anaerobic eukaryotes, receiving protection from these beings and offering energy to them. This hypothesis is called the endosymbiotic hypothesis on the origin of mitochondria.

The hypothesis is strengthened by some molecular evidences as the facts that mitochondria have own and independent DNA and protein synthesis machinery, with own RNA and ribosomes, and that they can self-replicate.

The endosymbiotic theory can be applied for chloroplasts too. It is supposed that these organelles were primitive photosynthetic prokaryotes because they have own DNA, RNA and ribosomes and they can self-replicate too.

 

24. Outline the basic morphology and function  of  mitochondria  

Mitochondria are the organelles in which the most important part of the cellular respiration occurs: the ATP production.

Draw a labelled diagram to show the internal structure of mitochondria as observed under electron microscope

Mitochondria are organelles delimited by two lipid membranes. The inner membrane invaginates to the interior of the organelle forming cristae that delimitate the internal space known as mitochondrial matrix and where mitochondrial DNA (mtDNA), mitochondrial RNA (mt RNA), mitochondrial ribosomes and respiratory enzymes can be found. Mitochondria are numerous in eukaryotic cells and they are even more abundant in those cells that use more energy, like muscle cells. Because they have their own DNA, RNA and ribosomes, mitochondria can self-replicate.

Mitochondria are the “power plants” of aerobic cells because within them the final stages of the cellular respiration process occurs. Cellular respiration is the process of using organic molecule (mainly glucose) and oxygen to produce carbon dioxide and energy. The energy is stored in the form of ATP (adenosine triphosphate) molecules and later used in other cellular metabolic reactions. In mitochondria the two last steps of the cellular respiration take place: the Krebs cycle and the respiratory chain.

25. State and explain the  Components  and  functions Of The Cytoskeleton

The cytoskeleton is a network of very small tubules and filaments distributed throughout the cytoplasm of eukaryotic cells. It is made of microtubules, microfilaments and intermediate filaments.

Microtubules are formed by molecules of a protein called tubulin. Microfilaments are made of actin, the same protein that participates in the contraction of muscle cells. Intermediate filaments are made of protein too.

As the name indicates, the cytoskeleton is responsible for the supporting of the normal shape of the cell; it also acts as a facilitator for substance transport across the cell and for the movement of cellular organelles. For example, the sliding between actin-containing filaments and the protein myosin creates pseudopods. In cells of the phagocytic defense system, like macrophages, cytoskeleton is responsible for the plasma membrane projections that engulf the external material to be interiorized and attacked by the cell.

26. Explain the function of Chloroplasts

Chloroplasts are organelles present in the cytoplasm of plant and algae cells. Likewise mitochondria, chloroplasts have two boundary membranes and many internal membranous sacs. Within the organelle there are own DNA, RNA and ribosomes and also the pigment chlorophyll, responsible for absorption of photic energy that is used in photosynthesis.

27. The main function of chloroplasts is photosynthesis:the production of highly energetic organic molecules (glucose) from carbon dioxide, water and light.

The chlorophyll molecules are the responsible for the absorption of the light energy for photosynthesis. These molecules are found on the internal membranes of chloroplasts.

Chlorophyll absorbs all other colors of the electromagnetic spectrum but it practically does not absorb the green. The green color is reflected and such reflection provides the characteristic color of plants. If the green light that reaches a plant is blocked and the exposition of the plant to other colours is maintained there would be no harm for photosynthesis. Apparent paradox: the green light is not important for photosynthesis.

There is difference between the optimun color frequency for the two main types of chlorophyll, the chlorophyll A and the chlorophyll B. Chlorophyll A has an absorption peak in approximately 420 nm wavelenght (anil) and chlorophyll B has its major absorption in 450 nm wavelenght (blue).

The energy source of photosynthesis is the sun, the unique and central star of our planetary system. In photosynthesis the solar energy is transformed into chemical energy, the energy of the chemical bonds of the produced glucose molecules (and of the released molecular oxygen). The energy of glucose then is stored as starch (a glucose polymer) or it is used in the cellular respiration process and transfered to ATP molecules. ATP is consumed in metabolic processes that spend energy (for example, in active transport across membranes).

28. State the constituents and functions of   cell wall

The plant cell wall is made of cellulose. Cellulose is a polymer whose monomer is glucose. There are other polymers of glucose, like glycogen and starch.

The plant cell wall has structural and protective functions. It plays important role in the constraint of the cell size, preventing the cell to break when it absorbs much water.

29. Explain  functions  of Plant Cell Vacuoles

Plant cell vacuoles are cell structures delimited by membranes within which there is an aqueous solution made of several substances like carbohydrates and proteins. In young plant cells many small vacuoles can be seen; within adult cells the most part of the internal area of the cell is occupied by a central vacuole.

The main function of the vacuoles is the osmotic balance of the intracellular space. They act as “an external space” inside the cell. Vacuoles absorb or release water in response to the cellular metabolic necessities by increasing or lowering the concentration of osmotic particles dissolved in the cytosol. Vacuoles also serve as storage place for some substances.

The membrane that delimits the vacuoles is called tonoplast, named after the osmotic function of the structure.

30. Describe and classify the cell membrane

Membrane is any delicate sheet that separates one region from other blocking or permitting (selectively or completely) the passage of substances. The skin, for example, can be considered a membrane that separates the exterior from the interior of the body; cellophane, used in chemical laboratories to separate solutions, acts as membrane too.

Membranes can be classified as impermeable, permeable, semipermeable or selectively permeable.

An impermeable membrane is that through which no substance can pass. Semipermeable membranes are those that let only solvent, like water, to pass through it. Permeable membranes are those that let solvent and solutes, like ions and molecules, to pass across it. There are still selectively permeable membranes, i.e., membranes that besides allowing the passage of solvent let only some specific solutes to pass blocking others.

The cell membrane is a selectively permeable membrane, i.e., it allows the passage of water and some selected solutes.

31. State and explain
    1. Diffusion

Diffusion is the movement of substances from a region of high concentration to low concentration. It is therefore said to occur down a concentration gradient. The diagram below shows the movement of dissolved particles within a liquid until eventually becoming randomly distributed.

Diffusion is a passive process which means it does not require any energy input. It can occur across a living or non-living membrane and can occur in a liquid or gas medium. Due to the fact that diffusion occurs across a concentration gradient it can result in the movement of substances into or out of the cell. Examples of substances moved by diffusion include carbon dioxide, oxygen, water and other small molecules that are able to dissolve within the lipid bilayer.

1. Concentration Gradient

Concentration gradient is the difference of concentration of a substance between two regions. Concentration is a term used to designate the quantity of a solute divided by the total quantity of the solution. Since water in general is the solvent in this situation it is not correct to refer to “concentration of water” in a given solution.

1. Osmosis

When the concentration of solutes in solution is low, the water concentration is high, and we say there is a high water potential. Osmosis is the movement of water from a region of higher water potential to a region of lower water potential across a semi-permeable membrane that separates the two regions. Movement of water always occurs down a concentration gradient, i.e from higher water potential (dilute solution) to lower potential (concentrated solution). Osmosis is a passive process and does not require any input of energy. Cell membranes allow molecules of water to pass through, but they do not allow molecules of most dissolved substances, e.g. salt and sugar, to pass through. As water enters the cell via osmosis, it creates a pressure known as osmotic pressure.

1. Facilitated diffusion

Facilitated diffusion is a special form of diffusion which allows rapid exchange of specific substances. Particles are taken up by carrier proteins which change their shape as a result. The change in shape causes the particles to be released on the other side of the membrane. Facilitated diffusion can only occur across living, biological membranes which contain the carrier proteins. A substance is transported via a carrier protein from a region of high concentration to a region of low concentration until it is randomly distributed. Therefore movement is down a concentration gradient. Examples of substances moved via facilitated diffusion include all polar molecules such as glucose or amino acids.

 

1. Active transport

Active transport is the movement of substances against a concentration gradient, from a region of low concentration to high concentration using an input of energy. In biological systems, the form in which this energy occurs is adenosine triphosphate (ATP). The process transports substances through a membrane protein. The movement of substances is selective via the carrier proteins and can occur into or out of the cell. The sodium-potassium pump shown below  is an example of primary active transport.

32. Distinguish between  between Osmosis And Diffusion?

Osmosis is the phenomenon of movement of solvent particles, in general water, from a region of lower solute concentration to a region of higher solute concentration. Diffusion, in the other hand, is the movement of solutes from a region of higher solute concentration to a region of lower solute concentration.

One can consider osmosis as movement of water (solvent) and diffusion as movement of solutes, both concentration gradient-driven.

33. Define  Osmotic Pressure

Osmotic pressure is the pressure created in a aqueous solution by a region of lower solute concentration upon a region of higher solute concentration forcing the passage of water from that to this more concentrated region. The intensity of the osmotic pressure (in units of pressure) is equal to the pressure that is necessary to apply in the solution to prevent its dilution by the entering of water by osmosis.

It is possible to apply in the solution another pressure in the contrary way to the osmotic pressure, like the hydrostatic pressure of the liquid or the atmospheric pressure. In plant cells, for example, the rigid cell wall makes opposite pressure against the tendency of water to enter when the cell is put under a hypotonic environment. Microscopically, the pressure contrary to the osmotic pressure does not forbid water to pass through a semipermeable membrane but it creates a compensatory flux of water in the opposite way.

The osmotic pressure of a solution does not depend on the nature of the solute, it depends only on the quantity of molecules (particles) in relation to the total solution volume. Solutions with same concentration of particles even containing different solutes exert same osmotic pressure.

Even when the solution contains a mixture of different solutes its osmotic pressure depends only on its total particle concentration regardless the nature of the solutes.

34. Classify solutions  According To Their Comparative Tonicity

Comparatively to other a solution can be hypotonic (or hyposmotic), isotonic (or isosmotic) or hypertonic (or hyperosmotic).

When a solution is less concentrated than other the adjective hypotonic is given and the more concentrated is called hypertonic. When two compared solutions have same concentration both receives the adjective isotonic. So this classification makes sense only for comparison of solutions.

35. A plasmolyzed  living plant cell is placed in a large volume of pure water . Explain the process that will take place

Plasmolysis is the process that takes place when the plant cell losses cytoplasmic water due to osmosis and shrinks in size.

It occurs when the plant’s cell is placed in a hypertonic solution or solution having a higher concentration of salts. Cytoplasm loses its water to the hypertonic solution to achieve equilibrium and shrinks.

When a plasmolysed cell (or the cells whose cytoplasm got shrunk) is placed in water, the cell absorbs water from the surrounding due to varied concentration gradient and tries to regain its original size and shape. Thus, the plant cell becomes turgid.

36. Explain the mechanisms through which molecules are transported  across the cell membrane

The key processes through which such movement occurs include diffusion, osmosis, facilitated diffusion and active transport.

37. List four factors that affect rate ofdiffusion
    1. Concentration Gradient

An increase in the concentration of molecules at one region results in a steeper concentration gradient which in turn increases the rate of diffusion.

1. Temperature

High temperature increases kinetic energy of molecules. They move faster hence resulting in an increase in rate of diffusion, and vice versa.

1. Size of Molecules or Ions

The smaller the size of molecules or ions, the faster their movement hence higher rate of diffusion.

1. Density
   The denser the molecules or ions diffusing, the slower the rate of diffusion, and vice versa.
2. Medium
   The medium through which diffusion occurs also affects diffusion of molecules or ions. For example, diffusion of molecules through gas and liquid media is faster than through a solid medium.
3. Distance

This refers to the thickness or thinness of surface across which diffusion occurs. Rate of diffusion is faster when the distance is small i.e., thin surface.

1. Surface Area to Volume Ratio

The larger the surface area to volume ratio, the faster the rate of diffusion. For example, in small organisms such as Amoeba the surface area to volume ratio, is greater hence faster diffusion than in larger organisms.

38. Identify  The Basic Constituents Of The Cell Membrane

The cell membrane is formed of lipids, proteins and carbohydrates.

The membrane lipids are phospholipids, a special type of lipid to which one extremity a phosphate group is bound thus assigning electric charge to this region of the molecule. Since phospholipids have one electric charged extremity and a long neutral organic chain they can organize themselves in two layers of associated molecules: the hydrophilic portion (polar) of each layer faces outwards in contact with water (a polar molecule too) of the extracellular and the intracellular space and the hydrophobic chains (non polar) faces inwards isolated from the water. Because this type of membrane is made of two phospolipid layers it is also called bilipid membrane.

Membrane proteins are embedded and dispersed in the compact bilipid structure. Carbohydrates appear in the outer surface of the membrane associated to some of those proteins under the form of glycoproteins or bound to phospholipids forming glycolipidis. The membrane carbohydrates form the glycocalix of the membrane.This description (with further explanations) is kown as the fluid mosaic model about the structure of the cell membrane.

39. State and explain the  functions of phospholipids, proteins and carbohydrates of the cell membrane

Membrane phospholipids have structural function they form the bilipid membrane that constitutes the cell membrane itself.

Membrane proteins have several specialized functions. Some of them are channels for substances to pass through the membrane, others are receptors and signalers of information, others are enzymes, others are cell identifiers (cellular labels) and there are still those that participate in the adhesion complexes between cells or between the internal surface of the membrane and the cytosketeleton. Membrane carbohydrates, associated to proteins or to lipids, are found in the outer surface of the cell membrane and they have in general labeling functions for recognition of the cell by other cells and substances (for example, they differentiate red blood cells in relation to the ABO blood group system), immune modulation functions, pathogen sensitization functions, etc.

40. State five functions of surface cell membranes
    1. The cell membrane supports and protects the cell. It controls the movement of substances in and out of the cells. It separates the cell from the external environment. The cell membrane is present in all the cells.
    2. The cell membrane is the outer covering of a cell within which all other organelles, such as the cytoplasm and nucleus, are enclosed. It is also referred to as the plasma membrane.
    3. By structure, it is a porous membrane (with pores) which permit the movement of selective substances in and out of the cell.  Besides this, the cell membrane also protects the cellular component from damage and leakage.
    4. It forms the wall-like structure between two cells as well as between the cell and its surroundings.
    5. Plants are immobile, so their cell structures are well-adapted to protect them from external factors. The cell wall helps to reinforce this function.
41. Explain  The Relationship  Between Concentration Gradient and  Active And Passive Transport

Passive transport is the movement of substances across membranes in favor of their concentration gradient, i.e., from a more concentrated region to a less concentrated region. Active transport, in the other hand, is the transport of substances across membranes against their concentration gradient, from a less concentrated to a more concentrated region. In passive transport, because it is spontaneous, there is no energy spending; the active transport however requires energy (work) to occur.

Active transport is a work to maintain or increase the concentration gradient of a substance between two regions while passive transport acts in a manner to reduce the concentration gradient.

The higher the concentration gradient of a substance the more intense its simple diffusion will be. If the concentration gradient diminishes the intensity of simple diffusion diminishes too.

Likewise simple diffusion facilitated diffusion is more intense when the concentration gradient of the substance increases and less intense when the gradient lessens. In facilitated diffusion however there is a limiting factor: the quantity of the permeases that facilitate the transport through the membrane. Even in a situation in which the concentration gradient of the diffusing substance increases, if there are not enough permeases to perform the transport there will be no increase in the intensity of the diffusion. This situation is called saturation of the transport proteins and it represents the point in which the maximum transport capacity of the substance across the membrane is achieved.

42. State Three Main Types Of Passive Transport?

The three main types of passive transport are simple diffusion, osmosis and facilitated diffusion.

43. State  Energy Source Used In Active Transport Through Biological Membranes

The energy necessary for active transport (against the concentration gradient of the transported substance) to occur comes from ATP molecules. The active transportation uses chemical energy from ATP.

44. Distinguish between  Simple And Facilitated Diffusion

Simple diffusion is the direct passage of substances across the membrane in favor of their concentration gradient. In facilitated diffusion the movement of substances is also in favor of their concentration gradient but the substances move bound to specific molecules that act as “permeabilizers”, i.e., facilitators of their passage through the membrane.

The action of facilitator proteins in facilitated diffusion makes this type of diffusion faster than simple diffusion under equal concentration gradients of the moved substance.

45. Compare Facilitated Diffusion Present Similarities With Enzymatic Chemical Reactions

One of the main examples of facilitated transport is the entrance of glucose from the blood into cells. Glucose from blood binds to specific permeases (hexose-transporting permeases) present in the cell membrane and by diffusion facilitated by these proteins it enters the cell to play its metabolic functions.

Facilitated diffusion resembles chemical catalysis because the transported substances bind to permeases like substrates bind to enzymes and in addition after one transport job is concluded the permease is not consumed and can perform successive other transports.

46. Give  Examples Of Biological Activities In Which Osmosis Plays Important Role?

Hemolysis (destruction of red blood cells) by entrance of water, the hydric regulation in plants and the entrance of water in the xylem of vascular plants are all examples of biological phenomena caused by osmosis.

Excessive dilution of the blood plasma makes, by osmosis, the entrance of too much water in red blood cells and then the destruction of these cells (hemolysis). Osmosis also is the main process for maintenance of the flaccid, turgid or plasmolytic states of plant cells. Osmosis is one of the forces responsible for the entrance of water in plant roots since root cells are hypertonic in comparison to the soil.

47. Compare and contrast  Facilitated Diffusion And Active Transport

Facilitated diffusion can be confused with active transport because in both processes there is participation of membrane proteins.

In active transport however the transported substance moves against its concentration gradient and with energy spending. Facilitated diffusion is a passive transport in favor of the concentration gradient and it does not require energy.

48. Describe the mechanism of The Sodium-potassium Pump Present In The Cell Membrane

Active transport is made by specific membrane proteins. These proteins are called “pumps” because they “pump” the moving substance through the membrane using energy from ATP molecules.

The sodium-potassium pump is the transport protein that maintains the concentration gradient of these ions between the intra and the extracellular spaces. This protein is phosphorylated in each pumping cycle and then it pumps three sodium ions outside the cell and puts two potassium ions inwards. The phosphorylation is made by the binding of a phosphate donated by one ATP molecule that then is converted into ADP (adenosine diphosphate).

The job of the sodium-potassium pump, also known as sodium-potassium ATPase, is fundamental to keep the characteristic negative electric charge in the intracellular side of the membrane of the resting cell and to create adequate conditions of sodium and potassium concentrations inside and outside the cell to maintain the cellular metabolism.

49. Explain Mass Transportation Across The Cell Membrane

Mass transportation is the entrance or the exiting of substances in or from the cell engulfed by portions of membrane. The fusion of internal substance-containing membranous vesicles with the cell membrane is called exocytosis. The entrance of substances in the cell after they have been engulfed by projections of the membrane is called endocytosis.

50. Distinguish between  endocytosis and exocytosis

Endocytosis and exocytosis are two cellular processes involved in the transport of substances across the cell membrane. They are opposite in nature and have distinct functions. Here’s how they can be distinguished:

Endocytosis:

Endocytosis is the process by which cells take in materials from the external environment into the cell. It involves the formation of vesicles from the cell membrane to engulf and internalize substances. There are three main types of endocytosis:

• Phagocytosis: In phagocytosis, large particles, such as bacteria or cellular debris, are engulfed by the cell membrane, forming a phagosome. The phagosome then fuses with lysosomes, leading to the degradation and breakdown of the engulfed material.
• Pinocytosis: Pinocytosis, also known as “cell drinking,” involves the internalization of fluid and dissolved solutes. The cell membrane forms small vesicles, known as pinocytic vesicles or endosomes, to bring in extracellular fluid and its contents.
• Receptor-Mediated Endocytosis: This type of endocytosis is highly specific and involves the internalization of specific substances that bind to receptors on the cell membrane. The substances are first recognized by receptors, which then initiate the formation of coated pits. The coated pits invaginate and pinch off from the membrane, forming coated vesicles that transport the specific substances into the cell.

 

Exocytosis:

Exocytosis is the process by which cells release substances from the cell to the external environment. It involves the fusion of vesicles containing cellular materials with the cell membrane, resulting in the secretion of the contents outside the cell. Exocytosis serves various functions, such as releasing hormones, neurotransmitters, enzymes, or waste products. It includes:

 

• Constitutive Exocytosis: Constitutive exocytosis occurs continuously and is responsible for the secretion of proteins and lipids to the extracellular space. It maintains the plasma membrane’s integrity and contributes to cell growth and maintenance.
• Regulated Exocytosis: Regulated exocytosis is a controlled process that occurs in response to specific signals or stimuli. It is involved in the release of neurotransmitters from nerve cells, hormone secretion, and the release of digestive enzymes from cells in the digestive system.

In summary, endocytosis is the process of internalizing materials into the cell through the formation of vesicles, while exocytosis is the process of releasing substances from the cell by fusing vesicles with the cell membrane. Endocytosis brings materials into the cell, while exocytosis expels materials out of the cell.

 

51. State and explain Two Main Types Of Endocytosis

Endocytosis is the entrance of materia in the cell engulfed by portions of the cell membrane.

Endocytosis can be classified as pinocytosis or phagocytosis. In pinocytosis small particles on the external surface of the membrane stimulate the invagination of the membrane inwards and vesicles full of that particles then detach from the membrane and enter the cytoplasm. In phagocytosis bigger particles on the external surface of the membrane induce the projection of pseudopods outwards enclosing the particles; the vesicle then detachs from the membrane and enter the cytoplasm receiving the name phagosome.

52. Give specific  examples of functions  carried out through exocytosis in living things

Exocytosis is an essential cellular process that occurs in various living organisms. It serves several functions, and here are some specific examples of the functions carried out through exocytosis:

• Neurotransmitter Release: In the nervous system, exocytosis is responsible for the release of neurotransmitters from nerve cells (neurons). When an action potential reaches the presynaptic terminal of a neuron, synaptic vesicles containing neurotransmitters fuse with the cell membrane, allowing the neurotransmitters to be released into the synaptic cleft. This facilitates the transmission of signals between neurons and enables communication within the nervous system.
• Hormone Secretion: Exocytosis is involved in the secretion of hormones by endocrine cells. Endocrine glands, such as the pituitary gland, adrenal glands, and pancreas, produce hormones that regulate various physiological processes. Hormones are synthesized and stored in secretory vesicles within the endocrine cells. Upon appropriate stimulation, these vesicles fuse with the cell membrane, releasing the hormones into the bloodstream. The hormones then travel to target cells or organs, where they elicit specific responses and help regulate bodily functions.
• Digestive Enzyme Release: Exocytosis plays a crucial role in the digestive system by facilitating the release of digestive enzymes. In the cells of the pancreas and salivary glands, zymogen granules contain inactive forms of digestive enzymes. When stimulated by food intake, these granules undergo exocytosis, releasing the enzymes into the digestive tract. The enzymes then help break down complex food molecules into simpler forms for absorption and digestion.
• Mucus Secretion: Goblet cells, found in various mucosal tissues such as the respiratory tract and digestive system, produce and secrete mucus. Mucus serves as a protective layer, lubricates surfaces, and helps trap foreign particles and pathogens. Exocytosis is involved in the release of mucus vesicles from goblet cells, allowing the mucus to be deposited onto the epithelial surfaces and provide their protective functions.

These examples highlight some of the vital functions carried out through exocytosis in living organisms, including neurotransmitter release for nerve cell communication, hormone secretion for regulation of bodily functions, digestive enzyme release for food digestion, and mucus secretion for protection of mucosal surfaces.

53. Describe the a cell  When  Placed Under Hypotonic Medium

The plant cell wall (the covering of the cell external to the cell membrane) is made of cellulose, a polymer of glucose.

When the cell is put under hypotonic medium it absorbs too much water through osmosis. In that situation the cell wall pressure acts to compensate the osmotic pressure thus forbiding excessive increase of the cellular volume and the cell lysis.

54. Explain the Suction Force Of The Plant Cell

The suction force (SF) is the osmotic pressure of the plant cell vacuole, i.e., of the vacuolar internal solution.

Since the vacuolar solution is hypertonic in comparison to cytosol it attracts water then increasing the cytosol concentration. With the osmotic action of the vacuole the cytosol becomes hypertonic in relation to the exterior and more water enters the cell.

55. Explain turgor pressure  Of Plant Cells

Wall resistance, or turgor pressure (TP), is the pressure made by the distension of the plant cell wall in opposition to the increase of the cell volume. The wall resistance works against the entrance of water in the cell, i.e., it acts forcing the exiting of water and compensating the entrance of the solvent by osmosis.

56. Explain  Deplasmolysis Of Plant Cells

The plant cell when placed under hypertonic medium loses a great amount of water and its cell membrane detaches from the cell wall. In that situation the cell is called plasmolysed cell. When the plasmolysed cell is placed under hypertonic medium it absorbs water and becomes a turgid cell. This phenomenon is called deplasmolysis.

57. Explain the principle behind use of Salt And Sugar Used In The Production Of Dried Meat And Dried Fruits

Substances that maintain highly hypertonic environment, like sugar and salt, are used in the production of dried meat, fish or fruits (for example, cod) because the material to be conserved is then dehydrated and the resulting dryness prevents the growth of populations of decomposer beings (since these beings also lose water and die).

58. State and explain  constituennts and functions of  Cytoskeleton.

Cytoskeleton is the cytoplasmic structure that supports the cell, keeps its shape and fixates and moves the cell organelles. It is made of an extensive network of fibers dispersed in the cytoplasm and anchored in the plasma membrane. Its components are microtubules, microfilaments and intermediate filaments.

59. Explain function of microtubules

Microtubules are made of consecutive dimers of the protein tubulin (each dimer has an alpha and a beta tubulin associated). Microtubules participate in cell division, they are constituents of cilia and flagella and they also form the centrioles.

60. Explain  How cell movements are  created

Cell movements are movements performed by cell structures, like the movements of cilia and flagella, the pseudopod movements (in amoeba, macrophages, etc.), the cyclosis of the cytoplasm and the sarcomere contraction in muscle cells.

Cell movements can be created by the citoskeleton action, by differences of viscosity among cytoplasmic regions and by intracellular contraction systems.

Cilia and flagella are structures found in some prokaryotes as well in some eukaryotic cells. They play defense, nutrition and movement roles for the cell. In eukaryotic cells of protists and animals they originate from centrioles that migrate towards the plasma membrane and differentiate into structures projected outside the cell. Each cilium or flagellum is made of nine peripheral pairs of microtubules and one central pair all covered by membrane. (In bacteria, flagella are made of a protein named flagellin and there can also be fimbria made of pilin.)

In the fixation base of each cilium or flagellum in the plasma membrane there are proteins that work as molecular motors providing movement for these structures with energy spending. Due to this energy spending ciliated or flagellated eukaryotic cells have a large number of mitochondria.

In humans ciliated cells can be found, for example, in the bronchial and tracheal epithelium. In these tissues the cilia have the defensive function of sweeping mucous and foreign substances that enter the airways. Sperm cells are typical example of flagellated cells their flagellum is the propulsion equipment for the movement towards the ovule.

 

61. Describe Amoeboid Movements

Amoeboid movements are created by cytoplasmic movements and plasma membrane projections called pseudopods. Their formation actively changes the external shape of some portions of the cell surface making it to move along a substratum. Pseudopods appear from differences of viscosity among neighboring regions of cytoplasm near the plasma membrane and from the contractile action of microfilaments.

Amoeboid movements occur, for example, in amoebas (a protozoan), organisms that use their movement to find food. The leukocytes, cells of the immune system, when attracted by chemical substances (immune mediators) use amoeboid movements to get out from capillaries in regions of tissue damage to participate in the inflammatory  

 

62. Explain Cellular Secretion

Cell secretion is the elimination to the exterior of substances produced by the cell (for example, hormones, mucous, sweat, etc.) . this process occur In secretory cells, like the secretory cells of endocrine glands, organelles related to production, processing and “exportation” of substances are widely present and well-developed. These organelles are the rough endoplasmic reticulum and the Golgi apparatus.

The nuclear membrane of the secretory cells generally has more pores to allow the intense traffic of molecules related to protein synthesis between the cytoplasm and the nucleus. Endocrine and exocrine pancreatic cells, thyroid and parathyroid endocrine cells, adenohypophysis, adrenal and pineal endocrine cells, the many types of gastric exocrine and endocrine cells, the mucous secretory cells of the lungs and of the bowels, the salivary gland cells, the lacrimal gland cells, the sebaceous gland cells, the secretory cells of the ovaries and testicles, etc., are all examples of secretory cells. Also are the Rough endoplasmic reticulum Golgi apparatus

63. Explain How The Rough Endoplasmic Reticulum And The Golgi Apparatus Act In The Production And Releasing Of Proteins

The rough endoplasmic reticulum has in its outer membrane numerous ribosomes, structures where translation of messenger RNA and protein synthesis occur. These proteins are stored in the rough endoplasmic reticulum and later they go to the Golgi apparatus. Within the Golgi apparatus proteins are chemically transformed and when ready they are put inside vesicles that detach from the organelle. These vesicles fuse with the plasma membrane (exocytosis) in the right place and its content is liberated outside the cell.

64. Distinguish between extracellular and intracellular digestion

Extracellular digestion is that in which food breaking into utile molecules that can be internalized by the cell is done in the extracellular space, i.e., outside the cell. In extracellular digestion the cells secret substances that break big molecules into smaller ones in the external environment. Later the cell can benefit from these products of the digestion.

Intracellular digestion, or cellular digestion, is the breaking in the interior of the cell of big molecules coming from outside or even from the own cell metabolism into smaller molecules. Products and residues of the intracellular digestion are used by the cell or excreted.

65. Intracellular digestion is classified into two types:
    1. heterophagic intracellular digestion
    2. autophagic intracellular digestion.

The organelles responsible for intracellular digestion are the lysosomes. Lysosomes are vesicles that contain digestive enzymes capable of breaking big molecules into smaller ones. These vesicles fuse with others that carry the material to be digested and then digestion takes place.

Heterophagic intracellular digestion is the breaking into smaller substances of external substances engulfed in the cell by pinocytosis or phagocytosis. Phagosomes or pinosomes fuse with lysosomes making the digestive vacuoles. Within the digestive vacuoles the molecules to be digested are hydrolyzed and the products of the digestion cross through the membrane and reach the cytoplasm or they are kept inside the vacuoles. The vacuole with residues from digestion is called residual body and by exocytosis it fuses with the plasma membrane and liberates its “waste” in the exterior space.

Autophagic intracellular digestion is the cellular internal digestion of waste and residual materials. In general it is done by lysosomes.

Autophagic intracellular digestion is intensified in situations of starvation because in such condition the cell tries to obtain from its own constituent materials the nutrients necessary to stay alive.

66. Give examples where  Lysosomic Enzymes Play Fundamental Role

The remodelation of the osseous tissue, the function of acrosomes in sperm cells and the elimination of the tadpole tail are examples of biological processes in which lysosomic enzymes are key factors.

The bone is a tissue made of osteoblast-containing matrix (osteoblasts are the secretory cells of the osseous matrix), osteocytes (mature bone cells) and osteoclasts (the remodeling cells). Osteoclasts are responsible for the the continual renovation of the osseous tissue since their lysosomic enzymes digest the osseous matrix.

The sperm acrosome, for carrying digestive enzymes within, is responsible for the perfuration of the egg cell membrane in the fertilization process. The acrosome, located in the anterior end of the sperm cell, is a specialized region of the Golgi apparatus that accumulates great amount of digestive enzymes.

In tadpoles the tail regresses while the organism develops into an adult frog. This tissue destruction is a digestion of the tail own cells and extracellular materials and it is made by lysosomes and their enzymes. The complete digestion of a cell by its own mechanisms is called autolysis, a type of apoptosis (celll suicide).

67. Explain Homologous Chromosomes

Chromosomes contain genes (genetic information in the form of nucleotide sequences) that command the protein synthesis thus regulating and controlling the activities of the cell. In the nucleus of somatic cells of diploid beings every chromosome has its correspondent homologous chromosome, both containing alleles of the same genes related to same functions. This occurs because one chromosome of one pair comes from the father and the other comes from the mother of the individual. The chromosomes that form a pair with alleles of the same genes are called homologous chromosomes. In humans, there are 22 pairs of homologous chromosomes plus the pair of sex chromosomes (the sex chromosomes are partially homologous).The only human cells that do not have homologous chromosomes are the gametes since during meiosis the homologous chromosomes are separated.

68. Distinguish  Between The Concepts Of Karyotype And Genome

Genome is the set of DNA molecules that characterizes each living being or each species. The concept then includes the specific nucleotide sequence of the DNA molecules of each individual or species. Karyotype is the set of chromosomes of individuals of a given individual or species concerning morphology and number of each chromosome or pair of homologous.

69. Differentiate between a somatic cells and  gamete cell

Somatic cells and gametes are two types of cells which are involved in asexual and sexual reproduction of organisms, respectively. Somatic cells can be found everywhere in the body whereas gametes are restricted to reproductive organs. Male gametes are called as sperms while female gametes are called as ova. The main difference between somatic cells and gametes is that somatic cells consist of diploid a genome whereas gametes consist of a haploid genome.

70. Differentiate between mitosis in plants and animals

The main difference between animal mitosis and plant mitosis is that the mitotic spindle in animal mitosis is formed with the help of two centrioles whereas mitotic spindle in plant mitosis is formed without any centrioles. Mitosis is followed by cytokinesis.

71. Differentiate between plants and animal cells  as seen under the light microscope

	Plant cell	Animal cell
Cell Shape	Square or rectangular in shape	Irregular or round in shape
Cell Wall	Present	Absent
Plasma/Cell Membrane	Present	Present
Endoplasmic Reticulum	Present	Present
Nucleus	Present and lies on one side of the cell	Present and lies in the centre of the cell
Lysosomes	Present but are very rare	Present
Centrosomes	Absent	Present
Golgi Apparatus	Present	Present
Cytoplasm	Present	Present
Ribosomes	Present	Present
Plastids	Present	Absent
Vacuoles	Few large or a single, centrally positioned vacuole	Usually small and numerous
Cilia	Absent	Present in most of the animal cells
Mitochondria	Present but fewer in number	Present and are numerous
Mode of Nutrition	Primarily autotrophic	Heterotrophic

 

72. State four functions of a nucleus of an animal cells
    1. The nucleus contains the hereditary material of the cell, the DNA.
    2. It sends signals to the cells to grow, mature, divide and die.
    3. The nucleus is surrounded by the nuclear envelope that separates the DNA from the rest of the cell.
    4. The nucleus protects the DNA  and is an integral component of a plant’s cell structure.
73. Outline the process of mitosis

Mitosis involves four major stages  i.e. prophase , matephase , anaphase and telophase

 

Mitosis is a process of cell division that occurs in eukaryotic cells to produce two genetically identical daughter cells. It is essential for growth, development, tissue repair, and asexual reproduction. The process of mitosis can be divided into several distinct stages:

 

• Interphase: This is the phase before mitosis where the cell prepares for division. The cell undergoes normal metabolic activities, DNA replication occurs during the S phase, and the cell grows in size. Interphase consists of three subphases: G1 phase (cell growth), S phase (DNA synthesis), and G2 phase (preparation for mitosis).
• Prophase:The first stage of mitosis, during which several key events occur:
• Chromatin condenses: The long, thin strands of DNA condense and become visible as chromosomes. Each chromosome consists of two sister chromatids held together by a centromere.
• Nuclear envelope breakdown: The nuclear membrane disintegrates, releasing the condensed chromosomes into the cytoplasm.
• Spindle formation: Microtubules called spindle fibers begin to form and radiate from structures called centrosomes, which move toward opposite poles of the cell.

(c ) Metaphase: In this stage, the condensed chromosomes line up along the equatorial plane, known as the metaphase plate. The spindle fibers attach to the centromeres of each chromosome, ensuring their alignment at the center of the cell.

• Anaphase: The sister chromatids separate at the centromeres and are pulled apart by the shortening of the spindle fibers. The separated chromatids, now called daughter chromosomes, move towards opposite poles of the cell.
• Telophase: As the daughter chromosomes reach the opposite poles, several events occur:
• Chromosomes decondense: The chromosomes start to uncoil and return to their extended chromatin form.
• Nuclear envelope formation: New nuclear envelopes begin to form around the separated sets of chromosomes at each pole, re-establishing separate nuclei.
• Spindle disassembly: The spindle fibers disassemble, and the microtubules are broken down.
• Cytokinesis: The final stage of cell division, during which the cytoplasm is divided to produce two daughter cells. Cytokinesis varies between animal and plant cells:
• Animal Cells: A cleavage furrow forms, which pinches the cell membrane inward until it divides the cytoplasm into two daughter cells.
• Plant Cells: A cell plate forms in the middle of the cell, gradually expanding outward to divide the cytoplasm. The cell plate develops into a new cell wall that separates the two daughter cells.
• After cytokinesis, each daughter cell enters its own interphase, starting the cell cycle again.

Mitosis ensures that each daughter cell receives an identical set of chromosomes and maintains the genetic stability of the organism. It is a tightly regulated process that involves the coordination of various cellular components and molecular events.

74. Differentiate between mitosis and meiosis

Mitosis and meiosis are two different processes of cell division that occur in eukaryotic cells. While both involve the division of genetic material, there are significant differences between the two. Here’s a comparison of mitosis and meiosis:

1. Purpose:

• Mitosis: The main purpose of mitosis is the growth, development, and repair of somatic (body) cells. It produces two identical daughter cells that are genetically identical to the parent cell.
• Meiosis: Meiosis, on the other hand, is specifically involved in the production of gametes (sex cells) for sexual reproduction. It produces four non-identical daughter cells with half the number of chromosomes as the parent cell.

1. Chromosome Number:

• Mitosis: Mitosis involves the division of diploid cells, which have two sets of chromosomes (one from each parent). The daughter cells produced in mitosis have the same number of chromosomes as the parent cell.
• Meiosis: Meiosis involves the division of diploid cells into haploid cells. Haploid cells have only one set of chromosomes. During meiosis, the chromosome number is halved, resulting in daughter cells with half the number of chromosomes as the parent cell.

1. Number of Divisions:

• Mitosis: Mitosis consists of a single division of the nucleus, resulting in two daughter cells.
• Meiosis: Meiosis involves two consecutive divisions of the nucleus: meiosis I and meiosis II. This results in the production of four daughter cells.

1. Genetic Variation:

• Mitosis: Mitosis produces genetically identical daughter cells because the chromosome number and genetic content are preserved. There is no recombination or exchange of genetic material.
• Meiosis: Meiosis generates genetic variation through the processes of crossing over and independent assortment. During meiosis I, homologous chromosomes pair up and exchange genetic material, leading to genetic recombination. In meiosis II, the sister chromatids separate, resulting in further shuffling of genetic material. These processes lead to the formation of genetically diverse gametes.

1. Occurrence:

• Mitosis: Mitosis occurs in somatic (non-reproductive) cells throughout an organism’s life for growth, tissue repair, and asexual reproduction.
• Meiosis: Meiosis occurs only in specialized cells called germ cells or germ line cells, which are involved in sexual reproduction. It occurs during the formation of gametes (sperm and eggs) in animals and spores in plants.
• Overall, mitosis is a process of cell division for growth and repair, producing genetically identical daughter cells. Meiosis is a specialized form of cell division that occurs in the production of gametes, resulting in genetic variation and the formation of haploid cells.

75. Explain the significance of mitosis
    • genetic stability– mitosis  produces  two nucleus  which have the same  number of chromosomes , the exact copy of genes  and DNA from the mother cell . Therefore the daughter cells is genetically identical
    • Growth – the number of cells within the organ is  increases  by mitosis . This is the basis of growth  in multicellular organism

(c)  Cell replacement – the cells are constantly   dying and being replaced by new ones . This replacement involves mitosis

(d)  Regeneration  –  some animals  e.g. lizards, crustacean  etc.  have the ability to regenerate  their lost parts or organs of the body  like tails , arms etc. . Also some internal cell organs e.g. liver can be regenerated back to their normal status . This process of regeneration involves mitosis

(e) Asexual reproduction – mitosis is the basis of reproduction  in cellular organisms 

76. (a) Define the term “genetic crossing over’

Crossing over is a cellular process that happens during meiosis when chromosomes of the same type are lined up. When two chromosomes — one from the mother and one from the father — line up, parts of the chromosome can be switched. The two chromosomes contain the same genes, but may have different forms of the genes.

• State the importance  of genetic crossing over

It creates gametes that contain new combinations of genes, which helps maximize the genetic diversity of any offspring that result from the eventual union of two gametes during sexual reproduction

77. Explain the role of meiosis in gamete formation

Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells.

78. Account for the shape of plant and animal cells when cells are placed in hypotonic, isotonic or hypertonic solutions;

The medium (solution) surrounding cells or organisms is described by the terms hypotonic, hypertonic and isotonic.

A solution whose solute concentration is more than that of the cell sap is said to be hypertonic. A cell placed in such a solution loses water to the surroundings by osmosis.

A solution whose solute concentration is less than that of the cell sap is said to be hypotonic. A cell placed in such a solution gains water from the surroundings by osmosis.

A solution which has the same solute concentration as the cell sap is said to be isotonic. When a cell is placed in such a solution there will be no net movement of water either into or out of the cell.

79. Name any four types of cells found in  areolor tissue

Areolar connective tissue contains  i) Fibroblasts and fibrocytes (ii) Macrophages or histocytes (iii) Mast cells (iv) Fat cells (v) Chromatophores (vi) Mesenchyme cells (vii) Plasma cells.

80. Describe the preparation of a stained onion epidermal tissue for examination under light microscope

The bulb of an onion is formed from modified leaves. While photosynthesis takes place in the leaves of an onion containing chloroplast, the little glucose that is produced from this process is converted in to starch (starch granules) and stored in the bulb.

81. Chlorophylland chloroplasts responsible for photosynthesis are therefore only present in the leafy part of the onion (above ground) and absent in the bulb (which grows below ground).

Unlike animal cells (such as cheek cells) the cell wall of an onion and other plants are made up of cellulose, which protects the cell and maintains its shape.

Differentiate between mitosis in plants and animals

Mitosis is a fundamental process of cell division that occurs in both plant and animal cells. While the basic steps of mitosis are similar in plants and animals, there are some key differences between mitosis in these two types of organisms. Here’s a comparison of mitosis in plants and animals:

• Cytokinesis:
• Animal Cells: In animal cells, cytokinesis occurs through the formation of a cleavage furrow. A contractile ring made of actin and myosin filaments contracts, constricting the cell membrane inward and eventually pinching the cell into two daughter cells.
• Plant Cells: In plant cells, cytokinesis involves the formation of a cell plate. During late stages of mitosis, vesicles containing cell wall materials are brought to the equatorial plane of the cell by the Golgi apparatus. These vesicles fuse together, forming a cell plate that gradually expands outward until it reaches the cell walls. The cell plate then matures into a new cell wall that separates the two daughter cells.
• Centrioles and Spindle Formation:
• Animal Cells: Animal cells typically have centrioles, which are involved in the formation of spindle fibers. Centrioles replicate during the cell cycle and migrate to opposite poles of the cell, organizing the spindle apparatus for chromosome movement.
• Plant Cells: Plant cells lack centrioles, and the spindle fibers are formed without their presence. The microtubules that make up the spindle fibers originate from structures called microtubule organizing centers (MTOCs) located near the nucleus.
• Chromosome Structure:
• Animal Cells: In animal cells, the chromosomes appear as condensed structures with a clearly visible centromere. The centromere is the point where sister chromatids are held together.
• Plant Cells: Plant cells also have condensed chromosomes with a centromere, but their chromosomes tend to be more linear and less easily distinguishable as individual structures due to the presence of a cell wall and other cellular components.
• Nuclear Envelope:
• Animal Cells: During mitosis in animal cells, the nuclear envelope breaks down into smaller vesicles during prophase, allowing the spindle fibers to interact with the chromosomes. In telophase, the nuclear envelope reforms around the separated sets of chromosomes.
• Plant Cells: In plant cells, the nuclear envelope persists throughout mitosis, even during prophase. The spindle fibers pass through nuclear pores, allowing interaction with the chromosomes. During telophase, new nuclear envelopes form around the separated sets of chromosomes at each pole.

These differences in mitosis between plants and animals reflect their distinct cellular structures and the presence of a cell wall in plant cells. Despite these variations, the underlying purpose of mitosis in both plants and animals remains the same: the production of genetically identical daughter cells for growth, development, and tissue repair.

 

77. distinguish between cytokinesis in plants and animals

Cytokinesis is the process by which the cytoplasm of a cell is divided into two daughter cells following nuclear division (mitosis or meiosis). While the overall goal of cytokinesis is the same in plants and animals, there are notable differences in the mechanisms and structures involved. Here are the key distinctions between cytokinesis in plants and animals:

1. Contractile Ring vs. Cell Plate Formation:

Animal Cells: In animal cells, cytokinesis is accomplished by the formation of a contractile ring made of actin and myosin filaments. The contractile ring contracts, constricting the cell membrane inward, leading to the formation of a cleavage furrow. The furrow deepens until it eventually pinches the cell into two daughter cells.

Plant Cells: In plant cells, cytokinesis involves the formation of a cell plate. During late stages of mitosis, vesicles containing cell wall materials are brought to the equatorial plane of the cell by the Golgi apparatus. These vesicles fuse together, forming a cell plate that gradually expands outward until it reaches the cell walls. The cell plate then matures into a new cell wall that separates the two daughter cells.

1. Presence of a Cell Wall:

Animal Cells: Animal cells lack a rigid cell wall, which allows for the formation of a contractile ring and subsequent cell membrane constriction during cytokinesis.

Plant Cells: Plant cells are surrounded by a rigid cell wall composed of cellulose. During cytokinesis, the formation of the cell plate allows for the partitioning of the cell contents and the eventual creation of a new cell wall between the daughter cells.

1. Vesicle Trafficking and Golgi Involvement:

Animal Cells: In animal cells, vesicle trafficking and Golgi involvement in cytokinesis are relatively limited. The contractile ring formation and constriction of the cell membrane primarily drive the process.

Plant Cells: In plant cells, vesicle trafficking and the Golgi apparatus play a crucial role in cytokinesis. The Golgi apparatus produces and transports vesicles containing cell wall materials to the equatorial plane of the dividing cell. These vesicles fuse together to form the cell plate, which eventually matures into a new cell wall.

1. Timing and Progression:

Animal Cells: Cytokinesis in animal cells typically occurs after nuclear division is complete (during late telophase), and it progresses rapidly.

Plant Cells: Cytokinesis in plant cells usually begins during late anaphase or early telophase, simultaneous to nuclear division. The formation and expansion of the cell plate take more time compared to animal cell cytokinesis.

These differences in cytokinesis mechanisms between plants and animals reflect their distinct cellular structures, including the presence of a cell wall in plants and the ability of animal cells to form a contractile ring.

 

82. Draw a labelled diagram of a single flame cell

A flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms, rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, removing waste materials. 

 

 

How to Obtain a Thin Layer of Onion Cells

83. An onion is made up of layers that are separated by a thin membrane. For this experiment, the thin membrane will be used to observe the onion cells. It can easily be obtained by peeling it from any layer of the onion using tweezers.
    How to Prepare a Wet Mount Slid

A thin onion membrane,

Microscopic glass slides,

Microscopic cover slips,

A needle,

Blotting paper,

Dropper,

Iodine Solution,

Water,

Microscope

84. **Note– In microscopy, wet mount refers to a glass slide holding a specimen suspended on a drop of liquid for examinatio

Moreover, to avoid breaking the slide and damage to the microscope objective lenses during observation, it’s important that the optical tube be lowered to the point that the objective lens is as close to the slide as possible.

This should be done starting with low power while looking from the side of the microscope rather than through the eye piece. From this point, it becomes easier to focus for clarity without any accidents.

Add a drop of water at the center of the microscopic slide

Having pulled of a thin membrane from the onion layer, lay it at the center of the microscopic slide (the drop of water will help flatten the membrane)

Add a drop of iodine solution on the onion membrane (or methylene blue)

Gently lay a microscopic cover slip on the membrane and press it down gently using a needle to remove air bubbles.

Touch a blotting paper on one side of the slide to drain excess iodine/water solution,

Place the slide on the microscope stage under low power to observe.

Adjust focus for clarity to observe.

Students can make another slide without adding the stain to see the difference between a stained slide and a non-stained slide.

Observations 

Large, rectangular interlocking cells,

Clearly visible distinct cell walls surrounding the cells,

Dark stained nucleus,

Large vacuoles at the center,

Small granules may be observed inside the cells (within the cytoplasm)

 

 

85. The layers of an onion contain simple sugars (carbohydrates) some of which are stored as starch (starch granules). Given that iodine tends to bind to starch, it stains the starch granules when the two come in to contact making them visible.

Although onions may not have as much starch as potato and other plants, the stain (iodine) allows for the little starch molecules to be visible under the microscope. Although onions are plants, students will not see any chloroplasts in their slides.

This is because of the fact that the chloroplast necessary for photosynthesis is largely present in the leafy part of the onion, which is exposed to the sun and absent in the bulb which is below ground and away from sunlight.

Unlike animal cells, students will also notice that the plant cells have a more regular shape. This is because they have a cell wall made up of cellulose which maintains its shape.

 

86. Describe  each of the following technique used in cell isolation for microscopy

• Teased preparation

A teased preparation is a technique used in microscopy to examine individual cells or fibers that have been separated and spread out in a thin, flat arrangement. It involves teasing or gently separating a tissue or cell sample using fine forceps or needles, allowing the individual components to be visualized and studied under a microscope. Teased preparations are commonly used in histology and cytology to study cellular morphology, structures, and arrangements.

The process of creating a teased preparation typically involves the following steps:

• Tissue or Cell Sample: Obtain a tissue or cell sample of interest. This can be obtained from a biopsy, tissue section, or cell culture.
• Fixation: Fix the tissue or cell sample using an appropriate fixative. Fixation helps preserve the cellular structures and prevents degradation.
• Dissection: Carefully dissect the tissue or cell sample to obtain a small portion for teasing. This can involve cutting or trimming the sample to a manageable size.
• Teasing: Using fine forceps or needles, gently tease or separate the tissue or cell sample. This can be done by pulling or spreading the sample apart, taking care not to damage or distort the individual components.
• Mounting: Transfer the teased sample onto a glass slide or a microscope slide. Place a drop of mounting medium, such as glycerin or mounting media, on the slide and carefully place the teased preparation onto the drop.
• Coverslipping: Cover the teased preparation with a coverslip, ensuring there are no air bubbles trapped underneath.
• Microscopic Examination: Place the slide on a microscope stage and observe the teased preparation under a light microscope. Adjust the focus and magnification to visualize the individual cells or fibers.

Teased preparations are particularly useful for studying the arrangement of cells or fibers in complex tissues, such as nerve cells in neural tissue or muscle fibers in skeletal muscle. By teasing apart the tissue, individual components can be examined more closely, allowing for detailed observations and analysis. This technique helps in understanding cellular organization, identifying abnormalities, and studying the structural characteristics of specific cell types within a tissue.

• Squash preparation

A squash preparation is a technique used in microscopy to create a flattened specimen for observation under a microscope. It is commonly used in cytology and histology to study the morphology and arrangement of cells. The process involves compressing a tissue or cell sample between a slide and coverslip to create a thin, single layer of cells. This allows for easier visualization and examination of cellular structures.

Here is a step-by-step guide to creating a squash preparation:

• Tissue or Cell Sample: Obtain a tissue or cell sample of interest. This can be obtained from a biopsy, tissue section, or cell culture.
• Fixation: Fix the tissue or cell sample using an appropriate fixative. Fixation helps preserve the cellular structures and prevents degradation.
• Transfer: Transfer a small portion of the tissue or a drop of cell suspension onto a clean glass slide.
• Spreading: Using a second glass slide or a coverslip, place it at an angle on top of the sample drop or tissue. Gently and slowly press down on the coverslip or slide to spread the sample and create a thin layer of cells. This will help separate the cells and create a single layer.
• Compress: Apply gentle pressure to the coverslip or slide to further compress the sample. This can be done using the edge of a book, a thumb, or a small weight. The pressure should be applied evenly to avoid damaging the cells.
• Removal of Excess Fluid: Remove any excess fluid around the edges of the coverslip or slide using absorbent paper or a tissue. Be careful not to disturb the sample while doing this.
• Fixation (Optional): If the sample was not previously fixed, it can be fixed at this stage by adding a fixative solution to the slide. This helps to preserve the sample and prevent deterioration.
• Coverslipping: Apply a drop of mounting medium, such as glycerin or mounting media, on top of the compressed sample. Carefully place a coverslip over the sample, starting from one edge and slowly lowering it onto the slide to avoid trapping air bubbles.
• Excess Medium Removal: Gently press on the coverslip to remove any excess mounting medium and ensure the coverslip is properly sealed.
• Microscopic Examination: Place the slide on a microscope stage and observe the squash preparation under a light microscope. Adjust the focus and magnification to visualize the flattened layer of cells.

Squash preparations are particularly useful for studying the shape, size, and arrangement of cells in tissues. By creating a thin, single layer, cellular details are more easily visible, facilitating the examination of cellular structures, nuclei, and other components. Squash preparations are commonly used in cytological studies, such as identifying abnormal cells or studying the progression of cell division.

• Macceration

Maceration is a technique used to soften or separate tissues by soaking them in a liquid. It is commonly used in laboratory settings for various purposes, such as the extraction of specific components from tissues or the preparation of specimens for microscopic examination. The process involves immersing the tissue in a suitable macerating solution for a period of time to allow the tissue to break down or become more pliable.

Here is a general overview of the maceration process:

• Tissue Selection: Choose the tissue or specimen that requires maceration. This can be plant material, animal tissues, or other biological samples.
• Macerating Solution: Prepare a macerating solution appropriate for the tissue or specimen. The macerating solution can vary depending on the specific purpose of maceration. It may include enzymes, acids, alkalis, detergents, or other chemical agents to facilitate tissue breakdown.
• Immersion: Place the tissue or specimen in a container or dish and immerse it in the macerating solution. Ensure that the tissue is fully submerged in the solution.
• Time and Temperature: Determine the appropriate maceration time and temperature based on the type of tissue and desired outcome. This can vary depending on the tissue’s density and composition. Generally, maceration can take several hours to several days.
• Periodic Agitation: Optionally, gently agitate or stir the macerating solution periodically to facilitate the breakdown of the tissue. This can help accelerate the maceration process by allowing the solution to penetrate the tissue more effectively.
• Monitoring: Monitor the progress of maceration regularly by observing the tissue’s texture and flexibility. The tissue should gradually soften or disintegrate, depending on the desired outcome.
• Completion: Once the tissue has reached the desired level of maceration, remove it from the macerating solution. Rinse the tissue thoroughly with water to remove any residual macerating solution.
• Further Processing: Depending on the purpose of maceration, the tissue may undergo additional processing steps. This can include staining, mounting on slides, or further analysis or experimentation.

Maceration is commonly used in various scientific disciplines, including botany, anatomy, and paleontology. It helps prepare tissues for histological studies, extraction of specific compounds from plant materials, or softening of specimens for further examination or dissection. By selectively breaking down or softening tissues, maceration allows for easier manipulation, analysis, and observation of biological samples.

• Touch preparation

A touch preparation, also known as a touch smear or touch imprint, is a technique used to obtain cells or particles from a tissue sample for immediate examination under a microscope. It is a quick and simple method that allows for the assessment of cellular morphology, identification of cell types, and detection of abnormalities or pathogens.

Here is a step-by-step guide to performing a touch preparation:

• Tissue Sample: Obtain a fresh tissue sample of interest. This can be a biopsy specimen or a surgically removed tissue.
• Slide Preparation: Take a clean glass microscope slide and label it with the necessary information. Ensure the slide is clean and free from any contaminants.
• Touching the Tissue: Gently touch the tissue surface of interest with the slide. Apply slight pressure and move the slide in a controlled manner to transfer cells or particles onto the slide. This can be done by touching the cut surface of a tissue, a lesion, or an area of interest.
• Smearing: Once the cells or particles are transferred onto the slide, use a second clean slide to smear and spread the material evenly across the surface of the first slide. This helps to create a thin and even layer of cells.
• Drying: Allow the slide to air-dry completely. Do not use heat or excessive airflow to speed up the drying process, as it may affect the cellular morphology.
• Fixation (Optional): If necessary, the slide can be fixed using an appropriate fixative. This helps to preserve the cellular structure and prevents artifacts.
• Staining: Depending on the purpose of the touch preparation, staining may be performed. Common stains used include hematoxylin and eosin (H&E), which provide general cellular details, or specific stains for special cell types or structures.
• Coverslipping: Once the slide is dried and stained (if applicable), place a coverslip over the sample. Ensure there are no air bubbles trapped underneath the coverslip.
• Microscopic Examination: Place the slide on a microscope stage and examine it under a light microscope. Adjust the focus and magnification to visualize the cells and particles collected from the touch preparation.

The touch preparation technique is particularly useful in situations where immediate assessment of cellular morphology is required, such as during intraoperative consultations or rapid on-site evaluations. It allows for quick visualization of cell types, identification of abnormal cells or pathogens, and immediate decision-making based on the findings. The simplicity and speed of the touch preparation make it a valuable tool in cytology and pathology for rapid diagnostic assessments.