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Functions of Cell

A cell performs major functions essential for the growth and development of an organism. Important functions of cell are as follows:

1. Provides Support and Structure

All the organisms are made up of cells. They form the structural basis of all the organisms. The cell wall and the cell membrane are the main components that function to provide support and structure to the organism. For eg., the skin is made up of a large number of cells. Xylem present in the vascular plants is made of cells that provide structural support to the plants.

1. Facilitate Growth Mitosis

In the process of mitosis, the parent cell divides into the daughter cells.

Thus, the cells multiply and facilitate the growth in an organism.

1. Allows Transport of Substances

Various nutrients are imported by the cells to carry out various chemical processes going on inside the cells. The waste produced by the chemical processes is eliminated from the cells by active and passive transport. Small molecules such as oxygen, carbon dioxide, and ethanol diffuse across the cell membrane along the concentration gradient. This is known as passive transport. The larger molecules diffuse across the cell membrane through active transport where the cells require a lot of energy to transport the substances.

1. Energy Production

Cells require energy to carry out various chemical processes. This energy is produced by the cells through a process called photosynthesis in plants and respiration in animals.

1. Aids in Reproduction

A cell aids in reproduction through the processes called mitosis and meiosis. Mitosis is termed as the asexual reproduction where the parent cell divides to form daughter cells. Meiosis causes the daughter cells to be genetically different from the parent cells. Thus, we can understand why cells are known as the structural and functional unit of life. This is because they are responsible for providing structure to the organisms and performs several functions necessary for carrying out life’s processes.

Major Physiological  processes

Diffusion, osmosis, facilitated diffusion, and active transport are all processes involved in the movement of substances across cell membranes.

1. Diffusion:  Diffusion refers to the process by which particles (such as ions, molecules, or other small substances) move across cell membranes from an area of higher concentration to an area of lower concentration. It is a fundamental mechanism that plays a vital role in various cellular processes, including nutrient uptake, waste removal, and cell signaling.

   Diffusion occurs due to the random thermal motion of particles. The kinetic energy of particles causes them to move in a zigzag pattern, leading to their spreading out and eventually becoming evenly distributed in a system. The concentration gradient, which is the difference in concentration between two regions, is the driving force behind diffusion.

   Cell diffusion can occur in two ways:

   1. Passive Diffusion: Passive diffusion is the spontaneous movement of particles across the cell membrane without the need for energy input. It is driven solely by the concentration gradient. Small, nonpolar molecules, such as oxygen and carbon dioxide, can passively diffuse across the lipid bilayer of the cell membrane. The rate of passive diffusion depends on factors like the size, charge, and lipid solubility of the molecules involved.

   2. Facilitated Diffusion: Facilitated diffusion is a type of diffusion that involves the assistance of specific transport proteins to facilitate the movement of larger, polar, or charged molecules across the cell membrane. These transport proteins can be either carrier proteins or channel proteins. Carrier proteins bind to the specific molecule they transport and undergo conformational changes to transport the molecule across the membrane. Channel proteins form hydrophilic channels that allow ions or small molecules to pass through. Facilitated diffusion still follows the concentration gradient and does not require energy expenditure.

   Cell diffusion is essential for maintaining the balance of molecules and ions inside cells. It enables the uptake of essential nutrients, such as glucose and amino acids, and the removal of waste products, such as carbon dioxide and metabolic byproducts. Additionally, diffusion is involved in cell signaling processes, where molecules diffuse across the extracellular space and bind to receptors on neighboring cells to initiate specific cellular responses.

   While diffusion is an important process, it has its limitations. Diffusion alone is not sufficient for efficient transport over long distances or for moving against a concentration gradient. In such cases, cells rely on other mechanisms such as active transport or osmosis to accomplish the desired movement of substances.

   Factors Affecting Diffusion 

   1. Concentration Gradient
   2. An increase in the concentration of molecules at one region results in a steeper concentration gradient which in turn increases the rate of diffusion.
   3. Temperature
      High temperature increases kinetic energy of molecules. They move faster hence resulting in an increase in rate of diffusion, and vice versa.
   4. Size of Molecules or Ions

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

   5. Density
      The denser the molecules or ions diffusing, the slower the rate of diffusion, and vice versa.
   6. 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.
   7. 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.

   8. 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.

   ​Role of Diffusion in Living Organisms 

   Some processes that depend on diffusion include the following:

   1. Gaseous exchange:Movement of gases through respiratory surfaces is by diffusion.
   2. Absorption of materials into cells: cells obtain raw materials and nutrients from the surrounding tissue fluid and blood through diffusion, e.g., glucose needed for respiration diffuses from blood and tissue fluid into cells.
   3. Excretion:Removal of metabolic waste products like carbon (IV) oxide, and ammonia out of cells is by diffusion.
   4. Absorptionof the end-products of digestion from the intestines is by diffusion

Osmosis:

Osmosis is a specific type of passive transport that involves the movement of water molecules across a selectively permeable membrane, such as a cell membrane or a semipermeable membrane, from an area of lower solute concentration to an area of higher solute concentration. It is driven by the concentration gradient of solute particles.

In osmosis, water molecules move across the membrane to equalize the concentration of solutes on both sides of the membrane. The movement of water continues until the concentration of solutes, or the osmotic pressure, is the same on both sides of the membrane. Osmosis is a vital process for maintaining the balance of water and solutes within cells and plays a crucial role in various biological systems.

To understand osmosis, let’s consider two solutions separated by a semipermeable membrane:

1. Hypertonic Solution: A hypertonic solution has a higher solute concentration compared to the other solution. When a hypertonic solution is separated from a hypotonic solution (lower solute concentration) by a semipermeable membrane, water molecules move from the hypotonic solution to the hypertonic solution through osmosis. This causes the hypertonic solution to gain water and dilute its solute concentration.

2. Hypotonic Solution: A hypotonic solution has a lower solute concentration compared to the other solution. When a hypotonic solution is separated from a hypertonic solution by a semipermeable membrane, water molecules move from the hypotonic solution to the hypertonic solution through osmosis. This results in the hypotonic solution losing water and causing the hypertonic solution to become more concentrated.

3. Isotonic Solution: An isotonic solution has an equal solute concentration compared to the other solution. When two solutions of equal solute concentration are separated by a semipermeable membrane, there is no net movement of water through osmosis. The concentrations of solutes remain balanced on both sides of the membrane.

Osmotic Pressure 

The term osmotic pressure describes the tendency of the solution with a high solute concentration to draw water into itself when it is separated from distilled water or dilute solution by a semi-permeable membrane.

Osmotic pressure is measured by an osmometer.

When plant cells are placed in distilled water or in a hypotonic solution, the osmotic pressure in the cells is higher than the osmotic pressure of the medium.

This causes the water to enter the cells by osmosis.

The water collects in the vacuole which increases in size. As a result the cytoplasm is pushed outwards and it in turn presses the cell membrane next to the cell wall. This builds up water pressure (hydrostatic pressure) inside the cell.

When the cell is stretched to the maximum, the cell wall prevents further entry of water into the cell. Then the cell is said to be fully turgid. The hydrostatic pressure developed is known as turgor pressure.

Plasmolysis 

When a plant cell is placed in a hypertonic medium, it loses water by osmosis.

The osmotic pressure of the cell is lower than that of the medium.

The vacuole decreases in size and the cytoplasm shrinks as a result of which the cell membrane loses contact with the cell wall.

The cell becomes flaccid. The whole process is described as plasmolysis.

Incipient plasmolysis is when a cell membrane just begins to lose contact with the cell wall.

Plasmolysis can be reversed by placing the cell in distilled water or hypotonic solution.However, full plasmolysis may not be reversed if cell stays in that state for long. 

​Wilting

 The term wilting describes the drooping of leaves and stems of herbaceous plants after considerable amounts of water have been lost through transpiration.

It is observed in hot dry afternoons or in dry weather.

This is when the amount of water lost through transpiration exceeds the amount absorbed through the roots.

Individual cells lose turgor and become plasmolysed and the leaves and stems droop. The condition is corrected at night when absorption of water by the roots continue while transpiration is absent.

Eventually, wilting plants may die if the soil water is not increased through rainfall or watering. 

​Haemolysis 

Haemolysis is the bursting of cell membrane of red blood cells releasing their haemoglobin. It occurs when red blood cells are placed in distilled water or hypotonic solution. This is because the cell membrane does not resist further entry of water by osmosis after maximum water intake. 

​Crenation 

Takes place when red blood cells are placed in hypertonic solution.They lose water by osmosis, shrink and their shape gets distorted.Animal cells have mechanisms that regulate their salt water balance (osmoregulation) to prevent above processes that lead to death of cells.An Amoeba placed in distilled water, i.e. hypotonic solution, removes excess water using a contractile vacuole.The rate of formation of contractile vacuoles increases.

Factors Affecting Osmosis 

1. Size of solute molecules-
2. Osmosis’ occurs only when solute molecules are too large to pass through a semi-permeable membrane.
3. Concentration Gradient.
4. Osmosis occurs when two solutions of unequal solute concentration are separated by a semi-permeable membrane.
5. High temperatures increase movement of water molecules hence influence osmosis. However, too high temperatures denature proteins in cell membrane and osmosis stops.
6. Pressure
   Increase in pressure affects movement of water molecules.
7. As pressure increases inside a plant cell, osmosis decreases.

Roles of Osmosis in Living Organisms 

The following processes depend on osmosis in living organisms:

Movement of water into cells from the surrounding tissue fluid and also from cell to cell.

1. Absorption of water from the soil and into the roots of plants.
2. Support in plants especially herbaceous ones, is provided by turgor pressure, which results from intake of water by osmosis.
3. Absorption of water from the alimentary canal in mammals.
4. Re-absorption of water in the kidney tubules.
5. Opening and closing stomata. 

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Active Transport:

Active transport is a cellular process that allows the movement of ions or molecules across a cell membrane against their concentration gradient, from an area of lower concentration to an area of higher concentration. Unlike passive transport processes like diffusion and facilitated diffusion, active transport requires the expenditure of energy, usually in the form of adenosine triphosphate (ATP).

In active transport, specific transport proteins called pumps are involved. These pumps are integral membrane proteins that span the cell membrane and undergo conformational changes to move molecules across the membrane. The energy required for these conformational changes is provided by ATP hydrolysis.

The  key features and examples of active transport are given below;

1. Energy Requirement: Active transport involves the direct use of energy to move molecules against their concentration gradient. ATP provides the necessary energy for the transport process.

2. Specificity: Active transport is highly specific, as different transport proteins are responsible for moving specific ions or molecules. For example, the sodium-potassium pump moves three sodium ions out of the cell and two potassium ions into the cell for each ATP hydrolyzed.

3. Primary Active Transport: In primary active transport, the energy for transport comes directly from ATP hydrolysis. Examples include the sodium-potassium pump, which plays a crucial role in maintaining the concentration gradients of sodium and potassium ions across the cell membrane.

4. Secondary Active Transport: In secondary active transport, the energy required for transport is obtained from the electrochemical gradient established by primary active transport. For instance, the sodium-glucose cotransporter uses the sodium gradient established by the sodium-potassium pump to transport glucose against its concentration gradient.

5. Ion Pumps: Many active transport processes involve the movement of ions across the cell membrane. Examples include the proton pump in the stomach lining that helps maintain gastric acidity and the calcium pump that maintains calcium ion homeostasis in cells.

6. Vesicular Transport: Some forms of active transport involve the movement of large molecules or particles through vesicles. Endocytosis and exocytosis are examples of vesicular transport mechanisms that require energy to transport substances into and out of the cell.

Factors Affecting Active Transport:

1. Availability of oxygen 

Energy needed for active transport is provided through respiration.An increase in the amount of oxygen results in a higher rate of respiration. If a cell is deprived of oxygen active transport stops. 

2. Temperature 

Optimum temperature is required for respiration, hence for active transport. Very high temperatures denature respiratory enzymes. While very low temperatures inactivate enzymes too and active transport stops. 

3. Availability of carbohydrates 

Carbohydrates are the main substrates for respiration. Increase in amount of carbohydrate results in more energy production during respiration and hence more active transport. Lack of carbohydrates causes active transport to stop. 

4. Metabolic poisons 

Metabolic poisons e.g. cyanide inhibit respiration and stops active transport due to lack of energy. 

​Role of Active Transport in Living Organisms 

1. Absorption of mineral salts from the soil into plant roots.
2. Absorption of end products of digestion e.g. glucose and amino acids from the digestive tract into blood stream.
3. Excretion of metabolic products e.g. Urea from the cells.
4. Re-absorption of useful substances and mineral salts back into blood capillaries from the kidney tubules.
5. Sodium-pump mechanism in nerve cells.
6. Re-absorption of useful materials from tissue fluid into the blood stream.