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Modes of Nutrition

The process of obtaining food and utilizing it to grow, stay healthy and repair any damaged body part is known as nutrition. Plants produce their food by taking raw materials from their surroundings, such as minerals, carbon dioxide, water and sunlight. There are two modes of nutrition:

1. Autotrophic– Plants exhibit autotrophic nutrition and are called primary producers. Plants synthesis their food by using light, carbon dioxide and water.
2. Heterotrophic– Both animals and human beings are called heterotrophs, as they depend on plants for their food.

Heterotrophic Plants

Listed below are different types of heterotrophic plants that are mainly classified based on their mode of nutrition:

1. Parasitic
2. Insectivorous
3. Saprophytic
4. Symbiotic

Parasitic Nutrition Some heterotrophic plants depend on other plants and animals for nutrition. Such plants are known as parasitic plants. However, the host is not benefitted from the parasite.

For eg., Cuscuta, Cassytha

Insectivorous Nutrition

Some plants have special structural features that help them to trap insects and are commonly known as carnivorous or heterotrophic plants. These plants digest the insects by secreting digestive juices and absorb the nutrients from them. These plants grow on the soil that lacks minerals.

For eg., Pitcher plant, Venus flytrap

Saprophytic Nutrition

The saprophytic plants derive nutrition from dead and decaying plants and animals. They dissolve the dead and decaying matter by secreting digestive juices and absorb the nutrients.

For eg., mushrooms, moulds.

Symbiotic Nutrition When two different plants belonging to two different categories show a close association, they are termed as symbiotic.

In this, both the plants are benefitted from each other. For example ., the association of fungi and trees.

Autotrophic Nutrition in Plants

Photosynthesis

Plants are able to produce their own food through a process called photosynthesis. The chloroplast is the site of photosynthesis.

Food production primarily is carried out in leaves. Water and minerals from the soil are absorbed by the root and transported to the leaves through vessels. Carbon dioxide reaches leaves through stomata – which are small pores on leaves surrounded by guard cells.

Chlorophyll is a green pigment present in leaves which helps the leaves capture energy from sunlight to prepare their food. This production of food which takes place in the presence of sunlight is known as photosynthesis. Hence, the sun serves as the primary source for all living organisms

During photosynthesis, water and carbon dioxide are used in the presence of sunlight to produce carbohydrates and oxygen.

Photosynthesis provides food to all living beings. Oxygen, one of the main components of life on earth is released by plants during photosynthesis.

Photosynthesis

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar. The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O₂) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.

Conditions necessary for Photosynthesis

1. Sunlight
2. Water
3. Carbon dioxide
4. Chlorophyll

The process

During photosynthesis, plants take in carbon dioxide (CO₂) and water (H₂O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Chlorophyll

Inside the plant cell are small organelles called chloroplasts, which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll, which is responsible for giving the plant its green color. During photosynthesis, chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Photosynthesis consists of both light-dependent reactions and light-independent reactions. In plants, the so-called “light” reactions occur within the chloroplast thylakoids, where the aforementioned chlorophyll pigments reside. When light energy reaches the pigment molecules, it energizes the electrons within them, and these electrons are shunted to an electron transport chain in the thylakoid membrane. Every step in the electron transport chain then brings each electron to a lower energy state and harnesses its energy by producing ATP and NADPH. Meanwhile, each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen (Figure 5).

The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH. The light-independent stage, also known as the Calvin Cycle, takes place in the stroma, the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light-independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like glucose, from carbon dioxide. 

Figure 5: The light and dark reactions in the chloroplast

The chloroplast is involved in both stages of photosynthesis. The light reactions take place in the thylakoid. There, water (H2O) is oxidized, and oxygen (O2) is released. The electrons that freed from the water are transferred to ATP and NADPH. The dark reactions then occur outside the thylakoid. In these reactions, the energy from ATP and NADPH is used to fix carbon dioxide (CO2). The products of this reaction are sugar molecules and various other organic molecules necessary for cell function and metabolism. Note that the dark reaction takes place in the stroma (the aqueous fluid surrounding the stacks of thylakoids) and in the cytoplasm.

Once the light reactions have occurred, the light-independent or “dark” reactions take place in the chloroplast stroma. During this process, also known as carbon fixation, energy from the ATP and NADPH molecules generated by the light reactions drives a chemical pathway that uses the carbon in carbon dioxide (from the atmosphere) to build a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). Cells then use G3P to build a wide variety of other sugars (such as glucose) and organic molecules. Many of these interconversions occur outside the chloroplast, following the transport of G3P from the stroma. The products of these reactions are then transported to other parts of the cell, including the mitochondria, where they are broken down to make more energy carrier molecules to satisfy the metabolic demands of the cell. In plants, some sugar molecules are stored as sucrose or starch.

C3 and C4 photosynthesis

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose. C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water.