CSEC Biology: Types of Nutrition and Photosynthesis

Nutrition can be described as the process by which living organisms obtain food and convert it into energy required for life.

There are three types of nutrition:

Autotrophic Nutrition- This is carried out by some bacteria and green plants known as autotrophs. They synthesize their complex food molecules (high in energy) from inorganic, simple molecules (low in energy) such as carbon dioxide and water and energy from sunlight. The most common form of autotrophic nutrition is photosynthesis, but other forms exist such as chemosynthesis.

Heterotrophic Nutrition- This occurs in most animals, fungi and bacteria, known as heterotrophs. They feed on other organisms which already contain 'ready-made' complex organic molecules to obtain food. Heterotrophic nutrition can be further broken down into holozoic, saprophytic and parasitic nutrition.

  • Holozoic Nutrition- In this mode of nutrition, the organism engulfs (ingests) the food into its body, where it is digested (i.e. broken down into simpler molecules) and the soluble products are absorbed. This occurs in most animals.

  • Saprophytic Nutrition- These organisms, known as saprophytes, do not ingest solid food. Rather, they secrete digestive enzymes onto dead or decaying organic matter, so the complex molecules are broken down outside of their bodies. They then absorb the simpler products of digestion. This is carried out by most bacteria and fungi, as well as houseflies.

  • Parasitic Nutrition- In this mode of nutrition, the organism, known as a parasite, lives on or inside the body of another organism known as the host. The parasite survives by absorbing liquid organic food materials from the host. In this 'relationship' (if you could call it that) the host is usually harmed. Examples of parasites include tapeworms, roundworms and certain bacteria.

Symbiotic Nutrition- CSEC doesn't require that you know this one, but it is still useful information. In this mode of nutrition two organisms live in association and receive food from each other. This interdependence is called mutualism. For example, E. coli lives in your large intestine. It synthesizes vitamin B12 which you use, and in return, E. coli gains simpler organic molecules from your digestive tract.


Photosynthesis is the process by which green plants and some bacteria convert the inorganic simple molecules of carbon dioxide and water using the energy from sunlight absorbed by chlorophyll in chloroplasts into glucose, with oxygen as a waste product.

This process can be summarized using the following equation:

6CO2 + 6H2O → C6H12O6 + 6O2 Sunlight energy

carbon dioxide + water → glucose + oxygen

The process of photosynthesis may occur in any part of a plant containing chlorophyll (photosynthesis occurs in the chloroplasts), but it mainly occurs in the leaves.

Photosynthesis has two stages, the light stage (or light-dependent stage) and the dark stage (or light-independent stage):

  • The light stage requires the presence of light to occur. The light energy from sunlight is absorbed by chlorophyll in the chloroplasts, where it is converted to chemical energy. Some of this energy is used to split water water molecules into hydrogen and oxygen (called photolysis) and some of it is stored as ATP. The oxygen evolved from photolysis is diffused out of the leaf as a waste product.

  • The dark stage doesn't require light to occur (as you probably guessed), but it does require enzyme activity, and the energy stored as ATP in the light stage. The hydrogen atoms produced from the light stage are used to reduce carbon dioxide to glucose (reducing is basically causing something to gain electrons). This is known as carbon fixation, and it is controlled by specific enzymes

What happens to the products of photosynthesis?

The products of photosynthesis, oxygen and glucose have different fates. The oxygen, as discussed previously, is diffused out of the leaf in the light stage. However, glucose can be used for several applications in the plant. Glucose can be broken down through respiration to generate ATP (energy). This energy can then be used in cell division to assist in plant growth. Glucose can be used in protein synthesis (eg chlorophyll) by combining it with nitrates.

However, glucose can also be stored in the cells of the plant as starch. Glucose must be converted to starch for storage because starch is insoluble and far less reactive than glucose. The starch can then be hydrolysed back into glucose when more is needed.

Glucose may also be converted to sucrose so that it can be transported throughout the plant. Sucrose is far less reactive than glucose, so it is easier to transport.

The structure of the leaf is adapted to complete photosynthesis as efficiently as possible. You may look at the thin structure of the average leaf and wonder how it is capable of synthesizing its own food. This flat part you see is known as the lamina. The lamina of each leaf is made up of several layers of cells.

Using the following diagrams, the adaptations of the leaf (external and internal) will be explained.

Diagram of the External Structure of a Leaf and the Adaptations
Diagram of the External Structure of a Leaf

The waxy cuticles on the upper and lower epidermis are waterproof, and so can prevent the unnecessary loss of water by the leaf.

The stomata, surrounded by the guard cells on the lower epidermis, allow carbon dioxide to diffuse in and oxygen to diffuse out.

The palisade mesophyll cells are located right underneath the upper epidermis, meaning they are closest to the sunlight. They, therefore, have large number of chloroplasts to maximize how much light is absorbed.

The air spaces in the spongy mesophyll allow carbon dioxide to diffuse up to the palisade cells and oxygen to diffuse from the palisade cells.

Xylem vessels and phloem sieve tubes in the veins running throughout the mesophyll cells supply them with materials such as water and transport the soluble sucrose converted from glucose produced in photosynthesis away to other parts of the plant.

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