Nutrition & Food

Plants produce food in all green parts, but mostly in their leaves. The greenness of leaves is due to the presence of a chemical called chlorophyll, contained in chloroplasts inside cells of the leaves. Chlorophyll is able to capture sunlight energy, and use it to combine carbon dioxide and water to produce oxygen and glucose (a simple sugar) through a chemical process called photosynthesis. 'Photo' means light and 'synthesis' means to make. So photosynthesis means making food using light energy. 

Like any manufacturing process, photosynthesis needs raw materials and energy in order to make the products:

• Water is absorbed from the soil via osmosis through the roots.

• Carbon dioxide is taken up by diffusion through very small pores on the undersides of its leaves called stomata

• Light energy is captured by chlorophyll in chloroplasts. 

Photosynthesis is a series of chemical reactions where atoms in the reactants (left side) are rearranged to make new products (right side), in the presence of light energy. The word equation and balanced symbol equation for photosynthesis is:

Glucose may be used immediately by the plant for cellular respiration. Cellular respiration is the chemical reaction that occurs in the mitochondria of most cells, in all organisms. During respiration, mitochondria in plant cells use oxygen to break chemical bonds stored between the atoms of glucose to produce energy in the form of ATP. Oxygen comes from the air, entering through the leaf stomata; or from oxygen that is in the soil air and water, entering through the root hairs. ATP is a simple form of energy that organelles can use to make other chemicals needed by the plant (proteins, enzymes, carbohydrates such as cellulose, chlorophyll and flower-colour pigments). 

Respiration produces carbon dioxide and water as well as energy. The carbon dioxide passes out through the stomata, while the water is used by plant or also passes out through the stomata.

Plant cells carry out photosynthesis only in the light, but respiration takes place all the time becacuse it doesn't need light energy to happen. Because there is more photosynthesis than respiration during the day, oxygen is released by plants. At night, while respiration continues, carbon dioxide is released and oxygen taken in. In darkness, starch stores have to be converted back into glucose before it can be used for respiration.

Some of the glucose that is not immediately used by the plant is stored as starch molecules. Starch is a polysaccharide, which is a complex carbohydrate chain made up of many glucose units. Starch is stored in various parts of the plant, including leaves, seeds, tubers and roots. 

When the plant's glucose levels decrease, starch is broken down into glucose, providing energy for plant growth and metabolism. 

We can test for the presence of starch by performing the iodine test. Iodine is an orange/brown solution that turns blue/black in the presence of starch. Other carbohydrates or molecules do not turn iodine blue/black.

Leaves have become very well designed to trap sunlight energy and convert it into stored chemical energy inside glucose molecules.

• Leaves are flat and thin to provide a large surface through which light can enter the cells where photosynthesis takes place.

• Leaves are thin to keep them lightweight. 

• Leaves are numerous and are spread out at the end of branches to maximise surface area.

• The network of veins carry water for photosynthesis, and minerals, from the roots into the leaf. Veins also carry glucose, and other chemicals made in the leaf, to other parts of the plant.

Leaves are always positioned to receive an adequate supply of sunlight without interfering with other leaves. As a result, size, shape and leaf arrangement vary between plants. For example, plants growing where there is little light, such as on the forest floor, often have very large leaves to trap as much sunlight as possible.

If you cut through a leaf and view the cut end under a microscope, you will see layers of cells. Each layer of cells has its own function.

• The thin waxy cuticle is waterproof and tightly packed cells in the epidermis on the upper and lower surface of the leaf allow light in but stop the entry and exit of water and bacteria. 

• The palisade mesophyll layer of cells is the main area for photosynthesis, containing long, thin, closely packed cells with many chloroplasts. This is the top layer of the leaf, where sunlight is strongest.

• Cells in the spongy mesophyll layer contain a few chloroplasts and are loosely packed so carbon dioxide, oxygen and water vapour can move between them.

• Stomata control the gases and water moving in and out of leaves. They do this by opening and closing.

  • • When the stomata are open, plants can get the gases they need into the leaves, but they lose water by transpiration.

    • When the stomata are closed, the plant doesn't lose any water, but it can't get the gases it needs to photosynthesise and grow.

How fast photosynthesis goes depends on the availability of the raw materials water and carbon dioxide. It also depends on the intensity of sunlight and the temperature of the air. (All reactions go faster in warmer conditions.) The rate of photosynthesis can be measured by the growth in size of plants or by the production of oxygen.

• • If a plant gets more water, carbon dioxide and sunlight it should be able to increase how much photosynthesis it does. If any of these three factors are limited (in short supply) the rate of photosynthesis will not increase.

  • If a plant can do more photosynthesis it will produce more glucose. This glucose is used by the plant to grow and reproduce. When a plant grows it produces more or larger leaves, which are able to do more photosynthesis, which leads to more growth.

  • If the plants leaves are used as food that means we get more food. If the plants fruits or seeds are eaten by us, then a plant doing more photosynthesis may produce more of these too.

If we want to grow more food, we need to make sure plants are grown in an area that gets at least 6 hours of direct sunlight or artificial light every day. We must also make sure the plants receive the right amount of water. Too much water and the plants roots will rot, too little water and the plant won’t be able to do photosynthesis.

For example a plant growing in the sun in a vegetable garden in summer, which is watered every day by a gardener will grow larger in size than a plant growing in the shade which is only watered once a week. This picture is of soybean leaves: the ones on the left were grown in the shade and on the right were grown in full sun.