Nutrition in plants is by a process called photosynthesis.
The process of photosynthesis is divided into two stages;
i) Light stage
ii) Dark stage
Photosynthesis is the process by which living plants manufacture their own food in form of carbohydrates (starch) from raw materials i.e. carbon dioxide and water using sunlight energy trapped by chlorophyll and give off oxygen as a bi-product.
In summary photosynthesis is a natural process that;
(i) requires two raw materials (carbon dioxide and water)
(ii) requires two conditions (i.e. chlorophyll and sunlight energy)
(iii) and forms two products namely (starch or carbohydrates & oxygen)
The process of photosynthesis occurs in all green plants in organs called chloroplast most of which are found in leaves.
Chloroplast contains chlorophyll which traps sunlight energy.
The process of photosynthesis is very complicated but it can be summarized by the equations below.
The insoluble starch (storage carbohydrates) is them converted to soluble glucose which is then transported by the phloem to different parts of the plant body especially storage organs like the roots (in cassava and sweat potatoes) stems (in sugarcanes and Irish potato) or transported to actively metabolizing parts of the plants e.g. the growing regions, and the respiring parts.
On reaching these storage organs, the soluble glucose is then reconverted back to insoluble starch for storage.
CONDITIONS NECESSARY FOR PHOTOSYNTHESIS TO TAKE PLACE
Chlorophyll is a green pigment that absorbs light energy from the sun. The amount of chlorophyll present in a leaf is directly related to the rate of photosynthesis.
2) Carbon dioxide:
It is absorbed from the atmosphere by terrestrial plants through their stomata. For aquatic plants like algae, they absorb the carbon dioxide as hydrogen carbonates which diffuse directly from the water in plant tissues. The use of carbon dioxide is to combine or react with hydrogen atoms to form carbohydrates. Thus CO2 is used as a raw material.
This is the source of energy necessary for the process of photosynthesis to take place. The rate of photosynthesis increases in light intensity, up to a maximum when it levels off. The energy of light is used for the following purposes:
i) Used to split water molecules into hydrogen atoms and oxygen. The oxygen is given off by the photosynthesizing plants. The hydrogen atoms combine with CO2 to form carbohydrates.
ii) Provides energy for photosynthesis. The process by which light energy splits water into H+ and oxygen is called photosynthesis of water.
Temperature influences the rate of chemical reactions which are controlled by enzymes which are protein in nature.
The rate of photosynthesis doubles for every ten degrees centigrade (100C) rise in temperature up to about 400C where the rate of photosynthesis drops drastically because the enzymes are denatured
Water is a raw material for the process of photosynthesis. It is absorbed by the root hairs from the soil and transported up the rot by the xylem vessels.
A decrease in the concentration of water lowers the rate of photosynthesis.
Oxygen is not necessary for the process of photosynthesis i.e. it is a bi-product of thus its accumulation instead lowers the rate of photosynthesis.