Question

How do plants make carbohydrates?

Answer 1

Plants produce substances from processes that act as a source of food for the entire plant. They produce substances that can be broken down with minimum wastage of energy.

Complete answer:
- Plants make carbohydrates from photosynthesis.
- Photosynthesis requires sunlight, carbon dioxide, and water as starting materials. The reaction forms glucose and oxygen. The glucose is further stored in the form of starch in the plant.
- Photosynthesis is the chemical combination or fixation of carbon dioxide and water, by using energy obtained from absorbing light.
- The process is carried out by membrane-covered organelles known as chloroplasts. The chloroplasts store pigments that efficiently absorb light, chlorophyll a and chlorophyll b.
- Photosynthesis is divided into two parts - the light-dependent reactions and the light-independent reactions (the Calvin cycle).
- The equation for photosynthesis is $6CO_{2} + 6H_{2}O \rightarrow C_{6}H_{12}O_{6} + 6O_{2}$.

Additional Information:
- Plants break down carbohydrates to gain energy during respiration.
- Respiration leads to the formation of carbon dioxide. Photosynthesis and cellular respiration function in a biological cycle, hence, both oxygen and carbon dioxide are required by plants.
- The oxygen obtained is formed from the oxidation of water, not carbon dioxide. This was proved from an experiment where photosynthesis was carried out in the presence of water labeled with $_{}^{18}\textrm{O}$ which showed $_{}^{18}\textrm{O}$ in the oxygen released, while carbon dioxide labeled with $_{}^{18}\textrm{O}$ did not show any labeled oxygen.

Note:
- Each gram of carbohydrate gives around four calories of energy to the body.
- In plants, carbohydrates form a part of cellulose, the polymer responsible for providing structure and strengths to the plant.
- In the human body, carbohydrates are stored in the form of glycogen or starch.