Why do we need plants?

The process by which plants make food is called photosynthesis. “Photo” means light and “synthesis” means putting together. Plants take in Carbon dioxide from the air, & water from the soil. Put them together in the presence of sunlight and chlorophyll to produce glucose and oxygen. Both the products are needed but not used up.

This process is called Oxygenic photosynthesis. In this process light energy transfers electrons from water and carbon-dioxide to produce carbohydrates. Carbon dioxide is reduced and water is oxidized. Carbon dioxide receives electrons and water loses electrons. Therefore, oxygen is produced along with carbohydrates.

6 molecules of carbon-dioxide and 12 molecules of water are required to form one molecule of carbohydrate, 6 molecules of oxygen and 6 molecules of water. Photosynthesis takes up carbon dioxide produced by all living organisms while breathing. Oxygen gas is given out during photosynthesis, which is an essential gas required by all the living things for breathing. In the absence of photosynthesis the atmosphere would be depleted of oxygen.

Oxygenic photosynthesis take place during the day time and requires sunlight and green coloured pigments chlorophyll present in the leaves. It is an anabolic, constructive process as glucose is being made in the leaves. Light energy is absorbed and stored during this process as glucose or chemical energy.  Therefore photosynthesis is an endothermic reaction. The dry weight of the plant increases during photosynthesis. During this process light energy is converted into potential energy.

Photosynthesis takes place even in algae, fungi and in certain bacteria also.  Ana oxygenic photosynthesis take place in certain bacteria, where they do not produce oxygen. During photosynthesis, the green colour pigment chlorophyll absorbs red colour light. Red, yellow, orange colour pigment absorbs bluish green colour light. Carotenoids and xanthophyll are red and yellow color pigments. Carrots are rich in these pigments. Therefore, carrots appear orange in colour. Phycobilins are red and blue pigments. They absorb light that is not absorbed by carotenoids and chlorophyll. This pigment is seen in Algae and Cyanobacteria.  

In plants, photosynthesis takes place in chloroplasts, which contain the chlorophyll. Chloroplasts are surrounded by a double membrane and contain a third inner membrane, called the thylakoid membrane that forms long folds within the organelle.  The green pigment chlorophyll is located within the thylakoid membrane, and the space between the thylakoid and the chloroplast membranes is called the stroma. Here the transfer of electrons take place.

The pigment molecules are associated with protein molecules. These protein molecules help the pigment molecules to move towards light. A large collection of pigment molecules captures light energy from sun in the form of photons. Light energy is converted into chemical energy in the protein complex.

Photosynthesis complements respiration. During respiration, oxygen is absorbed and carbon dioxide gas is given out. It is the reverse of photosynthesis. It takes place in all the living cells, both green and non- green cells. It also take place both in light and in dark. Respiration is a catabolic process because the food prepared is used up. Energy is released during respiration in the form of ATP. Respiration is an exothermic reaction. The dry weight of the plant decreases. During respiration, potential energy is converted into kinetic energy.

Photosynthesis and cellular respiration are connected through an important relationship. This relationship enables life to survive on this planet. The products of one process are the reactants of the other. Notice that the equation for cellular respiration is the direct opposite of photosynthesis:

·         Cellular Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

·         Photosynthesis: 6CO₂ +6H₂O → C₆H₁₂O₆+ 6O₂

 During photosynthesis glucose formed is used in cellular respiration to make ATP. The glucose is then turned back into carbon dioxide, which is used in photosynthesis.

Water is broken down to form oxygen during photosynthesis. In cellular respiration oxygen is combined with hydrogen to form water.

Photosynthesis requires carbon dioxide and releases oxygen, cellular respiration requires oxygen and releases carbon dioxide.

The released oxygen is used by us and most other organisms for cellular respiration. We breathe in that oxygen, which is carried through our blood to all our cells. In our cells, oxygen allows cellular respiration to proceed. Cellular respiration works best in the presence of oxygen. Without oxygen, much less ATP would be produced.

Cellular respiration and photosynthesis are important parts of the carbon cycle. The carbon cycle is the pathways through which carbon is recycled in the biosphere. While cellular respiration releases carbon dioxide into the environment, photosynthesis pulls carbon dioxide out of the atmosphere. The exchange of carbon dioxide and oxygen during photosynthesis and cellular respiration worldwide helps to keep atmospheric oxygen and carbon dioxide at stable levels.

The products of photosynthesis support respiration, and the products of respiration support photosynthesis. The plants can complete this cycle by themselves. We animals cannot complete the cycle because we cannot perform photosynthesis. Therefore we have to survive through respiration only. We cannot produce glucose by ourselves, we have to get it from eating plants. We produce carbon dioxide that the plants need, and the plants produce the oxygen that we need. We eat the plants to get the glucose we need. Therefore, we need the plants a lot more than they need us!