All animals including human beings depend on plants for their food. Green plants, in fact, have to make or rather synthesise the food they need and all other organisms depend on them for their needs. The green plants make or rather synthesise the food they need through photosynthesis and are therefore called autotrophs. The autotrophic nutrition is found only in plants and all other organisms that depend on the green plants for food are heterotrophs.
Green plants carry out ‘photosynthesis’, a physico-chemical process by which they use light energy to drive the synthesis of organic compounds. Ultimately, all living forms on earth depend on sunlight for energy. The use of energy from sunlight by plants doing photosynthesis is the basis of life on earth.
Photosynthesis is the process by which plants, some bacteria and some protistans use the energy from sunlight to produce glucose from carbon dioxide and water.
This glucose can be conerted into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed. Photosynthesis may be summarised by the word equation:
carbon dioxide + water ⟶ glucose + oxygen
The conversion of usable sunlight energy into chemical energy is associated with the action of the green pigment chlorophyll.
Stages of photosynthesis
When chlorophyll a absorbs light energy, an electron gains energy and is ‘excited’. The excited electron is transferred to another molecule (called a primary electron acceptor). The chlorophyll molecule is oxidized (loss of electron) and has a positive charge. Photoactivation of chlorophyll a results in the splitting of water molecules and the transfer of energy to ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP).
The chemical reactions involved include:
• condensation reactions – responsible for water molecules splitting out, including phosphorylation (the addition of a phosphate group to an organic compound)
• oxidation/reduction (redox) reactions involving electron transfer
Photosynthesis is a two stage process.
The light dependent reactions, a light-dependent series of reactions which occur in the grana, and require the direct energy of light to make energy-carrier molecules that are used in the second process:
• light energy is trapped by chlorophyll to make ATP (photophosphorylation)
• at the same time water is split into oxygen, hydrogen ions and free electrons:
2H2O → 4H+ + O2 + 4e- (photolysis)
• the electrons then react with a carrier molecule nicotinamide adenine dinucleotide phosphate
(NADP), changing it from its oxidised state (NADP+) to its reduced state (NADPH):
NADP+ + 2e- + 2H+ → NADPH + H+
The light-independent reactions, a light-independent series of reactions which occur in the stroma of the chloroplasts, when the products of the light reaction, ATP and NADPH, are used to make carbohydrates from carbon dioxide (reduction); initially glyceraldehyde 3-phosphate (a 3-carbon atom molecule) is formed.
Significance of Photosynthesis
1. Green plants possess the green pigment, chlorophyll which can capture, transform, translocate and store energy which is readily available for all forms of life on this planet.
2. Photosynthesis is a process in which light energy is converted into chemical energy
3. Except green plants, no other organism can directly utilise solar energy to synthesize food, hence they are dependent on green plants for their survival.
4. Green plants which can prepare organic food from simple inorganic elements are called autotrophic while all other organisms which cannot prepare their own food are called heterotrophic.
5. During photosynthesis, oxygen liberated into the atmosphere makes the environment livable for all aerobic organisms.
6. Simple carbohydrates produced in photosynthesis are transformed into lipids, proteins, nucleic acids and other organic molecules.
7. Plants and plant products are the major food sources of almost all organisms on the earth.
8. Fossil fuels like coal, gas, and oil represent the photosynthetic products of the plants belonging to early geological periods.
FAQ of Photosynthesis
1. What is photosynthesis?
Photosynthesis is the process by which green plants, in the presence of light combine water and carbon dioxide to form carbohydrates. Oxygen is released as a by product of photosynthesis