Question

Explain the three steps in the process of photosynthesis in plant. list four factors which influence this process

Answer 1

The process of photosynthesis occurs in two phases – the light reactions and the dark reactions . Light reactions – As the name suggests, this reaction takes place in the presence of light. Light energy is absorbed by chlorophyll molecules and is utilized for splitting water molecules into hydrogen and oxygen. Additionally in this phase, assimilatory power in the form of ATP and NADPH 2 are produced. Light reactions occur in the membranes of thylakoids. Events occurring during light reactions: Absorption of light energy by chlorophyll molecules Splitting of water molecules into hydrogen and oxygen atoms Formation of ATP and NADPH 2 Dark reaction s – This reaction does not require direct light and occurs in the stroma of chloroplasts. During this phase, ATP and NADPH 2 (formed during light reactions) are utilized for the reduction of CO2 to carbohydrates (food). Event occurring during dark reaction: Reduction of CO 2 to form carbohydrates The various factors that affect rate of photosynthesis are: Light Incident light ∝ CO 2 fixation rate; but at higher light intensities, the rate does not increase further as other factors become limiting Light is rarely a limiting factor (with exception of the shade plants or plants of dense forest) because light saturation occurs at 10% of the full sunlight. Beyond a point, if incident light is increased, then it leads to decrease in photosynthesis due to breakdown of chlorophyll. CO 2 Concentration Major limiting factor Usually low in atmosphere (0.03 − 0.04%) Up to 0.05% − increases rate of CO 2 fixation > 0.05% − damaging effect Though both C 3 and C 4 show increase in rate of photosynthesis at high light intensity accompanied by high CO 2 concentration, the saturation point for C 3 is obtained at higher concentrations as compared to C 4 . Therefore, CO 2 concentration is more of a limiting factor for C 3 plants. Increased CO 2 concentration is beneficial for greenhouse crops such as tomatoes and bell paper. Temperature Dark reactions are more sensitive to an increase in temperature. C 4 plants respond more to an increase in temperature as compared to C 3 plants. Adaptations according to habitat also effect temperature optima. Tropical plants have higher temperature optima compared to temperate. Water Water stress causes stomata to close and hence, less CO 2 is available. Water stress causes the leaves to wilt, thereby reducing their surface area and metabolic activity as well.