Photosynthesis

1: Photosynthesis

1. What are the main inputs (reactants) required for overall photosynthesis? Where are they used in the individual process?
2. Why do plants require light to perform photosynthesis? What is the mechanism plans have to utilize light energy in photosynthesis?
3. Which types of plants use the C4 cycle? Which types of plants utilize the Calvin Cycle?
4. Evolution led to the formation of Rubisco that is used by C3 plants to fix CO2. Why do you think that it led to an enzyme that also binds to 02 (which causes photorespiration that isn’t beneficial to the plant)?
5. Explain what is meant by the statement:
   “C4 plants separate C3 and C4 processes in space and CAM plants separate C3 and C4 processes in time.”

The Correct Answer and Explanation is :

1. Main Inputs (Reactants) for Photosynthesis:

The main reactants for photosynthesis are:

• Water (H₂O): This is used in the light-dependent reactions in the thylakoid membranes of the chloroplast. Water molecules are split to release oxygen and provide electrons and protons for the process.
• Carbon Dioxide (CO₂): This is used in the Calvin cycle (light-independent reactions) in the stroma of the chloroplast. CO₂ is fixed into organic molecules in the process of carbon fixation.
• Light Energy: Light is absorbed by chlorophyll in the chloroplasts and is used in the light-dependent reactions to produce ATP and NADPH, which are then used in the Calvin cycle.

2. Why Plants Require Light for Photosynthesis:

Plants require light to perform photosynthesis because light energy drives the light-dependent reactions of photosynthesis. In these reactions, light energy is absorbed by chlorophyll, which excites electrons, initiating the electron transport chain. This process results in the formation of ATP and NADPH, which are crucial for the Calvin cycle. The energy in the form of ATP and NADPH is used to convert CO₂ into glucose, a form of stored energy.

3. C4 and Calvin Cycle Plants:

• C4 Plants: These plants, such as maize, sugarcane, and sorghum, utilize the C4 cycle (also called the Hatch-Slack pathway). The C4 cycle helps to concentrate CO₂ in the bundle-sheath cells, minimizing photorespiration and increasing efficiency in hot, dry environments.
• Calvin Cycle Plants (C3 Plants): Most plants, including rice, wheat, and barley, use the Calvin cycle for carbon fixation. These plants are referred to as C3 plants because the first stable product of CO₂ fixation is a 3-carbon compound (3-phosphoglycerate).

4. Rubisco’s Dual Role in C3 Plants:

Rubisco is an enzyme that catalyzes the fixation of CO₂ in the Calvin cycle. However, it also has the ability to bind to O₂, leading to photorespiration, a process that reduces the plant’s efficiency by wasting energy. The reason evolution led to the formation of Rubisco with dual affinity (for both CO₂ and O₂) could be a trade-off. Early in plant evolution, the atmospheric concentration of O₂ was lower, and Rubisco may have evolved primarily to fix CO₂. As atmospheric O₂ increased, Rubisco’s ability to bind to O₂ led to photorespiration. However, the benefits of having an enzyme that can fix CO₂ outweigh the inefficiencies introduced by photorespiration, especially under conditions where CO₂ is abundant.

5. C4 and CAM Plants: Spatial and Temporal Separation:

The statement refers to how C4 plants and CAM plants manage carbon fixation and minimize photorespiration through spatial and temporal separation:

• C4 Plants: In C4 plants, the processes of CO₂ fixation (C4 cycle) and the Calvin cycle are separated spatially. The C4 cycle occurs in the mesophyll cells, where CO₂ is initially fixed into a 4-carbon compound. The CO₂ is then transported to the bundle-sheath cells, where the Calvin cycle occurs, and CO₂ is released for sugar production. This spatial separation reduces the chances of Rubisco interacting with O₂, thus minimizing photorespiration.
• CAM Plants: In CAM (Crassulacean Acid Metabolism) plants, such as cacti and succulents, the separation of the C4 and Calvin cycles occurs temporally. At night, CAM plants fix CO₂ into organic acids (C4 pathway), storing it as malate. During the day, when stomata are closed to conserve water, the CO₂ is released from these acids and enters the Calvin cycle to produce sugars. This temporal separation helps the plants avoid water loss while still fixing carbon.

In summary, C4 plants spatially separate the two processes by utilizing different cell types, while CAM plants do so by separating them based on the time of day. Both strategies enhance the efficiency of photosynthesis, particularly in arid conditions.