Question

Read the given statements and select the correct option. Statement 1: Photorespiration interferes with the successful functioning of Calvin cycle. Statement 2 : Photorespiration oxidises ribulose- 1,5 biphosphate which is an acceptor of \(\mathrm{CO}_{2}\) in Calvin cycle. (a) Both statements 1 and 2 are correct and statement 2 is the correct explanation of statement 1 . (b) Both statements 1 and 2 are correct but statement 2 is not the correct explanation of statement 1 . (c) Statement 1 is correct and statement 2 is incorrect. (d) Both statements 1 and 2 are incorrect.

Short answer

(a) Both statements 1 and 2 are correct and statement 2 is the correct explanation of statement 1.

Step-by-step solution

Understanding Statement 1

Analyze whether photorespiration interferes with the function of the Calvin cycle. Photorespiration is a process that consumes oxygen and releases carbon dioxide, thereby counteracting the photosynthetic process, specifically the Calvin cycle where carbon fixation happens. This implies that Statement 1 is indeed correct.

Understanding Statement 2

Determine if ribulose-1,5-bisphosphate (RuBP) is oxidized by photorespiration and if it is the CO2 acceptor in the Calvin cycle. RuBP is the molecule that reacts with CO2 during the Calvin cycle, initiating the process of carbon fixation. In photorespiration, RuBP is oxidized but not in a way that aids carbon fixation, therefore consuming the RuBP that would be used for CO2 fixation. Thus, Statement 2 is also correct.

Relating Both Statements

Assess if Statement 2 is the correct explanation of Statement 1. Photorespiration consumes RuBP and generates CO2, effectively reducing the efficiency of the Calvin cycle where RuBP is needed to accept CO2. Hence, Statement 2 is indeed the correct explanation of Statement 1.

Key concepts

Calvin Cycle

The Calvin cycle plays a pivotal role in the process of photosynthesis. It is the series of biochemical reactions that occur in the chloroplasts of photosynthetic organisms during which carbon dioxide is converted into organic compounds, particularly glucose. This cycle is also known as the 'dark reaction' because it does not require light to proceed, unlike the 'light reactions' of photosynthesis which capture energy from sunlight.

The Calvin cycle involves three main stages: carbon fixation, reduction, and regeneration of ribulose-1,5-bisphosphate (RuBP). During the first stage, carbon fixation, CO2 from the atmosphere is attached to a five-carbon sugar, RuBP, by the enzyme rubisco. This process eventually leads to the production of a three-carbon sugar, which can then be used to form glucose and other organic molecules. The reduction phase involves the use of ATP and NADPH, produced during light reactions, to convert the three-carbon compound into a usable form. Finally, the regeneration phase replenishes RuBP, allowing the cycle to continue.

However, the efficiency of the Calvin cycle can be impacted by photorespiration—an energetically wasteful pathway that competes with the Calvin cycle for substrates and reduces the net rate of photosynthesis.

Ribulose-1,5-Bisphosphate (RuBP)

Ribulose-1,5-bisphosphate (RuBP) is a critical compound in the photosynthetic process. It's a five-carbon sugar that serves as the carbon dioxide acceptor in the Calvin cycle. The enzyme rubisco catalyzes the reaction between RuBP and CO2, leading to the production of two molecules of 3-phosphoglycerate (3-PGA), which then participate in subsequent steps of the Calvin cycle.

However, when oxygen levels are high, rubisco can catalyze a reaction between RuBP and O2 instead of CO2, resulting in a process known as photorespiration that leads to the production of phosphoglycolate, a compound that can't directly enter the Calvin cycle. This 'mistake' leads to the consumption of RuBP without the production of useful sugars, and it is a factor that can significantly limit the efficiency of photosynthesis, especially under conditions where oxygen is more prevalent than carbon dioxide.

Carbon Fixation

Carbon fixation is a term given to the process of converting inorganic carbon dioxide into organic carbon compounds. It is the first step in the Calvin cycle and is crucial for the synthesis of glucose, which organisms use for energy and as a building block for growth. During this step, the enzyme rubisco facilitates the carboxylation of RuBP, which results in an unstable six-carbon intermediate that quickly breaks down into two molecules of 3-phosphoglycerate (3-PGA).

From an educational perspective, understanding carbon fixation is central to grasping the flow of carbon through ecosystems and the role of photosynthetic organisms in sequestering atmospheric CO2. The importance of carbon fixation extends beyond the biological realm as well; it directly impacts global carbon cycles and climate change discussions, as it pertains to the removal of CO2 from the atmosphere.

Photosynthesis

Photosynthesis is the biochemical process by which green plants, algae, and some bacteria convert light energy into chemical energy, using it to transform water and carbon dioxide into oxygen and glucose. This process is vital for life on Earth because it is the primary source of organic compounds for all organisms and it releases oxygen into the atmosphere.

Photosynthesis consists of two main stages: light-dependent reactions and the Calvin cycle, which operates as part of the light-independent reactions.

• The light-dependent reactions capture energy from sunlight, which powers the synthesis of ATP and NADPH.
• The light-independent reactions, or the Calvin cycle, use ATP and NADPH from the light-dependent reactions to convert CO2 into glucose, which can be used by the plant for energy and growth.

Understanding photosynthesis is not only fundamental for biology students but also for anyone interested in topics such as renewable energy, food security, and environmental sustainability. The process is analogous to solar panels capturing sunlight to produce electricity, but in this case, the end products support life on the planet.