Question

Which of the following statements about dark reactios! correct? (a) They occur in darkness. (b) They are not light dependent. (c) They are dependent upon the products synthese. during light reactions. (d) All of these.

Short answer

(b) and (c) are correct. Dark reactions are not directly light-dependent but do rely on the products synthesized during light reactions.

Step-by-step solution

Understanding Dark Reactions

The term 'dark reactions' refers to the part of photosynthesis that does not require light to occur. These reactions can technically happen in both light and dark conditions, as long as the necessary products from light reactions are available.

Evaluating the Given Statements

Assess each statement individually to determine which accurately describe the nature of dark reactions. (a) This is misleading, as dark reactions can occur in light as well, so long as the necessary products are present. (b) This is correct, they are not directly driven by light. (c) This is correct, they do rely on the ATP and NADPH produced in the light-dependent reactions.

Determining the Correct Statement

Based on the evaluation in step 2, statements (b) and (c) are correct in describing the dark reactions of photosynthesis. Statement (a) could be misleading. Therefore, the most comprehensive answer is that statements (b) and (c) are correct about dark reactions.

Key concepts

Photosynthesis

Photosynthesis is a crucial biological process used by plants, algae, and certain bacteria to convert light energy into chemical energy. This process enables these organisms to produce glucose, which acts as a fundamental source of energy to support their growth and physiological activities. The entire photosynthesis process is categorized into two main phases: the light-dependent reactions and the light-independent reactions, often referred to as the Calvin Cycle.

During the light-dependent reactions, which occur in the thylakoid membranes of the chloroplasts, sunlight is captured by the chlorophyll and converted into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These molecules carry the captured energy to the next stage of photosynthesis, the light-independent reactions, where the actual synthesis of glucose occurs.

Light-Independent Reactions

The light-independent reactions, also known as the dark reactions or the Calvin Cycle, take place in the stroma of the chloroplasts. Contrary to their name, they do not require darkness to occur; rather, they are independent of light, functioning as long as they are supplied with ATP and NADPH from the light-dependent reactions.

These reactions utilize carbon dioxide from the atmosphere, along with the ATP and NADPH, to produce glucose. The Calvin Cycle consists of three main steps: carbon fixation, reduction phase, and regeneration of the initial CO₂ acceptor. Notably, the cycle must be completed multiple times to synthesize a single molecule of glucose. This phase is crucial for the synthesis of various organic compounds essential for plant growth and survival.

ATP

ATP, or adenosine triphosphate, plays the role of an energy currency in the cell. It's created during the light-dependent reactions of photosynthesis and then utilized in the Calvin Cycle to provide the energy required for synthesizing glucose from carbon dioxide.

Structurally, ATP consists of an adenine base, a ribose sugar, and three phosphate groups. The energy stored in ATP is held within the bonds between these phosphate groups. When ATP is used in cellular processes, it loses one phosphate group and releases energy, becoming ADP (adenosine diphosphate). Organisms continually recycle ADP back into ATP, using energy from various sources, such as the sun in the case of photosynthetic organisms.

NADPH

NADPH stands for nicotinamide adenine dinucleotide phosphate, an essential coenzyme in both the anabolic reactions, such as the Calvin Cycle, and catabolic processes like the oxidative phase of the pentose phosphate pathway. In photosynthesis, NADPH functions as a powerful reducing agent, transferring high-energy electrons to carbon dioxide molecules to facilitate their transformation into glucose.

This molecule is the product of the light-dependent reactions and, alongside ATP, is transported to the Calvin Cycle where it delivers the electrons necessary for the synthesis of organic molecules. After donating its electrons, NADPH is oxidized to NADP⁺, which can then be recycled back into NADPH during the light-dependent reactions.