Which one of the following statements about the process of photosynthesis is correct?
Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide.
Understanding the Process of Photosynthesis
Photosynthesis is a vital process carried out by plants, algae, and some bacteria. It's how these organisms convert light energy into chemical energy in the form of glucose (sugar). This process uses carbon dioxide and water as raw materials and releases oxygen as a byproduct.
The overall equation for photosynthesis is often represented as:
\( 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \)
Let's break down the key events involved in photosynthesis and evaluate the given statements.
Stages of Photosynthesis
Photosynthesis occurs in two main stages:
- Light-dependent reactions: These happen in the thylakoid membranes of chloroplasts. Chlorophyll and other pigments absorb light energy. This energy is used to split water molecules (\(\text{H}_2\text{O}\)), a process called photolysis, which releases oxygen (\(\text{O}_2\)), electrons, and protons. The energy from light is converted into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are energy-carrying molecules.
- Light-independent reactions (Calvin cycle): These reactions take place in the stroma of the chloroplasts and do not directly require light, but they use the ATP and NADPH produced during the light-dependent reactions. In this stage, carbon dioxide (\(\text{CO}_2\)) from the atmosphere is 'fixed' and reduced using the energy from ATP and the reducing power of NADPH to synthesize glucose.
Analyzing the Statements about Photosynthesis
Let's look at each statement in the context of the photosynthesis process:
- Statement 1: "Light energy is directly converted to kinetic energy which oxidizes carbon dioxide."
- Light energy is converted to chemical energy (ATP and NADPH), not kinetic energy.
- Carbon dioxide (\(\text{CO}_2\)) is reduced, not oxidized, during photosynthesis. Oxidation involves losing electrons, while reduction involves gaining electrons. In the Calvin cycle, carbon from \(\text{CO}_2\) gains electrons as it becomes part of glucose.
- Therefore, this statement is incorrect.
- Statement 2: "Light energy is directly converted to chemical energy which causes reduction of water."
- Light energy is indeed converted to chemical energy (ATP and NADPH).
- However, water (\(\text{H}_2\text{O}\)) is split (oxidized), not reduced, during the light-dependent reactions. It loses electrons and splits into protons, electrons, and oxygen.
- Therefore, this statement is incorrect.
- Statement 3: "Chlorophyll absorbs the chemical energy of light which causes oxidation of carbon dioxide."
- Chlorophyll absorbs light energy, not the 'chemical energy of light'. Light energy is electromagnetic energy.
- Carbon dioxide (\(\text{CO}_2\)) is reduced, not oxidized, during photosynthesis.
- Therefore, this statement is incorrect.
- Statement 4: "Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide."
- Chlorophyll absorbs light energy.
- This absorbed light energy drives the splitting of water molecules (photolysis) in the light-dependent reactions.
- The energy (ATP) and reducing power (NADPH) produced from the light-dependent reactions are then used to reduce carbon dioxide (\(\text{CO}_2\)) in the light-independent reactions (Calvin cycle) to form glucose.
- This statement accurately describes key events in photosynthesis.
Based on the analysis, the fourth statement correctly describes the process of photosynthesis.
Revision Table: Key Events in Photosynthesis
| Event | Input | Location | Requires Light? | Role of Chlorophyll | Outcome |
|---|---|---|---|---|---|
| Light Absorption | Light energy | Thylakoid membranes | Yes | Absorbs light energy | Energizes electrons |
| Water Splitting (Photolysis) | \(\text{H}_2\text{O}\) | Thylakoid membranes | Yes (driven by light energy) | Indirect (via light energy) | \(\text{O}_2\) released, electrons & protons generated |
| Energy Conversion | Light energy | Thylakoid membranes | Yes | Direct (via light absorption) | ATP and NADPH produced (Chemical Energy) |
| Carbon Dioxide Reduction | \(\text{CO}_2\), ATP, NADPH | Stroma | No (directly), but uses products of light reactions | Indirect (via ATP/NADPH) | Glucose (\(\text{C}_6\text{H}_{12}\text{O}_6\)) synthesized |
Additional Information on Photosynthesis
- Chloroplasts: These are the organelles in plant cells where photosynthesis takes place. They contain chlorophyll.
- Chlorophyll: The primary pigment that absorbs light energy, mainly in the blue and red parts of the spectrum, reflecting green light (which is why plants look green).
- ATP: Adenosine triphosphate, an energy currency molecule used by cells.
- NADPH: Nicotinamide adenine dinucleotide phosphate, a molecule that carries electrons (reducing power) used to reduce \(\text{CO}_2\).
- Stomata: Small pores on the surface of leaves that allow \(\text{CO}_2\) to enter the leaf and \(\text{O}_2\) and water vapor to exit.
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