Prep Guide: Photosynthesis

Key Concepts (Recall & Understanding)

Photosynthesis: The process of converting light energy into chemical energy (glucose) using CO₂, water, and chlorophyll. The overall equation is 6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂.
Chloroplast: The site of photosynthesis, containing grana (stacks of thylakoids) and stroma (fluid-filled space).
Phases of Photosynthesis:
1. Light-Dependent Reactions (in Thylakoids):
  - Light energy activates chlorophyll.
  - Photolysis of water: Water is split to produce H+, electrons, and oxygen.
  - Energy is captured in the form of ATP and NADPH.
2. Light-Independent Reactions / Calvin Cycle (in Stroma):
  - CO₂ is "fixed" using the enzyme RuBisCO.
  - ATP and NADPH from the light reactions are used to convert CO₂ into glucose.
Leaf Adaptations: Large surface area, thinness, cuticle, stomata, palisade and spongy mesophyll layers, and vascular bundles are all adaptations for efficient photosynthesis.
Destarching: The process of removing starch from a plant by keeping it in the dark, to ensure that any starch detected in an experiment is a result of photosynthesis during the experiment.
Carbon Cycle: The movement of carbon between the atmosphere, oceans, land, and living organisms. Photosynthesis is a key component, removing CO₂ from the atmosphere.

Application Corner

An experiment is set up with an aquatic plant (Hydrilla) in a beaker of water under sunlight, and bubbles are observed rising from the plant. What gas are these bubbles composed of, and which specific process in photosynthesis produces this gas?
- Answer: The bubbles are oxygen (O₂). This gas is produced during the photolysis of water in the light-dependent reactions.
A leaf from a destarched plant is partially covered with black paper and then exposed to light. When the starch test is performed, only the uncovered parts turn blue-black. What does this experiment prove?
- Answer: This experiment proves that light is necessary for photosynthesis. Starch is only produced in the parts of the leaf that were exposed to light.

Analytical Thinking

Odd One Out: ATP, NADPH, Oxygen, Glucose.
- Odd One: Glucose.
- Category: The rest are products of the light-dependent reactions of photosynthesis. (Glucose is a product of the light-independent reactions).
Scenario: A plant is grown in an atmosphere that is rich in oxygen but has very low levels of carbon dioxide. How will this affect the plant's ability to produce glucose? Explain your answer in the context of the phases of photosynthesis.
- Answer: The plant's ability to produce glucose will be severely limited. The light-dependent reactions can still occur, producing ATP and NADPH. However, the light-independent reactions (Calvin Cycle) require carbon dioxide as the carbon source to build glucose molecules. Without sufficient CO₂, the Calvin Cycle cannot proceed, and glucose cannot be synthesized, regardless of how much ATP and NADPH are available.

Key Case Study

The Importance of Rainforests in the Global Carbon Cycle

Tropical rainforests, like the Amazon, are often called the "lungs of the planet." This is because they are home to a vast number of trees and plants that carry out photosynthesis on an enormous scale. Through this process, they absorb massive quantities of carbon dioxide (a major greenhouse gas) from the atmosphere and release oxygen. This makes them a critical carbon sink, helping to regulate global climate. However, widespread deforestation for agriculture and logging is destroying these rainforests. When trees are burned or decompose, the carbon they have stored is released back into the atmosphere as CO₂. This case study highlights the crucial role of photosynthesis on a global scale and how human activities like deforestation can disrupt the carbon cycle, contributing to climate change.

Assertion-Reason Practice

Assertion (A): The light-independent reactions of photosynthesis can occur in the absence of light. Reason (R): The light-independent reactions depend on the products of the light-dependent reactions, namely ATP and NADPH.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

Answer: (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). While the reactions themselves don't directly use light, they are dependent on the light reactions to provide them with energy (ATP) and reducing power (NADPH). Therefore, they will stop soon after the light source is removed. The name "light-independent" is somewhat misleading; "light-dependent product-requiring" would be more accurate.

HOTS (Higher-Order Thinking Skills) Question

Some herbicides work by inhibiting the electron transport chain in the thylakoid membranes of chloroplasts. Why is this an effective way to kill a plant? Trace the consequences of this inhibition on both the light-dependent and light-independent reactions.

Answer: Inhibiting the electron transport chain is highly effective because it strikes at the heart of energy conversion in the plant.
- Effect on Light-Dependent Reactions: If the electron transport chain is blocked, the plant cannot produce ATP through photophosphorylation, nor can it produce NADPH. The entire process of converting light energy into chemical energy comes to a halt. Photolysis of water might still occur initially, but the whole system would back up and fail.
- Effect on Light-Independent Reactions: The Calvin Cycle is entirely dependent on the ATP and NADPH supplied by the light reactions. Without this supply of energy and reducing power, the Calvin Cycle cannot fix carbon dioxide or produce glucose.
- Overall Consequence: The plant is unable to produce its own food (glucose). It will starve to death, making this a very effective herbicidal mechanism.

Specimen Prep - Photosynthesis