Photosynthesis andRespiration Model Answer Key
Introduction The relationship between photosynthesis and cellular respiration forms the backbone of most life‑supporting processes on Earth. While photosynthesis captures solar energy and stores it in the form of glucose, respiration releases that stored energy for cellular activities. Understanding how these two pathways complement each other is essential for students studying biology, ecology, and environmental science. This article provides a clear, step‑by‑step explanation of both processes, a model answer key for typical exam questions, and a set of frequently asked questions to reinforce learning. ---
H2 Understanding Photosynthesis
Photosynthesis occurs primarily in the chloroplasts of plant cells and some algae and bacteria. The overall reaction can be summarized 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 ]
Key Steps
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Light‑dependent reactions – Occur in the thylakoid membranes.
- Photon absorption by chlorophyll excites electrons. - Water molecules are split (photolysis), releasing oxygen as a by‑product.
- ATP and NADPH are generated through electron transport chains.
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Calvin Cycle (light‑independent reactions) – Takes place in the stroma.
- Carbon dioxide is fixed into a three‑carbon compound (3‑phosphoglycerate).
- ATP and NADPH drive the reduction of 3‑phosphoglycerate to glyceraldehyde‑3‑phosphate (G3P).
- G3P molecules are linked to form glucose and other carbohydrates.
Important Points
- Chlorophyll is the primary pigment that captures light.
- Oxygen produced during photosynthesis is released into the atmosphere, supporting aerobic life.
- The process is endothermic, requiring continuous input of light energy.
H2 Understanding Cellular Respiration
Cellular respiration is the set of metabolic pathways that break down glucose to release energy for cellular work. It occurs in three main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. The overall equation mirrors photosynthesis in reverse:
[ \text{C}6\text{H}{12}\text{O}_6 + 6 \text{O}_2 \rightarrow 6 \text{CO}_2 + 6 \text{H}_2\text{O} + \text{energy (ATP)} ]
Key Stages 1. Glycolysis – Takes place in the cytoplasm.
- One glucose molecule is split into two pyruvate molecules, yielding a net gain of 2 ATP and 2 NADH.
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Pyruvate Oxidation & Citric Acid Cycle – Occur in the mitochondrial matrix.
- Each pyruvate is converted to acetyl‑CoA, releasing CO₂.
- Acetyl‑CoA enters the cycle, producing NADH, FADH₂, GTP (or ATP), and additional CO₂.
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Oxidative Phosphorylation – Occurs across the inner mitochondrial membrane Easy to understand, harder to ignore..
- Electrons from NADH and FADH₂ travel through the electron transport chain, driving proton pumping.
- The resulting proton gradient powers ATP synthase to generate up to 34 ATP molecules.
Key Terms
- Mitochondria – The “powerhouse” organelles where most respiration steps occur.
- ATP – Adenosine triphosphate, the universal energy currency of cells. - NADH/FADH₂ – Electron carriers that shuttle high‑energy electrons to the electron transport chain.
H2 Comparative Overview
| Feature | Photosynthesis | Cellular Respiration |
|---|---|---|
| Primary Location | Chloroplasts (thylakoid membranes & stroma) | Mitochondria (matrix & inner membrane) |
| Energy Source | Light energy | Chemical energy stored in glucose |
| Main Reactants | CO₂, H₂O, light | Glucose, O₂ |
| Main Products | Glucose, O₂ | CO₂, H₂O, ATP |
| Direction of Energy Flow | Stores energy (endothermic) | Releases energy (exothermic) |
| Gas Exchange | Releases O₂, consumes CO₂ | Consumes O₂, releases CO₂ |
Both processes are tightly coupled: the oxygen produced by photosynthesis is used by organisms for respiration, while the CO₂ released by respiration is recycled by photosynthetic organisms to synthesize new glucose.
H2 Model Answer Key Below are sample exam questions along with concise, model answers that can be used as a reference key.
Question 1
Explain the role of chlorophyll in photosynthesis. Model Answer
Chlorophyll, located in the thylakoid membranes of chloroplasts, absorbs photons primarily in the blue and red wavelengths. This excitation of chlorophyll electrons initiates the light‑dependent reactions, leading to the splitting of water molecules and the generation of ATP and NADPH. Without chlorophyll, the capture of light energy would be impossible, halting the entire photosynthetic process The details matter here. Still holds up..
Question 2
Write the balanced chemical equation for photosynthesis and identify the organelle where it occurs.
Model Answer [
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
]
Photosynthesis occurs in the chloroplast, specifically within the thylakoid membranes and stroma.
Question 3
Describe the fate of pyruvate after glycolysis in aerobic respiration.
Model Answer
In the presence of oxygen, each pyruvate molecule enters the mitochondrion and is converted into acetyl‑CoA by the pyruvate dehydrogenase complex, releasing one molecule of CO₂. Acetyl‑CoA then combines with oxaloacetate to begin the citric acid cycle, generating NADH, FADH₂, GTP, and additional CO₂ Which is the point..
Question 4
Compare the energy yield of glycolysis in anaerobic versus aerobic conditions.
Model Answer - Anaerobic glycolysis yields a net gain of 2 ATP per glucose molecule, with pyruvate being reduced to lactate or ethanol to regenerate NAD⁺.
- Aerobic respiration continues beyond glycolysis, ultimately producing up to 36–38 ATP
