Ocr Biology Pag 4.2 Answers

OCR Biology A-Level: A Deep Dive into Chapter 4.2 – Cellular Respiration

This article provides comprehensive answers and explanations for OCR Biology A-Level, Chapter 4.2, focusing on cellular respiration. Understanding cellular respiration is crucial for grasping many biological processes, from energy production in muscles to the functioning of the entire organism. We'll break down the key concepts, providing detailed answers, and clarifying any potential areas of confusion. This guide aims to be more than just a simple answer key; it's designed to enhance your understanding and build a solid foundation in this vital topic.

Introduction: Understanding Cellular Respiration

Cellular respiration is the process by which cells break down glucose to release energy in the form of ATP (adenosine triphosphate). This energy is then used to power various cellular processes, such as muscle contraction, active transport, and protein synthesis. It's a fundamental process in all living organisms, and a thorough understanding is essential for success in OCR Biology A-Level. This chapter focuses on the specifics of this process, including glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation.

Glycolysis: The First Stage

Glycolysis is the initial stage of cellular respiration and occurs in the cytoplasm. It's an anaerobic process, meaning it doesn't require oxygen. The process can be summarized as follows:

1. Phosphorylation: Glucose is phosphorylated twice using ATP, making it more reactive. This forms fructose-1,6-bisphosphate.
2. Lysis: The six-carbon fructose-1,6-bisphosphate is split into two three-carbon molecules of glyceraldehyde-3-phosphate (G3P).
3. Oxidation: G3P is oxidized, releasing hydrogen atoms (H). These hydrogen atoms are accepted by NAD (nicotinamide adenine dinucleotide), forming NADH. This is a redox reaction; G3P is oxidized, and NAD is reduced.
4. ATP Formation: Substrate-level phosphorylation occurs, resulting in the net production of 2 ATP molecules per glucose molecule. This means 4 ATP are produced, but 2 are consumed in the earlier phosphorylation steps.
5. Pyruvate Formation: The end product of glycolysis is two molecules of pyruvate.

Key Points to Remember about Glycolysis:

• It's an anaerobic process.
• It produces a net gain of 2 ATP molecules per glucose molecule.
• It produces 2 NADH molecules per glucose molecule.
• It occurs in the cytoplasm.
• It's a relatively quick process.

The Link Reaction: Connecting Glycolysis to the Krebs Cycle

The pyruvate produced in glycolysis doesn't directly enter the Krebs cycle. Instead, it undergoes a link reaction in the mitochondrial matrix. This reaction involves:

1. Decarboxylation: One carbon dioxide molecule is removed from each pyruvate molecule, forming a two-carbon molecule called acetyl.
2. Oxidation: The acetyl group is oxidized, and the hydrogen atoms released are accepted by NAD, forming NADH.
3. Acetyl CoA Formation: The acetyl group combines with coenzyme A (CoA) to form acetyl CoA, which enters the Krebs cycle.

Crucial aspects of the Link Reaction:

• It occurs in the mitochondrial matrix.
• It is an aerobic process (though oxygen isn't directly involved in the reaction itself).
• It produces 2 NADH molecules per glucose molecule (one per pyruvate).
• It produces 2 CO2 molecules per glucose molecule.

The Krebs Cycle (Citric Acid Cycle): Central to Energy Production

The Krebs cycle, also known as the citric acid cycle, is a series of enzyme-catalyzed reactions that occur in the mitochondrial matrix. It's a cyclic pathway, meaning the end product regenerates a starting molecule. Each turn of the cycle involves:

1. Acetyl CoA entry: Acetyl CoA combines with oxaloacetate to form citrate (citric acid).
2. Decarboxylation: Two molecules of carbon dioxide are released.
3. Oxidation: Hydrogen atoms are released and accepted by NAD and FAD (flavin adenine dinucleotide), forming NADH and FADH2 respectively.
4. ATP Formation: One ATP molecule is produced per cycle via substrate-level phosphorylation.
5. Regeneration of Oxaloacetate: The cycle regenerates oxaloacetate, allowing the cycle to continue.

Important considerations for the Krebs Cycle:

• It's an aerobic process (although oxygen isn't directly involved in the reactions).
• It produces 2 ATP molecules per glucose molecule (one per pyruvate).
• It produces 6 NADH and 2 FADH2 molecules per glucose molecule.
• It produces 4 CO2 molecules per glucose molecule.
• It occurs in the mitochondrial matrix.

Oxidative Phosphorylation: The Electron Transport Chain and Chemiosmosis

Oxidative phosphorylation is the final stage of cellular respiration and is where the majority of ATP is produced. It involves two main processes:

1. Electron Transport Chain (ETC): Electrons from NADH and FADH2 are passed along a series of electron carriers embedded in the inner mitochondrial membrane. As electrons move down the chain, energy is released, which is used to pump protons (H+) from the matrix into the intermembrane space. This creates a proton gradient.

2. Chemiosmosis: The protons flow back into the matrix through ATP synthase, an enzyme that uses the energy from the proton gradient to synthesize ATP. This process is called chemiosmosis. Oxygen acts as the final electron acceptor in the ETC, combining with protons and electrons to form water.

Critical details about Oxidative Phosphorylation:

• It’s an aerobic process; oxygen is essential as the final electron acceptor.
• It occurs in the inner mitochondrial membrane.
• It produces the majority of ATP molecules (~32-34 per glucose molecule) through chemiosmosis.
• The process involves the electron transport chain and chemiosmosis.

Total ATP Yield: A Comprehensive Summary

The total ATP yield from cellular respiration varies slightly depending on the efficiency of the process and the shuttle system used to transport NADH from the cytoplasm into the mitochondria. However, a reasonable estimate is approximately 36-38 ATP molecules per glucose molecule. This breakdown shows the contribution of each stage:

• Glycolysis: 2 ATP + 2 NADH (approximately 5 ATP)
• Link Reaction: 2 NADH (approximately 5 ATP)
• Krebs Cycle: 2 ATP + 6 NADH + 2 FADH2 (approximately 20 ATP)
• Oxidative Phosphorylation: Approximately 28-34 ATP (from NADH and FADH2)

Factors Affecting Cellular Respiration

Several factors can influence the rate of cellular respiration, including:

• Temperature: Enzyme activity is temperature-dependent, with an optimal temperature for respiration. Too high or too low temperatures can decrease the rate.
• Substrate Concentration: The availability of glucose and oxygen affects the rate of respiration.
• Enzyme Inhibitors: Certain substances can inhibit enzymes involved in the process, reducing the rate of respiration. For example, cyanide inhibits cytochrome c oxidase, a crucial enzyme in the electron transport chain.
• pH: Changes in pH can affect enzyme activity, altering the rate of respiration.

Frequently Asked Questions (FAQ)

Q1: What is the difference between aerobic and anaerobic respiration?

A1: Aerobic respiration requires oxygen as the final electron acceptor in the electron transport chain, while anaerobic respiration does not. Anaerobic respiration produces significantly less ATP than aerobic respiration.

Q2: What is the role of oxygen in cellular respiration?

A2: Oxygen acts as the final electron acceptor in the electron transport chain, allowing the continuous flow of electrons and the generation of a proton gradient necessary for ATP synthesis. Without oxygen, the electron transport chain would stop, and ATP production would drastically decrease.

Q3: Why is ATP important?

A3: ATP is the primary energy currency of the cell. It provides the energy needed for various cellular processes, including muscle contraction, active transport, and biosynthesis.

Q4: What are the different types of anaerobic respiration?

A4: Two common types are alcoholic fermentation (producing ethanol and carbon dioxide) and lactic acid fermentation (producing lactic acid). Both occur in the absence of oxygen and produce far less ATP than aerobic respiration.

Q5: How does cellular respiration relate to photosynthesis?

A5: Cellular respiration and photosynthesis are complementary processes. Photosynthesis produces glucose and oxygen, which are used in cellular respiration to produce ATP. Cellular respiration produces carbon dioxide and water, which are used in photosynthesis.

Conclusion: Mastering Cellular Respiration

Cellular respiration is a complex but crucial process that underpins all life. Understanding its intricate stages – glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation – is fundamental to a strong grasp of OCR Biology A-Level. By breaking down the process step-by-step and focusing on the key concepts, you can build a confident understanding of this vital metabolic pathway. Remember to practice diagrams and work through example questions to solidify your knowledge. This detailed explanation, alongside diligent study, will equip you to excel in your examinations. Remember that consistent revision and application of the concepts are crucial for long-term retention and success.