What is the difference between aerobic and anaerobic respiration?

Aerobic respiration uses oxygen to fully oxidise glucose, producing a large amount of ATP (up to 32 per glucose), carbon dioxide, and water. Anaerobic respiration occurs without oxygen and only partially breaks down glucose, yielding just 2 ATP per glucose. In animals, anaerobic respiration produces lactate, while in plants and yeast, it produces ethanol and carbon dioxide.

Where does the Krebs cycle occur?

The Krebs cycle takes place in the mitochondrial matrix, the fluid-filled space inside the inner mitochondrial membrane. This location is crucial because it contains the necessary enzymes and is close to the electron transport chain embedded in the inner membrane.

How many ATP are produced in glycolysis?

Glycolysis produces a net gain of 2 ATP per glucose molecule. This is because 4 ATP are produced via substrate-level phosphorylation, but 2 ATP are used in the initial phosphorylation of glucose. Additionally, 2 NADH are produced, which can later yield more ATP if oxygen is present.

Why is oxygen important in respiration?

Oxygen acts as the final electron acceptor in the electron transport chain. Without oxygen, electrons cannot be passed down the chain, causing it to stop. This halts oxidative phosphorylation and ATP production, forcing cells to rely on anaerobic respiration, which is much less efficient.

What is the role of NAD and FAD in respiration?

NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are coenzymes that carry hydrogen atoms (electrons and protons) from glycolysis and the Krebs cycle to the electron transport chain. They are reduced to NADH and FADH₂, which then donate electrons to power ATP synthesis.

Can respiration occur without mitochondria?

Yes, glycolysis occurs in the cytoplasm and does not require mitochondria. However, the later stages (link reaction, Krebs cycle, oxidative phosphorylation) occur in mitochondria. Prokaryotes lack mitochondria but can still perform aerobic respiration using their cell membrane for the electron transport chain.

Respiration

OCR

A-Level

This topic covers the essential process of cellular respiration, where energy stored in complex organic molecules is transferred to ATP. It details the stages of aerobic respiration, including glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation, as well as the process of anaerobic respiration in eukaryotes.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Respiration is a fundamental biochemical process that releases energy from organic molecules, primarily glucose, to produce ATP (adenosine triphosphate). This process is essential for all living organisms as ATP powers cellular activities such as muscle contraction, active transport, and biosynthesis. In OCR A-Level Biology, respiration is studied in detail, covering both aerobic and anaerobic pathways, their locations within cells, and the regulation of these processes.

The topic is divided into four main stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Glycolysis occurs in the cytoplasm and does not require oxygen, while the remaining stages take place in the mitochondria. Understanding the precise sequence of reactions, the enzymes involved, and the energy yields is crucial. Additionally, students must appreciate how respiration integrates with other metabolic pathways, such as photosynthesis and lipid metabolism.

Mastering respiration is vital for success in A-Level Biology because it appears in multiple exam contexts, including data analysis, essay questions, and practical investigations. It also provides a foundation for understanding topics like energy transfer in ecosystems, the role of mitochondria in disease, and the impact of exercise on physiology. A deep grasp of respiration enables students to explain real-world phenomena, from why we breathe heavily during exercise to how certain poisons disrupt ATP production.

Key Concepts

Core ideas you must understand for this topic

→Glycolysis: The breakdown of glucose (6C) into two pyruvate molecules (3C) in the cytoplasm, producing a net gain of 2 ATP and 2 NADH. This process does not require oxygen and involves substrate-level phosphorylation.
→Link Reaction: Pyruvate is transported into the mitochondrial matrix, where it is decarboxylated and dehydrogenated to form acetyl CoA (2C), producing 1 NADH and releasing CO₂ per pyruvate.
→Krebs Cycle: Acetyl CoA combines with oxaloacetate (4C) to form citrate (6C), which undergoes a series of oxidation reactions, regenerating oxaloacetate. Per cycle, it produces 3 NADH, 1 FADH₂, and 1 ATP (via substrate-level phosphorylation), plus 2 CO₂.
→Oxidative Phosphorylation: The electron transport chain (ETC) uses electrons from NADH and FADH₂ to create a proton gradient across the inner mitochondrial membrane. ATP synthase uses this gradient to produce ATP (chemiosmosis). Oxygen is the final electron acceptor, forming water.
→Anaerobic Respiration: In the absence of oxygen, pyruvate is converted to lactate in animals (regenerating NAD⁺) or to ethanol and CO₂ in plants and yeast. This yields only 2 ATP per glucose, far less than aerobic respiration.

What You Need to Demonstrate

Key skills and knowledge for this topic

Structure of the mitochondrion (inner/outer membranes, cristae, matrix, DNA)
Glycolysis: phosphorylation of glucose, splitting into triose phosphate, oxidation to pyruvate, yield of ATP and reduced NAD
Link reaction and its site
Krebs cycle: formation of citrate, decarboxylation, dehydrogenation, reduction of NAD/FAD, substrate-level phosphorylation
Role of coenzymes (NAD, FAD, coenzyme A)
Oxidative phosphorylation: electron carriers, oxygen, mitochondrial cristae
Chemiosmotic theory: electron transport chain, proton gradients, ATP synthase
Anaerobic respiration in mammals and yeast

Marking Points

Key points examiners look for in your answers

Structure of the mitochondrion (inner/outer membranes, cristae, matrix, DNA)
Glycolysis: phosphorylation of glucose, splitting into triose phosphate, oxidation to pyruvate, yield of ATP and reduced NAD
Link reaction and its site
Krebs cycle: formation of citrate, decarboxylation, dehydrogenation, reduction of NAD/FAD, substrate-level phosphorylation
Role of coenzymes (NAD, FAD, coenzyme A)
Oxidative phosphorylation: electron carriers, oxygen, mitochondrial cristae
Chemiosmotic theory: electron transport chain, proton gradients, ATP synthase
Anaerobic respiration in mammals and yeast
Relative energy values of respiratory substrates (carbohydrates, lipids, proteins)
Respiratory quotient (RQ) calculation and interpretation

Examiner Tips

Expert advice for maximising your marks

💡Ensure you can clearly distinguish between aerobic and anaerobic respiration pathways
💡Practice calculating RQ values and interpreting what they indicate about the respiratory substrate
💡Be prepared to describe the role of ATP as the immediate energy source for biological processes
💡Use precise terminology when describing the roles of coenzymes and electron carriers
💡Review the interrelationship between photosynthesis and respiration as specified in 5.2.1
💡Tip 1: Always state the exact location of each stage. For example, glycolysis occurs in the cytoplasm, while the Krebs cycle takes place in the mitochondrial matrix. Marks are often awarded for precise locations.
💡Tip 2: When describing oxidative phosphorylation, use the term 'chemiosmosis' and explain the role of the proton gradient and ATP synthase. Avoid vague phrases like 'energy is released' – be specific about electron transfer and proton pumping.
💡Tip 3: For calculations of ATP yield, remember that the theoretical maximum is about 32 ATP per glucose in aerobic respiration, but this can vary. Show your working clearly, including the contributions from NADH and FADH₂.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the sites of different stages of respiration within the cell or mitochondrion
Incorrectly identifying the products and reactants of the Krebs cycle
Failing to explain the role of oxygen in oxidative phosphorylation
Misunderstanding the chemiosmotic theory and the role of the proton gradient
Incorrectly calculating or interpreting the respiratory quotient (RQ)
Misconception: Respiration is the same as breathing. Correction: Breathing (ventilation) is the mechanical process of moving air in and out of the lungs, while respiration is the cellular process that releases energy from glucose. Breathing supplies oxygen for aerobic respiration and removes carbon dioxide.
Misconception: Glycolysis requires oxygen. Correction: Glycolysis is anaerobic; it occurs in the cytoplasm without oxygen. Oxygen is only needed for the later stages (link reaction, Krebs cycle, oxidative phosphorylation).
Misconception: ATP is produced directly in the Krebs cycle. Correction: The Krebs cycle produces only 1 ATP per turn via substrate-level phosphorylation. Most ATP is generated during oxidative phosphorylation via chemiosmosis.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Cell Structure: Knowledge of mitochondria (cristae, matrix) and the cytoplasm is essential for understanding where respiration stages occur.
•Enzymes: Understanding enzyme action, including active sites, denaturation, and factors affecting rate (temperature, pH), is crucial for respiration reactions.
•Energy and ATP: A basic grasp of ATP as the energy currency and the concept of exergonic vs. endergonic reactions helps contextualise respiration.

Likely Command Words

How questions on this topic are typically asked

Describe

Explain

Calculate

Compare

Interpret

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Practice questions tailored to this topic

Respiration

Topic Overview

Key Concepts

What You Need to Demonstrate

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Examiner Tips

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Before You Start

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Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules

How to Revise Respiration — OCR A-Level Biology

Examiner Tips for Respiration

Common Mistakes in Respiration

Key Marking Points

Respiration

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between aerobic and anaerobic respiration?

Where does the Krebs cycle occur?

How many ATP are produced in glycolysis?

Why is oxygen important in respiration?

What is the role of NAD and FAD in respiration?

Can respiration occur without mitochondria?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules