Cell Biology
Cellular Respiration
5th Year · 6th Year (Leaving Cert)
- ✓Define cellular respiration and state its overall balanced chemical equation.
- ✓Outline the three main stages of aerobic respiration: glycolysis, the Krebs cycle, and the electron transport chain.
- ✓Describe the key events, inputs, and outputs of glycolysis, the Krebs cycle (Higher Level), and the electron transport chain (Higher Level).
- ✓Compare and contrast aerobic and anaerobic respiration (fermentation), including their products and ATP yield.
- ✓Calculate the theoretical maximum ATP yield from the complete oxidation of one glucose molecule during aerobic respiration.
Key concepts
The process by which living cells break down organic molecules, primarily glucose, to release energy in the form of ATP. It is a catabolic process essential for all life functions.
The universal energy currency of the cell. It stores and releases energy through the breaking and forming of phosphate bonds, providing energy for cellular activities.
The first stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm and involves the breakdown of one molecule of glucose (a 6-carbon sugar) into two molecules of pyruvate (a 3-carbon compound). It does not require oxygen.
(Higher Level) A transitional step occurring in the mitochondrial matrix, linking glycolysis to the Krebs cycle. Each pyruvate molecule is converted into an acetyl group, which combines with Coenzyme A to form Acetyl CoA. Carbon dioxide is released, and NAD⁺ is reduced to NADH.
(Higher Level) The second stage of aerobic respiration, occurring in the mitochondrial matrix. Acetyl CoA enters the cycle, where it is completely oxidised to carbon dioxide. This cycle generates ATP (or GTP), NADH, and FADH₂. It is a cyclic pathway involving a series of enzyme-catalysed reactions.
(Higher Level) The final stage of aerobic respiration, located on the inner mitochondrial membrane. Electrons from NADH and FADH₂ are passed along a series of protein complexes (electron carriers), releasing energy. This energy is used to pump protons (H⁺) into the intermembrane space, creating a proton gradient. Protons then flow back into the matrix through ATP synthase, driving the synthesis of large amounts of ATP (oxidative phosphorylation). Oxygen acts as the final electron acceptor, forming water.
Respiration in the absence of oxygen. It occurs in the cytoplasm and involves glycolysis followed by reactions that regenerate NAD⁺ from NADH, allowing glycolysis to continue. It yields much less ATP than aerobic respiration.
Occurs in animal muscle cells during strenuous exercise when oxygen supply is insufficient. Pyruvate is converted to lactic acid, regenerating NAD⁺.
Occurs in yeast and some plant cells. Pyruvate is converted to acetaldehyde, releasing CO₂, and then acetaldehyde is converted to ethanol, regenerating NAD⁺.
The total number of ATP molecules produced from the complete oxidation of one glucose molecule. Theoretical maximum for aerobic respiration is typically 30-32 ATP, while anaerobic respiration yields only 2 ATP.
Key facts to remember
- 1Cellular respiration is the process of releasing energy from food to produce ATP.
- 2ATP is the primary energy currency used by cells for all metabolic activities.
- 3Aerobic respiration requires oxygen and occurs mainly in the mitochondria, yielding a large amount of ATP (30-32 per glucose).
- 4Anaerobic respiration (fermentation) occurs in the cytoplasm without oxygen and yields only 2 ATP per glucose.
- 5Glycolysis is the initial stage of both aerobic and anaerobic respiration, breaking down glucose into pyruvate.
- 6The Krebs cycle (HL) and electron transport chain (HL) are the main ATP-generating stages of aerobic respiration.
- 7Oxygen acts as the final electron acceptor in the electron transport chain, forming water.
- 8Fermentation regenerates NAD⁺, allowing glycolysis to continue in the absence of oxygen.
Worked examples
Example 1
Outline the main events and products of glycolysis.
Answer
Glycolysis occurs in the cytoplasm. It begins with one molecule of glucose. Two ATP molecules are initially used to activate the glucose. The glucose is then broken down into two molecules of pyruvate. During this process, a net of two ATP molecules are produced, and two molecules of NAD⁺ are reduced to two NADH.
Glycolysis is an anaerobic process, meaning it does not require oxygen.
Example 2
Compare and contrast aerobic respiration and alcoholic fermentation in terms of oxygen requirement, location, final products, and ATP yield.
Answer
Oxygen Requirement: Aerobic respiration requires oxygen as the final electron acceptor in the electron transport chain. Alcoholic fermentation does not require oxygen; it is an anaerobic process. Location: Aerobic respiration begins with glycolysis in the cytoplasm, followed by the Link Reaction, Krebs cycle, and Electron Transport Chain in the mitochondria. Alcoholic fermentation occurs entirely in the cytoplasm. Final Products: The final products of aerobic respiration are carbon dioxide and water. The final products of alcoholic fermentation are ethanol and carbon dioxide. ATP Yield: Aerobic respiration yields a theoretical maximum of 30-32 ATP molecules per glucose molecule. Alcoholic fermentation yields only 2 ATP molecules per glucose molecule (from glycolysis).
Both processes begin with glycolysis, but their subsequent pathways and energy yields differ significantly due to the presence or absence of oxygen.
Example 3
(Higher Level) Explain the role of oxygen in the electron transport chain and its impact on ATP production.
Answer
In the electron transport chain (ETC), NADH and FADH₂ donate high-energy electrons to a series of protein complexes embedded in the inner mitochondrial membrane. As these electrons move along the chain, their energy is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient. Oxygen acts as the final electron acceptor at the end of the ETC. It combines with electrons and protons to form water (H₂O). This crucial role of oxygen ensures that the electron transport chain can continue to operate, preventing a build-up of electrons and allowing the continuous pumping of protons. The proton gradient drives ATP synthase, which uses the flow of protons back into the matrix to synthesise large amounts of ATP through oxidative phosphorylation. Without oxygen, the ETC would halt, the proton gradient would dissipate, and ATP production would cease, leading to a drastic reduction in energy yield.
Common mistakes
- ✗Confusing cellular respiration with breathing (ventilation). Respiration is a cellular process, while breathing is a macroscopic physiological process.
- ✗Incorrectly identifying the cellular locations of the different stages (e.g., Krebs cycle in cytoplasm instead of mitochondrial matrix).
- ✗Misunderstanding the role of oxygen as the final electron acceptor, not just a reactant.
- ✗Forgetting that fermentation's primary purpose is to regenerate NAD⁺ for glycolysis to continue, not to produce large amounts of ATP.
- ✗Stating an exact ATP yield (e.g., 38 ATP) instead of a range (30-32 ATP), as the actual yield can vary.
Exam tips
- ★Learn and be able to draw simplified diagrams of the overall process, highlighting the inputs, outputs, and locations of each stage.
- ★Pay close attention to the 'Higher Level' (HL) content for the Krebs cycle and electron transport chain, as these require more detailed explanations.
- ★Memorise the overall balanced chemical equation for aerobic respiration.
- ★Clearly distinguish between aerobic and anaerobic respiration, focusing on their oxygen requirements, products, and relative ATP yields.
Ready to practise?
Try a problem on this topic
Snap a photo or type a question — get step-by-step working instantly.