How do cells release the energy in glucose and transfer it to ATP?
The stages of aerobic respiration (glycolysis, the citric acid cycle and the electron transport chain), the role of ATP, dehydrogenase enzymes and NAD as a hydrogen carrier, and the use of alternative respiratory substrates.
An SQA Higher Human Biology answer on cellular respiration, covering glycolysis and the net yield of ATP, the citric acid cycle in the mitochondrial matrix, the electron transport chain and ATP synthase, the role of NAD and dehydrogenase enzymes, oxygen as final hydrogen acceptor, and alternative respiratory substrates.
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What this dot point is asking
The SQA wants you to describe the three stages of aerobic respiration (glycolysis, the citric acid cycle and the electron transport chain), state where each occurs and its ATP yield, explain the role of NAD and dehydrogenase enzymes, explain the role of oxygen, and describe the use of alternative respiratory substrates.
Glycolysis
Glycolysis has two phases. In the energy investment phase, two molecules of ATP are used to add phosphate to the intermediates (phosphorylation), making them more reactive. In the energy payoff phase, four molecules of ATP are generated. The result is a net gain of two ATP per glucose, and hydrogen is also removed from intermediates and passed to the carrier NAD, forming NADH.
The citric acid cycle
The two pyruvate molecules then move into the mitochondrion:
The job of the citric acid cycle is mainly to strip hydrogen and electrons from the respiratory substrate and load them onto NAD, ready for the final stage.
The electron transport chain
NADH delivers its hydrogen ions and high-energy electrons to the chain. As the electrons pass along the carriers, the energy they release is used to pump hydrogen ions across the inner membrane. The ions then flow back through the enzyme ATP synthase, and this flow drives the synthesis of ATP. Most of the ATP from respiration is made here.
Dehydrogenase enzymes and alternative substrates
Throughout respiration, dehydrogenase enzymes catalyse the removal of hydrogen from substrates and its transfer to NAD. The role of these enzymes can be shown in the laboratory using a hydrogen-accepting indicator that changes colour as hydrogen is released.
Glucose is the usual respiratory substrate, but it is not the only one. Fats and proteins can also be broken down and their products fed into the respiratory pathway at different points, releasing energy when glucose is in short supply. This is why the body can keep making ATP even during fasting or prolonged exercise.
Examples in context
Example 1. Why we breathe harder during exercise. Hard exercise raises the demand for ATP, so the electron transport chain runs faster and uses oxygen more quickly. Breathing and heart rate increase to deliver enough oxygen to act as the final acceptor, keeping aerobic respiration going.
Example 2. Respiring fat during a fast. When glucose is low, the body breaks down stored fat and feeds its products into the respiratory pathway. This shows the use of an alternative respiratory substrate to keep generating ATP when the usual substrate is scarce.
Try this
Q1. State the net number of ATP molecules produced per glucose in glycolysis. [1 mark]
- Cue. Two.
Q2. Explain the role of oxygen in the electron transport chain. [1 mark]
- Cue. It is the final hydrogen and electron acceptor, combining with them to form water so the chain can keep running.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20194 marksDescribe the stage of respiration that takes place in the cytoplasm, including the net yield of ATP, and state where the next two stages occur.Show worked answer →
A 4-mark answer needs glycolysis described with its ATP yield and the locations of the later stages.
The stage in the cytoplasm is glycolysis, the breakdown of glucose to two molecules of pyruvate. An energy investment phase first uses two ATP to phosphorylate intermediates, then an energy payoff phase generates four ATP, giving a net gain of two ATP per glucose. Hydrogen is also removed and passed to NAD.
The citric acid cycle then takes place in the matrix of the mitochondrion, and the electron transport chain takes place on the inner mitochondrial membrane.
Award (1) glycolysis breaks glucose into two pyruvate, (2) two ATP used and four produced, (3) net gain of two ATP, and (4) citric acid cycle in the matrix and electron transport chain on the inner membrane.
SQA Higher 20223 marksDescribe the role of NAD and the electron transport chain in the synthesis of ATP, and state the role of oxygen.Show worked answer →
This is a 3-mark question on the final stage of respiration.
NAD is a coenzyme that picks up hydrogen ions and high-energy electrons removed from respiratory substrates by dehydrogenase enzymes, becoming NADH. NADH delivers these to the electron transport chain on the inner mitochondrial membrane.
Along the chain, the electrons release energy that is used to pump hydrogen ions across the membrane; as the ions flow back through ATP synthase, ATP is generated. Oxygen acts as the final hydrogen and electron acceptor at the end of the chain, combining with hydrogen and electrons to form water.
Markers reward (1) NAD carries hydrogen and electrons to the chain, (2) energy from electrons drives ATP synthesis via ATP synthase, and (3) oxygen is the final acceptor, forming water.
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