AQA · A-Level · Biology

    Course version: 7402 · Version from Sept 2016

    Aerobic respiration in such detail as to show that: pyruvate is oxidised…

    Full specification wording

    Respiration (A-level only): Aerobic respiration in such detail as to show that: pyruvate is oxidised to acetate, producing reduced NAD in the process acetate combines with coenzyme A in the link reaction to produce acetylcoenzyme A acetylcoenzyme A reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters the Krebs cycle in a series of oxidation-reduction reactions, the Krebs cycle generates reduced coenzymes and ATP by substrate-level phosphorylation, and carbon dioxide is lost synthesis of ATP by oxidative phosphorylation is associated with the transfer of electrons down the electron transfer chain and passage of protons across inner mitochondrial membranes and is catalysed by ATP synthase embedded in these membranes (chemiosomotic theory) other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle.

    In the matrix, pyruvate is oxidised to acetate, producing reduced NAD and releasing carbon dioxide.

    Checked against the AQA 7402 specification (opens in a new tab) on .

    • Point 7 of 8
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    In the matrix, pyruvate is oxidised to acetate, producing reduced NAD and releasing carbon dioxide. Acetate combines with coenzyme A to form acetylcoenzyme A. This reacts with a 4-carbon molecule to form a 6-carbon molecule, entering the Krebs cycle. Through redox reactions, it is decarboxylated and dehydrogenated back to the 4-carbon molecule, producing ATP by substrate-level phosphorylation and reduced coenzymes. These release electrons to the electron transfer chain. Energy released pumps protons across the inner membrane. Protons diffuse back via ATP synthase, driving ATP synthesis (chemiosmosis). Oxygen is the final electron acceptor, forming water. Breakdown products of lipids and amino acids can also enter the Krebs cycle as alternative respiratory substrates.

    What earns the marks

    • Pyruvate being oxidised to acetate, producing reduced NAD and releasing carbon dioxide.
    • The Krebs cycle generating reduced coenzymes, losing carbon dioxide and producing ATP by substrate-level phosphorylation.
    • Protons diffusing across the inner membrane through ATP synthase to synthesise ATP (chemiosmosis).
    • Oxygen acting as the final electron acceptor to form water.
    • Stating that breakdown products of lipids and amino acids can enter the Krebs cycle.

    Common mistakes and how to put them right

    • Writing reduced NADP instead of reduced NAD in a respiration answer, which examiners reject.

    • Omitting water as the product formed when oxygen accepts electrons and protons at the end of the chain.

    • Describing the Krebs cycle ATP as coming from oxidative phosphorylation instead of substrate-level phosphorylation.

    Exam tips

    • Count the carbons through every step, because that catches most Krebs cycle errors before you write them down.
    • Clearly distinguish between substrate-level phosphorylation in the Krebs cycle and oxidative phosphorylation via ATP synthase.

    Try the lesson

    The start of the Respiration (A-level only): Aerobic respiration in such detail as to show that: pyruvate is oxidised to acetate, producing reduced NAD in the process acetate combines with coenzyme A in the link reaction to produce acetylcoenzyme A acetylcoenzyme A reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters the Krebs cycle in a series of oxidation-reduction reactions, the Krebs cycle generates reduced coenzymes and ATP by substrate-level phosphorylation, and carbon dioxide is lost synthesis of ATP by oxidative phosphorylation is associated with the transfer of electrons down the electron transfer chain and passage of protons across inner mitochondrial membranes and is catalysed by ATP synthase embedded in these membranes (chemiosomotic theory) other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle. lesson, made from checked teaching notes for this specification point.

    Have a go first

    In the link reaction, pyruvate is oxidised to acetate. Which molecule is produced as a result of this oxidation?

    Listen

    In the matrix, pyruvate is oxidised to acetate, producing reduced NAD and releasing carbon dioxide. Acetate combines with coenzyme A to form acetylcoenzyme A. This reacts with a 4-carbon molecule to form a 6-carbon molecule, entering the Krebs cycle. Through redox reactions, it is decarboxylated and dehydrogenated back to the 4-carbon molecule, producing ATP by substrate-level phosphorylation and reduced coenzymes. These release electrons to the electron transfer chain. Energy released pumps protons across the inner membrane. Protons diffuse back via ATP synthase, driving ATP synthesis (chemiosmosis). Oxygen is the final electron acceptor, forming water. Breakdown products of lipids and amino acids can also enter the Krebs cycle as alternative respiratory substrates. A strong answer earns marks for these points. pyruvate being oxidised to acetate, producing reduced NAD and releasing carbon dioxide. the Krebs cycle generating reduced coenzymes, losing carbon dioxide and producing ATP by substrate-level phosphorylation. protons diffusing across the inner membrane through ATP synthase to synthesise ATP (chemiosmosis). oxygen acting as the final electron acceptor to form water. Exam tip: Count the carbons through every step, because that catches most Krebs cycle errors before you write them down.

    See it

    A vertical flowchart. Pyruvate leads to acetate, which leads to acetylcoenzyme A. Acetylcoenzyme A leads to a six-carbon molecule, which leads to the Krebs cycle. The Krebs cycle branches into two outputs: reduced coenzymes and ATP by substrate-level phosphorylation. The reduced coenzymes lead to the electron transfer chain, which leads to protons across the inner mitochondrial membrane, which leads to ATP synthase, which leads to ATP by oxidative phosphorylation.
    It shows the sequence from pyruvate through the link reaction and Krebs cycle to ATP synthesis by oxidative phosphorylation via chemiosmosis.

    Key words

    Say what each word means before you turn the card.

    link reaction

    Try answering aloud or in your head before turning the card.

    Krebs cycle

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    oxidative phosphorylation

    Try answering aloud or in your head before turning the card.

    alternative respiratory substrates

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    See every definition
    link reaction
    Pyruvate is oxidised to acetate, producing reduced NAD, and acetate combines with coenzyme A to produce acetylcoenzyme A.
    Krebs cycle
    Acetylcoenzyme A reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters a series of oxidation-reduction reactions generating reduced coenzymes and ATP by substrate-level phosphorylation, with carbon dioxide lost.
    oxidative phosphorylation
    Synthesis of ATP associated with the transfer of electrons down the electron transfer chain and passage of protons across inner mitochondrial membranes, catalysed by ATP synthase embedded in these membranes.
    alternative respiratory substrates
    Other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A vertical flowchart. Pyruvate leads to acetate, which leads to acetylcoenzyme A. Acetylcoenzyme A leads to a six-carbon molecule, which leads to the Krebs cycle. The Krebs cycle branches into two outputs: reduced coenzymes and ATP by substrate-level phosphorylation. The reduced coenzymes lead to the electron transfer chain, which leads to protons across the inner mitochondrial membrane, which leads to ATP synthase, which leads to ATP by oxidative phosphorylation.

    • 1Box 1

    • 2Box 2

    • 3Box 3

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    Test your understanding of Respiration (A-level only): Aerobic respiration in such detail as to show that: pyruvate is oxidised to acetate, producing reduced NAD in the process acetate combines with coenzyme A in the link reaction to produce acetylcoenzyme A acetylcoenzyme A reacts with a four-carbon molecule, releasing coenzyme A and producing a six-carbon molecule that enters the Krebs cycle in a series of oxidation-reduction reactions, the Krebs cycle generates reduced coenzymes and ATP by substrate-level phosphorylation, and carbon dioxide is lost synthesis of ATP by oxidative phosphorylation is associated with the transfer of electrons down the electron transfer chain and passage of protons across inner mitochondrial membranes and is catalysed by ATP synthase embedded in these membranes (chemiosomotic theory) other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle.

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