Respiration

Overview

What you need to know (from the AQA specification)

Respiration produces ATP

What is respiration?

Remember in Photosynthesis, we saw that light energy was converted into chemical energy in the form of glucose.

But, cells cannot use glucose directly as a source of energy - they can only use ATP for their immediate energy needs.

Respiration is a process which breaks down the glucose, releasing energy to make this ATP

Why can't cells just use glucose directly instead of ATP?

Cells release energy from glucose in small, controlled steps rather than all at once. If the energy in glucose were released directly in a single reaction, it could release too much energy at once, leading to problems such as excess heat and damage to proteins or cell membranes. ATP acts as an energy transfer molecule, allowing energy to be released only when and where it is needed.

Why is respiration needed?

Since respiration produces ATP (immediate energy source for cells), it has many uses in cells for energy-requiring processes - see Hydrolysis & Synthesis ATP

Overview of Respiration

There are two main types of respiration

• Aerobic
  • Requires oxygen
  • Produces lots of ATP
  • Stages: Glycolysis, Link reaction, Krebs cycle & Oxidative Phosphorylation
• Anaerobic
  • Without oxygen
  • Produces minimal ATP
  • Stages: Glycolysis, Fermentation (Alcoholic for plants & yeast or Lactate for animals)

There are 4 main phases of aerobic respiration

• Glycolysis
• Link reaction
• Krebs cycle
• Oxidative phosphorylation

Glycolysis

What you need to know (from the AQA specification)

Glycolysis is the first stage of anaerobic and aerobic respiration. It occurs in the cytoplasm and is an anaerobic process. Glycolysis involves the following stages:

• phosphorylation of glucose to glucose phosphate, using ATP
• production of triose phosphate
• oxidation of triose phosphate to pyruvate with a net gain of ATP and reduced NAD.

Glycolysis occurs in the cytoplasm of the cell and doesn’t require oxygen (it’s a step in both aerobic and anaerobic). It produces pyruvate which can be transferred to the link reaction

Steps in Glycolysis

1. Phosphorylation to glucose phosphate
   • Glucose is phosphorylated using two ATP molecules to glucose phosphate
   • Note: Glucose phosphate is on the AQA specification, but the image here shows an additional step of hexose bisphosphate (with 2 phosphate groups on). This is just for understanding of ‘where did the other phosphate from ATP go?’
2. Hexose bisphosphate is highly reactive and unstable so splits into 2 molecules of triose phosphate
3. Triose phosphate is oxidised and forms 2 molecules of pyruvate
   • Hydrogen is removed and transferred to NAD to form NADH (reduced NAD)
4. ATP is produced
   • 4 ATP are produced
   • Net gain of 2 ATP (as 2 were used up in phosphorylation step)

Extra detail/not required: Why are 2 ATP produced in the oxidation of each triose phosphate, if they only start with 1 phosphate?

The diagram above is simplified for the exam specification, so it doesn’t show every intermediate step. Below is a full pathway diagram for reference (to help with overall understanding). You are not expected to know or understand this.

Image credit: Glycolysis pathway 2.png by Eunice Laurent is licensed under the Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license.

• Triose phosphate is oxidised and NAD+ is reduced to NADH. At the same time, an inorganic phosphate (Pi) from the cytosol is added to form 1,3-bisphosphoglycerate (1,3-BPG), which now has two phosphates.
• One phosphate is transferred from 1,3-BPG to ADP to form ATP (1,3-BPG to 3-phosphoglycerate).
• Later, the remaining phosphate is transferred to ADP to form a second ATP (PEP to pyruvate).

In aerobic respiration, the 2 molecules of pyruvate can be transported to the matrix of the mitochondria, for the link reaction

Active transport is required to transport pyruvate across the double membrane of the mitochondria.

Link Reaction

What you need to know (from the AQA specification)

If respiration is aerobic, pyruvate from glycolysis enters the mitochondrial matrix by active transport. 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

Steps in Link Reaction

• Pyruvate is oxidised to acetate, producing reduced NAD. Carbon dioxide is also removed (decarboxylation), so the 3C pyruvate becomes 2C acetate
• Acetate then combines with a coenzyme A, this forms acetyl coenzyme A (shortened to acetyl CoA)
• Products from the link reaction from 1 molecule of glucose (remember 1 molecule produces 2 pyruvates)
  • 2 acetyl CoA - this will feed into the Krebs cycle
  • 2 reduced NAD - this will go into oxidative phosphorylation
  • 2 carbon dioxide - produced as a waste product

Krebs Cycle

What you need to know (from the AQA specification)

• 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

The Krebs cycle takes place in the mitochondrial matrix. It’s a series of oxidation-reduction reactions (redox).

What do we mean when we say 'oxidation-reduction reactions?'

• Intermediates in the Krebs cycle (e.g. citrate, 5C & many others you don’t need to know about) are oxidised, meaning they lose electrons and hydrogen. The hydrogen is then used to reduce the coenzymes NAD/FAD
• NAD and FAD coenzymes are reduced, which means they gain electrons & hydrogen

Steps in Krebs Cycle

• Acetyl CoA (2C) reacts with a 4C molecule (Oxaloacetate), releasing coenzyme A and producing a six-carbon molecule (Citrate)

Where do you think the coenzyme A goes to?

• It gets recycled back to the link reaction

• The six-carbon molecule (Citrate) is converted to 5C molecule
  • Dehydrogenation = The hydrogen atoms are lost and are taken up by NAD to produce reduced NAD
  • Decarboxylation = carbon dioxide is removed (6C-> 5C)
• The 5C compound is converted to 4C (Oxaloacetate)
  • There are lots of intermediate compounds here (you don’t need to know these)
  • Decarboxylation = carbon dioxide is removed (5C->4C)
  • Dehydrogenation & oxidation of intermediate compounds
    • Hydrogen is removed and used to produce 2 x reduced NAD and 1 x reduced FAD
  • ATP is produced by substrate level phosphorylation
    • Phosphate group is directly transferred from one of the intermediates molecule to ADP to form ATP

Products from the Krebs Cycle

It’s important to note that for 1 molecule of glucose, this produces 2 x pyruvate, so 2 x Acetyl CoA. This means the Krebs cycle goes round twice

• 4 x carbon dioxide (2 for each turn)
• 2 x ATP (1 for each turn)
• 6 x reduced NAD (3 for each turn)
• 2 x reduced FAD (1 for each turn)

What is substrate level phosphorylation?

The phosphate group is directly transferred from a substrate molecule to ADP to form ATP.

• It occurs in:
  • Glycolysis (oxidation step, where ATP is produced)
  • Krebs cycle (from 5C to 4C Oxaloacetate)

Extra detail: substrate-level phosphorylation in glycolysis

Images below show an example of substrate level phosphorylation, where the phosphate is transferred from the intermediates in glycolysis to ADP to form ATP. You don’t need to know these intermediates this is just to illustrate how it works.

Image modified: Glycolysis pathway 2.png by Eunice Laurent is licensed under the Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license.

Oxidative Phosphorylation

What you need to know (from the AQA specification)

• 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)

Oxidative phosphorylation occurs in the inner mitochondrial membrane and produces lots of ATP! (much more than the other steps) Many enzymes and proteins involved with this process, e.g. ATP synthase, are present here.

This process is very similar to Light Dependent Reaction

Hydrogen atoms provided by reduced NAD and FAD

• The reduced NAD and FAD transfer hydrogen atoms, which split into hydrogen ions (H+) and electrons to the electron transport chain. They are oxidised to become NAD and FAD.

Where are the reduced NAD & FAD from?

All the other steps of respiration, including Glycolysis, Link reaction, and Krebs cycle.

Electrons move down the electron transport chain

• Electrons lose energy at each carrier (move from a higher energy level to a lower energy level)
• This energy is used to pump the H+ ions out of the mitochondrial matrix and into the intermembrane space.
• This establishes an electrochemical gradient, with a higher concentration of H+ ions in the intermembrane space than the matrix

What does electrochemical gradient mean?

• Combines two components (chemical and electrical) that drive proton flow
• Chemical gradient: Higher concentration of H+ ions in intermembrane space compared to matrix
• Electrical gradient: The accumulation of positively charged protons (H+) in intermembrane space creates a positive charge and a negative charge inside matrix

Chemiosmosis

• As the hydrogen ions move down their concentration gradient, they pass through an enzyme called ATP synthase. This movement of hydrogen ions drives the synthesis of ATP (from ADP + Pi)

Oxygen is the final acceptor of electrons

• At the end of the electron transport chain, electrons and protons (H+ ions) combine with oxygen to form water.

Anaerobic Respiration

What you need to know (from the AQA specification)

If respiration is only anaerobic, pyruvate can be converted to ethanol or lactate using reduced NAD. The oxidised NAD produced in this way can be used in further glycolysis.

In anaerobic respiration, there isn’t oxygen to be the final acceptor in the electron transport chain, so reduced NAD can’t be oxidised back to NAD there. Without NAD, glycolysis would stop. This is where the fermentation pathway comes in.

The first step is Glycolysis which breaks glucose down to pyruvate This pyruvate can be used in either a:

• Alcoholic fermentation (plants & yeast) - where pyruvate is converted to ethanol
• Lactate fermentation - where pyruvate is converted to lactate

In both pathways, pyruvate (or ethanal in yeast) is reduced, which means it accepts hydrogen from reduced NAD. This oxidises reduced NAD back to NAD, so glycolysis can continue.

Other respiratory substrates

What you need to know (from the AQA specification)

• other respiratory substrates include the breakdown products of lipids and amino acids, which enter the Krebs cycle.

The food you eat doesn’t just contain glucose, it contains other nutrients such as fats and proteins.

These can be broken down into:

• Fats: Glycerol & fatty acids
• Proteins: Amino acids

These can be converted into molecules that can enter the Krebs cycle (usually Acetyl CoA)

How this topic is tested

This analysis is based on past paper data from 2017 to 2025. It is intended for interest only and is not predictive of what will appear in future papers.

• Tested in 8 of 9 years (2017–2025): 28 question parts worth 60 marks.
• 9th most-examined topic overall by marks, 3rd in Unit 5.

Marks by year

2017

7 marks

2018

5 marks

2019

5 marks

2020

0 marks

2021

6 marks

2022

10 marks

2023

9 marks

2024

8 marks

2025

10 marks

Most-tested spec points

• Required Practical 9: tested in 12 parts (24 marks)
• Oxidative Phosphorylation: tested in 5 parts (14 marks)
• The Link Reaction & Krebs Cycle: tested in 6 parts (13 marks)
• Anaerobic Respiration: tested in 6 parts (12 marks)
• Glycolysis: tested in 2 parts (6 marks)
• Other Respiratory Substrates: tested in 1 part (3 marks)

Also links to: ATP Synthesis, Disaccharides & Glycosidic Bonds, Slow & Fast Skeletal Muscle Fibres.

Maths and practical skills

• Units and standard form (MS 0.1, 0.2): e.g. 2023 P3 Q2.2
• Area, volume and surface area of shapes (MS 4.1): e.g. 2024 P3 Q2.4
• Significant figures (MS 1.1): e.g. 2023 P3 Q2.2
• Using equations (MS 2.3): e.g. 2024 P3 Q2.4

Tips from examiner reports

What students commonly get wrong

• Pyruvate is reduced, not oxidisedWatch out: in anaerobic respiration, pyruvate is reduced (it gains hydrogen from reduced NAD) to lactate or ethanol. This regenerates NAD so glycolysis can continue. 2019 P2 Q4.3 · 2025 P2 Q1.3
• Why less oxygen is used: explain the chainWatch out: if something blocks the Krebs cycle or the electron transfer chain, fewer electrons reach the end of the chain. Oxygen is the final electron acceptor, so less oxygen is needed. 2021 P2 Q1.2 · 2025 P2 Q1.3
• Keep respiration and photosynthesis terms apartWatch out: respiration uses NAD and FAD, not NADP. RuBP, GP, photolysis and the Calvin cycle belong to photosynthesis. 2021 P2 Q1.1 · 2025 P2 Q1.2 · 2025 P2 Q1.3
• Glycolysis: net gain, and triose phosphate is oxidisedWatch out: say there is a net gain of ATP (4 made, 2 used). Say that triose phosphate is oxidised to pyruvate. 2021 P2 Q1.1
• Bacteria have no mitochondriaWatch out: bacteria don't have mitochondria. In prokaryotes, the electron transfer chain and ATP synthase are in the cell-surface membrane. 2025 P3 Q1.3
• Decarboxylation, not carboxylationWatch out: CO₂ is removed in the link reaction and the Krebs cycle. That's decarboxylation, not carboxylation. 2025 P2 Q1.2

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Exam Question Practice