Cellular respiration is how your body breaks down glucose to... Show more
Biology110 views·Updated Jun 9, 2026·3 pagesUnderstanding 3.3 Respiration in WJEC A-Level Biology
katie ♡@katielcnn
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Glycolysis, Link Reaction & Krebs Cycle
Glycolysis kicks off in your cell's cytoplasm because glucose can't actually squeeze through mitochondrial membranes. The process starts by adding two phosphate groups to glucose using ATP, creating hexose diphosphate that splits into two triose phosphate molecules.
The link reaction happens in the mitochondrial matrix where pyruvate gets modified. It loses a carbon (forming CO₂), gets stripped of hydrogen (making reduced NAD), and combines with coenzyme A to form acetyl CoA.
The Krebs cycle is where the real action happens - acetyl CoA joins with a 4-carbon acid to make a 6-carbon compound. Through two decarboxylation steps and four dehydrogenation reactions, each cycle produces 1 ATP, 3 reduced NAD molecules, 1 reduced FAD, and 2 CO₂ molecules.
Quick Tip: Remember that glucose produces two pyruvate molecules, so the Krebs cycle actually runs twice per glucose molecule, doubling all the products!
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Electron Transport Chain & Anaerobic Respiration
The electron transport chain on the inner mitochondrial membrane is like a series of energy-harvesting stations. Reduced NAD and FAD donate electrons that travel through carrier proteins, powering three proton pumps that push H⁺ ions into the intermembrane space.
This creates a proton gradient - loads of positive charges building up outside the matrix. These protons can only get back through special channels containing ATP synthase, which harnesses their flow to synthesise ATP from ADP.
When oxygen runs out, your cells switch to anaerobic respiration. In muscle cells during intense exercise, pyruvate becomes the hydrogen acceptor and forms lactate (causing that burning sensation). Microorganisms like yeast convert pyruvate to ethanol and CO₂ through alcoholic fermentation.
Key Difference: Lactate can be broken down when oxygen returns, but ethanol accumulates and becomes toxic to cells.
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ATP Production & Alternative Respiratory Pathways
A single glucose molecule can theoretically produce 38 ATP molecules through complete aerobic respiration - but real cells are less efficient due to energy costs of transport across membranes and proton leakage.
Lipids pack way more energy than carbohydrates. Fats get hydrolysed into glycerol and fatty acids - glycerol enters glycolysis whilst fatty acids break into 2-carbon fragments that feed directly into the Krebs cycle as acetyl CoA.
Proteins can also fuel respiration when needed. After deamination in the liver (removing the amino group as urea), the remaining carbon skeletons convert to pyruvate, acetyl CoA, or other Krebs cycle intermediates.
Energy Comparison: Fatty acids yield far more ATP per molecule than glucose because they contain more C-H bonds that can be oxidised.
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Biology110 views·Updated Jun 9, 2026·3 pagesUnderstanding 3.3 Respiration in WJEC A-Level Biology
katie ♡@katielcnn
Cellular respiration is how your body breaks down glucose to create the energy currency ATP that powers everything from muscle contractions to brain function. This complex process involves three main stages and can even work without oxygen when needed.
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Glycolysis, Link Reaction & Krebs Cycle
Glycolysis kicks off in your cell's cytoplasm because glucose can't actually squeeze through mitochondrial membranes. The process starts by adding two phosphate groups to glucose using ATP, creating hexose diphosphate that splits into two triose phosphate molecules.
The link reaction happens in the mitochondrial matrix where pyruvate gets modified. It loses a carbon (forming CO₂), gets stripped of hydrogen (making reduced NAD), and combines with coenzyme A to form acetyl CoA.
The Krebs cycle is where the real action happens - acetyl CoA joins with a 4-carbon acid to make a 6-carbon compound. Through two decarboxylation steps and four dehydrogenation reactions, each cycle produces 1 ATP, 3 reduced NAD molecules, 1 reduced FAD, and 2 CO₂ molecules.
Quick Tip: Remember that glucose produces two pyruvate molecules, so the Krebs cycle actually runs twice per glucose molecule, doubling all the products!
2
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Electron Transport Chain & Anaerobic Respiration
The electron transport chain on the inner mitochondrial membrane is like a series of energy-harvesting stations. Reduced NAD and FAD donate electrons that travel through carrier proteins, powering three proton pumps that push H⁺ ions into the intermembrane space.
This creates a proton gradient - loads of positive charges building up outside the matrix. These protons can only get back through special channels containing ATP synthase, which harnesses their flow to synthesise ATP from ADP.
When oxygen runs out, your cells switch to anaerobic respiration. In muscle cells during intense exercise, pyruvate becomes the hydrogen acceptor and forms lactate (causing that burning sensation). Microorganisms like yeast convert pyruvate to ethanol and CO₂ through alcoholic fermentation.
Key Difference: Lactate can be broken down when oxygen returns, but ethanol accumulates and becomes toxic to cells.
3
of 3Sign up to see the content. It's free!
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- Improve your grades
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ATP Production & Alternative Respiratory Pathways
A single glucose molecule can theoretically produce 38 ATP molecules through complete aerobic respiration - but real cells are less efficient due to energy costs of transport across membranes and proton leakage.
Lipids pack way more energy than carbohydrates. Fats get hydrolysed into glycerol and fatty acids - glycerol enters glycolysis whilst fatty acids break into 2-carbon fragments that feed directly into the Krebs cycle as acetyl CoA.
Proteins can also fuel respiration when needed. After deamination in the liver (removing the amino group as urea), the remaining carbon skeletons convert to pyruvate, acetyl CoA, or other Krebs cycle intermediates.
Energy Comparison: Fatty acids yield far more ATP per molecule than glucose because they contain more C-H bonds that can be oxidised.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content: Electron Transport Chain
3Most popular content in Biology
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha KlichAndroid user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
AnnaiOS user