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3 Jan 2026
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A-Level Biology: Clear Notes on Aerobic Respiration Processes
Isabelle
@isabelle_study
Glycolysis and the Link Reaction
Ever wondered how your body turns that glucose from your breakfast into usable energy? Glycolysis kicks off this process right in your cell's cytoplasm, splitting one glucose molecule into two smaller pyruvate molecules. It's like breaking a six-carbon chain into two three-carbon pieces.
The process starts with phosphorylation, where glucose gets energised by adding two phosphate groups from ATP molecules. Think of it as an investment - you spend 2 ATP to get the process rolling. Next comes oxidation, where those triose phosphate molecules get converted into pyruvate, and NAD picks up hydrogen atoms to become reduced NAD.
Here's the brilliant bit: whilst you invested 2 ATP at the start, you actually produce 4 ATP during oxidation, giving you a net gain of 2 ATP. Plus, you've made 2 reduced NAD molecules that will be crucial later.
Quick Tip: Remember that glycolysis happens in both aerobic AND anaerobic respiration - it's the universal starting point for energy release.
After glycolysis, pyruvate heads into the link reaction in the mitochondrial matrix. Each pyruvate loses a carbon (released as CO₂) and gets converted to acetate, which then combines with coenzyme A to form acetyl CoA. Since you started with 2 pyruvate molecules, this happens twice per glucose molecule.
Krebs Cycle and Oxidative Phosphorylation
The Krebs cycle is where the real energy extraction happens, taking place in the mitochondrial matrix like a perfectly choreographed dance. Your acetyl CoA from the link reaction combines with oxaloacetate to form a 6-carbon citrate molecule, kicking off this cyclical process.
As citrate moves through the cycle, it undergoes decarboxylation (losing CO₂) and dehydrogenation (losing hydrogen atoms). These reactions gradually break down the molecule whilst producing reduced NAD, reduced FAD, and some ATP. The clever part? Oxaloacetate gets regenerated at the end, ready to accept another acetyl CoA.
Oxidative phosphorylation is the grand finale where most of your ATP gets made. All those reduced NAD and reduced FAD molecules from previous stages release their hydrogen atoms, which split into protons and electrons. The electrons travel down the electron transport chain, losing energy that's used to pump protons into the intermembrane space.
Remember: Oxygen is the final electron acceptor - without it, this whole process grinds to a halt!
This creates an electrochemical gradient that drives chemiosmosis. Protons flow back through ATP synthase like water through a turbine, powering the conversion of ADP + Pi into ATP. Meanwhile, protons, electrons, and oxygen combine to form water, completing the cycle that keeps your cells energised.
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This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
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Sudenaz Ocak
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In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
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very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
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I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
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THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
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This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
A-Level Biology: Clear Notes on Aerobic Respiration Processes
Isabelle
@isabelle_study
Cellular respiration is how your body breaks down glucose to release energy for everything you do - from thinking to moving. This process happens in three main stages that work together like a well-oiled machine to produce ATP, the energy... Show more
Glycolysis and the Link Reaction
Ever wondered how your body turns that glucose from your breakfast into usable energy? Glycolysis kicks off this process right in your cell's cytoplasm, splitting one glucose molecule into two smaller pyruvate molecules. It's like breaking a six-carbon chain into two three-carbon pieces.
The process starts with phosphorylation, where glucose gets energised by adding two phosphate groups from ATP molecules. Think of it as an investment - you spend 2 ATP to get the process rolling. Next comes oxidation, where those triose phosphate molecules get converted into pyruvate, and NAD picks up hydrogen atoms to become reduced NAD.
Here's the brilliant bit: whilst you invested 2 ATP at the start, you actually produce 4 ATP during oxidation, giving you a net gain of 2 ATP. Plus, you've made 2 reduced NAD molecules that will be crucial later.
Quick Tip: Remember that glycolysis happens in both aerobic AND anaerobic respiration - it's the universal starting point for energy release.
After glycolysis, pyruvate heads into the link reaction in the mitochondrial matrix. Each pyruvate loses a carbon (released as CO₂) and gets converted to acetate, which then combines with coenzyme A to form acetyl CoA. Since you started with 2 pyruvate molecules, this happens twice per glucose molecule.
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Krebs Cycle and Oxidative Phosphorylation
The Krebs cycle is where the real energy extraction happens, taking place in the mitochondrial matrix like a perfectly choreographed dance. Your acetyl CoA from the link reaction combines with oxaloacetate to form a 6-carbon citrate molecule, kicking off this cyclical process.
As citrate moves through the cycle, it undergoes decarboxylation (losing CO₂) and dehydrogenation (losing hydrogen atoms). These reactions gradually break down the molecule whilst producing reduced NAD, reduced FAD, and some ATP. The clever part? Oxaloacetate gets regenerated at the end, ready to accept another acetyl CoA.
Oxidative phosphorylation is the grand finale where most of your ATP gets made. All those reduced NAD and reduced FAD molecules from previous stages release their hydrogen atoms, which split into protons and electrons. The electrons travel down the electron transport chain, losing energy that's used to pump protons into the intermembrane space.
Remember: Oxygen is the final electron acceptor - without it, this whole process grinds to a halt!
This creates an electrochemical gradient that drives chemiosmosis. Protons flow back through ATP synthase like water through a turbine, powering the conversion of ADP + Pi into ATP. Meanwhile, protons, electrons, and oxygen combine to form water, completing the cycle that keeps your cells energised.
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.
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Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.9/5
App Store
4.8/5
Google 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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user