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Oscar66
10/12/2025
Biology
Respiration notes (KREBS cycle etc) A Level Biology
57
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10 Dec 2025
•
Complete A Level Biology Notes: Glycolysis & Krebs Cycle
Oscar66
@oscar66
Overview of Cellular Respiration
Think of cellular respiration as your body's power station - it's constantly running to keep your cells energised. The process breaks down glucose through three connected stages, each happening in different parts of your cells.
Glycolysis kicks things off in the cytoplasm . This ancient process existed before mitochondria evolved, which explains why it happens outside these organelles. Here, glucose gets broken down into pyruvate, producing some ATP through substrate-level phosphorylation.
The real energy production happens in the mitochondrion. The Krebs cycle occurs in the matrix (think of it as the mitochondria's interior), whilst the electron transport chain operates in the cristae (folded inner membranes).
Here's the clever bit: high-energy electrons help pump protons across membranes, creating a gradient. When these protons flow back, their energy drives ATP synthase to make ATP through oxidative phosphorylation. Oxygen is only needed at the very end of the electron transport chain, but without it, everything grinds to a halt!
Key Insight: Cyanide is deadly because it blocks cytochrome oxidase, the enzyme that uses oxygen at the final step - showing just how crucial this process is for life.
The Three Stages in Detail
Glycolysis transforms one glucose molecule into two pyruvate molecules in your cytoplasm. This process uses and produces ATP, but more importantly, it generates NADH - think of this as an electron carrier that's like a rechargeable battery.
The Krebs cycle (also called the citric acid cycle) is where things get interesting. Pyruvate enters the mitochondria and gets converted into acetyl-CoA, which then goes through a series of chemical reactions. Each turn of the cycle produces CO₂, more NADH, FADH₂ (another electron carrier), and a bit more ATP.
The electron transport chain is your cellular powerhouse. NADH and FADH₂ donate their high-energy electrons to a series of protein complexes in the inner mitochondrial membrane. As electrons pass through these complexes, protons get pumped across the membrane, creating a proton gradient.
When protons flow back through ATP synthase, it's like water turning a turbine - the energy drives the production of loads of ATP. This is why the electron transport chain produces about 32-34 ATP molecules, compared to just 2 from glycolysis and 2 from the Krebs cycle.
Exam Tip: Remember the electron carriers - NAD⁺/NADH switching is crucial, and FAD/FADH₂ works similarly. These are your energy currency exchange systems!
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Basil
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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.
David K
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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
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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
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
Complete A Level Biology Notes: Glycolysis & Krebs Cycle
Oscar66
@oscar66
Cellular respiration is how your body converts glucose from food into usable energy (ATP) - it's literally what keeps you alive! This process happens in three main stages: glycolysis in the cytoplasm, the Krebs cycle in mitochondria, and the electron... Show more
Overview of Cellular Respiration
Think of cellular respiration as your body's power station - it's constantly running to keep your cells energised. The process breaks down glucose through three connected stages, each happening in different parts of your cells.
Glycolysis kicks things off in the cytoplasm . This ancient process existed before mitochondria evolved, which explains why it happens outside these organelles. Here, glucose gets broken down into pyruvate, producing some ATP through substrate-level phosphorylation.
The real energy production happens in the mitochondrion. The Krebs cycle occurs in the matrix (think of it as the mitochondria's interior), whilst the electron transport chain operates in the cristae (folded inner membranes).
Here's the clever bit: high-energy electrons help pump protons across membranes, creating a gradient. When these protons flow back, their energy drives ATP synthase to make ATP through oxidative phosphorylation. Oxygen is only needed at the very end of the electron transport chain, but without it, everything grinds to a halt!
Key Insight: Cyanide is deadly because it blocks cytochrome oxidase, the enzyme that uses oxygen at the final step - showing just how crucial this process is for life.
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The Three Stages in Detail
Glycolysis transforms one glucose molecule into two pyruvate molecules in your cytoplasm. This process uses and produces ATP, but more importantly, it generates NADH - think of this as an electron carrier that's like a rechargeable battery.
The Krebs cycle (also called the citric acid cycle) is where things get interesting. Pyruvate enters the mitochondria and gets converted into acetyl-CoA, which then goes through a series of chemical reactions. Each turn of the cycle produces CO₂, more NADH, FADH₂ (another electron carrier), and a bit more ATP.
The electron transport chain is your cellular powerhouse. NADH and FADH₂ donate their high-energy electrons to a series of protein complexes in the inner mitochondrial membrane. As electrons pass through these complexes, protons get pumped across the membrane, creating a proton gradient.
When protons flow back through ATP synthase, it's like water turning a turbine - the energy drives the production of loads of ATP. This is why the electron transport chain produces about 32-34 ATP molecules, compared to just 2 from glycolysis and 2 from the Krebs cycle.
Exam Tip: Remember the electron carriers - NAD⁺/NADH switching is crucial, and FAD/FADH₂ works similarly. These are your energy currency exchange systems!
We thought you’d never ask...
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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