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katie ♡
15/12/2025
Biology
3.3 Respiration - WJEC Biology A Level
79
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15 Dec 2025
•
Understanding 3.3 Respiration in WJEC A-Level Biology
katie ♡
@katielcnn
1 / 3
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!
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.
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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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.
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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 😍😁😲🤑💗✨🎀😮
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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
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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
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iOS user
Understanding 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.
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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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