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Caitlin Ellis
09/12/2025
Biology
WJEC A Level Biology Unit 3.1 & 3.3 Notes
164
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9 Dec 2025
•
WJEC A Level Biology: ATP and Respiration Notes (Units 3.1 & 3.3)
Caitlin Ellis
@caitlinellis0312
1 / 4
ATP Structure and Function
ATP (adenosine triphosphate) is literally the fuel that powers every single thing your cells do. It's made of adenine (a nitrogen base), ribose (a sugar), and three phosphate groups - that's where the magic happens.
When your cells need energy, ATP breaks down into ADP (adenosine diphosphate) plus a phosphate group, releasing exactly 30.6 kJ/mol of energy. This hydrolysis reaction is catalysed by ATPase enzymes. What makes ATP brilliant is that it releases energy in perfectly sized chunks - not too much, not too little.
Your cells use this energy for everything: active transport, nerve impulses, muscle contraction, and building complex molecules. The best part? ATP is water-soluble, so it zips around your cells effortlessly, delivering energy exactly where and when it's needed.
Quick Tip: Remember that making ATP from ADP requires energy input (endergonic), whilst breaking it down releases energy (exergonic) - it's like charging and using a battery!
Chloroplasts vs Mitochondria
Both chloroplasts and mitochondria are cellular powerhouses with surprising similarities. They've got double membranes, their own circular DNA, 70S ribosomes, and both contain electron transport chains that pump out ATP like energy factories.
Here's where they differ: chloroplasts (in plants) do photosynthesis using NADP, whilst mitochondria (in animals and plants) handle cellular respiration using NAD and FAD. Think of chloroplasts as solar panels and mitochondria as power stations.
Aerobic respiration can theoretically make 38 ATPs per glucose molecule, but you never quite hit that maximum due to leaky membranes and energy costs. Anaerobic respiration only manages 2 ATPs per glucose - much less efficient but crucial when oxygen runs low.
When oxygen's scarce, your muscle cells switch to making lactate (hello, muscle burn!), whilst plants and bacteria produce ethanol instead. The key difference? Your lactate can be recycled when oxygen returns, but ethanol production is permanent and can become toxic.
Remember: Aerobic = lots of ATP but needs oxygen; Anaerobic = quick energy but much less efficient!
Glycolysis and Early Respiration Stages
Glycolysis kicks off in your cytoplasm and doesn't need oxygen - making it your cells' emergency energy system. Glucose gets phosphorylated by 2 ATP molecules, splits into two triose phosphates, then gets converted into 2 pyruvate molecules through substrate-level phosphorylation.
The clever bit? You invest 2 ATPs upfront but get 4 back directly, plus 2 reduced NADs that'll make 6 more ATPs later - net gain of 8 ATPs total. Not bad for the opening act!
Next comes the link reaction in the mitochondrial matrix. Each pyruvate gets dehydrogenated and decarboxylated, losing CO₂ and forming acetyl coenzyme A. Since you've got 2 pyruvates, this happens twice, producing 2 reduced NADs (worth 6 ATPs).
The Krebs cycle then takes over, spinning acetyl coenzyme A through a series of reactions that produce 2 ATPs directly, 6 reduced NADs, and 2 reduced FADs. That's 24 ATPs worth of energy carriers heading to the final stage!
Memory Trick: Glycolysis = glucose splitting; Link reaction = pyruvate prepping; Krebs cycle = the big energy harvest!
Electron Transport Chain and Alternative Fuels
The electron transport chain on the inner mitochondrial membrane is where the real ATP magic happens. Reduced NAD and FAD get oxidised, releasing electrons that power pumps, forcing H⁺ ions into the intermembrane space like inflating a balloon.
When these H⁺ ions flow back through ATPase (the stalked particles), they drive ATP synthesis - it's called oxidative phosphorylation. Each reduced NAD makes 3 ATPs, whilst reduced FAD makes 2 ATPs. Oxygen acts as the final electron acceptor, combining with electrons and H⁺ to form water.
Your body's remarkably flexible with fuel sources. Lipids get chopped into 2-carbon fragments that enter as acetyl coenzyme A - they're actually more energy-dense than glucose. Proteins can also be used, but only during starvation after deamination removes the nitrogen groups.
The grand total? Up to 38 ATPs per glucose molecule through aerobic respiration - that's why oxygen is so vital for complex life. Without it, you're stuck with just 2 ATPs from glycolysis alone.
Key Point: The electron transport chain produces about 89% of your ATP - it's definitely the star of cellular respiration!
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David K
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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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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
WJEC A Level Biology: ATP and Respiration Notes (Units 3.1 & 3.3)
Caitlin Ellis
@caitlinellis0312
Ever wondered how your body gets energy from food? It all comes down to ATP - your cells' energy currency - and the incredible process of cellular respirationthat makes it all happen. Think of it as your body's power... Show more
ATP Structure and Function
ATP (adenosine triphosphate) is literally the fuel that powers every single thing your cells do. It's made of adenine (a nitrogen base), ribose (a sugar), and three phosphate groups - that's where the magic happens.
When your cells need energy, ATP breaks down into ADP (adenosine diphosphate) plus a phosphate group, releasing exactly 30.6 kJ/mol of energy. This hydrolysis reaction is catalysed by ATPase enzymes. What makes ATP brilliant is that it releases energy in perfectly sized chunks - not too much, not too little.
Your cells use this energy for everything: active transport, nerve impulses, muscle contraction, and building complex molecules. The best part? ATP is water-soluble, so it zips around your cells effortlessly, delivering energy exactly where and when it's needed.
Quick Tip: Remember that making ATP from ADP requires energy input (endergonic), whilst breaking it down releases energy (exergonic) - it's like charging and using a battery!
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Chloroplasts vs Mitochondria
Both chloroplasts and mitochondria are cellular powerhouses with surprising similarities. They've got double membranes, their own circular DNA, 70S ribosomes, and both contain electron transport chains that pump out ATP like energy factories.
Here's where they differ: chloroplasts (in plants) do photosynthesis using NADP, whilst mitochondria (in animals and plants) handle cellular respiration using NAD and FAD. Think of chloroplasts as solar panels and mitochondria as power stations.
Aerobic respiration can theoretically make 38 ATPs per glucose molecule, but you never quite hit that maximum due to leaky membranes and energy costs. Anaerobic respiration only manages 2 ATPs per glucose - much less efficient but crucial when oxygen runs low.
When oxygen's scarce, your muscle cells switch to making lactate (hello, muscle burn!), whilst plants and bacteria produce ethanol instead. The key difference? Your lactate can be recycled when oxygen returns, but ethanol production is permanent and can become toxic.
Remember: Aerobic = lots of ATP but needs oxygen; Anaerobic = quick energy but much less efficient!
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Glycolysis and Early Respiration Stages
Glycolysis kicks off in your cytoplasm and doesn't need oxygen - making it your cells' emergency energy system. Glucose gets phosphorylated by 2 ATP molecules, splits into two triose phosphates, then gets converted into 2 pyruvate molecules through substrate-level phosphorylation.
The clever bit? You invest 2 ATPs upfront but get 4 back directly, plus 2 reduced NADs that'll make 6 more ATPs later - net gain of 8 ATPs total. Not bad for the opening act!
Next comes the link reaction in the mitochondrial matrix. Each pyruvate gets dehydrogenated and decarboxylated, losing CO₂ and forming acetyl coenzyme A. Since you've got 2 pyruvates, this happens twice, producing 2 reduced NADs (worth 6 ATPs).
The Krebs cycle then takes over, spinning acetyl coenzyme A through a series of reactions that produce 2 ATPs directly, 6 reduced NADs, and 2 reduced FADs. That's 24 ATPs worth of energy carriers heading to the final stage!
Memory Trick: Glycolysis = glucose splitting; Link reaction = pyruvate prepping; Krebs cycle = the big energy harvest!
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Electron Transport Chain and Alternative Fuels
The electron transport chain on the inner mitochondrial membrane is where the real ATP magic happens. Reduced NAD and FAD get oxidised, releasing electrons that power pumps, forcing H⁺ ions into the intermembrane space like inflating a balloon.
When these H⁺ ions flow back through ATPase (the stalked particles), they drive ATP synthesis - it's called oxidative phosphorylation. Each reduced NAD makes 3 ATPs, whilst reduced FAD makes 2 ATPs. Oxygen acts as the final electron acceptor, combining with electrons and H⁺ to form water.
Your body's remarkably flexible with fuel sources. Lipids get chopped into 2-carbon fragments that enter as acetyl coenzyme A - they're actually more energy-dense than glucose. Proteins can also be used, but only during starvation after deamination removes the nitrogen groups.
The grand total? Up to 38 ATPs per glucose molecule through aerobic respiration - that's why oxygen is so vital for complex life. Without it, you're stuck with just 2 ATPs from glycolysis alone.
Key Point: The electron transport chain produces about 89% of your ATP - it's definitely the star of cellular respiration!
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.
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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