Maths
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Responding to change (a2 only)
Infection and response
Homeostasis and response
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Cell biology
Organisms respond to changes in their internal and external environments (a-level only)
Biological molecules
Organisation
Substance exchange
Bioenergetics
Genetic information & variation
Inheritance, variation and evolution
Genetics & ecosystems (a2 only)
Ecology
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Britain & the wider world: 1745 -1901
1l the quest for political stability: germany, 1871-1991
The cold war
Inter-war germany
Medieval period: 1066 -1509
2d religious conflict and the church in england, c1529-c1570
2o democracy and nazism: germany, 1918-1945
1f industrialisation and the people: britain, c1783-1885
1c the tudors: england, 1485-1603
2m wars and welfare: britain in transition, 1906-1957
World war two & the holocaust
2n revolution and dictatorship: russia, 1917-1953
2s the making of modern britain, 1951-2007
World war one
Britain: 1509 -1745
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Biology
11 Dec 2025
251
1 page
Ever wondered how massive trees get water from their roots all the way up to their highest leaves?... Show more
Think of phloem and xylem cells as the plant's motorway system - they're perfectly designed to move stuff around efficiently. These specialised cells link up end-to-end to create long tubes that stretch throughout the entire plant, from roots to leaves.
What makes these cells brilliant at their job is how they've adapted their structure. Xylem cells are completely hollow in the centre, creating clear pathways for water and minerals to flow through. Meanwhile, phloem cells have stripped away most of their internal bits to make room for transporting food and water.
The two systems work quite differently though. Xylem vessels are like one-way streets - they only transport water and minerals upwards from the roots. They've got thick walls strengthened with lignin (a tough substance) and no barriers between cells, so water flows freely.
Quick Tip Remember "Xylem = Up" - xylem only moves substances upward from roots to leaves, whilst phloem can go both ways!
Phloem vessels are more like two-way roads, moving food (mainly sugars made in photosynthesis) both up and down the plant. Unlike xylem, phloem cells keep their end walls, but these walls have tiny holes (perforations) that let substances pass through whilst maintaining some structure.
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.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
2
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Explore the mechanisms of gas exchange in mammals, fish, and insects. This study note covers specialized structures such as alveoli, gill filaments, and spiracles, detailing their roles in efficient oxygen and carbon dioxide exchange. Ideal for A-Level Biology students studying the Edexcel B curriculum.
BiologyExplore the intricate process of gas exchange in the lungs, detailing how oxygen is collected by blood and delivered to body cells. This summary covers the anatomy of the respiratory system, including the trachea, bronchi, bronchioles, and alveoli, highlighting their roles in efficient gaseous exchange. Ideal for students studying human biology and respiratory physiology.
BiologyExplore the intricate process of gas exchange in the lungs, focusing on the role of alveoli, blood capillaries, and the impact of exercise on breathing rates. This summary covers specialized exchange surfaces, diffusion pathways, and the physiological changes during physical activity. Ideal for biology students studying respiratory systems.
BiologyExplore the mechanisms of gas exchange in the lungs, focusing on the role of alveoli, blood capillaries, and the respiratory system. This summary covers key adaptations of alveoli for efficient oxygen and carbon dioxide exchange, highlighting the importance of surface area and moisture. Ideal for students studying respiratory physiology.
BiologyExplore the gas exchange systems of insects, bony fish, and mammals in this comprehensive AS and A-Level Biology summary. Understand key concepts such as diffusion, alveoli, gill arches, and the tracheal system. This resource highlights the importance of specialized exchange surfaces and the surface area-to-volume ratio in efficient respiration. Perfect for exam preparation and quick revision.
BiologyExplore the mechanisms of carbohydrate, lipid, and protein digestion, including the role of bile salts and enzymes. Understand how nutrients like glucose and fatty acids are absorbed in the small intestine, highlighting key processes and enzyme locations.
BiologyApp Store
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
Ever wondered how massive trees get water from their roots all the way up to their highest leaves? Plants have developed amazing specialised cells called phloem and xylemthat work like a sophisticated transport system, moving water, food, and minerals... Show more
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Think of phloem and xylem cells as the plant's motorway system - they're perfectly designed to move stuff around efficiently. These specialised cells link up end-to-end to create long tubes that stretch throughout the entire plant, from roots to leaves.
What makes these cells brilliant at their job is how they've adapted their structure. Xylem cells are completely hollow in the centre, creating clear pathways for water and minerals to flow through. Meanwhile, phloem cells have stripped away most of their internal bits to make room for transporting food and water.
The two systems work quite differently though. Xylem vessels are like one-way streets - they only transport water and minerals upwards from the roots. They've got thick walls strengthened with lignin (a tough substance) and no barriers between cells, so water flows freely.
Quick Tip: Remember "Xylem = Up" - xylem only moves substances upward from roots to leaves, whilst phloem can go both ways!
Phloem vessels are more like two-way roads, moving food (mainly sugars made in photosynthesis) both up and down the plant. Unlike xylem, phloem cells keep their end walls, but these walls have tiny holes (perforations) that let substances pass through whilst maintaining some structure.
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.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
2
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Explore the mechanisms of gas exchange in mammals, fish, and insects. This study note covers specialized structures such as alveoli, gill filaments, and spiracles, detailing their roles in efficient oxygen and carbon dioxide exchange. Ideal for A-Level Biology students studying the Edexcel B curriculum.
BiologyExplore the intricate process of gas exchange in the lungs, detailing how oxygen is collected by blood and delivered to body cells. This summary covers the anatomy of the respiratory system, including the trachea, bronchi, bronchioles, and alveoli, highlighting their roles in efficient gaseous exchange. Ideal for students studying human biology and respiratory physiology.
BiologyExplore the intricate process of gas exchange in the lungs, focusing on the role of alveoli, blood capillaries, and the impact of exercise on breathing rates. This summary covers specialized exchange surfaces, diffusion pathways, and the physiological changes during physical activity. Ideal for biology students studying respiratory systems.
BiologyExplore the mechanisms of gas exchange in the lungs, focusing on the role of alveoli, blood capillaries, and the respiratory system. This summary covers key adaptations of alveoli for efficient oxygen and carbon dioxide exchange, highlighting the importance of surface area and moisture. Ideal for students studying respiratory physiology.
BiologyExplore the gas exchange systems of insects, bony fish, and mammals in this comprehensive AS and A-Level Biology summary. Understand key concepts such as diffusion, alveoli, gill arches, and the tracheal system. This resource highlights the importance of specialized exchange surfaces and the surface area-to-volume ratio in efficient respiration. Perfect for exam preparation and quick revision.
BiologyExplore the mechanisms of carbohydrate, lipid, and protein digestion, including the role of bile salts and enzymes. Understand how nutrients like glucose and fatty acids are absorbed in the small intestine, highlighting key processes and enzyme locations.
BiologyApp Store
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
Sociology