Maths
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Organisms respond to changes in their internal and external environments (a-level only)
Biological molecules
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1l the quest for political stability: germany, 1871-1991
Britain & the wider world: 1745 -1901
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
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Britain: 1509 -1745
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Biology
2 Dec 2025
173
2 pages
eleanor ๐ @eleanor.calder
Cell membranes are like flexible security guards for your cells - they control what gets in and out... Show more
Think of cell membranes as a flexible sandwich made of phospholipids - molecules with water-loving heads and water-hating tails. These create a phospholipid bilayer that forms the basic structure of every cell membrane.
The membrane isn't just made of phospholipids though. Cholesterol acts like a temperature control system, keeping the membrane at just the right flexibility - not too rigid, not too melted. Various proteins float in this structure like icebergs, each with specific jobs like transporting molecules or recognising other cells.
Temperature and chemicals can seriously mess with membranes. Too cold and ice crystals pierce through them; too hot (above 45ยฐC) and the whole thing starts melting. Solvents like alcohol dissolve the lipids, which is why drinking too much alcohol damages your cells.
Quick Check Remember that polar or charged molecules can't just slip through membranes - they need special protein helpers to get across!
Moving stuff across cell membranes happens in several ways, and understanding these is key to biology success. Passive diffusion is the easiest - molecules simply slide from areas where there's loads of them to areas where there's fewer, like perfume spreading across a room.
Facilitated diffusion is for molecules that need a helping hand, like glucose and amino acids. They use carrier proteins (which change shape to shuttle molecules across) or channel proteins (which act like doorways) to cross the membrane.
Active transport is the real workhorse - it pumps substances uphill against the concentration gradient using ATP energy. Your gut uses this to absorb nutrients from food, and plant roots use it to grab minerals from soil.
Osmosis is just water moving to balance things out. Put a cell in pure water (hypotonic) and it swells up; put it in salty water (hypertonic) and it shrinks. Plant cells handle this better than animal cells because their cell walls prevent bursting.
Exam Tip Remember that active transport always needs energy (ATP) and moves against the gradient - that's what makes it "active"!
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 โ all at your fingertips.
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Smart Tools NEW
Transform this note into: โ 50+ Practice Questions โ Interactive Flashcards โ Full Mock Exam โ Essay Outlines
Flashcards for water (biological molecules) specifically designed from the specification
BiologyExplore the key processes of osmosis, diffusion, and active transport across cell membranes. This summary covers how water movement affects animal and plant cells, including concepts like turgidity and plasmolysis. Ideal for National 5 biology students seeking to understand cellular transport dynamics.
BiologyExplore the concepts of osmosis and water potential in this detailed summary. Understand the definitions of hypotonic, hypertonic, and isotonic solutions, and learn how water movement affects animal and plant cells. Key topics include water potential measurement, turgidity, and plasmolysis. Ideal for A Level Biology students (OCR).
BiologyExplore the concept of osmosis, including definitions, key differences between plant and animal cells, and the effects of various solutions on cell structure. This summary includes essential diagrams and questions to enhance your understanding of water movement across membranes, crucial for biology exams.
BiologyExplore key concepts of cell membrane transport, including osmosis, diffusion, and active transport. This resource covers the fluid mosaic model, tonicity, and the roles of carrier proteins in cellular transport. Ideal for A Level Biology revision.
BiologyExplore key concepts in NAT5 Biology with this comprehensive summary covering proteins, enzymes, DNA structure, cellular respiration, and genetic engineering. Understand the roles of organelles, protein synthesis, and transport mechanisms like diffusion and osmosis. Ideal for exam preparation and quick revision.
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 itโs 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 didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm 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 ๐๐๐ฒ๐ค๐โจ๐๐ฎ
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 itโs 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 didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm 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 ๐๐๐ฒ๐ค๐โจ๐๐ฎ
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
eleanor ๐
@eleanor.calder
Cell membranes are like flexible security guards for your cells - they control what gets in and out whilst keeping everything together. Understanding how these membranes work and how substances move across them is crucial for explaining how your body... Show more
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Think of cell membranes as a flexible sandwich made of phospholipids - molecules with water-loving heads and water-hating tails. These create a phospholipid bilayer that forms the basic structure of every cell membrane.
The membrane isn't just made of phospholipids though. Cholesterol acts like a temperature control system, keeping the membrane at just the right flexibility - not too rigid, not too melted. Various proteins float in this structure like icebergs, each with specific jobs like transporting molecules or recognising other cells.
Temperature and chemicals can seriously mess with membranes. Too cold and ice crystals pierce through them; too hot (above 45ยฐC) and the whole thing starts melting. Solvents like alcohol dissolve the lipids, which is why drinking too much alcohol damages your cells.
Quick Check: Remember that polar or charged molecules can't just slip through membranes - they need special protein helpers to get across!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Moving stuff across cell membranes happens in several ways, and understanding these is key to biology success. Passive diffusion is the easiest - molecules simply slide from areas where there's loads of them to areas where there's fewer, like perfume spreading across a room.
Facilitated diffusion is for molecules that need a helping hand, like glucose and amino acids. They use carrier proteins (which change shape to shuttle molecules across) or channel proteins (which act like doorways) to cross the membrane.
Active transport is the real workhorse - it pumps substances uphill against the concentration gradient using ATP energy. Your gut uses this to absorb nutrients from food, and plant roots use it to grab minerals from soil.
Osmosis is just water moving to balance things out. Put a cell in pure water (hypotonic) and it swells up; put it in salty water (hypertonic) and it shrinks. Plant cells handle this better than animal cells because their cell walls prevent bursting.
Exam Tip: Remember that active transport always needs energy (ATP) and moves against the gradient - that's what makes it "active"!
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 โ all at your fingertips.
3
Smart Tools NEW
Transform this note into: โ 50+ Practice Questions โ Interactive Flashcards โ Full Mock Exam โ Essay Outlines
Flashcards for water (biological molecules) specifically designed from the specification
BiologyExplore the key processes of osmosis, diffusion, and active transport across cell membranes. This summary covers how water movement affects animal and plant cells, including concepts like turgidity and plasmolysis. Ideal for National 5 biology students seeking to understand cellular transport dynamics.
BiologyExplore the concepts of osmosis and water potential in this detailed summary. Understand the definitions of hypotonic, hypertonic, and isotonic solutions, and learn how water movement affects animal and plant cells. Key topics include water potential measurement, turgidity, and plasmolysis. Ideal for A Level Biology students (OCR).
BiologyExplore the concept of osmosis, including definitions, key differences between plant and animal cells, and the effects of various solutions on cell structure. This summary includes essential diagrams and questions to enhance your understanding of water movement across membranes, crucial for biology exams.
BiologyExplore key concepts of cell membrane transport, including osmosis, diffusion, and active transport. This resource covers the fluid mosaic model, tonicity, and the roles of carrier proteins in cellular transport. Ideal for A Level Biology revision.
BiologyExplore key concepts in NAT5 Biology with this comprehensive summary covering proteins, enzymes, DNA structure, cellular respiration, and genetic engineering. Understand the roles of organelles, protein synthesis, and transport mechanisms like diffusion and osmosis. Ideal for exam preparation and quick revision.
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 itโs 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 didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm 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 ๐๐๐ฒ๐ค๐โจ๐๐ฎ
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 itโs 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 didnโt even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as Iโm 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 ๐๐๐ฒ๐ค๐โจ๐๐ฎ
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
