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Updated Mar 21, 2026
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AQA A-Level Transport Across Cell Membranes Comprehensive Notes
cait
@caittt
1 / 3
The Fluid-Mosaic Model
Think of your cell membrane as a flexible sandwich made mostly of phospholipids arranged in a double layer (bilayer). These phospholipids have water-loving heads and water-hating tails, which creates a perfect barrier that's both protective and functional.
Floating within this phospholipid bilayer are various proteins that act like doorways and transporters. Peripheral proteins sit on the surface, whilst integral proteins span right through the membrane, often serving as channels or pumps for moving substances across.
You'll also find glycoproteins and glycolipids decorating the outside surface like name tags - these help cells recognise each other and communicate. Cholesterol molecules scattered throughout act like membrane stabilisers, keeping everything at just the right consistency regardless of temperature changes.
Key Point: The membrane is "fluid" because molecules can move around, and "mosaic" because it's made up of many different components working together!
Transport Types and Diffusion
Cell transport comes in two main flavours: passive transport (no energy required) and active transport (energy needed). Diffusion is the star of passive transport - it's simply particles moving from crowded areas to less crowded ones, like people spreading out in a room.
The membrane's semi-permeability means it's picky about what passes through. Small, nonpolar molecules like oxygen slip through easily, whilst small polar molecules like water need a bit more effort. Large molecules and ions typically require special protein channels to get across.
Osmosis is diffusion's water-focused cousin - it's specifically about water moving through membranes towards areas with higher solute concentration. This creates different scenarios: isotonic (balanced), hypotonic (water rushes in), and hypertonic (water leaves the cell).
Remember: Plant cells handle water pressure better than animal cells because of their rigid cell walls - they can become turgid without bursting!
Active Transport and Diffusion Factors
Sometimes cells need to move substances against the concentration gradient - from low to high concentration areas. This is where active transport comes in, using energy from ATP to power the process through specialised protein pumps.
Co-transport is a clever system where two substances hitchhike together across the membrane. Symport moves them in the same direction, whilst antiport sends them in opposite directions - like the sodium-glucose transport in your intestines that helps absorb nutrients.
Several factors affect how quickly diffusion happens: concentration gradient (the driving force), surface area to volume ratio, distance, temperature, and the number of available channel proteins. Fick's law neatly summarises this - faster diffusion occurs with larger surface areas and concentration differences, but slows down over longer distances.
Exam Tip: Remember that active transport always requires energy and moves substances uphill against their natural flow - think of it as cellular weightlifting!
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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
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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
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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.
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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
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 Knowunity AI. 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
AQA A-Level Transport Across Cell Membranes Comprehensive Notes
cait
@caittt
Ever wonder how your cells decide what gets in and what stays out? Cell membranes are like incredibly smart bouncers at a club - they're selective about who passes through and use different methods to control the flow of substances... Show more
The Fluid-Mosaic Model
Think of your cell membrane as a flexible sandwich made mostly of phospholipids arranged in a double layer (bilayer). These phospholipids have water-loving heads and water-hating tails, which creates a perfect barrier that's both protective and functional.
Floating within this phospholipid bilayer are various proteins that act like doorways and transporters. Peripheral proteins sit on the surface, whilst integral proteins span right through the membrane, often serving as channels or pumps for moving substances across.
You'll also find glycoproteins and glycolipids decorating the outside surface like name tags - these help cells recognise each other and communicate. Cholesterol molecules scattered throughout act like membrane stabilisers, keeping everything at just the right consistency regardless of temperature changes.
Key Point: The membrane is "fluid" because molecules can move around, and "mosaic" because it's made up of many different components working together!
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Transport Types and Diffusion
Cell transport comes in two main flavours: passive transport (no energy required) and active transport (energy needed). Diffusion is the star of passive transport - it's simply particles moving from crowded areas to less crowded ones, like people spreading out in a room.
The membrane's semi-permeability means it's picky about what passes through. Small, nonpolar molecules like oxygen slip through easily, whilst small polar molecules like water need a bit more effort. Large molecules and ions typically require special protein channels to get across.
Osmosis is diffusion's water-focused cousin - it's specifically about water moving through membranes towards areas with higher solute concentration. This creates different scenarios: isotonic (balanced), hypotonic (water rushes in), and hypertonic (water leaves the cell).
Remember: Plant cells handle water pressure better than animal cells because of their rigid cell walls - they can become turgid without bursting!
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Active Transport and Diffusion Factors
Sometimes cells need to move substances against the concentration gradient - from low to high concentration areas. This is where active transport comes in, using energy from ATP to power the process through specialised protein pumps.
Co-transport is a clever system where two substances hitchhike together across the membrane. Symport moves them in the same direction, whilst antiport sends them in opposite directions - like the sodium-glucose transport in your intestines that helps absorb nutrients.
Several factors affect how quickly diffusion happens: concentration gradient (the driving force), surface area to volume ratio, distance, temperature, and the number of available channel proteins. Fick's law neatly summarises this - faster diffusion occurs with larger surface areas and concentration differences, but slows down over longer distances.
Exam Tip: Remember that active transport always requires energy and moves substances uphill against their natural flow - think of it as cellular weightlifting!
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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 Knowunity AI. 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 Knowunity AI. 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