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Organisms respond to changes in their internal and external environments (a-level only)
Biological molecules
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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
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Biology
15 Dec 2025
138
5 pages
Reuben Cowell @reubencowell
Ever wonder how stuff gets in and out of your cells? Your cells are constantly moving substances around... Show more
Your cells are like busy train stations with substances constantly moving in and out. Diffusion is the most straightforward process - it's simply molecules moving from where there's loads of them to where there's fewer.
Think of it like this if you spray perfume in one corner of a room, it'll spread out until the smell is everywhere. That's diffusion in action! The molecules are always jiggling about, but overall they move from the high concentration area to the low concentration area.
Diffusion happens across cell membranes and doesn't need any energy - it's completely natural. Once the molecules are spread out evenly, they keep moving but there's no net movement in any direction.
Quick Tip Diffusion always goes from high to low concentration - remember this for your exams!
Your lungs are brilliant examples of diffusion at work. Carbon dioxide diffuses from your blood (where there's loads of it) into your lung air sacs where there's less. Meanwhile, oxygen does the opposite - moving from your lungs into your blood.
Several factors control how fast diffusion happens. A bigger concentration gradient (difference between high and low areas) means faster diffusion. Higher temperatures speed things up because particles move more when they're hot.
Surface area is crucial too - more membrane surface means more space for molecules to cross. This is why smaller organisms are actually better at diffusion than larger ones - they've got a bigger surface area compared to their volume.
Remember Temperature, concentration difference, and surface area all affect diffusion speed!
Osmosis is just diffusion, but only for water molecules moving through partially permeable membranes. Water moves from where there's lots of it to where there's less of it.
In plant cells, osmosis creates some dramatic effects. Turgid cells are plump with water - the vacuole swells up and keeps the plant standing tall and strong. This happens when there's pure water outside the cell.
Flaccid cells are the opposite - they've lost water and gone a bit floppy. If things get really bad, plasmolysis occurs where the cytoplasm actually pulls away from the cell wall, leaving the plant droopy and weak.
Plant Tip Turgid = strong and upright, Flaccid = weak and droopy!
Animal cells don't have cell walls like plants, so they actually change size during osmosis. Put animal cells in pure water and they swell up; put them in salty water and they shrink. Too much change either way stops them working properly.
Sometimes cells need to move stuff the "wrong" way - from low to high concentration. This is called active transport and it's like pushing a boulder uphill. It needs energy from respiration and special enzymes to make it work.
Active transport is essential for things like getting mineral ions into plant roots and moving glucose through your intestines into your blood. Without it, your body couldn't get the nutrients where they need to go.
Key Difference Diffusion and osmosis are passive (no energy needed), active transport requires energy!
You'll likely need to calculate water uptake rates in practicals. The formula is Water uptake per hour = change in mass × (60 ÷ time in minutes). So if a plant gained 0.59g in 40 minutes, that's 0.59 × (60÷40) = 0.89g per hour.
Percentage change in mass shows you how much water was gained or lost compared to the starting point. Use ÷ starting mass × 100. This gives you positive numbers for water gain and negative for water loss.
These calculations pop up loads in osmosis experiments, so practice them! They help you compare different conditions and understand which setup caused the most water movement.
Exam Success Always show your working clearly and don't forget units in your final answers!
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.
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Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Anatomy Unit 1: Directional Terms (superior, inferior, ventral, dorsal, etc.) and Organ Systems
AP BiologyExplore the intricate functions of the kidney, focusing on the nephron's role in ultrafiltration and osmoregulation. This summary covers key components such as the glomerulus, Bowman's capsule, and the countercurrent multiplier mechanism, highlighting how the kidney maintains homeostasis and regulates water balance in the body. Ideal for students studying biology and human physiology.
BiologyAll notes from AP Classroom slides and videos
AP BiologyAP Bio topic 2.9
AP BiologyReviews homeostasis for AP Bio
AP BiologyPlant Organs
AP BiologyQuotes from every main character
English LiteratureApp 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
Reuben Cowell
@reubencowell
Ever wonder how stuff gets in and out of your cells? Your cells are constantly moving substances around - oxygen in, carbon dioxide out, water everywhere! There are three main ways this happens, and understanding them will help you ace... Show more
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Your cells are like busy train stations with substances constantly moving in and out. Diffusion is the most straightforward process - it's simply molecules moving from where there's loads of them to where there's fewer.
Think of it like this: if you spray perfume in one corner of a room, it'll spread out until the smell is everywhere. That's diffusion in action! The molecules are always jiggling about, but overall they move from the high concentration area to the low concentration area.
Diffusion happens across cell membranes and doesn't need any energy - it's completely natural. Once the molecules are spread out evenly, they keep moving but there's no net movement in any direction.
Quick Tip: Diffusion always goes from high to low concentration - remember this for your exams!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Your lungs are brilliant examples of diffusion at work. Carbon dioxide diffuses from your blood (where there's loads of it) into your lung air sacs where there's less. Meanwhile, oxygen does the opposite - moving from your lungs into your blood.
Several factors control how fast diffusion happens. A bigger concentration gradient (difference between high and low areas) means faster diffusion. Higher temperatures speed things up because particles move more when they're hot.
Surface area is crucial too - more membrane surface means more space for molecules to cross. This is why smaller organisms are actually better at diffusion than larger ones - they've got a bigger surface area compared to their volume.
Remember: Temperature, concentration difference, and surface area all affect diffusion speed!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Osmosis is just diffusion, but only for water molecules moving through partially permeable membranes. Water moves from where there's lots of it to where there's less of it.
In plant cells, osmosis creates some dramatic effects. Turgid cells are plump with water - the vacuole swells up and keeps the plant standing tall and strong. This happens when there's pure water outside the cell.
Flaccid cells are the opposite - they've lost water and gone a bit floppy. If things get really bad, plasmolysis occurs where the cytoplasm actually pulls away from the cell wall, leaving the plant droopy and weak.
Plant Tip: Turgid = strong and upright, Flaccid = weak and droopy!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Animal cells don't have cell walls like plants, so they actually change size during osmosis. Put animal cells in pure water and they swell up; put them in salty water and they shrink. Too much change either way stops them working properly.
Sometimes cells need to move stuff the "wrong" way - from low to high concentration. This is called active transport and it's like pushing a boulder uphill. It needs energy from respiration and special enzymes to make it work.
Active transport is essential for things like getting mineral ions into plant roots and moving glucose through your intestines into your blood. Without it, your body couldn't get the nutrients where they need to go.
Key Difference: Diffusion and osmosis are passive (no energy needed), active transport requires energy!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
You'll likely need to calculate water uptake rates in practicals. The formula is: Water uptake per hour = change in mass × (60 ÷ time in minutes). So if a plant gained 0.59g in 40 minutes, that's 0.59 × (60÷40) = 0.89g per hour.
Percentage change in mass shows you how much water was gained or lost compared to the starting point. Use: ÷ starting mass × 100. This gives you positive numbers for water gain and negative for water loss.
These calculations pop up loads in osmosis experiments, so practice them! They help you compare different conditions and understand which setup caused the most water movement.
Exam Success: Always show your working clearly and don't forget units in your final answers!
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.
10
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Anatomy Unit 1: Directional Terms (superior, inferior, ventral, dorsal, etc.) and Organ Systems
AP BiologyExplore the intricate functions of the kidney, focusing on the nephron's role in ultrafiltration and osmoregulation. This summary covers key components such as the glomerulus, Bowman's capsule, and the countercurrent multiplier mechanism, highlighting how the kidney maintains homeostasis and regulates water balance in the body. Ideal for students studying biology and human physiology.
BiologyAll notes from AP Classroom slides and videos
AP BiologyAP Bio topic 2.9
AP BiologyReviews homeostasis for AP Bio
AP BiologyPlant Organs
AP BiologyQuotes from every main character
English LiteratureApp 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