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M
Marisa Smith
09/12/2025
Biology
Biology Plant Responses Revision Notes
692
•
9 Dec 2025
•
Essential OCR A Level Biology: Plant Responses Revision Notes
M
Marisa Smith
@marisasmith_qauu
Plant Responses and Survival
Plants are tougher than they look! When faced with hungry insects or harsh weather, they fight back using chemical warfare. Alkaloids make leaves taste bitter and poisonous, while tannins mess with digestion - basically making plants unappetising to potential predators.
What's really clever is how plants communicate danger. When under attack, they release alarm pheromones that warn neighbouring plants to start producing toxic chemicals. Some plants even release chemicals that attract predators like wasps to deal with pests naturally. The mimosa pudica takes a different approach - it simply folds its leaves when touched to knock off insects.
Plants also handle environmental stress brilliantly. In cold conditions, they produce antifreeze proteins to prevent ice crystals forming inside their cells, which would otherwise cause fatal damage.
Quick Tip: Remember that plants use both chemical and physical responses - it's not just about producing toxins!
Tropisms - How Plants Move Towards What They Need
Tropisms are directional growth responses that help plants survive. Think of them as plant superpowers that ensure they get light, water, and proper positioning. Positive tropism means growing towards a stimulus, whilst negative tropism means growing away from it.
Phototropism (response to light) is probably the most important. Shoots are positively phototropic - they grow towards light for photosynthesis. Roots do the opposite, growing away from light to anchor the plant properly.
Geotropism (response to gravity) ensures plants grow the right way up. Shoots are negatively geotropic (grow upwards against gravity), whilst roots are positively geotropic (grow downwards with gravity). Other tropisms include hydrotropism (response to water) and thigmotropism (response to touch).
Apical Dominance and Energy Management
Apical dominance is like a plant's version of prioritising tasks. The main shoot tip (apical bud) inhibits growth of side shoots from lateral buds, preventing the plant from wasting energy on multiple branches.
This clever system means plants can focus energy on growing tall to outcompete others for sunlight. The hormone auxin controls this - its concentration decreases down the plant, so lower lateral buds experience less inhibition.
If you remove the apical bud (like when pruning), auxin levels drop and side shoots start growing. This is why gardeners pinch out growing tips to encourage bushier plants!
Growth Hormones - Plant Control Systems
Plant hormones are chemical messengers that control growth and development. Gibberellins kickstart seed germination and flowering, whilst abscisic acid (ABA) helps plants cope with stress by closing stomata to conserve water.
Cytokinins stimulate cell division and differentiation, and ethene triggers flowering and fruit ripening. These hormones work together in complex ways to coordinate plant responses.
Indoleacetic acid (IAA) is a type of auxin that's particularly important for responding to light and gravity. It works by entering cell nuclei and acting as a transcription factor, switching genes on or off to control cell elongation and growth.
Leaf Loss and Seasonal Changes
Deciduous plants drop their leaves in winter to conserve water and energy. This process is controlled by a hormone balance between auxin (which prevents leaf loss) and ethene (which promotes it).
As leaves age, they produce less auxin but more ethene. This hormonal shift creates an abscission layer at the base of the leaf stalk. Ethene then stimulates these cells to expand and break down cell walls, causing the leaf to fall off.
Commercial Applications
Understanding plant hormones has massive commercial benefits. Ethene is used in the fruit industry to ripen fruits by breaking down cell walls and converting starch to sugars. Auxins work as selective weedkillers - they make weeds grow too fast to sustain themselves, causing them to die.
ABA controls stomatal closure by binding to guard cell receptors, triggering calcium ion channels to open. This causes ions to leave the guard cells, water follows by osmosis, and the cells become flaccid, closing the stomata to prevent water loss.
Farmers use auxins as rooting hormones to help plant cuttings develop roots quickly. Gibberellins work with auxins either synergistically (together for bigger effects) or antagonistically (opposing each other) depending on the specific growth process.
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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
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
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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
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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
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
Essential OCR A Level Biology: Plant Responses Revision Notes
M
Marisa Smith
@marisasmith_qauu
Ever wonder how plants survive without being able to run away from danger? Plants have developed clever ways to protect themselves from being eaten and respond to their environment using chemical signals called hormones. Understanding these plant responses is crucial... Show more
Plant Responses and Survival
Plants are tougher than they look! When faced with hungry insects or harsh weather, they fight back using chemical warfare. Alkaloids make leaves taste bitter and poisonous, while tannins mess with digestion - basically making plants unappetising to potential predators.
What's really clever is how plants communicate danger. When under attack, they release alarm pheromones that warn neighbouring plants to start producing toxic chemicals. Some plants even release chemicals that attract predators like wasps to deal with pests naturally. The mimosa pudica takes a different approach - it simply folds its leaves when touched to knock off insects.
Plants also handle environmental stress brilliantly. In cold conditions, they produce antifreeze proteins to prevent ice crystals forming inside their cells, which would otherwise cause fatal damage.
Quick Tip: Remember that plants use both chemical and physical responses - it's not just about producing toxins!
Tropisms - How Plants Move Towards What They Need
Tropisms are directional growth responses that help plants survive. Think of them as plant superpowers that ensure they get light, water, and proper positioning. Positive tropism means growing towards a stimulus, whilst negative tropism means growing away from it.
Phototropism (response to light) is probably the most important. Shoots are positively phototropic - they grow towards light for photosynthesis. Roots do the opposite, growing away from light to anchor the plant properly.
Geotropism (response to gravity) ensures plants grow the right way up. Shoots are negatively geotropic (grow upwards against gravity), whilst roots are positively geotropic (grow downwards with gravity). Other tropisms include hydrotropism (response to water) and thigmotropism (response to touch).
Apical Dominance and Energy Management
Apical dominance is like a plant's version of prioritising tasks. The main shoot tip (apical bud) inhibits growth of side shoots from lateral buds, preventing the plant from wasting energy on multiple branches.
This clever system means plants can focus energy on growing tall to outcompete others for sunlight. The hormone auxin controls this - its concentration decreases down the plant, so lower lateral buds experience less inhibition.
If you remove the apical bud (like when pruning), auxin levels drop and side shoots start growing. This is why gardeners pinch out growing tips to encourage bushier plants!
Growth Hormones - Plant Control Systems
Plant hormones are chemical messengers that control growth and development. Gibberellins kickstart seed germination and flowering, whilst abscisic acid (ABA) helps plants cope with stress by closing stomata to conserve water.
Cytokinins stimulate cell division and differentiation, and ethene triggers flowering and fruit ripening. These hormones work together in complex ways to coordinate plant responses.
Indoleacetic acid (IAA) is a type of auxin that's particularly important for responding to light and gravity. It works by entering cell nuclei and acting as a transcription factor, switching genes on or off to control cell elongation and growth.
Leaf Loss and Seasonal Changes
Deciduous plants drop their leaves in winter to conserve water and energy. This process is controlled by a hormone balance between auxin (which prevents leaf loss) and ethene (which promotes it).
As leaves age, they produce less auxin but more ethene. This hormonal shift creates an abscission layer at the base of the leaf stalk. Ethene then stimulates these cells to expand and break down cell walls, causing the leaf to fall off.
Commercial Applications
Understanding plant hormones has massive commercial benefits. Ethene is used in the fruit industry to ripen fruits by breaking down cell walls and converting starch to sugars. Auxins work as selective weedkillers - they make weeds grow too fast to sustain themselves, causing them to die.
ABA controls stomatal closure by binding to guard cell receptors, triggering calcium ion channels to open. This causes ions to leave the guard cells, water follows by osmosis, and the cells become flaccid, closing the stomata to prevent water loss.
Farmers use auxins as rooting hormones to help plant cuttings develop roots quickly. Gibberellins work with auxins either synergistically (together for bigger effects) or antagonistically (opposing each other) depending on the specific growth process.
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?
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Is Knowunity really free of charge?
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Students love us — and so will you.
4.9/5
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4.8/5
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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
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Elisha
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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
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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.
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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
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