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Plant Hormones Explained: Essential Revision Guide for OCR A-Level Biology
O
Olivia
@oliviag
1 / 5
Plant Movement and Responses
Plants might seem stationary, but they're constantly moving and responding to their surroundings. Tropism is how plants grow directionally based on external stimuli - think of it as their way of navigating the world.
Phototropism makes shoots grow towards light (positive), whilst geotropism pulls roots down with gravity (positive) and pushes shoots up against it (negative). You'll also see chemotropism when pollen tubes follow chemical trails to reach eggs, and thigmotropism when climbing plants like ivy wrap around supports.
Plants need these responses to avoid being eaten, reduce stress from harsh conditions, and survive long enough to reproduce successfully. It's basically their survival toolkit.
Key Point: All tropisms can be positive (towards stimulus) or negative (away from stimulus) - this determines the plant's growth direction.
Plant Growth Centres and Hormones
Plant growth happens in specific areas called meristems - clusters of young, dividing cells that haven't yet specialised. Apical meristems at root and shoot tips control length, whilst lateral meristems handle width growth.
There are five major plant hormones working like chemical messengers: auxins, cytokinins, gibberellins, abscisic acid, and ethene. They travel through the plant's transport system and bind to specific receptors on target cells.
Each hormone has different jobs throughout the plant's life cycle. Some promote germination, others control flowering, and several manage fruit development and leaf drop.
Remember: Plant hormones work at incredibly low concentrations, making them tricky for scientists to study - that's why we're still discovering how they work!
Auxins and Gibberellins in Action
Auxins are the superstars of plant growth, controlling shoot elongation by making cell walls stretchy. They create acidic conditions that activate enzymes to loosen cellulose bonds, allowing cells to expand as they absorb water.
These hormones maintain apical dominance - keeping the main shoot growing whilst suppressing side shoots. This helps plants compete for light effectively. Auxins also prevent leaf fall in young leaves and promote root growth at low concentrations.
Gibberellins work alongside auxins to cause stem elongation and trigger flowering. They're essential for breaking down food stores during seed germination and stimulating pollen tube growth.
Study Tip: Remember that auxins and gibberellins often work together (synergistically) to help plants grow tall and reach sunlight.
Ethene and Abscisic Acid Functions
Ethene is the plant's aging hormone, triggering fruit ripening and leaf fall in autumn. It creates an abscission layer at the base of leaf stalks that eventually breaks, causing leaves to drop off deciduous trees.
Abscisic acid (ABA) is the plant's stress manager and growth inhibitor. It prevents seeds from germinating at the wrong time and closes stomata during drought by making guard cells lose water and become flaccid.
Plant hormones often work against each other - auxins prevent leaf loss whilst ethene promotes it, and auxins inhibit side shoots whilst gibberellins stimulate them. This creates a balanced control system.
Exam Focus: Understanding how ABA closes stomata through ion movement and water loss is a common exam question - learn the step-by-step process.
Gibberellins and Growth Control
Gibberellins are particularly important for stem elongation, working through both cell division and cell elongation mechanisms. Research on dwarf plants has shown that gibberellins are essential for normal height development.
These experiments have helped scientists understand how plant hormones coordinate growth responses, though the complete picture is still being pieced together through ongoing research.
Quick Revision: Gibberellins = growth and germination, making them essential for a plant's development from seed to mature plant.
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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.
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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 😍😁😲🤑💗✨🎀😮
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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
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
Plant Hormones Explained: Essential Revision Guide for OCR A-Level Biology
O
Olivia
@oliviag
Ever wondered how plants know to grow towards light or why their roots dig deep into soil? It's all down to clever chemical messengers called plant hormones that help plants respond to their environment and survive.
Plant Movement and Responses
Plants might seem stationary, but they're constantly moving and responding to their surroundings. Tropism is how plants grow directionally based on external stimuli - think of it as their way of navigating the world.
Phototropism makes shoots grow towards light (positive), whilst geotropism pulls roots down with gravity (positive) and pushes shoots up against it (negative). You'll also see chemotropism when pollen tubes follow chemical trails to reach eggs, and thigmotropism when climbing plants like ivy wrap around supports.
Plants need these responses to avoid being eaten, reduce stress from harsh conditions, and survive long enough to reproduce successfully. It's basically their survival toolkit.
Key Point: All tropisms can be positive (towards stimulus) or negative (away from stimulus) - this determines the plant's growth direction.
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Plant Growth Centres and Hormones
Plant growth happens in specific areas called meristems - clusters of young, dividing cells that haven't yet specialised. Apical meristems at root and shoot tips control length, whilst lateral meristems handle width growth.
There are five major plant hormones working like chemical messengers: auxins, cytokinins, gibberellins, abscisic acid, and ethene. They travel through the plant's transport system and bind to specific receptors on target cells.
Each hormone has different jobs throughout the plant's life cycle. Some promote germination, others control flowering, and several manage fruit development and leaf drop.
Remember: Plant hormones work at incredibly low concentrations, making them tricky for scientists to study - that's why we're still discovering how they work!
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Auxins and Gibberellins in Action
Auxins are the superstars of plant growth, controlling shoot elongation by making cell walls stretchy. They create acidic conditions that activate enzymes to loosen cellulose bonds, allowing cells to expand as they absorb water.
These hormones maintain apical dominance - keeping the main shoot growing whilst suppressing side shoots. This helps plants compete for light effectively. Auxins also prevent leaf fall in young leaves and promote root growth at low concentrations.
Gibberellins work alongside auxins to cause stem elongation and trigger flowering. They're essential for breaking down food stores during seed germination and stimulating pollen tube growth.
Study Tip: Remember that auxins and gibberellins often work together (synergistically) to help plants grow tall and reach sunlight.
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Ethene and Abscisic Acid Functions
Ethene is the plant's aging hormone, triggering fruit ripening and leaf fall in autumn. It creates an abscission layer at the base of leaf stalks that eventually breaks, causing leaves to drop off deciduous trees.
Abscisic acid (ABA) is the plant's stress manager and growth inhibitor. It prevents seeds from germinating at the wrong time and closes stomata during drought by making guard cells lose water and become flaccid.
Plant hormones often work against each other - auxins prevent leaf loss whilst ethene promotes it, and auxins inhibit side shoots whilst gibberellins stimulate them. This creates a balanced control system.
Exam Focus: Understanding how ABA closes stomata through ion movement and water loss is a common exam question - learn the step-by-step process.
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Gibberellins and Growth Control
Gibberellins are particularly important for stem elongation, working through both cell division and cell elongation mechanisms. Research on dwarf plants has shown that gibberellins are essential for normal height development.
These experiments have helped scientists understand how plant hormones coordinate growth responses, though the complete picture is still being pieced together through ongoing research.
Quick Revision: Gibberellins = growth and germination, making them essential for a plant's development from seed to mature plant.
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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 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