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Biology
BIOLOGICAL MOLECULES,, OCR A level biology
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4 Dec 2025
•
Comprehensive Guide to Biological Molecules for OCR A-Level Biology
M
@iamjungkook
1 / 10
Building Blocks of Life: Monomers and Polymers
Think of biological molecules like construction sets - monomers are the individual bricks, whilst polymers are the finished structures built from identical pieces joined together. This simple concept explains how your body creates everything from energy stores to muscle fibres.
Two key reactions control this molecular construction site. Condensation reactions join monomers together by removing water molecules, like welding pieces with the loss of steam. Hydrolysis reactions do the opposite - they break apart large molecules by adding water back, which is exactly what happens when you digest food.
Carbohydrates follow the CHO formula and serve as your body's primary energy source. Monosaccharides like glucose are single sugar units that dissolve easily in blood for quick transport around your body.
Quick Tip: Remember that alpha glucose (used in starch) has hydrogen above the first carbon, whilst beta glucose (used in cellulose) has the OH group above - this tiny difference completely changes their properties!
Sugar Chemistry: From Simple to Complex
Ribose and deoxyribose might sound complicated, but they're just five-carbon sugars with a crucial difference. Ribose (in RNA) keeps all its OH groups, whilst deoxyribose (in DNA) loses one oxygen atom - making DNA more stable for long-term information storage.
When two monosaccharides join through condensation reactions, they form glycosidic bonds and create disaccharides like table sugar. These double sugars remain soluble and sweet, perfect for quick energy hits.
Polysaccharides are the heavy-duty storage units - long chains of sugars linked by glycosidic bonds. Most are insoluble due to their massive, branched structures, which makes them ideal for storing energy without affecting your cells' water balance.
Exam Focus: Each glycosidic bond formation releases one water molecule - count the bonds to work out how much water is produced in synthesis reactions!
Energy Storage: Starch and Its Components
Plants face the same problem you do - they need to store excess energy for later use. Starch solves this perfectly by being completely insoluble, so it won't mess with the plant's water balance while storing massive amounts of glucose.
Amylose forms unbranched chains that coil into tight spirals, packing efficiently like a compressed spring. Its compact structure makes it brilliant for storage but slow to break down. Amylopectin takes a different approach with its branched design, allowing enzymes to attack multiple points simultaneously for faster glucose release.
The branching happens through different bond types - straight sections use 1-4 glycosidic bonds, whilst branch points use 1-6 bonds. This creates a tree-like structure that balances storage efficiency with accessibility.
Memory Trick: Amylose = "A-my-coiled" (coiled structure), Amylopectin = "A-my-branches" (branched structure)!
Animal Storage and Structure: Glycogen and Cellulose
Glycogen is basically amylopectin on steroids - it has branches every 8-12 glucose units compared to starch's longer straight sections. This ultra-branched structure reflects animals' higher metabolic demands and need for rapid energy release during movement.
Cellulose breaks all the rules by using beta glucose, creating straight, unbranched chains where every other glucose molecule flips 180°. This alternating pattern allows adjacent molecules to form hydrogen bonds, creating rope-like microfibrils of incredible strength.
Triglycerides represent a completely different storage strategy. Made from glycerol plus three fatty acids joined by ester bonds, they pack almost twice the energy of carbohydrates. Their hydrophobic nature means they won't absorb water and swell up like a sponge.
Real-world Connection: Cellulose's strength-to-weight ratio rivals steel - that's why it's perfect for plant cell walls and why we use it to make paper!
Fats and Membrane Structure
Saturated fatty acids pack together tightly like straight pencils in a box, whilst unsaturated fatty acids have kinks from double bonds that push molecules apart. More kinks mean lower melting points - think of butter (saturated) versus olive oil (unsaturated) at room temperature.
Phospholipids are the molecular architects of cell membranes. With hydrophilic phosphate heads and hydrophobic fatty acid tails, they automatically arrange into bilayers - the heads face outward toward water whilst the tails hide inside.
This bilayer structure creates a selectively permeable barrier. Small, non-polar molecules can slip through the fatty acid centre, but water-soluble substances get blocked. It's like having a security system that only lets certain molecules pass.
Visual Learning: Picture phospholipids as matchsticks with magnetic heads - they'll always orient themselves with heads touching water and tails clustering together!
Membrane Fluidity and Protein Basics
Cholesterol acts like a membrane thermostat, keeping cell membranes at just the right consistency. In hot conditions, it prevents excessive fluidity; in cold conditions, it stops the membrane from becoming too rigid and cracking.
Proteins are the workhorses of your cells, built from chains of amino acids linked by peptide bonds. Each amino acid has the same basic structure except for its R group - this variable side chain gives each amino acid its unique personality.
The condensation reaction between amino acids is straightforward: the hydrogen from one amino acid's amine group combines with the hydroxyl group from another's carboxyl group, releasing water and forming a covalent peptide bond.
Study Strategy: Focus on the R groups when learning amino acids - they determine everything about how the protein will fold and function!
Protein Bonds and Structure Levels
Four types of bonds hold proteins in their final shapes. Hydrogen bonds form between slightly charged atoms but break easily with heat. Ionic bonds create strong attractions between oppositely charged R groups. Disulfide bridges form the strongest covalent bonds between sulfur-containing amino acids.
Primary structure is simply the sequence of amino acids - change even one, and you change the entire protein. Secondary structure introduces basic folding patterns like alpha helixes (tight spirals) and beta pleated sheets (parallel chains with hydrogen bonds between them).
Tertiary structure creates the final 3D shape through complex interactions between R groups. Hydrophobic R groups cluster inward whilst hydrophilic ones face outward, stabilised by all four bond types working together.
Exam Essential: Temperature and pH changes break hydrogen bonds first, which is why proteins denature (lose their shape) when you cook them!
Protein Folding and Quaternary Structure
Quaternary structure only exists in proteins with multiple polypeptide chains, called subunits. Each subunit has its own tertiary structure, but they work together like different instruments in an orchestra to create the protein's final function.
The four structural levels build upon each other logically. Primary determines secondary, secondary influences tertiary, and tertiary affects how multiple chains interact in quaternary structures. It's like following a recipe where each step depends on getting the previous one right.
Understanding these levels helps explain why protein folding is so critical. A single amino acid change in the primary structure can cascade through all levels, potentially destroying the protein's function - this is what happens in genetic diseases like sickle cell anemia.
Connection Point: Computer modelling now predicts protein shapes from amino acid sequences, revolutionising drug design by showing how medicines might interact with target proteins!
Globular Proteins: Specialists in Action
Globular proteins are the specialists of the protein world - compact, spherical, and designed for specific jobs. Their hydrophilic R groups face outward for solubility, whilst hydrophobic groups cluster inside, making them perfect for transport in body fluids.
Haemoglobin exemplifies sophisticated protein design with four polypeptide chains, each carrying an iron-containing haem group. This quaternary structure allows cooperative oxygen binding - when one chain grabs oxygen, it makes the others more likely to bind too.
Insulin and pepsin show globular proteins' versatility. Insulin's compact, soluble structure lets it travel through blood to regulate glucose levels, whilst pepsin's stable tertiary structure survives stomach acid to digest other proteins.
Key Insight: Prosthetic groups like haem aren't made of amino acids but are essential for protein function - removing iron from haemoglobin makes it useless for oxygen transport!
Fibrous Proteins: The Body's Construction Materials
Fibrous proteins are your body's building materials - long, strong, and perfectly designed for structural roles. Their repetitive sequences and cross-linked chains create incredible tensile strength, like biological steel cables.
Collagen forms the framework for skin, tendons, and bones. Its flexibility allows movement whilst providing strength, and its ability to bind minerals makes it essential for bone formation. Keratin varies from flexible (skin) to rigid (nails) depending on how many disulfide bridges cross-link its structure.
Elastin solves the problem of tissues that need to stretch and spring back. Found in skin, blood vessels, and lung tissue, it allows these structures to expand under pressure then return to their original size.
Real-world Application: Understanding collagen structure helps explain why vitamin C deficiency causes scurvy - without it, collagen can't form properly, leading to weak connective tissues!
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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
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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 😍😁😲🤑💗✨🎀😮
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
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
Comprehensive Guide to Biological Molecules for OCR A-Level Biology
M
@iamjungkook
Ever wonder what makes your DNA different from a piece of toast? It all comes down to biological molecules - the essential building blocks that make life possible. You'll discover how simple monomers join together like LEGO bricks to create... Show more
Building Blocks of Life: Monomers and Polymers
Think of biological molecules like construction sets - monomers are the individual bricks, whilst polymers are the finished structures built from identical pieces joined together. This simple concept explains how your body creates everything from energy stores to muscle fibres.
Two key reactions control this molecular construction site. Condensation reactions join monomers together by removing water molecules, like welding pieces with the loss of steam. Hydrolysis reactions do the opposite - they break apart large molecules by adding water back, which is exactly what happens when you digest food.
Carbohydrates follow the CHO formula and serve as your body's primary energy source. Monosaccharides like glucose are single sugar units that dissolve easily in blood for quick transport around your body.
Quick Tip: Remember that alpha glucose (used in starch) has hydrogen above the first carbon, whilst beta glucose (used in cellulose) has the OH group above - this tiny difference completely changes their properties!
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Improve your grades
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Sugar Chemistry: From Simple to Complex
Ribose and deoxyribose might sound complicated, but they're just five-carbon sugars with a crucial difference. Ribose (in RNA) keeps all its OH groups, whilst deoxyribose (in DNA) loses one oxygen atom - making DNA more stable for long-term information storage.
When two monosaccharides join through condensation reactions, they form glycosidic bonds and create disaccharides like table sugar. These double sugars remain soluble and sweet, perfect for quick energy hits.
Polysaccharides are the heavy-duty storage units - long chains of sugars linked by glycosidic bonds. Most are insoluble due to their massive, branched structures, which makes them ideal for storing energy without affecting your cells' water balance.
Exam Focus: Each glycosidic bond formation releases one water molecule - count the bonds to work out how much water is produced in synthesis reactions!
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Energy Storage: Starch and Its Components
Plants face the same problem you do - they need to store excess energy for later use. Starch solves this perfectly by being completely insoluble, so it won't mess with the plant's water balance while storing massive amounts of glucose.
Amylose forms unbranched chains that coil into tight spirals, packing efficiently like a compressed spring. Its compact structure makes it brilliant for storage but slow to break down. Amylopectin takes a different approach with its branched design, allowing enzymes to attack multiple points simultaneously for faster glucose release.
The branching happens through different bond types - straight sections use 1-4 glycosidic bonds, whilst branch points use 1-6 bonds. This creates a tree-like structure that balances storage efficiency with accessibility.
Memory Trick: Amylose = "A-my-coiled" (coiled structure), Amylopectin = "A-my-branches" (branched structure)!
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Animal Storage and Structure: Glycogen and Cellulose
Glycogen is basically amylopectin on steroids - it has branches every 8-12 glucose units compared to starch's longer straight sections. This ultra-branched structure reflects animals' higher metabolic demands and need for rapid energy release during movement.
Cellulose breaks all the rules by using beta glucose, creating straight, unbranched chains where every other glucose molecule flips 180°. This alternating pattern allows adjacent molecules to form hydrogen bonds, creating rope-like microfibrils of incredible strength.
Triglycerides represent a completely different storage strategy. Made from glycerol plus three fatty acids joined by ester bonds, they pack almost twice the energy of carbohydrates. Their hydrophobic nature means they won't absorb water and swell up like a sponge.
Real-world Connection: Cellulose's strength-to-weight ratio rivals steel - that's why it's perfect for plant cell walls and why we use it to make paper!
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Fats and Membrane Structure
Saturated fatty acids pack together tightly like straight pencils in a box, whilst unsaturated fatty acids have kinks from double bonds that push molecules apart. More kinks mean lower melting points - think of butter (saturated) versus olive oil (unsaturated) at room temperature.
Phospholipids are the molecular architects of cell membranes. With hydrophilic phosphate heads and hydrophobic fatty acid tails, they automatically arrange into bilayers - the heads face outward toward water whilst the tails hide inside.
This bilayer structure creates a selectively permeable barrier. Small, non-polar molecules can slip through the fatty acid centre, but water-soluble substances get blocked. It's like having a security system that only lets certain molecules pass.
Visual Learning: Picture phospholipids as matchsticks with magnetic heads - they'll always orient themselves with heads touching water and tails clustering together!
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Membrane Fluidity and Protein Basics
Cholesterol acts like a membrane thermostat, keeping cell membranes at just the right consistency. In hot conditions, it prevents excessive fluidity; in cold conditions, it stops the membrane from becoming too rigid and cracking.
Proteins are the workhorses of your cells, built from chains of amino acids linked by peptide bonds. Each amino acid has the same basic structure except for its R group - this variable side chain gives each amino acid its unique personality.
The condensation reaction between amino acids is straightforward: the hydrogen from one amino acid's amine group combines with the hydroxyl group from another's carboxyl group, releasing water and forming a covalent peptide bond.
Study Strategy: Focus on the R groups when learning amino acids - they determine everything about how the protein will fold and function!
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Protein Bonds and Structure Levels
Four types of bonds hold proteins in their final shapes. Hydrogen bonds form between slightly charged atoms but break easily with heat. Ionic bonds create strong attractions between oppositely charged R groups. Disulfide bridges form the strongest covalent bonds between sulfur-containing amino acids.
Primary structure is simply the sequence of amino acids - change even one, and you change the entire protein. Secondary structure introduces basic folding patterns like alpha helixes (tight spirals) and beta pleated sheets (parallel chains with hydrogen bonds between them).
Tertiary structure creates the final 3D shape through complex interactions between R groups. Hydrophobic R groups cluster inward whilst hydrophilic ones face outward, stabilised by all four bond types working together.
Exam Essential: Temperature and pH changes break hydrogen bonds first, which is why proteins denature (lose their shape) when you cook them!
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Protein Folding and Quaternary Structure
Quaternary structure only exists in proteins with multiple polypeptide chains, called subunits. Each subunit has its own tertiary structure, but they work together like different instruments in an orchestra to create the protein's final function.
The four structural levels build upon each other logically. Primary determines secondary, secondary influences tertiary, and tertiary affects how multiple chains interact in quaternary structures. It's like following a recipe where each step depends on getting the previous one right.
Understanding these levels helps explain why protein folding is so critical. A single amino acid change in the primary structure can cascade through all levels, potentially destroying the protein's function - this is what happens in genetic diseases like sickle cell anemia.
Connection Point: Computer modelling now predicts protein shapes from amino acid sequences, revolutionising drug design by showing how medicines might interact with target proteins!
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Globular Proteins: Specialists in Action
Globular proteins are the specialists of the protein world - compact, spherical, and designed for specific jobs. Their hydrophilic R groups face outward for solubility, whilst hydrophobic groups cluster inside, making them perfect for transport in body fluids.
Haemoglobin exemplifies sophisticated protein design with four polypeptide chains, each carrying an iron-containing haem group. This quaternary structure allows cooperative oxygen binding - when one chain grabs oxygen, it makes the others more likely to bind too.
Insulin and pepsin show globular proteins' versatility. Insulin's compact, soluble structure lets it travel through blood to regulate glucose levels, whilst pepsin's stable tertiary structure survives stomach acid to digest other proteins.
Key Insight: Prosthetic groups like haem aren't made of amino acids but are essential for protein function - removing iron from haemoglobin makes it useless for oxygen transport!
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Fibrous Proteins: The Body's Construction Materials
Fibrous proteins are your body's building materials - long, strong, and perfectly designed for structural roles. Their repetitive sequences and cross-linked chains create incredible tensile strength, like biological steel cables.
Collagen forms the framework for skin, tendons, and bones. Its flexibility allows movement whilst providing strength, and its ability to bind minerals makes it essential for bone formation. Keratin varies from flexible (skin) to rigid (nails) depending on how many disulfide bridges cross-link its structure.
Elastin solves the problem of tissues that need to stretch and spring back. Found in skin, blood vessels, and lung tissue, it allows these structures to expand under pressure then return to their original size.
Real-world Application: Understanding collagen structure helps explain why vitamin C deficiency causes scurvy - without it, collagen can't form properly, leading to weak connective tissues!
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4.9/5
App Store
4.8/5
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