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20 Dec 2025
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AQA A-level Notes on Digestion and Absorption: Detailed Overview
cait
@caittt
1 / 4
Breaking Down Food: The Digestive Process
Your digestive system uses two main strategies to tackle food. Mechanical digestion physically breaks food into smaller pieces through chewing, stomach churning, and the wave-like muscle contractions called peristalsis that push food along your digestive tract.
Chemical digestion is where the real magic happens - enzymes slice large molecules into tiny, absorbable units. Your mouth kicks things off with amylase in saliva, which starts breaking down carbohydrates whilst mucus lubricates the food and bicarbonate creates the perfect alkaline environment for the enzyme to work.
In your stomach, gastric juice containing hydrochloric acid and pepsin begins protein digestion, creating a soupy mixture called chyme. After 1-2 hours, this moves into the small intestine where pancreatic enzymes and bile from the liver finish the job.
Quick Fact: Your small intestine's ileum absorbs 95% of nutrients thanks to its massive surface area created by length, villi, and microvilli - like having a football pitch folded inside your belly!
Carbs and Proteins: Getting the Good Stuff Out
Carbohydrate digestion follows a clear pathway: amylase converts starch to maltose, then specific enzymes like maltase, lactase, and sucrase break disaccharides into simple sugars. Glucose and galactose enter intestinal cells via co-transport with sodium, whilst fructose uses facilitated diffusion.
About 65% of adults produce minimal lactase, explaining why many people struggle with unfermented dairy products. Your genetics determine whether you'll be reaching for lactose-free milk or not!
Protein digestion relies on two enzyme types: endopeptidases (like pepsin and trypsin) slice proteins internally, whilst exopeptidases nibble amino acids from the ends. This tag-team approach ensures proteins become individual amino acids ready for absorption.
Amino acids enter cells through sodium-dependent transporters - several different types handle various amino acids. Some even transport small peptide chains that get their final breakdown inside the cell.
Study Tip: Remember that infants produce rennin to curdle milk - this disappears as we grow up, which is why babies handle milk better than many adults!
Fats: The Tricky Customer
Lipid digestion presents a unique challenge because fats and water don't mix - like trying to dissolve butter in a glass of water. Your body solves this with bile salts that emulsify fats, breaking large droplets into tiny ones that lipase enzymes can actually attack.
The digestion products - monoglycerides and fatty acids - form microscopic packages called micelles that can approach intestinal cells. Unlike other nutrients, these lipids enter enterocytes through simple diffusion rather than special transporters.
Inside the intestinal cells, something fascinating happens. The endoplasmic reticulum and Golgi apparatus reassemble the fats into triglycerides and package them with proteins into particles called chylomicrons. These are too large for blood capillaries, so they enter the lymphatic system through special vessels called lacteals.
This explains why fat absorption takes longer than carbs or proteins - your body needs time for all this repackaging and rerouting through the lymph system before fats finally reach your bloodstream.
Remember: Fat digestion is the only nutrient absorption that bypasses direct entry to blood - it takes the scenic route through your lymphatic system first!
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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.
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Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
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Best app on earth! no words because it’s too good
Thomas R
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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
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The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
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In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
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
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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
AQA A-level Notes on Digestion and Absorption: Detailed Overview
cait
@caittt
Digestion breaks down your food from massive chunks into tiny molecules that your blood can actually absorb and use. Think of it as your body's ultimate recycling system - turning a cheese sandwich into glucose, amino acids, and fatty acids... Show more
Breaking Down Food: The Digestive Process
Your digestive system uses two main strategies to tackle food. Mechanical digestion physically breaks food into smaller pieces through chewing, stomach churning, and the wave-like muscle contractions called peristalsis that push food along your digestive tract.
Chemical digestion is where the real magic happens - enzymes slice large molecules into tiny, absorbable units. Your mouth kicks things off with amylase in saliva, which starts breaking down carbohydrates whilst mucus lubricates the food and bicarbonate creates the perfect alkaline environment for the enzyme to work.
In your stomach, gastric juice containing hydrochloric acid and pepsin begins protein digestion, creating a soupy mixture called chyme. After 1-2 hours, this moves into the small intestine where pancreatic enzymes and bile from the liver finish the job.
Quick Fact: Your small intestine's ileum absorbs 95% of nutrients thanks to its massive surface area created by length, villi, and microvilli - like having a football pitch folded inside your belly!
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Carbs and Proteins: Getting the Good Stuff Out
Carbohydrate digestion follows a clear pathway: amylase converts starch to maltose, then specific enzymes like maltase, lactase, and sucrase break disaccharides into simple sugars. Glucose and galactose enter intestinal cells via co-transport with sodium, whilst fructose uses facilitated diffusion.
About 65% of adults produce minimal lactase, explaining why many people struggle with unfermented dairy products. Your genetics determine whether you'll be reaching for lactose-free milk or not!
Protein digestion relies on two enzyme types: endopeptidases (like pepsin and trypsin) slice proteins internally, whilst exopeptidases nibble amino acids from the ends. This tag-team approach ensures proteins become individual amino acids ready for absorption.
Amino acids enter cells through sodium-dependent transporters - several different types handle various amino acids. Some even transport small peptide chains that get their final breakdown inside the cell.
Study Tip: Remember that infants produce rennin to curdle milk - this disappears as we grow up, which is why babies handle milk better than many adults!
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Improve your grades
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Fats: The Tricky Customer
Lipid digestion presents a unique challenge because fats and water don't mix - like trying to dissolve butter in a glass of water. Your body solves this with bile salts that emulsify fats, breaking large droplets into tiny ones that lipase enzymes can actually attack.
The digestion products - monoglycerides and fatty acids - form microscopic packages called micelles that can approach intestinal cells. Unlike other nutrients, these lipids enter enterocytes through simple diffusion rather than special transporters.
Inside the intestinal cells, something fascinating happens. The endoplasmic reticulum and Golgi apparatus reassemble the fats into triglycerides and package them with proteins into particles called chylomicrons. These are too large for blood capillaries, so they enter the lymphatic system through special vessels called lacteals.
This explains why fat absorption takes longer than carbs or proteins - your body needs time for all this repackaging and rerouting through the lymph system before fats finally reach your bloodstream.
Remember: Fat digestion is the only nutrient absorption that bypasses direct entry to blood - it takes the scenic route through your lymphatic system first!
Sign up to see the contentIt's free!
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Improve your grades
Join milions of students
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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?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
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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
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