The digestive system explains how food is broken down into small molecules that can be absorbed by the body. It brings in terminology from other areas of the GCSE Biology curriculum, including enzymes, organ systems, and absorption.
This article explores the structure and function of the digestive system. You will see the breakdown of food into smaller molecules, how food is moved through each organ, the importance of enzymes like amylase, and how villi in the small intestine help to maximise absorption. Images and quiz questions are provided to help you learn the topic and test your understanding. It is suitable for GCSE Biology and GCSE Combined Science across all major exam boards, including AQA.
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Human Digestive System in GCSE Biology
Digestion is how the body turns food into nutrients that it can use for energy, healing, and growth. Many foods have large molecules that need to be broken down so they can pass through the walls of the intestine. They include starch, protein, and fat.
Digestion ensures that nutrients in food are absorbed by our bodies. Carbohydrate is converted into simple sugars, proteins into amino acids, and lipids into fatty acids and glycerol. These molecules are absorbed into the bloodstream and transported to cells, where they are used for respiration, growth, repair of tissues in the body, and the creation of biological molecules.
Understanding digestion helps with other areas of the GCSE curriculum. Enzymes control the chemical reactions during digestion, which improves our understanding of enzyme action and active sites. Absorption is linked to the topics of diffusion, active transport, and surface area adaptations.
1
Why are large food molecules digested before they enter the bloodstream?
Digestion of food through the body
The various functions of the digestive system are interlinked. Every organ has a specific role in the process of digestion.
Mouth and salivary glands
Digestion starts when food enters our mouths. The teeth perform mechanical digestion by breaking down food, cutting and grinding it into smaller pieces. This increases the surface area for enzymes, making chemical digestion more effective. Saliva contains the enzyme amylase, which breaks the starch down into simpler sugars. It is released from the salivary glands when we eat, and helps to moisten the food, turning it into a bolus - a ball of food - that can be safely swallowed.
The tongue has an important role. It positions the food between the teeth and pushes the bolus towards the back of the throat.
Oesophagus and peristalsis
The food passes into the oesophagus after it is swallowed. This tube connects our mouth to our stomach. The food is moved down the oesophagus using peristalsis, which is a series of rhythmic muscle contractions. Circular muscles contract behind the bolus, and muscles ahead relax. This moves the food downwards regardless of how we are standing, sitting, or even hanging.
Peristalsis is a coordinated muscular action. It isn't part of digestion itself, but is vital to ensure food reaches the stomach without requiring conscious control.
Stomach
The stomach is used to store and process food. It is a thick-walled organ that uses strong muscle contractions to churn the food. Gastric juice is mixed with the food, which contains hydrochloric acid, creating an acidic environment. This acid kills harmful microorganisms in the food. It also provides the optimum pH for protease enzymes, which break down proteins into amino acids. This includes pepsin.
Mucus lines the stomach wall to stop the acid from damaging its own tissues. This mixture of partly digested food and gastric juice is called chyme. It is slowly released into the small intestine through a muscular valve.
Liver, gallbladder and pancreas
The liver, gallbladder, and pancreas are organs that don't transport food. However, they do release substances to aid digestion:
- Liver - Creates bile, which is an alkaline liquid to neutralise the acidic chyme entering from the stomach. Bile emulsifies fats and breaks fat globules into smaller droplets, increasing the surface area for lipase enzymes.
- Gallbladder - Bile is stored in the gall bladder. It is released when fatty food is present.
- Pancreas - Digestive enzymes from the pancreas are key to breaking down food: pancreatic amylase digests carbohydrate; protease digests protein; lipase breaks down lipids into fatty acids and glycerol.
Small intestine (digestion and absorption)
The majority of digestion and nutrient absorption takes place in the small intestine. Enzymes from the pancreas and the intestinal lining complete the breakdown of carbohydrates into glucose, proteins into amino acids, and lipids into fatty acids and glycerol.
The inside of the small intestine is covered with tiny finger-like projections called villi. They significantly increase surface area, letting nutrients diffuse or be actively transported into the bloodstream. Each villus has a blood supply and a thin wall, so the nutrients don't need to travel far during absorption.
Large intestine and egestion
The large intestine is important for material that can't be digested. Water is absorbed into the bloodstream, which ensures hydration and electrolyte balance. The bacteria inside the large intenstive produces vitamins and can help break down this material.
When water is removed, the waste solidifies, turning into faeces. They are temporarily stored in the rectum and then removed from the body through the anus. This is known as egestion, which is not the same as excretion, which means removing metabolic waste products from the body.
2
What is the role of bile in the digestive system?
Mechanical vs Chemical Digestion
Digestion happens through mechanical digestion and chemical digestion. We have explored both below. Put simply, mechanical digestion prepares food by increasing surface area. Chemical digestion completes the breakdown of food into smaller molecules that can be absorbed.
You are expected to differentiate between these processes during your GCSE Biology examinations.
Mechanical Digestion
This means physically breaking food into smaller pieces. It doesn't involve changing its chemical structure. Mechanical digestion starts in the mouth, as our teeth cut and grind food. It continues in the stomach, where the food is churned by strong muscular contractions and mixed with digestive juices.
This process aims to increase the surface area. Food in small pieces has more surface for enzymes to work on, speeding up chemical digestion. While it doesn't release nutrients directly, mechanical digestion makes the following chemical processes much more efficient.
Chemical Digestion
Enzymes are used to break down large, insoluble molecules. Every enzyme is specific to a certain type of food molecule:
- Carbohydrases - These break down carbohydrates into sugars.
- Proteases - These break proteins down into amino acids.
- Lipases - These break lipids down into fatty acids and glycerol.
This process happens at several stages of the digestive system. Amylase starts in the mouth through saliva, protease occurs in the stomach, and a variety of enzymes aid digestion in the small intestine. Chemical digestion changes the structure of the molecules to allow them to be absorbed into the bloodstream.
3
What is the difference between mechanical and chemical digestion?
Digestive Enzymes You Need to Know for GCSE Biology
Enzymes speed up chemical reactions in the breakdown of food. Every enzyme has a specific substrate and only works on certain types of food molecules. We have explored the key enzymes below, including where they are produced and their substrates.
Amylase - Carbohydrase enzyme
Amalyse is a type of carbohydrase. It breaks down starch into smaller sugars. This is a type of complex carbohydrate. This process begins in the mouth, where the salivary glands release amylase into the saliva. The pancreas also produces amylase, continuing carbohydrate digestion in the small intestine.
Glucose is the final product of carbohydrate digestion. They are a key source of energy for respiration.
Protease - Protein enzyme
Proteases break proteins into amino acids. In the stomach, they work well in hydrochloric acid. Protease is also released by the pancreas into the small intestine, where protein digestion is completed.
This process results in amino acids, which are absorbed to build new proteins in the body. This includes enzymes, hormones, and structural tissues.
Lipase - Lipid enzyme
Lipase is responsible for breaking lipids (fats) down into fatty acids and glycerol. It's created in the pancreas and released into the small intestine.
Breaking down lipids takes longer than carbohydrate digestion because fats don't mix well with water. Despite not being an enzyme, bile helps with this process. It emulsifies fat into smaller droplets, which increases the surface area for lipase to work on.
Bile
Bile is created in the liver and stored in the gallbladder. It is then released into the small intestine. bill has two key functions:
- It neutralises the stomach acid, creating the right pH for enzymes in the small intestine.
- It emulsifies lipids, breaking large fat globules into smaller droplets.
4
Which enzyme breaks down lipids into fatty acids and glycerol?
Absorption in the small intestine
After food has been broken down, the soluble molecules need to be absorbed. This happens in the small intestine, where nutrients pass through the intestinal wall to enter the bloodstream. This absorption needs to be efficient as the glucose, amino acids, and fatty acids must be transported around the body and used by cells.
Villi
The small intestine is covered in millions of tiny finger-like projections on the inside. These are called villi. They greatly increase the surface area, allowing more nutrients to be absorbed at once. Each villus has a network of capillaries that carry the nutrients away, maintaining a concentration gradient to support diffusion.
The walls of the villi are very thin, with only one cell of thickness. This lets substances move quickly from the intestine into the blood. Nutrients like glucose can also be absorbed by active transport - this needs energy and allows absorption even if the concentration gradient is low.
What is absorbed?
Different products of digestion are stored in different ways:
- Glucose and amino acids enter the bloodstream through capillaries. They are sent to the liver through the hepatic portal vein.
- Fatty acids and glycerol enter specialised vessels called lacteals. They eventually join the bloodstream.
5
How do villi help nutrients move quickly through the small intestine?
Final thoughts - Revision of Digestion System
The digestive system enables different types of food to be broken down into smaller pieces. The nutrients are absorbed for energy, growth, and repair. The organs of the digestive system play a crucial role in both mechanical and chemical digestion. Enzymes are produced to break down carbohydrates, proteins and lipids, including amylase, protease and lipase. Finally, structures like villi in the small intestine strengthen absorption.
As we have summarised above, it helps when learning about the digestive system to see the various parts and functions as a continuous pathway where structure and function are all interlinked. For further reading, this article by Revision Science gives an overview of enzyme action and metabolism, which is linked closely to digestive enzymes. You can also test yourself with questions on digestion by BBC Bitesize.
If you need support learning this topic, TeachTutti has top GCSE Biology tutors. They can support specific subtopics you are struggling with in the process of digestion, such as the digestion of starch.
