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The digestive system is an example of an organ system in which several organs work together to digest and absorb food. For AQA GCSE Biology, you need to know the structure and function of each organ in the digestive system, how they work together, and how the products of digestion are absorbed into the bloodstream. This lesson takes you through the entire alimentary canal from mouth to anus.
The digestive system is essentially a long muscular tube called the alimentary canal (or gut), along with several associated organs. Food enters at the mouth and passes through each organ in turn:
graph TD
A[Mouth] --> B[Oesophagus]
B --> C[Stomach]
C --> D[Small Intestine — Duodenum then Ileum]
D --> E[Large Intestine — Colon]
E --> F[Rectum]
F --> G[Anus]
H[Liver] -->|Produces bile| D
I[Gall Bladder] -->|Stores and releases bile| D
J[Pancreas] -->|Produces enzymes| D
K[Salivary Glands] -->|Produce saliva and amylase| A
Digestion begins in the mouth (also called the oral cavity or buccal cavity):
| Process | Details |
|---|---|
| Mechanical digestion | Teeth break food into smaller pieces, increasing the surface area for enzymes to act on |
| Chemical digestion | Salivary amylase (an enzyme from the salivary glands) begins the breakdown of starch into simple sugars (maltose) |
| Saliva | Moistens food to form a bolus (a ball of chewed food) that can be swallowed easily |
Exam Tip: Distinguish between mechanical digestion (physical breakdown by teeth, churning, bile emulsification) and chemical digestion (breakdown by enzymes or acid). Both increase surface area, but only chemical digestion breaks chemical bonds.
The oesophagus (or gullet) is a muscular tube that connects the mouth to the stomach. No digestion occurs here. Its sole function is to transport the bolus to the stomach by a process called peristalsis.
Peristalsis is the rhythmic contraction and relaxation of muscles in the wall of the alimentary canal. Circular muscles behind the bolus contract to push it forward, while muscles ahead of the bolus relax to allow it through. This wave-like motion moves food all the way through the digestive system.
The stomach is a muscular, J-shaped organ that plays a key role in both mechanical and chemical digestion:
| Function | How It Works |
|---|---|
| Mechanical digestion | The muscular walls of the stomach churn and mix food with digestive juices, breaking it into smaller pieces |
| Chemical digestion — Protease (pepsin) | The stomach lining produces the protease enzyme pepsin, which begins the digestion of proteins into shorter chains of amino acids |
| Hydrochloric acid (HCl) | The stomach produces strong hydrochloric acid (pH ~2) which: (a) kills bacteria and pathogens in food, (b) provides the optimum pH for pepsin to work |
The stomach stores food for several hours while it is mixed into a thick, acidic liquid called chyme. The chyme is then gradually released into the small intestine through a ring of muscle called the pyloric sphincter.
As covered in Lesson 1, the stomach contains:
Exam Tip: If asked about the role of hydrochloric acid, give both functions: killing pathogens AND providing the correct acidic pH for pepsin. Giving only one may lose you a mark.
The small intestine is the longest part of the digestive system (approximately 6-7 metres in an adult). It has two main regions:
The duodenum is the first section of the small intestine, approximately 25 cm long. This is where the most intensive chemical digestion occurs:
| Substance | Source | Role in the Duodenum |
|---|---|---|
| Bile | Produced by the liver, stored in the gall bladder | Emulsifies fats (increases surface area for lipase) and neutralises stomach acid |
| Pancreatic amylase | Pancreas | Continues the digestion of starch into sugars |
| Pancreatic protease (trypsin) | Pancreas | Continues the digestion of proteins into amino acids |
| Pancreatic lipase | Pancreas | Digests lipids (fats) into glycerol and fatty acids |
The ileum is the longest part of the small intestine and is the main site of absorption of digested food molecules into the bloodstream.
The inner surface of the ileum is covered in millions of tiny finger-like projections called villi (singular: villus). Each villus is further covered in even smaller projections called microvilli (on the surface of the epithelial cells). Together these create an enormous surface area for absorption.
graph TD
A[Villi — Adaptations for Absorption] --> B[Very large surface area]
A --> C[Thin walls — only one cell thick]
A --> D[Rich blood supply — dense network of capillaries]
A --> E[Lacteals — absorb fatty acids and glycerol]
B --> F[Maximises rate of diffusion of nutrients]
C --> F
D --> F
E --> G[Fatty acids and glycerol enter lymphatic system]
| Adaptation of Villi | How It Helps Absorption |
|---|---|
| Large surface area (millions of villi and microvilli) | More area for absorption of digested food molecules |
| Thin walls (one cell thick) | Short diffusion distance — nutrients pass quickly into blood |
| Rich blood supply (dense capillary network) | Blood carries absorbed nutrients away quickly, maintaining a steep concentration gradient |
| Lacteals (lymph vessels inside each villus) | Absorb fatty acids and glycerol into the lymphatic system |
Exam Tip: When describing how villi are adapted for absorption, remember to explain why each feature helps. For example, do not just say "thin walls." Say "the villi are only one cell thick, providing a short diffusion distance so nutrients can be absorbed quickly."
The liver is the largest internal organ and performs many functions. In the context of digestion:
The liver also processes absorbed nutrients — for example, it converts excess glucose into glycogen for storage and breaks down excess amino acids in a process called deamination (producing urea as a waste product).
The pancreas is a gland located behind the stomach. It produces and secretes digestive enzymes (amylase, protease and lipase) that are released into the duodenum to aid chemical digestion. The pancreas also produces insulin and glucagon for blood glucose regulation (covered in a separate topic).
The large intestine (or colon) is shorter but wider than the small intestine. Its main function is to absorb water from the undigested food material. As water is removed, the remaining material becomes more solid, forming faeces.
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