The digestive system (AQA GCSE Biology): Revision Notes
The digestive system
What are digestive enzymes?
An enzyme is a biological catalyst. This means it's a protein that speeds up chemical reactions in your body.
Digestive enzymes break down large food molecules into smaller, soluble molecules. These smaller molecules can then be absorbed into your bloodstream.
The key point to understand is that without digestive enzymes, the food molecules would be too large to pass through the intestine wall and enter your bloodstream. Enzymes are essential for making nutrients available to your body.
The three main digestive enzymes
Your body uses three main types of digestive enzymes:
Remember the "Three Cs, Ps, and Ls": Carbohydrase breaks down Carbohydrates, Protease breaks down Proteins, and Lipase breaks down Lipids. Each enzyme is specific to its substrate - they cannot break down other types of molecules.
1. Carbohydrase enzymes
- What they break down: Carbohydrates → simple sugars
- Key example: Amylase
- Where they're made: Salivary glands, pancreas, and small intestine
- What they do: Break down starch in your mouth and small intestine
2. Protease enzymes
- What they break down: Proteins → amino acids
- Where they're made: Stomach, pancreas, and small intestine
- What they do: Break down proteins in your stomach and small intestine
3. Lipase enzymes
- What they break down: Lipids (fats) → fatty acids + glycerol
- Where they're made: Pancreas and small intestine
- What they do: Break down fats and oils in your small intestine
How the digestive system works
The digestive system is an organ system. This means several organs work together to digest and absorb food.
Teamwork in digestion: No single organ can complete digestion on its own. Each organ has a specialised role, and they must work in sequence to break down food completely. This is why the digestive system is called an "organ system" - it's multiple organs functioning as one coordinated unit.
Key organs and their jobs:
- Salivary glands: Produce digestive enzymes (amylase)
- Stomach: Some digestion happens here, produces protease enzymes
- Liver: Produces bile (not an enzyme, but helps break down fats)
- Pancreas: Produces digestive enzymes (amylase, protease, lipase)
- Small intestine: Digestion completed here, soluble substances absorbed
- Large intestine: Water absorbed, faeces formed
Why do we need digestive enzymes?
Food molecules are often too big to be absorbed directly into your blood. Digestive enzymes break them down into smaller pieces that can:
- Be absorbed through the intestine wall
- Enter your bloodstream
- Be used by your body for energy and growth
Worked Example: From Food to Energy
Let's trace how a piece of bread becomes energy:
Step 1: Bread contains starch (large carbohydrate molecules) Step 2: Amylase breaks down starch → simple sugars (glucose) Step 3: Glucose is small enough to pass through intestine wall Step 4: Glucose enters bloodstream and travels to cells Step 5: Cells use glucose for respiration to release energy
For example:
- Simple sugars from carbohydrates can be used for respiration
- Amino acids from proteins can build new proteins
- Fatty acids and glycerol can make new lipids
Levels of organisation
The digestive system shows how your body is organised:
- Cell - Basic building blocks (e.g. muscle cells)
- Tissue - Groups of similar cells (e.g. muscular tissue)
- Organ - Different tissues working together (e.g. stomach)
- Organ system - Different organs working together (e.g. digestive system)
Building complexity: Notice how each level builds upon the previous one. This hierarchical organisation allows your body to perform complex functions like digestion through the coordinated action of simpler components.
Remember!
Key Points to Remember:
- Enzymes are biological catalysts that speed up reactions in your body
- Three main digestive enzymes: carbohydrase (breaks down carbs), protease (breaks down proteins), lipase (breaks down lipids)
- Different organs produce different enzymes - salivary glands, stomach, pancreas, and small intestine all help
- The digestive system is an organ system where multiple organs work together
- Food must be broken down into small, soluble molecules before it can be absorbed into your bloodstream