The Human Digestive System 
Digestion begins inside the mouth, where chewing does 2 things - makes the food easier to swallow and increases the surface area (this helps to speed up digestion). Also inside the mouth, amylase (for digesting starch) can be found, produced by the salivary glands. The gullet then carries food from the mouth to the stomach by its muscular squeezing action called peristalsis.
The stomach does a number of things, including:
- pummels and churns the food
- produces protease enzymes to digest protein
- produces hydrochloric acid which kills bacteria and gives the ideal pH for protease to work.
The liver produces bile which emulsifies fats i.e. breaks them down into small droplets for a larger surface area. This will increase the rate at which the fat is digested by lipase. Bile also neutralizes the acid produced by the stomach to provide ideal alkaline conditions for enzymes in the small intestine. The gall bladder stores the bile made by the liver until it is needed.
The small intestine produces amylase, lipase and protease. The pancreas, a pistol shaped organ, produces the enzymes amylase, lipase and protease and releases them into the small intestine when needed. The enzymes are used to fully digest the food molecules, so that they are small enough to diffuse into the bloodstream. Food molecules diffuse in the small intestine, which is ideal for this purpose. The small intestine walls are very thin so that there is a short diffusion pathway. It is long and folded with villi to increase surface area. There is a rich blood supply to maintain a steep concentration gradient for diffusion.
When all the useful products have diffused into the blood, the remaining waste reaches the large intestine where the majority of the remaining water is absorbed into the blood stream. Finally the waste products leave the body in the form of faeces through the anus.
It is useful to know that the small intestine consists of the duodenum, jejunum and ileum and the large intestine consists of the caecum, colon and rectum in that order. The appendix is a small cul-de-sac at the base of the caecum. Only the terms in italics are needed by most GCSE boards.
How digestion works 
The working of the digestive system is based on two things:
- Muscular Action – The gut is a muscular tube and its squeezing action is very important. It helps to break up the food into very small pieces with a large surface area for enzyme action to take place. It also helps the chemical breakdown of the food, mixing it with various digestive juices, which break down its molecules. The muscles also move the food along the gut from one area of chemical digestion to another.
- Enzyme Action – enzymes are chemicals which speed up (catalyse) other reactions. They do not actually take part in the reaction or change it in any way except to make it happen faster. Each type of enzyme acts on a specific type of food.
Enzymes are proteins. They are made inside cells. Most of them work inside the cell, but some (e.g. digestive enzymes) work outside the cell. Enzymes are biological catalysts. A catalyst speeds up a chemical reaction. It does not change the final products. A catalyst is not changed by the reaction – therefore it can be used over and over again.
How Enzymes Work 
In an enzyme the reactants (things on the left of the equation) are called substrates. An enzyme is a large molecule. It has an indentation on the surface called the active site. The active site has a specific shape which is exactly the right shape for the substrates. When the substrates fit into the active site, they bind to it forming an enzyme/substrate complex. The reaction occurs, and the products are released. This is known as the lock and key hypothesis – the enzyme is the lock and the substrate is the key.
Key features of enzymes 
- They change substrates to products.
- They are proteins.
- They are catalysts. Tiny amounts of enzyme can be used over and over again.
- They have specificity, acting on particular substrate molecules.
- They are pH sensitive, most work best at pH 7. Proteins can be damaged by high or low pH.
- Enzyme catalysed reactions go faster as the temperature increases.
- Enzymes give the fastest reaction at an optimum temperature.
- They are inactivated at temperatures over 40 degrees C because proteins are denatured at these temperatures so the active site is misshapen.
Enzymes in Digestion 
Enzymes are very important in digestion. They help act as catalysts to break down the big molecules (which cannot pass through the gut wall into the blood) into smaller ones (which can diffuse into the blood). Starch, proteins and fats are big molecules. Sugars, amino acids and fatty acids/ glycerol are much smaller.
Amylase enzyme converts starch into simple sugars e.g. glucose.
Protease enzyme converts proteins into amino acids.
Lipase enzyme converts fats into fatty acids and glycerol.
Bile is not an enzyme. However, it emulsifies the fats, breaking it down into tiny droplets. This provides a much larger surface area for lipase enzyme to work on, so that fat is digested more quickly.
Other enzymes 
Catalase is an enzyme found in cells that breaks down hydrogen peroxide into water and oxygen.
Key Ideas 
- Large food molecules must be broken down (using enzymes as a catalyst) so that they are small enough to diffuse through the gut wall into the blood.
- The small intestine is long, filled with bile, has thin walls and a rich blood supply - these features all speed up diffusion.
Test Yourself 
- Name 2 ways in which chewing helps digestion.
- What organ carries food from mouth to stomach, and how does it do this?
- Where is bile a) produced? and b) stored?
- Name the 3 large molecules that cannot diffuse into the blood.
- Name the 4 small molecules that can diffuse into the blood.
- Name the 3 enzymes involved in digestion.
- Chewing aids digestion by making food easier to swallow and providing a larger surface area for amylase (produced by the salivary glands) to work on.
- The gullet carries food from the mouth to the stomach by peristalsis, a muscular squeezing action.
- Bile is produced in the liver and stored in the gall bladder
- Starch, proteins and fats cannot diffuse into the blood
- Glucose (or another simple sugar), amino acids, fatty acids and glycerol.
- Amylase (for starch), protease (for protein) and lipase (for fats)