AQA GCSE Biology (Digestion)

The digestive system is also known as the alimentary canal or the gut.

It has two jobs:

• Digestion - breaking down food into smaller pieces and smaller molecules
• Absorption - passing the small soluble food molecules through the gut wall and into the blood

Digestion means breaking down food

Our gut digests food so that it is small and soluble so it can pass into the blood

Mechanical digestion happens in the mouth when we chew and in the intestines by peristalsis

Peristalsis is when the muscles in the gut wall squeeze on the food as well as pushing it along.

Chemical digestion is carried out by enzymes, these break down large molecules into small ones.

Chewing and peristalsis chop and mash ip food into a "soup" called chyme. This increases the surface area of the food.

Bile contains an emulsifier. This breaks down big lipid molecules into small ones. This increases the surface area of the lipids.

The above are examples of physical digestion. They help the action of the enzymes in chemical digestion by increasing the surface area of the food so that the enzymes have more surface area to work on.

Apart from emulsifying lipids, bile also helps in digestion by altering the pH of the acidic liquid from the stomach into a slightly alkaline liquid in the small intestine. As food comes from the stomach into the small intestine, bile is squirted onto it. This neutralises the stomach acid and makes the digested food alkaline. This provides the ideal conditions for the enzymes in the small intestine.