I/GCSE Biology - Exchange of substances in the gut

• The food we eat is broken down in the gut.
• It forms simple sugars, such as glucose, amino acids, fatty acids and glycerol.
• These products are of no use in the gut, and if they stayed there would simply be removed with faeces.
• This is why, via a combination of active transport and diffusion, the molecules from food enter the bloodstream.
• After being broken down, the food molecules are small enough to pass through the walls of the small intestine and into the blood vessels.
• They can move this way because there is a very high concentration of food molecules in the gut, and a very low concentration in the blood, so the process here is diffusion. They move along a very steep concentration gradient.

Diagram: The labelled diagram of a villus

• The lining of the small intestine is folded into thousands of tiny villi.
• These greatly increase the uptake of digested food by diffusion because only a certain amount of digested food molecules can diffuse over a certain surface area at a given time – so the villi increase the surface area dramatically.

• The blood is found in minute small vessels known as capillaries. The capillaries are found protruding into the villi. The blood approaches the villus, picks up the absorbed food molecules and then leaves.

• Diffusion is very rapid and efficient in the gut, because, as with the lungs, it has a rich blood supply – so digested food molecules are carried away the second it diffuses from one side to the other.

• Therefore, a steep concentration gradient is constantly maintained

That's the end of the topic!

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Reference:

https://getrevising.co.uk/resources/biology_revision_notesexchange_of_materials

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