GCSE Biology: Food and digestion

Justina: The average person will spend between four and six years of their life eating food.

Georgina: So today, we’re going to be looking at food and digestion.Food is a vital source of nutrients and vitamins, fibre, water and much more, that our bodies need.

Justina: There are three main types of nutrients we’re going to look at: carbohydrates, proteins and lipids.The digestive system allows for these nutrients to be absorbed into the body.

Georgina: Let’s take a closer look.When we eat, the food we chew consists of large insoluble molecules.For the body to be able to absorb the nutrients contained in the food into our bloodstream, they need to be broken down into smaller soluble molecules.This happens in the digestive system through the process of digestion.

Justina: Carbohydrates need to be broken down into simple sugars, such as glucose, which is used to release energy through respiration.Proteins are broken down into amino acids, which are used to make new proteins that carry out essential roles in our bodies, such as enzymes and antibodies.And lipids are broken down into fatty acids and glycerol, which are used for energy sources and the development of cell membranes.

Georgina: Once the food has been chewed and mixed with saliva in the mouth, the pharynx (part of the throat), propels the food down to the oesophagus where the wave-like contractions of the muscles push the food onward into the stomach.

Justina: The stomach is where the partly digested food is mixed and churned with acidic gastric juices and protein-digesting enzymes to form chyme.

Georgina: This chyme then enters the small intestine, where most of the chemical digestion and absorption of nutrients occurs.And this final stage of digestion is enabled by enzymes secreted primarily by the pancreas, liver and in the small intestine.

Justina: An enzyme is a protein that acts as a biological catalyst.They speed up chemical reactions without being consumed or changed in the process.They act on specific molecules called substrates.In the digestive system, this substrate would be either a carbohydrate, a protein or a lipid.

Georgina: The shape of an enzyme’s active site and the shape of the shape of the substrate molecule are complementary.In other words, each type of enzyme is specific to one type of substrate.This is called the ‘lock and key model’.

Justina: In the first part of the small intestine, the duodenum, enzymes and substrates collide to form enzyme substrate complexes.And this is where the enzyme gets to work, speeding up the breakdown of the substrate into smaller molecules before releasing them.The enzyme is now free to act again on another substrate molecule.

Georgina: For example, protease enzymes break down proteins into simple amino acids.These are then absorbed more quickly into the bloodstream, through the wall of the small intestine.

Justina: The small intestine is about five and a half metres long and it’s folded many times to be able to fit into the abdomen.

Georgina: Not particularly small then!And it looks bigger than the large intestine.

Justina: Yes.It is much longer than the large intestine but is called ‘small’ due to its narrower diameter.

Georgina: Aha! And the length means there’s a large surface area for absorption to occur on.

Justina: And the small width means molecules don’t have to travel very far to reach the permeable walls.These walls are covered in small finger-like projections called villi, which are covered in even smaller hairs, called micro villi.

Georgina: All of which further increases the surface area.The majority of the absorption of molecules occurs in the latter part of the small intestine, the ileum.There is a large network of capillaries throughout the small intestine, which maintain a good blood supply.For example, once the blood has been loaded up with absorbed amino acids, it is immediately transported away, to be replaced by blood with a lower concentration of amino acids.Maintaining a concentration gradient between the small intestine and the bloodstream allows diffusion and absorption to occur.

Justina: The whole process is incredible!Those little enzymes.

Georgina: Enzymes are known to catalyse more than 5,000 biochemical reactions and it’s not just in our bodies.They are also used in the manufacture of cheeses, paper, medicines and so much more.

Justina: Handy little biochemical assistants.

Georgina: Something like that.