Digestion and the Use of Nutrients

Why food molecules need to be digested

Food provides materials for growth and repair as well as substances that can release energy. However, large molecules such as starch, proteins and lipids cannot simply pass through the wall of the gut into the blood. They must first be broken down into smaller, soluble molecules. This chemical breakdown is digestion.

The digestive system is an organ system: a group of organs working together to digest food and absorb useful substances. Digestive enzymes speed up the reactions that break food molecules apart. An enzyme is a biological catalyst, so it speeds up a reaction without being used up. Different types of digestive enzyme act on different food molecules.

The small, soluble products are absorbed through the wall of the small intestine into the blood. This movement is called absorption. The blood then transports the dissolved molecules to cells throughout the body. Digestion and transport therefore work together: digestion makes molecules absorbable, and the blood delivers them to where they are needed.

Breaking down molecules and building them again

Large food molecules are made from smaller building blocks. Starch is a long chain of glucose molecules, proteins are long chains of amino acids, and lipids contain fatty acids joined to glycerol.

Digestion separates these building blocks. Inside cells, they can then be used in respiration or joined together to make new large molecules. Building larger molecules from smaller ones is called synthesis. The body does not simply move the proteins and fats in a meal unchanged into its own tissues: it breaks them down and reuses their components.

Carbohydrates: glucose for respiration and storage

Starch is a carbohydrate made from many glucose molecules linked in a long chain. Starch is insoluble, whereas glucose is water-soluble. Digestion by carbohydrase enzymes breaks starch down to glucose, which can be absorbed and transported in the blood.

Cells use glucose in respiration, a series of chemical reactions that releases energy for their activities. This is a chemical process in cells, not another name for breathing.

Sugars can also be used to synthesise larger carbohydrates. For example, human cells can join glucose molecules together to make glycogen, an energy reserve. This links breakdown and synthesis: a carbohydrate in food supplies small sugar molecules that cells can either use immediately or rebuild into a storage carbohydrate.

Lipids: reusable components of an energy reserve

Lipids are fats and oils. During digestion, lipase enzymes break lipids down into fatty acids and glycerol. These small products can be absorbed and transported to cells.

Cells can join fatty acids and glycerol together again to synthesise fats. Stored fat acts as an energy reserve because cells can later break it down and use it in respiration. Fatty acids and glycerol therefore matter both as products of lipid breakdown and as the building blocks for new lipids.

Proteins: amino acids for growth and repair

A protein is a long-chain molecule made from many amino acids linked together. Protease enzymes digest proteins into amino acids, which are absorbed into the blood and carried to cells.

Cells join amino acids together to synthesise the proteins they need. These proteins provide materials for growth and repair, and include molecules such as enzymes. For example, repairing damaged tissue requires cells to make new proteins from amino acids delivered by the blood.

Digestion breaks an existing protein apart; protein synthesis builds a new protein from the resulting small molecules. Keeping these two processes distinct helps explain why amino acids are essential nutrients.

Removing unwanted amino acids

Not all absorbed amino acids are needed to make new proteins. The liver breaks down unwanted amino acids and produces urea. Urea is a waste substance that must be removed from the body.

The urea dissolves in the blood and is carried to the kidneys. The kidneys excrete it in solution as urine. The sequence is therefore: unwanted amino acids → liver produces urea → blood transports urea → kidneys excrete it in urine.

This connects digestion with excretion. Digestion supplies useful small molecules, while excretion removes waste produced when the body processes those molecules.

Digestion and transport

Large, insoluble food molecules → enzyme digestion → small, soluble molecules → absorption through the gut wall → blood → body cells.

Enzymes and products