Carbohydrates, proteins, lipids (or fats) are major nutrients that we need in large quantities.
We get these by eating them. They are broken down first and then reassembled into our own carbohydrates, proteins and lipids. This is because:
- most of the molecules in food are too large to pass through the absorbing surface of the gut wall
- the carbohydrates, proteins and lipids are reassembled in the form required, rather than other animal or plant versions
| Nutrient | Major function | Major sources |
| Carbohydrates | Source of energy, glucose is the main respiratory substrate | Starch: potatoes, rice and wheat products, bread, cereals and pasta. Sugars: fruit, smoothies, fizzy drinks, chocolate and sweets |
| Proteins | Growth and repair | Meat, eggs, cheese, beans, nuts and seeds |
| Lipids | Energy, make up part of cell membranes so essential for normal growth | Butter and margarine, meat and processed meat, plant oils, oily fish, nuts and seeds |
Carbohydrates
Carbohydrates in our diet include sugars and starches.
The glucose molecule is small enough to be absorbed directly through the walls of the digestive system.
Starch is a polymer of glucose. It must be broken down into glucose molecules – it is too large to pass through the gut.
Cellulose is also made up of glucose molecules. It makes up plant cell walls. It is therefore a fundamental part of our diet. It cannot be broken down by the digestive system, so is egested from the gut.
Once absorbed by the body, glucose molecules are transported to cells and:
- used for respiration
- reassembled into the storage form of carbohydrate in animals – glycogen
In plant metabolism, the glucose produced by photosynthesis is converted into starch for storage, and cellulose, for cell wall synthesis.
In humans and animals glucose is stored in glycogen. It is not converted into starch.
Proteins
Proteins are made up of amino acids.
Proteins are big molecules that are too large to pass through the gut wall. They must first be broken down into amino acids.
Once inside the body, the amino acids are reassembled into the proteins the individual requires – the process of protein synthesis.
Excess amino acids are broken down in the liver.
Lipids
Lipids are esters of fatty acids and glycerol.
Lipid molecules are too large to pass through the gut wall and must be digested first.
In the body’s cells, they are reassembled into the lipids the cell needs, for instance, for the cell membranes.
The human digestive system
The human digestive system has two functions:
- breaks down complex food substances
- provides the very large surface area for maximum absorption of food
The structure of the digestive system
Regions of the digestive system are adapted to the digestion and absorption of food:
Digestion
| Region | Function |
| Mouth | Begins the digestion of carbohydrates |
| Stomach | Begins the digestion of protein; small molecules such as alcohol absorbed |
| Small intestine -Duodenum | Continues the digestion of carbohydrate and protein; begins the digestion of lipids |
| Small intestine -Ileum | Completes the digestion of carbohydrates and proteins into single sugars and amino acids; absorption of single sugars, amino acids and fatty acids and glycerol |
| Large intestine | Absorption of water; egestion of undigested food |
Digestive enzymes are used to break down food in the gut into small, soluble molecules that can be absorbed through the gut wall.
Absorption
The surface of the small intestine wall is folded, and has projections called villi.
Key fact
Villi is the plural of villus.
The epithelial cells that cover each villus themselves have projections called microvilli.
These all increase the surface area over which digested food – now simple molecules – is absorbed.
Most of the digested food passes through the epithelial cells of the gut wall and is carried by blood to the liver. Digested lipids pass through the gut wall and enter the lacteals.
