Proteins Carbohydrates Lipids Vitamins Minerals Fibre

All living things need food to provide the nutrients for the renewal and growth of cells, and also as an energy source.

Plants (producers) can make their own food using carbon dioxide from the air, water and minerals from the soil, and sunlight.

Animals (consumers) eat these foods and break them down into small, simple building blocks in their digestive systems. These nutrients can then pass out of the digestive system into the bloodstream to be transported to all the living, working cells of the body, where they are used.

Let's look more closely at the different types of foods.

Proteins
These are large complex chemicals needed by an organism for growth and repair. (Milk and eggs are high in protein as they provide growth materials for the developing young.)

Proteins are made up of chains of small units called amino acids. There are twenty different types of amino acids which, when joined in different sequences, make the many different proteins needed by the organism. Our bodies break down the proteins in the food we eat to amino acids, so they can be absorbed into the blood stream, and then our cells use them to build the proteins we need.

Some cells make structural proteins for growth, like muscle (meat and fish are excellent sources of protein). Amino acids can also be linked together to form enzymes which are proteins that help speed up the chemical reactions of life.

Carbohydrates
The sugars and starchy foods we eat are also made up of smaller units, called simple sugars. Our bodies break down complex sugars and starch into simple sugars so they can be absorbed. These provide most of the energy the body needs. Seeds and grains (cereals) are high in carbohydrates as they represent the energy store for the embryonic plant in the seed. Similarly, plant food stores such as potatoes are high in carbohydrates.

In the cells, these simple sugars combine with oxygen in a chemical reaction called cellular respiration to release energy which is then available for the organism's needs. In humans, when the blood sugar level is too high, the hormone insulin stimulates the liver to store excess sugar for later use. Longer-term storage is created by conversion to fat.

Lipids
Fats and oils are taken into the body as large complex chemicals which are digested and broken into smaller units. They can then be used as immediate energy sources, or as building blocks in structures such as cell membranes, or deposited as fat which provides an energy-rich store for later use.

Vitamins
These organic nutrients, which have originated from once living things, help regulate chemical reactions in the cells. If sufficient fresh fruit and vegies are eaten to supply the bodies' needs, then vitamin supplements are not required.

Minerals
A supply of inorganic minerals is needed for building many important substances in the body. For example, iron (from red meat, grains and green vegetables) is essential to make haemoglobin, the red substance in the blood which carries oxygen. Bones and teeth require calcium and phosphorus (from dairy foods, eggs, leafy vegetables) for strength. Sodium (table salt) and potassium (from fruit and vegetables) help regulate water balance. Magnesium (dairy foods, cereals, vegetables) helps nerves send messages and muscles to contract.

Minerals from the soil are taken in by plants through their roots and become part of the plants' cells. As a consequence, minerals needed by animals become available to them when they eat and digest plant materials, or when they eat other animals which have digested plant material.

Some plants (e.g. sundew and Venus fly trap) can survive in poor soils which lack the minerals needed to support life. These insectivorous plants trap insects, secrete digestive enzymes over the insects' bodies, and then absorb the nutrients and make use of the required minerals.

Fibre
Plant cell walls contain the complex carbohydrate, cellulose (fibre). Humans cannot digest cellulose but it helps push food quickly through the intestine. Fresh fruits, vegetables and grains are therefore important to keep the waste material moving and thereby may help prevent bowel cancer.

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.