Carbohydrates are the planets most common organic compounds. They provide energy and structure for both plant and animal life, we commonly call them sugars.

Carbohydrates are organic compounds composed of the elements hydrogen, carbon and oxygen. These elements can be arranged in many structures which give them a variety of biochemical properties and give rise to many forms of carbohydrates.

The simplest carbohydrates are called monosaccharides. Glucose, also called dextrose is the sugar present in the bloodstream as blood sugar. More complex sugars are polymers of glucose molecules and include sucrose, lactose, starches and cellulose.

From a dietary point of view carbohydrates can be classified as simple and complex. The simple carbohydrates can be transformed into energy more quickly than the complex carbohydrates. This can be measured by a glycaemic index (GI) which ranks the effect on blood sugar levels over two hours compared with the effect of glucose on blood sugar over the same period.

High GI foods break down quickly and provide a rapid increase in blood sugar whereas low GI foods increase blood sugar at a slower steady pace. Foods with a GI over 70 are considered high, those with a GI between 56 and 69 are considered medium, and those with a GI less than 55 are considered low. Fruit and vegetables are generally considered low GI although there is considerable variation – carrots and kiwi fruit are low GI, beetroot and mango are medium GI, and pumpkin and watermelon are high GI. There can also be a difference in the GI of fruit depending on the ripeness; ripe or over ripe fruit has a higher GI than unripe or just ripe fruit. There are other factors such as food combinations, the presence of fat, protein and the way food is cooked, prepared and stored which can effect food GI.     

Once glucose migrates from the bloodstream to cells, the glucose is burned to produce energy, excess glucose is converted to glycogen in a process called glycogenesis, it is stored in muscle tissue. If glucose is not used immediately or converted into glycogen it is converted to fat and stored in the body.

In plants, starch is a major form of stored carbohydrates, this is a polysaccharide composed of amylase and amylopectin. Another important carbohydrate for plants is cellulose which forms the structural component of the cell walls of green plants.

Animals need to consume plants for nutrition however cellulose is extremely difficult to digest because the molecules are bound very strongly to each other. Most animals cannot digest cellulose. Animals which are able to digest cellulose, such as ruminants, horses, kangaroos, koalas harbour micro organisms in their digestive system which ferment the cellulose to break down the bonds. Fungi which are responsible for recycling nutrients from plants are also able to break down cellulose.

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