The Role of Carbohydrates in our Bodies

Carbohydrates (specifically glucose) provide the main source of energy in the body.  Immediately after eating, blood sugar (glucose) levels rise. In response to these increased levels the pancreas (in higher animals and humans) releases insulin (a hormone), which increases the rate of absorption of glucose f\out of the blood and into cells.  Glucose may then be used as energy, or in the liver and muscles, converted into glycogen for storage.

A series of chemical reactions which changes glucose into glycogen.

Glycogen is a molecule which can be stored in the body, whereas high levels of glucose cannot be stored. As glycogenesis occurs, the level of blood sugar drops back to normal levels.

A series of chemical reactions which turns glycogen back into glucose.

If blood sugar levels become too low, the pancreas stops releasing insulin, and begins releasing glucagon.  Glucagon, like insulin is a hormone, but it acts in opposition to insulin, stimulating the conversion of stored glycogen into glucose by the liver, and the release of the glucose by the liver into the blood stream.  The blood then supplies glucose molecules to cells which transport them inside and utilise them for energy to drive their biochemical processes.

A series of chemical reactions that convert organic molecules other than sugar into glucose.  Pyruvate, lactate, glycerol and some amino acids can be converted into glucose by this general pathway.  This is the pathway which allows the body to use both fat and protein as an energy source when carbohydrates are not available.

Abnormally high blood sugar levels.

Different animals have different levels of blood sugar which they can tolerate.  Human kidneys, for example, can tolerate only 160 mg of glucose per 100 ml of blood.

Abnormally low blood sugar levels.

This means cells are being starved of energy and is particularly serious for the brain, for which glucose it the only food source.  Neurons cannot store glucose or glycogen at all, and therefore rely on a constant supply of it from the blood stream.

What is Carbohydrate Oxidisation?
Glucose moves from blood into cells of the body. Glucose in body tissues undergoes cellular respiration in an intracellular organelle known as the mitochondrion (plural = mitochondria). This involves glucose being oxidised to form carbon dioxide, water and ATP).  This is an oxygen dependent reaction, where the cell takes in fresh oxygen, and at the end of the process delivers waste carbon dioxide back the circulation for expulsion via the lungs.

The energy produced by cellular respiration is stored in the body in the form of ATP.  When the cell is ready to use the energy, ATP is converted to ADP and this reaction provides the energy to drive an array of biochemical reactions in the cell.

How are Carbohydrates Stored in the Body?

Very little carbohydrate is stored in the body of humans or animals. Excess glucose is converted into carbohydrate called glycogen and this is stored in the liver where it acts as a reservoir which can be converted back into glucose and fed into the bloodstream when required. However, only a comparatively small amount of glycogen can be stored in the liver. Additional carbohydrate is normally converted into fat which is stored under the skin, inside the muscles and around certain organs of the body (such as the kidneys).  

How are Carbohydrates Absorbed?

Carbohydrates need to be broken down into monosaccharides before they are absorbed into cells. As monosaccharides they will move through the apical (free) surface of intestinal cells by either:

Facilitated Diffusion
This is where a substance which is not normally soluble in lipids is transported through a
membrane by combining with a transporter (ie. carrier); or

Active Transport
Active transport is where substances move through a membrane against a concentration
gradient (against osmotic pressure). This process requires the expenditure of energy
(ATP) to occur to make it work. An example is glucose which is moved into a cell against its concentration gradient (from an area where it is in low concentration – the blood, to an area where it is in higher concentration – the cell) using the movement of sodium ions with their concentration gradient as the energy source.

Monosaccharides move from the epithelial cells lining the intestine by facilitated diffusion and then enter the capillaries in the villi (in the digestive tract).