Metabolism

About metabolism

Metabolism is the term given to chemical reactions performed by cells that the body needs to live. Your body is constantly breaking down chemicals and building new ones from the pieces.

Metabolism rearranges atoms (eg, carbon in glucose) into other carbon containing molecules. All the chemical components within the body are first absorbed from food (which is why not getting enough iron from your diet will cause anaemia).

An example of the consequences of metabolism can be seen with weight gain and weight loss:

• Surplus food eaten will be digested and stored as fatty acid chains, that can be broken down if needed later.

• Not enough food eaten will lead to these fatty acid chains being broken into pieces, releasing energy and being lost from the body as carbon dioxide and water.

For more complicated reactions, several enzymes will work together like a production line. Each one performs a specific task in a chain of events, converting a molecule from one type to another in a metabolic pathway.

In this reaction molecules are repeatedly added to a starting group in a metabolic pathway.

Adenosine TriPhosphate (ATP)

Energy is needed to make and break chemical bonds. This is provided by the high energy bonds of phosphates. These phosphates are carried around the body in the form of ATP to places where it is needed.

ATP is the energy currency of the cell and the breakdown of ATP into ADP (adenosine diphosphate) provides free energy to allow other chemical reactions to take place.

ATP is regenerated by adding an extra phosphate group onto ADP, this uses energy released from breaking down other chemicals, such as glucose.

Cell

A cell contains many sub units, organelles, encased in an inner and outer lipid membrane.

Metabolism of sugars in cells

This project is interested in energy (ATP) production by sperm. Every cell is capable of releasing energy from single sugar units, eg, glucose and fructose, made from carbon, hydrogen and oxygen. A source of glucose and fructose is from sucrose, common table sugar, that is digested by the gut into the smaller units.

Longer carbohydrates made from single sugar units joined together can also be broken down in the gut to generate fuel for energy.

Pathways of single sugar unit metabolism in cells

There are two main pathways that release energy with (aerobic) and without (anaerobic) oxygen:

1. With oxygen – Oxidative Phosphorylation takes place in mitochondria, the power house of the cell.

2. Without oxygen – Glycolysis happens in the cytoplasm, a jelly like substance that fills the cell.

When oxygen is present, cells are much more efficient at releasing energy, but sometimes there is insufficient oxygen to do this so cells rely on anaerobic processes (eg in exercising muscles).

During the break down of glucose there are many new chemicals formed:

• Glucose is the starting sugar.

• Glucose gets split into two pyruvate molecules.

• Pyruvate without oxygen will be turned into lactate.

• With oxygen pyruvate is metabolised into acetyl-CoA.

• Acteyl-CoA can be metabolised and stored as fats- or completely broken down releasing all the energy from glucose.

Krebs cycle (turning Acetyl-CoA to CO₂ and H₂O)

After the formation of pyruvate within the cytoplasm, it is converted into acetyl-CoA and enters the Krebs cycle. The Krebs cycle breaks down acetyl-CoA to carbon dioxide and water using other molecules. This break down generates new molecules that are used to produce more ATP.

The Krebs cycle, also known as the citric acid cycle, was fully described by Hans Adolf Krebs whilst he was at the University of Sheffield.

Breakdown of sugars and fats

Despite different chemical structures to start with different chemicals get broken down by different enzymes into the same components, water and carbon dioxide. The amount of energy released is higher from fat than sugar.