Organs are made up of cells that are arranged into tissues.

In the liver, these cells help to remove toxins from the blood, recycle worn-out red blood cells, store glucose and produce urea for excretion by the kidneys.

These jobs need highly specialised cells all working together in the liver tissue. Even a small piece of liver will contain millions of cells.

Your body has many different systems. Each one performs a specific job. For example, your digestive system digests and absorbs the nutrients from your food. Other body systems include:

• Immune system - fights off infections.
• Skeletal and muscular systems - give support and generate movement.
• Reproductive system - reproduction.
• Nervous and endocrine (hormone) systems - control and co-ordination.
• Respiratory and circulatory systems - transport oxygen and other nutrients around the body.
• Excretory system - control the level of water in the body.

DNA is a long molecule. Base-pairs join the two strands of a double helix. The bases are like the letters of an alphabet. They are arranged to spell out the instructions that control the cell.

Sections of the DNA make up genes. Each gene has the information needed to make a particular protein, for example an enzyme. These genes direct the production of the structures and enzymes that make up the cell.

The bases are always arranged in these pairs: adenine-thymine and guanine-cytosine. This allows the DNA to copy itself when the cell grows and divides.

Each chromosome in the cell contains one long molecule of DNA. There is about
1.8 metres of DNA in every human cell.

DNA is a long molecule. It is coiled and packed into chromosomes inside the nucleus. Humans have 23 pairs of chromosomes. One of each pair comes from the mother and the other from the father.

You have about 1.8 metres of DNA coiled inside the nucleus inside each of your cells.

The nucleus contains the cell's DNA and genes. This controls the cell's activities by directing the proteins and enzymes that the cell makes.

Every cell in your body contains exactly the same DNA. It is a copy of the DNA that was first made when egg and sperm joined to make the fertilised egg that you developed from.

Different types of specialised cells are made by switching on or off different combinations of genes. For example, in a liver cell, the genes needed to make all the structures and enzymes found in a liver cell will be switched on. All other genes will be switched off.

Cells make up all the tissues and organs of the body. They are specialised to do a particular job. Skin cells join tightly together to make sure your skin does not tear.

Liver cells contain large numbers of mitochondria to generate energy to drive the chemical reactions happening in their cytoplasm. They are packed with enzymes to catalyse these reactions.

Each system contains several different organs. These each do a particular job.

The liver is part of the digestive system. It helps to process and store foods such as glucose.

The liver is a very important organ as it also helps to make bile (used in digestion) and remove poisonous toxins from the body. It also recycles damaged red blood cells and produces urea from the breakdown of excess protein in the diet.

Your body is made up of more than a 1000 billion cells. These all came from a single fertilised egg cell.

This cell started to grow, copied itself and divided into two new ones. Each of these cells did the same to make four cells. This repeats time and time again to make up the billions of cells that now make your body. It is still happening to grow new cells and repair any damaged body tissues.

Each stage of growth and splitting into two new cells is called cell division or mitosis.