Bacterial cells are smaller and simpler than plant and animal cells.

Bacterial cells do not contain chloroplasts (like animal cells but unlike plant cells). In addition, unlike animal and plant cells, bacterial cells lack:

• A 'true' nucleus
• Mitochondria

Instead of a cell nucleus containing chromosomes, the DNA in bacterial cells is arranged in a single circular strand in the cytoplasm.

In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. Eukaryotes usually have a single nucleus, but a few cell types have no nuclei, and a few others have many. It contains genetic material, which controls the activities of the cell.

In cell biology, the cytoplasm is the material within a living cell, excluding the cell nucleus. It comprises cytosol and the organelles – the cell's internal sub-structures. All of the contents of the cells of prokaryotic organisms are contained within the cytoplasm. Within the cells of eukaryotic organisms the contents of the cell nucleus are separated from the cytoplasm, and are then called the nucleoplasm. The cytoplasm is about 80% water and usually colorless.

It controls the movement of substances in and out of the cell.

Mitochondria are structures in cells where respiration happens.

Protein synthesis happens here.

It strengthens the cell.

They contain chlorophyll, which absorbs light energy for photosynthesis

It is filled with sap to help keep the cell turgid.

Plant cells can be seen using a light microscope.

You should be able to describe how to prepare a microscope slide of stained onion cells. Here is a typical method:

1. Cut open an onion
2. Use forceps to peel a thin layer of epidermis from the inside
3. Lay the layer of epidermis on a microscope slide
4. Add a drop of iodine solution to the layer
5. Carefully place a cover slip over the layer

The iodine solution stains starch in the cells blue-black, making the cell features easier to see.

Growth involves cell division followed by cells becoming specialised. Growth can be measured as an increase in:

• Height
• Wet mass
• Dry mass

Height and wet mass can be measured when an organism is alive, but dry mass can only be measured when an organism has had all its water removed and is dead. However,dry mass is the best measure of growth.

An enzyme is a protein that functions as a biological catalyst – a substance that speeds up a chemical reaction without being changed by the reaction.

Lock and key model

Enzymes are folded into complex shapes that allow smaller molecules to fit into them. The place where these molecules fit is called the active site.

In the lock and key model, the shape of the active site matches the shape of its substrate molecules. This makes enzymes highly specific – each type of enzyme can catalyse only one type of reaction (or just a few types of reactions).

The diagram shows how this works. In this example, the enzyme splits one molecule into two smaller ones, but other enzymes join small molecules together to make a larger one.

the shape of the enzyme changes, its active site may no longer work. We say that the enzyme has been denatured. Enzymes can be denatured by high temperatures or extremes of pH.

Tansport system is a means by which materials are moved ('transported') from an exchange surface or exchange surfaces to cells located throughout the organism.

A nutrient is a substance which is needed for growth, repair and metabolism. The three main nutrients are:

• carbohydrates
• proteins
• lipids (fats and oils)

These nutrients are all examples of organic chemicals. This means that they all contain carbon atoms, covalently bonded to the atoms of other elements.

any of a large group of organic compounds occurring in foods and living tissues and including sugars, starch, and cellulose. They contain hydrogen and oxygen in the same ratio as water (2:1) and typically can be broken down to release energy in the animal body: "water-soluble carbohydrates".  The basic units of carbohydrates are simple sugars, such as glucose and fructose. These are also called monosaccharides.

Lipids are fats and oils. Lipids are large molecules made from smaller units of fatty acids and glycerol.

Proteins are large molecules made from smaller units of amino acids. There are only about 20 different naturally-occurring amino acids. However, each protein molecule has hundreds, or even thousands, of them joined together in a unique sequence and folded into the correct shape. This gives each protein its own individual properties.

The human genome project was launched in 1990 and resovled to sequence 95% of the DNA in human cells in just 15 years. John Sulston, for several years, had been mapping the genome of a nematode worm and worked out that mapping the whole sequence was feasible. To sequence the genome as accurately as possible, scientists came up with the 'heirarchical shotgun' method. Researchers eventually broke many copies of the genome into fragments, each around 150, 000 letters of code (or base- pairs) long. Using special enzymes, researchers could cut the individual clones into diagnostic 'fingerprint' of fragments defined by each ones clone sequence. Then they could search among millions of fingerprints for shared fragments that would reveal overlaps among the clones. Researchers then assembled the clones into longer continuous regions and mapped these onto the human chromosomes. Scientists needed to break the cloned fragments into smaller chunks, each around 1000 to 200 base - pairs long. By 1944, the Sanger Institute had produced its first 100,000 bases of human DNA. In 1995, researchers from the Sanger located the BRCA2 gene, associated with increased risk of breast cancer. Other scientists from around the world found MSH2 gene, which increases the risk of colon cancer for carriers, canadien scientists found the FAD gene, which together confer an almost 100% risk of develping Alzhiemer's disease.