AQA Combined science Specification based revision

During the cell cycle, the genetic material is doubled and then divided into two identical cells.

1. Before the cell can divide it needs to grow and increase the number of sub-cellular structures such as ribosomes and mitochondria. The DNA replicates to form two copies of each chromosome.

2. In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides.

3. Finally, the cytoplasm and cell membrane divide to form two identical cells.

Cell division by mitosis is important in the growth and development of multicellular organisms.

The nucleus of a cell contains chromosomes made of DNA molecules.

Each chromosome carries a large number of genes.

In body cells, the chromosomes are normally found in pairs.

A stem cell is an undifferentiated cell of an organism which is capable of producing more stem cells and also turning into a differentiated cell.

Stem cells from human embryos can be cloned and made to differentiate into most types of human cells. Stem cells from adult bone marrow can form many types of cells including blood cells.

Meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant. These cells can be used to produce clones of whole plants quickly and cheaply which is useful for:

1. growing crops of identical plants that have desired features for farmers.

2. Growing more plants of rare species.

Treatment with stem cells may be able to help conditions such as diabetes and paralysis. It can differentiate into nerve cells to replace dead or damaged nerve cells. It can differentiate into insulin-producing cells to replace faulty cells in diabetics.

In therapeutic cloning, an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the parent's body so they may be used for medical treatment.

The use of stem cells has potential risks such as the transfer of a viral infection, and some people have ethical or religious objections.

Against:

Each one is a potential human life.

They believe scientists should find an alternative solution.

For:

Curing patients who already exist are more important than the rights of embryos. 

Embryos used in research are usually unwanted ones from fertility clinics, would have been destroyed either way. (However, those supporting the rights of embryos want this banned too.)

Substances move in and out of cells across the cell membrane via diffusion.

Diffusion is the spreading out of the particles of any substance in solution, or particles in a gas, resulting in a net movement from an area of higher concentration to an area of a lower concentration.

Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea (produced from breaking down of proteins) from cells into the blood plasma for excretion in the kidney. 

Factors which affect the rate of diffusion:

The difference in concentrations ( the concentration gradient). The higher the concentration gradient the higher the rate of diffusion. This is because the net movement from one side is greater.

The temperature, this is because the particles have more energy to move around faster.

The surface area of the membrane, this is because more particles can pass through at once.

A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism. Enough substances can be exchanged across the membrane to supply the volume of the cell.

Multi-cellular organisms have a smaller surface area compared to their volume- not enough substances can diffuse through the outside surface to supply their entire volume. This means they need some sort of exchange surface for efficient diffusion. 

A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism. Enough substances can be exchanged across the membrane to supply the volume of the cell.

Multi-cellular organisms have a smaller surface area compared to their volume- not enough substances can diffuse through the outside surface to supply their entire volume. This means they need some sort of exchange surface for efficient diffusion.