All living things are made of cells. ~ Cells can be prokaryotic or eukaryotic. ~ Eukaryotic cells are complex and include all animal and plant cells. ~ Prokaryotic cells are smaller and simpler. ~ Eukaryotes are organisms made up of eukaryotic cells. ~ A prokaryote is a single-celled organism.

Nucleus - contains genetic material that controls the activities of the cell
Cytoplasm - gel-like substance where most of the chemical reactions happen. It contains enzymes that control these chemical reactions.
Cell Membrane - holds the cell together

Plant cells usually have all the bits that animal cells have, plus a few extra:
Rigid cell wall - made of cellulose. It supports the cell and strengthens it.
Permanent Vacuole - contains cell sap, a weak solution of sugar and salts.

Bacterial cells are much smaller. Bacteria are prokaryotes. They don't have chloroplasts or mitochondria or a true nucleus. Instead, they have a singular circular strand of DNA that floats freely in the cytoplasm. It has a cell membrane and wall. They may also contain one or more small rings of DNA called plasmids.

Prokaryotic cells (like bacteria) can reproduce using a type of simple cell division called binary fission. It is virtually the same as mitosis but with the organelles that make up the prokaryotic cell.

Cells differentiate to become specialised.
Differentiation is the process by which a cell changes to become specialised for its job. As cells change, they develop different subcellular structures and turn into different types of cells. This allows them to carry out specific functions. Most differentiation occurs as an

Sperm cells are specialised for reproduction: The function of a sperm cell is to get the male DNA to the female DNA. It has a long tail and a streamlined head to help it swim to the egg. There are lots of mitochondria to give it energy. It also carries enzymes in its head to help it digest through the egg cell membrane.

Nerve cells are specialised for rapid signalling: The function of the nerve cell is to carry electrical signals from one part of the body to another. The cells are long (to cover more distance) and have branched connections at their ends to connect with other nerve cells and form a network throughout the body.

Phloem and Xylem cells are specialised for transporting substances: Phloem and Xylem cells form phloem and xylem tubes, which transport substances such as food and water around plants. To form tubes, the cells are long and joined end to end.

What happens when a cell becomes specialised?
They become specialised through cell differentiation. This is the process where a cell changes to become specialised for a certain job. As they change, they develop different subcellular structures, and turn into different types of cells.

__Chromosomes__
Most cells in the body have a nucleus. This contains genetic material in the form of chromosomes. These are coiled up lengths of DNA molecules. Each chromosome carries a large number of genes. Different genes control the development of different characteristics, eg: hair colour.

In an undivided cell, the DNA is spread out in long strings. The cell has to grow and and increase the amount of subcellular structures (such as mitochondria and ribosomes) Then it duplicates its DNA (one copy for each new cell) The copied DNA forms x-shaped chromosomes. Each 'arm' is an exact duplicate of the other

__Stem Cells__
Animals: Stem cells are found in early human embryos and bone marrow. Embryonic stem cells can differentiate to become any type of cell. Adult stem cells can only change into certain cells, such as blood cells.

__Stem Cell Ethics__
Pros of Adult:
Doesn't kill donor. -/- Permission can be given. -/- Tried and tested method.
Cons of Adult:
Painful procedure to remove. -/- Fewer diseases can be treated. -/- Risk of operation.