B3.1 Growth and differentiation · KS3 Science

Eukaryotic and prokaryotic cells

Eukaryotic cells (animal, plant and fungal cells) have a nucleus that contains their DNA.
Prokaryotic cells (bacteria) are much smaller and have no nucleus. Their DNA is a single loop in the cytoplasm, and they often have plasmids (small rings of DNA). They have a cell wall and may have a flagellum (tail) to move.
Electron microscopes magnify far more than light microscopes and show much finer detail (higher resolution), such as the inside of mitochondria.
Magnification = image size ÷ actual size. 1 mm = 1000 µm.

Bacteria reproduce by binary fission (splitting in two), as often as every 20 minutes when it is warm and there are nutrients. The number doubles each time: 1, 2, 4, 8, 16…
Aseptic technique stops unwanted microbes getting in: sterilise the inoculating loop in a Bunsen flame, use sterile agar plates, lift the lid only a little, and tape the lid on without sealing it completely.
In school, cultures are incubated at no more than 25 °C, to reduce the chance of growing bacteria that cause disease in humans.

Diffusion is the spreading out of particles from where they are more concentrated to where they are less concentrated. It needs no energy. Oxygen and carbon dioxide move in and out of cells by diffusion.
Osmosis is the diffusion of water across a partially permeable membrane, from a dilute solution to a more concentrated one.
In pure water, potato pieces gain mass as water moves in by osmosis. In a strong sugar solution, they lose mass as water moves out.
Active transport moves substances against a concentration gradient (from low to high concentration), using energy from respiration. Root hair cells take in mineral ions this way.

Mitosis makes two genetically identical cells, for growth and repair. Before a cell divides, it copies its DNA.
Cancer is caused by uncontrolled cell division, which forms a lump called a tumour. Malignant tumours can spread around the body.
Stem cells are undifferentiated: they can become many types of cell (differentiation). Embryonic stem cells can become any type of cell; adult stem cells, such as those in bone marrow, can become only a few types.
In plants, meristems at the tips of roots and shoots contain stem cells throughout the plant's life.
Stem cells could treat diseases such as leukaemia or diabetes, but using embryos raises ethical concerns.

Eukaryotic cell: A cell with a nucleus, such as an animal or plant cell.
Prokaryotic cell: A cell with no nucleus, such as a bacterium.
Plasmid: A small ring of DNA found in bacteria.
Binary fission: How bacteria reproduce, by splitting in two.
Aseptic technique: Ways of working that stop unwanted microbes getting into a culture.
Diffusion: The spreading of particles from a high concentration to a low concentration.
Concentration gradient: The difference in concentration between two areas.
Osmosis: The diffusion of water across a partially permeable membrane.
Partially permeable membrane: A membrane that lets some substances through but not others.
Active transport: Moving substances against a concentration gradient, using energy.
Mitosis: Cell division that makes two identical cells.
Tumour: A lump of cells made by uncontrolled cell division.
Stem cell: An undifferentiated cell that can become other types of cell.
Differentiation: A cell becoming specialised for a particular job.
Meristem: Plant tissue at root and shoot tips that contains stem cells.