Unit 1: 1.4 Cellular Differentiation Higher Biology Unit 1: 1.4 Cellular Differentiation

Learning Intentions To be able to describe cell differentiation in plants and animals To be able to discuss the ethical issues surrounding the use of stem cells

Success Criteria I can explain cellular differentiation in plants and animals I can give examples of the ethical issues surrounding the use of stem cells I can explain the pros and cons of stem cell research

Cellular differentiation Multi-cellular organisms are comprised of a large number of cells. These are specialised to carry out specific roles in the body. Differentiation is the process by which unspecialised cells become altered and adapted to form a special function in the body.

Selective gene expression Every cell in your body has all the genes necessary for constructing the whole organism. Genes can be switched on or switched off when they are required (see Unit 2) otherwise the body would waste energy producing proteins in cells where they are not needed. e.g. insulin produced in pancreas cells only, not in brain or heart cells.

Differentiation in animals Each human life begins life as a fertilised egg (zygote) and divides to become approximately 37.2 trillion (37 200 000 000 000) with almost 200 different cell types.

In the embryonic cells all the genes are switched on, or have the potential to be switched on. As development proceeds, the cells undergo differentiation and become specialised. Once a cell has become specialised it only expresses the genes that code for proteins specific to the role of the cell.

Stem cells Stem cells are unspecialised cells that can: Reproduce themselves by repeated mitosis and cell division. Differentiate into specialised cells when required to do so.

Embryonic stem cells

Embryonic stem cells come from very early embryos (5-14 days after fertilisation) called blastocysts.

Embryonic stem cells have the potential to differentiate into any type of cell found in the human body

Tissue (Adult) stem cells Tissue (Adult) stem cells involved in maintaining and repairing the body. They have a narrower differentiation potential than embryonic stem cells as many of their genes are already switched off.

Adult stem cells are found at several locations around the body. Hematopoietic stem cells give rise to different blood cells. Mesenchymal stem cells give rise to bone, cartilage, adipose tissue.

Ethical Issues The use of adult stem cells is widely accepted as a medical treatment Controversy surrounds the use of embryonic stem cells This is because when embryonic stem cells are harvested the embryo is prevented from developing any further Some people believe this shouldn’t be allowed to happen

Growth and differentiation in plants In multicellular plants growth is restricted to regions called meristems. These are groups of unspecialised plant cells capable of dividing through out the life of the plant. They are a source of all tissue types in a plant.

Types of meristem Apical meristem at the tip of the root and the shoot – adding length to the plant. Lateral meristems allow the stems to thicken.

Apical meristem Found at the tip of the root and shoot.

2. Lateral meristem Inside the plant. Called cambium. Produces new vascular bundles (xylem and phloem) to transport food and water in the stems of perennial plants. Each year it produces an annual ring of xylem. The number of rings indicates the age of the plant.

Transverse section of a woody stem

Success Criteria I can explain cellular differentiation in plants and animals I can give examples of the ethical issues surrounding the use of stem cells I can explain the pros and cons of stem cell research