2.1.5 Stem Cells in Animals and Plants — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

Section 1 of 9Differentiation and stem cells

A fertilised egg divides by mitosis to form a ball of identical cells.

Differentiate

As an embryo grows, most of these cells differentiate: they become specialised for a particular job,

such as a nerve cell, red blood cell or muscle cell.

Specialisation involves switching particular genes on or off so that different proteins are made and the cell gets different structures.

A stem cell is an undifferentiated cell that can divide by mitosis to produce more of itself and can differentiate into one or more types of specialised cell. Stem cells are therefore a supply of new cells for growth and repair.

Section 2 of 9Stem cells in animals

Embryonic stem cells

Embryonic stem cells are found in the early embryo and can develop into almost any cell type in the body.

Adult stem cells

Adult stem cells are found in tissues such as bone marrow.

They can only form a limited range of cell types, for example bone marrow stem cells make the different types of blood cell.

In an adult these cells replace worn-out cells, for example the cells of the skin, gut lining and blood. Because stem cells can be grown in the laboratory, they could provide replacement cells for people with damaged tissues.

Section 3 of 9Stem cells in plants

Plants keep stem cells for their whole lives in regions called meristems, found at the tips of roots and shoots and in the cambium between xylem and phloem.

Stem cells in plants

Meristem cells divide and differentiate into cells such as xylem, phloem, epidermis and root hair cells,

so plants can keep growing throughout life, unlike most animals.

Plant stem cells can therefore produce any cell type needed.

Uses in agriculture

stem cells from meristem tissue can be grown on agar containing nutrients and hormones to produce many identical plants (clones).
This is useful to:

Stem cells in plants

Produce large numbers of disease-free plants quickly, because shoot-tip meristem is often virus-free.

Grow plants that are rare or endangered and protect them from extinction.

Produce crop plants with a desired characteristic, for example high yield or disease resistance, for growers to plant in bulk.

Section 4 of 9Medical uses of stem cells

Stem cell treatments already include bone marrow transplants for leukaemia and other blood disorders: the healthy donor's stem cells replace those destroyed by treatment and make new blood cells. Research aims to use stem cells to replace cells lost in conditions such as diabetes (insulin-producing cells), paralysis after spinal injury, and some forms of blindness.

Therapeutic cloning involves making an embryo that is genetically identical to the patient, so that stem cells from it will not be rejected by the patient's immune system.

Section 5 of 9Risks and issues

Risks and issues

Viruses may be transferred to the patient in donor stem cells, causing infection.

Stem cells can divide out of control and form tumours.

Rejection if cells come from a donor who is not a match.

Ethical views are covered in the next lesson. In plants, cloning produces a population with the same genes, so a single disease could affect all of them, which reduces genetic variation.

Section 6 of 9Plant tissue culture step by step

The commercial cloning of plants is called micropropagation or tissue culture.

1

A small piece of meristem is cut from the parent plant with sterile tools.

2

It is placed on sterile agar jelly containing nutrients, sugars and plant hormones that stimulate cell division.

3

Sterile conditions are essential because microorganisms would grow rapidly on the nutrient-rich agar and kill the tissue.

4

The mass of dividing cells is then transferred to a medium with different hormones that encourages roots and shoots to form.

5

The small plantlets are finally moved to compost and hardened off.

Compared with growing from seed, this method gives many plants of identical genotype, all year round, and in a small space. The disadvantages are cost, the need for skill, and the lack of genetic variation, which makes the whole crop vulnerable to the same disease.

Animals do not have equivalent meristemsmost animal stem cells are restricted to repair and replacement, and animal tissues cannot be grown into whole organisms in this way, which is why cloning an animal needs a different technique.

Section 7 of 9Exam-style questions

Explain why meristem tissue is useful for growing many identical plants. [3]3 marks
Describe what is meant by differentiation. [2]2 marks
Suggest one risk of treating a patient with donated stem cells. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Stem cell	An undifferentiated cell that can divide and develop into specialised cells.
Differentiation	The process by which a cell becomes specialised.
Embryonic stem cell	A stem cell from an early embryo able to form almost any cell type.
Adult stem cell	A stem cell in a mature tissue with a limited range of possible cell types.
Meristem	A region of plant tissue containing stem cells that divide throughout life.
Clone	A genetically identical organism or cell produced by mitosis.
Therapeutic cloning	Making an embryo identical to a patient to obtain stem cells that will not be rejected.

Section 9 of 9Exam technique

1

Saying stem cells are specialised; they are unspecialised until they differentiate.

2

Thinking all stem cells can become any cell; adult stem cells have a more limited range than embryonic ones.

3

Stating that plants lose stem cells; meristems keep them throughout life.

4

Writing that clones are produced by meiosis; mitosis makes genetically identical cells.