Differentiation- cell specialisation and cellular organisation

Stem CellsLearning Objectives

• Define a stem cell as able to turn into any type of cell

• Know where stem cells are found - in early human embryos

• State that stem cells are found in adults, in the bone marrow.

• Explain how stem cells can be used to cure diseases

Learning outcomes

You should be able to:

• Explain the term DIFFERENTIATION

• Describe how the structure of some specialised plant and animal cells relates to their functions

- sperm- red blood cells - nerve cells (neurones

- palisade leaf cells

Key Words

• Photosynthesis guard cells

• Chloroplasts vacuole ribosomes

• Mitochondria cell wall / membrane

• Stomata cytoplasm nucleus

Sponges

• Sponges are amongst the earliest multicellular animals on Earth (600 million years old)

• Some cells are tubular, providing pores through which the water can circulate

Sponges

• Other sponge cells secrete calcium carbonate or silica, which makes a skeleton for the sponge

• Sponges are multicellular animals like us but...

• ... the different types of cells are scattered throughout their bodies

• They have no tissues and no organs

A silica ‘spicule’

Sponges

• Recently drugs companies have become very interested in sponges!

• Unable to move sponges cannot run away from predators

• Instead, they have evolved chemical defenses

• Some other organisms carry sponges because of this

A North Wales sponge crab

Sponges

• Some of these chemicals show promise as drugs to fight human diseases

• For example, a compound called halichondrin, derived from Lissodendoryx is being trialled as an anti-cancer drug

• Another sponge produces a chemical called avarol, which could be used to treat psoriasis

Differentiation and cell specialisation• Multicellular organisms consist of many cells – all

derived from the single fertilised egg formed from the fusion of egg and sperm

• Some cells become specialised to perform certain functions = differentiation

• Cells can differentiate in a number of different ways:

- the number of specialised parts- the shape / size of the cell

- some of the contents of the cell

Differentiation and cell specialisation• Once a human cell has differentiated it cannot

change into another type of cell

• The cells of the early embryo– they can become any of the different types of cells in a human

• These are stem cells

• Stem cells:

- are unspecialised- can divide repeatedly to make new cells- can differentiate into several kinds of specialised

cells

Differentiation and cell specialisation• So far, all of the stem cells identified are only

able to differentiate into a limited range of cells

• For example, stem cells in bone marrow can form white or red blood cells but not neurones, or any other kind of cell

• There is much interest in stem cells, as they could cure many diseases

• For example, Parkinson’s disease is caused by the death of a particular group of cells in the brain, which could be replaced using stem cells

Specialised animal cells• Red blood cells transport oxygen in the blood

• They have a very short life-span: every second, 10 million old erythrocytes are destroyed in your spleen and 10 million new ones are made

• They are made from stem cells in the bone marrow, especially in the ribs, vertebrate, pelvis and skull

• These cells also make white blood cells (leucocytes)– cells that attack and destroy invading microorganisms

• Red blood cells transport O2 and CO2

• They are very small, to fit through capillaries and get close to cells in body tissues

• They are packed with haemoglobin, which combines with O2 or CO2

• There is no nucleus, to make room for more haemoglobin

Red blood cells

• The ‘biconcave’ (refresher) shape of erythrocytes provides a high surface area: volume ratio

• This increases the amount of oxygen and CO2 that can pass into and out of the cell in a certain period of time

White blood cells• leucocytes (white cell)

• They destroy bacteria and other foreign material

• They move toward bacteria

• They are the predominant cell in pus!

Multilobed nucleus

Cytoplasm

Specialised plant cells• Plants do not have stem cells – most of their cells

retain the ability to differentiate throughout their lives