Multicellular organisms contain many different types of cells, which are specialised to carry out particular functions. A large number of specialised cells together make up tissues and various tissues together make up an organ.
Specialised animal cells
There are numerous specialised cells in animals which perform particular functions.
Some examples of specialised animal cells and their functions are described below.
Muscle cells
These are long, thin cells which contract to cause movement. The cells contain a large number of mitochondria to produce the large amount of energy required for this contraction.
Sperm cells
A sperm cells particular function is to fertilise an egg cell. It has a specialised tail to allow it to swim towards an egg and the head of the sperm cell contains enzymes to digest the exterior of the egg, which allows the sperm to enter the egg and fuse with it.
Tail
Head contains nucleus and enzymes
Nerve cells
These cells form connections with other nerve cells to create a huge network which is used to transmit impulses throughout the body; this allows us to co-ordinate movement and thoughts. They have long projections which allow them to make connections with other cells; these connections can be made with cells some distance away.
Projection to connect with other cells

Blood cells
Red blood cells are small and have a ‘bi-concave’ shape to increase their surface area and to allow them to squeeze through tiny blood capillaries. Their function is to transport oxygen around the body; to achieve this, they contain a pigment called haemoglobin which binds with oxygen and they lack a nucleus to maximise the amount of pigment they can carry.
White blood cells fight infection in the body. They have an irregular shape to squeeze through blood vessels and their shape allows them to surround a microbe; they also contain enzymes which digest the microbe into harmless waste substances.
Specialised plant cells
Phloem cells
These cells are involved in the transport of sugar in plants; they have perforated end walls which allow sugars to be transported to the next phloem cell. They also possess companion cells with a large nucleus to control the phloem cells.
Root hair cells
These cells are responsible for absorbing water (for photosynthesis) from soil around the roots. Each cell has a large surface area to maximise the volume of water it can absorb.
Palisade mesophyll cells (leaf cells)
These are found in the upper layer of a leaf and have the main function of carrying out photosynthesis; they contain a very large number of chloroplasts to maximise photosynthesis.

Tissues
Tissues are groups of specialised cells which will carry out a particular function.
Examples of animal tissues include:
 Muscle
 Blood
 Nerves
Examples of plant tissues include:
 Vascular tissue (i.e. transport tissues such as xylem and phloem)
 Palisade tissue (layer of palisade cells within the leaf)
 Root tissue
Organs
Organs are made up of several types of tissue grouped together, for example, the small intestine is made up of muscle tissue, connective tissue, epithelial lining tissue and also nerve tissue to control its action. Organs can carry out specific function due to the large variety of specialised cells within them.
Other examples of animal organs include:
 Heart (pumps blood around the body)
 Lungs (site of gas exchange)
 Eyes
Examples of plant organs include:
 Leaves
 Reproductive organs (e.g. anther)
Examples of the link between cells, tissues and organs are shown below:

Part a: Stem cells
Stem cells are unspecialised cells which are found in all multicellular organisms. Stem cells have the potential to become many different types of cells and so are described as the site of production of specialised cells. As with other body cells, stem cells undergo mitosis and therefore increase the number of cells in an organism; specialised cells produced by division of stem cells can be used for growth or repair.
The diagram shows a few of the possible cell types which can be produced by stem cells.
Potential uses of stem cells
Stem cells are currently being used after cancer treatment to restore destroyed stem cells; bone marrow (which contains stem cells) is transplanted into a patient and the cells it contains will begin to produce blood cells.
There is a vast range of possible uses of stem cells in medical science which may improve health. The diagram outlines some of these potential stem cell uses:

Due to the unspecialised nature of stem cells, they could potentially be used to cure or repair damage caused by a wide range of conditions.
Stem cell technology could be used to:
 repair damaged heart tissue after a heart attack
 grow new skin cells to repair skin damage after burns
 grow complete organs (removing the need for waiting lists for transplants)
Stem cell technology is a promising but ethically controversial procedure due to the source of some stem cells; stem cells can be derived from adult tissues (and other sources) or they can be sourced from embryos. The use of embryonic stem cells has been criticised by many groups due to the ethics of creating, using and destroying potential embryos.
Part b: Meristems
Within plants, only particular areas can undergo mitosis and therefore contribute to growth, these areas are called meristems. Meristems produce non-specialised cells in plants and these non-specialised cells can potentially become any type of plant cell (e.g. leaf mesophyll cells, phloem cells etc). meristem
Root hair young leaves meristem
Shoot tip Root tip
Areas which undergo growth, such as root tips and shoot tips, will contain meristems.
Root cap