Site author Richard Steane
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Cell specialisation

All eukaryotic cells start out with the components set out in previous units.

See Standard features of cells, below

However as cells become specialised some of these parts of cells become modified and emphasised, and others may even become lost.

Within an organism there may be many groups of similar cells which are collectively known as tissues.

These may be collected together (organised) into organs which have a particular function in an organism. Several organs may work together as organ systems within the organism.

Click or tap after the > prompt when you have thought about your answers
Some answers respond temporarily to mouseover or tapping
And several diagrams also respond in different ways to mouseover or tapping

Standard features of cells

You should be able to name the following parts of cells based on their roles:

The outermost part of a cell, fairly flexible {actually in most types of organisms}
> The cell membrane
(inside the cell wall in plant cells/bacteria)

The main inner part of cells, enclosed inside the above
> The cytoplasm

A single structure that controls activities within (eukaryotic) cells
> The nucleus
(not in bacteria)

Small structures that produce the majority of energy used inside cells.
> Mitochondria
(not in bacteria)

Very small structures that are used to produce proteins needed within cells.
> Ribosomes

Other features only found in some types of organisms

The rather rigid non-living outer layer on the outside of the cell

> The cell wall
(in plant cells/bacteria)

A large fluid-filled space on the inside of the cell {only in some types of organisms}
> The vacuole
(in plant cells)

Small green structures that convert solar energy into chemical energy (by photosynthesis)
> Chloroplasts
(in plant cells)

{Cyanobacteria (single celled 'blue-green algae') are about the same size as chloroplasts - and have chlorophyll 'loose' in their cytoplasm.}

A human cheek cell
seen under the light microscope
(mouseover for labels) cheek_cell (71K)

Some cell components cannot be seen above.
Explain why the following cannot be seen:

mitochondria > - They are smaller than nuclei - electron microscope gives greater resolution (and magnification)
- not many in the cell as it is not very active
- could be above or below plane of focussing - which concentrates on the nucleus
- need to be stained for the light microscope
- Some of the dots might be mitochondria, anyway

chloroplasts > - It is not a plant cell
- It does not carry out photosynthesis!

ribosomes > - They are far too small to be seen with a light micoscope
- Only visible using an electron microscope

A diagram of a leaf mesophyll cell
as seen using an electron microscope (but coloured)
(mouseover for labels)

plantcellmod (41K)
Chloroplasts are only found in plant cells that are specialised to carry out photosynthesis.
In other words, not all plant cells contain chloroplasts.

Ribosomes might be seen as small clouds of dots, e.g. just under the nucleus, on the left.

Specialised animal cells

Look at the pictures of these specialised cells, and explain the structural features ('specialisations') that enable them to carry out their functions

Sperm cell

sperm_cell (25K)

Function: > Male reproductive cells
- to move of their own accord towards an egg cell and fertilise it

Working from right to left
> flagellum/'tail' for swimming to meet egg cell
Mouseover diagram for action!

> many mitochondria in middle section to produce ATP from aerobic respiration to power swimming activity

> nucleus in head contains DNA/(a single set of) chromosomes (23 in humans) to join - at fertilisation - with another nucleus in the egg cell, also containing DNA/(a single set of) chromosomes

Sperm cells are produced in the testes following a special form of nuclear division (meiosis) which halves the number of chromosomes in the cells - producing the haploid number, instead of the diploid number of chromosomes in most ordinary cells.

> head also contains enzymes (in an 'acrosome') to assist entry into egg cell

Although they are produced within the reproductive system, sperms do not form a tissue, as each cell in independent of and effectively competing with other sperms. They are produced in large numbers, but each one is 'working on its own'!

Nerve cell (neurone)

Motor neurone disease is a degenerative condition where spinal nerves do not pass nerve impulses to muscles in the body
There are three main types of neurones: sensory neurones act as receptors and detect information, usually from the environment, relay neurones pass on the resulting impulse and motor neurones pass impulses to effector organs.
A motor neurone
Mouseover for extra information

unmyelinated (7K)

Click here to see extra details of accompanying cells - and mouseover above to return to original image

Function:> To provide a pathway to carry nervous impulses from cell to cell, perhaps some distance apart.
Nerve cells working collectively form nervous tissue, and in different parts of the body they are organised into organs like the brain and spinal cord, functioning as parts of the nervous system.

Left to right this time

> Cell body has dendrites - fibres (like tree branches) to receive incoming impulses from other nerve cells.

> The axon (main fibre) is an elongated section leading away from cell body, conducting nerve impulses to another nerve cell or muscle. Cell membrane forms a cylindrical section which transmits electrochemical impulses by allowing ions (Na+ and K+) to enter or leave.

In a myelinated neurone, the axon is surrounded by many Schwann cells which wind round it, and several layers of fatty cell membrane make a myelin sheath which acts as insulation, speeding up movement of impulses as they jump from gap to gap along the axon.

> End of axon has terminal or 'end plate' to pass impulses to another cell on the other side of a small gap called a synapse, using a chemical neurotransmitter.

What is likely to be next to the axon terminal?
> a muscle fibre / possibly a gland
(sensory and relay neurones pass impulses to another neurone)

Muscle cells

There are three types of muscle tissues:

skeletal_muscle (181K)
Mouseover above for (contr)action
Skeletal muscle - striped or striated muscle - is made up of many parallel cylindrical muscle fibres. It is described as multinucleate, as cells fuse leaving nuclei on the edges of muscle fibres. It causes movement of limbs by contracting and pulling on bones under 'voluntary control' (the brain).
Each muscle generally has a partner which pulls in the other direction, also by contracting.

smooth_muscle (190K)
Smooth muscle cells are spindle shaped, have a single, centrally located nucleus (also spindle-shaped), and lack stripes (striations). Smooth muscle causes movements within internal organs, not under voluntary control.

cardiac_muscle (159K)
Cardiac (heart) muscle has branching fibres showing striations, one nucleus clearly visible per cell. It contracts rhythmically of its own accord, but the frequency of its contractions can be affected by impulses from the autonomic nervous system, not under voluntary control.

Function (all 3 types): > To cause movement by contracting - shortening in length


> Cytoplasm has fibres of proteins (actin and myosin) which pull against one another in a sliding action

> Cytoplasm contains many mitochondria - sites of aerobic respiration - to provide energy (using the molecule ATP) to power contraction

> Muscle cells are tubular in shape and join up with other muscle cells and transmit electrochemical impulses to co-ordinate contractions.

Not seen on the diagrams above, but giving a pink colour:
> Myoglobin - a protein which absorbs oxygen from (oxy-) haemoglobin in the blood so it can be supplied to mitochondria
Important in aerobic respiration to generate energy for contraction

Other examples

Egg cell (ovum)

egg_cell_unlabelled (143K)
Function: > Female reproductive cell
- to be fertilised by a sperm cell to form a zygote, then to divide and develop into an embryo, a foetus, a baby!

Working from centre outwards
> Single large cell - diameter about 120 µm - 0.12 mm
Cytoplasm contains normal cell components e.g. mitochondria and a store of nutrients for the resulting ball of cells until it embeds ('implants') into the wall of the uterus.

> Surrounded by an thick transparent glycoprotein matrix ('zona pellucida'). For fertilisation, this needs to be acted on by enzymes from the sperm (inside the acrosome in its head) so that it attaches and the sperm nucleus passes through the cell membrane beneath it and into the cell where it fuses with the egg nucleus.
After fertilisation this layer changes to prevent other sperms entering.

> Nucleus (blue) contains DNA/(a single set of) chromosomes (23 in humans) to join at fertilisation with the sperm cell nucleus, also containing DNA/(a single set of) chromosomes
Human egg cells are produced (singly) in the ovaries following a special form of nuclear division (meiosis) which halves the number of chromosomes in the cells - producing the haploid number, instead of the diploid number of chromosomes - 46 - in most ordinary cells in humans.

>{Not technically part of the egg cell itself - but good to see their relative size!}
On the outside (in purple) is a protective layer of - many - (smaller) cells (from the follicle in the ovary) - forming the 'corona radiata'.

Egg cells are not able to move of their own accord, but see below.

Ciliated epithelium cells

cilcells (7K)
Function: > Form a protective lining layer in various parts of the body: bronchioles, bronchi and trachea in the respiratory system, also the oviducts in the female reproductive system
Also to function as a transport system.


> Constantly beating cilia - hair-like filaments - on the exposed surface create a current in the liquid above, transporting trapped microbes and other particles (upwards) to the back of the throat, or moving the egg cell from the ovary towards the uterus.

> Cells are attached to a basement membrane which provides a tubular foundation. Individual cells are columnar in shape, tightly joined to others on either side.

Other cells between these cells secrete a runny mucus secretion onto the surface of the epithelium.

Specialised plant cells

Root hair cells

Function: > to absorb water and dissolved minerals from soil

rhaircsoil (22K)
The root is composed of several layers of cells and the epidermis is the outer one


> tubular extension of cell wall (and underlying cell membrane) - projecting out from root epidermis, giving increased surface area for intake of water by osmosis and dissolved mineral salts e.g. nitrates from soil water via ion channels (specialised transport proteins) - also growing out to penetrate between soil particles

> No chloroplasts - not photosynthetic - no light beneath soil!

> (possibly increased numbers of) mitochondria to respire aerobically and provide ATP for active transport of mineral ions

Xylem cells

Function: > to take water and dissolved minerals, e.g. nitrates, up from root through stem to leaves
- also to give strength to support stems and tree trunks (wood is xylem, built up in layers over time)

xylem (65K)


> Many cylindrical 'cells' connecting with 'cells' above, forming long tubular sections, with others alongside - xylem tissue.

> Thickened cell walls (at sides) reinforced with lignin to resist negative pressure - "suction" - from above, due to transpiration stream (caused by water evaporating from leaf cells).
Cell walls may have spiral thickening giving strength but allowing for upward growth.

> No end walls (giving reduced resistance to water flow). What were cells are actually dead ('vessel elements'), with no cytoplasm, no nucleus, no end cell walls, just forming hollow tubes

Phloem cells

Function: > to take water and products of photosynthesis (sugars etc) - mostly down from leaves through stem to rest of plant - but some will be taken upwards

phloem (44K)


> Many cylindrical cells connecting with cells (above and) below - phloem tissue.

> End walls have holes ('sieve plates'), through which strands of cytoplasm pass from cell to cell, no nucleus, but cells are not thought to be dead

> Companion cells alongside have nuclei and are active in powering transport


Notes on words:

The word sperm can be used as a collective term or we can refer to sperms which reminds us that they are usually in large numbers.
A single sperm cell may be described as a spermatozoon, and the plural is spermatozoa. This is derived from the Greek for seeds, and animals. (Zoology is the study of animals)

Xylem pronounced zy -lem
Phloem pronounced flow -em
Xylem is derived from the Greek for wood, and phloem from the Greek for bark, emphasising their roles as inner and outer vascular tissues (ducts to convey water and dissolved substances) in tree trunks and ordinary plant stems
This topic has connections with other units on this site:
Cell biology
Eukaryotes and prokaryotes
Animal cells
Plant cells
Yeast cells
Bacterial cells

Cell structure and specialisation - covering similar material to this unit - and I am quite pleased with the section on nuclear transplantation (at the end)

Higher level
Nerve cells, nerve impulses
Skeletal muscle
Mass transport in plants

External web references

Welcome to Histology at SIU

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