CELL STRUCTURE

Cells are the individual units of which organisms are made up, and in larger organisms they are specialised to perform many functions. There is quite a lot of difference between cell types, but most cells have certain features.
Animal cells seem simpler to study than plant cells, because they are not made up of so many parts. Most animal cells have three main components.

You should be able to label the 3 main components of this typical animal cell, on the labelling lines.

simple animal cell, e.g.cheek epithelium

Points marked with an asterisk below are important connections with the next topic. Cytoplasm is a fairly transparent jelly-like substance consisting mainly of water* and protein as well as many other dissolved substances (solutes)*. It may contain granules, globules etc, e.g. glycogen, fat. It seems shapeless and it may appear to flow about. Under high magnification we may see small structures or organelles with their own functions, e.g. nucleus, mitochondria. Most of the biochemical processes of the cell, e.g. respiration, take place here.

N.b. Animal cells are unlikely to contain many or large vacuoles, as shown in some textbooks.

The nucleus [plural- nuclei] is round in shape, with a nuclear membrane on the outside. It controls the development and activities of the cell, especially cell division. It contains structures called chromosomes made of protein and nucleic acids (DNA and RNA) which carry genetic information from cell to cell, and from generation to generation.

N.b. There is always a film of moisture* on the outside of an actively growing living cell. The cell membrane (plasma membrane) is a very thin, flexible and rather elastic layer around the outside of the cell. It is made of fats and proteins. It is said to be selectively permeable* as it holds in the cell contents, as well as allowing useful substances to pass into and waste substances out of the cell.
Movement of substances into cells is partly by diffusion* and partly by active selective processes (active transport).

Name some (specific) substances useful to the typical (animal) cell, which may enter via the cell membrane.
> oxygen
> glucose

Name a waste substance from a typical (animal) cell.
> carbon dioxide

Give the name and function of the 2 special animal cells below. Label and explain the parts of each cell concerned with that special function.

cell name: [motor] neurone (nerve cell)
cell function : to transmit nervous impulses [to muscle/gland] [fatty sheath - insulates & speeds up messages]

cell name:sperm cell
cell function : to fertilise egg cell

Both animal cells and plant cells share the same 3 features. However, plant cells usually have the following features in addition:

A cell wall - normally composed of cellulose- on the outside of the cell. This is rigid, so it gives the plant cell its shape. It is made of several layers of parallel fibres, and the gaps are filled in with jelly-like substances, e.g. pectin. This means it is usually freely permeable to many molecules, especially gases and water. In some parts of a plant it may be impregnated with other substances e.g. lignin for strength in wood, or suberin for waterproofing in bark.

The cell wall gives most plant cells a fairly regular many-sided outline. The cellulose is secreted by the cytoplasm when a cell divides and it gradually thickens up as more materials are added. It stays soft and extensible until the cell stops growing, then it becomes tougher. It is non-living and not part of the cytoplasm. Normally, the cell membrane is pushed tightly against the inside of it, so that it is practically invisible, but it may be seen when the cytoplasm shrinks.

A vacuole is a cavity, usually in the centre of the cell, containing a liquid, known as cell sap. This may be a store of water and dissolved substances, e.g. sugars and metabolic by-products. The vacuolar membrane is similar to the cell membrane. Sometimes it is said that animal cells have a vacuole, but these are usually much smaller and only temporary.

Because it has a vacuole, the cytoplasm in a plant cell is usually pushed to the outside of the cell, so that it forms a thin layer inside the cell wall. The nucleus is also displaced to the edge.

Chloroplasts may be present in the cytoplasm, in different numbers, e.g. cells of leaves contain more than stems, and roots usually contain none at all. Chloroplasts are green, due to the pigment chlorophyll, and are said to be the site of photosynthesis, which only goes on in green plants. Label the diagrams below, and colour in as appropriate.
Name and note the functions of the cells, and explain their specialisations.

Cell name : palisade mesophyll cell

Function : to carry out photosynthesis

Specialisations : many chloroplasts containing chlorophyll - regular columnar shape enbling close packing for efficient absorption of light and water

Cell name : root hair cell

Function : to absorb water and mineral ions from soil

Specialisations : projecting section has large surface area for absorption of water and minerals from soil water - no chloroplasts

CELL SPECIALISATION

In both plants and animals, individual cells cannot get much bigger than a certain size. Therefore, in order to grow, an organism must increase the number of cells it is made up from. This is done by cell division, i.e. by individual single cells dividing repeatedly so that each one gives rise to two cells.

At the same time, cells may become specialised according to the tasks they have to perform. These specialisations usually involve some modifications of the structure described above, and the process of changing is called differentiation.
Many similar cells may be grouped together to form tissues. These may then be combined to make organs which have major functions , and several of these work together to form organ systems.
So an individual living organism (plant or animal) is said to be made up from all these sub-units.
Name at least 3 types of cells from the human body, and briefly mention any special characteristics that enable them to perform their functions. One has been done for you.

Cell type	Specialisation	Function
lining cell (epithelium) e.g. from inside cheek	flattened shape interlocking edges	to fit together to make a thin covering layer
> `red blood cell`	> `disc shaped`	> `to carry oxygen`
>`white (blood) cell`	> `can change shape`	>`to attack invading microorganisms ("germs")`
> `nerve cell (neuron)`	>`long thin fibres`	> `to conduct nervous impulses`
> `muscle cell`	>`can contract (tighten) or relax`	> `cause movement`

Name 2 types of cells from a green plant, and briefly describe their structural specialisations and the functions they perform. One has been done for you.

Cell type	Specialisation	Function
leaf palisade layer (photosynthetic tissue)	many chloroplasts - closely packed	to make food by photosynthesis (trapping the energy of sunlight)
> `stem (xylem is composed not of typical cells but vessel elements to be more precise)`	> `no end walls leaving hollow lumen (channel through middle) - also thick cell walls to withstand pressure`	> `carry water`
> `stem (phloem)`	>`vertical strands of cytoplasm`	> `carry products of photosynthesis` (`"foods"`)

What is the difference between an organ and an organelle? > an organ is a part of the body of an organism whereas
> an organelle is part of a cell

NUCLEUS TRANSPLANTATION EXPERIMENTS

frog spawn nucleus transplantation expts

In a series of experiments, nuclei were removed by being sucked out carefully from fertilised frogs' eggs (at the one cell stage), and in some cases they were either replaced with nuclei from other frogs' eggs, or their original nuclei were restored. Some of the transplanted nuclei were from frogs of different colour varieties. After this, the eggs were usually able to develop normally, and tadpoles hatched out and grew into adult frogs. Some of the results are shown graphically alongside.

What is another name for a fertilised egg (one cell stage)? > zygote

What are the very next couple of stages (in normal development) composed of?
> 2 cells
> 4 cells

Why does this progression not continue evenly after a while?
> Different cells are needed in different quantities in embryo

What do the results above tell you about the role of the nucleus?
> necessary for development
> controls colour of organism - i.e. expression of characteristic

What contribution do you think the cytoplasm makes in the normal development of fertilised cells turning into frogs?
> provides nutrition, energy,etc, for growth

What was the experimental reason for replacing the nucleus inside its own cell?
> to show that the process itself could be survived

Frogs' blood is usually red, due to a pigment called haemoglobin, which carries oxygen.
What colour do you think a grey frog's blood is?
> red
What colour do you think an albino frog's blood is?
> red

In humans, red blood cells have no nuclei.
What process are they therefore unable to do?
> divide

This topic has connections with other units on:-

HUMAN BODY - ORGANS AND SYSTEMS
HOW SUBSTANCES GET INTO AND OUT OF CELLS - 1 (OSMOSIS)
HOW SUBSTANCES GET INTO AND OUT OF CELLS - 2 (PLASMOLYSIS + DIALYSIS)
CELL DIVISION and CHROMOSOMES
THE BRAIN AND NERVOUS SYSTEM
RESPONSE AND CO-ORDINATION & DRUGS
THE HUMAN REPRODUCTIVE SYSTEM
PREGNANCY AND BIRTH
PLANT NUTRITION
TRANSPORT & SUPPORT IN PLANTS
PLANT MINERAL NUTRITION
THE STRUCTURE OF THE LEAF