CELLS: STRUCTURE AND FUNCTION
Chapter 1, The anatomy of a cell

On this page, you will find the following topics:

Cell theory
Cell size
Cell complexity
Cell cycle
Prokaryotic cells
Eukaryotic cells
Table of comparison of plant and animal cells
Cell organelles
Table of structure and function of sub-cellular organelles
Cell specialisation
Viruses: Cells or not?

CELL THEORY

The combined observations and conclusions of cell biologists over many years led to the development of the cell theory, which states:

� All living things are composed of cells.
� Cells are the basic units of structure and function in living things.
� All cells come from preexisting cells.

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VARIATION IN CELLS

CELL SIZE

Cells vary enormously in their sizes and their complexity. The smallest cells are less than 1 micrometer in diameter. A micrometer is one millionth of a meter. On the other end of the cell spectrum, the yolks of birds' eggs are a single cell, and are quite visible to the naked eye, and some of the nerve cells in your body run between your toes and your spine. However, most cells are in the range of 5 - 50 micrometer in diameter.

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CELL COMPLEXITY

Cells are usually very complex structures with variation in the assortment of structures inside which limit them to, or suit them to, particular functions.

All living cells have some basic features in common. They all have some structure which separates the inside of the cell from the outside. They all have a jelly-like substance inside. They all have, at least at some stage in their lives, some genetic material. They all have structures which respond to the genes and produce proteins.

According to the above description of what all cells possess, it is possible to divide cells into two major groups:

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CELL CYCLE

Since all cells come from preexisting cells, it follows that any one cell has a finite 'life', that is the time between when it is first formed until it either divides, or dies, or if it is a sex cell, fuses with another sex cell.

This is known as the cell cycle and the time taken for it to be completed varies enormously between different types of cells. Cells such as those linig the intestines live only for a few days, others, such as nerves, last a lifetime.

The stages in the cell cycle, starting with a newly formed cell, can be simply divided into:

Growth and normal functioning. This is the Interphase of mitosis. This is generally the longest part of a cell's life cycle.
Either replication. The division phases of mitosis - prophase, metaphase, anaphase and telophase. This gives new cells to enter interphase.
Or cell death. In this case, enzymes dismantle the dead cell's molecules so that they may be reused by living cells.

What triggers a normal cell to die or divide, and why the trigger sometimes fails to work correctly, as in cancer, where cell division becomes much more frequent and the cells do not die, is the subject of interesting research in Biochemistry.

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TYPES OF CELLS

PROKARYOTIC CELLS

These are the bacteria and other very simple cells. They have the following features:

� Possess an external cell membrane and often a cell wall.
� Possess cytoplasm.
� May possess structures for mobility, flagellae or cilia.
� Contain free floating genetic material, usually DNA, in the cytoplasm.
� Contain ribosomes.
� Do not contain membrane bound structures within the cell.

Check your text book for diagrams and photos of prokaryotic cells, or click below.

Diagram of a prokaryotic cell

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EUKARYOTIC CELLS

These are much more complex than prokaryotic cells.
Examples of eukaryotic cells are found in animals, plants, fungi and protists.
They have all or some of the following features:

� All have a cell membrane, some have cell walls.
� All have cytoplasm.
� May have flagellae or cilia, but many do not.
� Contain membrane bound organelles, but types and numbers of organelles vary widely.
� All have a distinct membrane bound nucleus at some stage.
� Most have a nucleus through the cell life.

Both plants and animals are eukaryotic, but there are some notable differences which help divide them into these two groups. The Table which follows summarises these structural differences.

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**COMPARISON OF PLANT AND ANIMAL CELLS**
PLANT CELLS	ANIMAL CELLS
Cell wall present (Therefore relatively rigid structure)	No cell wall (Therefore relatively flexible.)
May possess chloroplasts, containing chlorophyll (Cells with chloroplasts look green.)	No chloroplasts, no chlorophyll.
Large vacuole usually present	Vacoule, if present, usually small.
Aster the structure giving rise to spindles in mitosis.	Centrioles precursor on mitotic spindles.

Check your textbook for diagrams and photos of plant and animal cells. You should be familiar enough with the structural differences between them to identify a cell as either animal or plant.

You can click below to see diagrams of plant and animal cells.

Plant cell - line drawing

Animal cell - line drawing

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CELL ORGANELLES

The separate structures which can be found in cells are called organelles, or "little organs". And just like the organs and systems which interact to allow our bodies to function normally, the cell's organelles combine together to allow the cell to carry out the function it is designed to do.

It is important to realise that different cells have different numbers and types of organelles. Cells are specialised for a particular function.

The Table which follows lists the most commonly found organelles, features of their structures and their functions. It might be a good idea to print this and stick it somewhere you will look at it often. A bedroom mirror, above your desk, or even the toilet wall!

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**STRUCTURE AND FUNCTION OF SUB-CELLULAR ORGANELLES**
ORGANELLE	In bacteria?	In plant?	In animal?	STRUCTURE & FUNCTION
Cell membrane(Plasma membrane)	Yes	Yes	Yes	Boundary between intracellular & extracellular environments. Regulates entry/exit of substances.
Cell wall	Yes, usually	Yes	No	Rigid structure providing support for cell.
Cytoplasm	Yes	Yes	Yes	Jelly-like substance filling intracellular space, contains dissolved substances.
Cyto-skeleton	No	Yes	Yes	Network of fine tubes and threads. Provides internal structural support.
Centrioles	Rarely	No, but aster is similar.	Yes	Paired rods which help organise microtubules during mitosis.
Nucleus	No	Yes	Yes	Membrane-bound structure containing cells' genetic information (DNA) and support molecules.
Nucleolus	No	Yes	Yes	Small structure within nucleus. Site of production of ribosomal RNA.
Nuclear Membrane	No	Yes	Yes	Boundary between nucleus and cytoplasm. Regulates passage of materials between the two.
Flagella, pili or cilia	Sometimes flagella or pili	Rarely, but some specialised cells may.	Only present in some specialised cells.	Structures used to enable movement of cells or sometimes to propel substances across outer surface of the cell. Predominantly protein in composition.
Mitochondria	No	Yes	Yes	Membrane bound organelles. Folded membranes within contain enzymes for aerobic respiration. (A little DNA in here too.)
Chloroplasts	No	Only in photosynthetic cells	No	Membrane bound organelles. Folded membranes within contain chlorophyll and enzymes for photosynthesis. (A little DNA in here too.)
Vacuole	No	Yes, often large	Unusual, and small if present.	Membrane bound area filled with water and assorted solutes. Role in maintenance of water balance of the cell.
Ribosomes	Yes	Yes	Yes	Small organelles at which protein synthesis occurs. May be free floating or membrane-bound.
Endoplasmic reticulum (ER) -smooth	No	Yes	Yes	Network of flattened membranes forming tunnels. Enzymes assisting synthesis of some lipids and final processing of proteins found here.
Endoplasmic reticulum -rough	No	Yes	Yes	Similar to smooth ER, but with ribosomes embedded in membrane. Proteins to be exported from cell produced here.
Golgi apparatus (aka Golgi Body)	No	Yes	Yes	Stacks of saucer shaped membranes where export proteins are modified and stored prior to entering secretory vesicles for exocytosis.
Lysosomes	No	Yes	Yes	Membrane bound structure containing enzymes which break down toxic or unwanted molecules.
Plastids	No	Yes	No	Membrane bound structures with varied functions. Leucoplasts - starch storage. Chromoplasts - coloured pigments within (eg flower petals).
Vesicles	No	Rare	Yes	Packages for storage (eg fat droplets) or temporary transport associated with endocytosis/exocytosis.

Felix

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CELL SPECIALISATION

� Shapes of cells, and the number and variety of cellular organelles vary according to the function of the cell.
� The assortment and relative number of organelles in a cell should enable you to predict accurately the main function of that cell.
� You should be familiar with a range of cell types and be able to identify the structures seen with both light and electron microscopes in this variety of cells.
� Examples you might review would include; root hair cell, guard cell, neurone, red and white blood cells, a pancreatic insulin secretory cell.

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VIRUSES: CELLS OR NOT?

There is debate about whether or not viruses should be classified as living things, or cells at all.
Because viruses can survive but not reproduce outside a host cell, they fall into a group of their own.
However, viruses do have some features common with living cells, and some characteristics of which you should be aware:

� Protein coat with varying characteristics depending on the type of virus.
� Nucleic acid inside. Can be either DNA or RNA, but it's role is to take over the metabolism of the infected host cell and force it to produce and release millions more virus particles.
� Different viruses known to infect all types of cells: bacteriophages infect bacteria, plant and animal viruses also well researched.

Viruses come in a wide variety of shapes. Check your textbook for diagrams or pictures of viruses.

Here is a diagram of the HIV-AIDS virus. Because this virus contains RNA as its nucleic acid, it also carries the enzyme reverse transcriptase. Once inside the host cell, this enzyme forces the cell to manufacture DNA, using the viral RNA as the template. This new copy DNA (cDNA) then takes over the host's metabolism. HIV

HIV

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This page is maintained by Jenny Herington, who can be contacted at bio_cat@bioserve.latrobe.edu.au by email, Last update : 21 Feb. 97