Sample-Cells/The Cell

Introduction
Add brief introduction to chapter including: (beginnings of guiding narrative)
 * Rationale and importance to practice
 * Relationship to previous and next topics
 * Suggested approach to study of the chapter

Objectives
After completing this section, you should know:
 * that cells can be different shapes and sizes
 * the role and function of the plasma membrane; cytoplasm, ribosomes, rough endoplasmic reticulum; smooth endoplasmic reticulum, mitochondria, golgi bodies, lysosomes, centrioles and the nucleus
 * the structure of the plasma membrane
 * that substances move across the plasma membrane by passive and active processes
 * that passive processes include diffusion, osmosis and facilitated diffusion and active processes include active transport, pinocytosis, phagocytosis and exocytosis
 * what the terms hypotonic, hypertonic isotonic and haemolysis mean
 * that the nucleus contains the chromosomes formed from DNA
 * that mitosis is the means by which ordinary cells divide
 * the main stages of mitosis
 * that meiosis is the process by which the chromosome number is halved when ova and sperm are formed

The Cell
The cell is the basic building block of living organisms. Bacteria and the parasite that causes malaria consist of single cells, while plants and animals are made up of trillions of cells. Most cells are spherical or cube shaped but some are a range of different shapes (see diagram 3.1).

Most cells are so small that a microscope is needed to see them, although a few cells, eg the ostrich’s egg, are so large that they could make a meal for several people.

A normal cell is about 0.02 of a millimetre (0.02mm) in diameter. (Small distances like this are normally expressed in micrometres or microns (μm). Note there are 1000 μms in every mm).

Diagram 3.1. A variety of animal cells

When you look at a typical animal cell with a light microscope it seems quite simple with only a few structures visible (see diagram 3.2).



Diagram 3.2 An animal cell

Three main parts can be seen:


 * an outer cell wall or plasma membrane,
 * an inner region called the cytoplasm and
 * the nucleus

However, when you use an electron microscope to increase the magnification many thousands of times you see that these seemingly simple structures are incredibly complex, each with its own specialized function. For example the plasma membrane is seen to be a double layer and the cytoplasm contains many special structures called organelles (meaning little organs) which are described below. A drawing of the cell as seen with an electron microscope is shown in diagram 3.3.



Diagram 3.3 An animal cell as seen with an electron microscope