All human beings develop from just two cells: an egg cell (ovum) from their mother and a sperm cell from their father. Egg cells are the largest human cells and can just be seen without a microscope. Most of the other cells can only be seen through very powerful microscopes.
What is a cell made of?
Although there are differences between the various types of cell, most have the same basic structure and they all need certain substances, such as food and oxygen, to stay alive and to work properly. Here are the basic parts of a cell:
- Membrane—This is a fine layer which holds the cell together and separates it from other cells. It is rather like the frontier of a country. It allows waste products made in the cell to pass out.
- Mitochondria—These are the cell’s power stations. Here food and oxygen react together to produce energy so that the cell can live and work.
- Ribosomes—Ribosomes are the cell’s factories. They manufacture proteins, including those from which the cell itself is made.
- Nucleus—The nucleus acts rather like a government headquarters, controlling and directing all the activities of the cell. It is in the center of the cell body. The nucleus contains special threads called chromosomes. These carry complex coded instructions for the workings of the cell, rather like a computer program. You inherit your chromosomes from your parents.
- Cytoplasm—This is a jelly-like substance, which makes up most of the cell, rather like a background landscape. It consists mainly of protein and water, especially water. Your cells are about two-thirds water.
- Endoplasmic reticulum—These channels are the cell’s industrial estates. They are where the ribosomes are found.
- Golgi complex—This acts as a storage depot. Some of the proteins made by the ribosomes are kept here until they are needed.
- Lysosomes—These are the cell’s secret police. They contain chemicals which destroy harmful foreign substances and any old or diseased parts of the cell.
Types of cell
You have many different types of cell in your body, each with different jobs to do:
- Nerve cells—have long fibers which send messages to other parts of the body. Some have special endings for feeling sensations.
- Muscle cells—are long and thin. They can shorten their length (contract) and then relax, which causes movement.
- Sperm cells—have long tails (from the male’s body). This helps them to swim towards the egg cells in the female’s body.
What is a tissue?
A group of cells of mainly the same type is called a tissue. The minute spaces among the cells are filled with a watery substance called a tissue fluid.
What are organs?
Different types of tissue are grouped together to form organs. An organ has a particular job to do in your body. For instance, your heart pumps blood, your stomach digests food and your eyes enable you to see.
What is a system?
A group of organs whose jobs are closely related is often referred to as a system. Examples are the circulatory system, which includes your heart and blood vessels, and the skeletal system.
How do cells reproduce?
Millions of cells die every second but new cells are constantly being made to take their place. Some cells live longer than others. The cells lining your intestines get worn away by food and live for only about six days. Red blood cells live for about four months, bone cells for up to 30 years, and nerve and muscle cells, which cannot reproduce, up to a lifetime. A cell reproduces by dividing into two to produce a pair of identical new cells.