Cells: the basic unit of life

Look at the skin on your hand, a leaf of a bean plant, or a drop of pond water from Lake Victoria under a microscope. However different they look, all of them are built from the same thing: cells.

By the end of these notes you should be able to:

• explain why the cell is called the basic unit of life;
• draw, label and give the functions of the parts of a typical animal cell and a typical plant cell;
• explain how specialised cells are adapted to their jobs;
• describe the levels of organisation from cell to organism.

What is a cell?

A cell is the smallest unit of a living organism that can carry out all the processes of life. That is why it is called the basic unit of life.

Every living thing is made of one or more cells:

• Unicellular organisms are made of a single cell that does everything. Examples: Amoeba, Paramecium, yeast and bacteria.
• Multicellular organisms are made of many cells that share the work. Examples: humans, cows, mango trees, maize plants and mushrooms.

Cells are very small. Most are between 0.01 mm and 0.1 mm across, so we need a microscope to see them. The English scientist Robert Hooke first used the word "cell" in 1665 after looking at thin slices of cork, which reminded him of small rooms.

The cell theory

Scientists summarise what we know about cells in the cell theory:

1. All living things are made of one or more cells.
2. The cell is the basic unit of structure and function in living things.
3. New cells come only from cells that already exist, by cell division.

The typical animal cell

A cheek cell from the inside of your mouth is a good example of a typical animal cell. It has three main parts, plus several smaller structures inside the cytoplasm.

The typical plant cell

A cell from the leaf of a plant, or from the thin skin of an onion, is a good example of a typical plant cell. It has all the parts of an animal cell, plus three extra structures:

Comparing plant and animal cells

Drawing cells in biology

When you draw cells in an exam or in your practical book:

• use a sharp pencil and draw clear, continuous lines, with no shading;
• make the drawing large, at least half a page;
• draw label lines with a ruler, keep them parallel to the page, and do not let them cross;
• write labels in pencil, on the right-hand side where possible;
• give the drawing a title, and the magnification if you know it.

Specialised cells

In multicellular organisms, cells are specialised: their structure is adapted to do one particular job well. This is called division of labour. Here are cells you must know.

Specialised animal cells

Specialised plant cells

Levels of organisation

Cells do not work alone in multicellular organisms. They are organised in levels, from simple to complex:

1. Cell: the basic unit of life. Example: a muscle cell.
2. Tissue: a group of similar cells working together to perform a particular function. Example: muscle tissue; xylem tissue in plants.
3. Organ: a structure made of different tissues working together to perform a particular function. Example: the heart, made of muscle, nerve and connective tissue; a leaf.
4. Organ system: a group of organs working together to perform a major function. Example: the circulatory system (heart, arteries, veins and capillaries); the digestive system.
5. Organism: a complete living thing, made of several organ systems working together. Example: a human being; a maize plant.

Why cells matter in everyday life

Understanding cells explains many things around you:

• Medicine. Malaria parasites invade and destroy red blood cells, which is why malaria causes anaemia. Antibiotics work by attacking structures that bacterial cells have but human cells do not.
• Food and farming. Yeast is a single-celled fungus. Its respiration produces the carbon dioxide that makes bread rise and the alcohol in traditional brews.
• Wilting. When a crop like sukuma wiki lacks water, its vacuoles shrink, the cells become soft (flaccid), and the leaves wilt.

Summary

• The cell is the basic unit of structure and function in all living things.
• Animal and plant cells both have a cell membrane, cytoplasm, nucleus, mitochondria and ribosomes.
• Plant cells also have a cellulose cell wall, a large permanent vacuole and (in green parts) chloroplasts.
• Specialised cells have structures adapted to their function, such as red blood cells, nerve cells, sperm cells, root hair cells and palisade cells.
• Cells form tissues, tissues form organs, organs form organ systems, and systems form an organism.

Practice questions

1. Define the term cell and explain why it is called the basic unit of life.
2. Draw and label a typical plant cell. Beside each label, state the function of that part.
3. State four differences between a plant cell and an animal cell.
4. A red blood cell has no nucleus. Explain how this helps it do its job.
5. Explain why a root hair cell has no chloroplasts.
6. Arrange in order, from simplest to most complex: organ, cell, organism, tissue, organ system. Give a human example of each.
7. Activity of integration. Your village health worker says some children are tired and pale because of malaria. Using what you know about red blood cells, write a short explanation for a parents' meeting of why malaria can make children weak, and suggest two ways families can prevent it.