Chapter 1. Cells as the Basic Units of Life FREE
Chapter 1: Cells as the Basic Units of Life
Every living thing you can name, from a towering bluegum tree to the bacteria living on your skin, is built from cells. A cell is the smallest unit that can carry out all the processes we call life. In this opening chapter of Grade 9 Natural Sciences we look closely at what a cell is made of, how plant cells differ from animal cells, and how cells join together to build the tissues, organs and systems of a living body. This knowledge is the foundation for the Life Sciences you may choose in Grade 10.
1.1 The discovery of cells
The cell was first seen in 1665, when the English scientist Robert Hooke looked at a thin slice of cork through an early microscope. The tiny empty boxes he saw reminded him of the small rooms, or cells, in which monks lived, and the name stayed with us. Modern biology rests on three ideas that together form cell theory:
- All living organisms are made of one or more cells.
- The cell is the basic unit of structure and function in living things.
- All cells arise from cells that already exist.
Some organisms, such as bacteria and amoebae, are made of a single cell. Others, such as a human being, are made of many millions of millions of cells working together.
1.2 The structure of an animal cell
A cell is not just a bag of jelly. Inside it are tiny working parts called organelles, each with its own job. Study the labelled animal cell below.
The main organelles and what they do are set out in the table.
| Organelle | Function |
|---|---|
| Cell membrane | A thin outer layer that controls which substances enter and leave the cell. We say it is selectively permeable. |
| Cytoplasm | The jelly-like fluid where the organelles float and where many chemical reactions take place. |
| Nucleus | The control centre of the cell. It holds the genetic material (DNA) and directs all cell activities. |
| Mitochondrion | Releases energy from food during cellular respiration. A very active cell, such as a muscle cell, has many mitochondria. |
| Ribosomes | Tiny structures where proteins are built. |
| Vacuole | A storage space for water, food or waste. In animal cells vacuoles are small. |
1.3 How plant cells are different
Plant cells contain the same organelles as animal cells, but they also have three extra features that animal cells do not have. Look carefully at the plant cell below and compare it with the animal cell above.
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present, made of strong cellulose, gives the cell a fixed shape | Absent |
| Chloroplasts | Present, contain the green pigment chlorophyll that traps light energy for photosynthesis | Absent |
| Vacuole | One large central vacuole filled with cell sap | Small vacuoles, if any |
| Shape | Regular, often box-like | Rounded and more variable |
Because a plant cell can trap light energy in its chloroplasts and make its own food, plants stand at the base of almost every food chain on Earth.
1.4 Looking at cells with a microscope
Cells are far too small to see with the naked eye, so we use a light microscope. A microscope magnifies an object, which means it makes the image look larger than the real object. The total magnification is worked out by multiplying the magnification of the eyepiece lens by the magnification of the objective lens.
Worked example 1: total magnification
A learner views onion cells using an eyepiece lens of 10 times and an objective lens of 40 times. What is the total magnification?
Total magnification = eyepiece x objective
Total magnification = 10 x 40
Total magnification = 400 times
The cells therefore appear 400 times larger than they really are.
1.5 Why are cells so small?
A cell takes in food and oxygen, and removes waste, across its surface (the cell membrane). The amount of activity inside the cell depends on its volume. As a cell grows, its volume grows faster than its surface area, so a large cell cannot move materials in and out fast enough. Cells stay small to keep a large surface area compared with their volume. We measure this with the surface-area-to-volume ratio.
Worked example 2: surface-area-to-volume ratio
Imagine a cube-shaped cell with sides of 2 mm. Work out its surface-area-to-volume ratio.
A cube has 6 square faces.
Surface area = 6 x (2 mm x 2 mm) = 6 x 4 = 24 mm^2
Volume = 2 mm x 2 mm x 2 mm = 8 mm^3
Surface-area-to-volume ratio = 24 : 8 = 3 : 1
A smaller cube of side 1 mm would give 6 mm^2 to 1 mm^3, a ratio of 6 : 1. The smaller cell has the larger ratio, which is why being small helps a cell survive.
1.6 From cells to systems
In a large organism, cells do not work alone. They are organised into levels, with each level built from the one below it:
- Cells of the same kind group together to form a...
- Tissue (for example, muscle tissue). Several tissues form an...
- Organ (for example, the heart). Organs that work together form an...
- Organ system (for example, the circulatory system). All the systems together make up the whole...
- Organism (for example, a human being).
So the correct order from smallest to largest is: cell, tissue, organ, system, organism. You will meet these human systems in the chapters that follow.
Summary
- The cell is the basic unit of life, and cell theory explains that all cells come from existing cells.
- Important organelles include the cell membrane, cytoplasm, nucleus, mitochondria and ribosomes.
- Plant cells also have a cell wall, chloroplasts and a large central vacuole.
- Total magnification = eyepiece magnification x objective magnification.
- Cells stay small to keep a high surface-area-to-volume ratio.
- Levels of organisation run from cell to tissue to organ to system to organism.