Introduction
Your body is made up of trillions of tiny cells. Although these cells are microscopic, each one is an incredibly complex and highly organised living system. Every animal cell contains specialised structures called organelles, each with a specific job that helps the cell survive and function.
Just as a city has buildings, roads, power stations, and waste disposal systems that work together, the organelles inside a cell cooperate to carry out all the processes needed for life. Understanding the structure and function of animal cells provides the foundation for studying tissues, organs, body systems, genetics, and disease.
What Is an Animal Cell?
An animal cell is the basic structural and functional unit of animals.
Animal cells:
- are eukaryotic cells, meaning they contain a nucleus,
- contain membrane-bound organelles,
- carry out all the life processes needed for survival,
- vary in shape depending on their function.
Although animal cells come in many forms, they all share several important organelles.
Definition:
An animal cell is a eukaryotic cell that contains a nucleus and specialised organelles which work together to support life.
The Major Organelles of an Animal Cell
The five organelles you need to know are:
- nucleus
- cell membrane
- cytoplasm
- mitochondria
- ribosomes
Each organelle performs a different function.
| Organelle | Main Function |
|---|---|
| Nucleus | Controls the activities of the cell and contains DNA |
| Cell membrane | Controls what enters and leaves the cell |
| Cytoplasm | Site of many chemical reactions |
| Mitochondria | Release energy by respiration |
| Ribosomes | Make proteins |
The Nucleus
The nucleus is often called the control centre of the cell.
It contains:
- DNA (genetic material),
- chromosomes,
- instructions for building proteins.
The nucleus controls:
- growth,
- reproduction,
- protein production,
- all major cell activities.
Without a nucleus, most animal cells cannot survive for long.
Everyday Analogy
Think of the nucleus as the head office of a company.
It stores all the important information and sends instructions to every part of the cell.
The Cell Membrane
The cell membrane is a thin, flexible layer surrounding the cell.
Its main job is to control:
- what enters the cell,
- what leaves the cell.
The membrane allows useful substances to enter, such as:
- oxygen,
- water,
- nutrients.
It allows waste products to leave, such as:
- carbon dioxide,
- excess water,
- other wastes.
Because it controls movement into and out of the cell, it is described as selectively permeable.
The Cytoplasm
The cytoplasm is the jelly-like material filling most of the cell.
It:
- supports the organelles,
- allows materials to move around the cell,
- is where many chemical reactions occur.
Most of the cell's activities take place within the cytoplasm.
The Mitochondria
The mitochondria are often called the powerhouses of the cell.
Their job is to carry out cellular respiration.
Respiration releases energy from food.
This energy is used for:
- movement,
- growth,
- repair,
- active transport,
- reproduction.
Cells that require lots of energy, such as muscle cells, contain many mitochondria.
Ribosomes
Ribosomes are tiny structures that make proteins.
Proteins are needed for:
- growth,
- repairing damaged tissues,
- enzymes,
- hormones,
- muscles,
- antibodies.
Ribosomes may be:
- free in the cytoplasm,
- attached to the rough endoplasmic reticulum (which you will study later).
Although ribosomes are extremely small, they are among the busiest structures in the cell.
How Organelles Work Together
No organelle works alone.
Each one depends on the others.
For example:
- The nucleus contains the instructions for making proteins.
- The ribosomes build those proteins.
- The mitochondria provide the energy needed.
- The cytoplasm is where many reactions occur.
- The cell membrane allows materials to enter and leave.
Together, these organelles keep the cell alive and functioning properly.
Visualising an Animal Cell
A typical animal cell should include and clearly label:
- cell membrane
- cytoplasm
- nucleus
- mitochondria
- ribosomes
Drawing and Labelling an Animal Cell
When drawing an animal cell:
- Draw a roughly circular or irregular outline.
- Label the cell membrane around the outside.
- Fill the inside with cytoplasm.
- Draw a large nucleus.
- Add several bean-shaped mitochondria.
- Scatter many tiny ribosomes throughout the cytoplasm.
Remember:
Scientific diagrams should be:
- neat,
- large,
- clearly labelled,
- drawn in pencil,
- labelled with straight ruler lines.
Worked Example 1
A student says:
"The mitochondria control everything the cell does."
Question
Is this correct?
Solution
No.
The nucleus controls the activities of the cell.
The mitochondria release energy through respiration.
Worked Example 2
Which organelle is responsible for each function?
| Function | Organelle |
|---|---|
| Controls cell activities | Nucleus |
| Makes proteins | Ribosomes |
| Releases energy | Mitochondria |
| Controls entry and exit | Cell membrane |
| Site of many reactions | Cytoplasm |
Relating Structure to Function
The structure of each organelle helps it perform its job.
| Organelle | Structure | Function |
|---|---|---|
| Nucleus | Large, membrane-bound | Stores DNA and controls the cell |
| Cell membrane | Thin and flexible | Controls movement of substances |
| Cytoplasm | Jelly-like fluid | Supports organelles and chemical reactions |
| Mitochondria | Folded inner membrane | Efficient energy release |
| Ribosomes | Tiny particles | Protein synthesis |
Scientists often say:
Structure determines function.
This means that the shape and design of an organelle help it perform its specific role.
Real-World Connection
Doctors and scientists study animal cells to understand health and disease. For example, cancer develops when cells divide uncontrollably, while inherited diseases may result from changes in the DNA stored inside the nucleus. Researchers also investigate mitochondria because problems with energy production can contribute to muscle weakness and certain genetic disorders.
Did You Know?
The human body contains more than 200 different types of specialised cells. Although a nerve cell, a muscle cell, and a skin cell look very different and perform different functions, they all contain the same basic organelles and the same DNA. Their different roles arise because different genes are switched on or off in each cell type.
Key Terms
- Animal cell — the basic structural and functional unit of animals.
- Organelle — a specialised structure inside a cell that performs a specific function.
- Nucleus — the organelle that contains DNA and controls the activities of the cell.
- Cell membrane — the selectively permeable barrier that controls what enters and leaves the cell.
- Cytoplasm — the jelly-like material where many chemical reactions occur.
- Mitochondrion (plural: mitochondria) — the organelle where cellular respiration releases energy.
- Ribosome — the organelle responsible for making proteins.
- Selective permeability — allowing some substances to pass through while blocking others.
Key Takeaways
- Animal cells are eukaryotic cells that contain several specialised organelles.
- The nucleus controls cell activities and stores genetic information.
- The cell membrane regulates the movement of substances into and out of the cell.
- The cytoplasm is where many chemical reactions take place.
- Mitochondria release energy through cellular respiration.
- Ribosomes produce proteins needed for growth, repair, and normal cell function.
- All organelles work together to enable the cell to carry out the processes of life.
- Understanding animal cell structure provides the foundation for studying tissues, organs, body systems, genetics, and human health.