Introduction to Cells
All living organisms are made of cells. Some organisms consist of one cell, while others are made of many specialised cells. Learn the main ideas of cell theory, explore the structures found in plant and animal cells, and discover how cells are adapted for different jobs.
Learning time: approximately 30 minutes · Beginner level
What You Will Learn
- Understand what a cell is and describe cell theory.
- Identify the main structures in animal and plant cells.
- Explain the basic jobs of common organelles.
- Compare plant and animal cells.
- Describe examples of specialised cells.
1. What Is a Cell?
A cell is the basic structural and functional unit of life. It is the smallest unit that can carry out the processes needed for life.
Some living things, such as many bacteria, are single-celled. Plants, animals and many other organisms are multicellular, meaning they are made of many cells.
2. The Basic Ideas of Cell Theory
Cell theory is a central idea in biology. Its three main statements are:
- 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 come from pre-existing cells.
3. Common Cell Structures
| Structure | Main function |
|---|---|
| Cell membrane | Controls what enters and leaves the cell. |
| Cytoplasm | Jelly-like material where many cell reactions take place. |
| Nucleus | Contains genetic material and helps control cell activities in eukaryotic cells. |
| Mitochondria | Site of most aerobic respiration; releases energy usable by the cell. |
| Ribosomes | Build proteins. |
| Cell wall | Provides support and protection in plant cells and some other organisms. |
| Chloroplasts | Contain chlorophyll and are the site of photosynthesis in plants and algae. |
| Large permanent vacuole | Stores cell sap and helps maintain pressure in many plant cells. |
4. Animal Cells
An animal cell is a eukaryotic cell. It has a cell membrane, cytoplasm, a nucleus, mitochondria and ribosomes. Animal cells do not have a cellulose cell wall or chloroplasts.
Cell membrane · Cytoplasm · Nucleus · Mitochondria · Ribosomes
Different animal cell types can have different shapes and numbers of organelles.
5. Plant Cells
Plant cells have many structures found in animal cells, but typical plant cells also have a cellulose cell wall, chloroplasts in photosynthetic tissues and a large permanent vacuole.
Cell wall · Cell membrane · Cytoplasm · Nucleus · Chloroplasts · Large permanent vacuole · Mitochondria · Ribosomes
Not every plant cell contains chloroplasts. For example, many root cells are not exposed to light and do not photosynthesise.
6. Plant and Animal Cells: Similarities and Differences
| Feature | Typical animal cell | Typical plant cell |
|---|---|---|
| Cell membrane | Yes | Yes |
| Cytoplasm | Yes | Yes |
| Nucleus | Usually present | Usually present |
| Mitochondria | Yes | Yes |
| Cellulose cell wall | No | Yes |
| Chloroplasts | No | In photosynthetic cells |
| Large permanent vacuole | Not typical | Usually present in mature plant cells |
7. Unicellular and Multicellular Organisms
A unicellular organism is made of one cell that carries out all the life processes it needs. A multicellular organism is made of many cells that often specialise in different tasks.
- Examples of unicellular organisms include many bacteria and some protists, such as Amoeba.
- Humans, trees and most animals are multicellular.
8. Specialised Cells
In multicellular organisms, some cells have structures that help them perform particular jobs.
| Cell type | Adaptation | Function |
|---|---|---|
| Red blood cell | Contains haemoglobin and has a shape that helps exchange gases | Transports oxygen around the body |
| Nerve cell (neurone) | Has a long fibre and branched connections | Carries electrical signals |
| Root hair cell | Has a long extension that increases surface area | Absorbs water and mineral ions from soil |
| Muscle cell | Contains contractile proteins | Contracts to produce movement |
| Sperm cell | Has a tail and many mitochondria in its midpiece | Moves towards and fertilises an egg cell |
9. Looking at Cells with a Microscope
Microscopes make small structures appear larger so they can be observed. A simple school light microscope uses lenses and light.
- Place a prepared slide on the stage and secure it.
- Start with the lowest-power objective lens.
- Use the focus controls carefully to make the image clear.
- Adjust the light so the specimen can be seen.
- Move to a higher magnification only when needed, and refocus carefully.
10. Common Mistakes to Avoid
11. Practice Exercises
Try these before revealing the answers.
- What is the basic structural and functional unit of life?
- Name two structures found in both typical plant and animal cells.
- What is the main function of the cell membrane?
- Which structure contains chlorophyll and carries out photosynthesis?
- Why do root hair cells have a long extension?
- What is one difference between a unicellular and a multicellular organism?
- Which specialised cell carries electrical signals?
Show practice answers and explanations
- The cell.
- Examples include cell membrane, cytoplasm, nucleus, mitochondria and ribosomes.
- It controls what enters and leaves the cell.
- The chloroplast.
- It increases surface area for absorbing water and mineral ions.
- A unicellular organism has one cell; a multicellular organism has many cells.
- A nerve cell, or neurone.
12. Test Your Understanding
Choose one answer for each question, then select Check Answers. Each question is worth one point.
13. Free Printable Worksheet
Print this worksheet or save it as a PDF for extra practice.
Learnora — Grade 7 Science
Introduction to Cells Worksheet
Name: __________________________ Date: _______________
Instructions: Answer in complete sentences where needed.
- State one part of cell theory.
- Name two structures found in both plant and animal cells.
- What does the nucleus contain?
- What is the function of a mitochondrion?
- Why do many plant cells have a large permanent vacuole?
- How is a root hair cell adapted to its function?
- Which specialised cell carries signals around the body?
Keep practising — you are making progress!
Learnora — Learn Better. Practice Smarter.
Teacher / Parent Answer Key
- Any one: all living things are made of cells; cells are the basic unit of life; cells come from pre-existing cells.
- Examples: cell membrane, cytoplasm, nucleus, mitochondria or ribosomes.
- Genetic material (DNA).
- It is the site of most aerobic respiration and releases usable energy for cell activities.
- It stores cell sap and helps maintain pressure in the cell.
- Its long extension increases surface area for absorbing water and mineral ions.
- A nerve cell (neurone).
14. Lesson Summary
- Cells are the basic units of life.
- Cell theory explains that living organisms are made of cells and new cells come from existing cells.
- Plant and animal cells share many structures, including a cell membrane, cytoplasm and mitochondria.
- Typical plant cells also have a cellulose cell wall and a large permanent vacuole; photosynthetic cells have chloroplasts.
- Specialised cells have adaptations that help them perform particular functions.
Educational note: This is a curriculum-neutral Grade 7 science lesson. Teachers and parents can adapt examples and terminology to their local curriculum.