Cells and Body Systems: Year 8 Study Guide
All living things are made of cells, and in complex organisms cells form tissues, organs and systems.
The cell is the basic unit of life. All cells have a cell membrane that controls what enters and leaves, cytoplasm where chemical reactions happen, a nucleus that holds DNA and directs the cell, and mitochondria that release energy. Plant cells also have a cell wall, chloroplasts for photosynthesis and a large vacuole.
Cells with similar jobs form tissues, tissues form organs, and organs working together form body systems: for example the digestive, respiratory, circulatory, nervous and musculoskeletal systems. A microscope's total magnification = eyepiece × objective.
Organelles
| Part | Function | Plant | Animal |
|---|---|---|---|
| Cell membrane | controls entry and exit | ✓ | ✓ |
| Nucleus | holds DNA | ✓ | ✓ |
| Mitochondria | release energy | ✓ | ✓ |
| Cell wall | support | ✓ | ✗ |
| Chloroplast | photosynthesis | ✓ | ✗ |
| Vacuole | storage | large | small |
Body systems working together
The respiratory system takes in oxygen and removes carbon dioxide. The circulatory system (heart, blood vessels, blood) carries oxygen and nutrients to cells. The digestive system breaks food down so nutrients can be absorbed. The nervous system sends signals. During exercise, the heart beats faster and breathing speeds up to deliver more oxygen to muscles. Systems cooperate to keep the body in balance.
Oxygen enters the lungs.
Pumped by the heart.
To release energy from food.
And is breathed out.
Diffusion and osmosis
Substances move into and out of cells by diffusion, from a region of higher concentration to lower concentration. Osmosis is the diffusion of water across a membrane. A potato slice placed in salty water loses water and becomes limp; in pure water it takes up water and becomes firm. This explains why plants wilt and why salty food makes you thirsty.
Worked example and common misconceptions
Example: Muscle cells contain many mitochondria because they need a lot of energy. Common misconceptions: thinking plant cells have no mitochondria (they do), that organs work alone, and that cells are the same shape everywhere. This guide is general science, not medical advice; see a doctor for personal health questions.
Check your understanding: model answers
1. Cell wall and chloroplasts. 2. It carries oxygen and nutrients around the body. 3. The diffusion of water across a membrane.
✨ Key Takeaways
- Cells are the basic unit of life.
- Plant cells have a wall and chloroplasts.
- Organs form systems.
- Diffusion moves substances down a gradient.
💬 Questions for Discussion & Reflection
- Name two structures only in plant cells.
- What does the circulatory system do?
- What is osmosis?
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