Sciences · Biology
2.1 Cells & Organisation
All living things are made of cells — the basic unit of life. In this topic you will learn how cells are structured, what each part does, and how cells work together to form entire organisms.
Why Cells Matter
Every function your body performs — breathing, digesting food, thinking, growing — starts at the cell level. Understanding cells is the foundation of all biology. Without cells, there is no life.
What You Will Learn
- The three statements of cell theory
- The difference between prokaryotic and eukaryotic cells
- The structures found in plant and animal cells
- How cells organise into tissues, organs, and organ systems
- How to use a microscope to observe cells
Sciences · Biology
Cell Theory
Cell theory is one of the most important ideas in biology. It was developed in the 1800s by scientists including Schleiden, Schwann, and Virchow.
The Three Statements of Cell Theory
- All living things are made of one or more cells.
- The cell is the basic unit of life. All life processes happen inside cells.
- All cells come from pre-existing cells. New cells are made by existing cells dividing.
Two Types of Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | No membrane-bound nucleus | Has a membrane-bound nucleus |
| DNA location | Floats freely in the cytoplasm | Inside the nucleus |
| Organelles | No membrane-bound organelles | Has membrane-bound organelles (mitochondria, ER, etc.) |
| Size | Smaller (1–5 µm) | Larger (10–100 µm) |
| Examples | Bacteria | Plant cells, animal cells, fungi |
Key Distinction: The word “eukaryote” comes from Greek meaning “true nucleus.” If a cell has a membrane around its nucleus, it is eukaryotic. If not, it is prokaryotic (“before nucleus”).
Sciences · Biology
Cell Structure
Cells contain tiny structures called organelles, each with a specific job. Think of them like organs inside a miniature body.
Key Organelles and Their Functions
| Organelle | Function |
|---|---|
| Cell membrane | Controls what enters and exits the cell; acts as a protective barrier |
| Nucleus | Contains DNA (genetic material); controls cell activities and cell division |
| Cytoplasm | Jelly-like substance where chemical reactions take place |
| Mitochondria | Carries out aerobic respiration — converts glucose and oxygen into energy (ATP). The “powerhouse of the cell” |
| Ribosomes | Make proteins by reading instructions from DNA |
| Cell wall | Rigid outer layer made of cellulose; provides support and shape (plant cells only) |
| Chloroplast | Contains chlorophyll; carries out photosynthesis — converts light energy into glucose (plant cells only) |
| Vacuole | Large permanent vacuole in plant cells stores cell sap and keeps the cell firm; animal cells may have small temporary vacuoles |
Memory Tip
Think of the cell as a factory: the nucleus is the manager (gives instructions), ribosomes are the workers (make products), mitochondria are the power plant (supply energy), and the cell membrane is the security gate (controls entry and exit).
Sciences · Biology
Plant vs Animal Cells
Both plant and animal cells are eukaryotic, but they have important differences that you need to know.
Comparison Table
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Yes (cellulose) | No |
| Chloroplasts | Yes | No |
| Large permanent vacuole | Yes | No (small temporary ones) |
| Cell membrane | Yes | Yes |
| Nucleus | Yes | Yes |
| Mitochondria | Yes | Yes |
| Cytoplasm | Yes | Yes |
| Ribosomes | Yes | Yes |
| Shape | Regular, box-like | Irregular, rounded |
Critical Rule: Chloroplasts and cell walls are found in plant cells only, not animal cells. Mitochondria are found in both. This is one of the most common exam mistakes — do not mix them up!
Why the Differences?
- Chloroplasts — Plants make their own food through photosynthesis; animals eat food instead.
- Cell wall — Gives plants rigidity so they can stand upright without a skeleton.
- Large vacuole — Stores water and keeps plant cells turgid (firm). When a plant wilts, its vacuoles have lost water.
Sciences · Biology
Levels of Organisation
In multicellular organisms, cells do not work alone. They are organised into groups that carry out specific functions together.
From Smallest to Largest
Cell → Tissue → Organ → Organ System → Organism
| Level | Definition | Example |
|---|---|---|
| Cell | The basic unit of life | A red blood cell |
| Tissue | A group of similar cells working together | Muscle tissue (many muscle cells) |
| Organ | A group of different tissues working together | The heart (muscle tissue + nerve tissue + blood tissue) |
| Organ System | A group of organs working together for a major function | The circulatory system (heart + blood vessels + blood) |
| Organism | A complete living thing made of all its organ systems | A human being |
Key Organ Systems
- Digestive system — breaks down food into nutrients (mouth, stomach, intestines)
- Respiratory system — takes in oxygen and removes carbon dioxide (lungs, trachea)
- Circulatory system — transports blood around the body (heart, blood vessels)
- Nervous system — sends electrical signals to control the body (brain, spinal cord, nerves)
- Skeletal system — supports the body and protects organs (bones, joints)
Sciences · Biology
Using Microscopes
Microscopes allow us to see cells and structures that are too small for the naked eye. You need to know how to use a light microscope safely and correctly.
Parts of a Light Microscope
| Part | Function |
|---|---|
| Eyepiece lens | The lens you look through (usually 10x magnification) |
| Objective lenses | Lenses near the specimen that magnify at different powers (4x, 10x, 40x) |
| Stage | The platform where you place the slide |
| Light source / Mirror | Shines light through the specimen so you can see it |
| Focus knobs | Coarse focus (big movements) and fine focus (small adjustments) to sharpen the image |
Calculating Magnification
Formula
Total magnification = Eyepiece magnification × Objective magnification
Example: If the eyepiece is 10x and the objective lens is 40x:
Total magnification = 10 × 40 = 400x
This means the specimen appears 400 times larger than its actual size.
Total magnification = 10 × 40 = 400x
This means the specimen appears 400 times larger than its actual size.
Lab Rule: Always start with the lowest power objective lens first, then increase magnification. This helps you find the specimen before zooming in and prevents damage to the lens or slide.
Sciences · Biology
Worked Examples
These examples show the kind of structured answers expected in IB MYP assessments. Notice how each answer uses evidence and scientific vocabulary.
EXAMPLE 1A student looks at an onion cell under a microscope. Identify the organelles they would see and explain the function of each.
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Full Solution
An onion cell is a plant cell, so the student would see:
Cell wall — a rigid outer layer made of cellulose that provides structural support and gives the cell its box-like shape.
Cell membrane — a thin layer inside the cell wall that controls what enters and exits the cell.
Nucleus — contains the cell’s DNA and controls all cell activities.
Cytoplasm — the jelly-like substance where chemical reactions take place.
Large vacuole — filled with cell sap; keeps the cell turgid (firm).
Note: The student would NOT see chloroplasts because onion cells are underground and do not carry out photosynthesis (they are not green).
Cell wall — a rigid outer layer made of cellulose that provides structural support and gives the cell its box-like shape.
Cell membrane — a thin layer inside the cell wall that controls what enters and exits the cell.
Nucleus — contains the cell’s DNA and controls all cell activities.
Cytoplasm — the jelly-like substance where chemical reactions take place.
Large vacuole — filled with cell sap; keeps the cell turgid (firm).
Note: The student would NOT see chloroplasts because onion cells are underground and do not carry out photosynthesis (they are not green).
EXAMPLE 2Using the levels of organisation, explain how your body digests food — from the cell level to the organ system level.
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Full Solution
Cell level: Individual cells in the stomach lining produce digestive enzymes and acid.
Tissue level: These similar cells group together to form epithelial tissue (lining tissue) and glandular tissue (secreting tissue).
Organ level: The stomach is an organ made of muscle tissue (to churn food), glandular tissue (to produce enzymes), and epithelial tissue (to line and protect the stomach).
Organ system level: The stomach works with other organs — the mouth, oesophagus, small intestine, large intestine, and liver — to form the digestive system, which breaks down food into nutrients the body can absorb.
Organism level: The digestive system works with all other systems (circulatory, respiratory, etc.) to keep the whole organism alive.
Tissue level: These similar cells group together to form epithelial tissue (lining tissue) and glandular tissue (secreting tissue).
Organ level: The stomach is an organ made of muscle tissue (to churn food), glandular tissue (to produce enzymes), and epithelial tissue (to line and protect the stomach).
Organ system level: The stomach works with other organs — the mouth, oesophagus, small intestine, large intestine, and liver — to form the digestive system, which breaks down food into nutrients the body can absorb.
Organism level: The digestive system works with all other systems (circulatory, respiratory, etc.) to keep the whole organism alive.
EXAMPLE 3A student claims that bacteria are not alive because they do not have a nucleus. Evaluate this claim.
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Full Solution
The student’s claim is incorrect. Bacteria are alive — they are prokaryotic cells.
While bacteria do not have a membrane-bound nucleus, they still contain DNA which floats freely in their cytoplasm. Cell theory states that all living things are made of cells — and bacteria ARE cells. They carry out all life processes: they grow, reproduce (by binary fission), respond to their environment, and metabolise nutrients.
The absence of a membrane-bound nucleus is what makes them prokaryotic rather than eukaryotic — it does not mean they are not alive.
While bacteria do not have a membrane-bound nucleus, they still contain DNA which floats freely in their cytoplasm. Cell theory states that all living things are made of cells — and bacteria ARE cells. They carry out all life processes: they grow, reproduce (by binary fission), respond to their environment, and metabolise nutrients.
The absence of a membrane-bound nucleus is what makes them prokaryotic rather than eukaryotic — it does not mean they are not alive.
EXAMPLE 4A plant is left in a dark cupboard for two weeks. Predict what would happen to its cells and explain why.
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Full Solution
Prediction: The plant would become pale, wilted, and eventually die.
Explanation: Without light, the chloroplasts in the plant cells cannot carry out photosynthesis (converting light energy into glucose). The plant would run out of glucose for energy and food. The chlorophyll in the chloroplasts would break down, making the leaves lose their green colour.
The cells would also lose water from their vacuoles over time without proper function, causing them to become flaccid (soft) instead of turgid (firm). This would make the plant wilt. Eventually, without energy from photosynthesis, the cells would die.
Explanation: Without light, the chloroplasts in the plant cells cannot carry out photosynthesis (converting light energy into glucose). The plant would run out of glucose for energy and food. The chlorophyll in the chloroplasts would break down, making the leaves lose their green colour.
The cells would also lose water from their vacuoles over time without proper function, causing them to become flaccid (soft) instead of turgid (firm). This would make the plant wilt. Eventually, without energy from photosynthesis, the cells would die.
EXAMPLE 5Calculate the total magnification when using a 15x eyepiece lens with a 40x objective lens. Then explain why you would start on the lowest objective lens.
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Full Solution
Calculation: Total magnification = eyepiece × objective = 15 × 40 = 600x
Explanation: You should always start with the lowest power objective lens (e.g., 4x) because:
1. It provides a wider field of view, making it easier to locate the specimen on the slide.
2. It prevents the lens from crashing into the slide — higher power lenses are longer and sit closer to the specimen.
3. Once you have found and centred the specimen, you can switch to a higher power to see more detail.
Explanation: You should always start with the lowest power objective lens (e.g., 4x) because:
1. It provides a wider field of view, making it easier to locate the specimen on the slide.
2. It prevents the lens from crashing into the slide — higher power lenses are longer and sit closer to the specimen.
3. Once you have found and centred the specimen, you can switch to a higher power to see more detail.
EXAMPLE 6Explain why mitochondria are found in both plant and animal cells, but chloroplasts are only in plant cells.
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Full Solution
Mitochondria are found in both plant and animal cells because all living cells need energy. Mitochondria carry out aerobic respiration (glucose + oxygen → energy + CO₂ + water), which is how cells release the energy stored in glucose. Both plants and animals need this process to power their life functions.
Chloroplasts are found only in plant cells because only plants make their own food through photosynthesis (light + CO₂ + water → glucose + oxygen). Animals do not photosynthesise — they obtain glucose by eating other organisms.
Key insight: Plant cells have BOTH mitochondria and chloroplasts. The chloroplasts make glucose using light, and the mitochondria then break down that glucose to release energy.
Chloroplasts are found only in plant cells because only plants make their own food through photosynthesis (light + CO₂ + water → glucose + oxygen). Animals do not photosynthesise — they obtain glucose by eating other organisms.
Key insight: Plant cells have BOTH mitochondria and chloroplasts. The chloroplasts make glucose using light, and the mitochondria then break down that glucose to release energy.
Sciences · Biology
Practice Q&A
Attempt each question before revealing the model answer.
DESCRIBEDescribe two differences between prokaryotic and eukaryotic cells.
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Model Answer
First, eukaryotic cells have a membrane-bound nucleus that contains their DNA, while prokaryotic cells have no true nucleus — their DNA floats freely in the cytoplasm. Second, eukaryotic cells contain membrane-bound organelles (such as mitochondria and endoplasmic reticulum), while prokaryotic cells do not have these structures.
IDENTIFYIdentify the function of the mitochondria.
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Model Answer
Mitochondria are the site of aerobic cellular respiration. They convert glucose and oxygen into ATP (energy) that the cell can use for its activities. Mitochondria are often called the “powerhouse of the cell.”
EXPLAINExplain why plant cells have chloroplasts but animal cells do not.
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Model Answer
Plant cells contain chloroplasts because plants make their own food through photosynthesis — the process of converting light energy, carbon dioxide, and water into glucose. Animal cells do not need chloroplasts because animals obtain their energy by eating food (other organisms) rather than making it from sunlight.
EXPLAINExplain the relationship between cells, tissues, and organs using one example.
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Model Answer
Individual muscle cells group together to form muscle tissue, which can contract. Muscle tissue then combines with other tissues (such as nerve tissue and connective tissue) to form an organ like the heart. So cells are the building blocks of tissues, and tissues are the building blocks of organs.
CALCULATEA microscope has a 10x eyepiece and a 40x objective lens. What is the total magnification?
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Model Answer
Total magnification = eyepiece × objective = 10 × 40 = 400x. The specimen appears 400 times larger than its actual size.
DESCRIBEDescribe the function of the cell wall in a plant cell.
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Model Answer
The cell wall is a rigid outer layer made of cellulose that surrounds the cell membrane in plant cells. Its function is to provide structural support and maintain the cell’s shape, giving the plant its rigidity. Without cell walls, plants would not be able to stand upright.
IDENTIFYIdentify which parts of a microscope you would adjust to get a clearer image.
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Model Answer
To get a clearer image, you would adjust the focus knobs. Use the coarse focus first for large adjustments to bring the specimen roughly into view, then use the fine focus for small, precise adjustments to sharpen the image. You might also adjust the light source to ensure the specimen is well illuminated.
EXPLAINExplain what happens to a plant cell when it loses water.
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Model Answer
When a plant cell loses water, the vacuole shrinks as it loses cell sap. The cell membrane pulls away from the cell wall — this is called plasmolysis. The cell becomes flaccid (soft and limp) instead of turgid (firm). If many cells lose water, the whole plant wilts and droops because the cells no longer have enough internal pressure to hold the plant upright.
Biology · Review
Flashcard Review
Tap each card to reveal the answer. Try to answer from memory first.
What are the three statements of cell theory?
(1) All living things are made of cells; (2) The cell is the basic unit of life; (3) All cells come from pre-existing cells.
Tap to reveal
What is the difference between a prokaryotic and a eukaryotic cell?
Prokaryotic cells have NO membrane-bound nucleus (e.g., bacteria). Eukaryotic cells HAVE a membrane-bound nucleus (e.g., plant and animal cells).
Tap to reveal
What is the function of the nucleus?
Contains DNA (genetic material) and controls cell activities, including growth and reproduction.
Tap to reveal
What is the function of mitochondria?
Site of aerobic respiration — converts glucose and oxygen into ATP (energy). The “powerhouse of the cell.”
Tap to reveal
Name two organelles found in plant cells but NOT animal cells.
Cell wall (made of cellulose) and chloroplasts (for photosynthesis).
Tap to reveal
What is the function of the cell membrane?
Controls what enters and exits the cell. It is a selectively permeable barrier found in ALL cells.
Tap to reveal
What are the five levels of organisation in order?
Cell → Tissue → Organ → Organ System → Organism
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What is a tissue?
A group of similar cells working together to perform the same function (e.g., muscle tissue).
Tap to reveal
What is the function of chloroplasts?
Carry out photosynthesis — convert light energy, CO&sub2;, and water into glucose. Found only in plant cells.
Tap to reveal
How do you calculate total magnification?
Total magnification = eyepiece magnification × objective lens magnification.
Tap to reveal
What is the function of the cell wall?
Provides rigid support and shape to plant cells. Made of cellulose. Not found in animal cells.
Tap to reveal
Why should you start with the lowest power objective lens?
To find the specimen easily in a wider field of view before zooming in, and to prevent damage to the lens or slide.
Tap to reveal
What is the function of ribosomes?
Make proteins by reading instructions from DNA. Found in both prokaryotic and eukaryotic cells.
Tap to reveal
What is the function of the large vacuole in plant cells?
Stores cell sap (water and dissolved substances) and keeps the cell turgid (firm). When it loses water, the plant wilts.
Tap to reveal
Name three organ systems in the human body.
Digestive system, respiratory system, circulatory system (also: nervous system, skeletal system, muscular system).
Tap to reveal
Biology · Practice Test
Practice Test
20 questions covering cell theory, cell structure, plant vs animal cells, organisation, and microscopes.
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