Cells & Microscopy
Explore the building blocks of life — prokaryotic and eukaryotic cells, key organelles and their functions, and how microscopes allow us to see the incredibly small world inside living things.
The Cell — The Basic Unit of Life
All living organisms are made of cells. The cell theory states that: (1) all living things are composed of one or more cells; (2) the cell is the basic structural and functional unit of life; and (3) all cells come from pre-existing cells.
There are two fundamental types of cells: prokaryotic (no true nucleus) and eukaryotic (with a true nucleus enclosed by a membrane).
Prokaryotic Cell (e.g. Bacteria)
- No membrane-bound nucleus
- DNA is circular, floating in cytoplasm
- No membrane-bound organelles
- Has cell wall (usually peptidoglycan)
- Much smaller than eukaryotic cells (~1–10 µm)
- Examples: bacteria, archaea
Eukaryotic Cell (e.g. Animal)
- True nucleus with nuclear membrane
- Linear DNA within nucleus
- Many membrane-bound organelles
- Much larger (~10–100 µm)
- Examples: plant cells, animal cells, fungi, protists
Key Organelles and Their Functions
Organelles are specialised structures within eukaryotic cells, each with a specific job — like tiny organs within the cell.
| Organelle | Found in | Function |
|---|---|---|
| Nucleus | Animal & plant | Controls cell activities; contains DNA (genetic instructions) |
| Cell membrane | All cells | Controls what enters and leaves the cell (selectively permeable) |
| Cytoplasm | All cells | Jelly-like fluid where chemical reactions occur; holds organelles in place |
| Mitochondria | Animal & plant | Site of cellular respiration — converts glucose to ATP (energy). "Powerhouse of the cell." |
| Ribosomes | All cells | Site of protein synthesis — assembles amino acids into proteins using mRNA instructions |
| Endoplasmic Reticulum (ER) | Eukaryotes | Rough ER: transports proteins. Smooth ER: synthesises lipids and detoxifies substances |
| Golgi apparatus | Eukaryotes | Sorts, packages, and sends proteins to where they are needed (like a post office) |
| Cell wall | Plant cells, fungi, bacteria | Provides structural support and shape. Made of cellulose in plants. |
| Chloroplasts | Plant cells (and algae) | Site of photosynthesis — converts light energy, CO₂, and water into glucose and oxygen |
| Large vacuole | Plant cells | Stores water, maintains cell turgidity (firmness), stores nutrients and waste |
Plant Cell Only
Cell wall, chloroplasts, large central vacuole
Animal Cell Only
Centrioles (for cell division), no cell wall, no chloroplasts
Both (Eukaryotic)
Nucleus, mitochondria, ER, Golgi, ribosomes, cell membrane
Using a Microscope
A light microscope uses visible light and glass lenses to magnify specimens. It can magnify up to about ×1000 with good resolution. An electron microscope uses beams of electrons and can magnify over ×100,000, revealing organelles like ribosomes.
Parts of a Light Microscope
- Eyepiece lens (ocular): The lens you look through. Typically ×10 magnification.
- Objective lenses: Sit above the stage. Usually ×4, ×10, and ×40. Always start on lowest power.
- Stage: Flat platform where you place the microscope slide.
- Coarse focus: Large knob for initial rough focusing.
- Fine focus: Small knob for precise, sharp focusing.
- Light source / mirror: Illuminates the specimen from below.
- Diaphragm (iris): Controls the amount of light entering.
Calculating Magnification
e.g. ×10 eyepiece × ×40 objective = ×400 total
M = I ÷ A
When making a scientific drawing of a cell, you must:
- Use a sharp pencil and ruler for label lines
- Label all visible structures
- Include a scale bar or magnification
- No shading — use stippling or line hatching
Key safety reminder: Always carry the microscope with two hands (one on the arm, one under the base). When switching to higher magnification, never use the coarse focus — use only the fine focus to avoid cracking the slide or scratching the lens.
Key Vocabulary
Prokaryote
An organism whose cells lack a membrane-bound nucleus. DNA floats freely in the cytoplasm. Bacteria are prokaryotes. They were the first life forms on Earth (~3.8 billion years ago).
Eukaryote
An organism whose cells have a membrane-bound nucleus and other membrane-bound organelles. Includes animals, plants, fungi, and protists.
Organelle
A specialised structure within a cell that carries out a specific function. Examples include the nucleus, mitochondria, ribosomes, and chloroplasts.
Magnification
How many times larger an image is compared to the actual object. Calculated as: magnification = image size ÷ actual size. Total microscope magnification = eyepiece × objective.
Worked Examples
A cell is viewed with a ×10 eyepiece and a ×40 objective. The image of the cell is 2 cm wide. What is the actual size of the cell?
Step 1: Total magnification = 10 × 40 = ×400
Step 2: Rearrange M = I ÷ A, so A = I ÷ M
Step 3: Convert image size: 2 cm = 20,000 µm
Answer: A = 20,000 ÷ 400 = 50 µm. Typical for a eukaryotic cell.
State three differences between a plant cell and an animal cell.
1. Cell wall: Plant cells have a rigid cell wall made of cellulose; animal cells do not.
2. Chloroplasts: Plant cells contain chloroplasts for photosynthesis; animal cells do not.
3. Vacuole: Plant cells have a large, permanent central vacuole for water storage; animal cells have only small, temporary vacuoles (if any).
Why do bacteria not have mitochondria, yet they are still alive?
Bacteria are prokaryotes and lack membrane-bound organelles, including mitochondria.
However, bacteria still carry out cellular respiration — the enzymes needed are located in the cell membrane itself rather than in a separate organelle.
Mitochondria are only needed in eukaryotic cells as a dedicated structure for this process. Bacteria use a simpler, more direct approach to generate energy.
Knowledge Check
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Key Concepts Summary
- ✓ All living things are made of cells (cell theory). Cells are the basic unit of life.
- ✓ Prokaryotes (bacteria) have no membrane-bound nucleus; eukaryotes (animals, plants) do.
- ✓ Key organelles: nucleus (control), mitochondria (energy), ribosomes (proteins), chloroplasts (photosynthesis).
- ✓ Plant cells have a cell wall, chloroplasts, and a large vacuole; animal cells do not.
- ✓ Total microscope magnification = eyepiece magnification × objective magnification.
- ✓ Actual size = image size ÷ magnification. Always start on the lowest power objective lens.