The cell as a living factory
A cell is like a busy factory. It takes in raw materials, makes products, uses energy, ships things out, and follows instructions stored in a central office. Each part of the factory is an organelle with a specific job.
Two basic cell types
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No; DNA floats in the cytoplasm | Yes; DNA is enclosed in a nucleus |
| Membrane-bound organelles | No | Yes |
| Size | Small (1–10 µm) | Larger (10–100 µm) |
| Examples | Bacteria, archaea | Plants, animals, fungi, protists |
| Ribosomes | Yes (smaller) | Yes (larger) |
Prokaryotes appeared on Earth first, about 3.5 billion years ago. Eukaryotes appeared about 2 billion years ago.
Organelles and their functions
| Organelle | Function | Factory analogy |
|---|---|---|
| Nucleus | Holds DNA; controls cell activities | Head office |
| Cell membrane | Controls what enters and leaves | Security gate |
| Cytoplasm | Gel-like fluid where reactions happen | Factory floor |
| Ribosomes | Build proteins | Assembly workers |
| Rough endoplasmic reticulum | Studded with ribosomes; makes and transports proteins | Assembly line |
| Smooth endoplasmic reticulum | Makes lipids; detoxifies | Specialty workshop |
| Golgi apparatus | Modifies, packages and ships proteins | Shipping department |
| Mitochondria | Release energy through cellular respiration | Power plant |
| Lysosomes | Digest wastes and worn-out parts | Recycling crew |
| Vacuole | Stores water, food and wastes | Warehouse |
| Cytoskeleton | Gives shape; helps movement | Steel frame |
Plant cells vs. animal cells
| Structure | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Yes (cellulose) | No |
| Chloroplast | Yes | No |
| Large central vacuole | Yes | No (small vacuoles only) |
| Shape | Usually rectangular | Usually rounded or irregular |
| Centrioles | Usually absent | Present |
The cell wall gives plants rigid support. The central vacuole stores water and pushes against the wall, keeping the plant firm. When a plant wilts, its vacuoles have lost water.
The cell membrane
The cell membrane is made of a phospholipid bilayer: two layers of molecules with water-loving heads facing outward and water-fearing tails facing inward. Proteins float in it like boats, which is why it’s called the fluid mosaic model.
The membrane is selectively permeable: it lets some substances through and blocks others.
Moving materials across membranes
| Process | Energy needed? | Direction | Example |
|---|---|---|---|
| Diffusion | No (passive) | High to low concentration | Oxygen entering cells |
| Osmosis | No (passive) | Diffusion of water across a membrane | Water entering root cells |
| Facilitated diffusion | No (passive) | High to low, through protein channels | Glucose entering cells |
| Active transport | Yes (ATP) | Low to high concentration | Sodium–potassium pump |
| Endocytosis / exocytosis | Yes | Large particles in or out in vesicles | White blood cells engulfing bacteria |
Osmosis and tonicity
- Hypotonic solution (less solute outside): water enters; animal cells can burst, plant cells become firm.
- Isotonic solution (equal solute): no net movement.
- Hypertonic solution (more solute outside): water leaves; cells shrink.
This is why you shouldn’t drink seawater: it’s hypertonic and pulls water out of your cells.
Why are cells small?
As a cell grows, its volume increases faster than its surface area. A large cell would not have enough membrane to move in nutrients and move out wastes fast enough. That’s why large organisms are made of many small cells rather than a few giant ones.
Common mistakes
- Thinking plant cells don’t have mitochondria. They have both chloroplasts and mitochondria.
- Confusing the cell wall and the cell membrane. Plant cells have both; the membrane is inside the wall.
- Thinking diffusion needs energy. It is passive.
Try it yourself
- Place a peeled egg (shell dissolved in vinegar) in corn syrup overnight, then in water overnight. Describe and explain the changes.
- Draw and label a plant cell and an animal cell, highlighting three differences.
- Explain which organelle would be most common in a muscle cell, and why.
Quick recap
- Prokaryotic cells lack a nucleus; eukaryotic cells have one.
- Organelles each do a specific job, like departments in a factory.
- Plant cells have cell walls, chloroplasts and a large central vacuole.
- The cell membrane is selectively permeable.
- Materials move by diffusion, osmosis and active transport.