Cells
What Is a Cell?
A cell is the basic building block of all living things. Every organism — from a tiny bacterium to a blue whale — is made of cells. Cells carry out all the processes needed to keep an organism alive, including respiration, growth, and reproduction.
The cell theory states:
- All living organisms are made of one or more cells
- The cell is the basic unit of life
- All cells come from pre-existing cells
There are two main types of cells:
- Prokaryotic cells — simple cells with no true membrane-bound nucleus (bacteria are prokaryotes)
- Eukaryotic cells — complex cells with a nucleus enclosed in a membrane (plants, animals, and fungi are eukaryotes)
Cell Organelles
Organelles are structures inside the cell, each with a specific function. Think of them as the cell's "organs."
| Organelle | Job |
|---|---|
| Nucleus | Controls cell activity; contains DNA (genetic instructions) |
| Cell membrane | Controls what enters and leaves the cell (selectively permeable) |
| Cytoplasm | Jelly-like fluid filling the cell; where many chemical reactions occur |
| Mitochondria | Produces energy (ATP) through aerobic respiration — the "powerhouse" |
| Ribosome | Makes proteins by reading instructions from the nucleus |
| Vacuole | Stores water, waste, or food (small and temporary in animal cells) |
Plant vs Animal Cells
Both plant and animal cells are eukaryotic and share many structures, but plant cells have three additional structures:
| Structure | Animal cell | Plant cell |
|---|---|---|
| Cell wall | No | Yes — made of cellulose; gives rigidity and support |
| Chloroplast | No | Yes — contains chlorophyll; carries out photosynthesis |
| Large permanent vacuole | No | Yes — filled with cell sap; maintains turgor pressure |
| Nucleus | Yes | Yes |
| Mitochondria | Yes | Yes |
| Cell membrane | Yes | Yes |
| Cytoplasm | Yes | Yes |
| Ribosomes | Yes | Yes |
The cell wall is not the same as the cell membrane — the cell wall is rigid and made of cellulose, providing structural support. The cell membrane lies just inside the cell wall in plant cells.
Specialised Cells
Not all cells look the same. Cells can be specialised — adapted to carry out a specific function. Their shape and internal structure reflect their job:
| Cell type | Adaptation | Function |
|---|---|---|
| Red blood cell | Biconcave disc shape, no nucleus | Carries oxygen efficiently |
| Sperm cell | Long tail (flagellum), many mitochondria | Swims to fertilise egg |
| Root hair cell | Long thin extension | Increases surface area to absorb water and minerals |
| Nerve cell | Long axon | Transmits electrical signals quickly |
| Muscle cell | Contains protein fibres | Contracts to produce movement |
Levels of Organisation
In multicellular organisms, cells do not work alone. They are organised into a hierarchy:
Cells → Tissues → Organs → Organ systems → Organism
- Tissue — a group of similar cells working together (e.g., muscle tissue, nervous tissue)
- Organ — a group of different tissues working together (e.g., the heart contains muscle tissue, nervous tissue, and connective tissue)
- Organ system — a group of organs working together (e.g., the circulatory system: heart, blood vessels, blood)
- Organism — a complete living thing made of organ systems working together
Cell Division
Cells reproduce by dividing. There are two main types of cell division:
Mitosis — produces two identical daughter cells with the same number of chromosomes as the parent cell. Used for growth, repair, and asexual reproduction.
Meiosis — produces four genetically different cells, each with half the number of chromosomes. Used to produce gametes (sex cells — sperm and eggs).
Before any cell division, the DNA is replicated so each new cell receives a full copy of the genetic instructions.
Diffusion and the Cell Membrane
The cell membrane is selectively permeable — it allows some substances through but not others. Substances move in and out of cells by:
- Diffusion — movement of particles from high to low concentration (passive, no energy needed). Example: oxygen diffuses into cells for respiration.
- Osmosis — diffusion of water across a selectively permeable membrane from high water concentration to low water concentration.
- Active transport — movement of substances against the concentration gradient using energy (ATP). Example: root hair cells absorb mineral ions from the soil.
Key Terms
| Term | Definition |
|---|---|
| Cell | Basic structural and functional unit of all living organisms |
| Organelle | Specialised structure within a cell that performs a specific function |
| Prokaryote | Organism whose cells lack a membrane-bound nucleus |
| Eukaryote | Organism whose cells have a membrane-bound nucleus |
| Mitochondria | Organelle where aerobic respiration occurs; produces ATP |
| Chloroplast | Organelle in plant cells where photosynthesis occurs |
| Cellulose | Structural carbohydrate making up the plant cell wall |
| Selective permeability | Property of the cell membrane allowing only certain substances through |
Worked Example
Q: A student looks at a cell under a microscope and sees a nucleus, mitochondria, ribosomes, a cell wall, and chloroplasts. Is this a plant or animal cell? Give two reasons.
Answer: This is a plant cell.
Reason 1: It has a cell wall — animal cells do not have a cell wall; only plant cells (and some other organisms) do.
Reason 2: It has chloroplasts — these are only found in plant cells (and algae) as they are needed for photosynthesis, which animals do not perform.
Common Mistakes
- Confusing the cell wall and cell membrane — the cell wall is rigid and external (plants only); the cell membrane is flexible and found in all cells.
- Saying chloroplasts are found in all plant cells — they are only found in green parts of the plant (not in root cells, which have no light).
- Thinking the nucleus stores energy — energy is produced by mitochondria, not the nucleus. The nucleus stores DNA.
- Mixing up prokaryotic and eukaryotic cells — bacteria are prokaryotes; plant and animal cells are eukaryotes.
Tips and Tricks
- Mnemonic for plant cell extras: Chloroplast, Cell wall, Vacuole (large permanent) — the three things plant cells have that animal cells do not.
- Remember: mitochondria = powerhouse (ATP/energy); ribosomes = protein factories; nucleus = control centre.
- If a question asks about a cell with no nucleus, think red blood cells (specialised animal cell with no nucleus) OR prokaryotic cells (bacteria).
- The number of mitochondria in a cell reflects how much energy it needs — muscle cells and sperm cells have many mitochondria.