Key Idea: Every living thing is built from cells, and the cell is the smallest unit that is alive — so each cell must carry out all the functions of life for itself. Cells come in just two basic kinds — prokaryotic (simple, no nucleus) and eukaryotic (has a nucleus and membrane-bound organelles) — but all cells share four structures: DNA, cytoplasm, a plasma membrane and ribosomes. This topic is one of the most-tested in the course. It appears on Paper 1A (quick identify/structure MCQs) and heavily on Paper 2 (comparing cell types, and reading, deducing or drawing cells from micrographs).
🔬 Cell theory & the functions of life
Cell theory says three things: all living organisms are made of cells; the cell is the basic unit of life; and cells come only from pre-existing cells (which disproved spontaneous generation — see Pasteur's swan-neck flask). Because a cell is the smallest living thing, every cell must carry out all seven functions of life for itself — this matters most for a single-celled organism, which has no other cells to help it.
| Function (MRS GREN) | What it means |
|---|---|
| Metabolism | the chemical reactions inside the cell (e.g. respiration) |
| Response | reacting to changes in the surroundings |
| Reproduction | producing offspring / new cells |
| Growth | getting bigger / making more living material |
| Excretion | removing toxic metabolic waste (e.g. CO₂) |
| Nutrition | obtaining food / nutrients for energy and raw materials |
| Homeostasis | keeping internal conditions stable |
🔭 Microscopy & imaging cells
Most cells are far too small to see by eye, so we use microscopes. Judge them by magnification (how much bigger) and, more importantly, resolution (how much fine detail). Magnifying a blurry image just makes it a bigger blur — only better resolution shows more. Newer imaging methods (such as cryogenic / freeze-fracture and fluorescence techniques) let scientists snapshot membranes and tag specific molecules, advancing our picture of cell structure.
| Microscope | Uses | Resolution / detail | Living cells? |
|---|---|---|---|
| Light microscope | a beam of light | lower — shows whole cells, large structures | yes |
| Electron microscope | a beam of electrons | much higher — reveals organelles & ultrastructure | no (sample is dead) |
Magnification = how big the image is. Resolution = how much detail you can see. The electron microscope mattered because of its much higher resolution, not just bigger images.
🧬 Structures common to all cells
However different cells look, every single living cell contains the same four structures. A handy hook is D-C-M-R: DNA, Cytoplasm, plasma Membrane, Ribosomes. An organelle is a structure inside a cell with its own specialised job. Be precise: a nucleus is NOT common to all cells — prokaryotes have none.
| Universal structure | Its job |
|---|---|
| DNA | the genetic material — stores the instructions for the cell |
| Cytoplasm | the watery interior where the cell's reactions happen |
| Plasma membrane | controls what enters and leaves the cell |
| Ribosomes | the site of protein synthesis (building proteins) |
Only DNA, cytoplasm, plasma membrane and ribosomes are in every cell. A nucleus, mitochondria and a cell wall are not universal — so they are wrong answers to this question.
🦠 Prokaryotic vs eukaryotic cells
Every cell is one of two basic types. Prokaryotes (all bacteria) are small and simple: no nucleus and no membrane-bound organelles — their naked, circular DNA lies free in a region called the nucleoid. Eukaryotes (animals, plants, fungi) are larger and compartmentalised, with a true nucleus and membrane-bound organelles, and their DNA is wound around histone proteins into chromosomes. Both types still share the four universal structures — plasma membrane, cytoplasm, DNA and ribosomes.
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | absent — DNA free in the nucleoid | present — DNA enclosed in a nucleus |
| DNA organisation | one circular loop, naked (no histones) | linear chromosomes wound on histones |
| Membrane-bound organelles | none | present (e.g. mitochondria) |
| Cell size | small (≈ 1–5 μm) | larger (≈ 10–100 μm) |
| Extras | may have flagella; ribosomes (smaller) | ribosomes (larger); various organelles |
Histones are proteins that DNA wraps around, packaging long eukaryotic DNA neatly into chromosomes. Prokaryotic DNA is naked — no histones.
🌱 Animal, plant & fungal cells; atypical cells
Animal, plant and fungal cells are all eukaryotic — don't confuse this with the prokaryote/eukaryote split. You tell the three eukaryote types apart by a few key structures. Most cells have one nucleus, but the IB calls a few atypical: anucleate cells (no nucleus, e.g. mature red blood cells) and multinucleate cells (several nuclei, e.g. skeletal muscle fibres).
| Structure | Animal | Plant | Fungal |
|---|---|---|---|
| Cell wall | none | yes — cellulose | yes — chitin |
| Chloroplasts | no | yes | no |
| Large permanent vacuole | no | yes | small/variable |
| Overall shape | irregular (flexible membrane) | regular, fixed (rigid wall) | regular (rigid wall) |
Plant wall = cellulose; fungal wall = chitin; animal cells have no wall (so they look irregular). Chloroplasts are found in plant cells only.
🖼️ Identifying & drawing cells from micrographs
A micrograph is a photo taken down a microscope. Don't recall a picture — read the evidence. Ask, in order: (1) Is there a nucleus? A nucleus → eukaryotic; no nucleus with naked DNA → prokaryotic. (2) If eukaryotic, which kind? Use the clues below. For drawings, work from what the electron micrograph actually shows, and label clearly with straight lines that touch each structure.
| Clue in the micrograph | What it tells you |
|---|---|
| No nucleus, naked DNA, no organelles | prokaryotic cell |
| A clear nucleus | eukaryotic cell |
| Cell wall + chloroplasts (+ big vacuole) | plant cell |
| No wall, irregular outline | animal cell |
| Cell wall but no chloroplasts | fungal cell |
When asked to draw a eukaryotic nucleus from an electron micrograph, show and label: the double membrane (nuclear envelope), nuclear pores, chromatin (DNA) and the nucleolus.
✍️ Worked examples
IB-style question — why a single cell must excrete
A unicellular alga lives in pond water and carries out all seven functions of life. Explain why this single cell must also carry out excretion. [2]
Model answer:
Start from metabolism. The cell's chemical reactions (metabolism), such as respiration, produce waste products — for example carbon dioxide (CO₂).
Say why it matters. If this waste is not removed it builds up and becomes toxic, which would harm the cell — and no other cell can remove it for a single-celled organism.
Answer the command term (Explain). So the cell must carry out excretion (removing metabolic waste) to stay alive.
Metabolism (e.g. respiration) makes waste such as CO₂; if not removed it builds up and becomes toxic, so a single cell must excrete to survive.
IB-style question — compare a bacterium with a liver cell
A scientist examines a bacterium and a human liver cell. Give one similarity and two differences in their structure. [3]
Model answer:
Similarity (1 mark). Both cells have a plasma membrane, cytoplasm, DNA and ribosomes — name any one shared structure.
Difference 1 (1 mark). The liver cell has a nucleus enclosing its DNA; the bacterium has no nucleus (its DNA is a naked loop free in the cytoplasm).
Difference 2 (1 mark). The liver cell has membrane-bound organelles (e.g. mitochondria); the bacterium has none.
Similarity: both have a plasma membrane, cytoplasm, DNA and ribosomes. Differences: the liver cell has a nucleus and membrane-bound organelles; the bacterium has neither.
IB-style question — deduce the cell type from a micrograph
An electron micrograph shows a small cell about 3 μm across, with its DNA lying free in the cytoplasm and no nucleus or mitochondria visible. Deduce whether this is a prokaryotic or eukaryotic cell, giving a reason. [2]
Model answer:
Pick out the evidence. There is no nucleus and no membrane-bound organelles, and the DNA is free in the cytoplasm — all prokaryotic features.
Answer the command term (Deduce). State the type and link it to the evidence: this is a prokaryotic cell because it has no nucleus (its DNA lies free in the cytoplasm).
Prokaryotic — because there is no nucleus (the DNA lies free in the cytoplasm) and no membrane-bound organelles.
✅ Quick self-check
Tap each card to check yourself.
What are the three points of cell theory? All organisms are made of cells; the cell is the basic unit of life; and cells come only from pre-existing cells.
Why must a single cell carry out all seven functions of life? It is one cell living on its own, with no other cells to help — so it must do nutrition, metabolism, growth, response, excretion, homeostasis and reproduction itself.
Magnification vs resolution? Magnification = how much bigger the image is; resolution = how much fine detail you can see. The electron microscope wins on resolution.
What four structures are in EVERY cell? DNA, cytoplasm, a plasma membrane and ribosomes (D-C-M-R). A nucleus is not universal — prokaryotes have none.
Biggest difference between prokaryotes and eukaryotes? Prokaryotes have no nucleus and no membrane-bound organelles (DNA is a naked loop in the nucleoid); eukaryotes have both, and their DNA is wound on histones into chromosomes.
How do you tell plant, animal and fungal cells apart? Cell wall material: plant = cellulose, fungal = chitin, animal = no wall. Chloroplasts are in plant cells only.
Exam Tips
- 'Common to all cells' means EXACTLY four structures: DNA, cytoplasm, plasma membrane, ribosomes. A nucleus is not on the list.
- Prokaryote vs eukaryote answers need PAIRED points — compare the same feature in both cells (e.g. nucleus present vs absent).
- Prokaryotic DNA is naked, circular and in the nucleoid; eukaryotic DNA is on histones, in chromosomes, inside a nucleus.
- Tell cell types apart by wall material: plant = cellulose, fungal = chitin, animal = no wall. Chloroplasts = plant only.
- In a micrograph, check for a nucleus FIRST — nucleus = eukaryotic, no nucleus with naked DNA = prokaryotic.
- The electron microscope advanced biology through higher RESOLUTION (more detail), not just higher magnification.
- A drawn eukaryotic nucleus needs four labels: double membrane, nuclear pores, chromatin and nucleolus.