Cell Structure & Function
Middle SchoolThe cell is the smallest unit that is unambiguously alive. Everything else in biology is either inside a cell, made by a cell, or a community of cells. Understanding the cell's parts and their division of labour is the foundation for all that follows.
The cell theory and two blueprints
Modern cell theory states three things: (1) all living organisms are composed of one or more cells; (2) the cell is the basic unit of structure and function in life; and (3) all cells arise from pre-existing cells (omnis cellula e cellula). Cells come in two fundamental designs:
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Examples | Bacteria, Archaea | Animals, plants, fungi, protists |
| Nucleus | None — DNA free in cytoplasm (nucleoid) | True membrane-bound nucleus |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, Golgi…) |
| DNA | Single circular chromosome + plasmids | Multiple linear chromosomes |
| Ribosomes | Small (70S) | Large (80S); 70S in mitochondria/chloroplasts |
| Size | ~1–10 µm | ~10–100 µm |
Key eukaryotic organelles and their jobs
- Nucleus — stores DNA, controls gene expression; the nucleolus inside makes ribosomes.
- Mitochondrion — site of aerobic respiration; makes ATP. Has its own DNA and double membrane.
- Ribosome — reads mRNA and builds proteins (translation). Free or bound to rough ER.
- Rough ER — studded with ribosomes; folds and processes proteins for export.
- Smooth ER — synthesises lipids, stores Ca²⁺, detoxifies.
- Golgi apparatus — modifies, sorts and packages proteins and lipids into vesicles.
- Lysosome — digestive sac full of hydrolytic enzymes; recycles worn components.
- Chloroplast (plants/algae) — captures light for photosynthesis. Double membrane + own DNA.
- Cell wall — rigid outer layer: cellulose in plants, chitin in fungi, peptidoglycan in bacteria.
- Cytoskeleton — protein filaments giving shape, enabling movement and internal transport.
Mitochondria and chloroplasts have their own circular DNA, 70S ribosomes, and a double membrane, and they divide by binary fission. The best explanation: they descend from free-living bacteria engulfed by an ancestral eukaryote roughly 1.5–2 billion years ago. Two organisms became one — the origin of complex life.
Plant cell
- Rigid cellulose cell wall outside the membrane
- Has chloroplasts — makes its own food by photosynthesis
- One large permanent central vacuole (turgor)
- Fixed, box-like shape; stores starch
Animal cell
- No cell wall — flexible membrane only
- No chloroplasts — must eat to obtain food
- Small, temporary vacuoles (if any)
- Varied shape; has centrioles; stores glycogen
A cell is a compartmentalised factory
Why bother with organelles at all? Prokaryotes thrive without them. The answer is compartmentalisation: membranes let a eukaryotic cell run incompatible chemistry side by side. Digestive enzymes that would destroy the cell are sealed in lysosomes; the delicate reading of DNA happens in the protected nucleus; energy-releasing reactions are concentrated in mitochondria where their proton gradients can be harnessed. Each organelle is a specialised room with its own conditions.
Trace the protein secretory pathway and the logic becomes vivid: a gene is transcribed in the nucleus → mRNA exits to a ribosome on the rough ER → the new protein is folded and threaded into the ER → packaged into a vesicle → shipped to the Golgi for finishing and address-labelling → sent in another vesicle to the cell membrane and secreted. It is a literal assembly line, with membranes as conveyor belts and vesicles as shipping containers.
Worked example: identifying a cell and calculating its true size
Under a light microscope you see a cell with a cell wall, a large central vacuole, and chloroplasts. First identify it, then calculate real dimensions.
- Identify. Cell wall + central vacuole + chloroplasts = a plant cell. (No chloroplasts and no wall would suggest animal; wall but no chloroplast and no nucleus would suggest bacterium.)
- Recall the magnification formula. magnification = image size ÷ actual size. Rearranged: actual = image ÷ magnification.
- Measure the image. The cell measures 60 mm across in a photograph taken at ×150 magnification.
- Convert units first. 60 mm = 60 000 µm (×1000).
- Compute the real size. actual = 60 000 µm ÷ 150 = 400 µm. That is a large but plausible plant cell.
- Sanity-check. Typical eukaryotic cells are 10–100 µm; 400 µm is at the large end (many plant cells are big because of the vacuole), so the answer is reasonable — always test magnitude against known ranges.
Test yourself
Q1 Give three lines of evidence for the endosymbiotic origin of mitochondria.
(1) Mitochondria contain their own circular DNA, resembling a bacterial chromosome. (2) They have 70S ribosomes, the prokaryotic type, not the 80S ribosomes of the eukaryotic cytoplasm. (3) They are bounded by a double membrane — the inner one from the ancestral bacterium, the outer from the host's engulfing vesicle — and they divide by binary fission independently of the cell. Antibiotics that target bacterial ribosomes can also affect mitochondria, further supporting the link.
Q2 A cell secretes large amounts of protein. Which organelles would be especially abundant, and why?
Expect abundant rough endoplasmic reticulum (ribosomes synthesising and folding export proteins), a large Golgi apparatus (modifying, sorting and packaging them), many secretory vesicles, and numerous mitochondria (protein synthesis and secretion are energy-expensive, needing ATP). This profile fits cells such as pancreatic acinar cells or antibody-secreting plasma cells.
Q3 Why is compartmentalisation an advantage that outweighs the cost of building membranes?
Compartments let a cell maintain different chemical conditions simultaneously — acidic lysosomes, high-Ca²⁺ ER, proton-gradient mitochondria — and keep dangerous reactions (digestion, oxidation) separate from vulnerable machinery (DNA). This raises efficiency by concentrating enzymes with their substrates, allows tighter regulation, and enables far larger, more complex cells. The metabolic cost of membranes is repaid by the ability to run many specialised processes at once without interference.
Q4 Distinguish the resolution and magnification of a microscope. Why can't a light microscope resolve a ribosome?
Magnification is how much larger the image appears; resolution is the smallest distance between two points that can still be seen as separate. Resolution is limited by the wavelength of the illumination. Visible light (~400–700 nm) cannot resolve structures much smaller than ~200 nm, so a ribosome (~20 nm) blurs into nothing no matter how much you magnify — magnifying a blurry image just gives a bigger blurry image ("empty magnification"). Electron microscopes use electron beams with far shorter wavelengths, resolving down to ~0.1 nm, which is why ribosomes and membranes are seen only with them.
Q5 Both a bacterium and a mitochondrion have 70S ribosomes and circular DNA. Does this make a mitochondrion "alive"? Defend your answer.
By the criteria of independent life, no. Although a mitochondrion is bacterially descended and semi-autonomous, most of the ~1000+ proteins it needs are now encoded by the nuclear genome and imported. It cannot survive or reproduce outside the host cell — the endosymbiont surrendered its autonomy over evolutionary time, transferring genes to the nucleus. It is an organelle, not an organism: a striking illustration that the boundary of "an individual" is a matter of degree, set by evolutionary history.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1A cell appears 40 mm wide under x400 magnification. Find its real size.
- Real size = image size / magnification
- 40 mm / 400 = 0.1 mm
Example 2A cell has a wall, chloroplasts and a large vacuole. Plant or animal?
- Chloroplasts and a cell wall are plant features
- Animal cells lack these
Now you try
Work each one out first, then tap to reveal the worked answer.