Every cell is built from the same four parts. A eukaryote just has a lot more room — and has to divide that room up.
Cell theory says life comes in cells. Strip any cell — bacterium, leaf, neuron — down to what it cannot lose, and you always find the same four things.
All living things are made of cells (or cell products). The cell is the smallest unit of life. Cells arise only from pre-existing cells.
Plasma membrane (a border), cytoplasm (a reaction medium), DNA (instructions), ribosomes (protein makers).
Some eukaryotic structures bend the rules: many nuclei in one membrane, or no nucleus at all. The theory still holds — but it has edges.
Where cell theory frays. Striated muscle fibres and aseptate fungal hyphae are multinucleate with no internal partitions, challenging the idea of cells as discrete, autonomous units. Mature red blood cells eject their nucleus and mitochondria to pack in haemoglobin — they can't replicate, so bone marrow must keep making them. Phloem sieve tube elements go further still: no nucleus, few organelles, kept alive by companion cells.
Cell theory waited on the compound microscope. Every figure in this lesson depends on one number: how much bigger the picture is than the thing.
1 mm = 1000 µm; 1 µm = 1000 nm. Cells live in µm, organelles and membranes in nm. Convert before you calculate, every time.
M = image size / actual size. With a scale bar: measure the bar in mm, convert to µm, divide by the length it claims to represent.
Calibrate the eyepiece graticule against a stage micrometer at each objective, then read cells in graticule units. A ruler is only good to ± its smallest division.
Same units on top and bottom. The single most common lost mark: dividing millimetres by micrometres. Convert the measured bar to µm first, then the magnification is unitless.
Type any image and actual size in whatever units you measured them in. The bench converts both to µm before dividing — copy that habit.
Electrons have a far shorter wavelength than light, so an electron microscope resolves ~0.1 nm against ~200 nm. Higher magnification and higher resolution — but specimens are dead, in a vacuum, in false colour.
Freeze a cell, snap it, and the fracture runs between the two layers of a membrane. The exposed face shows proteins studding a lipid sheet — the fluid-mosaic evidence.
Flash-freeze molecules in vitreous ice and average thousands of images: protein structures to atomic detail without crystals. Nobel Prize 2017.
Dyes and antibodies tagged with fluorophores (immunofluorescence) light up one protein in a living cell. Green fluorescent protein lets you watch it move.
The headline difference is a nucleus. The useful one is size — because a tenfold jump in width is a thousandfold jump in volume, and that is what forces eukaryotes to compartmentalise.
Cell wall, plasma membrane, cytoplasm, naked DNA in a loop (the nucleoid), 70S ribosomes. No membrane-bound organelles. Divides by binary fission.
Nucleus with a double membrane and pores, DNA bound to histones, 80S ribosomes, mitochondria, ER, Golgi, vesicles & lysosomes, a cytoskeleton.
Walls of cellulose (plant) or chitin (fungus) or none (animal). Big sap vacuole and plastids only in plants. Centrioles, cilia & flagella in animals, not plants.
Why the eukaryote needs organelles. Prokaryotes run at 1–5 µm, eukaryotes at 10–100 µm. At that size, diffusion from the surface cannot serve the interior — so a eukaryote folds membrane inside itself to bring surface area to where the chemistry happens.
| Mnemonic | Prokaryote | Eukaryote |
|---|---|---|
| DDNA | Naked (not on histones), one circular chromosome in a nucleoid; often plasmids | Bound to histones, several linear chromosomes inside a nuclear envelope |
| OOrganelles | None membrane-bound; 70S ribosomes | Nucleus, mitochondria, ER, Golgi, lysosomes, vesicles; 80S ribosomes |
| RReproduction | Binary fission | Mitosis (or meiosis for gametes) |
| AAverage size | 1–5 µm | 10–100 µm |
Careful with two rows. Prokaryotic DNA is called "naked" because it isn't wound on histones — bacteria do have proteins near the nucleoid. And a prokaryote isn't "haploid": that word describes chromosome sets, which a single loop doesn't have.
No spindle, no mitosis. One loop, copied, pulled apart by the growing membrane. At twenty minutes a division, one bacterium is over 4 × 10²¹ after a day — if nothing ran out.
Tap a part. Notice how many eukaryotic organelles are really one membrane system seen at different stages.
Tap the plasma membrane, nucleus, nucleolus, rough or smooth ER, Golgi, mitochondrion, lysosome, vesicles, centrioles, ribosomes or cytoskeleton.
A unicellular organism has to do everything a body does — inside one membrane. Tap a part of Paramecium to see which function of life it carries out.
Homeostasis, metabolism, nutrition, excretion, growth, response, movement, reproduction. Living things do all of these; a rock does none.
Paramecium eats: cilia sweep food in. Chlamydomonas photosynthesises: a chloroplast, an eyespot to find light, two flagella to swim to it.
Tap the cilia, membrane, oral groove, food vacuoles, contractile vacuoles, nuclei or the anal pore.
Mitochondria and chloroplasts look like bacteria because they were bacteria. Step through the engulfment and watch the second membrane appear.
The membrane count is the giveaway. The bacterium's own membrane becomes the inner one; the host's membrane wraps it during endocytosis and becomes the outer one. Both organelles kept their loop of DNA, their 70S ribosomes, and their habit of dividing by binary fission — so a mitochondrion is never built from scratch, only inherited.
Multicellularity is the eukaryote's second big move after compartments — and it evolved more than once.
Cells of a multicellular organism carry the same genome but express different genes, so they can specialise — and specialised cells group into tissues that do one job well.
Multicellularity evolved independently in animals, plants, fungi and several algal lines. Colonial algae like Volvox show a plausible halfway house.
Being many small cells beats being one big one: SA:V stays high, cells can specialise, and losing a cell isn't fatal. Picked up in B2.3.
Tap a node to light up its links. The pink dashed link is the idea that connects size to structure.
Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.
Single best answer. You get the reasoning as soon as you commit. HL items are marked.