A eukaryotic cell is not a bag of soup. It is a building with rooms. Each room has its own walls, its own conditions and its own job, and that is the single biggest structural difference between your cells and a bacterium’s. This page is about those rooms and what happens inside them.
📚 What you need to know
Eukaryotic cells have a more complex ultrastructure than prokaryotic cells.
The cytoplasm is divided into membrane-bound compartments called organelles, bound by either a single or a double membrane.
The cell wall, cytoskeleton and cytoplasm are not organelles, because they have no membrane.
Compartmentalisation lets the cell concentrate enzymes and substrates, isolate damaging substances, hold different conditions in different places, and change organelle numbers to suit its needs.
Organelle structure is adapted to its function, which is why they all look different.
NOS: progress in science often follows new techniques. Cell fractionation using the ultracentrifuge is what made studying individual organelles possible.
What counts as an organelle
The definition to work from
Organelle = a membrane-bound compartment inside the cell with its own specific function
The membrane is the important bit. It is what turns a region of cytoplasm into a separate room with its own contents and conditions. So three structures that students often list are ruled out straight away:
Cell wall — outside the plasma membrane, and not membrane-bound itself.
Cytoskeleton — a network of protein filaments, with no membrane around it.
Cytoplasm — the fluid the organelles sit in, not a compartment in its own right.
The ribosome question — worth getting straight. Ribosomes have no membrane. Some textbooks still call them organelles, and the usual compromise is to describe them as non-membrane-bound organelles. What matters for the exam is this: ribosomes are not compartments, so they cannot maintain their own internal conditions. If a question asks about compartmentalisation, do not use ribosomes as your example. Centrioles are in the same category.
The animal cell
The rough ER is rough because of those ribosomes. Smooth ER has none, which is the whole reason the two look different under a microscope and do different jobs.
The plant cell
Plant cells contain everything above except the centrioles, plus three structures animal cells do not have: a cell wall, chloroplasts and a large permanent vacuole.
Two details that earn marks: the membrane around the vacuole has its own name, the tonoplast, and the small channels through the wall are plasmodesmata, which connect neighbouring cells.
The organelles, and how many membranes each has
Structure
Membranes
What it does
Nucleus
Double (nuclear envelope, with pores)
Holds the DNA as chromatin; site of transcription; keeps mRNA separate from ribosomes until it has been modified
Nucleolus
None (a dense region inside the nucleus)
Makes ribosomal RNA and assembles ribosome subunits
Mitochondrion
Double, inner one folded into cristae
Aerobic respiration; the folded inner membrane gives a large surface area for the reactions
Chloroplast (plants)
Double, with internal stacked grana
Photosynthesis; the grana hold chlorophyll and give a large surface for light absorption
Rough ER
Single, studded with ribosomes
Transports proteins made by its ribosomes towards the Golgi
Smooth ER
Single, no ribosomes
Makes lipids and steroids, and stores calcium ions
Golgi apparatus
Single, a stack of flattened sacs
Modifies, sorts and packages proteins into vesicles
Vesicle
Single
Transports material around the cell and to the plasma membrane
Lysosome
Single
Contains digestive enzymes; the membrane stops them attacking the rest of the cell
Vacuole (plants)
Single, called the tonoplast
Holds cell sap, keeps the cell turgid and stores substances
Plasma membrane
Single
Controls what enters and leaves; forms the boundary of the cell
Ribosome
None
Site of translation; not a compartment, so it cannot hold its own conditions
Centrioles (animals)
None
Organise the spindle during cell division
Cell wall (plants)
None
Cellulose layer outside the membrane that stops the cell bursting; not an organelle
If you are ever asked which organelles have a double membrane, the answer is short: nucleus, mitochondrion, chloroplast. That is a memorable little list and it comes up more often than you would expect.
Why compartments are worth the effort
Building all those membranes costs the cell energy and materials. It is worth it for four reasons, and these are the ones the syllabus wants:
🧩 The four advantages of compartmentalisation
Higher concentrations. Enzymes and their substrates are localised in a small space instead of diluted through the whole cell, so reactions run faster.
Damaging substances are kept apart. Digestive enzymes are held inside lysosomes, so they cannot digest the cell itself.
Optimal conditions in each compartment. A lysosome can hold a low pH suited to its enzymes while the cytoplasm stays near neutral.
Flexibility. The number and position of organelles can be changed to suit the cell’s needs — a muscle cell packs in far more mitochondria than a skin cell.
Structure follows function
Organelles look different from each other because each one is adapted to its job. Two clean examples:
A mitochondrion’s inner membrane is folded into cristae, which packs a very large surface area into a small organelle. More surface means more space for the proteins that carry out respiration.
A chloroplast’s internal membranes are stacked into grana, giving a large area of chlorophyll-containing membrane to absorb light.
Notice both are the same trick: fold a membrane to get more area in the same volume. Whenever you are asked to explain an organelle’s adaptation, look for that pattern first.
NOS: new techniques come first, discoveries follow
We could not study organelles individually until we could physically separate them. That became possible with the invention of the ultracentrifuge and the development of cell fractionation. It is a neat example of the general point that progress in science often waits on a new technique rather than a new idea.
To study an organelle you need a pure sample containing only that organelle. Cell fractionation gets you there in three stages.
The sample is kept cold, buffered and in a solution of the right concentration throughout, so that enzymes do not damage the organelles and the organelles do not burst or shrivel.
🧩 The three stages, in order
Homogenisation. A homogeniser (a blender-like machine) breaks the cells open and releases the organelles into solution.
Filtration. The homogenate is filtered through gauze to remove whole cells and large debris.
Ultracentrifugation. The filtrate goes into a tube in a centrifuge, which separates the contents by spinning. The speed can be changed, so components are separated according to their density and mass — heaviest first.
Worked examples
WORKED EXAMPLE
Explain why the cytoskeleton and the cell wall are not classed as organelles. [2]
Step 1: state the definitionAn organelle is a membrane-bound compartment within the cell with its own function.Step 2: apply itNeither is surrounded by a membrane, so neither forms a separate compartmentThe cytoplasm fails the same test, for the same reason.
WORKED EXAMPLE
Explain two advantages to a cell of dividing its cytoplasm into membrane-bound compartments. [4]
Advantage 1: concentrationEnzymes and substrates are localised in a small volume rather than spread through the whole cell.Higher concentrations mean more frequent collisions, so the reaction proceeds faster.Advantage 2: isolation of harmful substancesLysosomes hold digestive enzymes behind a membrane.This prevents them digesting the cell’s own contents, and lets the lysosome hold a low pH the enzymes needEach advantage needs a statement plus a consequence. That is what gets you both marks.
WORKED EXAMPLE
A researcher fractionates liver tissue and spins the filtrate at a low speed, then removes the liquid and spins it faster. Explain what she obtains at each step and why. [3]
Step 1: the first, slow spinThe densest and heaviest organelles settle out first, so the pellet contains nuclei.Step 2: the second, faster spinThe liquid poured off still contains the lighter organelles. A faster spin brings down the next group, the mitochondria.Step 3: the principleSeparation depends on mass and density, so raising the speed step by step gives a pure sample of each group in turnLiver tissue has no chloroplasts, so mitochondria come second here.
💡 Exam tip
When defining an organelle, always include the words membrane-bound and compartment.
Learn the double-membrane list: nucleus, mitochondrion, chloroplast.
For adaptation questions, look for folded membranes giving a large surface area — cristae and grana are the two standard answers.
Give the four advantages of compartmentalisation with an example attached to each, not as a bare list.
Learn cell fractionation in order and by name: homogenisation, filtration, ultracentrifugation.
For the NOS point, say that the technique came first and the discoveries followed, and name the ultracentrifuge.
⚠ Common mix-up
Calling the cell wall an organelle. No membrane, and it is outside the cell membrane anyway.
Using ribosomes as an example of compartmentalisation. They have no membrane, so they hold no separate conditions.
Confusing rough and smooth ER. Rough has ribosomes and handles proteins; smooth has none and makes lipids.
Mixing up nucleus and nucleolus. The nucleolus is a region inside the nucleus that builds ribosomes.
Saying the vacuole membrane is just “the membrane”. It has a name: the tonoplast.
Getting the fractionation order backwards. Heaviest first, at the slowest speed. Ribosomes come last.
Writing that centrifuging separates by size alone. It separates by mass and density.
Up next: Cell Compartmentalisation — why keeping transcription and translation in separate rooms produces better proteins, and how a white blood cell eats a bacterium without poisoning itself.
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