A eukaryotic cell is a prokaryotic cell’s problem solved. Instead of running every reaction in one shared space, it splits itself into rooms. Each organelle is a room with its own conditions, its own enzymes and its own job.
📚 What you need to know
Eukaryotic cells divide their cytoplasm into membrane-bound compartments called organelles.
This compartmentalisation lets enzymes and substrates be concentrated, keeps damaging chemicals contained, and allows different conditions in different places.
The nucleus holds linear DNA wrapped around histones as chromatin, inside a double membrane with pores.
The nucleolus is where ribosomes are built.
Rough ER is covered in 80S ribosomes and processes proteins; smooth ER has none.
Mitochondria have a double membrane with folded cristae, and carry out aerobic respiration to make ATP.
The Golgi apparatus modifies and packages proteins into vesicles.
Lysosomes are vesicles of digestive enzymes. Microtubules form the cytoskeleton.
Plant cells add a cellulose cell wall, chloroplasts and a large permanent vacuole.
Why compartments are worth having
Splitting a cell up costs energy and membrane. It is worth it because of four things:
Higher concentrations. Enzymes and their substrates are packed into a small space, so reactions run faster.
Dangerous things stay contained. Digestive enzymes sit inside lysosomes, so they do not digest the cell itself.
Different conditions in different places. A compartment can hold the exact pH one process needs without affecting the rest of the cell.
Flexibility. The cell can make more of one organelle where it is needed — a muscle cell can carry far more mitochondria.
The animal cell
The rough ER is drawn joined to the nucleus for a reason — its membrane is continuous with the nuclear envelope, so proteins can move straight from one to the other.
The organelles, one by one
Organelle
Structure
Function
Nucleus
Double membrane (nuclear envelope) with pores; contains chromatin
Stores DNA and controls the cell
Nucleolus
Dark region inside the nucleus
Makes ribosomes
Rough ER
Folded membrane sacs (cisternae) covered in 80S ribosomes
Processes and transports proteins
Smooth ER
Same folded sacs but no ribosomes
Makes and transports lipids
Ribosomes
80S; rRNA plus protein; free or on the rough ER
Translation — building proteins
Mitochondrion
Double membrane; inner one folded into cristae; matrix inside
Aerobic respiration, producing ATP
Golgi apparatus
Stack of flattened cisternae with vesicles budding off
Modifies and packages proteins and lipids
Vesicles
Small membrane sacs
Transport and storage
Lysosome
Vesicle full of hydrolytic enzymes
Breaks down waste and worn-out organelles
Microtubules
Tubes of tubulin protein, about 25 nm across
Form the cytoskeleton: support and movement
There is a route running through several of these: ribosomes on the rough ER make a protein, vesicles carry it to the Golgi, the Golgi modifies and repackages it, and a final vesicle carries it to the membrane to be released. If you can tell that story in order, you can answer most protein-secretion questions.
A clue about where they came from. Mitochondria and chloroplasts both have their own small circular DNA and their own ribosomes, and both are surrounded by a double membrane. That is strong evidence they were once free-living prokaryotes taken inside a larger cell.
What plant cells add
The vacuole is so big it pushes everything else into a thin layer around the edge. That pressure against the cell wall is what keeps a plant standing up.
Chloroplasts
Larger than mitochondria, with a double membrane.
Inside are flattened sacs called thylakoids, containing chlorophyll, stacked into piles called grana and joined by lamellae.
The light-dependent stage happens in the thylakoids; the light-independent stage happens in the stroma.
They contain their own circular DNA and ribosomes.
The vacuole and the cell wall
The large permanent vacuole is a sac surrounded by a selectively permeable membrane called the tonoplast, filled with cell sap. Animal cells can have vacuoles, but they are small and temporary.
The cell wall is made of cellulose, sits outside the plasma membrane, and gives support and a fixed shape. It is freely permeable, unlike the membrane. Threads of cytoplasm called plasmodesmata pass through it to link neighbouring cells.
Worked examples
WORKED EXAMPLE
Explain one advantage of compartmentalisation in a eukaryotic cell. [2]
Step 1: state the advantage
Damaging substances can be kept separate from the rest of the cell.
Step 2: give a specific example
Hydrolytic enzymes are contained inside lysosomes, so they cannot digest the cell’s own contents.
Advantage plus a named example scores both marksa general statement with no example usually gets only one
WORKED EXAMPLE
A cell is found to contain an unusually large number of mitochondria and a lot of rough ER. Suggest what this cell does. [3]
Step 1: what mitochondria mean
Lots of mitochondria means a high rate of aerobic respiration, so the cell needs a lot of ATP.
Step 2: what rough ER means
Lots of rough ER means it makes and processes large amounts of protein.
Step 3: combine them
A cell that uses a lot of energy to make and export proteins — for example a secretory cell in the pancreas.
High energy demand + high protein output = a secretory cell
WORKED EXAMPLE
A typical plant cell is 100 µm across and a typical bacterium is 2 µm across. How many times wider is the plant cell?
Step 1: divide100 ÷ 2 = 5050 times widerboth are already in µm, so no conversion is needed – always check that first
💡 Exam tip
Learn organelles as structure + function pairs. Questions almost always want both halves.
Say “double membrane” for the nucleus, mitochondria and chloroplasts — it is a favourite one-mark detail.
Rough ER has ribosomes and handles proteins; smooth ER has none and handles lipids.
The plant extras are exactly three: cell wall, chloroplasts, large permanent vacuole.
If asked to deduce a cell’s job from a micrograph, argue from which organelle is unusually common.
Use cell wall for the cellulose layer and plasma membrane for the one underneath — never interchange them.
⚠ Common mix-up
Saying the nucleolus makes DNA. It makes ribosomes.
Confusing the Golgi with the ER. The ER makes and processes; the Golgi modifies, packages and ships.
Saying mitochondria “make energy”. They transfer energy into ATP — energy is not created.
Thinking animal cells never have vacuoles. They do, but small and temporary.
Calling the cell wall an organelle. It sits outside the membrane, so technically it is not one.
Forgetting the cell wall is freely permeable. It is the membrane underneath that controls entry.
Up next: Functions of Life — the seven things every living thing must do, and how a single-celled organism manages all of them at once.
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