A eukaryotic cell is not just a bigger bacterium. It is divided into rooms, and each room does a different job under its own conditions. Learn the organelles as a team rather than a list, and the whole topic gets easier — because exam questions almost always ask how two of them work together.
📚 What you need to know
Eukaryotic cells have a true nucleus holding linear DNA wrapped around histone proteins.
They are compartmentalised: membranes divide the inside into organelles with different conditions.
Key organelles: nucleus, rough and smooth ER, Golgi apparatus, mitochondria, 80S ribosomes, lysosomes, vesicles.
Proteins for export follow one route: nucleus → rough ER → vesicle → Golgi → vesicle → plasma membrane.
Plant cells also have a cellulose cell wall, chloroplasts and a large permanent vacuole.
Animal cells have centrioles and small temporary vacuoles, and no wall or chloroplasts.
Compartments allow different pH, enzymes and concentrations in different places at the same time.
The cell as a set of rooms
Picture a factory. There is an office with the plans in it, a production floor, a packing department, a power station and a bin store. Nobody would run all of those in one open room. A eukaryotic cell has the same idea: keep jobs apart, and each job runs better.
This is an animal cell. A plant cell would keep everything here and add a cellulose wall, chloroplasts and one huge central vacuole.
What each organelle does
Organelle
What it looks like
What it does
Nucleus
Large, round, wrapped in a double membrane with pores
Stores the DNA and controls the cell by deciding which genes are used
Nucleolus
A dark patch inside the nucleus
Makes the parts that ribosomes are built from
Rough ER
Flattened sacs covered in ribosomes
Makes proteins and moves them through the cell
Smooth ER
Tubes with no ribosomes on them
Makes lipids and steroids, and stores calcium
Golgi apparatus
A stack of curved flattened sacs
Modifies proteins, then sorts and packages them into vesicles
Vesicles
Small membrane bubbles
Carry substances between organelles and to the plasma membrane
Mitochondrion
Oval, with a folded inner membrane
Carries out aerobic respiration to release energy as ATP
Ribosomes (80S)
Tiny dots, free or attached to the rough ER
Join amino acids to build proteins
Lysosome
A small sac of digestive enzymes
Breaks down worn-out organelles, waste and material taken into the cell
Chloroplast
Green, with stacked internal membranes; plants only
Carries out photosynthesis
Vacuole
Large and permanent in plants, small in animals
Stores water and solutes, and keeps a plant cell firm
Cell wall
Rigid cellulose layer outside the membrane; plants only
Supports the cell and stops it bursting
The folds inside a mitochondrion are called cristae, and they exist for one reason: surface area. More folded membrane means more room for the proteins that make ATP. If a question gives you a cell with unusually many mitochondria, it wants you to say that cell needs a lot of energy — muscle, sperm, or something doing active transport.
The protein export line
This sequence appears in exams constantly, and it is much easier to remember as a journey than as separate facts.
Vesicles appear twice in this route, which is why “vesicle” on its own is rarely enough in an answer — say which two organelles it is travelling between.
A gene in the nucleus is copied into mRNA, which leaves through a nuclear pore.
Ribosomes on the rough ER read the mRNA and build the protein, which ends up inside the ER sacs.
A vesicle buds off the ER carrying the protein.
The vesicle fuses with the Golgi apparatus, where the protein is modified — sugars added, chains trimmed — then packaged.
Another vesicle carries it to the plasma membrane, fuses with it, and the protein is released outside. That release is exocytosis.
Animal cells and plant cells
Both are eukaryotic, so they share nearly everything. Three additions and one removal cover the difference.
Feature
Animal cell
Plant cell
Cell wall
Absent
Present, made of cellulose
Chloroplasts
Absent
Present in cells exposed to light
Vacuole
Small and temporary, if present
One large permanent central vacuole
Centrioles
Present
Absent from flowering plants
Shape
Rounded and flexible
Fixed and often box-like, held by the wall
Storage carbohydrate
Glycogen
Starch
🧠
Plants get three extras
Wall, Chloroplast, Vacuole. Everything else on a plant cell is also on an animal cell, so you only have to remember what gets added.
The reason for the nuclear membrane. Putting DNA in its own compartment means transcription and translation happen in different places. mRNA can be checked and edited on the way out before any protein is made from it. A prokaryote cannot do this — translation starts while the mRNA is still being copied.
Worked examples
WE 1
Trace a protein out of the cell
Outline the path taken by a protein that is made in a cell and then secreted from it. (4 marks)
Step 1: the instructions
mRNA is transcribed in the nucleus and leaves through a nuclear pore.
Step 2: building it
Ribosomes on the rough ER use the mRNA to build the protein.
Step 3: modifying it
A vesicle carries the protein to the Golgi apparatus, where it is modified and packaged.
Step 4: getting it out
A second vesicle carries it to the plasma membrane and fuses with it, releasing the protein by exocytosis.
Nucleus → rough ER → vesicle → Golgi → vesicle → outthe word “exocytosis” is nearly always worth a mark on its own
WE 2
Deduce a cell’s job from its organelles
A cell contains a great deal of rough ER, many Golgi bodies and unusually many mitochondria. Suggest the function of this cell. (3 marks)
Clue 1: lots of rough ER
Rough ER makes proteins, so this cell must be producing them in large amounts.
Clue 2: many Golgi bodies
The Golgi packages proteins for export, so the proteins are being secreted rather than kept.
Clue 3: many mitochondria
Making and exporting proteins needs a lot of ATP, which the mitochondria supply.
A secretory cell, such as one producing enzymes or hormoneslink each organelle to a job, then combine them — do not guess from one clue
WE 3
Compare a plant and an animal cell
State three structures found in a plant cell but not in an animal cell, and give a function for each. (3 marks)
Structure 1
Cellulose cell wall — supports the cell and stops it bursting when water enters.
Structure 2
Chloroplast — carries out photosynthesis to make glucose.
Structure 3
Large permanent vacuole — stores water and solutes and keeps the cell firm.
Wall, chloroplast, vacuolea wall alone is not enough — say cellulose, since bacteria and fungi have walls too
💡 Exam tips
Learn the export route as a chain. It answers a whole family of questions.
Say cellulose cell wall for plants, to separate it from peptidoglycan and chitin.
Rough ER has ribosomes and makes proteins; smooth ER has none and makes lipids.
Plenty of mitochondria means a high energy demand. Say what the energy is used for.
For compartmentalisation marks, mention separate enzymes and pH, and isolating harmful enzymes.
Use full names once — endoplasmic reticulum — then the abbreviation is fine.
⚠ Common mistakes
Saying the nucleus makes proteins. It holds the instructions; ribosomes do the building.
Swapping rough and smooth ER. Rough is rough because it is covered in ribosomes.
Writing that the Golgi makes proteins. It modifies and packages ones already made.
Saying mitochondria make energy. Energy is released and transferred to ATP, not created.
Giving plant cells no mitochondria. Plants respire too, in every cell.
Drawing chloroplasts in a root cell. No light there, so no chloroplasts.
Up next: Functions of Life — the jobs every living cell has to carry out, and how a single-celled organism manages all of them at once.
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