A bacterium and a liver cell look nothing alike, and one is about fifty times wider than the other. But strip both back to essentials and you find the same four things every time. Those four are worth learning first, because everything else in this topic is a variation on them.
📚 What you need to know
Every cell has a plasma membrane, cytoplasm, DNA and ribosomes.
The plasma membrane is the boundary, and it controls what goes in and out.
The cytoplasm is where most of the cell’s chemical reactions happen.
DNA carries the instructions for making proteins.
Ribosomes are where those instructions are used to build proteins.
Cells are split into two types: prokaryotic (no nucleus) and eukaryotic (DNA inside a nucleus).
Prokaryotic cells are much smaller and simpler; eukaryotic cells are larger and divided into compartments.
The four things every cell has
Whatever the organism, whatever the job, no cell manages without these.
In every single cell
plasma membrane • cytoplasm • DNA • ribosomes
The oval here is deliberately generic. It could be a bacterium or a cheek cell — at this level of detail you cannot tell them apart, and that is the point.
Plasma membrane
Every cell is wrapped in a thin membrane made mainly of phospholipids. It does two jobs at once. It holds the contents in, keeping the inside separate from the outside. And it decides what crosses, letting some substances through while blocking others. That second job is called being partially permeable.
Without a boundary, the chemicals a cell needs would simply drift away. This is why the membrane is the one structure you can never leave out of a definition of a cell.
Cytoplasm
The cytoplasm is the watery substance filling the cell. It is not just water — it is packed with dissolved enzymes, ions and small molecules, and it is where most of the cell’s chemical reactions take place. Respiration in a prokaryote starts here. So does protein synthesis, since ribosomes sit in it.
DNA
DNA holds the coded instructions for building proteins, and proteins do almost everything in a cell. Where the DNA sits is the single biggest difference between the two cell types:
In prokaryotic cells the DNA lies free in the cytoplasm, in a region called the nucleoid. There is no membrane around it.
In eukaryotic cells the DNA is enclosed in a nucleus with its own double membrane.
Ribosomes
Ribosomes are tiny structures that read the instructions from DNA and join amino acids together into proteins. They are not surrounded by a membrane, which is why even the simplest cells have them.
Two sizes, and the numbers matter. Prokaryotic ribosomes are 70S and eukaryotic ones are 80S. The S is a measure of how fast they settle when spun in a centrifuge, so it is a size measurement rather than a mass. Examiners like this detail because it is a quick way to tell the two cell types apart from a description.
The two types of cell
Every cell on Earth falls into one of two groups. The names tell you the difference: karyon is Greek for kernel or nut, and it means the nucleus here. Prokaryotic means “before a nucleus”. Eukaryotic means “true nucleus”.
Both cells contain the same four essentials. The eukaryotic one just adds walls inside itself so different jobs can happen in different places at once.
Feature
Prokaryotic cell
Eukaryotic cell
Nucleus
Absent — DNA sits in the nucleoid
Present, with a double membrane
DNA shape
Circular, and not wrapped around proteins
Linear, wound around histone proteins
Membrane-bound organelles
None
Many, such as mitochondria and Golgi
Ribosomes
70S, smaller
80S, larger
Typical size
1 to 5 µm
10 to 100 µm
Cell wall
Usually present, made of peptidoglycan
Present in plants and fungi only, and made of different material
Examples
Bacteria and archaea
Animals, plants, fungi and protists
🧠
Pro means before
Prokaryote came before the nucleus existed. Eukaryote has a proper (eu = true) nucleus. If you can remember which prefix means what, you never mix the two up.
Why compartments are worth having
This is the part that is usually skipped, and it explains the whole of the next few pages.
In a prokaryotic cell everything happens in the same open space. That works, but it limits you. Two reactions that need different conditions cannot both run well at once, and an enzyme that digests things cannot be let loose or it damages the cell itself.
A eukaryotic cell solves this by building membranes inside itself. Digestive enzymes get sealed in a lysosome. Respiration gets its own compartment in a mitochondrion. The DNA gets protected inside a nucleus. Each compartment can hold the exact conditions its reactions need, and dangerous chemicals stay where they are meant to be.
If a question asks for an advantage of compartmentalisation, give a concrete reason rather than saying “it is more efficient”. Say that reactions can be kept separate, that each compartment can keep its own pH and enzymes, and that harmful enzymes are isolated from the rest of the cell.
Worked examples
WE 1
State the features common to all cells
State four structures found in all living cells and give the function of each. (4 marks)
Structure 1
Plasma membrane — forms the boundary and controls what enters and leaves.
Structure 2
Cytoplasm — the site of most chemical reactions in the cell.
Structure 3
DNA — carries the coded instructions for making proteins.
Structure 4
Ribosomes — where amino acids are joined together to make proteins.
Membrane, cytoplasm, DNA, ribosomesthe question says “and give the function”, so a bare list of names scores half
WE 2
Identify a cell type from a description
An electron micrograph shows a cell 2 µm long, with no internal membranes and DNA lying free in the cytoplasm. Deduce the type of cell and justify your answer. (3 marks)
Step 1: the deduction
It is a prokaryotic cell.
Step 2: first reason
The DNA is not enclosed, so there is no nucleus, which rules out a eukaryotic cell.
Step 3: second reason
There are no membrane-bound organelles, and 2 µm fits the usual prokaryotic range of 1 to 5 µm.
Prokaryotic — no nucleus, no organelles, and the right size“justify” means give evidence from the stem, not general knowledge
WE 3
Explain the value of compartments
Explain one advantage to a eukaryotic cell of having membrane-bound organelles. (3 marks)
Point 1: what a membrane does
A membrane separates the inside of the organelle from the rest of the cytoplasm.
Point 2: what that allows
Each compartment can hold its own enzymes, pH and concentrations, so reactions run at their best rate.
Point 3: a concrete case
Digestive enzymes stay sealed inside lysosomes, so they break down waste without damaging the cell itself.
Separate conditions for separate reactions, and harmful enzymes kept apartone clear example is worth more than three vague statements
💡 Exam tips
Learn the four shared features as a set. They come up as a whole question and as one mark inside bigger ones.
Say partially permeable, not “semi-permeable”, when describing the membrane.
Quote ribosome sizes as 70S and 80S if you are asked to distinguish cell types.
Sizes are useful evidence: a few µm points to a prokaryote, tens of µm to a eukaryote.
For compartmentalisation, give a named organelle as an example.
The nucleoid is a region, not an organelle — it has no membrane.
⚠ Common mistakes
Saying prokaryotes have no DNA. They have plenty. It just is not inside a nucleus.
Calling the nucleoid a nucleus. No membrane, so it is not one.
Writing that prokaryotes have no ribosomes. They do — 70S ones.
Confusing cytoplasm with cytosol. At this level, treat cytoplasm as everything inside the membrane apart from the nucleus.
Saying the membrane keeps everything out. It is selective, not sealed.
Assuming all eukaryotic cells have a cell wall. Plants and fungi do; animal cells do not.
Up next: Prokaryotic Cell Structure — a proper look inside a bacterium, including the parts that only prokaryotes have.
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