Cell Structure: Eukaryotes and Prokaryotes

Every living thing is built from cells, and those cells fall into two groups defined by one feature: where the genetic material sits.

A eukaryotic cell keeps its genetic material (DNA) inside a nucleus — a membrane-bound compartment. Plant cells and animal cells are eukaryotic. A prokaryotic cell has no nucleus; its DNA floats freely in the cytoplasm. Bacteria are prokaryotic.

Eukaryote = genetic material enclosed in a nucleus. Prokaryote = genetic material not enclosed in a nucleus.

Size separates them too. Prokaryotic cells are much smaller than eukaryotic cells.

Cell type	Example	Nucleus?	Typical size
Eukaryotic	Plant, animal	Yes (DNA enclosed)	About 10–100 µm
Prokaryotic	Bacterium	No (DNA free)	About 1–10 µm

The unit µm is a micrometre: one millionth of a metre ($1\text{ µm}=10^{-6}\text{ m}$). A bacterium is roughly a tenth the length of an animal cell, so it can be hundreds of times smaller by volume. Remembering "pro = no [nucleus]" fixes the core distinction the exam tests first.

An animal cell is the baseline eukaryotic cell — learn its parts first, then add to them for plant and bacterial cells.

Each structure does a specific job. The nucleus controls the cell's activities and stores the DNA. The cytoplasm is the jelly-like fluid where most chemical reactions happen. The cell membrane is the thin outer boundary that controls what enters and leaves. Mitochondria (singular: mitochondrion) release energy through aerobic respiration — the oxygen-using reaction that powers the cell. Ribosomes are the tiny sites where proteins are made.

Structure	Function
Nucleus	Controls the cell and holds the genetic material (DNA)
Cytoplasm	Site of most chemical reactions; jelly-like fluid
Cell membrane	Controls movement of substances in and out of the cell
Mitochondria	Release energy by aerobic respiration
Ribosomes	Site of protein synthesis (making proteins)

A cell that uses a lot of energy — like a muscle cell — has many mitochondria. Structure follows demand.

These five parts are present in most animal cells. Plant cells have all of them too, plus extras covered next.

A plant cell contains everything an animal cell has, then adds three structures linked to making food and providing support.

Chloroplasts are where photosynthesis happens — the reaction that uses light energy to make glucose. They contain the green pigment chlorophyll, which absorbs the light. A permanent vacuole is a large fluid-filled sac containing cell sap; when full it pushes outward and keeps the cell firm. The cell wall, made of cellulose (a tough carbohydrate), surrounds the membrane and strengthens the cell so it holds its shape.

Structure (plant only)	Function
Chloroplasts	Site of photosynthesis; contain chlorophyll to absorb light
Permanent vacuole	Holds cell sap; keeps the cell firm and supported
Cell wall (cellulose)	Strengthens the cell and maintains its shape

Only cells exposed to light contain chloroplasts. Root cells are underground and dark, so they have none — a point examiners use to catch students who assume every plant cell is green.

So the cell wall and vacuole appear in all plant cells, but chloroplasts appear only where photosynthesis can occur.

A bacterium is prokaryotic, so its layout differs in ways the exam asks you to state precisely.

It has cytoplasm and a cell membrane, like other cells, and a cell wall surrounding the membrane. But its genetic material is not in a nucleus. Instead the DNA is a single loop floating in the cytoplasm. Many bacteria also carry plasmids — small extra rings of DNA, one or more per cell, separate from the main loop. Bacterial cells have no chloroplasts and no permanent vacuole.

The bacterial cell wall is not made of cellulose — that is a plant feature. Stating "cellulose cell wall" for a bacterium loses the mark.

The single DNA loop and the plasmids are the two features that most reliably identify a cell as prokaryotic.

When an exam gives you a list of structures, you decide whether the cell is animal, plant or bacterial by working through the distinguishing features in order.

Ask three questions:

1. Is the DNA in a nucleus? No → it is a bacterium (prokaryotic). Stop.
2. If yes, does it have a cellulose cell wall, vacuole or chloroplasts? Yes → plant.
3. If it has a nucleus but none of those plant extras → animal.

Walk-through 1. A cell has: cytoplasm, a cell membrane, a cell wall, a single loop of DNA and two plasmids. No nucleus is listed. Question 1 answers no nucleus, so this is prokaryotic — a bacterium. The cell wall alone does not make it a plant; the missing nucleus and the plasmids settle it.

Walk-through 2. A cell has: a nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, a cellulose cell wall and a permanent vacuole, but no chloroplasts. The nucleus rules out a bacterium. The cellulose wall and vacuole are plant features, so it is a plant cell — most likely one from a part of the plant that gets no light, such as a root, which is why no chloroplasts are present.

As an organism develops, its cells differentiate — they change to become a particular type, gaining the sub-cellular structures and shape needed for one job. The result is a specialised cell whose structure fits its function.

Specialised cell	Adaptation	Why it helps
Sperm cell	Tail (flagellum) and many mitochondria	Swims to the egg; mitochondria supply the energy
Nerve cell	Long, thin shape with branched ends	Carries electrical signals over long distances
Muscle cell	Packed with protein fibres and many mitochondria	Contracts and has energy to do so
Root hair cell	Long projection increasing surface area; no chloroplasts	Absorbs more water and mineral ions from soil
Xylem cell	Hollow dead tubes, no end walls, strengthened walls	Transports water up the plant; walls give support
Phloem cell	Living tubes with sieve-like end plates	Transports dissolved sugars around the plant

Walk-through — the sperm cell. Its job is to reach and fuse with an egg. The tail lets it swim; swimming costs energy, so it carries many mitochondria to release that energy by respiration. Every adaptation maps to the function: name the feature, then state what it lets the cell do.

Differentiation matters because it lets one organism build many cell types from the same DNA, each efficient at one task.

1. Saying prokaryotes have "no DNA"

Bacteria do have DNA — it simply is not enclosed in a nucleus. State it as a single loop in the cytoplasm, plus plasmids. The error is confusing no nucleus with no genetic material.

2. Giving a bacterium a cellulose cell wall

Bacterial cells have a cell wall, but it is not made of cellulose. Cellulose walls are a plant and algal feature. Write "cell wall" for a bacterium without claiming it is cellulose.

3. Assuming every plant cell has chloroplasts

Chloroplasts appear only in cells that receive light. Root cells and many internal cells have none. Describe a root hair cell as having no chloroplasts.

4. Listing a feature without linking it to a function

A 2-mark "explain" question wants the adaptation and its purpose. "It has many mitochondria" is half an answer; "many mitochondria release the energy needed to swim" is the full one.

5. Confusing differentiation timing in animals and plants

Most animal cells differentiate at an early stage, and in mature animals cell division is mainly for repair and replacement. Many plant cells keep the ability to differentiate throughout life. Do not state that all cells stay able to differentiate.

6. Mixing up units of size

A micrometre (µm) is $10^{-6}$ m and a nanometre (nm) is $10^{-9}$ m. Animal cells are measured in µm, not mm, so check the prefix before any size comparison.