Animals, plants and fungi are all eukaryotes, but they are not built the same way. And a handful of cells break the neat “one cell, one nucleus” picture completely — which is exactly why examiners like asking about them.
📚 What you need to know
Eukaryotes fall into four kingdoms: animals, plants, fungi and protists.
Cell walls: none in animals, cellulose in plants, chitin and glucans in fungi.
Vacuoles: small and temporary in animals and fungi; large and permanent in plants.
Chloroplasts: plants only.
Centrioles: animals only.
Cilia and flagella: animals yes; plants and true fungi no.
Storage carbohydrate: glycogen in animals and fungi, starch in plants.
Atypical cells that stretch cell theory: striated muscle fibres, aseptate fungal hyphae, red blood cells and phloem sieve tubes.
Animal, plant and fungal cells compared
Feature
Animal
Plant
Fungal
Cell wall
None
Cellulose
Chitin and glucans
Vacuole
Small, temporary, often several
One large, permanent
Small, non-permanent
Chloroplasts
None
Many
None
Centrioles
Present
Absent
Absent
Cilia and flagella
Present in some
Absent
Absent in true fungi
Carbohydrate store
Glycogen
Starch
Glycogen
Shape
Flexible — no rigid wall
Fixed by the wall
Has a wall but can still vary
A neat way to hold this: fungi look like plants because they have a wall and no movement, but chemically they are closer to us. Chitin, not cellulose. Glycogen, not starch. No chloroplasts, so they have to feed rather than make their own food.
Why animal cells need centrioles and plants do not. Centrioles organise the microtubules that pull chromosomes apart during cell division. Plant cells do the same job without them, using other structures — so their absence is a difference, not a missing ability.
Cells that bend the rules
Cell theory says a cell is one unit with its own membrane and one nucleus. Most cells fit. A few do not, and the syllabus expects you to know them by name and explain why each one is awkward.
Every one of these oddities has a purpose. Losing a nucleus makes room for haemoglobin; fusing cells lets a muscle contract as one unit.
Striated muscle fibres
Formed when many separate cells fuse together, so a single fibre contains many nuclei inside one membrane (the sarcolemma).
Far longer than a normal cell — up to 300 mm, compared with a cardiac muscle cell at 100–150 µm.
They challenge the idea that cells are separate units working independently.
Aseptate fungal hyphae
Fungi grow as long narrow threads called hyphae, which have membranes and walls.
Some hyphae are divided by cross walls called septa. Aseptate hyphae have none.
That leaves a continuous stretch of cytoplasm containing many nuclei with no dividing walls — it behaves as one enormous cell.
Red blood cells
Mature red blood cells have no nucleus at all.
Removing it frees up space for far more haemoglobin, so each cell carries more oxygen.
Their biconcave shape gives a large surface area for the volume, speeding up oxygen exchange.
The cost is that they cannot divide or repair themselves, so they have a limited lifespan.
Phloem sieve tubes
They transport dissolved sugars such as sucrose around a plant.
They have no end walls between them, forming a continuous tube.
They lack a nucleus, mitochondria and ribosomes, so they cannot maintain themselves.
They survive only because companion cells sitting alongside keep them going.
Worked examples
WORKED EXAMPLE
State two differences between a plant cell and a fungal cell. [2]
Difference 1
Plant cell walls are made of cellulose, whereas fungal cell walls are made mainly of chitin.
Difference 2
Plant cells contain chloroplasts, whereas fungal cells do not.
Use “whereas” and give both sides each timea difference needs two halves – “plants have chloroplasts” on its own is incomplete
WORKED EXAMPLE
Explain how striated muscle fibres challenge cell theory. [3]
Step 1: state what is unusual
A single muscle fibre contains many nuclei inside one membrane.
Step 2: explain why
It forms when many separate cells fuse together, so it is not one cell in the ordinary sense.
Step 3: link back to cell theory
This challenges the idea that a cell is a single independent unit with its own nucleus.
Unusual feature → cause → which part of cell theory it strains
WORKED EXAMPLE
Suggest one advantage and one disadvantage of a red blood cell having no nucleus. [2]
Advantage
More space inside the cell for haemoglobin, so it carries more oxygen.
Disadvantage
Without DNA it cannot make new proteins, repair itself or divide, so it has a short lifespan.
Space gained, self-repair lost
💡 Exam tip
Learn the three wall materials as a set: cellulose, chitin, none.
Answer “difference” questions with both sides and the word whereas.
For atypical cells, always finish by saying which part of cell theory is being challenged.
Cell theory is a generalisation, not a law — the exceptions do not disprove it.
Every atypical feature has an advantage. Being able to name it turns a 1-mark answer into a 2-mark one.
Remember protists as the fourth eukaryotic kingdom — it is easy to forget it exists.
⚠ Common mix-up
Saying fungal cell walls are cellulose. That is plants; fungi use chitin.
Claiming animal cells never have vacuoles. They do — small and temporary ones.
Saying atypical cells disprove cell theory. They challenge it; they do not overturn it.
Confusing aseptate and septate hyphae.Aseptate means absent septa.
Thinking all red blood cells lack a nucleus. Mature mammalian ones do; many other animals’ do not.
Forgetting companion cells when explaining how sieve tubes survive without organelles.
Up next: Cell Types & Structures (Skills) — how to look at an unlabelled micrograph and work out, step by step, exactly what you are looking at.
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