IMAT Biology · Cell Structure and Membranes
Prokaryotes, Eukaryotes, Plant and Animal Cells, and Viruses
Prokaryotes lack a nucleus and membrane-bound organelles; plant and animal cells differ in walls, chloroplasts and vacuoles; viruses are not cells at all.
Why this matters
The 2024 ministry paper asked which organisms have internal compartments. The older papers used tables of features, asking which belong to a bacterium, a plant cell or an animal cell, and which are shared by all of them.
Concept 1 of 3: Prokaryotic and eukaryotic cells compared
A prokaryotic cell is a single open room: DNA, ribosomes and enzymes all share one space. A eukaryotic cell is a house with many rooms, each sealed by membranes, so different jobs can run side by side without getting in each other's way. That division into compartments is the defining feature of eukaryotes.
Definition
Prokaryotes (bacteria and archaea) have no nucleus and no membrane-bound organelles. Eukaryotes (protists, fungi, plants, animals) have a nucleus and membrane-bound organelles: this internal compartmentalisation is the key difference.
- Shared by both: plasma membrane, cytoplasm, ribosomes, DNA as the genetic material, ATP. Both can store glycogen (bacteria and animals do).
- In a prokaryote the single circular chromosome lies in a region of the cytoplasm called the nucleoid. Many also carry plasmids, small extra rings of DNA that often carry antibiotic resistance genes.
- Eukaryotes also contain circular DNA, inside their mitochondria and chloroplasts.
- A eukaryotic cell can still be recognised in metaphase, when the nucleus has broken down, by its membrane-bound organelles such as mitochondria.
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Examples | Bacteria and archaea | Protists, fungi, plants, animals |
| Typical size | About 1 to 5 µm | About 10 to 100 µm |
| DNA | One circular chromosome in the nucleoid, often plasmids | Linear chromosomes with histones in a nucleus, plus circular DNA in mitochondria and chloroplasts |
| Membrane-bound organelles | None | Nucleus, mitochondria, ER, Golgi, lysosomes and others |
| Ribosomes | 70S | 80S in the cytoplasm; 70S in mitochondria and chloroplasts |
| Cell wall | Usually present; peptidoglycan in bacteria | In plants (cellulose) and fungi (chitin); none in animals |
| Cell division | Binary fission | Mitosis and meiosis |
Practice this conceptself-check · 3 quick reps
Circular DNA and ribosomes do not prove a cell is prokaryotic
Human cells contain circular DNA in their mitochondria, and every cell has ribosomes. To show a cell is eukaryotic, point to a membrane-bound organelle. To show it is prokaryotic, point to the absence of a nucleus, 70S ribosomes in the cytoplasm or a peptidoglycan wall.
Concept 2 of 3: Plant and animal cells compared
Plant and animal cells share almost all their organelles, because both are eukaryotic. The differences follow from how a plant lives: it stands still, makes its own food from light, and holds itself up with water pressure inside a stiff wall.
Definition
Both have a nucleus, mitochondria, ER, Golgi, ribosomes, plasma membrane, cytoskeleton and peroxisomes.
- Only plants (green cells) have chloroplasts; all plant cells have mitochondria.
- Plants have a cellulose cell wall, a large central vacuole, and store starch. Animals store glycogen.
- Animal cells have centrioles; most flowering plant cells do not.
- In plants the vacuole does much of the digestive work that lysosomes do in animal cells.
- Cytokinesis: an animal cell is pinched in two by a ring of actin; a plant cell builds a cell plate from Golgi vesicles between the two new nuclei.
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Cellulose wall outside the membrane | None |
| Chloroplasts | In green, photosynthetic cells | None |
| Vacuole | One large permanent central vacuole | Small, temporary vacuoles, if any |
| Centrioles | Absent in most flowering plants | A pair in the centrosome |
| Storage carbohydrate | Starch | Glycogen |
| Shape | Fixed, often box-like | Variable, often rounded |
| Cytokinesis | Cell plate | Cleavage furrow |
Practice this conceptself-check · 3 quick reps
Plant cells respire too
Plant cells have mitochondria as well as chloroplasts, and respire day and night. Only cells with chloroplasts photosynthesise; roots and other non-green parts only respire. An option saying all plant cells photosynthesise, or that plant cells lack mitochondria, is wrong.
Concept 3 of 3: Viruses: structure and why they are not cells
A virus is a set of instructions in a protective case. It has no machinery of its own: no ribosomes, no enzymes for making energy, no cytoplasm. Outside a cell it does nothing at all. Inside one, it takes over the cell's machinery to make copies of itself.
Definition
A virus is a particle of nucleic acid inside a protein coat (the capsid). Most are 20 to 300 nm across.
- Its genome is DNA or RNA, never both; it may be single- or double-stranded.
- Some viruses (HIV, influenza) have an envelope: a phospholipid membrane taken from the host cell as they leave, studded with viral glycoproteins.
- Retroviruses such as HIV carry reverse transcriptase, which copies their RNA into DNA.
- Viruses are not cells: no cytoplasm, no ribosomes, no metabolism. They reproduce only inside a living host cell (obligate intracellular parasites). They are not prokaryotes.
- Viruses that infect bacteria are bacteriophages.
- Antibiotics do not work on viruses, because they target bacterial walls, ribosomes and enzymes that viruses do not have.
| Part | Made of | Note |
|---|---|---|
| Genome | DNA or RNA | HIV and influenza use RNA; herpes viruses use DNA |
| Capsid | Protein subunits | Protects the genome; helical or many-sided shapes |
| Envelope (some viruses) | Host phospholipid bilayer with viral glycoproteins | Present in HIV and influenza; absent in bacteriophages |
| Enzymes (some viruses) | Proteins such as reverse transcriptase | Carried in the particle, used once inside the host |
Practice this conceptself-check · 3 quick reps
Viruses are not prokaryotes
Prokaryotes are cells: they have a membrane, cytoplasm and ribosomes. Viruses have none of these, so they are not prokaryotes and not cells. They have no internal compartments either, so an option linking compartmentalisation to viruses is wrong.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Prokaryotic and eukaryotic cells compared7 rows
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Examples | Bacteria and archaea | Protists, fungi, plants, animals |
| Typical size | About 1 to 5 µm | About 10 to 100 µm |
| DNA | One circular chromosome in the nucleoid, often plasmids | Linear chromosomes with histones in a nucleus, plus circular DNA in mitochondria and chloroplasts |
| Membrane-bound organelles | None | Nucleus, mitochondria, ER, Golgi, lysosomes and others |
| Ribosomes | 70S | 80S in the cytoplasm; 70S in mitochondria and chloroplasts |
| Cell wall | Usually present; peptidoglycan in bacteria | In plants (cellulose) and fungi (chitin); none in animals |
| Cell division | Binary fission | Mitosis and meiosis |
Plant and animal cells compared7 rows
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Cellulose wall outside the membrane | None |
| Chloroplasts | In green, photosynthetic cells | None |
| Vacuole | One large permanent central vacuole | Small, temporary vacuoles, if any |
| Centrioles | Absent in most flowering plants | A pair in the centrosome |
| Storage carbohydrate | Starch | Glycogen |
| Shape | Fixed, often box-like | Variable, often rounded |
| Cytokinesis | Cell plate | Cleavage furrow |
Viruses: structure and why they are not cells4 rows
| Part | Made of | Note |
|---|---|---|
| Genome | DNA or RNA | HIV and influenza use RNA; herpes viruses use DNA |
| Capsid | Protein subunits | Protects the genome; helical or many-sided shapes |
| Envelope (some viruses) | Host phospholipid bilayer with viral glycoproteins | Present in HIV and influenza; absent in bacteriophages |
| Enzymes (some viruses) | Proteins such as reverse transcriptase | Carried in the particle, used once inside the host |
Watch out for (3)
- Circular DNA and ribosomes do not prove a cell is prokaryotic→ Prokaryotic and eukaryotic cells compared
- Plant cells respire too→ Plant and animal cells compared
- Viruses are not prokaryotes→ Viruses: structure and why they are not cells
Drill the past-year questions on this topic
Open the bank with this topic pre-filtered.