IMAT Biology · Cell Structure and Membranes
Mitochondria, Chloroplasts, the Cytoskeleton and the Cell Wall
Mitochondria and chloroplasts handle energy and carry their own DNA; the cytoskeleton shapes and moves the cell; walls give plants, fungi and bacteria a rigid outer case.
Why this matters
The 2023, 2024 and 2025 ministry papers all asked about these structures: which have a double membrane, what the mitochondrial membranes are like, which organelles contain DNA, and what the cytoskeleton and centrioles are made of. The older papers asked which cells have the most mitochondria.
Concept 1 of 3: Mitochondria and chloroplasts: double membranes and their own DNA
Mitochondria release energy from food; chloroplasts capture energy from light. Both look like bacteria living inside the cell, and that is very likely what they once were. They still have two membranes, their own small circular DNA and bacterial-sized ribosomes, and they divide by splitting in two.
Definition
Only three structures in a eukaryotic cell have a double membrane: the nucleus, mitochondria and plastids (chloroplasts, and the starch-storing amyloplasts of roots).
- Mitochondrion: a permeable outer membrane and a folded, highly selective inner membrane (folds called cristae) around the matrix. Aerobic respiration: the Krebs cycle runs in the matrix, and most ATP is made on the inner membrane.
- Chloroplast: outer and inner membranes around the stroma, plus a third internal system of thylakoids, stacked into grana. Light-dependent reactions on the thylakoids; sugar made in the stroma (Calvin cycle, enzyme RuBisCO). Starch grains are stored there.
- Endosymbiotic theory: both came from bacteria engulfed by an early eukaryote. Evidence: double membrane, circular DNA without histones, 70S ribosomes, division by binary fission.
- So DNA is found in the nucleus, mitochondria and chloroplasts, and nowhere else in the cell. Mitochondrial DNA is inherited from the mother.
- Cells with high energy demand have the most mitochondria: heart muscle, sperm (midpiece), kidney tubule and liver cells. Mature red blood cells have none.
- Plant cells have mitochondria as well as chloroplasts, and only their green cells have chloroplasts. A root cell respires but cannot photosynthesise.
| Feature | Mitochondrion | Chloroplast |
|---|---|---|
| Found in | Almost all eukaryotic cells | Green cells of plants and algae |
| Membranes | Outer membrane plus a folded inner membrane (cristae) | Outer and inner membrane, plus thylakoids stacked in grana |
| Fluid inside | Matrix | Stroma |
| Job | Aerobic respiration, making most of the cell's ATP | Photosynthesis, making sugar from and water |
| Own genetic system | Circular DNA and 70S ribosomes | Circular DNA and 70S ribosomes |
Practice this conceptself-check · 4 quick reps
Two membranes, not three
A mitochondrion has an outer and an inner membrane, with an intermembrane space between them. The space is not a third membrane, and the outer membrane is a normal bilayer, not a monolayer. In a chloroplast the thylakoids are a separate internal system, not part of the envelope.
Sugar units are in almost every organelle
Nucleotides contain a sugar (ribose in RNA and ATP, deoxyribose in DNA). So any structure with DNA, RNA or ATP contains carbohydrate monomers: the nucleolus, mitochondria and chloroplasts all do, as do the glycoproteins of every membrane and the cellulose of the wall.
Concept 2 of 3: The cytoskeleton: microtubules, microfilaments and intermediate filaments
The cytoplasm is not a loose soup. A network of protein fibres holds the cell's shape like tent poles and ropes, and also acts as rail tracks along which motor proteins drag vesicles and chromosomes. Different jobs use fibres of different thickness.
Definition
The cytoskeleton has three kinds of protein fibre:
- Microfilaments: the thinnest, made of actin. Cell shape, muscle contraction (with myosin), amoeboid movement, pinching an animal cell in two.
- Intermediate filaments: middle thickness, made of several proteins (keratin in skin, lamins in the nuclear lamina). Mechanical strength.
- Microtubules: the thickest, hollow tubes of tubulin. The mitotic spindle, tracks for vesicles, and the core of centrioles, cilia and flagella.
- Motor proteins walk along the fibres: myosin on actin, kinesin and dynein on microtubules. They are not fibres themselves.
- A centriole is a ring of nine triplets of microtubules. Animal cells have a pair in the centrosome, which organises the spindle; most flowering plants have none.
- Eukaryotic cilia and flagella have nine pairs of microtubules round a central two (9 + 2). Bacterial flagella are different: a rotating protein filament with no microtubules.
- Collagen is outside the cell (extracellular matrix), not part of the cytoskeleton.
| Fibre | Protein | Diameter | Main jobs |
|---|---|---|---|
| Microfilament | Actin | About 7 nm | Shape, muscle contraction, cell movement, cytokinesis in animals |
| Intermediate filament | Keratin, lamins and others | About 10 nm | Strength; anchors desmosomes; supports the nucleus |
| Microtubule | Tubulin | About 25 nm, hollow | Spindle, vesicle tracks, centrioles, cilia and flagella |
Practice this conceptself-check · 3 quick reps
Myosin, kinesin and dynein are motors, not cytoskeleton fibres
The three fibres of the cytoskeleton are microtubules, microfilaments and intermediate filaments. Myosin, kinesin and dynein are motor proteins that move along them, and collagen is an extracellular protein. Lists built from motors or collagen are wrong.
Concept 3 of 3: Cell walls in plants, fungi and bacteria
A wall is a rigid case outside the plasma membrane. It stops a cell bursting when water rushes in, and gives it a fixed shape. It is full of holes, so it lets almost everything through; the membrane inside it still does the choosing.
Definition
A cell wall lies outside the plasma membrane. Its material depends on the group.
- Plants: cellulose microfibrils set in other polysaccharides (pectins, hemicelluloses). Neighbouring walls are glued by a pectin layer, the middle lamella. Woody cells add lignin.
- Fungi: mainly chitin.
- Bacteria: peptidoglycan (murein). Penicillin stops the cross-links of peptidoglycan forming, so growing bacteria burst; human cells have no peptidoglycan, so they are unharmed.
- Animals have no cell wall; their cells sit in an extracellular matrix of collagen and other proteins.
- The wall is freely permeable; the plasma membrane is the selectively permeable barrier.
| Group | Main wall material | Key point |
|---|---|---|
| Plants | Cellulose | Resists turgor pressure; crossed by plasmodesmata |
| Fungi | Chitin | Same polysaccharide as insect exoskeletons |
| Bacteria | Peptidoglycan | Target of penicillin |
| Archaea | No peptidoglycan; other polymers or protein | Not affected by penicillin |
| Animals | No wall | Extracellular matrix of collagen instead |
Practice this conceptself-check · 3 quick reps
The outermost selective layer of a plant cell is the membrane, not the wall
A plant cell's wall is its outermost layer, but water and solutes pass through it freely. The selectively permeable barrier is the plasma membrane just inside it. An option calling the plant cell's outermost layer selectively permeable 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)
Mitochondria and chloroplasts: double membranes and their own DNA5 rows
| Feature | Mitochondrion | Chloroplast |
|---|---|---|
| Found in | Almost all eukaryotic cells | Green cells of plants and algae |
| Membranes | Outer membrane plus a folded inner membrane (cristae) | Outer and inner membrane, plus thylakoids stacked in grana |
| Fluid inside | Matrix | Stroma |
| Job | Aerobic respiration, making most of the cell's ATP | Photosynthesis, making sugar from and water |
| Own genetic system | Circular DNA and 70S ribosomes | Circular DNA and 70S ribosomes |
The cytoskeleton: microtubules, microfilaments and intermediate filaments3 rows
| Fibre | Protein | Diameter | Main jobs |
|---|---|---|---|
| Microfilament | Actin | About 7 nm | Shape, muscle contraction, cell movement, cytokinesis in animals |
| Intermediate filament | Keratin, lamins and others | About 10 nm | Strength; anchors desmosomes; supports the nucleus |
| Microtubule | Tubulin | About 25 nm, hollow | Spindle, vesicle tracks, centrioles, cilia and flagella |
Cell walls in plants, fungi and bacteria5 rows
| Group | Main wall material | Key point |
|---|---|---|
| Plants | Cellulose | Resists turgor pressure; crossed by plasmodesmata |
| Fungi | Chitin | Same polysaccharide as insect exoskeletons |
| Bacteria | Peptidoglycan | Target of penicillin |
| Archaea | No peptidoglycan; other polymers or protein | Not affected by penicillin |
| Animals | No wall | Extracellular matrix of collagen instead |
Watch out for (4)
- Two membranes, not three→ Mitochondria and chloroplasts: double membranes and their own DNA
- Sugar units are in almost every organelle→ Mitochondria and chloroplasts: double membranes and their own DNA
- Myosin, kinesin and dynein are motors, not cytoskeleton fibres→ The cytoskeleton: microtubules, microfilaments and intermediate filaments
- The outermost selective layer of a plant cell is the membrane, not the wall→ Cell walls in plants, fungi and bacteria
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