IMAT Biology · Free notes

Cell Structure and Membranes

Cell Structure and Membranes has 45 past questions since 2011, and the ministry papers from 2023 on have asked 15 of them. The ministry items are short recall: one fact about an organelle, the membrane or a transport protein, chosen from five close options. The older papers asked the same facts inside "which statements are correct" lists, and added magnification sums and concentration data. The difficulty is in near misses: a structure with RNA but no DNA, a pH on the wrong side of 7, a membrane counted once too often, a carrier protein that is not always active.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

IMAT home All formulas on one page

Subtopic notes

Formula & revision sheet

4 formulas · 18 reference tables · 24 gotchas across all subtopics — the exam-eve cheat-sheet

Cell Theory, Cell Size and Microscopes

Formulas (2)

Magnification: image size, actual size and scale bars · Magnification $M = \frac{I}{A}$
Surface area to volume ratio and the limit on cell size · Ratio for a cube $\frac{\text{surface area}}{\text{volume}} = \frac{6L^2}{L^3} = \frac{6}{L}$

Reference tables (2)

Cell theory and the features of living things4 rows

Statement	What it means	Exam angle
All living things are made of cells	One cell (unicellular) or many (multicellular)	Viruses are not made of cells, so they are not covered
The cell is the basic unit of life	It is the smallest thing that shows every feature of life	An organelle taken out of a cell cannot live on its own
Cells come only from cells	New cells form when existing cells divide	Rules out life arising from non-living matter today
Cells carry hereditary information	All cells use DNA as their genetic material	Some viruses use RNA, which is one reason they are not cells

Sizes of cells, organelles and viruses9 rows

Structure	Typical size	Seen with
Ribosome	About 25 nm	Electron microscope only
Most viruses	About 20 to 300 nm	Electron microscope only
Bacterium such as E. coli	About 2 µm long, under 1 µm wide	Light microscope, but no inner detail
Mitochondrion	About 0.5 to 1 µm wide, 1 to 5 µm long	Light microscope as a speck; inner membranes need EM
Nucleus	About 5 to 10 µm	Light microscope
Human red blood cell	About 7 to 8 µm across	Light microscope
Typical animal cell (liver cell)	About 20 to 30 µm	Light microscope
Plant cell (onion epidermis)	A few hundred µm long	Light microscope, low power
Human egg cell	About 0.1 mm	Just visible to the naked eye

Rough order to remember: ribosome < virus < bacterium ≈ mitochondrion < nucleus < red blood cell < most animal cells < plant cells.

Watch out for (4)

A cell does not need a nucleus to be a cell
→ Cell theory and the features of living things
More magnification does not mean more detail
→ Sizes of cells, organelles and viruses
Divide by the magnification; do not multiply
→ Magnification: image size, actual size and scale bars
The ratio falls as a cell grows
→ Surface area to volume ratio and the limit on cell size

The Plasma Membrane and the Fluid Mosaic Model

Reference tables (4)

Phospholipids and the lipid bilayer4 rows

Part	Made of	Behaviour in water	Position in the bilayer
Head	Phosphate group (plus glycerol, often choline)	Hydrophilic, polar	Faces the water on both surfaces
Tails	Two fatty acid chains	Hydrophobic, non-polar	Point inward, forming the core
Saturated tail	Only C-C single bonds, so straight	Hydrophobic	Packs tightly; makes the membrane less fluid
Unsaturated tail	One or more C=C double bonds, each making a kink	Hydrophobic	Packs loosely; makes the membrane more fluid

The fluid mosaic model: proteins, cholesterol and carbohydrates5 rows

Component	Where it sits	Main job
Phospholipids	The bilayer itself	Barrier to ions and polar molecules; gives fluidity
Integral proteins	Embedded in the bilayer, often spanning it	Channels, carriers, pumps, receptors
Peripheral proteins	On the inner or outer surface	Enzymes, links to the cytoskeleton, signalling
Cholesterol	Between phospholipids in both layers (animal cells)	Keeps fluidity within limits
Glycoproteins and glycolipids	Outer surface, sugar chains facing out	Cell recognition, receptors, blood group antigens

Membrane fluidity: temperature, unsaturated tails and cholesterol5 rows

Factor	Effect on fluidity	Why
Higher temperature	More fluid	Molecules have more kinetic energy
More unsaturated tails (C=C)	More fluid	Kinks stop the tails packing closely
Shorter fatty acid tails	More fluid	Fewer attractions between neighbouring tails
Cholesterol, at body or higher temperature	Less fluid	Its rigid rings restrict tail movement
Cholesterol, at low temperature	Stops the membrane setting solid	It spaces the tails so they cannot pack

Cell junctions and cell signalling4 rows

Junction	Structure	Job	Example
Tight junction	Rows of proteins that fuse neighbouring membranes	Seals the gap so fluid cannot leak between cells	Gut lining, kidney tubules, brain capillaries
Desmosome	Protein plaques joined across the gap, anchored to intermediate filaments	Holds cells together under stretching	Skin, heart muscle
Gap junction	Channels of connexin proteins linking two cytoplasms	Lets ions and small molecules pass straight between cells	Heart muscle, smooth muscle
Plasmodesma (plants)	Membrane-lined channel through two cell walls	Joins the cytoplasm of neighbouring plant cells	Almost all living plant tissues

Watch out for (4)

Phospholipids, not triglycerides, and two layers, not one
→ Phospholipids and the lipid bilayer
The membrane is not a protein sandwich
→ The fluid mosaic model: proteins, cholesterol and carbohydrates
Cholesterol regulates fluidity; it is not an energy store or a universal hormone source
→ Membrane fluidity: temperature, unsaturated tails and cholesterol
Where the receptor is depends on the signal
→ Cell junctions and cell signalling

Diffusion, Osmosis and Active Transport

Formulas (2)

Osmosis, osmolarity and tonicity · Osmolarity of a solution $\text{osmolarity} = i \times c$
Active transport and the sodium potassium pump · One cycle of the sodium potassium pump $3\,\mathrm{Na^+_{in}} + 2\,\mathrm{K^+_{out}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na^+_{out}} + 2\,\mathrm{K^+_{in}} + \mathrm{ADP} + \mathrm{P_i}$

Reference tables (1)

Simple and facilitated diffusion: channels and carriers4 rows

Mechanism	Example of what crosses	Membrane protein?	ATP?	Direction
Simple diffusion	$\mathrm{O_2}$ , $\mathrm{CO_2}$ , steroid hormones	No	No	Down the gradient
Facilitated diffusion by channel	$\mathrm{Na^+}$ , $\mathrm{K^+}$ , $\mathrm{Cl^-}$ , water through aquaporins	Yes, channel	No	Down the gradient
Facilitated diffusion by carrier	Glucose into red blood cells	Yes, carrier	No	Down the gradient
Active transport	$\mathrm{Na^+}$ out and $\mathrm{K^+}$ in by the pump	Yes, carrier (pump)	Yes	Against the gradient

Watch out for (4)

"Uses a carrier protein" does not mean "active"
→ Simple and facilitated diffusion: channels and carriers
Water moves toward the higher solute concentration
→ Osmosis, osmolarity and tonicity
Count particles, not moles
→ Osmosis, osmolarity and tonicity
Secondary active transport still depends on ATP
→ Active transport and the sodium potassium pump

Nucleus, Ribosomes, ER, Golgi and Lysosomes

Reference tables (5)

The nucleus, nuclear envelope and nucleolus4 rows

Part	Structure	Job
Nuclear envelope	Two membranes; the outer one joins the rough ER	Separates DNA from the cytoplasm
Nuclear pores	Protein complexes through both membranes	Control traffic: mRNA out, proteins in
Chromatin	DNA wrapped round histones	Carries the genes
Nucleolus	Dense region with no membrane, containing DNA and RNA	Makes rRNA and assembles ribosome subunits

Ribosomes: what they are made of, their types and where they sit4 rows

Feature	Eukaryotic cytoplasm	Prokaryotes, mitochondria and chloroplasts
Whole ribosome	80S	70S
Subunits	60S and 40S	50S and 30S
Made of	rRNA and protein	rRNA and protein
Where found	Free in cytosol, or on rough ER and the outer nuclear membrane	Free in the cytoplasm, or in the mitochondrial matrix and chloroplast stroma

Endoplasmic reticulum and Golgi: the route of a secreted protein5 rows

Step	Structure	What happens
1	Ribosome on the rough ER	mRNA is translated; the growing chain is threaded into the ER lumen
2	Rough ER lumen	The protein folds and first sugar chains are added
3	Transport vesicle	Buds off the ER and carries the protein to the Golgi
4	Golgi apparatus	Modifies, sorts and packs the protein
5	Secretory vesicle	Moves to the plasma membrane and fuses with it, releasing the protein

Amino acids → ribosome → rough ER → vesicle → Golgi → vesicle → plasma membrane. The nucleus is not on the route.

Lysosomes, peroxisomes and vacuoles4 rows

Organelle	Membrane	Contents	Job
Lysosome	Single	Hydrolytic enzymes at about pH 5	Digestion of engulfed material and old organelles
Peroxisome	Single	Oxidases and catalase	Breaks down $\mathrm{H_2O_2}$ and fatty acids
Central vacuole (plants)	Single, the tonoplast	Cell sap	Turgor and storage
Contractile vacuole (protists)	Single	Water	Removes excess water

Endocytosis and exocytosis: moving large cargo in vesicles4 rows

Process	Direction	What moves	Example
Phagocytosis (cell eating)	Into the cell	Large particles or whole cells	Macrophages and neutrophils engulfing bacteria
Pinocytosis (cell drinking)	Into the cell	Small droplets of extracellular fluid	Most cells, continuously
Receptor-mediated endocytosis	Into the cell	Specific molecules bound to surface receptors	LDL (cholesterol) uptake by liver cells
Exocytosis	Out of the cell	Contents of secretory vesicles	Insulin from pancreas cells; neurotransmitter from neurons

Watch out for (5)

The nucleolus contains DNA and has no membrane
→ The nucleus, nuclear envelope and nucleolus
Ribosomes contain RNA but not DNA, and do not transcribe
→ Ribosomes: what they are made of, their types and where they sit
The Golgi modifies and ships proteins; it does not make them
→ Endoplasmic reticulum and Golgi: the route of a secreted protein
Lysosomal enzymes work below pH 7, not above
→ Lysosomes, peroxisomes and vacuoles
Neurons secrete too
→ Endocytosis and exocytosis: moving large cargo in vesicles

Mitochondria, Chloroplasts, the Cytoskeleton and the Cell Wall

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 $\mathrm{CO_2}$ 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

Prokaryotes, Eukaryotes, Plant and Animal Cells, and Viruses

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

PYQ weightage by concept

22 concepts · 0 PYQs — where the marks actually sit, so you know what to drill first

Cell Theory, Cell Size and Microscopes0 PYQs · 0%

Concept	PYQs	Share
Cell theory and the features of living thingsfoundation	—	—
Sizes of cells, organelles and virusesfoundation	—	—
Magnification: image size, actual size and scale barsfoundation	—	—
Surface area to volume ratio and the limit on cell sizefoundation	—	—

The Plasma Membrane and the Fluid Mosaic Model0 PYQs · 0%

Concept	PYQs	Share
Phospholipids and the lipid bilayerfoundation	—	—
The fluid mosaic model: proteins, cholesterol and carbohydratesfoundation	—	—
Membrane fluidity: temperature, unsaturated tails and cholesterolfoundation	—	—
Cell junctions and cell signallingfoundation	—	—

Diffusion, Osmosis and Active Transport0 PYQs · 0%

Concept	PYQs	Share
Simple and facilitated diffusion: channels and carriersfoundation	—	—
Osmosis, osmolarity and tonicityfoundation	—	—
Active transport and the sodium potassium pumpfoundation	—	—

Nucleus, Ribosomes, ER, Golgi and Lysosomes0 PYQs · 0%

Concept	PYQs	Share
The nucleus, nuclear envelope and nucleolusfoundation	—	—
Ribosomes: what they are made of, their types and where they sitfoundation	—	—
Endoplasmic reticulum and Golgi: the route of a secreted proteinfoundation	—	—
Lysosomes, peroxisomes and vacuolesfoundation	—	—
Endocytosis and exocytosis: moving large cargo in vesiclesfoundation	—	—

Mitochondria, Chloroplasts, the Cytoskeleton and the Cell Wall0 PYQs · 0%

Concept	PYQs	Share
Mitochondria and chloroplasts: double membranes and their own DNAfoundation	—	—
The cytoskeleton: microtubules, microfilaments and intermediate filamentsfoundation	—	—
Cell walls in plants, fungi and bacteriafoundation	—	—

Prokaryotes, Eukaryotes, Plant and Animal Cells, and Viruses0 PYQs · 0%

Concept	PYQs	Share
Prokaryotic and eukaryotic cells comparedfoundation	—	—
Plant and animal cells comparedfoundation	—	—
Viruses: structure and why they are not cellsfoundation	—	—