Cell Membranes & Transport

1. The Phospholipid Bilayer

Key idea: ALL membranes are organised in a phospholipid bilayer – not just the cell surface membrane (CSM).

• Mitochondria – outer membrane (OM) + inner membrane (IM, folded into cristae)
• Nucleus – double nuclear membrane (with nuclear pores for transport)
• Golgi body – stack of flattened membranous sacs = cisternae (not interconnected)
• RER / SER – stacks of interconnected membranes (cisternae), continuous with each other
• Lysosomes / Golgi vesicles – single-membrane-bound vesicles; the CSM is the cell’s outer boundary

Triglyceride vs phospholipid

• Triglyceride = glycerol + 3 fatty acids → no polar part → non-polar, water-insoluble
• Phospholipid = glycerol + 2 fatty acids + phosphate group bonded to the 3rd –OH of glycerol
• Phosphate carries a charge (both O⁻) → charged/polar species attract water (H-bonds) → hydrophilic head
• Fatty-acid tails = only C–H bonds → non-polar (no uneven charge, no ions) → hydrophobic

Drawing the phospholipid

Drawing the whole structure every time is slow, so use the standard symbol: ○ = phosphate head (hydrophilic), ‖ = two fatty-acid tails. One ○‖ = one phospholipid molecule.

• Outside the cell = tissue fluid (aqueous). Inside = cytoplasm (aqueous). Both are mostly water.
• So heads (hydrophilic) always point towards the aqueous environment on both sides; tails (hydrophobic) always point away from water → sandwiched in the middle.
• Sandwich picture: phosphate heads = the two slices of bread, fatty-acid tails = the filling.
• Membrane width ≈ 7 nm = also called the diffusion distance (the thickness a particle must cross).

What can cross the bilayer directly?

• Non-polar molecules (O₂, CO₂, fat-soluble vitamins, steroid/lipid hormones) slip through – the bilayer is mostly lipid (tails make up most of the membrane), so they overcome the small repulsion from the heads.
• Polar / charged / large molecules (glucose, amino acids, ions) cannot – polar molecule can pass the polar heads but the non-polar tails repel it.
• Water is the exception: polar but tiny (M_r = 18) → crosses directly (osmosis), but slowly because of repulsion; faster via aquaporins.

2. The Fluid Mosaic Model

Proposed by: Singer & Nicolson. The membrane = a fluid phospholipid bilayer with a mosaic of different molecules scattered through it.

Component	Where / what it is
Phospholipid bilayer	two layers; heads out, tails in; the basic fabric
Integral (intrinsic) protein	embedded in the bilayer – spans all/part of the thickness
Peripheral (extrinsic) protein	only on the surface (periphery) of the bilayer
Glycolipid	phospholipid + carbohydrate chain (only SOME phospholipids)
Glycoprotein	protein + carbohydrate chain (only SOME membrane proteins)
Cholesterol	small molecules wedged between phospholipid tails/heads (sandwiched, not on the boundary)

“Why FLUID?”

• Not because it contains water! Compare solid (tightly packed, fixed, only vibrate) and liquid (particles slide past each other).
• Phospholipids are free to move via diffusion within their monolayer (swap places sideways) – like a liquid.
• Catch: movement is within one monolayer; proteins move too but much less than phospholipids.

“Why MOSAIC?”

• A mosaic = many different stones, shapes, sizes, colours in a scattered pattern that still forms one picture.
• Membrane = phospholipids + integral & peripheral proteins + glycoproteins + glycolipids + cholesterol: different molecules of different sizes, scattered (no fixed pattern) → mosaic-like appearance.

3. Membrane Fluidity & Cholesterol

Definition: Membrane fluidity is a parameter of the freedom of movement of phospholipids and protein molecules (mostly we talk about phospholipids – they diffuse more than proteins).

• More fluid = more / easier movement (like water flowing freely). Less fluid = restricted movement (like honey – viscous).
• Fluidity depends on the kinetic energy of the molecules and how much room they have.

Factors affecting fluidity

① Length of fatty-acid tail

Longer tail → more C–H bonds → larger M_r (heavier molecule) → moves more slowly → fluidity ↓. (thin runner vs heavier runner – the lighter one reaches the finish first)

② Number (ratio) of unsaturated FA tails

Saturated = straight, close packing (like a narrow lane – hard for a car to pass). Unsaturated = C=C double bond gives a kink → loose packing, bigger gaps (like a main road) → fluidity ↑.

③ Temperature

↑ temp → ↑ kinetic energy → ↑ vibration/movement → fluidity ↑. ↓ temp → KE ↓ → fluidity ↓. (temp is the factor that changes KE)

④ Cholesterol – the regulator / stabiliser

• Structure: –OH head = polar (δ⁻ on O, δ⁺ on H) → hydrophilic, interacts with phosphate heads; rest of molecule is all C–H = non-polar → hydrophobic, interacts with fatty-acid tails.
• Both too fluid and too rigid are bad (like an enzyme’s optimum): need a balance. Cholesterol stabilises (regulates) membrane fluidity – it does the OPPOSITE job depending on temperature:

Condition	Membrane without cholesterol	What cholesterol does
Low temp	less KE → PLs pack closely → ↓ fluid	pushes PLs apart (like tickling a baby so it wriggles) → disrupts close packing → fluidity ↑
High temp	too much KE → PLs far apart, loose → too fluid	OH group + ring attract heads & tails → pulls PLs closer together → fluidity ↓

How much cholesterol? (animal vs plant vs bacteria)

Cell type	Cholesterol	Why
Animal	PL : cholesterol ≈ 1 : 1 (≈50 PL : 40–50 cholesterol)	no cell wall → cholesterol is the main stabiliser
Plant	ratio PL : C is less (relatively little C)	high-tensile cellulose cell wall gives extra stability/protection
Bacteria (prokaryote, unicellular)	none	peptidoglycan cell wall

Cholesterol, insulation & nerves

• Lipids give thermal insulation (adipose/fat tissue under skin, round kidneys, heart – fat people feel less cold) and electrical insulation.
• Myelin sheath (many cholesterol-containing membranes around the axon) insulates the nerve: impulses are carried by ions (Na⁺, K⁺, Ca²⁺ – Cl⁻ too) and cannot cross myelin, so they jump between nodes of Ranvier.
• Cholesterol keeps membranes from being too fluid so important ions don’t leak out and unwanted polar/charged substances don’t enter – and not too rigid, so needed substances can still move (A2: coordination).

4. Membrane Proteins

Two types – together called membrane proteins:

	Integral = INTRINSIC	Peripheral = EXTRINSIC
Position	inside the bilayer – span the entire or part of the internal thickness	only on the surface / periphery (edge); face outside only
Memory	intrinsic = inside	extrinsic = on the outside; periphery = boundary (like a cricket ground’s boundary)
Types	Transmembrane (entire thickness) – syllabus; intra-membrane (part) – not needed	—

Why do we need transmembrane proteins?

Large, polar or charged molecules (glucose, amino acids) and ions cannot diffuse through the bilayer (tails repel them). They need a “road” – a channel or a carrier protein.

	CHANNEL protein	CARRIER protein
Pore	water-filled pore / channel	no water-filled pore
Binding site	none (shape of pore not important)	binding site complementary to the molecule (specific, like enzyme–substrate)
Shape	does not change (no conformational change)	changes shape (conformational change) when molecule binds → releases it on other side
Gradient / energy	only down the gradient → passive, no ATP	down gradient = passive (no ATP) OR against gradient = active (ATP needed)
Examples	Na⁺, Cl⁻, K⁺, Ca²⁺ channels; aquaporin (water)	GLUTs (glucose), AATs (amino acids); carrier pumps e.g. Na⁺–K⁺ pump

• Water in channels: water is polar – attracts ions/polar molecules by H-bonding, so the water-filled pore lets them pass.
• Carrier proteins that do active transport are called carrier pumps (like a balloon pump – needs effort/energy).
• Similarities: both transmembrane · both transport polar molecules & charged ions · both specific for what they transport (Na⁺ won’t go through a Cl⁻ channel).

5. Glycolipids & Glycoproteins

Carbohydrate chains stick out on the outer surface (towards tissue fluid). Different chains = different shapes → each shape is recognised by a specific molecule.

• Receptors: different cells have different receptors depending on function. e.g. ADH only binds to cells of the kidney collecting duct (they have ADH receptors) – not liver or heart cells.
• Analogy: hormone = courier, receptor = house address/nameplate – the message is delivered only to the matching “address” (complementary shape).
• Signalling receptors recognise messenger molecules (hormones, neurotransmitters); when the messenger binds, a series of chemical reactions starts inside the cell.

6. Transport Across the Membrane – Overview

	PASSIVE	ACTIVE
Energy	no ATP needed	ATP needed (ATP = the cell’s energy currency)
Direction	down the concentration gradient (high → low) – like walking down a hill (no effort)	against the gradient (low → high) – like climbing a hill
Processes	simple diffusion · facilitated diffusion · osmosis	active transport · bulk transport (endo-/exocytosis)

• Concentration gradient = unequal distribution of substances (a “slope”); Δ conc. gradient = OUT − IN (or IN − OUT) – it is a difference; gradient = 0 means equal distribution.
• Diffusion always: down the gradient · passive · no energy. Two types → simple (direct through the bilayer) and facilitated (via channel/carrier).
• Osmosis = special/water-specific case of simple diffusion.

7. Simple Diffusion

Definition: the net movement of molecules down the concentration gradient – passive, no ATP, directly through the bilayer with no protein/mediator.

• Molecules move randomly both ways; net movement is high → low until the gradient is gone.
• Only small, simple, non-polar molecules: O₂, CO₂ (non-polar: equal pull on both sides cancels, net dipole = 0), fat-soluble vitamins, lipid-based / steroid hormones (testosterone, oestrogen, progesterone), and H₂O → called osmosis.
• Can’t: polar molecules, ions (Na⁺, K⁺, Ca²⁺, Cl⁻), large molecules. Peptide (protein) hormones are polar → can’t cross.
• Barrier analogy: a child ducks under a closed barrier effortlessly (small non-polar); a truck needs the barrier lifted (needs a protein + energy).

Factors affecting the rate of simple diffusion

Factor	Relationship	Why
① Temperature	directly related	↑ KE → faster movement
② Concentration gradient	directly related	bigger difference → faster net movement (NOT just concentration)
③ Surface area of membrane	directly related	folding gives more places to cross (flat = one dimension only)
④ Molecular mass / size	inversely related	smaller particles diffuse quicker
⑤ Membrane thickness	inversely related	thickness = diffusion distance; longer to cross

Temperature is left out of Fick’s expression – the law looks at the other factors at constant temperature.

8. Facilitated Diffusion

Definition: movement of molecules down the concentration gradient through carrier / channel proteins (the “facilitators”).

• Passive – no ATP. Same 3 properties as all diffusion: down gradient · passive · no energy.
• Carrier proteins used here = the passive carriers (NOT carrier pumps).
• For polar molecules & charged ions – they can’t go through the bilayer so they need a facilitator.

Transport protein	Moves
GLUTs – glucose transporters	glucose (needed for respiration – must get into cells)
AATs – amino acid transporters	amino acids
Na⁺ / Cl⁻ / K⁺ (and Ca²⁺) channels	that specific ion only
Aquaporin (aqua = water, porin = pore)	H₂O (water-filled pore)

Which factors affect FD?

Factor	SD	FD
Temperature	yes	yes (KE of molecules moving through channel/carrier)
Concentration gradient	yes	yes – but only up to a point (see graph)
Surface area	yes	yes (more membrane = more proteins/entry angles)
Size / Mr	yes	yes (big thing in narrow channel = slow)
Membrane thickness	yes	NO – substance doesn’t cross the thickness, it goes through the protein
No. of carrier / channel proteins	n/a	YES – extra factor!

• SD: straight line – bigger gradient always gives faster rate.
• FD: part 1 – rate rises with gradient (gradient = limiting factor); part 2 – plateau: all carriers/channels are saturated/busy, so number of carrier/channel proteins = limiting factor. (Like enzymes: more substrate doesn’t help once all active sites are occupied.)
• Analogy: door of capacity – 40 students outside, one door, they enter one at a time; adding more outside doesn’t speed it up. Or a 50-seater bus carrying 100 people.
• If you also add more proteins, you get a straight line again.

Simple vs facilitated – comparison

Similarities (both)	Difference
① passive transport ② movement down conc. gradient ③ due to kinetic energy of the molecules	FD: through channel/carrier proteins SD: through the bilayer

9. Active Transport

The 2 energy-requiring processes: active transport and bulk transport (endocytosis / exocytosis). Active transport moves individual molecules/ions; bulk moves large molecules in bulk.

• Cells with a high rate of active transport: ① abundant mitochondria (make ATP) ② adequate supply of O₂ (aerobic respiration).
• Mature RBCs lose nucleus + mitochondria (more room for haemoglobin) → only anaerobic respiration (glycolysis in cytoplasm, net 2 ATP vs 36–38 aerobic) → active transport is slow, but still possible. O₂ is only needed at the last stage which happens in mitochondria.

Why is ATP needed?

Carrier must change shape to move the substance. ATP is the “energy currency”: the enzyme ATPase hydrolyses it:

The energy powers the conformational change; the phosphate attaches to the pump. (Don’t confuse: ATP synthase makes ATP in mitochondria; ATPase breaks it.) The high-energy bonds between phosphates are like a bag of stored energy – break the bond, energy is released.

The Na⁺–K⁺ pump (must know)

• A transmembrane carrier protein / pump. Inside the cell: Na⁺ low, K⁺ high. Outside: Na⁺ high, K⁺ low. Always.
• Pumps 3 Na⁺ OUT and 2 K⁺ IN – both against their gradients – using ATP.

1. 3 Na⁺ bind (pump open to inside). ATP binds its own site → ATPase makes ADP + P_i; P stays attached, energy released.
2. Energy → shape change: pump now opens outwards → 3 Na⁺ released outside.
3. 2 K⁺ bind from outside; the pump is “choked” – the attached phosphate acts like a tight screw keeping the mouth shut.
4. P_i is removed (screw loosened) → pump flips back, opens inwards → 2 K⁺ released into cytoplasm.

10. Bulk Transport

Definition: process involving the uptake or release of large molecules using energy (ATP) – active process, no pump/carrier; works by membrane-bound vesicles.

Endocytosis – substance moves INTO the cell

Active uptake of large molecules by inward folding of the cell membrane to form a membrane-bound endocytic vesicle. (endo = into).

1. Particle/fluid reaches CSM and presses it inwards.
2. Membrane folds inward more and more until a complete vesicle forms.
3. Vesicle pinches off and moves into the cytoplasm (it is membrane-bound because the membrane itself folded).

Type	What is taken in	Vesicle
Phagocytosis	solid particle (e.g. bacterium)	phagosome (food vacuole)
Pinocytosis	soluble fluid (water + dissolved solutes – not “water movement”, water uses osmosis/aquaporins)	pinosome / vesicle
Receptor-mediated	specific molecules: they bind to receptors first (glycoproteins on CSM), then endocytic vesicle forms with receptor still attached	coated endocytic vesicle

Exocytosis – substance moves OUT of the cell

Active secretion (release) of large molecules by fusion of membrane-bound vesicles with the cell surface membrane. (exo = out)

• Eg. protein/enzyme made via transcription (nucleus) → translation (ribosome) → folded in RER → modified in Golgi (e.g. carbohydrate added → glycoprotein) → packed in Golgi vesicle → vesicle fuses with CSM → contents secreted (e.g. extracellular digestive enzymes).
• Vesicle membrane becomes part of the CSM – no problem, both are the same phospholipid bilayer.
• Eg. WBC (phagocyte): engulfs bacterium (phagocytosis) → digestive enzymes digest it → waste left in the same vesicle → vesicle fuses with CSM → waste released by exocytosis. Cells must get rid of toxic metabolic waste.

11. Cell Signalling

Definition: the molecular mechanism by which cells detect and respond to external stimuli, including communication between cells – it explains how organisms control and coordinate their bodies.

• Stimulus = any change from normal conditions (e.g. ↑ blood glucose after sweets).
• Ligand = the signalling (messenger) molecule. Two pathways: electrical – neurotransmitters (travel between neurons); chemical – hormones (travel in the bloodstream).
• Cell A = receptor cell = site of production of the ligand → Cell B = target / effector cell that carries out the response.

The 4 steps

1. A stimulus causes a cell to secrete a specific chemical = ligand.
2. Ligand (signalling molecule) is transported to the target cell (it must find the right cell among millions).
3. Ligand binds to cell surface membrane receptors on the target cell. Receptor is specific and complementary to the ligand’s shape.
4. This binding results in transduction.

Example – insulin & G-protein receptor (surface level)

• Insulin (ligand) made by pancreas (islets of Langerhans; beta cells – A2) when blood glucose is high. Target cells = liver & muscle → they convert glucose → glycogen (store) → blood glucose falls.
• Ligand does not enter the cell – it just binds to the CSM receptor. Receptor (with G protein) changes shape (= transduction) → enzyme makes second messengers inside.
• Courier analogy: courier (ligand) knocks at the gate (receptor); a family member (second messenger) inside carries the message to the person it is for.
• 1 ligand → many second messengers (amplification) → activate enzymes → enzyme cascade (rxn A →[enz A]→ rxn B →[enz B]→ rxn C) → response: secretion, transcription, movement, metabolic change.

Hormone type decides where the receptor is

• Lipid-based (steroid) hormones: hydrophobic → diffuse through bilayer → cytoplasmic receptor.
• Peptide-based hormones: hydrophilic, can’t cross → bind CSM receptors (glycolipids / glycoproteins – “gL, gP”).

12. Osmosis & Water Potential

Definition: osmosis is the movement of water molecules from a region of high water potential to a region of low water potential across a selectively permeable membrane.

• Passive process (no ATP). A special case of simple diffusion (water-specific); no carrier/channel needed.
• Always say water potential, never “water concentration”. Two conditions: water molecules + selectively permeable membrane.
• Only water moves – solutes are usually too large to cross.
• Movement is both ways; the net movement is high → low ψ (like net force: 14 N one way vs 10 N the other → net 4 N).

Like a person’s “potential” (capability): more water molecules relative to solute → greater tendency/capability to leave.

Quantifying water potential

• Symbol: ψ (Greek letter psi); unit: kPa (kilopascal – a unit of pressure, because moving water exerts pressure).
• Pure water has the highest ψ = 0 kPa (an arbitrary value; no solute, most water molecules).
• Every other solution: ψ < 0 (negative): cell cytoplasm, vacuole sap, any solution.
• More negative ψ = lower water potential (−40 kPa < −10 kPa). Less negative = relatively higher ψ.

Factors affecting ψ

	SOLUTE	PRESSURE
Effect	greater solute conc. → lower ψ (more −ve)	greater pressure → higher ψ
Contribution =	SOLUTE POTENTIAL ψs	PRESSURE POTENTIAL ψp
Sign	ψs ≤ 0 always negative	ψp ≥ 0 always positive (0 if no pressure)

• Pure water: no solute, no pressure → ψ_s = 0, ψ_p = 0, ψ = 0.
• Temperature is not used here – it affects solute and water KE, not just water’s tendency to leave.

Osmosis in animal cells (RBC)

Outside	Water moves	Observation (RBC)
0.5% NaCl – hypotonic (less solute)	IN	swells; loses biconcavity (swollen/thicker); osmotic lysis (bursts)
0.9% NaCl – isotonic	no NET movement	no change in shape, size, volume
1.3% NaCl – hypertonic (more solute)	OUT	shrinks (volume ↓); loses biconcavity (edges thin); crenates (spiky)

• Both anomalies lose biconcavity, but for different reasons: swelling vs shrinking. Biconcave shape = ↑ SA:V for O₂ uptake.
• Animal cell: no cell wall → ψ_p is always 0 → ψ = ψ_s.
• Write observations in exam: ① swells ② loses biconcavity ③ lysis (bursts) – word is osmotic lysis.

Osmosis in plant cells

• Protoplasm = living contents inside the cell = nucleus + cytoplasm (not plasma!).
• Completely plasmolysed (hypertonic outside): water leaves cytoplasm + vacuole → membrane pulls away from wall (only a few points still attached), vacuole shrinks. ψ_p = 0 ⇒ ψ = ψ_s. Plasmolysis = contraction of the protoplast due to water loss; nothing is pushing, water is just being pulled out.
• Completely turgid (hypotonic): water enters, vacuole ↑ volume, membrane pushes against the cell wall (turgor pressure). Cell doesn’t burst – cellulose wall is high-tensile. ψ = 0; ψ_p = −ψ_s (e.g. ψ_p = +x, ψ_s = −y).
• Incipient plasmolysis (isotonic = equilibrium): membrane just touching the cell wall; point at which plasmolysis is about to occur – protoplast exerts no pressure on the wall. ψ_p = 0, ψ = ψ_s.

State	Outside	ψp	ψ
Completely turgid	hypotonic	= −ψs (positive)	0
Incipient plasmolysis	isotonic (equilibrium)	0	ψs
Completely plasmolysed	hypertonic	0	ψs

13. Surface Area : Volume Ratio

Why does size matter for diffusion? Work out the SA:V of 3 cubes (sides 1, 2, 3 cm):

• Smallest cube = highest SA:V (SA ∝ l², V ∝ l³, so SA:V ∝ 1/l).
• As size ↓ → SA:V ↑ → shorter diffusion distance → higher rate of diffusion.
• Larger object → SA:V ↓ → diffusion distance ↑ → slower diffusion (why big organisms need transport systems).

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