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Gas Exchange, Cell Membranes & Transport

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Edexcel IAL Biology · Unit 2

Gas Exchange, Cell Membranes & Transport

The big idea: living things need to move stuff (gases, water, nutrients) in and out of cells across membranes — and the whole chapter is really just different answers to "how does that movement happen, and how do organisms make it efficient?"

Summary — the whole chapter in one scan

Read this first. If every line makes sense, you're in great shape. If any line confuses you, jump to that section below.

Exchange surfaces (like alveoli, gills, leaves) are built to maximise diffusion — big surface area, thin/short diffusion pathway, steep concentration gradient.
As organisms get bigger, their surface area : volume (SA:V) ratio drops, so simple diffusion alone stops being enough — hence specialised exchange organs evolved.
Fick's Law gives the maths: rate of diffusion depends on surface area, concentration difference, and (inversely) the thickness of the membrane.
The mammalian lung is a real-world example of an exchange surface — trachea → bronchi → bronchioles → alveoli, all adapted for efficient gas exchange.
Cell membranes are built from a phospholipid bilayer — hydrophilic heads out, hydrophobic tails in — plus proteins, cholesterol, and glycoproteins/glycolipids. This whole structure is called the fluid mosaic model.
Scientific models of the membrane evolved over time (Gorter-Grendel → Davson-Danielli → Singer-Nicolson) as new evidence and technology appeared — a great example of how science actually works.
Osmosis is just a special, water-only case of diffusion — water moves from high to low water potential through a partially permeable membrane.
Substances cross membranes by four main routes: simple diffusion (small/non-polar, passive), facilitated diffusion (large/polar/charged, passive, via channel or carrier proteins), active transport (against the gradient, needs ATP, uses carrier proteins), and endocytosis/exocytosis (bulk transport of huge molecules via vesicles).
The Core Practical uses beetroot pigment leakage to investigate how temperature (and alcohol) affects membrane permeability.

Topic 1

Properties of Gas Exchange Surfaces

Why every exchange surface in biology "looks" similar, no matter which organism you're looking at.

Surface Area : Volume (SA:V) Ratio

Every organism needs to exchange gases (O₂ in, CO₂ out for respiration; CO₂ in, O₂ out for photosynthesis). This exchange happens by diffusion across an exchange surface.

Think of it like a warehouse and its loading dock. The "volume" is how much stuff is stored inside (how much oxygen the organism's cells need). The "surface area" is the loading dock — the only place goods can get in or out. A tiny warehouse has a huge dock relative to its stock, so deliveries are effortless. A giant warehouse with the same-shaped dock gets overwhelmed — not enough door space for how much needs to move in and out. That's exactly the SA:V problem organisms face as they grow.

As an organism's linear size increases, volume increases much faster than surface area (volume scales with length³, surface area only scales with length²). So SA:V ratio always falls as size increases:

1 cm cube: SA = 6 cm² Volume = 1 cm³ SA:V = 6:1 2 cm cube: SA = 24 cm² Volume = 8 cm³ SA:V = 3:1 3 cm cube: SA = 54 cm² Volume = 27 cm³ SA:V = 2:1 → Same shape, bigger size = SMALLER SA:V ratio

Common Mistake

"Surface area" and "surface area : volume ratio" are NOT the same thing, and examiners love catching students who mix them up. A blue whale has a way bigger surface area than a bacterium — but the whale's SA:V ratio is far smaller. Always be precise about which one a question is asking about.

Single-celled organisms have a high SA:V ratio, so simple diffusion across the cell surface membrane is enough to supply everything they need — and the small volume means nothing inside is ever far from the surface (short diffusion distance to organelles).

Large multicellular organisms have a low SA:V ratio relative to their needs, so they've evolved specialised exchange surfaces that boost the effective surface area available for exchange:

Alveoli — increase surface area of mammalian lungs
Gill lamellae — provide huge surface area in fish
Spongy mesophyll — exposes lots of leaf cell surface to air

Diffusion Pathway

Exchange surfaces are kept as thin as possible — often just one layer of flattened (squamous) epithelial cells — so the diffusion distance is as short as possible. Short distance = faster diffusion.

Concentration Gradient

A steep concentration gradient means faster diffusion. Exchange surfaces maintain this gradient by constantly "refreshing" both sides:

A good blood supply constantly removes the diffused gas from one side (e.g. blood removes O₂ from the alveoli side, so O₂ concentration there stays low, keeping the gradient steep)
Ventilation (breathing in/out) constantly refreshes the air side, replacing "used" air with fresh air

Practice Question

A student says "a large animal has a bigger surface area than a small animal, so gas exchange should be easier for it." Explain why this reasoning is flawed.

Topic 2

Fick's Law of Diffusion

The maths behind everything you just read.

Fick's Law formalises the three factors above into one relationship:

Fick's Law (proportionality)

rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of membrane

In plain words: diffusion goes faster with more surface area, a bigger concentration difference, or a thinner membrane — and slower if any of those get worse.

Fick's Law (as a calculable equation)

Rate = P × A × ((C₁ − C₂) ÷ T)

P = permeability constant (how easily this specific molecule crosses this specific membrane)
A = surface area
C₁ − C₂ = concentration difference between the two sides
T = thickness of the exchange surface

Key Insight

Proportionality means if you double the surface area or the concentration difference, the rate of diffusion doubles. If you halve the diffusion pathway (thickness), the rate doubles too. This is exactly why exchange surfaces are thin, large, and constantly refreshed — each adaptation directly speeds up diffusion according to this law.

Worked Example

A sample of alveolar epithelium is 1.5 μm thick with a surface area of 3 μm². Oxygen concentration in the alveolus is 1.8 × 10⁻¹⁶ mol μm⁻³ and in the blood is 7.5 × 10⁻¹⁷ mol μm⁻³. The permeability constant for oxygen is 0.012 molecule s⁻¹. Find the rate of diffusion.

Step 1 — Write out the equation and substitute:
Rate = P × A × ((C₁ − C₂) ÷ T)
Rate = 0.012 × 3 × ((1.8×10⁻¹⁶ − 7.5×10⁻¹⁷) ÷ 1.5)

Step 2 — Work out the concentration difference:
1.8×10⁻¹⁶ − 7.5×10⁻¹⁷ = 1.05×10⁻¹⁶

Step 3 — Divide by thickness:
1.05×10⁻¹⁶ ÷ 1.5 = 7×10⁻¹⁷

Step 4 — Multiply by P and A:
0.012 × 3 × 7×10⁻¹⁷ = 2.52×10⁻¹⁸ molecules μm⁻² s⁻¹

Exam Tip

You'll always be given the equation in the exam — you don't need to memorise it. What you DO need is to understand what each part means, so you can explain (in words) why a change to A, C, or T changes the rate — that's the part examiners actually test.

Practice Question

Using Fick's Law, explain why emphysema (a lung disease that destroys alveoli walls, merging many small alveoli into fewer larger air spaces) causes breathlessness.

Topic 3

The Lung & Gas Exchange

A real, worked example of "exchange surface theory" applied to the human body.

Air travels: Nasal cavity → Trachea → Bronchi → Bronchioles → Alveoli. Each structure has a specific job:

Structure	Key Features	Why
Trachea	C-shaped cartilage rings, mucus lining, cilia	Cartilage keeps tube open; C-shape prevents friction with oesophagus and allows flexibility when swallowing; mucus + cilia trap and remove dust/pathogens
Bronchi	Thinner walls, smaller diameter than trachea, cartilage rings/plates	Support the smaller tubes while still keeping them open
Bronchioles	Narrow, self-supporting, thin walls; larger ones have smooth muscle + elastic fibres	Smooth muscle allows the airway diameter to be adjusted (bronchoconstriction/dilation)
Alveoli	Single layer of squamous (flattened) epithelium, surrounded by capillaries and elastic fibres, moist lining	Thin + permeable = short diffusion pathway; capillary network maintains steep concentration gradient; elastic fibres allow stretch on inhalation; moisture lets gases dissolve for easier diffusion

AIR IN/OUT (ventilation → steep gradient) | [ALVEOLUS] Low CO2 <----- ----- High O2 | || | Alveolar wall (1 cell thick) | || | Capillary wall (1 cell thick) | || | High CO2 -----> <----- Low O2 | GOOD BLOOD SUPPLY (→ steep gradient)

Common Mistake

Never call the alveolar lining a "cell wall" — cell walls only exist in plants, fungi, and bacteria. The correct term is the alveolar wall or alveolar epithelium. Examiners specifically dock marks for this mix-up.

Put together, the alveoli tick every box from Fick's Law: huge surface area (millions of alveoli), short diffusion pathway (one cell thick wall + one cell thick capillary wall), and steep concentration gradient (maintained by ventilation and blood flow).

Topic 4

Cell Membranes: Phospholipid Structure & the Fluid Mosaic Model

What membranes are actually made of, and why they behave the way they do.

Phospholipids — the building block

A phospholipid has:

A glycerol backbone
A phosphate head — polar, so it's hydrophilic ("water-loving")
Two fatty acid tails — non-polar, so they're hydrophobic ("water-hating")

Think of phospholipids like tiny matchsticks with a magnetic head and a greasy tail. The head is drawn to water (like a magnet to metal); the tail actively avoids water (like oil avoiding water). Put loads of these matchsticks in water and they'll automatically arrange themselves so all the "water-loving" heads face outward into the water and all the "water-hating" tails huddle together away from it. That self-arranging behaviour is exactly why a bilayer forms — nobody has to build it; physics does it automatically.

Because of this dual nature, phospholipids spontaneously arrange into a bilayer in water: two layers, tails pointing inward toward each other (away from water on both sides), heads pointing outward (into the watery cytoplasm on the inside, and the watery extracellular fluid on the outside).

OUTSIDE (water) o o o o o o o o o o o o <- hydrophilic heads | | | | | | | | | | | | <- hydrophobic tails | | | | | | | | | | | | <- hydrophobic tails o o o o o o o o o o o o <- hydrophilic heads INSIDE / CYTOPLASM (water)

The other components of the membrane

Component	Role
Intrinsic (integral) proteins	Embedded within the bilayer — form channels/carriers for transport
Extrinsic (peripheral) proteins	Sit on the inner or outer surface only — often involved in cell signalling/support
Cholesterol	Sits between phospholipids; regulates fluidity — stops the membrane getting too rigid at low temperatures and stops it getting too fluid at high temperatures; adds mechanical strength
Glycoproteins	Protein + carbohydrate chain; act as cell markers/antigens and receptors, e.g. ABO blood group antigens
Glycolipids	Lipid + carbohydrate chain; same cell-recognition/communication role as glycoproteins

Why "Fluid Mosaic"?

Mosaic — because the scattered pattern of different-sized proteins studded through the phospholipid sea looks like a mosaic pattern when viewed from above.

Fluid — because phospholipids and most proteins aren't fixed in place; they can drift and move sideways within their layer by diffusion (though phospholipids mainly stay within their own layer — up/down "flip-flopping" between layers is rare).

Key Insight — Models Change Over Time

This is a favourite "How Science Works" exam theme. The membrane model has evolved as technology improved:

1920s — Gorter & Grendel: showed phospholipids form a bilayer (extracted lipids from red blood cells = double the membrane's surface area). Problem: didn't explain proteins or how lipid-insoluble molecules crossed.
1930s — Davson & Danielli: proposed protein layers above and below the bilayer (like a sandwich), based on electron micrographs showing two dark lines with a lighter band between (proteins appear darker than lipids). Problem: freeze-etched micrographs later showed globular structures scattered throughout, not in flat layers.
1970s — Singer & Nicolson: proposed the modern fluid mosaic model — proteins are globular, varied in size, and embedded (both peripheral and integral) rather than forming flat layers. Supported by freeze-etch micrographs showing proteins extending into the membrane's centre, and biochemical evidence that proteins can move within the bilayer.

The takeaway: models are our best current interpretation of the evidence — when new evidence or technology (like freeze-etch electron microscopy) appears, models get revised or replaced.

Practice Question

Explain why a phospholipid bilayer forms spontaneously when phospholipids are mixed with water, using the terms hydrophilic and hydrophobic.

Topic 5 · Core Practical 3

Investigating Membrane Structure & Permeability

The beetroot experiment — know this method well, it comes up constantly.

The logic: beetroot cells store a dark purple-red pigment (betalain) inside their vacuole. If the cell membrane (and tonoplast) is damaged/more permeable, more pigment leaks out into the surrounding water. We measure "how much pigment leaked out" using a colorimeter, which shines light through a coloured sample and measures absorbance — more pigment = darker solution = higher absorbance.

Method (temperature investigation)

Use a cork borer to cut equal-diameter beetroot cylinders, then cut to equal length with a scalpel/ruler — this controls surface area and volume of tissue used.
Rinse the pieces to remove any pigment released during cutting (this pigment isn't from membrane permeability, so it would be a confounding variable).
Add one beetroot piece to each of five test tubes, all containing the same volume of water.
Place tubes in water baths at different temperatures (e.g. 10°C, 20°C, 30°C, 40°C, 50°C) for the same length of time (e.g. 30 minutes).
Remove the beetroot pieces, leaving just the coloured liquid.
Use a fresh pipette per sample to transfer liquid into colorimeter cuvettes (avoids cross-contamination of pigment between samples).
Zero/calibrate the colorimeter first using distilled water, then measure absorbance of each sample.

Result Pattern

Higher temperature → higher absorbance → more pigment leaked → higher membrane permeability

Why: higher temperature gives phospholipids more kinetic energy, so they move around more and pack less tightly (more gaps for pigment to escape through). At very high temperatures, membrane proteins also denature, and the water inside the cell expands, physically damaging the membrane.

Membrane Permeability ^ | \ / | \ / | \ / | \ / | \___ _____/ | \__________________/ +------------------------------------------> Temp (°C) -10 0 20 40 Both VERY LOW temps (ice crystals pierce membrane) AND VERY HIGH temps (proteins denature, phospholipids move too freely) INCREASE membrane permeability.

Watch Out — U-shaped Curve

Students often assume permeability just keeps rising smoothly with temperature. Actually, at temperatures below 0°C, ice crystals can form and physically pierce the membrane — so permeability can be high at very low temperatures too, once the cells thaw. The full relationship (if you go below freezing) is roughly U-shaped, not a straight line.

Limitations of the practical

Limitation	Solution
Cuvettes may differ in thickness/scratches, affecting absorbance readings	Use the same cuvette every time, or repeat and take a mean
Beetroot pieces may not be perfectly identical in size/shape	Cut discs as accurately as possible with scalpel + ruler; repeat and average
Different parts of the beetroot contain different amounts of pigment	Repeat using different parts of the beetroot and average

Variation: the same method can test alcohol concentration instead of temperature. As alcohol concentration increases, permeability increases, because alcohol dissolves the lipids in the membrane, destroying its structure.

Practice Question

In this experiment, why must a different pipette be used for each beetroot sample, and why must all beetroot pieces be rinsed before being placed into water?

Topic 6

Osmosis

A special case of diffusion — but only for water.

Definition — learn this word-for-word

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

Water potential is a measure of the number of free (unbound) water molecules in a solution. The more solute dissolved in water, the more water molecules get "tied up" surrounding those solute particles — meaning fewer free water molecules — meaning lower water potential.

Think of water molecules like party guests, and solute molecules like clingy friends. Each solute particle "grabs" a few water molecules and won't let them wander off freely. The more solute particles in a room, the fewer "free" water guests there are wandering around able to leave through the door (the membrane). Pure water has zero clingy friends — every single water molecule is completely free, so pure water has the highest possible water potential.

Because osmosis is water moving down its own concentration gradient (from more free water molecules to fewer), it's really just diffusion — with the special condition that only water crosses (solute molecules are too big to fit through the partially permeable membrane).

Situation	Animal Cell	Plant Cell
Placed in pure water (highest water potential)	Water moves in by osmosis → cell swells → can burst (no cell wall to resist)	Water moves in by osmosis → cell swells → cell wall prevents bursting → cell becomes turgid (firm, rigid) once fully inflated
Placed in concentrated solution (lower water potential than cytoplasm)	Water moves out by osmosis → cell shrivels	Water moves out by osmosis → vacuole/cytoplasm shrink, protoplast pulls away from cell wall → cell is plasmolysed; the rigid cell wall keeps the overall outer shape

Exam Tip — Vocabulary Precision

Examiners specifically want the correct terms: turgid (firm/full of water, plant cell), flaccid (has lost some water, no longer firm), plasmolysed (protoplast has pulled fully away from the cell wall). Vague answers like "the cell got smaller" lose marks compared to naming the actual state.

Practice Question

A red blood cell and a plant cell are both placed into pure distilled water. Predict and explain what happens to each.

Topic 7

Diffusion, Facilitated Diffusion & Active Transport

The four ways substances actually cross a membrane — and how to tell them apart in an exam question.

Simple Diffusion

Definition

The net movement of a substance from a region of higher concentration to a region of lower concentration, down a concentration gradient.

Passive — no ATP required. Continues until equilibrium (equal concentration on both sides).

Only works directly through the phospholipid bilayer for molecules that are small (can fit between phospholipids) and non-polar (can interact with the hydrophobic tails) — e.g. O₂, CO₂.

Facilitated Diffusion

Large, polar molecules (like glucose, amino acids) and charged ions (like Na⁺, Cl⁻) can't cross the bilayer directly — they need help from transport proteins. This is still passive (still down the concentration gradient, still no ATP needed) — the protein is just a "doorway," not a pump.

	Channel Proteins	Carrier Proteins
Shape	Fixed pore through the membrane	Can switch between two shapes/conformations
What crosses	Charged substances, e.g. ions	Larger polar molecules, e.g. glucose
Mechanism	Many are "gated" — part of the protein can move to open/close the pore	Binding site opens to one side, then the protein changes shape to open to the other side, releasing the molecule
Direction	Down the concentration gradient	Down the concentration gradient

Active Transport

Definition

The movement of molecules/ions through a membrane from a region of LOWER concentration to a region of HIGHER concentration, using carrier proteins and ATP.

Active — requires energy because it's moving substances AGAINST their concentration gradient (uphill, like pushing a boulder up a hill rather than letting it roll down).

ATP (produced by respiration) is hydrolysed to provide the energy that changes the carrier protein's shape, transferring the substance across.

Real examples of active transport:

Reabsorbing useful molecules/ions back into the blood in kidney tubules
Absorbing some digestion products from the gut into the blood
Loading sugar into phloem tissue in plants
Loading inorganic ions from soil into root hair cells

Common Mistake

Active transport uses carrier proteins, NOT channel proteins. Channel proteins can't change conformation to actively "pump" something against a gradient — only carrier proteins can do that (with ATP). If a question mentions ATP or "against the concentration gradient," the protein involved must be a carrier protein.

Endocytosis & Exocytosis — bulk transport

Some things are simply too big to pass through any membrane protein — whole proteins, lipids, large carbohydrates, even bacteria. These move by bulk transport, using vesicles:

Endocytosis — the cell surface membrane wraps around a substance and pinches off inside the cell to form a vacuole/vesicle. (Phagocytosis — a white blood cell engulfing a bacterium — is a classic example.)
Exocytosis — vesicles (often from the Golgi apparatus) move to the cell surface membrane and fuse with it, releasing their contents outside the cell. Used to secrete hormones, enzymes, and lipids.

Both endocytosis and exocytosis are active processes — they require energy, because the membrane has to physically deform and vesicles need to be moved and fused.

How to identify which transport method a question is describing

1️⃣ Is it moving with or against the concentration gradient?
With → diffusion/facilitated diffusion. Against → active transport (or bulk transport if the particle is huge).

2️⃣ Is ATP / energy mentioned?
Yes → active transport, endocytosis, or exocytosis. No → simple or facilitated diffusion.

3️⃣ Is the molecule small & non-polar, or large/polar/charged?
Small & non-polar → can use simple diffusion. Large/polar/charged → needs a protein (facilitated diffusion or active transport) or a vesicle (bulk transport) if it's huge.

Practice Question

Glucose is absorbed from the gut into intestinal cells even when the glucose concentration in the cell is already higher than in the gut lumen. Name the type of transport involved and explain why.

Practice Question

Explain, in terms of the phospholipid bilayer, why oxygen can diffuse directly across a cell membrane but glucose cannot.

What to Memorise

The core vocabulary and formulas — quiz yourself on these before the exam.

Surface Area : Volume (SA:V) ratio

The relationship between an organism's exposed surface area and its total internal volume; decreases as organisms get larger.

Fick's Law

rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of membrane; equation form: Rate = P × A × ((C₁−C₂) ÷ T)

Hydrophilic / Hydrophobic

Hydrophilic = "water-loving," attracted to water (polar). Hydrophobic = "water-hating," repelled by water (non-polar).

Fluid mosaic model

The current accepted model of membrane structure (Singer & Nicolson, 1972): a phospholipid bilayer with proteins scattered throughout that can move by lateral diffusion ("fluid") in a scattered pattern ("mosaic").

Water potential

A measure of the number of free water molecules in a solution; pure water has the highest possible water potential.

Osmosis

The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

Diffusion

The net movement of a substance from a region of higher concentration to a region of lower concentration; a passive process (no ATP needed).

Facilitated diffusion

Diffusion of large/polar/charged molecules through channel or carrier proteins; still passive, still down the concentration gradient.

Active transport

Movement of molecules/ions against their concentration gradient (low → high), using carrier proteins and ATP.

Endocytosis / Exocytosis

Bulk transport of large substances into/out of a cell via vesicles formed from (or fusing with) the cell surface membrane; both require energy.

Turgid / Plasmolysed

Turgid = plant cell firm and full of water (osmosis in, cell wall resists). Plasmolysed = protoplast has pulled away from the cell wall (osmosis out).

Concepts Checklist

Tick off each idea only once you can explain it out loud, without notes, to someone else.

Gas Exchange Surfaces

I can explain why SA:V ratio decreases as organisms get bigger, using the cube example

I can list the three properties of a good exchange surface (large SA, short diffusion pathway, steep concentration gradient)

I can use Fick's Law to explain how changing A, C, or T affects diffusion rate

I can substitute numbers into the Fick's Law equation and calculate a rate

I can label the structures of the human gas exchange system (trachea, bronchi, bronchioles, alveoli) and describe their adaptations

Cell Membranes

I can describe the structure of a phospholipid and explain hydrophilic vs hydrophobic

I can explain why phospholipids form a bilayer spontaneously in water

I can name and describe every component of the fluid mosaic model (proteins, cholesterol, glycoproteins, glycolipids)

I can explain why "fluid" and why "mosaic" in the model's name

I can describe how membrane models evolved (Gorter-Grendel → Davson-Danielli → Singer-Nicolson) and why

Core Practical & Osmosis

I can describe the full beetroot practical method, including why each control step matters

I can explain why membrane permeability increases at both very high and very low temperatures

I can state the definition of osmosis word-for-word, using "water potential"

I can predict/explain what happens to animal and plant cells in solutions of different water potential

I can correctly use the terms turgid, flaccid, and plasmolysed

Transport Mechanisms

I can distinguish simple diffusion, facilitated diffusion, and active transport by direction and energy use

I can explain the difference between channel proteins and carrier proteins

I can give real biological examples of active transport

I can describe endocytosis and exocytosis and explain why they need energy

Exam Tips & Common Traps

These are the exact places examiners catch students out — read this section last, right before your exam.

"Surface area" ≠ "surface area : volume ratio"

Larger organisms have a bigger surface area but a smaller SA:V ratio. Always check which one the question is actually asking about.

Never say "cell wall" for the alveoli

It's the "alveolar wall" or "alveolar epithelium." Cell walls only exist in plants, fungi, and bacteria — using this term for an animal structure loses marks.

Cell surface membrane vs plasma membrane

Both terms are acceptable for the outer membrane, but always distinguish it clearly from a cell's internal membranes (e.g. nuclear envelope, mitochondrial membranes) when describing structure.

Direction is everything for transport questions

If a question describes movement against a concentration gradient, it cannot be diffusion or facilitated diffusion — full stop. It must be active transport (or bulk transport for huge molecules). Check the direction before naming the mechanism.

Active transport = carrier proteins, not channel proteins

Channel proteins are fixed pores and can only allow passive facilitated diffusion. Only carrier proteins can change shape to actively pump substances against a gradient, using ATP.

Use the correct osmosis definition — every time

"Water moves from a dilute solution to a concentrated solution" is a common shorthand but is imprecise and can lose marks. Always frame it as "water potential" and "partially permeable membrane" using the full formal definition.

Explain WHY temperature affects permeability, not just THAT it does

Don't just state "permeability increases with temperature" — explain the mechanism: phospholipids gain kinetic energy and move more, packing less tightly; at very high temperatures, membrane proteins denature too. Mechanism marks are separate from "trend" marks.

Models can be wrong — and that's fine

When asked about how the membrane model developed, don't just recite facts about each model. Explain the "How Science Works" angle: new technology (freeze-etch electron microscopy) provided new evidence that didn't fit the old model, so the model was revised. This is the actual skill being tested.

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