SPEC 4.1.3.1 • TRANSPORT IN CELLS
Diffusion
The first transport mechanism. The exact mark-scheme definition the exam wants — and the alveoli adaptations that come up nearly every paper.
The Preposition That Loses The Mark
Diffusion appears on almost every Higher Tier Paper 1 — most often as a 2-mark definition question. The mark scheme rewards precise wording, and one preposition decides whether you score full marks or half.
The Trap
Mark schemes credit "down the concentration gradient" and "from higher to lower concentration." They explicitly reject phrases like "across the gradient", "along the gradient", or "from a concentration gradient". Same idea, different preposition — half the marks.
Why This Lesson
One definition, three factors, four alveoli adaptations. That's the whole spec for this section — but each item has mark-scheme strict wording. In the next 10 minutes, we'll get every word right.
By The End Of This Lesson
Three skills — every one tested directly on Paper 1.
1. Define
Define diffusion using mark-scheme wording, including the words "net movement" and the correct preposition ("down", not "across").
2. Identify
Identify the three factors that affect rate of diffusion (concentration gradient, temperature, surface area) — and explain how each works.
3. Apply
Apply diffusion to gas exchange in the alveoli — listing the four mark-scheme adaptations and explaining how each speeds up diffusion.
Spec Anchor
This is spec 4.1.3.1, Paper 1, Higher Tier. Spec 4.1.3.1 also includes SA:V calculations and other exchange surfaces (gills, roots) — those parts are covered in Lesson 1.09. Here we focus on diffusion fundamentals + the lungs as the spec's main exchange-surface example.
What Is Diffusion?
The mark-scheme definition. Two distinct points — both required for full marks.
Diffusion — Spec 4.1.3.1
Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
Two Marking Points
Net movement of particles / molecules — particles spread out (or "move").
From higher concentration to lower concentration — or equivalently, "down the concentration gradient."
The Word "Net"
Mark schemes increasingly require "net movement" — not just "movement." Why? Because particles move randomly in both directions. The net direction (overall direction) is from high to low. Saying just "particles move from high to low" misses that they also bounce back occasionally — the mark scheme rewards precision.
Three Key Features Of Diffusion
Understand these three, and you can answer almost any diffusion question.
1. Passive — No Energy Needed
Diffusion is a passive process. The cell does not need to spend any energy to make it happen — the random kinetic motion of particles drives it. (Compare: active transport, which DOES need energy — that's spec 4.1.3.3, coming in Lesson 1.08.)
2. Net Direction — Down The Gradient
Particles always move from higher to lower concentration (from "more crowded" to "less crowded"). The concentration gradient is the difference in concentration between two areas — diffusion goes DOWN the gradient.
3. Continues Until Equilibrium
Diffusion stops being a net movement when concentrations are equal on both sides — though particles still move randomly in both directions equally (no net direction).
Examples Of Diffusion In Cells
The spec names two specific examples of diffusion in the human body. Memorise both.
Example 1 — Gas Exchange
Oxygen and carbon dioxide move by diffusion in the lungs:
Oxygen diffuses from the alveolar air (high O₂) into the blood (low O₂).
Carbon dioxide diffuses from the blood (high CO₂) into the alveolar air (low CO₂).
Example 2 — Excretion of Urea
Urea diffuses from cells (where it's produced as a waste product) into the blood plasma — for excretion in the kidney.
Water Moves By Osmosis — NOT Diffusion
A common error: writing "water moves by diffusion." Water moves by osmosis — a special form of diffusion through a partially permeable membrane. We'll cover osmosis in Lesson 1.07. For now: if the question is about water across a membrane, the answer is osmosis.
Three Factors That Affect Rate
The spec names exactly three factors. Each factor + WHY = a likely exam mark.
Concentration Gradient
Steeper gradient = faster diffusion.
(Bigger difference = more particles moving in net direction.)
🌡️
Temperature
Higher temperature = faster diffusion.
(Particles have more kinetic energy, so move faster.)
Surface Area
Larger surface area = faster diffusion.
(More space for particles to cross at once.)
Mark-scheme phrase
Spec wording: "the difference in concentrations (concentration gradient)", "the temperature", "the surface area of the membrane." Use these exact terms.
Why These Matter
These three factors underpin most exchange surface adaptations — large surface area directly enhances factor 3, while good blood supply and ventilation maintain a steep gradient (factor 1). One additional effectiveness mechanism — thin membranes / short diffusion path — also matters but isn't in the three named factors. Slide 10 ties this together for the alveoli.
Why Big Organisms Need Exchange Surfaces
Single-celled organisms diffuse molecules straight through their cell membrane. Multicellular organisms can't — and the reason is geometry.
Single-Celled = Large SA:V
A single-celled organism has a large surface area to volume ratio. The spec says: "this allows sufficient transport of molecules into and out of the cell to meet the needs of the organism." No specialised exchange surface needed.
Multicellular = Small SA:V
A large organism has a small SA:V. Diffusion across the body surface alone cannot supply enough oxygen / nutrients to inner cells. They need specialised exchange surfaces — alveoli (lungs), villi (small intestine), gills (fish), root hair cells (plants).
Spec Boundary
You don't need SA:V calculations here — those are also part of spec 4.1.3.1 but covered in Lesson 1.09 along with other exchange surfaces (gills, roots, leaves). For this lesson, you just need to know that single-celled organisms manage by diffusion alone, and big organisms need specialised exchange surfaces. The lungs are the spec's main example.
Alveoli — Four Mark-Scheme Adaptations
The lungs are the spec's primary example of an exchange surface. The alveoli have four named adaptations — every one in the mark scheme.
1. Large Surface Area
Millions of alveoli give a huge total surface area for diffusion. (Estimated: ~70 m² of alveolar surface in adult human lungs — about the size of a tennis court.)
2. Walls One Cell Thick
The walls of the alveoli are one cell thick — and so are the capillary walls. This gives a short diffusion path for gases. Note the mark-scheme wording: say "walls are one cell thick" — NOT "alveoli are thin."
3. Good Blood Supply
A rich capillary network surrounds each alveolus. Blood arriving has low O₂ and high CO₂; blood leaving has the opposite. This maintains a steep concentration gradient for both gases.
4. Ventilated
Breathing in fresh air with high O₂ and breathing out air with high CO₂ also maintains the gradient. (Mark schemes accept "ventilation" or "breathing in/out brings fresh air".)
How Each Adaptation Speeds Diffusion
The four adaptations aren't random — each one enhances diffusion through a specific mechanism. Mark schemes reward this causal chain: adaptation → why it speeds diffusion.
AdaptationMechanism — why it speeds diffusion
Large surface areaMore space for gases to diffuse at once → directly enhances factor: surface area
Walls one cell thickProvides a short diffusion path → less distance for molecules to travel
Good blood supplyRemoves O₂ / brings CO₂ → maintains a steep concentration gradient
VentilationBrings fresh O₂ / removes CO₂ → also maintains steep concentration gradient
Note On Diffusion Path
Three of the adaptations (large SA, blood supply, ventilation) work by enhancing the named rate factors from Slide 7. "Walls one cell thick" works through a different mechanism — providing a short diffusion path. The spec lists this as a separate effectiveness criterion for exchange surfaces ("a membrane that is thin, to provide a short diffusion path").
The "Moist" Misconception
A long-standing myth in school biology: "alveoli are moist, which helps gas exchange." Examiner reports state plainly: this does NOT score. Mark schemes explicitly say ignore "moist surfaces". Stick to the four adaptations above.
Worked Example — 2-Mark Definition
The classic Higher Tier definition question. Two marks. Two distinct mark-scheme points required.
What is meant by the term "diffusion"?2 marks
📋 Pattern: 2-mark definition — TWO distinct points
Strategy
Don't just write a sentence and hope. Plan two specific points: (1) what's moving, and (2) where it's moving from/to. Each point = 1 mark. Combine them into one clean sentence using mark-scheme verbs and prepositions.
Worked Example — Two-Point Solution
Each point lifted directly from the spec wording. Word the answer to scoop both marks.
MP1 — What Moves
"Diffusion is the net movement of particles (or molecules) of a gas or substance in solution..."
Key word: net movement — not just "movement."
MP2 — Where It Moves
"...from an area of higher concentration to an area of lower concentration" (or equivalently: "down the concentration gradient").
Key prepositions: " from...to " or " down " — never "across" or "along".
Full mark answer
"Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration."
Worked Example — Mark Scheme Tick-Off
Two marking points. Each maps to one tick.
✓
MP1: (net) movement of molecules / particles
✓
MP2: from higher concentration to lower concentration (or "down the concentration gradient")
Mark-Scheme "Do Not Accept" List
Mark schemes for this question pattern explicitly reject: "movement across the gradient", "movement along the gradient", "movement to the gradient", "movement from the gradient". Same idea — wrong preposition — no mark.
The Big Lesson
Mark schemes reward specific words and specific prepositions. "Net" is required. "Down" or "from...to" is required. Vague phrasing scoops 1/2, not 2/2. Memorise the exact wording.
Practice Question
The alveoli in the lungs are adapted for the efficient diffusion of gases.
Describe TWO adaptations of the alveoli, and explain how each adaptation increases the rate of diffusion.4 marks
A1
Adaptation 1: the walls of the alveoli are one cell thick. Why it helps: this gives a short diffusion path for oxygen and carbon dioxide to cross. (2 marks)
A2
Adaptation 2: the alveoli have a large surface area (millions of alveoli). Why it helps: more space for gas molecules to diffuse simultaneously → faster overall diffusion. (2 marks)
⚠️
Other valid alternatives: good blood supply (maintains gradient), or ventilation (maintains gradient). Lose-the-mark traps: writing "alveoli are thin" (must be walls are one cell thick); writing "moist surface" (mark schemes ignore this).
Three Examiner Alerts
The three traps that cost the most marks on diffusion questions.
Alert 1 — Down The Gradient
Mark schemes credit "down the concentration gradient" or "from higher to lower concentration." They explicitly reject "across the gradient", "along the gradient", "to/from a gradient". The preposition decides whether your answer scores. "Down" is the safe choice.
Alert 2 — Walls One Cell Thick
The mark-scheme phrase is "walls are one cell thick" — NOT "alveoli are thin." Examiner reports flag this directly: "the cells are thin, not the alveoli." The alveoli are spherical sacs; their walls are one cell thick. Use the precise wording.
Alert 3 — "Moist" Does Not Score
A long-held school-biology misconception: that the alveoli being moist aids gas exchange. Examiner reports state this is not correct and does not gain credit. Mark schemes explicitly "ignore moist." Use the four spec adaptations: large SA, walls one cell thick, good blood supply, ventilation.
Common Errors That Cost Marks
Four wrong-vs-right pairs lifted directly from mark-scheme guidance.
✗ Wrong
"Diffusion needs energy"Confused with active transport.
✓ Right
"Diffusion is passive — no energy needed"Active transport needs energy; diffusion does not.
✗ Wrong
"Particles move across the gradient"Mark schemes reject "across".
✓ Right
"Particles move down the gradient"Or: "from higher to lower concentration".
✗ Wrong
"Alveoli are thin and moist"Both errors in one sentence.
✓ Right
"Walls are one cell thick" (ignore moist)Walls are thin, not the alveoli themselves.
✗ Wrong
"Water moves by diffusion"Wrong process for water.
✓ Right
"Water moves by osmosis"Diffusion through partially permeable membrane.
Three Things To Remember
The whole lesson, distilled to three takeaways.
1. The Definition
Diffusion = net movement of particles from higher to lower concentration (or down the gradient). Passive — no energy needed. The two key words: net and down.
2. Three Factors
Concentration gradient (steeper = faster), temperature (higher = faster), surface area (larger = faster). Memorise all three names + the WHY behind each.
3. Alveoli Adaptations
Large surface area, walls one cell thick, good blood supply, ventilated. Ignore "moist" — it doesn't score. Use the precise wording every time.
Final Word
Diffusion questions reward precision: "net" not "movement", "down" not "across", "walls one cell thick" not "alveoli are thin". The marks live in the prepositions and the named adaptations. Memorise the exact phrases.
Next: Lesson 1.07 — Osmosis (RP3)
Diffusion's specialised cousin — water moving across a partially permeable membrane. The exact mark-scheme definition matters even more here, plus Required Practical 3 on osmosis in plant tissue.
Coming Up
The spec definition of osmosis — and the "do not accept" line that catches most students out.
Required Practical 3: investigating osmosis in plant tissue (potato cylinders).
How to process the data from RP3 — % change in mass, calculating concentration of solute.
Before You Move On
Practise writing the diffusion definition from memory, including the words "net" and "down" or "from higher to lower concentration". Then take the practice questions and end-of-lesson test. Definition + alveoli adaptations are the two highest-yield to master.
Next: The Preposition That Loses The Mark