Diffusion — net movement down a gradient
Diffusion is the net movement of particles (molecules or ions) from a region of higher concentration to a region of lower concentration — down a concentration gradient — as a result of random particle motion. Both elements are needed for the marks. It is passive: no ATP is used, only the kinetic energy the particles already have. Net movement stops at equilibrium, when the concentrations are equal.
Where diffusion matters in living organisms
Gases and dissolved solutes cross membranes and thin exchange surfaces by diffusion. In the alveoli, passes from air into blood and from blood into air; at respiring tissues both directions reverse. Glucose and amino acids diffuse from the gut lumen into the blood at the villi. In a leaf, diffuses in and out through the stomata. Respiration, photosynthesis and digestion constantly consume or produce these molecules, so the gradient — and the diffusion — is maintained.
Four factors set the rate of diffusion
Rate rises with a larger surface area (more particles cross at the same time), a higher temperature (more kinetic energy, faster random motion) and a steeper concentration gradient (greater net movement per unit time); it falls as the distance particles must travel increases. Rate itself = quantity of substance ÷ time, e.g. per minute. Exchange organs — lungs, villi, leaves, gills — are adapted to maximise surface area and minimise diffusion distance.
Drawn from real examiner reports.
Diffusion ≠ osmosis (water only)
Diffusion applies to any particle — a gas or a dissolved solute — moving down a concentration gradient, and does not need a membrane. Osmosis is the special case: water molecules only, through a partially permeable membrane, down a water potential gradient. Never write that oxygen enters by osmosis, or that water diffuses through a membrane where the mark scheme wants osmosis.
Flagged s23 P11 Q6 — "osmosis" written for gas movement, "diffusion" written for water movement
"Particles spread out" earns nothing
The mark scheme wants both halves of the definition: net movement from higher to lower concentration, and random particle motion. "Particles spread out" describes the appearance and gives neither. "Movement due to random motion" omits the direction. "Particles move from high to low concentration" omits the mechanism. Write both halves every time.
Dropping the word "net"
Particles move in both directions the whole time. There are simply more particles on the high-concentration side, so more of them cross toward the low side — the overall, net, movement is down the gradient. Writing only "movement from high to low" risks the precision mark, and at equilibrium it is the net movement that stops, not particle motion itself.
Diffusion ≠ active transport
Diffusion is passive and moves substances down the gradient. Active transport moves them against the gradient, from lower to higher concentration, using energy from respiration — mineral ions into root hair cells, glucose reabsorbed in the kidney tubule. Marks are lost by saying diffusion needs energy, or that active transport works down a gradient.
Flagged w22 P21 Q5; P22 Q5; P23 Q5 — active transport said to need no energy, or to move down the gradient
Naming an adaptation without the mechanism
"The alveolus has a large surface area" identifies the feature and stops — usually one mark of two. Add the mechanism: "…so more molecules can cross at the same time, increasing the rate of diffusion." The same applies to a wall one cell thick (short diffusion distance) and a rich blood supply (keeps the concentration gradient steep).
Concentration gradient ≠ water potential
For diffusion of gases or solutes the term is concentration gradient. Reserve water potential for osmosis questions about water movement, where the mark scheme demands the term explicitly — and "higher water potential", not "more dilute". Swapping the two terms is penalised in both directions.
Flagged s23 P41 Q2a ii · w22 P41 Q3c · s22 P31 Q1
One link of the chain, one mark
Marks are usually awarded per completed link, so a two-mark "Explain" is safest written as three short steps — one sentence each. Extra sentences cost nothing; a missing mechanism costs a mark.
Write the three-link chain
Every "Explain" question on diffusion rate wants [structure or change] → [factor it affects] → [effect on rate]. For example: folded villi increase the surface area → more molecules can cross at the same time → the rate of absorption by diffusion increases. Never skip the middle link.
Rate = quantity ÷ time — show the unit
Write the formula, the substitution and then the answer with its unit: of gas in minutes gives . A bare number can lose the unit mark even when the arithmetic is right.
Check what is moving before naming the process
Read the substance first. A gas or a dissolved solute moving down a gradient → diffusion. Water through a partially permeable membrane → osmosis. Anything moving against the gradient → active transport. This single check settles most of the trap questions in Section 3.
Diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration (down a concentration gradient) as a result of random particle motion. It is a passive process — no energy (ATP) is required.
Definition trap: writing "particles spread out" or "particles move randomly" does not score. The mark scheme demands both elements: (1) net movement from higher to lower concentration and (2) because of random particle motion.
Concentration gradient: the difference in concentration of a substance between two regions. The steeper the gradient (greater the difference), the faster the rate of diffusion.
Net movement: particles move in both directions randomly, but there are more collisions on the high-concentration side carrying particles outward, so the overall (net) movement is down the gradient. At equilibrium (equal concentrations) there is no further net movement.
Passive: diffusion does not require ATP. It relies on the kinetic energy already possessed by particles. This is the key distinction from active transport.
Define diffusion.
(a) Define diffusion.
(b) State whether diffusion requires energy from respiration. Explain your answer.
