1.3.1: Diffusion
Not started yet — this one needs some love.
Diffusion: net movement of particles from higher to lower concentration (down the concentration gradient); passive (no energy required).
Occurs in gases and solutions; does not require energy.
Examples: O₂ and CO₂ across alveoli; urea from cells into blood plasma.
Factors affecting rate of diffusion:
Concentration gradient: larger difference → faster diffusion.
Temperature: higher → particles have more kinetic energy → faster diffusion.
Surface area of membrane: larger SA → faster diffusion.
SA:V ratio: large SA:V ratio (e.g. single-celled organisms) → sufficient diffusion to meet needs.
As organisms grow larger, SA:V ratio decreases → need specialised exchange surfaces (alveoli, villi, gills, leaf surfaces) and transport systems (circulatory system).
Adaptations of exchange surfaces:
Large surface area.
Thin membrane (short diffusion path).
Efficient blood supply (animals) to maintain the concentration gradient.
Ventilation (gaseous exchange in animals) to maintain the concentration gradient.
Common exam mistakes
Movement 'along' or 'across' a concentration gradient is insufficient — always specify direction: particles move from high to low concentration.
It is particles, molecules, or ions that diffuse — not cells. Never write that cells move.
In questions about alveoli, use the word diffusion not absorption — gases diffuse across the alveolar wall; 'absorption' implies active uptake and loses marks in this context.
Alveolar and villi walls are one cell thick — do not write 'thin cell walls' (a plant structure term) or 'the alveoli have a thin membrane' (the cell membrane is not the same as the wall). The wall is a layer of cells.
The moist lining of alveoli is not an adaptation for gas exchange — it is a consequence of evaporation from cells, not a feature that increases the rate of diffusion. Do not include it as an adaptation.
Mitochondria are found in the epithelial cells of the villi, not in the villi themselves — phrase this carefully in extended answers.
When calculating SA:V, divide surface area by volume — do not give both values as a raw ratio without dividing.