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4.2.14 · Cell Membranes and Transport

Diffusion — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 8 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

4.2.1 Diffusion — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Define diffusion and facilitated diffusion, explain the factors that affect their rate and apply Fick's law to exchange surfaces.

  • Passive process
  • Net movement
  • Dynamic equilibrium
  • Small non-polar molecules
  • Very small uncharged polar molecules
  • Rate pattern

Section 1 of 9What diffusion is

1Diffusion
Diffusion the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient,
  • as a result of their random kinetic energy.
2Passive process
  • Passive process the cell supplies no metabolic energy (no ATP).
3Net movement
  • Net movement particles move randomly in all directions,
    • but more leave the high-concentration region than arrive,
    • so overall movement is down the gradient.
4Dynamic equilibrium
  • Dynamic equilibrium particles become evenly spread, and they still move but there is no net change in concentration.
5Limit
  • Limit diffusion is efficient only over short distances, so larger organisms need exchange surfaces and transport systems.
  • A particle takes roughly four times as long to diffuse twice as far.

Section 2 of 9Simple diffusion across the bilayer

Small non-polar molecules
  • Small non-polar molecules oxygen, carbon dioxide and steroid hormones dissolve in the hydrophobic core and pass directly through.
Very small uncharged polar molecules
  • Very small uncharged polar molecules water and urea can slip through slowly.
Rate pattern
  • Rate pattern the rate of simple diffusion increases in proportion to the concentration gradient, giving a straight line through the origin on a graph.

Section 3 of 9Facilitated diffusion

Why it is needed
  • Why it is needed large polar molecules and charged ions (Na+, Cl-) cannot cross the hydrophobic core.
    • (glucose, amino acids)
Passive
  • Passive it still goes down a concentration gradient and uses no ATP, but needs a specific transport protein.
Channel proteins
  • Channel proteins water-filled hydrophilic pores with a fixed shape that are specific to particular ions.
  • Most are gated, because part of the protein on the inside surface can move to open or close the pore.
Carrier proteins
  • Carrier proteins they bind a specific molecule, change shape and release it on the other side.
  • The binding site is open to one side of the membrane first and then to the other.
Saturation
  • Saturation the rate levels off once all channels or carriers work at maximum speed, giving a curve that plateaus on a graph.

Section 4 of 9Factors affecting the rate of diffusion

FactorEffect on rateReason
Concentration gradientSteeper gradient, fasterMore particles move down than back up
TemperatureHigher, fasterParticles have more kinetic energy and move faster
Surface areaLarger, fasterMore space for particles to cross at once
Diffusion distanceShorter, fasterParticles have less far to travel
Size of moleculeSmaller, fasterSmall particles move faster and pass more easily
Lipid solubilityMore soluble, fasterIt dissolves in the bilayer more readily
Number of transport proteinsMore, faster (facilitated only)More routes across the membrane
1Surface area and cell size
  • Surface area and cell size as a cell grows, its surface area to volume ratio falls, which slows diffusion because the distance becomes too great.
2Increasing area
  • Increasing area folding the membrane, as in microvilli and cristae, raises the surface area.

Section 5 of 9Fick's law

Fick's law
  • Fick's law rate of diffusion is proportional to ÷ diffusion distance.
    • (surface area × concentration difference)
Linking adaptations
  • Linking adaptations each adaptation of an exchange surface relates to one term: surface area, gradient or distance.

Section 6 of 9Exchange surface adaptations

Alveoli
  • Alveoli they have a very large total surface area because they are numerous and folded.
  • They are one cell thick and flattened, giving a short diffusion distance.
  • A dense capillary network and ventilation keep the oxygen and carbon dioxide gradients steep.
Maintaining the gradient
  • Maintaining the gradient blood flow removes oxygen as it enters and delivers carbon dioxide, and breathing replaces the air.
Other surfaces
  • Other surfaces gills, root hairs, intestinal villi and the thin, flat leaf blade follow the same principles.

Section 7 of 9More from the specification

Diffusion and Fick's Law
  • Diffusion is the net passive movement of particles down a concentration gradient across a selectively permeable membrane, proportional to surface area and concentration gradient.
Osmosis and Water Potential (Ψ)
  • Osmosis is the passive movement of water molecules from a region of higher water potential to lower water potential across a selectively permeable membrane.
Investigating Osmosis in Potato Tissue
  • Immersing potato cylinders in graded sucrose solutions allows determination of internal tissue water potential where zero percentage change in mass occurs.
Active Transport Mechanisms
  • In contrast to passive diffusion, active transport moves solutes against a concentration gradient into cells using carrier proteins and ATP energy.
Indicator
  • A chemical that changes colour depending on pH.
Endocytosis
  • Bulk transport into a cell using a membrane vesicle.
Selectively permeable
  • Allowing only some substances through.
Active transport
  • Movement of particles against a concentration gradient using energy from respiration.
Phospholipid bilayer
  • Two layers of phospholipids with hydrophobic tails inwards
Dilute solution
  • Solution with a high water concentration and low solute concentration.
Animal cell
  • A cell with a nucleus, cytoplasm, cell membrane and mitochondria but no cell wall, chloroplasts or large vacuole.
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.
Permeable
  • Allowing substances to pass through.
Sucrose
  • The sugar mainly transported in phloem.

Section 8 of 9Key terms

TermMeaning
DiffusionNet movement of particles from a higher to a lower concentration, down a concentration gradient, using their own kinetic energy.
Concentration gradientThe difference in concentration of a substance between two regions.
Facilitated diffusionPassive movement of polar molecules or ions across a membrane through channel or carrier proteins.
Channel proteinMembrane protein with a hydrophilic pore through which specific ions pass.
Carrier proteinMembrane protein that binds a specific molecule and changes shape to move it across.
Dynamic equilibriumState in which particles still move but there is no net movement.
Fick's lawRate of diffusion is proportional to surface area × concentration difference ÷ diffusion distance.

Section 9 of 9Exam technique

1
Saying molecules move because they want to reach equilibrium — they move randomly, and net movement down the gradient is a statistical result.
2
Stating that facilitated diffusion uses ATP — it is passive and only needs a transport protein.
3
Writing that oxygen diffuses from high to low concentration without saying net movement — at equilibrium particles still move.
4
Treating a steeper gradient as a longer diffusion distance — gradient and distance are separate factors in Fick's law.

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