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2.4Cells

Membrane Permeability — AP BIOLOGY Revision Notes & Topical Questions

5 min read 6 parts Lesson 6 of 12 1 quiz

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Lesson summary

2.4 Membrane Permeability — Notes — AP BIOLOGY notes: Explain how the structure of biological membranes influences selective permeability.Barrier — plasma membranes separate the internal environment from the external environment, and their hydrophobic interior makes them selectiv…

  • Transport proteins
  • Nonpolar interaction
  • Blocked substances
  • Cell walls
  • Channel protein
  • Carrier protein

Part 1 of 4Selective permeability

Barrier
  • Barrier plasma membranes separate the internal environment from the external environment,
    • and their hydrophobic interior makes them selectively permeable
Role
  • Role selective permeability lets the cell control which substances cross and so build and maintain concentration gradients
Transport proteins
  • Transport proteins they are specific for the substances they move,
    • so the types of substances that cross depend on the number and type of transport proteins
SubstanceHow it crossesExamples
Small, nonpolar moleculesPass freely between the phospholipids by simple diffusionN₂, O₂, CO₂, steroid hormones
Small, polar, uncharged moleculesPass between phospholipids in small amounts; water relies mainly on aquaporinsH₂O, NH₃
Large polar molecules and ionsCannot pass between phospholipids and need channel or carrier proteinsGlucose, sucrose, Na⁺, Ca²⁺
Very large molecules and particlesVesicles (endocytosis and exocytosis)Proteins such as insulin

Part 2 of 4Why the membrane filters substances and role of cell walls

Nonpolar interaction
  • Nonpolar interaction small nonpolar molecules interact with the nonpolar fatty acid tails and are small enough to pass between them
Blocked substances
  • Blocked substances charged ions and polar substances cannot interact with the hydrophobic interior,
    • and the tightly packed phospholipids block large molecules
Cell walls
  • Cell walls bacteria, archaea, fungi and plants have a wall in addition to the membrane
  • It gives structure and some rigidity.
  • It is freely permeable to small molecules but can be a barrier to some large molecules.
  • It limits how much water a cell can take up, preventing osmotic lysis.
  • Plant cell walls contain a mesh of cellulose and other complex carbohydrates.

Part 3 of 4Predicting how a molecule crosses

1
Identify the size of the molecule.
2
Identify whether it is polar, nonpolar or charged.
3
Small nonpolar molecules diffuse directly through the bilayer.
4
Water crosses in small amounts through the bilayer and mostly via aquaporins.
5
Ions and large polar molecules require a specific channel or carrier protein.

Part 4 of 4More from the specification

1Selective membrane permeability in cells
  • Selective membrane permeability in cells phospholipid bilayer permeability and transport proteins regulate solute entry and osmoregulation inside living biological cells.
2Active transport
  • Movement of particles against a concentration gradient using energy from respiration.
3Pressure potential
  • Physical pressure on a solution.
4Facilitated diffusion
  • Passive movement through transport proteins.
5Passive transport
  • Movement down a gradient without energy input.
6Solute
  • A substance dissolved in a solvent.
7Movement
  • An action by an organism or part of it causing a change of position or place.
8Tonicity
  • The ability of a solution to change the water content of a cell.
9Hydrophilic
  • Water-loving; polar or charged and able to dissolve in water.
10Water potential
  • A measure of the tendency of water to move; pure water has the highest.

Key terms

TermMeaning
Selective permeabilityAllows some substances to cross more easily than others.
Hydrophobic interiorNonpolar fatty acid core that blocks ions and polar molecules.
Channel proteinHydrophilic pore for specific ions or water.
Carrier proteinProtein that binds a substance and changes shape to move it across.
AquaporinChannel protein that allows rapid water movement.
Cell wallRigid layer outside the membrane in plants, fungi, bacteria and archaea.

Avoid these mistakes

1
Saying ions and polar molecules pass through the bilayer freely — they need transport proteins.
2
Thinking water cannot cross the bilayer at all — small amounts cross and most uses aquaporins.
3
Saying cell walls are selectively permeable like the membrane — they are largely freely permeable but give support.
4
Forgetting that selective permeability comes from the hydrophobic interior.

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