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2.3.2Additional lessons

Membrane Proteins — IB Biology SL Revision Notes & Topical Questions

5 min read 4 parts Lesson 21 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 4Types of Membrane Protein

2.3.2 Membrane Proteins
Integral proteins
  • Embedded in the bilayer and amphipathic, with hydrophobic regions in the core; some span both layers.
Peripheral proteins
  • Hydrophilic proteins attached to one surface of the membrane, either inside or outside the cell.
Transmembrane
  • Proteins crossing the full bilayer have hydrophobic regions in the core and hydrophilic regions in contact with water.
Mobility
  • Many membrane proteins can drift within the bilayer, which contributes to the fluid mosaic model.
Variety
  • A single membrane may contain many different proteins, each specific to a particular molecule or task.

Part 2 of 4Functions of Membrane Proteins

A useful memory aid is that proteins transport, receive signals, catalyse reactions, attach cells and identify cells.

Functions of Membrane Proteins
Transport
  • Channel and carrier proteins move specific ions and molecules across the bilayer.
Receptors
  • Proteins with binding sites for hormones or neurotransmitters pass the signal into the cell.
Enzymes
  • Membrane-bound enzymes are held in position next to their substrates and catalyse reactions on the membrane surface.
Adhesion and recognition
  • Proteins link cells together and identify cells to others, using glycoproteins as markers.
Cell-to-cell communication
  • Receptor proteins let a cell respond to chemical messages from other cells, which coordinates activity in a tissue.

Part 3 of 4Membrane Transport Processes

1
Simple diffusion moves small non-polar molecules, such as oxygen, down a concentration gradient with no energy or protein.
2
Facilitated diffusion uses channel or carrier proteins to move ions and polar molecules down their concentration gradient, without ATP.
3
Osmosis is the passive movement of water across a partially permeable membrane from lower to higher solute concentration.
4
Active transport uses carrier proteins, called pumps, to move substances against the concentration gradient.
5
ATP is hydrolysed in active transport, and the energy changes the shape of the pump so that the substance is moved across.
ProcessNeeds protein?Needs ATP?Direction
Simple diffusionNoNoDown the gradient
Facilitated diffusionYesNoDown the gradient
Active transportYesYesAgainst the gradient

Part 4 of 4Exam Checklist & Tips

2.3.2 – Key Points for the Exam

  • Integral protein A protein embedded in the phospholipid bilayer.
  • Peripheral protein A protein attached to the membrane surface.
  • Channel protein A protein forming a pore through which specific ions pass by facilitated diffusion.
  • Carrier protein A protein that changes shape to move a specific substance across the membrane.
  • Active transport Movement of substances against a concentration gradient using energy from ATP.

Common Mistakes

  • Saying facilitated diffusion needs ATP; it is passive.
  • Describing active transport as movement down the concentration gradient.
  • Stating that peripheral proteins pass through the membrane; that describes transmembrane integral proteins.
  • Forgetting that each transport protein is specific for one type of ion or molecule.

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