Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.
• Amphipathic phospholipids have hydrophilic and hydrophobic properties.
• Membrane proteins are diverse in terms of structure, position in the membrane and function.
• Cholesterol is a component of animal cell membranes.
Applications and skills:
• Application: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.
• Skill: Drawing of the fluid mosaic model.
• Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.
• Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.
• Drawings of the fluid mosaic model of membrane structure can be two
dimensional rather than three dimensional. Individual phospholipid
molecules should be shown using the symbol of a circle with two parallel lines attached. A range of membrane proteins should be shown including glycoproteins.
Na+ K+ pump
Application: Structure and function of sodium–potassium pumps for active
transport and potassium channels for facilitated diffusion in axons.
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.
Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.
Design your experiment!