Membrane structure
Davson-Danielli model (a)
Singer-Nicolson model (b)
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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. Can you create a membrane, in the time frame allowed, that will work correctly ? Can you identify organelles with lightning speed? Click below to show your skills! |
Membrane transport
Na+ K+ pumpApplication: Structure and function of sodium–potassium pumps for active
transport and potassium channels for facilitated diffusion in axons. |
Passive transportParticles 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. |
OsmosisApplication: 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. (Practical 2) Design your experiment! 
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