03Feb_9 - Biological Science I Tuesdays and Thursdays...

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Biological Science I Tuesdays and Thursdays 8:00-9:15, HCB 102 Mondays 5:15-6:15, KIN 1024 03 February 2011 – Lecture 9 At the end of this lecture you should be able to: - Define the following terms: amphipathic molecules, aquaporins, diffusion - Explain how membrane fluidity is influenced by temperature and membrane composition - Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions - Explain how transport proteins facilitate diffusion - Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps - Explain how large molecules are transported across a cell membrane 1. The Plasma Membrane The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others a. Phospholipids a.i. Phospholipids are the most abundant lipid in the plasma membrane a.ii. Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions a.iii. The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it b. History b.i. In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins b.ii. Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions b.iii. In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water b.iv. Freeze-fracture studies of the plasma membrane supported the fluid mosaic model b.v. Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer c. The Fluidity of Membranes c.i. Phospholipids in the plasma membrane can move within the bilayer c.ii. Most of the lipids, and some proteins, drift laterally c.iii. Rarely does a molecule flip-flop transversely across the membrane c.iv. As temperatures cool, membranes switch from a fluid state to a solid state c.v. The temperature at which a membrane solidifies depends on the types of lipids c.vi. Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids c.vii. Membranes must be fluid to work properly; they are usually about as fluid as salad oil 1
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c.viii. The Role of Cholesterol c.viii.1. The steroid cholesterol has different effects on membrane fluidity at different temperatures c.viii.2. At warm temperatures (such as 37°C), cholesterol restrains movement of
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03Feb_9 - Biological Science I Tuesdays and Thursdays...

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