BIOSC 0100 Lecture Notes - Lecture 10: Fluid Mosaic Model, Lipid Bilayer, Electrochemical Gradient

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6 Feb 2017

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Chapter 6 Cont.
Cell membranes:
-Contain almost as much protein as phospholipids
-Early models of the cell membrane suggested that the phospholipid bilayer was sandwiched
between proteins
-Singer and Nicolson challenged the sandwich model and proposed that amphipathic proteins
could span the membrane
-Fluid Mosaic Model (1972) current model of cell membrane structure
Integral Membrane Proteins
-Embedded within the hydrophobic interior of lipid bilayers
-Amphipathic with a hydrophobic region that may cross the entire membrane or extend only
part way into the bilayer
-If the integral proteins span the entire membrane and have hydrophilic inside and outside the
cell is called a transmembrane protein
-Can be isolated from membranes with detergents (amphipathic molecules)
-Detergents break up the membrane
Peripheral Membrane Protein
-Proteins bind to the membrane without passing into the membrane
-Found only on one side of the membrane
-Non-covalent interactions often attached to integral membrane proteins
Transport proteins:
-Transmembrane proteins that transport ions and molecules
-Three classes: channels, carrier proteins/transporters, and pumps
Channel Proteins:
-Act as tunnels for ions or small molecules to cross a membrane
-Highly selective allow only a particular type of ion or molecule to pass through
-Electrochemical gradients occur when ions build up on one side of a plasma membrane
-Ion channels are specialized merae proteis that iruvet the plasa erae’s
impermeability to small, charged compounds
-Diffuse through channels down their electrochemical gradients
CFTR example of an ion channel
-Aquaporins water channels that allow water to cross the cell membrane faster than it would
by simple diffusion (kidneys, eyes = water needs to move faster)
-Flow of ions and small molecules through membrane channels is carefully controlled
-Gated channels: open and close in response to a signal
-Movement of substances through channels does not require an input of energy (high to low
-Passive transport: diffusion of a substance across a membrane along an electrochemical
gradient with no energy investment
-Facilitated diffusion: passive transport of substances that would not otherwise cross the
membrane can occur through channels or through carrier proteins
Carrier proteins
-Can change shape during the transport process
-Carrier proteins only move molecules down a concentration gradient
-GLUT-1 is a glucose transporter that increases membrane permeability to glucose
-Carrier/transporter proteins are passive transporters high to low concentration
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