Post on 16-Jan-2016
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Membrane Structure & Function
Ch. 7
Membrane & Function
Lipid Bilayer Minimizes number of hydrophobic groups exposed to
water Fatty acid tails don’t have to spend energy fighting
water molecules
Fluid Mosaic Model
Mosaic – because cell membranes have a mixed composition (phospholipids, glycoplipids, sterols, proteins)
Fluid – motions and interactions of the parts of the membrane
Hydrophobic interactions in membrane are weaker than covalent bonds
Most phospholipids and some proteins are free to drift sideways
Membrane Proteins
Transport proteins – allow water-soluble substances to move through Bind molecules or ions on one side/release on the other
Receptor proteins – Bind extracellular substances like hormones that change cell activity
Recognition Proteins – “Molecular Fingerprint” Ex: “Self” sign for immune system
Adhesion Proteins – Cells of same type stick together (tissue)
Crossing Cell Membranes
Selective Permeability
Allows some substances to cross, but not others.
Water, oxygen, carbon dioxide, small nonpolar molecules can cross cell membrane
Glucose, ions, other large polar molecules do not move freely across bilayer
Concentration Gradients
Concentration – number of molecules or ions of a substance in a region
Gradient – number in one region is different than in another
Concentration Gradient – difference in number of molecules or ions in two adjoining regions
Things will naturally travel from
high concentration low concentration
Diffusion – movement of like molecules or ions down a concentration gradient (high low)
Factors that influence rate
Steepness of gradient ↑ Diffusion ↑
Temperature ↑ Diffusion ↑
Molecule size ↓ Diffusion ↑
Electric gradient (opposite charges attract)
Osmosis
Osmosis – Diffusion of water due to a water concentration gradient between two regions through a selectively permeable membrane
The side with more solute has a lower water concentration
Hypertonic Solution – more solutes Hypotonic Solution – less solutes Water goes: hypotonic hypertonic Isotonic Solution – solutes the same, no net movement
Protein Mediated Transport
Passive Transport
Transport proteins allow solutes to move both ways across cell membrane
Concentration ↑ binding ↑ transport ↑
Travels from higher concentration to lower concentration until even
Active Transport
Moves things AGAINST concentration gradient
Requires ENERGY in order to “force” it against the concentration gradient (usually ATP)
Donates phosphate group to a transport protein
Binding site on other side improvesSolute bindsProtein changes shape, solute moves to
other side, binding site becomes less attractive
Solute and phosphate releasedProtein goes back to original shape
Exocytosis & Endocytosis
Exocytosis – cytoplasmic vesicle moves to cell surface & fuses with plasma membrane. Vesicle loses shape, but contents released to surroundings
Endocytosis – cell takes in substances next to its surface Small indentation, balloons inward, pinches off
Three types:
Receptor MediatedMembrane receptors chemically recognize &
bind substances Bulk Phase
Vesicle forms around small volume of extra-cellular fluid regardless of what’s in it
PhagocytosisActive form of Endocytosis where cell takes
in microorganisms and other debris
** Membrane cycling – maintain surface area of membrane