Biology 1202B Lecture Notes - Paramecium, Transmembrane Protein, Pinocytosis

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- The proteins in the plasma membrane may provide a variety of major cell functions
- Transport, enzymatic activity, signal transduction, intercellular joining, cell-cell recognition and attachment to
the cytoskeleton and extracellular matrix (ECM)
- Main molecule in a membrane is the phospholipids (phospholipid bilayer - membrane)
- Polar head (hydrophilic; ) with non-polar tail (hydrophobic; fatty acids)
- To study cell membranes you freeze the cell - prefracture
- Model of membrane - fluid mosaic model - created by S.J. Singer and G. Nicolson
- Model proposed that the membrane proteins are dispersed and individual inserted into the phospholipid
bilayer
Membranes are Fluid
- membrane molecules are held in place by relatively weak hydrophobic interactions
- Most of the lipids and some proteins can drift laterally in the plane of the membrane
- Rarely flip-flop from one layer to the other
- The lateral movements of the phospholipids are rapid - 2 micros per second
- Many larger membrane proteins move more slowly but do drift.
- Some proteins move in very directed manner, perhaps guided/driven by the motor proteins attached to the
cytoskeleton
- Other proteins never move, anchored by the cytoskeleton
- Membrane fluidity is influenced by temperature and by its constituents
- Cooling, membranes switch from a fluid state to a solid state as the phospholipids are more closely packed
- Membranes rich in unsaturated fatty acids are more fluid than those dominated by saturated fatty acids
Cholesterol
- Amount may vary with the type of membrane
- Plasma - one cholesterol per phospholipid molecule
- Other membranes (like those around bacteria) have no cholesterol
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