BIOL 1010 Lecture Notes - Transmembrane Protein, Cellular Respiration, Integral Membrane Protein

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12 Oct 2012
School
Department
Course
Membrane Structure and Function
Lecture 13-14
Structure
1. Membrane Models
2. Fluid mosaic of lipids, proteins, and carbohydrates
3. Selective permeability
4. Passive transport. Osmosis.
5. Active Transport
6. Exocytosis and endocytosis
The cell membrane
- The plasma membrane is the boundary that separates the living cell from its nonliving
surroundings.
- Membranes are of crucial importance to life, because a cell must spate itself from the
outside environment for two major reasons
o It must keep its molecules of life (DNA, RNA. Proteins) from dissipating away.
o It must keep out foreign molecules that damage or destroy the cells components
and molecules.
1. Membrane Models
- Two generations of membrane models
o A) The Davidson-Danielli model. Sandwiched phospholipid bilayer between two protein
layers.
Widely accepted until 1970.
o B) The fluid mosaic model disperses the proteins and immerses them in the
phospholipid bilayer, which is in a fluid state.
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Singer and Nicolson proposed
- Proteins are individually embedded in the phospholipid bilayer, rather than forming a solid
coat spread upon the surface.
- Hydrophilic portions of both proteins and phospholipids are maximally exposed to water
resulting in a stable membrane structure
- Hydrophobic portions of proteins and phospholipids are in the nonaqueous environment
inside the bilayer.
- Membrane is a mosaic of proteins inserted in a fluid bilayer of phospholipids.
The cell membrane
- The cell membrane functions as a semi-permeable barrier, allowing very few molecules
across it while fencing the majority of organically produced chemicals inside the cell.
- The most common molecule in the model is the phospholipid which is polar
(hydrophilic)head and two (hydrophobic) tails
Artificial membranes
- (a) water can be coated with a single layer of phospholipid molecules
- The hydrophilic heads of phospholipids are immersed in water, and the hydrophobic tails
are excluded from water.
- (b) a bilayer of phospholipids forms a stable boundary between two aqueous
compartments.
- This arrangement exposes the hydrophilic parts of the molecules to water and shields the
hydrophobic parts from water.
2. The fluid mosaic of lipids, proteins and carbohydrates
- A membrane is held together primarily by the hydrophobic interactions, which are much
weaker than covalent bonds.
Movement of phospholipids
- Most of the lipids and some of the proteins can
drift in the plane of the membrane, but not from
one layer to another.
- Phospholipids move quickly along the
membrane’s plane averaging 2 MicroM per second. Proteins drift more slowly.
Evidence for the drifting of membrane proteins
- When researchers fuse a human cell with a mouse cell, it takes
less than an hour for the membrane proteins of the two
species to completely mix in the membrane of the hybrid cell.
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Membrane fluidity
- Tails with kinks are keeping molecules from packing together, enhancing membrane fluidity.
Cholesterol within the membrane
- Cholesterol reduces membrane fluidity by reducing phospholipid
movement at the moderate temperatures and also hinders
solidification at low temperature: it make the membrane less fluid
at warm temps and more fluid at lower temp.
Sidedness of the plasma membrane
- The membrane has distinct cytoplasmic and extracellular sides.
- The bifacial quality determined when the membrane is first synthesized and modifies by the
ER and Golgi
- The side facing the inside of the ER, Golgi and vesicles is topologically equivalent to the
extracellular surface of the plasma membrane,
- The other side always faces the cytosol, from the time the
membrane is made by the ER to the time it is added to the plasma
membrane by fusion of a vesicle.
- The small green “trees” represent the membrane carbs that are
synthesized in the ER and modifies by the Golgi
- Vesicle fusion with the plasma membrane is also responsible for
secretion of cell products (purple).
Cell recognition by membrane carbohydrates
- Cell-cell recognition- the ability of a cell to determine if other cells
its encounters are alike or different from itself.
- Cell-cell recognition is crucial in the functioning of an organism. It
is the basis for
o Sorting of an animal embryo’s cells into tissues and organs
o Rejection of foreign cells by the immune system
- Because of their diversity and location, likely candidates are
membrane carbohydrates:
o Branched oligosaccharides
o Some covalently bonded to lipids(glycolipids)
o Most covalently bonded proteins(glycoproteins)
o Vary from species to species, between individuals of the same
species and among cells in the same individual.
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