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BI110 Lecture Notes - Fluid Mosaic Model, Lipid Bilayer, Passive Transport

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Diane Williams

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5.1 An Overview of the Structure of Membranes
plasma membrane a selectively permeable membrane that allows for the uptake or the key
nutrients and elimination of waste products while maintaining a protected environment in which
metabolic processes can occur
5.1 The Fluid Mosaic Model of Membranes
the Fluid Mosaic Model proposes that membranes are not rigid with molecules locked into
place but rather consist of fluid lipid molecules in which proteins are embedded and float freely
the lipid molecules of all biological membranes exist in a double layer, called a bilayer, that is less
than 10nm thick. Lipids in either side of the bilayer can move, but not switch between layers. Very
membranes contain a wide variety of proteins, each with a specific function
the relative proportions of lipid and protein within a membrane vary considerably depending on
the type of membrane (ie. myelin sheaths are mostly lipid, inner mitochondial membrane is mostly
protein for the electron transport chain)
the proteins and other components of one half of the lipid bilayer are different from those that
make up the other half of the bilayer -> membrane asymmetry. reflects the differences in the
functions performed by each side of the membrane
5.1b Experimental Evidence in Support of the Fluid Mosaic Model
Membranes are Fluid - mouse proteins + human proteins on a cell, they moved around the cell
and mixed
Membrane Asymmetry - cryo freeze a cell and slice the bilayer and look at with electron
microscope to see the bilayer
5.2 The Lipid Fabric of a Membrane
5.2a Phospholipids are the Dominant Lipids in Membranes
phospholipids consist of two fatty acid “tails” linked to one of several types of alcohols or amino
acids by a phosphate group
they are amphipathic, they each contain a region that is hydrophobic (tail) and hydrophilic
when added into an aqueous solution, phospholipids associate with each other and form into a
bilayer spontaneously because it represents the lowest energy state (most stable)
5.2b Membrane Fluidity
the fluidity of the lipid bilayer is dependent on how densely the individual lipid molecules can
pack together. Influenced by two factors:
1.1. Composition of the Lipid Molecules
more unsaturated fats on the tails results in a more fluid membrane
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1.2. Temperature
the lower the temperature the more densely the molecules are packed (less movement)
5.2c Organisms Can Adjust Fatty Acid Composition
exposure to low temperatures may result in membrane viscosity to decrease to the point where
normal membrane permeability is inhibited.
if the membrane solidifies, electron transport ceases to operate
membranes may become too fluid and liquid due to the increase in molecular motion, which can
result in membrane leakage, irreversible disruption of cellular ion balance can lead to cell death
most organisms can adjust the fatty aid composition of their membranes such that proper fluidity
is maintained over a relatively broad range of temperatures
desaturases are enzymes that produce unsaturated fatty acids through fatty acid synthesis. Act
on saturated fatty acides by catalyzing a reaction that removes two hydrogen atoms from
neighbouring carbon atoms and introduces a double bond.
the abundance of desaturases increases as the temperature is lowered to maintain membrane
Sterols act as membrane buffers: at high temperatures, they help restrain the movement of lipid
molecules, thus reducing the fluidity of the membrane
5.3 Membrane Proteins
5.3a The Key Function of Membrane Proteins
1. TRANSPORT - of substances into and out of the cell through channels, or transport proteins
2. ENZYMATIC ACTIVITY - enzyme membrane proteins such as those associated with the
electron transport chain
3. SIGNAL TRANSDUCTION - receptors that bind to signal molecules such as hormones that
4. ATTACHMENT/RECOGNITION - attachment points for a range of cytoskeleton elements, as
well as components involved in cell to cell recognition
5.3b Integral Membrane Proteins
integral membrane proteins are proteins that are embedded in the phospholipid bilayer, most
are transmembrane proteins, spanning the entire membrane bilayer
composed of both hydrophobic (nonpolar) and hydrophilic (polar) domains to interact with the
bilayer and the external and internal aqueous environments
generally 17-20 amino acids long to span the entire bilayer, predominantly nonpolar, and often
coiled into alpha helices
5.3c Peripheral Membrane Proteins
peripheral membrane proteins are on the surface of the membrane and thus do not interact
with the hydrophobic core.
bound to the surface by noncovalent bonds, hydrogen or ionic bonds, usually by interacting with
the exposed portions of integral proteins a well as directly with membrane lipid molecules
mostly on the cytoplasmic side of the membrane
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