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Chapter 5

BIOL 1000 Chapter Notes - Chapter 5: Aquaporin, Lysosome, Symporter

Course Code
BIOL 1000
Nicole Nivillac

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BIOL 1000 – Chapter 5: Cell Membranes & Signalling
5.1 – Overview Structure of Membrane
Plasma membrane is a selectively permeable barrier that allows for the uptake of key
nutrients and elimination of waste products while maintaining a protected environment
for cellular processes to occur.
Membrane structure is based on the fluid mosaic model which proposes that
membranes are not rigid with molecules locked into place but rather consist of proteins
that move around within a mixture of lipid molecules
-Most membranes contain an assortment of types of proteins
-Proportion of lipid and protein within a membrane depend on the type of membrane
Fluid Mosaic Model PROPOSES…
1. Membranes are fluid
-Lipid molecules of bilayer vibrate, flex back and forth, move sideways and exchange
places within the same bilayer half.
-Proteins (b/c they are larger than lipids) move more slowly in the fluid environment of
the membrane
2. Membrane asymmetry
-Reflects differences in the functions performed by each side of the membrane
5.2 - Lipid Portion of Membranes
Lipid refers to the diverse group of water-insoluble molecules that includes fats;
phospholipids (dominant lipids in membrane) and steroids.
-Phospholipids consist of two parts;
1. Hydrophilic (polar)
Side chain, phosphate & glycerol
2. Hydrophobic (non-polar)
Two fatty acid chains
Phospholipids possess a property, they are all amphipathic – the molecule contains a
region that is hydrophobic and a region that is hydrophilic. (laundry detergents)
Phospholipids spontaneously form these structures in aqueous environments due to the
hydrophobic effect.
-This arrangement is favored because they represent the lowest energy state and are
therefore more likely to occur over any other arrangement.
Membrane Fluidity
Lipid bilayer fluidity is primarily influenced by two factors:
1. Type of fatty acid that makes up lipid molecules (sat/unsat.)
Fully saturated FA are linear and more easily pack tightly together
Unsaturated FA don’t pack as easily because they have double bonds that
introduce kinks to the structure
2. Temperature
As temperature drops, molecular motion slows down until fluidity is lost and a
semisolid gel is formed.
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At high temperatures, molecular motion is increased resulting in membranes
becoming too fluid resulting in a loss of structure.
Keeping membranes in optimal state of fluidity is essential to cell function.
-Too low temps; decreased membrane permeability and inhibit the function of
enzymes and receptors attached to the bilayer
-Too high temps; membrane leakage, ions diffuse freely into and out of cell
Group of compounds called sterols (ex. Cholesterol) also influence membrane fluidity
-They act as membrane buffers;
At high temps, they help restrain the movement of lipid molecules therefore
reducing fluidity.
At low temps, they disrupt FA from associating by occupying space between
lipid molecules therefore slowing transition to gel state.
Unsaturated fatty acids are produced during fatty acid biosynthesis with the help of a
group of enzymes called desaturases.
-All FA start off saturated; these enzymes catalyze reaction that removes 2 H- and
introduces a double bond
5.3 – Membrane Proteins
Proteins associated with membrane determines its function and makes each unique.
There are two major types of proteins; integral and peripheral membrane proteins.
1. Integral Proteins; proteins embedded in the phospholipid bilayer
-Many of these transverse the entire lipid bilayer and are called transmembrane
-Transmembrane proteins interact with both the aqueous environment on both sides
of the membrane and with the hydrophobic core therefore they have distinct regions
called domains.
oDomain that interacts with hydrophobic region consists of nonpolar amino
acids that form a type of secondary structure (alpha-helix)
oDomain that are exposed are composed of primarily polar amino acids
2. Peripheral Proteins; positioned on surface of membrane, do NOT interact with
hydrophobic core of membrane.
-Held to membrane surface by hydrogen and ionic bonds
-Many are found on the cytoplasmic side of the membrane and form part of the
cytoskeleton (ex. Linking microtubule to membrane)
Membrane proteins can be separated into four major functional categories:
1. Transport; protein provides a hydrophilic channel that allows movement of specific
molecule into/out of cell. (shape of protein may change during this transfer)
2. Enzymatic activity; many enzymes are membrane proteins
3. Signal transduction; receptor proteins lay on the outer surface of membrane that
binds to specific chemicals (hormones). This binding triggers changes on the inside
surface of the membrane that lead to transduction of the signal through the cell.
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