BISC 101 Lecture Notes - Lecture 5: Exocytosis, Endocytosis, Electrochemical Gradient

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BISC 101 – Lecture 5 – Cell Membrane Structure and Function
Cell Membrane Structure & Function
Cellular membranes are fluid mosaics of lipids and proteins
The plasma membrane or cell membrane separates life from nonlife
It is a layer of molecules that surrounds the cell interior and separates it from the
external environment
The plasma membrane serves as a selective barrier: it keeps damaging
compounds out of the cell and allow entry of compounds needed by the cell
Plasma membrane keeps appropriate chemicals in an enclosed area, causing
reactants to collide more frequently, allowing chemical reactions to occur more
efficiently
The Structures of Membrane Lipids
Not all lipids can form membranes
Membrane – Forming Lipids: Must have both a polar hydrophilic region and a
non – polar hydrophobic region
The charges and polar bonds in the head region interact with water molecules
The fatty acid tails of a phospholipid are nonpolar and hydrophobic
Amphipathic: Compounds that contain both hydrophilic and hydrophobic
elements
oMost steroids and phospholipids are amphipathic molecules
The amphipathic nature of phospholipids is their most important feature, as they
are responsible for plasma membranes
Phospholipid Bilayers
Phospholipids are the most abundant lipid in the plasma membrane
Phospholipids are amphipathic molecules
Phospholipids do not dissolve when they are placed in water
Water molecules interact with the hydrophilic heads of the phospholipids but not
with their hydrophobic tails
oThe hydrophobic tails begin to interact with each other
Phospholipid bilayers are created when two sheets of phospholipid molecules
align
No input of energy is required for phospholipids to form
Amphipathic molecules are much more stable in aqueous solution when their
hydrophobic tails avoid water
Selective Permeability of Lipid Bilayers
A cell must exchange materials with its surroundings, a process controlled by the
plasma membrane
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Permeability: A structure’s tendency to allow a given substance to pass across it
Lipid bilayers are highly selective
Selective Permeability: Some substances cross a membrane more easily than
other substances can
oPolar molecules, such as sugars, do not cross the membrane
oCharged substances are virtually unable to pass through a lipid bilayer
oHydrophobic or Non – polar molecules can dissolve in the lipid bilayer
and pass quickly
Large charged polar molecules cannot pass through the nonpolar hydrophobic
tails of a lipid bilayer
Ions that are electrically charged or polar are more stable in solution where they
interact with water than they are in membrane’s interiors which are neutral
High to low permeability:
oSmall, nonpolar:
oSmall uncharged polar
oLarge uncharged polar: Cannot pass
oIons: Cannot pass
Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer
Most of the lipids and some proteins, drift laterally
Barely does a molecule flip – flop transversely across the membrane
Bond Saturation and Hydrocarbon Chain Length Change Membrane Permeability
Hydrophobic tails are packed in the center of a lipid bilayer
If unsaturated lipids (double bonds) are present then more kinks will appear
oThese spaces produced from the kinks will reduce the strength of
hydrophobic interactions between the tails
oInteractions between the tails are stronger among saturated hydrocarbon
tails
Hydrophobic interactions also become stronger as saturated hydrocarbon tails
increase in length
Lipid bilayers become more permeable and fluid when they consist of short,
unsaturated hydrocarbon tails
oThe interior is held together less tightly, allowing more materials to pass
Cholesterol Change Membrane Permeability
Steroid rings in cholesterol are bulky
High cholesterol concentration in cell membranes lead to decrease in membrane
fluidity
Low cholesterol concentration leads to an increase in membrane fluidity
Steroid cholesterol with differences in temperature, affects fluidity
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