Cell Membranes Flashcards
Cell Membranes
• Plasma membrane (plasmalemma) is the
boundary that separates a cell from its
surroundings and controls traffic into and
out of the cells
• It exhibits selective permeability, due to its
structure (lipids, carbs and proteins)
o Function is fundamental to life
• The hydrophobic interior of the bilayer is
one reason membranes are selectively
permeable
Eukaryotic cell membrane
• Plasma membrane (plasmalemma) is an
external feature of a eukaryotic cell
• 8nm thick
• Ensures a steady internal state
Composition of cellular membranes
• Lipids, proteins and carbohydrates are the staple ingredients of membranes • Most abundant lipids à phospholipids o Ability to form membranes is inherent in their structure
Phospholipids
• Similar to fat • Constructed of two parts: o Two fatty acids joined to glycerol -> hydrophobic backbone (tail) that is non- polar
o Phosphorylated alcohol (phosphate) on
last hydroxyl group -> Hydrophilic head on
molecule that is polar
• Amphipathic molecule -> has a hydrophilic
and a hydrophobic region
• Unsaturated hydrocarbons (double carbon
bond) in molecular structure causes a kink in
structure to form
• Two phospholipids can form a bilayer
o Stable boundary between aqueous
compartments
o Molecular arrangement allows the
hydrophobic tail to be sheltered from
water while exposing hydrophilic head
Proteins
• Like lipids are amphipathic
• Reside in the phospholipid bilayer with
hydrophilic regions protruding
• Maximizes contact with water while keeping
hydrophobic areas dry
This model is known as the fluid mosaic model ->
membrane is a mosaic of protein molecules
bobbing in a fluid bilayer of phospholipids
Fluid Mosaic model
• Fluid refers to pattern of proteins
interspersed amongst phospholipids
• Membranes must be fluid to work properly
as it affects cell permeability and the ability to
move proteins to where they are needed
• Fluid refers to the fact that components
(proteins and phospholipids) are not static
and are capable of moving in plasma
membrane
o Phospholipids move faster than
proteins however due to size
Effects of Temperature of fluidity
• Higher temperatures would allow
phospholipids to move further away from
each other
• Lower temperatures would move them
closer until they settle into a closely packed
arrangement and the membrane solidifies
• The more kinks in the phospholipid tails the
lower the solidifying temperature
• Kinks in tails of the phospholipids prevent
them from getting close together
Cholesterol -> fluidity buffer
• Assists in retaining integrity or structure of the
phospholipid layer
• They are wedges between phospholipid
molecules of membrane
• At high temperatures makes membrane less
fluid by restraining movement
• Low temperatures: cholesterol hinders close
packing of phospholipids -> lowers the
required temp for the membrane to solidify
Structure of proteins
• Proteins have levels of organization
o All are made from a sequence of amino acids
o Although proteins are diverse polymers, they are
all still made from the same 20 amino acids
o Polymers of amino acids (monomers, building
blocks) -> polypeptides
• Primary structure:
o R group -> side chain
o Can carry a charge
o Determines how the polypeptide will fold
• Secondary structure:
o Polypeptide chains twisting due to H bonds in the
backbone (can be helix or pleating)
• Tertiary structure: represents overall shape of the
polypeptides
o Interaction between R groups
o Interactions reinforced by disulphide bridges
• Quaternary structure: represents the overall protein
structure
o Results from the aggregation of the polypeptide
subunits
Shape of protein determines the function
• Structure of protein is vulnerable -> can be
destroyed (often irreversibly) by:
o Heat
o pH
o Chemicals
• Results in proteins losing folding ability, as
a result might not be able to function
properly
Possible test question -> If a generic mutation
changes primary structure, how might it
destroy the proteins function.
Movement of substances across
membrane
• Hydrophobic molecules -> those that are soluble in lipids, can easily pass through the membrane. • Small molecules like O2 can sneak between the phospholipids of the membrane • On the other hand, some hydrophilic molecules (like water and glucose) can pass only slowly • Ions (such as sodium ions and hydrogen ions) cannot pass directly through the phospholipids of the plasma membrane
Membrane Proteins and their functions
• Mosaic aspect of fluid mosaic model
o Membrane is a collage of different proteins, embedded in the fluid matrix of the fluid bilayer
• Phospholipids form the main fabric of the membrane but proteins determine most of the
membrane’s functions
• Different cells each have a unique set of proteins in their membranes
• There are two major populations of proteins
o Integral proteins
o Peripheral proteins
Integral Proteins
• Integral proteins penetrate the hydrophobic
interior of the lipid bilayer
• Majority are transmembrane proteins (span
entire membrane)
• Others extend only partway in hydrophobic
region
• Hydrophobic parts are coiled into helices and
nonpolar
• Hydrophilic areas are exposed to aqueous
solutions either side of membrane
• Some proteins have channels that allow
passage through the membrane of
molecules
Peripheral Proteins
• Not embedded in the lipid bilayer
• Appendages loosely bound to the surface of
the membrane, often to exposed parts of
integral proteins
Features and Functions
• On cytoplasmic side, proteins are held in
place by cytoskeleton
• On extracellular side, certain proteins are
attached to fibres of extracellular matrix
o Provides cell with stronger
framework
• Surface proteins carry out different
functions whilst a single membrane protein
may carry out different functions itself
• Makes the membrane not only a structural
mosaic but also a functional mosaic
• See diagrams for the six functions of
membrane proteins
