5.1 Structure of Cell Membranes Flashcards
Not only do membranes physically separate cells from their external environment, they also:
define spaces within many cells that allow them to carry out their diverse functions
what are the main component of cell membranes?
lipids because they have properties that allow them to form a barrier in an aqueous environment
also has proteins and carbohydrates embedded or associated
what is the major type of lipid found in cell membranes?
phospholipids (made up of a glycerol backbone attached to a phosphate-hydrophilic group and two fatty acids-hydrophobic)
amphipathic
having both hydrophilic and hydrophobic regions
how do amphipathic molecules arrange themselves in aqueous environment? (in water)
they arrange themselves so hydrophilic side faces outwards to interact with water and non polar groups face away from water
polar molecules like water tend to….
exclude nonpolar molecules or nonpolar groups of molecules (useful for healing cell membranes)
what is the shape of the structure determined by?
the bulkiness of the head group relative to the hydrophobic tails
micelle
a spherical structure in which lipids with bulky heads and a single hydrophobic tail are packed
bilayer (formed by lipids with less bulky head gouts and two hydrophobic tails)
a two-layered structure of the cell membrane with hydrophilic “heads” pointing outward toward the aqueous environment and hydrophobic “tails” oriented inward, away from water
the bilayer structure forms spontaneously and without the action of an enzyme as long as:
the concentration of free phospholipids is high enough and the pH of the solution is similar to that of a cell
the pH of the environment is important for cell membranes/phospholipids because :
it ensure that the head groups are in their ionized (charged) form and thus hydrophilic
liposome
an enclosed spherical bilayer structure spontaneously formed by phospholipids in environments with neutral pH, like water
lipids freely associate with one another because of:
extensive van der Waals forces between their fatty acid tails (very weak forces, can easily be broken and re-formed)
fluid
describes lipids that are able to move in the plane of the cell membrane
the degree of membrane fluidity depends on:
which types of lipid make up the membrane
in a single layer of the lipid bilayer, the strength of van der Waals interactions between the lipids’ tails depends on:
the length of the fatty acid tails and the presence of double bonds between neighbouring carbon atoms
the longer the fatty acid tails, the more surface is available for van der Waals interactions resulting in:
tighter packing and reduced mobility
saturated fatty acid tails have no double bonds which has what result on mobility?
reduced mobility because of tighter packing
unsaturated fatty acid tails have double bonds and kinks resulting in:
enhanced lipid mobility due to reduced tightness of packing
cholesterol
an amphipathic lipid that is a major component of animal cell membranes
the amphipathic structure of cholesterol allows it to:
pack tightly with phospholipids: hydrophilic hydroxyl group interacts with polar head of phospholipids, ring structure participates in van der Waals interactions with the fatty acid chains
cholesterol increases or decreases membrane fluidity depending on:
temperature
at temperatures typically found in a cell, cholesterol:
decreases membrane fluidity because of interactions between rings of cholesterol and phospholipid fatty acid tail (van der Waals forces)
at low temperatures, cholesterol:
increases membrane fluidity because it prevents phospholipids from packing tightly with other phospholipids
cholesterol helps maintain a consistent state of membrane fluidity by:
preventing dramatic transitions from a fluid to solid state.
lipid raft
lipids assembled in a defined patch of the cell membrane
membranes are not always a uniform fluid bilayer, but instead can contain regions with:
discrete components
what is a lipid flip-flop?
the spontaneous transfer of a lipid between layers of the bilayer (VERY RARE)
there is little exchange of components between the two layers of the membrane which results in:
the two layers to differ in composition; different types of lipids are present primarily in one layer or the other
transporters
Membrane proteins that move ions or other molecules across the cell membrane.
receptor
A molecule on cell membranes that detects critical features of the environment. Receptors detecting signals that easily cross the cell membrane are sometimes found in the cytoplasm.
enzyme
A protein that functions as a catalyst to accelerate the rate of a chemical reaction; enzymes are critical in determining which chemical reactions take place in a cell.
anchor
A membrane protein that attaches to other proteins and helps to maintain cell structure and shape.
membrane proteins can be classified into two groups depending on:
how they associate with the membrane
what are the two groups of proteins?
integral membrane proteins and peripheral membrane proteins
integral membrane protein
A protein that is permanently associated with the cell membrane and cannot be separated from the membrane experimentally without destroying the membrane itself.
peripheral membrane protein
A protein that is temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions. easily separated from the lipid membrane
transmembrane proteins
Proteins that span the entire lipid bilayer; most integral membrane proteins are transmembrane proteins
transmembrane proteins are composed of three regions:
two hydrophilic regions, one protruding from each face of the membrane, and a connecting hydrophobic region that spans the membrane.
structure of transmembrane proteins allows for:
separate functions and capabilities of each end of the protein
Peripheral membrane proteins may be associated with either the internal or external side of the membrane, they interact with either:
the polar heads of lipids or with integral membrane proteins by weak noncovalent interactions such as hydrogen bonds.
how can one prove protein mobility experimentally
fluorescence recovery after photobleaching, or FRAP
fluid mosaic model
A model that proposes that the lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane and is a mosaic of two types of molecules, lipids and proteins.
