structure of cell membranes Flashcards
what is the plasma membrane
the cell surface membrane which separates the cell from its external environment
what are membranes formed from
a phospholipid bilayer
describe the phospholipid bilayer
the hydrophillic phosphate heads of the phospholipids from both the inner and outer surface of a membrane sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane.
where do cells normally exist
in aqueous environments. the insides of cells and organelles are also usually aqueous environments
why are phospholipid bilayers perfectly suited as membranes
because the outer surfaces of the hydrophilic head interact with water
what do plasma membranes contain
various proteins and lipids - the type and number are particular to each cell type.
why do the components of the plasma membrane play an important role in the membrane
they play in the functions of the membrane and the cell or organelle they are part of
why are the two types of membrane proteins
intrinsic and extrinsic
what are intrinsic proteins
transmembrane proteins that are embedded through both layers of a membrane. they have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane keeping them in place.
what are the two types of intrinsic proteins
carrier and channel proteins
what are channel proteins
they provide a hydrophillic channel that allow the passive movement of polar molecules and ions down a concentration gradient through membranes. they are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R groups on the outside of the proteins.
what are carrier proteins
have an important role in passive and active transport. involve the shape of the protein changing.
what are glycoproteins
a type of intrinsic protein. they are a protein with a chain of carbohydrate molecules attached.
where are glycoproteins found
they are embedded in the cell surface membrane with attached carbohydrate chains of varying shapes and lengths
what are the roles of glycoproteins
play a role in cell adhesion and as receptors for chemical signals.
what is cell adhesion
when cells join together to form tight junctions in certain tissues
what is the process of cell signalling/cell communication
when a certain chemical binds to the receptor it elecits a response from the cell. this may cause a direct response or set off a cascade of events inside the cell.
what is an example of cell signalling
receptors for neurotransmitters at nerve cel synapses. the binding of the neurotransmitters triggers or prevents an impulse in the next neurone.
another example is receptors for peptide hormones, including glucagon and insulin which affect the uptake and storage of glucose by cells. some drugs act by binding to cell receptors.
what are beta blockers used for
to reduce the response of the heart to stress
what are glycolipids
they are similar to glycoproteins. they are lipids with attached carbohydrate chains. these are cell markers or antigens and can be recognised by the cells of the immune system as self or non self.
what are extrinsic proteins
present in one side of the bilayer. normally have hydrophillic R groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins. they can be present in either layer and some move between layers.
what is cholesterol
a lipid with a hydrophillic end and a hydrophobic end like a phospholipid. it regulates the fluidity of membranes.
where are cholesterol molecules positioned
between phospholipids in a membrane bilayer with the hydrophillic end interacting with the heads and the hydrophobic end interacting with the tails pulling them together. in this way, cholesterol adds stability to membranes without making them too rigid. they prevent the molecules from becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.
why do proteins in the membranes have to be in particular positions
for chemical reactions to take place
why dont all cell membranes have the same composition
cells become differentiated and specialised so their membranes have particular distributions of proteins in order to enable them to carry out their specific functions
what do membranes do in neurones
the protein channels and carriers in the plasma membrane covering the long axon allow entry and exit of ions to bring about the conduction of electrical impulses along their length
what does the myelin sheath do on neurones
the myelin sheath, made of flattened cells, wraps around them several times giving several layers of cell membrane
what % of protein and lipid forms the myelin sheath membrane
20% protein and 76% lipid
what do the plasma membranes of white blood cells have
they contain special protein receptors that enable them to recognise the antigens on foreign cells, usually from invading pathogens but also from tissue or organ transplants
what do root hair cells have a lot of
many carrier proteins to actively transport nitrate ions from the soil into the cells
what % of lipid and protein are in mitochondria membranes
76% protein
24% lipid
their inner membranes contain many electric carriers that are made of protein and hydrogen ion channels associated with ATP synthase enzymes
why do cell membranes need to be partially permeable
because they form a barrier and separate the cell contents from the cell’s exterior environment separate organelles from the cytoplasm, they need to allow some molecules through into or out of the cell. some organelles also have membrane within them and these form barriers too
what is permeability
the ability to let substances pass through
how do small molecules pass through the membrane
some very small molecules simply diffuse through the cell membrane in between its structural molecules
how do other substances pass through the membrane
some substances dissolve in the lipid layer and pass through
other substances pass through special protein channels or are carried by carrier proteins
