membranes Flashcards
how many membranes do eukaryotes have?
many - they have organelles
how many membranes do prokaryotes have?
one - they have no organelles
describe a membrane
partially permeable (selective) barrier that separate the contents of cells from their environment and different areas within cells (organelles)
describe compartmentalisation
formation of separate membrane-bound areas in a cell
vital to a cell because metabolism includes many different (sometimes incompatible) reactions
containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions
how are membranes formed?
phospholipid bilayer
hydrophilic phosphate heads form the inner and outer surface of a membrane
hydrophobic core of fatty acid (tails)
where do cells usually exist?
in aqueous environments
describe the fluid mosaic model
phospholipids are free to move within the layer relative to each other (fluid)
which gives the membrane flexibility
proteins are embedded in the bilayer and vary in size, shape and position (mosaic)
describe intrinsic proteins (integral proteins)
transmembrane proteins embedded through both layers of membrane
have amino acids with hydrophobic r-groups on external surfaces
which interact with the hydrophobic core of a membrane, keeping them in place
how do channel proteins carry out their function?
intrinsic protein
provide a hydrophilic channel
allowing passive movement of polar molecules & ions down a concentration gradient through membrane
what is the function of a carrier protein?
have a role in passive transport (down conc. gradient)
and active transport (against the gradient) into cells
involves the shape of a protein changing
describe glycoproteins
instrinsic proteins
embedded in cell-surface membrane
carb chains (glycocalyx) attached to it with varying shape
glycoproteins play a role in cell adhesion
and as receptors for chemical signals
how does chemical signalling work in glycoproteins
when a chemical binds to the receptor it elicits a response from the cell
may cause a direct response or set off a cascade of events inside the cell (cell communication/cell signalling)
describe glycolipids
lipids with attached carb chains
cell markers or antigens and are recognised by cells of the immune system as self (of the organism) or non-self (cells belong to another organism)
describe extrinsic proteins (peripheral proteins)
present in one side of the bilayer
usually have hydrophilic r-groups on their outer surfaces and interact with polar heads of phospholipids or with intrinsic proteins
some move between layers
describe and outline the function of cholesterol in the membrane
lipid
has hydrophilic end and hydrophobic end
regulates fluidity of membrane
positioned between phospholipids with hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails pulling them together
which adds stability to membranes without making them too rigid
cholesterol prevents membranes becoming too solid by stopping the phospholipids from grouping too closely and crystallising
what do enzymes have to do in membranes forming organelles for chemical reactions to take place
have to be in particular positions
what kind of molecules are able to rapidly diffuse into the membrane, diffuse slower, or are unlikely to diffuse across the membrane?
small, non-polar - diffuse quickly
small, polar - diffuse slowly
charged particles - unlikely to diffuse
how is membrane stability ensured by phospholipids
phospholipids are amphipathic
interaction between hydrophobic and hydrophilic ends helps give membrane stability
what kind of molecules are able to diffuse through the membrane
lipid soluble (hydrophobic) molecules
hydrophilic molecules cannot diffuse through
what gives the membrane fluidity
lipids move laterally within membrane
exchange position with each other
what is the degree of cell membrane fluidity determined by
length of fatty acid side chains
(longer chains, lower fluidity)
proportion of fatty acids which are saturated
(higher the % of fatty saturated fats, lower fluidity)
steroid content (cholesterol)
(higher the steroid content, lower the fluidity)
what are the four main functions of intrinsic proteins?
act as channels
transport
receptors
enzymes
what do intrinsic proteins and the lipids have in common?
have hydrophilic and hydrophobic regions which interact
confers stability on the membrane
describe the method to which glucose is transported across the cell surface membrane
facilitated diffusion
transport protein binds to glucose
(transport protein is specific to a molecule)
protein changes shape
glucose transported to the other side of the membrane
glucose detaches from transporter protein
protein reverts back to original shape
glucose is phosphorylated
(maintains diffusion/conc gradient)
