lecture 2- membranes Flashcards
membranes and their environment
membranes can act independently of their environment, but also with it- needs to receive and interpret signals from outside environment to know what to do, has to communicate with its surroundings
structural components of cell membranes
phospholipid & protein
- lipids cannot dissolve in water, so lipids make a good barrier (water can’t pass through), cell membrane made up of only lipids cannot exist in aqueous environment –> so convert lipid to phospholipid, substitute phosphate group for fatty acid (add some polarity)
describe structural changes made to phospholipid in membrane
add choline to the phosphate group, since nitrogen has 4 bonds, it is positive (have put a phosphocholine group on one glycerol carbon), so have added a polar component to the lipid
- phosphocholine is the hydrophilic head (allows this area to be dissolved in water, can exist in water)
- the twin fatty acid tails are the hydrophobic tails, water insoluble, form the barrier of the membrane of the inside and outside
phospholipids are ___, meaning they are both __ and __, part likes __ and part likes __
amphipathic
hydrophilic and hydrophobic
water and lipid
which sides of the membrane do the hydrophilic heads face and hydrophobic tails face
hydrophilic layer faces outside the cell and inside the cell
hydrophobic tails buried inside membrane and forms water insoluble layer
membrane is fluid (always in motion), lipids form lipid bilayer
membranes made only of lipids have disadvantages-
fairly rigid, impermeable
proteins can exist in different configurations in the lipid bilayer, name 2 kinds of proteins and the one subset
1- peripheral proteins
2- integral proteins
3- transmembrane proteins
describe peripheral proteins as a part of the membrane
on the surface, peripheral proteins exist on the inside or outside of membrane
- usually small & loosely connected (no strong covalent bonding)
- act as surfactants (surface-acting proteins)- they lower the rigidity of the membrane and make it more flexible
peripheral proteins act as ___, they lower the rigidity of the membrane and make it more ___
surfactants
flexible
describe integral proteins as a part of the membrane
permanent anchored part of the membrane, have many functions- function as receptors, or cell-surface recognition proteins, or as enzymes
describe transmembrane proteins as a part of the membrane
subset of integral proteins that span the whole width of the membrane
- can act as receptors and can form a channel in the membrane, can act as transport proteins to bind substances and carry across membrane , can also act as pumps (transport substance across membrane against a gradient)
proteins are arranged in such a way in the membrane to be referred to as…
fluid mosaic model (arranged in mosaic tile fashion)
describe the distribution of amino acids of the proteins in a membrane
within membrane: nonpolar amino acids (9 nonpolar aa’s, aromatic aa’s), hydrophobic- no charge at all
- anchors protein into membrane/lipid bilayer
on outer surfaces of membrane in fluid- polar amino acids, hydrophilic
- extend into extracellular fluid & cytosol
describe the distribution of amino acids as a part of the pores that span the membrane
amino acids on the outside of the pore are nonpolar, interact with fatty acid tails
- the inside of the pore has polar aa’s, allows water and soluble substances to pass through
describe transport proteins as simple channels & aquaporins
some transport proteins form simple channels, just a hole in the membrane
water channel (aquaporin)- allows free passage of water in and out of cell (always open) –> the cell regulates the number of these channels inserted into membrane in order to regulate the amount of water in/out
describe an example of a very large & complex transmembrane protein
acetylcholine receptor- large transmembrane protein made of 15 subunits, arranged in a circular ring, middle of ring creates a channel in the membrane
- when acetylcholine binds to receptor, opens the channel
some transmembrane proteins form pumps…
- pumping goes on against the gradient
sodium pump: splits ATP (energy), pumps 3 sodiums outward and 2 potassium inward
some transmembrane proteins span the membrane between 2 adjacent cells…
form pores that allow communication between one cell to the other through movement of small substances like ions
- these are called gap junctions
name 4 mechanisms of membrane transport
1- diffusion
2- facilitated diffusion
3- endocytosis/exocytosis
4- active transport
define diffusion
the passive, random movement of a substance through a membrane from an area of high to an area of low concentration (downhill)
- passive- does not require energy on the part of the cell
describe the process of diffusion in terms of ions
ions always going to be moving due to thermal energy, as they move, going to collide with one another – every once in a while, big enough collision that it will push an ion through the membrane
- the energy is from collisions due to thermal energy, more collisions there are –> greater probability it will push one through the membrane from high to low conc.
- also collisions going on on the inside b/c some ions on inside –> every once in a while, can push an ion back out (so theoretically can move in both directions)
in the process of diffusion, the energy is from ___ due to ___
net movement will always be from __ to __ concentration
collisions
thermal energy
high to low
what can diffuse through the lipid bilayer without a protein?
anything that is nonpolar:
respiratory gases- O2, CO2
small uncharged molecules (not polar)
anything water soluble, amino acids, ions, glucose, urea cannot diffuse- needs protein carrier
what is more permeable- Na or K?
K is 30x more permeable than Na
diffusion can be explained with this equation
Fick’s law of diffusion ds/dt = Ds x As x dc/dx
rate of diffusion = conc grad x SA x diffusion coeff / membrane thickness
ds/dt = the rate of diffusion
the change in concentration of substance / change in
time
Ds = diffusion coefficient (measure of the permeability of a substance across that membrane) – nonpolar substance has higher permeability –> higher rate of diffusion
As - stands for surface area of membrane that’s available for the substance to diffuse across (nonpolar like oxygen diffuse faster than Na+ b/c surface area)
dc/dx
dc = the larger the conc gradient, faster the diffusion
dx = area that has to be crossed (thicker the membrane, slower the diffusion)
describe facilitated diffusion
diffusion with the aid of a carrier molecule (still passive, still goes from high to low conc), just needs some type of protein carrying molecule that can bind a substance and carry across
- usually speeds things up, increases rate of diffusion
- ex: myoglobin (transport of oxygen across membrane from cell into mitochondria by carrying it)
which is faster- simple or facilitated diffusion
facilitated is faster
- simple diffusion is like everyone walking thru a door, have to open and close the door each time (slower)- ion diffusion through a channel is simple diffusion
example of facilitated diffusion & simple
facilitated- myoglobin transports oxygen
simple- ion diffusion through a channel
define exocytosis
the quantitative release of a substance from a vesicle; vesicle opens and releases all the substance at once, takes energy to do this but not necessarily against the concentration gradient
define endocytosis
membrane invaginates and buds off and forms vesicle and engulfs something from outside to bring inside
do exocytosis and endocytosis require energy?
yes
define active transport
the net movement of a substance against the concentration gradient (low to high conc) requiring the expenditure of energy on the part of the cell (energy usually in the form of splitting ATP)
- protein pump has ability to split ATP and use that energy to transport substances across membrane against gradient
the major lipids of the plasma membrane are ___ and ___
phospholipids and phosphoglycerides
the sterol molecule ___ is is a critical component of lipid bilayer, found in both leaflets and serves to stabilize the membrane at normal body temp
cholesterol
as temperature increases, fluidity of the membrane ___
increases
a minor lipid component of the plasma membrane is ___, an example of this called ___ plays an impt role in anchoring proteins to the outer leaflet of the membrane
glycolipid
GPI
both cholesterol & glycolipids are ___, just like phospholipids, oriented with their polar groups on the outer surface and their hydrophobic portion on the interior of the membrane
amphipathic
transmembrane proteins have both hydrophilic and hydrophobic regions…the hydrophobic region is often in the form of ___ and spans the membrane; hydrophilic amino acid residues are then exposed to the aqueous environment on either side of the membrane
alpha helix
the superfamily of membrane proteins that serve as receptors for many hormones, neurotransmitters, and numerous drugs , these receptors are coupled to ___ and are called ___
these proteins span the membrane with _____ domains
heterotrimic G proteins
G protein- coupled receptors
7 alpha-helical domains
membrane transporters are divided into 3 groups based on their mode of transport:
1- uniporter (facilitated transporter)- transports a single molecule across the membrane
2- symporter (cotransporter)- couples the movement of 2 or more molecules/ions across the membrane
3- antiporter (exchange transporter)- couples the movement of 2 or more molecules/ions across the membrane, but in opposite directions
the ATPase ion transporters are are subdivided into…
1- P-type ATPases
2- V-type ATPases
describe P-type ATPases
they are phosphorylated during the transport cycle
ex: Na+/K+ ATPase
- with the hydrolysis of each ATP molecule, it transports 3 Na+ out of cell and 2 K+ into cell
describe V-type ATPases
found in the membranes of several intracellular organelles (aka vacuolar H+ ATPases)
- the H-ATPase in the plasma membrane plays impt role in urinary acidification
a specific type of endocytosis is ___, which consists on the nonspecific uptake of small molecules and water into the cell
pinocytosis
the movement of water across cell membranes occurs by the process of ___
the movement of water is passive with the driving force for this movement being the ___ difference across the cell membrane
osmosis
osmotic pressure
osmotic pressure is determined by the number of ___ dissolved in ___
it is not dependent on factors such as…
solute molecules
the solution
size of molecules, their mass, or chemical nature
what is the misspelled word on p.5 of textbook
imbedded/embedded
