CH 8 - transport across membranes Flashcards
membrane transport
the use of integral proteins to control movement of substances across a membrane
E. coli
- non-photosynthetic bacterium
- takes nutrients from outside itself
- can’t engulf chunks of matter via phagocytosis because its cell wall surrounds the membrane
- takes in all nutrients through membrane transport
surface area to volume ratio for eukaryotic cells vs prokaryotic cells
prokaryotic cells have a higher surface area to volume ratio
solutes move across membrane via three types of transport
- simple diffusion
- movement through channels
- carrier mediated transport
- facilitated diffusion
- primary active transport
- secondary active transport
simple diffusion
diffusion through the phospholipid bilayer, unaided by transport proteins
diffusion
when random movements of individual molecules cause them to spread out. NOT movement of whole volumes of fluid (gas or liquid)
occurs at molecular level
no good real world example because it occurs at such a small scale
SPONTANEOUS AND EXERGONIC BECAUSE IT INCREASES ENTROPY
evidence of what can and cannot pass through pure phospholipid bilayers
- agitate water with phospholipids
- trap solutes in liposomes
- measure how fast solutes diffuse out
- gases diffuse out immediately
- ions remain trapped for days
partition coefficient vs. membrane permeability (slide 8-5)
more hydrophobic, more easily cross phospholipid bilayer
larger molecule, less permeant
below line = larger, less permeable
above line = smaller, more permeable
three things affect a substance’s ability to move across a pure phospholipid bilayer:
- size
- polarity - polar bonds allow formation of hydrogen with water
- charge - net charges allow formation of ion-dipole with water
“To move such solutes into the membrane requires that the water be stripped off… which is a highly endergonic process.”
what would move easily across a phospholipid bilayer?
anything small and lipophilic
Ex. gases like O2, CO2; steroids; ethyl alcohol
rate of simple diffusion depends on the concentration gradient
rate of diffusion inward = PdeltaS (concentration gradient)
v inward = inward flux of substance, ie number of molecules moving in per unit area per time
P = permeability coefficient, depends on membrane and solute
deltaS = [S] out - [S] in
rate of simple diffusion is a linear function of the concentration gradient
movement via transport proteins
transport proteins transport things like ions, molecules with lots of polar bonds, water –>all things that do not pass a pure phospholipid bilayer easily
transport proteins include
- channels - open pathway
2. carrier proteins - grabbing on one side, changing, releasing
movement through channels
hydrophilic pores lined with hydrophilic amino acids
gated channels open and close - others are always open
channels generally carry small things like ions or just water.
channels large enough to carry sugars or whole proteins would lose their selectivity
open pores with very brief, weak binding
porins
large channels found in the outer membranes of mitochondria, chloroplasts, and gram-negative bacteria.
where is their selectively permeable membrane? inner membrane
can open and close to control what enters
ion channels
channels that pass ions
aquaporins
channels that pass water but not ions - unrelated to porins
what makes small channels selective?
size
selective binding on inside of channel
maltoporin
porin that passes maltose and longer glucose polymers
Michaelis-Menten equation can be applied whenever there is reversible binding
v=(Vmax(deltaS))/(Km+deltaS)
Km = concentration gradient at 1/2 Vmax Vmax = max transport rate deltaS = concentration gradient
enzyme saturation occurs why?
binding sites are always occupied
ion channel saturation…
indicates that ions bind channels as they pass through
binding with high Km?
high Km = hard to saturate –> binding is very weak –> binding is very brief
osmosis
=diffusion of a solvent through a semi-permeable barrier from an area of low solute concentration (hyposmotic) to an area of high solute concentration (hyperosmotic)
2 solutions with different amounts of solutes
bio-solute = water
solvent is able to pass through barrier, solute unable
Hypo –> Hyper
hyperosmotic
higher concentration of solute than water
hyposmotic
lower concentration of solute than water
osmolarity
total moles of solute of solution/liter solution
isosmotic
same as original solution
Tonicity
–only applied to solution surrounding cell
Hypotonic: cell swelling - lysis
Hypertonic: cell shrinking - crenation
Agre’s frog egg experiments
some cells are especially permeable to water due to aquaporins
–injected frog eggs with either aquaporin mRNA or control solution, eggs with aquaporins swelled 40% and burst after placed in hypotonic medium while control eggs only swelled a little
low permeability of eggs without aquaporins
carrier-mediated transport
aka “carrier-facilitated transport” via “carrier proteins” “transporters” or “permeases”
proteins bind on one side, changes configuration, releases on other side
uniport
transporter that transports just one substance
symport
transporter that transports more than one substance, in the same direction
antiport
transporter that transports more than one substance, in opposite directions
carrier-mediated transport can be divided into 4 types:
- facilitated diffusion
- primary active transport
- secondary active transport
- light-drived active transport
facilitated diffusion
=a substance can bind on one side and be released on the other and that’s all there is to it - net direction down concentration gradient
direction of primary, secondary, light-driven active transport…
something is transported up its gradient, endergonically –> requires coupling to an energy source
energy source for primary active transport
ATP hydrolysis; the transporters are described as being ATPases
four classes of primary active transport proteins
P-type ATPases
V-type ATPases
F-type ATPases
ABC-type ATPases
P-type ATPases (phosphorylation)
reversibly phosphorylated
most are found in plasma membrane
different types transport various different ions
Ex: Na/K-ATPase (sodium pump) –> keeps [Na+] low and [K+] high within cell, maintains membrane potential; exergonic: breaking down ATP, endergonic: Na/K transport
V-type ATPases (vacuole)
acidify intracellular compartments; transport H ions
F-type ATPases (factor)
inner membrane of mitochondria, thylakoid membrane of chloroplasts, plasma membrane of some prokaryotes
work backwards
energy source = H ions moving exergonically
endergonic proces = ATP synthesis
ABC-type ATPases (ATP-binding-cassette)
transport all sorts of things, including large molecules
a subclass is the multidrug resistance-MDR –> they pump out toxins, drugs, etc. and can make cancer cells resistant to chemotherapy
secondary active transport
one substance moved endergonically
immediate, direct energy source is another one moving exergonically (Na or H)
If the immediate direct energy source is Na influx, what prevents it from accumulating inside? Na/K pump
Thus secondary or indirect… indirectly, energy source is ATP hydrolysis
light-driven active transport
found in some archaebacteria
low oxygen or nutrient levels –> low ATP –> can’t maintain H gradient with primary active transport –> produce purple patches full of bacteriorhodopsin –> H gradient that’s produced is used to make ATP via F-type ATPases
sunlight –> APT but no carbon fixation or reduction. not photosynthesis.
