lecture 22 - transport across cell membranes Flashcards
non-mediated transport
doesn’t use transport protein, substances move directly through the lipid bilayer.
mediated transport
requires transport proteins (membrane is otherwise a barrier to this material)
passive transport
moves substances down concentration gradient using only their kinetic energy
active transport
uses energy to drive substances against concentration gradient or electrochemical gradient
vesicular transport
move materials across membrane in small lipid vesicles by either exocytosis or endocytosis
transport involved with absorption of nutrients and excretion of waste
non-mediated transport
What is transported via “non-mediated transport”?
- non-polar hydrophobic molecules
- e.g. O2, N2, small alcohols, fatty acids, steroids, fat soluble vitamins AEDK
fat soluble vitamins are…
A, E, D, and K
ion channels are _______ filled pores
ion channels are water filled pores
which type of amino acids line the inside and the outside of the ion channels
hydrophobic amino acids coiled into alpha helices on the outside. hydrophilic amino acids on the inside of the channel
do the ions interact and bind with the ion channel? what does this allow?
NO, this allows for rapid transport (1million per second)
in ion channels what determines direction of the ion flow?
down electrochemical gradient
ion selectivity filter = ?
selectivity of the channel to specific ions, achieved using various amino acids. Allows only specific ion gradient’s energy to be harnessed.
why do ion channels have gates
if they were always open you would not be able to store energy in ion gradients - they would always equilibriate
5 stimuli for opening/closing gates
- voltage
- ligand binding
- cell volume
- pH
- phosphorylation
patch clamp technique
measures the current travelling through an individual channel. measures ion channel function.
diffusion of over 1 _______ ions per second generates current of ____A
diffusion of over 1 million ions per second generates current of _10^-12_A
what do current fluctuations indicate
opening and closing of gates
main way carrier-mediated transport differs to ion channels. implications?
the substrate being transported binds directly to the channel, inducing a conformational change, meaning the rates of transfer are much slower
how are carrier transport proteins like enzymes?
- specificity
- inhibition (covalent bonds form)
- competition (glucose/glactose compete for same site)
- saturation (transport maximum)
carrier-mediated can be ______ or _______
carrier-mediated can be Passive or Active
passive mediated transport called…
facilitated difsussion
3 steps to facilitated diffusion of glucose
- glucose binds to GLUT
- Glut changes shape. Glucose moves across cell membrane DOWN concentration gradient
- Kinase reduces glucose concentration (tranforms to glucose-6-phosphate) otherwise you reach an equilibrium g
primary active transport
energy comes straight from the hydrolysis of ATP.
what percentage of a cell’s ATP is used for primary active transport?
~30%
secondary active transport
energy indirectly comes from ATP (the energy stored in ionic gradients is used)
Ca/K ATPase
muscle SR
H/K ATPase
stomach acidity
electrogenic =
net current generated
Na/K ATPase 4 steps
- Na+ binding
- ATP hydrolysed, P binds and 3Na+ pushed out
- K+ binding, P released
- 2K+ pushed in
Na/K ATPase important for
- maintaining resting membrane potential
- electrical excitability
- muscle contraction
- maintain cell volume
- uptake of nutrients via secondary active transport
- pH by secondary active transport
Pump-leak hypothesis
Na and K are constantly leaking back in, thus the pump must work consistently
Na+ antiporters (or exchanges)
Na+ rushes in causing Ca2+ or H+ to be pushed out
Na+ symporters (or cotransporters)
glucose or amino acids (MUST BIND FIRST) then rush inwards together with Na+
endocytosis
large particles into the cell. Substance transported is inside a vesicle and therefore doesn’t make contact with the hydrophobic core and whatnot
phagocytosis
cell eating - foreign particle binds to receptor proteins. pseudopods extend to form phagosome
pinocytosis
cell drinking - no receptor proteins, no pseudopods form, non-selective
exocytosis
substances are exported from cells by fusion of vesicle with cell membrane
vesicles necessary to…
prevent things from reacting that shouldn’t be e.g. protease in the pancreas
neurotransmitters and enzymes transported via
exocytosis
receptor-mediated endocytosis for?
uptake of specific substances or LIGANDS
receptor-mediated endocytosis steps
- desired substance binds receptor protein in CLATHERIN-COATED PIT REGION of membrane
- causes membrane to fold inwards, forms vesicle
- vesicles become uncoated and bind with ENDOSOME
- receptor proteins seperate from ligands and return to surface
- ligands digested by lysosomes or transported across cell
