Lesson 1 - Transport Mechanisms Flashcards
1
Q
what is the cell membrane composed of and what molecules can diffuse readily
A
- Cell membrane = composed of Phospholipid bilayer
- Lipid-soluble molecules and gases diffuse through readily
- Water-soluble molecules cannot cross without help
- Impermeable to organic anions (proteins)
–> selective permeability
2
Q
what does cell membrane permeability depend on? (3)
A
Permeability depends on molecular size, lipid solubility, and charge
3
Q
explain simple diffusion
- what molecules pass through?
- what direction does it move in?
- what is the rate of diffusion
- what energy is needed, if any?
A
- Small, lipid-soluble molecules and gases (e.g. O2, CO2, ethanol, urea etc…) pass either directly through the phospholipid bilayer or through pores
- Movement of substance is down its [ ] gradient
- The relative rate of diffusion is roughly proportional to the [ ] gradient across the membrane
- Passive: No energy input required from ATP
4
Q
explain facilitated diffusion
- what is it?
- what molecules pass through?
- what direction does it move in?
- what energy is required, if any?
A
- Process of diffusion, where molecules diffuse across membrane, with the assistance of carrier protein
- Carrier protein aid the movement of polar molecules (e.g. sugars and amino acids) across cell membrane
- Movement of substance is down its [ ] gradient
- The energy comes from the [ ] gradient of the solute
- Passive: No energy input required from ATP
5
Q
how do channel proteins work?
A
- one gate is open
- molecule enters
- undergoes a conformational change
- first gate closes and second opens to release the molecule
6
Q
what is active transport?
- what is it?
- how does it work
- what energy is needed and how is it acquired?
A
- Active Transport is a mechanism to move selected molecules across cell
membranes, against their [ ] gradient - Substance binds to protein carrier that changes conformation to move substance across membrane
- Active Requires energy from ATP hydrolysis
7
Q
what are ATPases and what is a famous example?
A
- ATPases are a group of enzymes that catalyze the hydrolysis of a phosphate bond in adenosine triphosphate (ATP) to form adenosine diphosphate (ADP)
- the sodium potassium pump is a famous example of how ATP is used to transport Na out and K in
8
Q
what is secondary active transport?
- what is it?
- how does it work?
- what does sequential binding do?
A
- when a substance is transported up/against its concentration gradient without using ATP
- it works by harnessing the kinetic energy that one substance generates from moving down its [ ] gradient to move its substance against the [ ] gradient
- Sequential binding of a substance and ions to specific sites in the transporter protein induces a conformational change in the protein
9
Q
what are channels?
- what is it made of?
- what does it look like in the membrane?
- is it selective? if so how?
- what are pores also called?
A
- membrane spanning protein (around 4-5 protein subunits) creates a central pore for which specific ions can diffuse
- These ‘Pore loops’ of the protein molecules dangle inside the channel
- Physical properties of the pore loops create a selectivity filter
–> Only specific molecules can diffuse through (by means of size and electric charge) - These ‘pores’ are called Membrane Channels
10
Q
what are gated channels
- what does the protein shape of gated channels mean for the diffusion of a substance?
- what are the two types of gated channels that determine protein shape?
A
- Membrane channels (these holes in the membrane) generally are not kept open
- Channels can be closed off by a branch of the protein structure = ‘Gate’
- gate closes = no diffusion, gate open = diffusion (remember that it is still selective)
- The protein components switch between 2 shapes; one creates an open pore, the other blocks the pore
- Factors determining channel protein shape (aka when it opens and when its closed):
– ligand gates channels = binding of chemical agent
– voltage gated channels = voltage across the membrane
11
Q
ligand gates channels
- what body system are cell membrane receptors part of?
- what does the channel need in order to change shape?
A
- Cell membrane receptors are part of the body’s chemical signaling system
- The binding of a receptor with its ligand usually triggers events at the membrane, such as activation of an enzyme
12
Q
voltage gated channels
- explain membrane potential and what it usually is like in a cell?
- how does voltage gated channels work depending on the membrane potential
- what is the voltage sensing mechanisms on these gated channels?
A
- all cells generate a membrane potential
- usually the voltage inside the cell is negative and outside the cell is positive
- Some membrane channels are sensitive to the potential difference across the membrane (e.g. depolarization) - polarized = gate is open, depolarized = back to natural position - closed
- The voltage sensing mechanism is in the 4th transmembrane domain of the protein, the S4 segment. the S4 segment is what changes its shape
13
Q
voltage gated channels - S4 segment
- where is the s4 segment and what does it look like naturally and in a polarized cell?
- what does the s4 segment look like in a depolarized cell?
A
- S4 sticks out to the side of the protein (like a wing)
Naturally:
* The natural position of the S4 ‘wing’ is up towards the outer surface of the cell membrane.
Polarized:
- When the membrane is polarized, the positively charged wing is attracted downwards to the negatively charged inner surface of the membrane
Depolarized:
- when the membrane is depolarized (around -50 mV) (inside is more positive), inner region no longer provides sufficient electrical attraction to hold the S4 wing downwards, so it migrates back-up
* In the up position, S4 removes a structural occlusion from the pore (gate is open) such that ions can now diffuse through it (from inside of the cell to outside)
14
Q
define endo/exocytosis generally
A
- Endocytosis (outside to in): inward ‘pinching’ of membrane to create a vesicle; usually receptor- mediated to capture proteins, from outside to inside.
- Exocytosis (inside to out): partial or complete fusion of vesicles with cell membrane for bulk trans-membrane transport of specific molecules, from inside to outside.
15
Q
how many types of exocytosis are there?
A
two
exocytosis 1: the more rapid mechanism had been dubbed the ‘Kiss and Run’
exocytosis 2: full exocytosis
16
Q
explain exocytosis 1 / kiss and run method
- where does this occur specifically?
- what is this method?
- when is this method used?
A
- The secretory vesicles dock and fuse with the plasma membrane at specific locations called ‘fusion pores’
- Vesicle connects (kiss) and disconnects (run) several times
- Since only part of the contents are emptied in one ‘Kiss’, the process can be repeated several times before the vesicle is depleted
- This exocytosis is used for low rate of signaling
17
Q
explain exocytosis 2 / full exocytosis
- what is it
- how do proteins play a role
- how is the action that proteins play become counterbalanced
- when is this method used?
A
- This involves complete fusion of the vesicle with the membrane, leading to total release of vesicle contents at once
- sometimes the vesicle that will be performing exocytosis has proteins attached to it which will then fuse with the membrane
- However, this must be counterbalanced by endocytosis to stabilize membrane surface area (so it doesn’t become too loose)
- used for high levels of signaling
18
Q
what is membrane potential ?
what happens if the MP is 0 on the inside?
A
- All cells in the body generate Membrane Potential (MP) (potential difference in a cell)
- if the membrane potential is 0 inside the cell and not negative (-70 mV), it is likely to be dead
19
Q
what are the two conditions needed to create an Membrane potential?
A
- To generate MP we need 2 conditions:
1. A concentration gradient: created via an enzyme ion pump (functions as an ATPase) – this pump actively transports certain ion species across the membrane to create the concentration gradient
- Semi-permeable membrane: allows one ion species to diffuse across the membrane more than others
* Diffusion of that ion species down its conc. gradient creates an electrical gradient
20
Q
Sodium potassium pumps (Na+/K+)
- where are they found
- what does it do
- how much energy does it need in body vs neuronal cells
- what value potential difference does it create?
A
- all cells are loaded with Na+/K+ pumps – staple for all living cells
- Na+/K+ dependent ATPase is enzyme that moves 3 Na+ out of cell, and 2 K+ into cell for each ATP molecule that is broken down (concentration gradient)
- takes energy to do so (1/3 of energy needs in body cells and 2/3 of energy in neurons)
- Na/K inequality –> potential difference of -10 mV (more neg on inside vs outside – extra cations on the outside)
21
Q
why is the resting membrane potential not -10 mV as indicated by the numerous Na+/K+ pumps everywhere?
A
- the resting membrane potential is closer to -70 mV
- this is due to the semi-permeable membrane of cells which contains K+ leakage channels along its membrane (at rest the membrane is permeable to K+ ions - not others)
- these channels allow K+ to travel along its concent gradient (in to out - more to less) thus making the inside of the cell more negative (loosing positive charge)
22
Q
why doesn’t the membrane potential decrease forever
A
- This efflux of K+ ions leaving the cell will occur until there is such a build up of “+” charge on the outside of the membrane that further diffusion of K+ is repelled by the electromagnetic force (more K+ cant leave because the positive charge around the membrane will repel the leaving ions, making it stay in) = i.e. we reach an equilibrium situation
23
Q
define what the equilibrium potential is
thus, what is membrane potential based on then?
what formula helps us calculate this?
A
- when the electrical force of repulsion is equal in magnitude by the chemical force of diffusion down the [] gradient
- in other words, at equilibrium, electrical work to repel outward cation diffusion equals chemical work of diffusion down conc. gradient
- keep in mind these forces are equal in mag but opposite in direction
- Membrane potential at equilibrium is determined by the concentration gradient - because without it there will be no movement of K+ ions in the first place
- Can be calculated using the Nernst Equation
24
Q
what does the nernst equation describe
when is this only valid?
A
- In the ideal situation, the Nernst equation, describes the balance between the chemical work of diffusion with electrical work of repulsion
- The equation gives the potential difference across the membrane, inside with respect to outside, at equilibrium
- The result is valid if and only if one ion species (ex. K+ in this case) is diffusing across the membrane (ideal situation)
