L1 Flashcards
(91 cards)
1
Q
What is the cell membrane composed of?
A
- a phospholipid bilayer
- not an inert bag holding the cell together (not like a rubber balloon but an active structure)
2
Q
What factors determine membrane permeability?
A
- molecular size
- lipid solubility
- charge
3
Q
How do lipid-soluble molecules and gases interact with the cell membrane?
A
They diffuse through the membrane readily/easily
4
Q
Why can’t water-soluble molecules cross the membrane independently?
A
they cannot pass through the hydrophobic core of the phospholipid bilayer
5
Q
What is the definition of membrane permeability?
A
If a substance can cross the membrane by any means, the membrane is permeable to that substance
6
Q
What do polar molecules and ions require to cross the membrane?
A
They need the help of proteins (channels or carriers) to cross the membrane
7
Q
What is simple diffusion in cell membranes?
A
When small, lipid-soluble molecules and gases (e.g., O₂, CO₂, ethanol, urea) pass either directly through the phospholipid bilayer or through pores
8
Q
What physical phenomenon is involved in simple diffusion?
A
- It involves Brownian motion
- Where small lipid-soluble molecules and gases move randomly
9
Q
What is the direction of movement in simple diffusion?
A
- Down (along) the concentration gradient
- From areas of high concentration to areas of low concentration
10
Q
What determines the rate of diffusion across a membrane?
A
- The relative rate of diffusion is roughly proportional to the concentration gradient across the membrane
- The greater the concentration gradient, the greater the rate of diffusion
11
Q
Does simple diffusion require energy input?
A
No, it’s passive and requires no energy input from ATP breakdown
12
Q
What is facilitated diffusion, and how does it differ from simple diffusion?
A
- A process where molecules diffuse across the membrane with the assistance of carrier proteins
- Unlike simple diffusion, where molecules pass directly through the membrane without protein assistance
13
Q
What types of molecules typically require facilitated diffusion?
A
Polar molecules (e.g., sugars and amino acids)
14
Q
What is the direction of movement in facilitated diffusion?
A
- Down or along its concentration gradient
- From high to low concentration
15
Q
Does facilitated diffusion require energy from ATP?
A
No, it’s passive and requires no ATP breakdown or energy input
16
Q
How does the carrier protein function in facilitated diffusion?
A
- The solute binds to the transporter, causing a conformational change in the transporter
- Opening the other end and allowing translocation of the solute
17
Q
What is a key limitation of facilitated diffusion?
A
- The number of transporters is finite
- So the system will saturate whenever the concentration of molecules exceeds the number of available transporter proteins
18
Q
Why is facilitated diffusion described as having a “non-continuous passage”?
A
- One end of the transporter is always closed
- Unlike channels, which have continuous pores
19
Q
What is active transport and when is it necessary?
A
- A mechanism to move selected molecules across cell membranes against their concentration gradient
- Necessary when we want to move molecules from an area of low concentration to high concentration
20
Q
What energy source is required for active transport?
A
energy from ATP hydrolysis (ATP breakdown)
21
Q
How does the protein carrier function in active transport?
A
- The substance binds to a protein carrier that changes conformation to move the substance across the membrane against its concentration gradient
22
Q
What is the most famous example of primary active transport?
A
Na⁺/K⁺ pump
23
Q
What is the role of the Na⁺/K⁺ pump in living tissues?
A
plays a crucial role in all living tissues by maintaining ion gradients necessary for cellular function
24
Q
What distinguishes primary active transport from other transport mechanisms?
A
- In primary active transport, energy from the hydrolysis of ATP is directly coupled to the movement of a specific substance across the membrane against its concentration gradient
25
How does primary active transport operate in relation to other substances in the cell?
- It operates independently of any other species
- It doesn't matter what other substances are inside or outside the cell, it simply moves the substance of interest
26
How do facilitated diffusion and active transport differ in terms of energy requirements?
- Facilitated diffusion requires no energy input (passive)
- Active transport requires energy from ATP hydrolysis (active process)
27
What is Secondary Active Transport?
- A mechanism where a substance is carried up its concentration gradient without ATP catabolism
- Using the kinetic energy from another substance moving down its concentration gradient
28
What drives the conformational change in Secondary Active Transport?
Sequential binding of a substance and ions to specific sites in the transporter protein induces a conformational change that enables transport
29
What is the energy source for Secondary Active Transport?
The chemical energy in the substance diffusing down its concentration gradient
- Used to "push" another substance against its concentration gradient
30
What are membrane channels?
- Pores formed by membrane-spanning proteins that create continuous passages from outside to inside the cell
- Allowing specific ions to diffuse through
31
How is the structure of a membrane channel organized?
4-5 protein subunits that fit together to create a central pore through the membrane, with pore loops dangling inside the channel
32
What creates the selectivity of membrane channels?
The physical properties of the pore loops create a selectivity filter that allows only specific molecules to diffuse through based on size and electric charge
33
How do gated channels differ from regular channels?
- Not always open
- They can be closed off by a branch of the protein structure that functions as a "gate."
34
What are the two conformational states of gated channels?
- one creates an open pore allowing diffusion
- the other blocks the pore, preventing diffusion
35
What are the two main types of gated channels based on the opening mechanism?
- Ligand-gated channels (opened by binding of a chemical agent)
- Voltage-gated channels (opened by changes in voltage across the membrane)
36
What happens when the gate of a channel is open?
diffusion is allowed through the channel, though it remains selective for specific ions or molecules
37
What are ligand gated channels part of in the body?
They are part of the body's chemical signaling system
38
What happens when a receptor binds with its ligand?
Usually triggers events at the membrane, such as activation of an enzyme
39
What role do membrane receptors play in synaptic transmission?
They play an important role in synaptic transmission by binding neurotransmitters once they are released from the presynaptic axon terminal
40
What is the sequence of events in ligand-gated channel activation?
- An extracellular signal molecule binds to a cell membrane receptor
- This binding triggers rapid cellular responses, including the opening of the channel
41
What causes voltage gated channels to open?
- They open when membrane polarity changes
- They sense and react to the potential difference across the membrane
42
Where is the voltage sensing mechanism located in voltage gated channels?
It's located in the 4th transmembrane domain of the protein, known as the S4 segment
43
How is the S4 segment described structurally?
The S4 segment sticks out to the side of the protein like a wing and is positively charged
44
What is the natural position of the S4 'wing' and what affects it?
- The natural position of the S4 'wing' is up towards the outer surface of the cell membrane
- But when the membrane is polarized, the positively charged wing is attracted downwards to the negatively charged inner surface of the membrane
45
What is the typical resting membrane potential of a neuron?
-70 millivolts on the inside compared to the outside
46
What is the state of voltage gated channels when the membrane is at resting potential?
When the membrane is at resting potential (-70 mV), the channel is shut with the S4 wings pulled down toward the negatively charged inner surface
47
What do all cells in the body generate?
a membrane potential
48
What is true about the membrane potential in eukaryotic cells?
- Virtually all eukaryotic cells maintain a non-zero membrane potential
- Usually showing negative voltage on the inside compared to the outside
49
What happens to voltage gated channels when the membrane depolarizes to about -50 mV?
- It makes the membrane more positive on the inside
- Which no longer provides sufficient electrical attraction to hold the S4 wing downwards
- Causing it to migrate back up
50
What is the relationship between membrane depolarization and the S4 wing position?
When the membrane depolarizes (becomes more positive on the inside), it can't hold the S4 wings down, so they migrate upward
51
What happens when the S4 segment moves to the up position?
In the up position, S4 removes a structural occlusion from the pore, allowing ions to diffuse through the channel
52
At what membrane potential do voltage gated channels typically begin to open?
Voltage gated channels typically begin to open when the membrane depolarizes to about -50 mV
53
What is endocytosis and what direction does it transport molecules?
- The inward 'pinching' of membrane to create a vesicle
- Usually receptor-mediated to capture proteins
- Transporting molecules from outside to inside the cell
54
What is exocytosis and what direction does it transport molecules?
- The partial or complete fusion of vesicles with the cell membrane for bulk trans-membrane transport of specific molecules
- Moving them from inside to outside the cell
55
Why is exocytosis important for synaptic transmission?
it's the mechanism used to release neurotransmitters from the presynaptic neuron
56
What are the two different types of exocytosis?
1) "Kiss and Run" exocytosis (the more rapid mechanism)
2) Full exocytosis
57
What is "Kiss and Run" exocytosis?
- A transient form of exocytosis where secretory vesicles dock and fuse with the plasma membrane at specific locations called fusion pores
- Releasing only part of their contents before disconnecting
58
How long does a typical "Kiss and Run" fusion event last?
A few hundred milliseconds
59
What happens to the vesicle in "Kiss and Run" exocytosis after content release?
The vesicle can connect and disconnect several times before its contents are completely emptied, allowing for multiple release events
60
What type of signaling is "Kiss and Run" exocytosis typically used for?
Low rate of signaling, as only part of the vesicle contents diffuse into the interstitial fluid
61
How does full exocytosis differ from "Kiss and Run"?
Full exocytosis involves complete fusion of the vesicle with the membrane, leading to total release of vesicle contents at once
62
What cellular functions require full exocytosis?
Delivery of membrane proteins and high levels of signaling
63
What challenge does full exocytosis create for the cell?
- It keeps adding to the membrane by fusing
- Could result in a very long and loose membrane if not regulated
64
How does the cell maintain membrane stability during full exocytosis?
must be counterbalanced by endocytosis to stabilize membrane surface area
65
What indicates a dead cell when measuring membrane potential?
If the membrane potential measures zero (not negative on inside), the cell is likely dead
66
What are the two conditions needed to generate membrane potential?
1) Creation of a concentration gradient via an enzyme ion pump (Na⁺/K⁺ pump)
2) A semi-permeable membrane that allows one ion species to diffuse across more than others
67
How does ion diffusion contribute to membrane potential?
Diffusion of ion species down their concentration gradient creates an electrical gradient
68
Where are Na+/K+ pumps found in the body?
- everywhere
- they are loaded in all cell membranes and are the staple of all living cells.
69
What is the Na+/K+ dependent ATPase and what does it do?
- It's an enzyme that moves Na+ out of the cell and K+ into the cell by breaking down ATP
70
What is the ion exchange ratio of the Na+/K+ pump?
For each ATP molecule broken down, 3 Na+ ions are pumped out and 2 K+ ions are pumped in
71
How much of the body's energy needs does the Na+/K+ pump consume?
- It consumes 1/3 of the energy needs of the body in general
- In neurons it's 2/3, making it a huge consumer of energy
72
What electrical effect does the Na+/K+ pump have on the cell membrane?
The Na/K inequality creates a potential difference of approximately -10 mV (more negative on inside relative to the outside)
73
Why does the Na+/K+ pump create an electrical inequality?
- Both Na+ and K+ are cations (ions with positive charge)
- Since 3 Na+ ions are pumped out while only 2 K+ ions are pumped in, it creates an electrical inequality from the beginning (a small potential difference)
74
Why is the resting membrane potential (-70 mV) different from the potential created directly by the Na+/K+ pump (-10 mV)?
The additional negative potential is due to the diffusion of K+ ions outward through K+ leak channels, which makes the inside of the cell more negative.
75
What is the membrane most permeable to in the resting state?
The resting membrane is most permeable to K+ ions and not much to other ions due to semi-permeability
76
How does K+ movement affect the membrane potential?
As K+ moves from inside to outside, it makes the membrane more negative on the inside because it's taking positive charges away from the inside to the outside
77
Why does K+ diffuse out of the cell?
K+ diffuses out of the cell down its concentration gradient via K+ channels because there is more K+ on the inside compared to outside due to the action of the sodium-potassium pump
78
What happens to cations during the establishment of resting membrane potential?
Cations (K+) accumulate on the outside of the membrane, leaving a net negativity inside the membrane
79
What happens if potassium leakage channels remain open in the resting state?
they will allow K+ to leak out of the membrane down or along its concentration gradient
80
What stops the continuous efflux of K+ ions from the cell?
The efflux 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, reaching an equilibrium situation
81
How is equilibrium achieved in the resting membrane potential?
Equilibrium is reached when the chemical force (which depends on the concentration gradient outwards) is matched by the electrical force (which is directed inward due to K+ accumulation outside)
82
What principle explains why K+ ions stop leaving the cell?
K+ ions that left will accumulate on the outside (positive repels positive). This efflux will occur only until there is such a buildup of positive charge on the outside that further diffusion of K+ is repelled by the positively charged K+ ions sitting outside
83
What is the relationship between the Na+/K+ pump and K+ leakage channels in establishing resting membrane potential?
The Na+/K+ pump creates the concentration gradient with more K+ on the inside, while the K+ leakage channels allow K+ to flow down this gradient, creating the full resting potential of approximately -70 mV
84
What is equilibrium potential in terms of electrical and chemical work?
At equilibrium, the electrical work to repel outward cation diffusion equals the chemical work of diffusion down the concentration gradient. The forces are equal in magnitude but opposite in direction.
85
What determines the membrane potential at equilibrium?
The membrane potential at equilibrium is determined by the concentration gradient of ions across the membrane
86
What equation is used to calculate equilibrium potential?
The Nernst Equation
87
What does the Nernst equation describe?
the balance between the chemical work of diffusion and the electrical work of repulsion across a membrane
88
What does the Nernst equation calculate specifically?
it gives the potential difference across the membrane, inside with respect to outside, at equilibrium
89
When is the Nernst equation valid for calculating membrane potential?
The result is valid if and only if one ion species (like K+ in neurons) is diffusing across the membrane
90
What does it mean when we say "K+ wants the membrane to be sitting at -90 mV"?
If only K+ ions were involved and all K+ channels were open, the membrane potential would stabilize at -90 mV, which is the equilibrium potential for K+
91
Why is the membrane most permeable to K+ at rest?
At rest, K+ leakage channels are open while most other ion channels are closed, making the membrane more permeable to K+ than to other ions
