Membrane physiology Flashcards
Name the two main types of transport proteins in cell membranes and explain their functions
Channel proteins: Allow water and selected molecules/ions to diffuse through
Carrier proteins: Bind with molecules/ions to be transported and undergo conformational changes to move substances across the membrane (with or without energy)
What substances can diffuse directly through the lipid bilayer of a cell membrane
Lipid-soluble substances such as oxygen, nitrogen, carbon dioxide, and alcohols can diffuse directly through the lipid bilayer.
Explain facilitated diffusion and give an example
Facilitated diffusion is a passive transport process where carrier proteins aid transport by binding chemically with molecules.
An example is glucose transport via GLUT membrane proteins (like GLUT4 which is activated by insulin).
What are aquaporins and what is their function
Aquaporins are specialized channel proteins that allow selective passage of water through cell membranes
What determines the selectivity of channel proteins
Selectivity is determined by features of the channel including diameter, shape, electrical charges, and chemical bonds on the inside surfaces
Describe the mechanism of a potassium channel’s selectivity
Pore loops form a selectivity filter.
Carbonyl oxygens lining this filter dehydrate potassium ions to allow their passage through the channel.
What are voltage-gated channels and what is their importance
Voltage-gated channels open or close in response to electrical potential across the cell membrane through molecular conformation changes.
They are the basic mechanism for action potentials in nerve and muscle cells.
Explain ligand-gated channels and give an example
Ligand-gated channels open when a chemical substance (ligand) binds to the protein, causing a conformational or chemical bonding change.
An example is the acetylcholine receptor channel
What is Vmax in facilitated diffusion
Vmax refers to the maximum rate of diffusion possible in facilitated diffusion, which occurs when all carrier proteins are saturated with the substance being transported
What is the Nernst potential
The Nernst potential is the electrical difference across a membrane that will exactly balance a given concentration difference of ions (such as Na+), resulting in no net movement of those ions.
Define osmosis and osmotic pressure
Osmosis is the net movement of water caused by a concentration difference across a membrane.
Osmotic pressure is the amount of pressure required to stop osmosis from occurring.
What determines osmotic pressure, and what is the relationship between osmolality and osmotic pressure?
Osmotic pressure is determined by the molar concentration (number of particles), not the mass of particles.
1 milliosmole per litre concentration causes 19.3 mmHg osmotic pressure.
Describe the sodium-potassium ATPase pump mechanism
The Na+/K+ ATPase pump has carrier protein with α and β subunits.
The α subunit has 3 binding sites for Na+ inside the cell, 2 binding sites for K+ outside, and ATPase activity.
When ions bind, ATP is cleaved to ADP, causing a conformational change that moves 3 Na+ out and 2 K+ in.
How is a low calcium concentration maintained inside cells
Low Ca²⁺ concentration is maintained by two calcium pumps:
- one in the cell membrane that pumps Ca²⁺ out of the cell,
- another that pumps Ca²⁺ into sarcoplasmic reticulum or mitochondria.
Explain secondary active transport
Secondary active transport uses energy stored in ion gradients (created by primary active transport) to move substances against their concentration gradients.
It requires carrier proteins and can be either co-transport (substances move in same direction) or counter-transport (substances move in opposite directions).
Provide an example of co-transport in secondary active transport
Sodium-glucose co-transporters move glucose against its concentration gradient by coupling its movement to the downhill movement of sodium ions.
These transporters will not move Na+ until glucose is attached.
How does active transport occur through cellular sheets
Active transport through cellular sheets (like intestinal epithelium or renal tubules) occurs via active transport through the cell membrane on one side of the cell, followed by simple or facilitated diffusion on the opposite side.
What is the resting membrane potential (RMP) and what is its typical range in neurons?
RMP is the voltage difference across the membrane of a neuron at rest. The average voltage ranges from -70 mV to -90 mV.
What are the three main mechanisms that maintain the resting membrane potential?
1) Na+/K+ ATPases (Push 3 Na+ out and 2K+ in)
2) Leaky K+ channels (allow K out of cell)
3) Leaky Na+ channels (allow Na into cell)
How do Na+/K+ ATPases contribute to the resting membrane potential?
Na+/K+ ATPases push 3 Na+ out and 2 K+ in, creating a net negative charge. They require ATP to work and contribute approximately -5 mV of the typical -70 mV RMP.
Why do leaky K+ channels have a significant impact on the resting membrane potential?
Leaky K+ channels are always open, allowing K+ to move freely down its concentration gradient out of the cell.
This makes the inside of the cell more negative (around -90 mV).
Resting cells are more permeable to K+ than Na+, so these channels play the largest role in determining RMP.
What does the Nernst potential describe and how is it calculated for K+
The Nernst potential describes the relationship of diffusion potential to ion concentration difference across a membrane.
For K+, it is calculated as:
Equilibrium potential of K+ = 61.5 × log(K+ in/K+ out) = -90 mV.
What is the Goldman potential used for
The Goldman potential is used to calculate the diffusion potential when the membrane is permeable to several different ions.
It considers the polarity of ions, membrane permeability to each ion, and the concentration of ions on each side of the membrane.
What are graded potentials and what types exist
Graded potentials are the sum of potentials needed to approach threshold potential in the axon hillock.
They include excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).
