Cell Membrane

Structure of Cell Membrane

• Phospholipid bilayer- contains hydrophobic tails and hydrophilic heads

• Composed of glycerophospholipids

Function of Cell Membrane

• Selectively permeable and has a fluid structure

• The thicker the membrane the more selective

Decreasing Membrane Fluidity

• Cholesterol embedded among the lipids restricts the movements of acyl chains

Increasing Membrane Fluidity

• Cholesterol prevents the close packing of lipids (prevents their crystallization)

This is an example of:

Transverse diffusion (flip-flop)

This is an example of:

Lateral diffusion

Integral membrane proteins

• Span the bilayer

• They serve as carriers, channels, and transporters

Peripheral membrane proteins

• More loosely associated with the membrane via interactions with lipid head groups or integral proteins

Simple diffusion

• The movement of a molecule down its concentration gradient (high to low) without needing for energy or a protein

Passive transport diffusion

• The movement of a molecule down its concentration gradient (high to low) with the assistance of a protein, but without the need for energy

Common carrier proteins in passive transport diffusion

• leak channels

• gated channels

• stretch channels

This is an example of:

• Leak channel

• Pore is continuously open

This is an example of:

• Gated Channel

• Pore is open, but has a physical barrier

This is an example of:

• Stretch channel

• As membrane pulls apart, membrane opens and vice versa

Active transport diffusion

• The movement of a molecule across a membrane with the assistance of a protein and the need for energy (ATP)

• Concentration gradient of low to high

Secondary active transport diffusion

• The movement of a molecule across a membrane with the assistance of a protein and this protein does not directly need energy, but the mechanism leverages energy invested by another cellular process

Chemicals that can diffuse across the phospholipid bilayer

• Lipid soluble

• Uncharged

Chemicals that require a channel to diffuse across the phospholipid bilayer

• Lipid insoluble

• Charged

Sodium-potassium ATPase pump

• Membranes have a greater K⁺ permeability (compared to Na⁺), thus the membrane relies on this pump.

• Pumps 2 K⁺ into the cell and 3 Na⁺ out for each ATP hydrolyzed

• Maintains the Na/K gradients and membrane potential

• Na is found in higher concentrations on outside of cell

• K is found in higher concentrations on inside of the cell

Approximate concentrations of sodium

• Inside cell is low (12mM)

• Outside cell is high (145mM)

Approximate concentrations of potassium

• Inside cell is high (155mM)

• Outside cell is low (4mM)

Approximate concentration of chloride

• Inside cell is low (4.2mM)

• Outside cell is high (123mM)

Approximate concentration of calcium

• Inside cell is low (0.1mM)

• Outside cell is high (1.5mM)

How are membrane channels gated?

• Channels only see size and charge. If the hole in the protein that conducts cations is big enough to allow calcium, sodium, and potassium through the channel

• General movement through the channels is passive diffusion

Predict how movement of an ion across the membrane would impact membrane potential.

look at notes

How is an action potential conducted along the cell membrane?

• To achieve an action potential, the membrane must depolarize to “threshold”

• Therefore, depolarization can lead to an action potential (excitation)

• Hyper-polarization can make it harder to accomplish an action potential (inhibition).

Predict how changes in extracellular concentration of potassium, calcium, and sodium impact membrane potential.

Practice in notes

Excitatory cues influence membrane potential by…

Depolarize (hypo-polarize) the membrane

Inhibitory cues influence membrane potential by…

Hyper-polarize the membrane

Spatial summation to threshold

Two dendritic signals are added together to reach the threshold

Temporal summation to threshold

One dendritic signal is needed to reach the threshold

Action potential refractory period

• Channel is closed at resting membrane potential

• Channel opens by depolarization (action potential)

• Channel inactivates so that no action potential can occur, it allows for action potential to go in one direction

Refractory period importance

It’s important that the action potential only ‘travels’ in one direction.

Conductance

Flow of ions

Cardiac Action potential

Na+ conductance (Nerve and Cardiac)

Ca2+ conductance (Cardiac)

K+ conductance (Cardiac)

Nerve Action potential

Ca2+ conductance (Nerve)

K+ conductance (Nerve)

Describe how an action potential is conducted along the cell membrane

• Trigger at the start: A stimulus makes the inside of the cell less negative. If it reaches the threshold voltage (around -55 mV), voltage-gated sodium channels open.

• Sodium influx: Sodium ions rush into the cell, making the inside positive (depolarization).

• Local current spread: The positive charge inside flows sideways to the next neighboring patch of the membrane.

• Reaching threshold ahead: This local current brings the next patch up to its threshold, opening its sodium channels and repeating the spike.

• Refractory period: Behind the moving wave, sodium channels close and potassium channels open to reset the membrane voltage, ensuring the signal only moves forward and cannot go backward

SNARE proteins

• Activated by the binding of calcium

• Pull synaptic vesicles filled with acetylcholine down and fuse it with the presynaptic membrane to allow neurotransmitter to be released into the synaptic cleft