Cell Membrane and Permeability Quiz

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Questions and Answers

What happens to the S4 segment of the voltage-gated channel during depolarization?

It migrates downwards towards the inner membrane

It fully retracts into the cell

It remains unchanged and does not affect the pore

It moves upwards and allows ion diffusion (correct)

What initiates the movement of the S4 'wing' in voltage-gated channels?

Increased external pressure on the membrane

Decreased temperature outside the cell

Chemical signals from neighboring cells

Change in the electrical charge of the membrane (correct)

In the context of endocytosis, what is the primary mechanism used to capture proteins?

Passive diffusion through the lipid bilayer

Receptor-mediated inward pinching of the membrane (correct)

Direct fusion of the protein with the membrane

Active transport through protein channels

What occurs during exocytosis in a cell?

Vesicles merge with the membrane and release their contents Signup and view all the answers

What characteristic of the S4 segment helps it in the voltage sensing mechanism?

Its positively charged designation Signup and view all the answers

What is the primary function of the 'Kiss and Run' mechanism in exocytosis?

To allow repeated partial emptying of vesicle contents. Signup and view all the answers

How does full exocytosis differ from the Kiss and Run method?

It involves the total release of vesicle contents at once. Signup and view all the answers

What must occur alongside full exocytosis to stabilize membrane surface area?

Endocytosis. Signup and view all the answers

What is essential for generating membrane potential in cells?

A concentration gradient created by an enzyme ion pump. Signup and view all the answers

What primarily determines the permeability of the cell membrane to a substance?

Molecular size, lipid solubility, and charge Signup and view all the answers

Why is a semi-permeable membrane necessary for generating membrane potential?

It permits selective diffusion for specific ions. Signup and view all the answers

What type of signaling is associated with the Kiss and Run mechanism?

Low rate of signaling due to partial content release. Signup and view all the answers

Which type of transport does not require energy input from ATP?

Facilitated Diffusion Signup and view all the answers

What is the primary function of carrier proteins in facilitated diffusion?

To aid the movement of polar molecules Signup and view all the answers

What role does ATPase play in generating membrane potential?

It actively transports ions across the membrane. Signup and view all the answers

What characterizes the contents released during full exocytosis?

All contents are released at once. Signup and view all the answers

How do lipid-soluble molecules primarily cross the cell membrane?

By simple diffusion directly through the phospholipid bilayer Signup and view all the answers

What characterizes simple diffusion?

Occurs down the concentration gradient Signup and view all the answers

Which ion transport mechanism contributes to the resting membrane potential?

Na+/K+ pump Signup and view all the answers

What feature of the cell membrane makes it impermeable to organic anions?

Charge Signup and view all the answers

What happens at equilibrium regarding potassium ions (K+)?

Electrical work to repel outward cation diffusion equals chemical work of diffusion. Signup and view all the answers

What distinguishes secondary active transport from primary active transport?

Relies on the gradient of another molecule Signup and view all the answers

What equation is used to calculate the equilibrium potential of an ion?

Nernst Equation Signup and view all the answers

What is the equilibrium potential for K+ ions calculated using the Nernst Equation?

-90 mV Signup and view all the answers

Why is the resting membrane potential in typical neurons not equal to -90 mV?

Multiple ion species contribute to the resting membrane potential. Signup and view all the answers

What determines the membrane potential at equilibrium?

The concentration gradient of the specific ion involved. Signup and view all the answers

What defines facilitated diffusion?

Movement of molecules assisted by a carrier protein. Signup and view all the answers

Which process requires energy to move substances across a membrane?

Active transport Signup and view all the answers

What is the primary function of a channel protein?

To create a selective pathway for specific ions. Signup and view all the answers

What describes secondary active transport?

Uses kinetic energy from one substance to transport another. Signup and view all the answers

What mechanism do ligand-gated channels employ to open?

Binding of chemical agents to receptors. Signup and view all the answers

Which characteristic is true about gated channels?

They can close or open based on specific conditions. Signup and view all the answers

What role does the Na+/K+ pump play in cellular function?

It moves sodium ions out and potassium ions into the cell against their gradients. Signup and view all the answers

What is the primary energy source for active transport mechanisms?

ATP hydrolysis. Signup and view all the answers

How do voltage-gated channels operate?

They respond to changes in the membrane's electric potential. Signup and view all the answers

In facilitated diffusion, how do substances move?

Down their concentration gradient without energy. Signup and view all the answers

Study Notes

Cell Membrane

The cell membrane is not a simple container; it plays a crucial role in cell function.
It is composed of a phospholipid bilayer.
It allows lipid-soluble molecules and gases (like oxygen, carbon dioxide, ethanol, and urea) to diffuse easily.
Water-soluble molecules require assistance to cross the membrane.
The membrane is impermeable to organic anions, such as proteins.
Permeability depends on the size, lipid solubility, and charge of the molecule.

Membrane Permeability

If a substance can cross the membrane, the membrane is permeable to that substance.
Gases can diffuse freely across the membrane.
Polar molecules and ions need the help of proteins (channels or carriers) to cross.

Simple Diffusion

Small, lipid-soluble molecules and gases move across the membrane directly through the phospholipid bilayer or through pores.
Movement occurs down the concentration gradient.
The rate of diffusion is proportional to the concentration gradient.
It's a passive process, requiring no energy input from ATP.

Facilitated Diffusion

Facilitated diffusion relies on carrier proteins to help molecules cross the membrane.
This is used for polar molecules like sugars and amino acids.
Movement occurs down the concentration gradient.
The energy comes from the concentration gradient of the solute.
It's a passive process, requiring no energy input from ATP.

Active Transport

Active transport moves molecules against their concentration gradient, requiring energy expenditure.
A specific protein carrier binds to the substance, changes conformation, and transports it across the membrane.
It uses energy from ATP hydrolysis, typically via ATPases like the Na+/K+ pump.

Secondary Active Transport

This type of transport moves molecules against their concentration gradient without directly using ATP hydrolysis.
It harnesses the kinetic energy of one substance moving down its concentration gradient to drive the transport of another against its gradient.
It relies on the energy built up by primary active transport, like the Na+/K+ pump.
Sequential binding of substances and ions to the transporter protein induces conformational changes.

Channels

Channels are formed by membrane-spanning proteins creating pores through the membrane.
These pores are composed of multiple protein subunits, allowing specific ions to diffuse.
The 'pore loops' inside the channel act as selectivity filters, allowing only specific molecules to pass based on their size and charge.

Gated Channels

Most channels are not continuously open.
They possess a 'gate', which can open and close under specific conditions.
These gates are controlled by different factors:
- Ligand-gated channels are controlled by the binding of a chemical agent.
- Voltage-gated channels are controlled by the voltage difference across the membrane.

Ligand Gated Channels

These receptors are part of the body's chemical signaling system.
Binding of a ligand to its receptor often triggers events at the membrane, such as enzyme activation.

Voltage Gated Channels

These channels are sensitive to the potential difference across the membrane.
Changes in voltage, such as depolarization, cause conformational changes in the channel subunits, leading to the formation of a diffusion pore.
The voltage-sensing mechanism is located in the S4 segment of the protein's fourth transmembrane domain.

Endo/Exocytosis

Endocytosis is the 'pinching in' of the membrane to form a vesicle, typically through receptor-mediated processes to capture proteins from outside the cell.
Exocytosis is the process of partially or completely fusing vesicles with the cell membrane, transporting molecules from the inside to the outside.
Exocytosis can be divided into two types:
- Kiss and Run: Rapid exocytosis.
- Full Fusion: Complete exocytosis.

Exocytosis 1 (Kiss and Run)

Secretory vesicles dock and fuse with the plasma membrane at specific locations called 'fusion pores'.
The vesicle can connect and disconnect several times before releasing its contents.
Only a portion of the contents is released in each 'Kiss', allowing for repeated releases.
This type of exocytosis is used for low-rate signaling.

Exocytosis 2 (Full Fusion)

This involves complete fusion of the vesicle with the membrane, releasing all contents at once.
It's necessary for delivery of membrane proteins and high-level signaling.
It must be balanced by endocytosis to maintain stable membrane surface area.

Membrane Potential

All cells in the body generate a Membrane Potential (MP).
To create MP, two conditions are needed:
- Concentration Gradient: An enzyme ion pump (acting as an ATPase) actively transports specific ions across the membrane to create a concentration gradient.
- Semi-permeable Membrane: Allows one ion species to diffuse across the membrane more than others, creating an electrical gradient.

Resting Membrane Potential

This potential is established by the balance of ion movement across the membrane.
The flow of potassium (K+) ions out of the cell, driven by its concentration gradient, creates a negative charge inside the cell.
This negative charge opposes further K+ efflux, until an equilibrium is reached.

Equilibrium Potential

At equilibrium, the electrical force opposing ion movement is balanced by the chemical force driving it.
The membrane potential at equilibrium is determined by the concentration gradient of the ion.
The Nernst equation can be used to calculate the equilibrium potential.

Nernst Equation

The Nernst equation describes the balance between the chemical forces and electrical forces acting on ions.
It provides the potential difference between the inside and outside of the membrane at equilibrium.
It's accurate only if one ion species is diffusing across the membrane.

K+ Equilibrium Potential

The equilibrium potential for potassium (EK+) can be calculated using the Nernst equation.
In a typical neuron, it is around -90 mV.
This is the potential if only K+ ions were involved, but the resting membrane potential is usually around -70 to -80 mV.
This difference is due to the presence of other ions like sodium (Na+).

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Description

Test your knowledge on the structure and function of the cell membrane. This quiz covers key concepts such as membrane permeability, diffusion processes, and the types of molecules that can traverse the membrane. Enhance your understanding of cell biology fundamentals with this interactive quiz.