NEUR 303 1st

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Questions and Answers

What characteristic of phospholipids contributes to their ability to form bilayers?

They are entirely hydrophilic.

They are hydrophobic only.

They form micelles at low concentrations.

They are amphipathic. (correct)

What is the role of transport systems in cellular membranes?

They maintain lipid packing.

They allow free diffusion of all ions.

They control internal ion concentrations. (correct)

They create a completely impermeable barrier.

Which factor does NOT influence membrane fluidity?

Cholesterol content.

Oxygen levels. (correct)

Lipid packing.

Temperature.

According to Fick’s Law, the flux of solute is determined by which of the following factors?

Permeability coefficient of solute. Signup and view all the answers

What does it mean for a membrane to be semipermeable?

It selectively allows certain ions and molecules to pass. Signup and view all the answers

What primarily drives the passive transport of solutes across a membrane?

Chemical or electrochemical gradients. Signup and view all the answers

What happens to phospholipids in water at low concentrations?

They form micelles. Signup and view all the answers

Which of the following best describes simple diffusion?

The random motion of solutes from high to low concentration. Signup and view all the answers

What is the primary mechanism driving the movement of charged solutes across a cell membrane?

The combined effect of concentration gradient and electrical potential difference Signup and view all the answers

Which type of transport does not require energy and involves solutes moving downhill?

Facilitated diffusion Signup and view all the answers

What happens when there is no net driving force acting on a solute across a cell membrane?

Equilibrium is reached and there is no net transport Signup and view all the answers

Which type of proteins enables facilitated diffusion across cell membranes?

Integral membrane proteins Signup and view all the answers

How do pores differ from channels in membrane transport?

Pores are always open, while channels can be opened or closed Signup and view all the answers

What defines passive transport compared to active transport?

Passive transport does not require energy and moves substances down their gradient Signup and view all the answers

Which statement is true about the movement of ions across a membrane without proteins?

Cell membranes are impermeable to ions and water without proteins Signup and view all the answers

Which component contributes to the electrical potential energy difference across a membrane?

The difference in voltage between the outside and inside compartments Signup and view all the answers

What is the primary function of porins in cell membranes?

Form aqueous transmembrane conduits that are always open Signup and view all the answers

What regulates the transport of materials into and out of the nucleus?

Nuclear pore complex (NPC) Signup and view all the answers

Which of the following correctly describes the selectivity filter in channel proteins?

Determines types of ions or molecules that can access the pore Signup and view all the answers

What differentiates facilitated diffusion from simple diffusion?

Facilitated diffusion involves protein carriers or channels Signup and view all the answers

What happens to the flux of a solute during simple diffusion as the concentration gradient increases?

It increases linearly without saturation Signup and view all the answers

Which of the following is true regarding the channel functional components?

Sensors respond to changes in external signals to open or close the channel Signup and view all the answers

What is a characteristic of passive carrier-mediated transporters?

They have a specific affinity for a limited number of solutes Signup and view all the answers

What primarily distinguishes aquaporins from other channel proteins?

They are exclusively responsible for transporting water molecules Signup and view all the answers

What is a key characteristic of ion channels compared to pumps?

Ion channels have a continuous aqueous pathway for ion conduction. Signup and view all the answers

What happens to the ion composition of a cell when exposed to ouabain?

Increased intracellular Na+ concentration. Signup and view all the answers

Which statement accurately describes carrier-mediated transport?

It has a selectivity filter preventing the passage of certain ions. Signup and view all the answers

Why are ion channels not classified as secondary active transporters?

They do not require energy input. Signup and view all the answers

What is the role of the Na-K pump in the context of ion gradients in mammalian cells?

It generates a concentration difference by moving Na+ out and K+ in. Signup and view all the answers

What is the primary function of the Na+/K+ pump?

To move 3 Na+ ions out of the cell and 2 K+ ions into the cell Signup and view all the answers

What type of transport does the sodium-glucose transporter (SGLT) primarily utilize?

Secondary active transport using Na+ gradient Signup and view all the answers

During glucose uptake via the sodium-glucose cotransporter, what happens in the presence of Na+?

Glucose accumulates above equilibrium level Signup and view all the answers

Which statement correctly describes secondary active transport?

It relies on existing ion gradients to facilitate transport Signup and view all the answers

What distinguishes exchangers (antipoers) from symporters?

Exchangers utilize an existing chemical gradient to move ions in opposite directions Signup and view all the answers

Which transport mechanism is primarily responsible for establishing the ionic gradients for Na+ and K+?

Primary active transport Signup and view all the answers

What happens to glucose transport when Na+ is absent from the external medium?

Glucose enters the vesicle only by passive diffusion Signup and view all the answers

How does the Na/K pump contribute to the overall electrical charge across the cell membrane?

It is electrogenic, moving more positive charges out than in Signup and view all the answers

Study Notes

The Structure of Biological Membranes

Phospholipids are amphipathic with hydrophilic heads and hydrophobic tails.
At low concentrations, phospholipids form a monolayer, but at higher concentrations, they form micelles and eventually bilayers.
Phospholipid membranes form an impermeable barrier to diffusion of ions.
Cells require transport systems (pumps, channels, and transporters) to control internal ion concentrations and the flow of ions between the inside and outside of the cell.
The plasma membrane is considered semipermeable because it has transport systems.

Membrane Fluidity

Membrane fluidity describes the freedom of movement of proteins and lipids within the cell membrane.
Fluidity influences several cellular processes, including the activity of membrane-associated enzymes.
Lipid packing can influence membrane fluidity, especially cholesterol.

Simple Diffusion

Movement from one location to another as a result of random thermal motion.
Simple diffusion of glucose occurs between two compartments of equal volume separated by a barrier permeable to glucose.

Fick's Law

The simplest description of diffusion.
The flux of solute X (JX) is determined by the permeability coefficient of X (PX) multiplied by the difference in external and internal concentrations ([X]o – [X]i).
Fick's Law only applies to uncharged particles.

Driving Forces for Solute Transport

The driving force for passive transport of solutes across a membrane is the chemical, electrochemical gradient, or potential energy difference acting on the solute between the two compartments.
For charged solutes (e.g., Na+, Cl−), the electrochemical potential energy difference includes contributions from the concentration gradient (chemical potential) and the voltage difference between the compartments (electrical potential).
When no net driving force acts on X, we say that X is at equilibrium across the cell membrane, and there is no net transport of X across the membrane.

Types of Transport Across a Membrane

Simple diffusion
Passive transport (facilitated diffusion)
Active transport (primary and secondary)

Passive Transport by Facilitated Diffusion

Facilitated transport occurs when proteins enable solutes to cross biological membranes down their concentration gradient.
This type of transport is passive because it does not require energy.

Integral Membrane Proteins and Facilitated Diffusion

Integral membrane proteins form pores, channels, or carriers that facilitate passive transport.
Pores are always open channels.
Channels can open or close due to specific mechanisms.
Carriers facilitate passive transport through membranes.

Examples of Pores

Porins are found in outer membranes of gram-negative bacteria and mitochondria.
Perforin is used by cytotoxic T lymphocytes to kill target cells.
Nuclear pore complex (NPC) regulates traffic into and out of the nucleus.
Aquaporins (AQPs) are channels that allow water molecules to pass through.

Channels

Channels are gated pores.
Channels have a gate that determines whether the channel is open or closed.
Channels have sensors that respond to signals like changes in membrane voltage, second messengers, or ligands.
Channels have a selectivity filter determining the classes of ions (e.g., anions or cations) or particular ions (e.g., Na+, K+, Ca2+) that have access to the channel pore.
Open channel pores allow ions to flow through passively by diffusion until the channel closes again.

Carriers

Carriers are never an open gate through the membrane.
They transfer a broad range of ions and organic solutes.
They have specific affinity for binding one or a small number of solutes.
The simplest passive carrier-mediated transporter mediates facilitated diffusion.
No ATP requirement is needed for carriers.

Flux Across the Membrane: Simple vs. Facilitated Diffusion

In simple diffusion, flux increases linearly with increases in [X]o, with no maximal rate of transport (O2, CO2, NO).
In facilitated diffusion, there is a fixed number of carriers or channels, each with a limited speed.
As the extracellular X concentration increases, flux of X will eventually reach a maximum once all the carriers have become loaded with X (saturation).

Active Transport

Active transport requires energy to move ions across biological membranes against their concentration gradient.
Active transport can be primary or secondary.

Primary Active Transport: The Na+/K+ Pump

The Na+/K+ pump is a primary active transport system that moves 3 Na+ ions out of the cell and 2 K+ ions into the cell.
It is electrogenic because it moves more positive charges out of the cell than into the cell, contributing to the membrane potential.

Secondary Active Transport: Symporters

Symporters are co-transporters that utilize an existing gradient (e.g., Na+ or K+) to move another ion across the membrane.
They rely on the sodium concentration gradient generated by the sodium–potassium ATPase as a source of chemical potential.

Examples of Symporters

Sodium-glucose cotransporters (SGLTs) use the Na+ gradient to transport glucose against its concentration gradient.
Most symporters use the Na+ gradient.

Secondary Active Transport: Exchangers

Exchangers or antiporters use an existing gradient (e.g., Na+ or K+) to move one ion to the side of the membrane of lower concentration in exchange for another ion (usually of the same charge) that is moving to the opposite side of the membrane where it is present in higher concentration.

Example of an Exchanger

The Na/Ca exchanger uses the inward Na+ gradient to move Ca2+ out of the cell.

Functional Differences Between Channels and Carriers:

Ion channels have a continuous aqueous pathway for ion conduction across the membrane, which can be occluded by the closing of a gate.
Ion pumps and transporters have two gates in series that control ion flux. These gates are never open simultaneously, but both can close to trap one or more ions in.

Maintenance of Ion Gradients in a Typical Cell

Active transport is responsible for maintaining differences in the concentrations of key ions inside and outside of mammalian cells.
Active transport creates and maintains the gradients of key ions that play crucial roles in various cellular processes, including nerve impulse propagation, muscle contraction, and maintaining cell volume.

Self-Testing Questions

Name 3 different transport mechanisms for Na+ to cross the membrane:
- Simple diffusion
- Facilitated diffusion through Na+ channels
- Active transport via the Na+/K+ pump
Carrier-mediated transport is a form of secondary active transport. (False)

Carrier-mediated transport can mediate facilitated diffusion (passive), or be part of secondary active transport.

Carrier-mediated transport is mediated by facilitated diffusion, just like voltage-gated ion channels. (False) - Although both involve proteins, they differ in mechanism and energy requirements. Voltage-gated ion channels allow ions to move passively down their electrochemical gradient, while carrier-mediated transport can facilitate both passive and active transport.
Do carriers have a selectivity filter? (Yes)
Describe the effect of ouabain on the Na-K pump. - Ouabain is a potent inhibitor of the Na+/K+ pump. It binds to the pump and prevents it from transporting ions, leading to a decrease in the Na+ gradient across the membrane.
What is the most likely consequence (regarding ion composition) of ouabain on a given cell? - Ouabain would cause an increase in intracellular Na+ concentration and a decrease in intracellular K+ concentration.
Explain why the Na-K pump is electrogenic. - It moves three positively charged sodium ions out of the cell for every two potassium ions moved into the cell, contributing to the negative charge inside the cell.
Knowing that ion channels move ions using the concentration gradient generated by pumps as electromotive force, why aren’t they considered secondary active transport? - Ion channels do not directly utilize the energy provided by the pump to move ions. Instead, they allow ions to move passively down the existing gradient. Secondary active transport requires the transporter to couple the movement of one ion down its gradient to the movement of another ion against its gradient.

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Description

Explore the key concepts of biological membranes, including the structure and fluidity of phospholipid bilayers, the importance of transport systems, and the mechanisms of simple diffusion. This quiz will test your understanding of how these factors influence cellular processes.