OSU Physio 6101 - Lecture 2 Transport Mechanisms

Comprehensive review of lecture 2.

What are the four major membrane transport mechanisms?

Simple Diffusion; Facilitated Diffusion; Primary Active Transport; Secondary Active Transport.

What does the plasma (cell) membrane consist of?

An amphipathic phospholipid bilayer.

Is theinterior of the plasma membrane hydrophilic or hydrophobic?

Hydrophobic.

Is the exterior of the plasma membrane polar or nonpolar?

Polar.

What determines membrane fluidity and stability?

The proportion of cholesterol.

What are integral proteins?

Proteins tightly associated with the bilayer.

What are peripheral proteins?

Proteins located at the membrane surface.

What compartments does the plasma membrane separate?

The ECF/ISF and ICF compartments.

What is permeability?

The ease with which the molecule crosses the barrier.

What kind of property is permeability of a barrier to a molecule?

An intrinsic property of the barrier.

What is simple diffusion?

The movement of molecules from one location to another as a result of their random thermal motion.

Is simple diffusion a passive process?

Yes. No energy expenditure is involved.

What direction do molecules tend to move during simple diffusion?

From an area of higher concentration to an area of lower concentration until reach equilibrium.

What is net flux?

The sum of two opposed unidirectional fluxes.

How is net flux related to concentration gradient for passive diffusion?

For passive diffusion the net flux is linearly related to the concentration gradient.

According to Fick’s law, net flux is directly proportional to what?

Concentration Gradient (ΔC), Permeability Coefficient (P), and Area of the membrane (A).

What is the Fick’s law equation for net solute flux?

Jₛ^net = (PₛA/Δx)(ΔCₛ).

What is the concentration gradient shown in Fick’s law?

ΔC = C¹ − C².

What does the permeability coefficient P reflect?

The ease with which a molecule is able to move across a given membrane at a given temperature.

How does lipid solubility affect the permeability coefficient?

The permeability coefficient is directly proportional to the lipid solubility of the solute.

How do lipophilic substances move across cell membranes?

They move easily across the lipid bilayer of cell membranes.

What are examples of nonpolar molecules that diffuse rapidly through membranes?

Oxygen, carbon dioxide, fatty acids, and steroid hormones.

How do polar and hydrophilic molecules diffuse through membranes?

They do not diffuse readily through the membranes.

How does solute size affect the permeability coefficient?

The permeability coefficient is inversely proportional to the size of the solute.

What is the relationship between net flux and distance?

Net flux is indirectly proportional to distance.

What is the diffusion time across a mitochondrion (1.0 µm diameter)?

0.5 msec.

What is the diffusion time across a mammalian cell (10 µm diameter)?

50.0 msec.

What is the diffusion time across a muscle fiber (cell) (100 µm diameter)?

5.0 sec.

How close are almost all cells to a capillary?

Within 25–50 µm.

How long does it take for a substance to diffuse between a capillary and a cell?

Only a few seconds.

What are the three types of gated channels?

Ligand gated; mechanically gated; voltage gated.

What activates ligand-gated channels?

Binding of a chemical signaling agent to the ligand.

What activates mechanically gated channels?

Physical deformation of the surrounding plasma membrane.

What activates or inactivates voltage-gated channels?

Changes in the membrane potential.

What do ion channels control?

The movement of ions into and out of a cell.

What property do ion channels show?

Selectivity for the type of ion(s).

What is membrane potential?

A voltage gradient caused by a separation of electrical charges across plasma membranes of cells.

What does membrane potential provide?

An electrical force that influences the movement of ions across the membrane.

Why do many molecules such as glucose require help to enter cells?

Many molecules are either too large and charged to get into the cell without help.

What proteins mediate the movement of these molecules?

Integral membrane proteins—transporters also known as carriers.

How do protein transporters bring molecules into and out of cells?

By conformation changes.

What are the three mediated-transport mechanisms?

Facilitated diffusion; primary active transport; secondary active transport.

What occurs in facilitated diffusion?

An integral protein binds to a specific solute or family of solutes; occurs down a concentration gradient; movement is bi-directional; not coupled to energy (ATP).

When is steady state reached in facilitated diffusion?

When the ICF and ECF concentrations are equal; the unidirectional fluxes are equal and opposite.

What are examples of solutes moved by facilitated diffusion?

Glucose (through transporters), and organic ions (through ion channels).

What is the approximate glucose facilitated-diffusion rate shown?

10²–10³ molecules/sec.

What is the single most important property of an active transport system?

It uses energy to move an ion or solute against its electrochemical gradient.

What do active transport processes make possible?

The creation of large differences in the concentrations of substances in the intracellular and extracellular compartments.

What are the two means of coupling energy to transporters?

Primary active transport uses ATP; secondary active transport uses an electrochemical gradient.

What is the most important active transport mechanism in mammalian cells?

The Na+/K+-ATPase (or Sodium-Potassium pump).

Where is the Na+/K+-ATPase present?

In All cells.

What does the Na+/K+-ATPase pump?

3 Na+ out and 2 K+ in.

What energy does the Na+/K+-ATPase use?

The energy of ATP.

What happens in step 1 of the Na+/K+-ATPase cycle?

ATP binds directly to the Na+/K+-Pump. Three Na+ ions bind to the intracellular surface of the transporter.

What happens in step 2 of the Na+/K+-ATPase cycle?

This turns on the ATPase activity of the transporter, ATP is broken down and the transporter is phosphorylated.

What happens in step 3 of the Na+/K+-ATPase cycle?

This changes the shape of the transporter and Na+ is released outside of the cell.

What happens in step 4 of the Na+/K+-ATPase cycle?

K+ ions bind to the extracellular surface of the transporter resulting in de-phosphorylation of the transporter.

What happens in step 5 of the Na+/K+-ATPase cycle?

This induces a conformational change, changing the shape of the transporter and K+ is released inside the cell.

What characterizes secondary active transport?

Transport proteins utilize the energy gradient for one solute (or ion) to facilitate the “uphill” transport of a second solute.

How does ATP provide energy for secondary active transport?

ATP was used to set up the gradient for the first solute, such as Na+ gradient; the energy comes indirectly from ATP.

What happens first in the secondary-active-transport cycle shown?

Na+ and the solute bind to the transporter, which changes shape.

What happens second in the secondary-active-transport cycle shown?

Both Na+ and the solute come off on the other side.

What does the Na+/K+-ATPase maintain for Na+?

A relatively low ICF Na+ concentration compared to the ECF.

What electrical condition exists inside the cell in the secondary-active-transport example?

There is a negative electrical potential inside the cell.

What two gradients favor Na+ entry into cells?

Both the Na+ concentration and the voltage gradient.

What is the Na+ gradient often used to do?

“Power” secondary active transport systems.

What is symport?

The co-transport of coupled solutes in the same direction across the membrane.

What do Na+-driven symporters move inside the cell?

Glucose, amino acids and other ions.

How does Na+ movement facilitate uphill transport in symport?

Na+ movement down its gradient facilitates the uphill transport.

What must offset inward movement of Na+?

Na+ transport out of the cell by the Na+/K+ pump.

What is the example of symport?

Na+/Glucose Transporter.

What is antiport?

The transport of a solute out of a cell, in opposite direction than Na+ across the membrane.

What is the example of antiport?

Na+/Ca2+ Exchanger.

What are the vesicle-mediated transport mechanisms?

Endocytosis and Exocytosis.

What do membrane-bound vesicles allow?

Passage of molecules.

What is endocytosis?

Allows molecules to enter the cell.

What are the three types of endocytosis?

Pinocytosis; phagocytosis; receptor-mediated endocytosis.

What is pinocytosis?

Vesicles engulf extracellular fluid.

What is phagocytosis?

Immune cells engulf bacteria or large debris.

What is receptor-mediated endocytosis?

Specific proteins on the outer surface of the plasma membrane recognize a ligand and activate membrane invagination.

What is exocytosis?

Allows molecules to exit the cell.

What is exocytosis used for?

To secrete membrane-impermeable molecules and/or replace portions of the plasma membrane.

What is the function of epithelial transport?

Regulate the absorption or secretion of substances in hollow organs.

What is the apical membrane?

Membrane that typically faces a hollow chamber.

What is the basolateral membrane?

Membrane that typically faces blood vessels.

What holds epithelial cells together along their lateral surfaces?

Tight junctions.

What is transcellular transport?

Movement into a cell, through the cytosol, and exit across the opposite membrane.

What is special about apical and basolateral membranes?

They contain different ion channels and different transporters for mediated transport.

What is paracellular transport?

Diffusion through the paracellular pathway.

What limits paracellular diffusion?

The presence of tight junctions between adjacent cells.

What do tight junctions form?

A seal around the apical ends of the cells.

In the intestinal glucose-transport example, what occurs at the apical membrane?

Secondary active transport moves Na+ and glucose from the intestinal lumen into the epithelial cell.

In the intestinal glucose-transport example, what occurs at the basolateral membrane?

The Na+/K+ pump performs primary active transport, while glucose moves toward the blood by facilitated diffusion.

What is the overall direction of glucose transport in the intestinal example?

Intestinal lumen → epithelial cell → blood vessel.

In the kidney epithelial example, what occurs at the apical membrane?

Na+ enters from the kidney lumen; solute S is transported with Na+ by secondary active transport.

In the kidney epithelial example, what occurs at the basolateral membrane?

Na+ is transported into the blood by the Na+/K+ pump; solute S moves into the blood by facilitated diffusion.

What is capillary endothelium?

The wall of a capillary composed of a single layer of epithelial cells.

What does capillary endothelium separate?

The plasma from the ISF.

What is the permeability of capillary endothelium?

It has a high permeability to solutes and water.

How do plasma and ISF compositions generally compare?

With the exception of the much higher protein content of plasma, the differences in the composition of the plasma and the ISF are, in general, very small.