Physiology Chapter 1 Costanzo

Pages 1-17

Which ion is the major cation in extracellular fluid (ECF), and which is the major cation in intracellular fluid (ICF)?

ECF: Na+ (sodium); ICF: K+ (potassium)

What are the major anions in ECF and ICF?

ECF: Cl- and HCO3-; ICF: phosphates and proteins

A cell is placed in a solution containing a high concentration of Na+ and a low concentration of K+. Which compartment does this solution most closely resemble: ECF or ICF?

ECF because ECF has a high Na+ concentration and a low K+ concentration.

Why can the ICF and ECF have different ionic compositions while each compartment remains electrically neutral?

The cell membrane separates the compartments and selectively regulates ion movement. Each compartment remains electrically neutral because the total concentration of positive charges equals the total concentration of negative charges.

What does electroneutrality mean?

The total number of positive charges equals the total number of negative charges in a solution.

A patient’s plasma Na+ concentration increases substantially. What happens to plasma osmolality, assuming the other major osmoles remain unchanged?

Plasma osmolality increases. Na+ is the major determinant of ECF osmolality, so an increase in plasma Na+ increases plasma osmolality.

Which body-fluid compartment contains the greatest concentration of protein?

ICF contains the highest concentration of protein.

Why are the proteins particularly important contributors to the osmotic pressure of plasma?

Plasma proteins are large, charged molecules that do not readily cross the capillary membrane. They therefore contribute to the colloid osmotic pressure (oncotic pressure) of plasma and help retain water in the ECF.

What property of the phospholipid bilayer makes it an effective barrier to ions?

The phospholipid bilayer has a hydrophobic core, which prevents most ions and polar molecules from crossing it.

Which substances can cross the lipid bilayer most easily without membrane proteins?

Lipid-soluble substances and small nonpolar molecules cross most easily. Examples include O2 and CO2.

Which substances generally require memberane proteins to cross the cell membrane?

Ions and polar or large molecules generally require membrane proteins.

What is the difference between a channel and a carrier?

A channel provides a hydrophillic pathway through a membrane. A carrier binds a substance and changes conformation to move it across the membrane.

A membrane protein transports a solute down its concentration gradient without directly using ATP. What type of transport is occuring?

Facilitated diffusion

Define simple diffusion.

The movement of a solute from an area of high concentration to an area of low concentration directly through the lipid bilayer, without a transport protein.

What determines the net direction of simple diffusion of a solute?

The concentration gradient determines the net direction; substances move from high concentration to low concentration.

How does increasing the concentration gradient affect the rate of diffusion?

Increasing the concentration gradient increases the rate of diffusion.

What is facilitated diffusion, and how does it differ from simple diffusion?

Facilitated diffusion is passive movement down an electrochemical gradient through a membrane protein. Simple diffusion occurs directly through the lipid bilayer.

Why can facilitated diffusion become saturated?

There are a limited number of membrane transport proteins. At high concentrations, all available transporters are occupied.

A substance is transported from an area of low concentration to an area of high concentration. Can this occur by simple diffusion? Why or why not?

No, simple diffusion only occurs down a concentration gradient. Movement from low to high concentration requires active transport.

What is the difference between primary active transport and secondary active transport?

Primary active transport directly uses ATP. Secondary active transport uses the energy stores in an ion concentration gradient.

Which energy source directly powers primary active transport?

ATP hydrolysis directly provides the energy.

The Na+/K+ ATPase moves 3 Na+ out of the cell and 2 K+ into the cell per ATP hydrolyzed. Is the pump electrogenic or electroneutral? Explain.

The Na+/K+ ATPase is electrogenic because it moves 3 Na+ out of every 2 K+ moved in, resulting in a net movement of one positive charge out of the cell.

What are the major physiological consequences of the Na+/K+ ATPase?

It maintains high intracellular K+ and low intracellular Na+. It contributes to the negative resting membrane potential and provides the Na+ gradient needed for secondary acive transport.

If the Na+/K+ ATPase suddenly stops working, what happens to intracellular Na+ and K+ concentrations over time?

Intracellular Na+ increases and intracellular K+ decreases. The concentration gradients eventually disappear.

Why is the Na+/K+ ATPase considered a primary active transporter?

It directly uses ATP hydrolysis to transport Na+ and K+ against their electrochemical gradients.

What is the difference between cotransport (symport) and countertransport (antiport)?

Cotransport/symport: substances move in the same direction. Antiport/countertransport: substances move in opposite directions.

A transporter uses the downhill movement of Na+ to drive glucose uptake against its concentration gradient. What type of transport is this?

Secondary active transport (symport/cotransport)

Why is secondary active transports ultimately dependent on ATP even though the transporter itself does not directly hydrolyze ATP?

It depends indirectly on ATP because ATP is used by the Na+/K+ ATPase to establish the Na+ gradient that drives the secondary transporter.

If the Na+ gradient across the cell membrane were eliminated, what would happen to Na+ dependent secondary active transport?

Na+-dependent secondary active transport would decrease and eventually stop because there would be no Na+ electrochemical gradient to provide energy.

What is osmosis?

The movement of water across a selectively permeable membrane in response to a difference in effecive solute concentration.

In which direcion does water move during osmosis?

Water moves toward the compartment with the higher concentration of impermeant solute.

What determines the direction of water movement across a semipermeable membrane?

It is determined by the difference in effective osmolarity (tonicity) across the membrane.

A cell is placed in a solution with higher effecive osmolarity than the cell. What happens to cell volume?

The cell loses water and shrinks.

A cell is placed in a hypotonic solution. What happens to the cell?

The cell gains water and swells. If sufficiently hypotonic, it may lyse.

Why is an impermeant solute more important than a freely permeant solute in determining sustained cell volume?

An impermeant solute cannot cross the membrane, so it maintains a lasting osmotic gradient. Freely permeant solutes can cross and therefore cannot maintain a sustained osmotic gradient.

What is a diffusion potential?

A voltge difference across a membrane that results when a charged ion diffuses down its concentration gradient.

Can a diffusion potential be generated if the membrane is permeable to an ion but there is no concentration gradient for that ion?

No, a concentration gradient is required for net movement and therefore for generation of a diffusion potential.

A membrane is permeable only to K+. K+ concentration is higher inside the cell than outside. Which direction does K+ initially move?

K+ moves out of the cell because its concentration is higher inside.

As K+ leaves the cell, what happens to the electrical charge inside the cell?

The inside of the cell becomes more negative as positive K+ leaves.

What two forces eventually oppose further movement of K+ across the membrane?

The chemical force of driving K+ out and the electrical force pulling K+ back into the negatively charged cell

What is the equilibrium potential of an ion?

The membrane potential at which the electrical force exactly balances the chemical force for an ion.

At an ion’s equilibrium potiential, is there net movement of that ion across the membrane?

No net movement of the ion occurs.

Does “no net movement” mean that individual ions stop crossing the membrane? Explain.

No, individual ions continue to move in both directions. At equilibrium, movement in one direction equals movement in the other, so there is zero net movement.