Guyton Chap 4-5

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Questions and Answers

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

To synthesize ATP from ADP

To regulate cell volume and ion concentrations (correct)

To convert glucose into energy

To facilitate the diffusion of glucose

How does the Na+-K+ pump affect water movement within the cell?

It prevents any water movement across the membrane

It creates an osmotic force that moves water out of the cell (correct)

It increases water intake by drawing it into the cell

It allows water to flow freely in and out of the cell

What is produced as a result of ATP cleavage by the ATPase function?

Two molecules of high-energy phosphate

Adenosine diphosphate and a high-energy phosphate bond (correct)

Sodium ions and potassium ions

Adenosine triphosphate and energy

What initiates the activation of the Na+-K+ pump when a cell begins to swell?

The physiological response to increased cell volume Signup and view all the answers

Which statement accurately describes co-transport?

It uses the diffusion energy of sodium to pull other substances along in the same direction Signup and view all the answers

What happens to the rate of transport at high concentrations of the substance being transported?

It is limited by the binding and release rates Signup and view all the answers

What is the result of the Na+-K+ pump transporting three sodium ions for every two potassium ions?

It results in a net movement of positive charges out of the cell Signup and view all the answers

Counter-transport and co-transport are both forms of what type of transport?

Secondary active transport Signup and view all the answers

What happens to a cell when there is a concentration difference of water across its membrane?

The cell shrinks or swells. Signup and view all the answers

What does osmotic pressure depend on in a solution?

The number of particles per unit volume. Signup and view all the answers

Which statement about osmolality is correct?

300 milliosmoles per kilogram is typical for extracellular fluids. Signup and view all the answers

How many grams of glucose are equivalent to 1 osmole?

180 grams of glucose. Signup and view all the answers

Which process is described as moving a substance against an electrochemical gradient?

Active transport. Signup and view all the answers

What is the osmotic pressure of normal extracellular and intracellular fluids?

5500 mm Hg. Signup and view all the answers

What is the relationship between osmotic pressure and particle size?

Osmotic pressure is unaffected by particle size. Signup and view all the answers

What is meant by the term 'osmole'?

It reflects the number of undissociated solute particles in solution. Signup and view all the answers

What happens during sodium co-transport of glucose and amino acids?

A protein carrier is required for transport. Signup and view all the answers

What is required for the conformational change in transport proteins during sodium co-transport?

Both sodium and glucose or amino acid must attach to the carrier. Signup and view all the answers

In the sodium counter-transport mechanism, what is the movement of calcium ions?

Calcium ions move to the exterior of the cell. Signup and view all the answers

Where does hydrogen ion counter-transport primarily occur?

In the proximal tubules of the kidneys. Signup and view all the answers

What type of cells are considered 'excitable'?

Nerve and muscle cells. Signup and view all the answers

What is primarily determined by the concentration gradient of sodium ions?

The energy available for transport mechanisms. Signup and view all the answers

What occurs when a sodium ion moves into a cell during counter-transport?

Hydrogen ions are simultaneously moved out. Signup and view all the answers

What characterizes the membrane potentials of nerve and muscle cells?

They generate electrical impulses during action potentials. Signup and view all the answers

What is the primary ion responsible for causing depolarization during an action potential?

Sodium ions Signup and view all the answers

What must occur for an action potential to be initiated?

The threshold potential must be reached Signup and view all the answers

During which period can an action potential not be elicited regardless of the stimulus strength?

Absolute refractory period Signup and view all the answers

What is the threshold membrane potential that must be reached for an action potential to occur?

−65 millivolts Signup and view all the answers

Which of the following statements is true regarding the inactivation of sodium channels?

Sodium channels become inactivated shortly after action potential initiation Signup and view all the answers

How many impulses can be transmitted per second in large myelinated nerve fibers?

2500 impulses Signup and view all the answers

What occurs during the relative refractory period?

Stronger than normal stimuli are required to generate an action potential Signup and view all the answers

What occurs during the positive-feedback mechanism of action potential generation?

More sodium channels open leading to greater depolarization Signup and view all the answers

What is the primary function of voltage-gated sodium channels during an action potential?

To enable rapid depolarization of the neuronal membrane Signup and view all the answers

What factor contributes to the greater leakage of potassium ions compared to sodium ions during the resting state?

Higher conductance for potassium ions Signup and view all the answers

How does the activation of voltage-gated sodium channels affect sodium permeability during an action potential?

It increases sodium permeability by up to 5000-fold Signup and view all the answers

What role do voltage-gated potassium channels play during the action potential?

They contribute to the rapid repolarization of the membrane Signup and view all the answers

What is the effect of rising membrane potential on voltage-gated sodium channels?

They open and allow sodium to rapidly enter the cell Signup and view all the answers

What happens to the voltage-gated sodium channels shortly after they are activated?

They immediately close due to inactivation Signup and view all the answers

What is the primary reason for the negative resting membrane potential before the action potential begins?

Predominance of potassium conductance Signup and view all the answers

What triggers the beginning of the action potential?

A sufficient increase in membrane voltage Signup and view all the answers

Study Notes

Water Movement and Osmotic Pressure

Under normal conditions, water molecules diffuse equally in both directions across a cell membrane, resulting in no net movement and constant cell volume.
A concentration difference for water can develop, causing net movement and potential cell swelling or shrinking.
Osmotic pressure, the pressure required to stop osmosis, depends on particle concentration per unit volume rather than mass.
The osmole quantifies concentration based on the number of particles: 1 osmole equals 1 gram molecular weight of undissociated solute.
180 grams of glucose represents 1 osmole, as glucose does not dissociate when dissolved.
A solution with 1 osmole of solute per kilogram of water has an osmolality of 1 osmol/kg; 1/1000 osmole per kilogram is 1 milliosmole/kg.
Normal osmolality for intracellular and extracellular fluids is about 300 milliosmoles/kg, with osmotic pressure around 5500 mm Hg.

Active Transport Mechanisms

Active transport can move substances against their electrochemical gradient using energy, typically from ATP.
The Na+-K+ pump transports three sodium ions outside the cell for every two potassium ions moved inside, contributing to osmotic forces that regulate cell volume.
If a cell swells, the Na+-K+ pump is activated to extrude additional ions and water, maintaining balance.
Active transport mechanisms can saturate like facilitated diffusion, limited by binding/release reactions and carrier conformation.

Co-Transport and Counter-Transport

Co-transport allows sodium ions' passive diffusion to drive the active transport of other substances into the cell.
Counter-transport moves sodium ions into the cell while expelling calcium or hydrogen ions using the same transport protein.
Glucose and amino acids can enter most cells via sodium co-transport, requiring both sodium and the nutrient to bind to the transporter.

Membrane Potentials and Action Potentials

All cell membranes, especially in excitable cells like nerves and muscles, maintain electrical potentials across them.
Potassium ions' rapid diffusion reestablishes the negative resting membrane potential after action potentials.
Voltage-gated sodium and potassium channels are critical for the progression of action potentials, affecting depolarization and repolarization phases.
Initially, potassium conductance is much greater than sodium, but during action potentials, sodium conductance surges.
Voltage-gated sodium channels open rapidly with an increase in membrane potential, facilitating high sodium ion influx—a positive-feedback cycle that continues until channels become inactivated.

Threshold and Refractory Periods

An action potential only occurs when the threshold potential (approx. -65 mV) is reached, marked by more sodium ions entering than potassium exiting.
During the absolute refractory period, no new action potential is possible even with strong stimuli, lasting about 1/2500 seconds in large myelinated fibers.
Following the absolute refractory period, a relative refractory period occurs where stronger stimuli are required to elicit a new action potential.

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Description

Explore the principles of osmosis and diffusion as they relate to cell membranes. Understand how concentration differences affect the movement of water molecules across the cell membrane, leading to cell swelling or shrinking. Test your knowledge on how these processes maintain cellular volume.