Transmembrane Transport of Ions & Molecules

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Questions and Answers

What characterizes selective permeability in biomembranes?

Size is the only factor determining permeability.

All substances can freely pass through.

Hydrophilic substances are always impermeable.

Only certain types of substances can move through. (correct)

What type of transport does not require energy expenditure?

Simple Diffusion (correct)

Primary Active Transport

Secondary Active Transport

Facilitated Diffusion

Which factor does NOT influence the rate of diffusion across a membrane?

Free energy of reactants (correct)

Concentration gradient

Size of the gradient

Molecular weight of diffusing particles

Which statement about hydrophobicity is correct?

A substance with a high partition coefficient (K) is more lipid-soluble. Signup and view all the answers

Which transport mechanism uses energy directly from ATP?

Primary Active Transport Signup and view all the answers

What characterizes facilitated diffusion?

Involves movement from higher concentration to lower concentration Signup and view all the answers

Which type of transport protein uses ATP to move substances against their concentration gradient?

Pumps Signup and view all the answers

How does secondary active transport differ from primary active transport?

It does not directly require ATP Signup and view all the answers

What distinguishes gated ion channels from non-gated ion channels?

Gated channels require a stimulus to open, while non-gated channels are always open Signup and view all the answers

Which of the following correctly describes uniport transport?

It is a form of facilitated diffusion for small, hydrophilic molecules Signup and view all the answers

Which ion is the most critical for maintaining the membrane potential?

K+ Signup and view all the answers

What causes the activation of the Na+/H+ antiporter?

Decrease in cytosolic pH Signup and view all the answers

What is generated by the activity of the Na+/K+ ATPase on the basolateral surface membrane?

Concentration gradients of Na+ and K+ Signup and view all the answers

Which transport mechanism helps to decrease cytosolic pH?

Cl-/HCO3- antiporter Signup and view all the answers

What occurs during the outward movement of K+ ions through nongated K+ channels?

It generates an inside-negative membrane potential Signup and view all the answers

What is the function of the Na+/glucose symporter in the context of rehydration therapy?

To facilitate the uptake of glucose along with sodium ions Signup and view all the answers

What is the role of proton pump inhibitors in the stomach?

To inhibit acid production by blocking the proton pump Signup and view all the answers

How does ouabain affect muscle contraction?

It increases cytosolic Ca2+ levels, enhancing muscle contractions Signup and view all the answers

Which of the following best describes the mechanism of the Na+/K+ pump as influenced by ouabain?

It is blocked, leading to increased cytosolic Na+ levels Signup and view all the answers

What condition is primarily treated by rehydration therapy involving water, sugar, and salt?

Intense dehydration Signup and view all the answers

Which of the following substances may be implicated in hypertension?

Ouabain Signup and view all the answers

What is a key characteristic of uniport transport compared to simple diffusion?

It exhibits a maximum rate (Vmax). Signup and view all the answers

What effect does the acid produced in the stomach have on digestion?

It aids in the breakdown of food for easier digestion Signup and view all the answers

Which glucose uniport transporter is known to be expressed in erythrocytes?

GLUT 1 Signup and view all the answers

What role does the Na+/Ca2+ antiporter play in muscle function?

It regulates cytosolic Ca2+ levels for muscle relaxation Signup and view all the answers

What does osmotic pressure relate to in terms of water movement across membranes?

The hydrostatic pressure needed to stop water movement. Signup and view all the answers

What role do aquaporins play in cell membranes?

They increase the permeability of membranes to water. Signup and view all the answers

Which class of pumps is primarily responsible for generating and maintaining ion gradients across membranes?

V-class pumps Signup and view all the answers

What is a major function of the MDR1 pump in the ABC superfamily?

Transporting lipid-soluble substances out of cells. Signup and view all the answers

What distinguishes gated channels from ungated (leaky) channels?

Gated channels require a stimulus to open. Signup and view all the answers

What is a characteristic of the flippase mechanism used by the ABC superfamily pumps?

It is utilized for transporting lipid-soluble substances. Signup and view all the answers

What factor significantly influences a substance's hydrophobicity when moving through a lipid bilayer?

The proportion of uncharged regions in the substance Signup and view all the answers

Which statement correctly describes the process of simple diffusion across a membrane?

It occurs spontaneously from higher to lower concentration without energy expenditure. Signup and view all the answers

How does the partition coefficient (K) relate to a substance's ability to diffuse through a lipid bilayer?

A higher K indicates greater lipid solubility and faster diffusion rates. Signup and view all the answers

Which of the following factors contributes least to the rate of diffusion across a membrane?

The specific type of membrane lipid present Signup and view all the answers

Which mechanism allows the transport of small molecules against their concentration gradient?

Secondary active transport Signup and view all the answers

What is the main feature of facilitated diffusion?

It involves the movement of substances from higher to lower concentration. Signup and view all the answers

Which type of transport mechanism uses pumps to move substances?

Primary active transport Signup and view all the answers

What distinguishes secondary active transport from primary active transport?

It involves the movement of a substance against its concentration gradient coupled with the movement of an ion down its gradient. Signup and view all the answers

What role do gated ion channels play in cellular transport?

They open in response to specific stimuli to regulate ion flow. Signup and view all the answers

Which statement accurately describes the function of uniport transporters?

They facilitate the movement of small hydrophilic molecules through a membrane. Signup and view all the answers

Which ion plays the most significant role in maintaining the membrane potential?

K+ Signup and view all the answers

How do Na+/H+ and Cl-/HCO3- antiporters affect cytosolic pH under specific conditions?

Both are activated at low cytosolic pH. Signup and view all the answers

What process generates a negative membrane potential across the plasma membrane?

Outward movement of K+ ions Signup and view all the answers

What type of transport mechanism is represented by the Na+/glucose symporter?

Secondary active transport Signup and view all the answers

Which statement regarding membrane transport proteins is correct?

Specific membrane channels regulate resting potential. Signup and view all the answers

What is the role of the Na+/glucose symporter in the uptake of glucose from the intestinal lumen?

Uses the Na+ concentration gradient to facilitate glucose uptake Signup and view all the answers

Which substances are used in rehydration therapy to stimulate the Na+/glucose symporter?

Water, sugar, and salt Signup and view all the answers

How do proton pump inhibitors affect stomach acid production?

They completely block the production of stomach acid Signup and view all the answers

What effect does ouabain have on muscle contraction?

Increases cytosolic Ca2+ levels, enhancing contraction Signup and view all the answers

What is the full name of the system inhibited by proton pump inhibitors?

Hydrogen-potassium adenosine triphosphate enzyme system Signup and view all the answers

What is the potential consequence of inhibiting the Na+/K+ pump with ouabain?

Altered resting membrane potential of muscle cells Signup and view all the answers

Why is the Na+/glucose symporter essential in managing dehydration?

It requires high sodium levels for glucose absorption Signup and view all the answers

What effect does sodium have in the function of the Na+/Ca2+ antiporter?

Accelerates calcium removal from muscle cells Signup and view all the answers

What is a key aspect of GLUT transporters in humans?

Each GLUT transporter has a distinct tissue distribution. Signup and view all the answers

What does Vmax represent in the context of uniport transport?

The highest rate of transport achievable. Signup and view all the answers

Which statement accurately describes osmotic pressure?

It is the hydrostatic pressure needed to halt water movement across a membrane. Signup and view all the answers

How do aquaporins influence cell membranes?

They increase water permeability. Signup and view all the answers

What is the primary function of ATP-powered ion pumps?

They generate and maintain ion gradients across membranes. Signup and view all the answers

What distinguishes gated channels from ungated channels?

Gated channels open in response to stimuli. Signup and view all the answers

Which statement best describes the flippase mechanism?

It is involved in the movement of lipid-soluble substances. Signup and view all the answers

Which of the following accurately describes the function of MDR1 in the ABC superfamily?

It secretes lipid-soluble substances out of cells. Signup and view all the answers

Study Notes

Transmembrane Transport of Ions & Small Molecules

Biomembranes are selectively permeable due to their lipid bilayer structure.
This means only certain types of substances can move through.
Gases (CO2, N2, O2), small uncharged polar molecules (ethanol), and water are permeable.
Urea and glucose/fructose are slightly permeable.
Ions (K+, Mg2+, Ca2+, Cl-, HCO3-, HPO42-), charged polar molecules (amino acids, ATP, proteins, and nucleic acids) are impermeable.

Mechanisms of Transmembrane Transport

Simple (passive) diffusion: Movement of a substance across a membrane from a region of higher concentration to one of lower concentration, without energy expenditure.
Facilitated diffusion: Movement of a substance across a membrane from a region of higher concentration to one of lower concentration with the help of a transport protein; still no energy expenditure.
Primary active transport: Movement of a substance from a region of lower to higher concentration across a membrane, requiring energy expenditure and using proteins called pumps (ATPases).
Secondary active transport (cotransport): Energetically unfavorable transport of a substance against its concentration gradient is coupled to the energetically favorable movement of an ion down its electrochemical gradient (symport or antiport).

Simple Diffusion

Diffusion is spontaneous due to increasing entropy as a substance moves from high to low concentration.
The change in free energy (ΔG) is negative because products have more entropy and less free energy than reactants.
ΔG = ΔH – TΔS (where ΔH is enthalpy and ΔS is entropy, and T is temperature in Kelvin).

What Determines Diffusion Rate Across a Membrane?

Concentration gradient across the membrane.
Size of the diffusing particles.
Electric potential across the membrane (for charged particles).
Hydrophobicity of the substance.

Hydrophobicity

Hydrophobicity measures the degree to which a substance is hydrophobic.
Measured by the substance's partition coefficient (K).
K is the equilibrium constant for its partition between oil and water.
Higher K = more lipid-soluble and faster rate of movement across the bilayer.
Hydrophobicity is influenced by the proportion of uncharged regions in a substance (e.g., long-chain fatty acids are more hydrophobic than shorter-chained ones).

How do Impermeable Substances Move Through Membranes?

Impermeable substances require transport proteins: pumps, channels or transporters
These protein-mediated processes may or may not require energy.
Multiple transport proteins can work together

Transport Protein Mediated Processes

Facilitated Diffusion: Passive movement across membranes, no energy needed. typically with channel or uniporters.
Primary Active Transport: Active movement from a low concentration to high, requiring ATP hydrolysis through pump proteins.
Secondary Active Transport (Cotransport): Coupled transport of substances, one substance (e.g ion) going down its electrochemical gradient drives the movement of another substance against its gradient.

Membrane Transport Proteins

ATP-powered pumps: 100-10^3 ions per second, move substances against their concentration gradient, requiring energy.
Ion channels: 10^7-10^8 ions per second, allow passive transport of ions based on electrochemical gradient.
Transporters (uniporter, symporter, antiporter): 10^2-10^4 molecules per second, facilitate passive or active transport of substances based on concentration.

Mechanisms for Transporting Ions and Small Molecules across Cell Membranes

Simple diffusion: no protein required, high to low concentration gradient.
Facilitated transport: protein required, high to low concentration gradient.
Active transport: protein required, low to high concentration gradient.
Cotransport: coupled to energetically favorable movement of another substance.

Comparing Simple Diffusion & Uniport Transport

Uniport transport is a form of facilitated diffusion involving small hydrophilic molecules through a membrane and is faster than simple diffusion.
K is irrelevant as the molecule doesn't enter the hydrophobic membrane core.
Uniport transport has a maximal rate (Vmax).
It is reversible (direction based on concentration gradient).
It is specific.

Glucose Uniport Transporters (GLUT)

Humans have 12 GLUT isoforms.
GLUTs are tissue-specific: GLUT1 in erythrocytes, GLUT2 in liver cells.
GLUT transport is a key mechanism for glucose entry.

Osmotic Pressure and Water Movement

Osmotic pressure causes water to move across membranes from a region of lower solute concentration to a region of higher solute concentration.
Water moves to equalize solute concentration across a semipermeable membrane.
Hydrostatic pressure is required to prevent net water flow.

Aquaporins

Aquaporins increase water permeability of cell membranes.
Aquaporins are water channel proteins.

4 Classes of ATP-Powered Pumps

P-class: transport ions
V-class: acidify organelles
F-class: used in ATP synthesis
ABC: transport large molecules, lipids, etc

Model of Ca2+ ATPase

Ca2+ ATPase is a protein that pumps calcium ions across membranes.
It involves binding and conformational changes involving ATP.
It is an important regulatory protein in muscle contraction and other cellular processes.

Structure of Catalytic a-subunit of Muscle ATPase

Shows how a-subunit is conformational involved with calcium and phosphorylation.

Na+/K+ ATPase

A pump that moves Na+ out of and K+ into the cell, maintained by ATP hydrolysis.

Effects of V-class H+ pumps on H+ concentration and electric potential gradients

V-class pumps change the pH gradient across a membrane.
This gradient creates an electric potential across the membrane.
The effects depend on whether or not other membrane channels are also present.

ATP-powered ion pumps generate and maintain ion gradients.

Illustrates the concentration differences of ions inside and outside a mammalian cell (e.g. K+, Na+, CI-).
Provides context for the importance of ion gradients in cellular function and homeostasis.

ABC Superfamily

About 50 ABC superfamily pumps in mammals.
Transport of lipids, sugars, other molecules.
e.g., MDR1 to expel drugs, CFTR for Cl- transport and other substances.
Present in bacteria, too

Selected Human ABC Proteins

Table including protein, tissue expression, function and disease caused by defective protein

Flippase Mechanism

Mechanism used by ABC superfamily pumps to transport lipid-soluble substances across membranes involving ATP binding.

Gated & Ungated Channels

Gated channels open and close in response to stimuli (chemical or voltage).
Non-gated (leaky) channels are always open, allowing ion flow to maintain membrane resting potential.
e.g. Na+, K+, Cl-, Ca2+ channels.

Generation of Transmembrane Electric Potential

Diagram showing the role of selective permeable membranes.
Na+ and K+ permeability differences create membrane potential.
Illustrates how differences in ion concentrations and membrane permeability generate electric potential across a cell membrane.

Structure of Resting Bacterial K+ Channel

Illustrates the structure of a potassium channel, highlighting the selectivity filter, P segment, and membrane helices.

K+ and Na+ ions in the pore of a K+ channel

The figure illustrates why potassium channel selectively transports K+ over Na+.

K+ ions in the pore of a K+ channel (side view).

Ion concentration gradient and Membrane electric potential

Illustrates the free-energy change during Na+ transport across a membrane based on the ion concentration gradient and the membrane electric potential.

Cotransport

Na+/H+ antiporter, Cl-/HCO3- antiporter, 2Na+/glucose symporter, 3Na+/Ca2+ antiporter are examples
Cotransport mechanisms that affect cytosolic pH of mammalian cells.

Figure 11-26: 2 Na+/1 Leucine Symporter

Shows structural details of the leucine symporter.

Co-transporters and pH

The activity of membrane transport proteins regulating cytosolic pH in mammalian cells depends on pH.
Na+/H+ and Na⁺HCO₃⁻/Cl⁻ antiporters increase cytosolic pH when it decreases.
CI⁻/HCO₃⁻ antiporter reduces cytosolic pH at high pH.

Transport Across Epithelial Membranes: Transcellular Glucose Transport

Illustrates transport of glucose across epithelial cells in the small intestine.
Importance of Na+/K+ ATPase and transport proteins.

Rehydration Therapy

Ingestion of water, sugar, and salt can stimulate the Na+/glucose symporter, facilitating rehydration.
Principle for drinks like Gatorade.

Transport Across Epithelial Membranes: Acidification of Stomach Lumen

Illustrates the acidification of the stomach using the H+/K+ ATPase and CI-/HCO3- antiporter.

Proton Pump Inhibitors

Block stomach acid production through inhibition of the proton pump (hydrogen-potassium adenosine triphosphate enzyme system).

Ouabain, A Cardiac Glycoside

Endogenous steroid hormone.
Blocks Na+/K+ pump.
Increased cytosolic Ca2+ levels and stronger muscle contraction.

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Description

Explore the mechanisms of transmembrane transport including simple diffusion, facilitated diffusion, and active transport. This quiz covers the selectively permeable nature of biomembranes and the types of substances that pass through them. Test your knowledge on the permeability of ions and small molecules.