Transmembrane transport of ions & small molecules

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Questions and Answers

What is the primary driving force behind simple diffusion across a membrane?

Concentration gradient (correct)

Carrier proteins

Energy from ATP hydrolysis

Electrical gradient

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

Electric potential across the membrane

Concentration gradient

Time of exposure (correct)

Size of the diffusing particles

What is the primary mechanism of substances moving from a region of lower concentration to a region of higher concentration?

Facilitated diffusion

Simple diffusion

Passive transport

Primary active transport (correct)

How is hydrophobicity measured for a substance?

Partition coefficient (K) Signup and view all the answers

Which type of transport protein remains always open and is often referred to as a 'leaky' channel?

Non-gated channels Signup and view all the answers

What characterizes substances that move easily through the lipid bilayer?

They have high hydrophobicity. Signup and view all the answers

Which type of transport does NOT require energy expenditure?

Facilitated diffusion Signup and view all the answers

In secondary active transport, the movement of a target substance is coupled with what type of ion movement?

Down its electrochemical gradient Signup and view all the answers

Which term describes the process in which substances move from an area of higher concentration to an area of lower concentration without the use of energy?

Facilitated diffusion Signup and view all the answers

Which of the following transport proteins are specifically characterized as ATPases?

Pumps Signup and view all the answers

How does the Na+/glucose symporter function in the context of rehydration therapy?

By using the Na+ concentration gradient to drive glucose uptake Signup and view all the answers

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

To block the secretion of stomach acid Signup and view all the answers

Which action describes the mechanism of ouabain in muscle cells?

It blocks the Na+/K+ pump, increasing cytosolic Na+ levels. Signup and view all the answers

What is Gatorade primarily designed to do in terms of hydration?

Stimulate the Na+/glucose symporter for fluid absorption. Signup and view all the answers

In what way can intense dehydration be treated according to the described methods?

By ingesting a combination of water, sugar, and salt. Signup and view all the answers

When the Na+/K+ pump is inhibited by ouabain, what happens to cytosolic Ca2+ levels?

They increase due to impaired Na+/Ca2+ exchange. Signup and view all the answers

What effect does the acid produced by parietal cells in the stomach have initially?

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

How does the Na+/Ca2+ antiporter function in normal conditions?

It allows Na+ to enter the cell and Ca2+ to exit. Signup and view all the answers

Which ion is most crucial in maintaining the membrane resting potential?

K+ Signup and view all the answers

What effect does the Na+/H+ antiporter have when cytosolic pH is decreased?

Increases Na+ concentration Signup and view all the answers

What generates the inside-negative membrane potential across the plasma membrane?

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

Which cotransporter is correctly matched with its function?

3Na+/Ca2+ antiporter - moves calcium ions out Signup and view all the answers

What initiates the generation of sodium and potassium concentration gradients in the basolateral surface membrane?

Activity of Na+/K+ ATPase Signup and view all the answers

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

It can reach a maximum transport rate (Vmax). Signup and view all the answers

How many glucose uniporters (GLUT) are encoded in humans?

12 Signup and view all the answers

What is osmotic pressure?

The pressure required to stop water flow across a membrane separating solutions of different concentrations. Signup and view all the answers

What role do aquaporins play in cell membranes?

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

Which of the following best describes V-class pumps?

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

What do ABC superfamily pumps primarily transport?

Lipids, sugars, and other molecules. Signup and view all the answers

Which statement regarding gated channels is true?

They remain closed until activated by a specific stimulus. Signup and view all the answers

What is a characteristic behavior of GLUT transporters?

They often have high specificity for glucose transport. Signup and view all the answers

Study Notes

Membrane Transport

The Na+ concentration gradient and membrane potential are used to drive the uptake of glucose from the intestinal lumen by the 2 Na+/1 glucose symporter located on the apical surface membrane
The Na+/glucose symporter is the principle behind the ingredients of Gatorade
Proton pump inhibitors completely shut down the production of stomach acid by inhibiting the hydrogen-potassium adenosine triphosphate enzyme system
Endogenous ouabain is found in mammals and acts as a steroid hormone
Ouabain blocks the Na+/K+ pump of muscle which consequentially causes cytosolic Na+ levels to increase and the Na+/Ca2+ antiporter to function less efficiently, causing cytosolic Ca2+ levels to increase.

Biomembranes are Selectively Permeable

The lipid bilayer structure results in selective permeability meaning only specific substances can move through

Mechanisms of Transmembrane Transport

Simple Diffusion: The movement of permeable substances across a membrane from a higher concentration to a lower concentration. No energy is expended.
Facilitated Diffusion: The movement of substances from a higher concentration to a lower concentration with the use of channel proteins or uniport transporters
Primary Active Transport: The movement of a substance from a lower concentration to a higher concentration with an expenditure of energy. Requires use of proteins called pumps, which are ATPases.
Secondary Active Transport (Cotransport): Energetically unfavorable transport of a target substance against its concentration gradient is coupled to the energetically favorable movement of an ion down its electrochemical gradient. This can be symport or antiport cotransport.

Simple Diffusion

Diffusion occurs spontaneously because entropy increases as a substance moves from high concentration to low concentration.

What Determines Diffusion Rate Across a Membrane?

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

Hydrophobicity

Hydrophobicity is measured by the partition coefficient (K), which is the equilibrium constant for a substance's partition between oil and water.
The higher the K, the more lipid-soluble the substance and the faster its rate of movement across a bilayer.

Transport Protein mediated Processes

Facilitated Diffusion: Substance moves from a higher concentration to a lower concentration with no expenditure of energy. Usually involves channel proteins or uniport transporters.
Primary Active Transport: Substance moves from a lower concentration to a higher concentration with the expenditure of energy. Usually involves pumps, which are ATPases.
Secondary Active Transport: Energetically unfavorable transport of a target substance against its concentration gradient is coupled to the energetically favorable movement of an ion down its electrochemical gradient.

Membrane Transport Proteins

ATP Powered Pumps: Use energy from ATP to pump substances against their concentration gradient
Ion Channels: Transport ions, water, and small hydrophilic molecules. May be gated which are closed until opened by a stimulus, or non-gated which are always open.
Transporters: Bind to specific solutes and undergo conformational changes to move them across membranes

Glucose Uniport Transporters (GLUT)

Humans have 12 glucose uniporters (GLUT 1-12), which vary in their tissue specificity.

Osmotic Pressure

Osmotic pressure is the hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different water concentrations.

Aquaporins

Aquaporins increase the water permeability of cell membranes.

4 Classes of ATP Powered Pumps

P-type pumps: phosphorylated during the transport cycle
V-type pumps: pump protons into organelles.
F-type pumps: synthesize ATP using proton gradients
ABC Superfamily pumps: Use ATP to transport a wide range of molecules.

ABC Superfamily

About 50 pumps in mammals that transport lipids, sugars, and other molecules.
Examples include MDR1 which transports lipid-soluble substances out of cells and CFTR which transports Cl-.

Flippase Mechanism

Used by superfamily pumps that transport lipid-soluble substances

Gated and Ungated Channels

Gated Channels are closed until opened by a stimulus which can be chemical(ligand) or voltage.
Non-gated Channels are always open, also known as "leaky" channels. Important for maintaining membrane resting potential.

Cotransport

Examples include: Na+/H+ antiporter, Cl-/HCO3- antiporter, 2Na+/glucose symporter, and 3Na+/Ca+2 antiporter.

Transport Across Epithelial Membranes: Transcellular Glucose Transport from Small Intestine Lumen

The Na+/K+ ATPase on the basolateral surface membrane generates Na+ and K+ concentration gradients
The outward movement of K+ ions through nongated K+ channels generates an inside-negative membrane potential across the plasma membrane.

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Description

Explore various membrane transport mechanisms including the Na+/glucose symporter and the selective permeability of biomembranes. Understand how these processes impact functions such as glucose uptake and the role of proton pump inhibitors. This quiz will challenge your knowledge on essential transport mechanisms in cellular biology.