Regulation of Membrane Transport

Flashcards testing concepts, mechanisms, kinetics, and tissue distributions for passive, primary active, and secondary active membrane transport based on Module 2 Lecture 2.

What primary function does the plasma membrane perform in maintaining cellular homeostasis?

It acts as a selectively permeable barrier between the cell and the extracellular environment, ensuring an internal environment is maintained while essential molecules enter.

Why is simple diffusion prevented for both hydrophobic and hydrophilic molecules across the plasma membrane?

Hydrophobic molecules are stopped by the polar head groups on the outer leaflet before reaching the core, whereas hydrophilic molecules can access polar head groups but are stopped by the hydrophobic core.

What is osmosis and what channel facilitates it across the cell membrane?

Osmosis is the passage of water across a semi-permeable membrane from higher water concentration to lower water concentration, facilitated by water channels called aquaporins.

How do red blood cells react when placed in hypotonic vs. hypertonic solutions?

In hypotonic solutions, cell volume increases and cells can burst; in hypertonic solutions, cell volume decreases (cell shrinkage).

What are two key characteristics of passive transport (facilitated diffusion)?

It uses ion channels and membrane transport proteins, and it requires no direct energy source.

How do channel proteins enable charged ions to cross the plasma membrane?

Hydrophilic amino acid residues inside the channel structure form a pathway that shields the ion from the hydrophobic core of the phospholipid bilayer.

How do membrane transport proteins (transporters) differ from simple diffusion in terms of kinetics?

Transporters move substrates across membranes in a saturable manner characterized by $V_{\max}$ and $K_m$, whereas simple diffusion rate increases linearly with solute concentration.

What defines active transport?

Active transport is the movement of substances against a concentration gradient, requiring energy from ATP hydrolysis either directly or indirectly.

What is the mechanism of primary active transport?

The energy derived directly from ATP hydrolysis moves molecules across biological membranes against their electrochemical gradients.

What is the stoichiometry and direction of ion movement catalyzed by the $\text{Na}^+\text{-K}^+\text{ ATPase}$ pump?

In each cycle powered by ATP hydrolysis, $3\,\text{Na}^+$ ions are transported out of the cell and $2\,\text{K}^+$ ions are transported into the cell.

How does secondary active transport utilize energy without directly hydrolyzing ATP?

It relies on ion gradients established by primary active transport to drive the transport of other solutes against their concentration gradients.

What is the functional difference between a symporter and an antiporter?

A symporter transports two substrates in the same direction across the membrane, whereas an antiporter transports two substrates in opposite directions.

What are the location, function, and $K_m$ value of GLUT 1?

GLUT 1 is located in most tissues, provides basal glucose uptake, and has a $K_m$ of $1\,\text{mM}$.

What are the location, function, and $K_m$ value of GLUT 2?

GLUT 2 is located in the liver, kidneys, and pancreas, removes excess glucose from blood, and has a $K_m$ of $15\text{--}20\,\text{mM}$.

What are the location, function, and $K_m$ value of GLUT 3?

GLUT 3 is located in most tissues, provides basal glucose uptake, and has a $K_m$ of $1\,\text{mM}$.

What are the location, function, and $K_m$ value of GLUT 4?

GLUT 4 is located in muscle and fat cells, removes excess glucose from blood, and has a $K_m$ of $5\,\text{mM}$.

What substrate is transported by GLUT 5, and where is it located?

GLUT 5 transports fructose and is located in the small intestine and testes.

Why is a high $K_m$ ($15\text{--}20\,\text{mM}$) functionally advantageous for GLUT 2 in liver cells?

The low affinity allows the rate of glucose uptake into liver cells to increase proportionally as blood glucose levels rise, rather than remaining saturated at $V_{\max}$.

Which glucose transporter is insulin-sensitive, and how does insulin alter its function?

GLUT 4 is insulin-sensitive; insulin binding to its receptor promotes the recruitment of GLUT 4 transporters from an intracellular vesicle storage pool to the cell membrane.

How is dietary glucose transported across the apical membrane of intestinal epithelial cells?

Glucose is actively cotransported into the cell against its gradient alongside $\text{Na}^+$ (moving down its gradient) via the sodium-glucose symporter (SGLT).

Based on this transport model of intestinal epithelial cells, which proteins are located on the basolateral membrane to transport glucose into the blood and maintain the sodium gradient?

The basolateral membrane contains a glucose uniporter (moving glucose into blood) and the $\text{Na}^+\text{-K}^+\text{ ATPase}$ (pumping excess $\text{Na}^+$ out and $ Kan^+$$ in).

What structural feature in the brain creates a barrier preventing many drugs from passively entering central nervous system tissue?

Tight junctions between endothelial cells forming the blood-brain barrier.