Transport Across Membranes: Overcoming the Permeability Barrier

Flashcards covering key concepts of transport across cell membranes, including simple diffusion, facilitated diffusion, active transport, and the proteins/mechanisms involved.

Why is overcoming the permeability barrier of cell membranes crucial?

It allows specific molecules and ions to be selectively moved into and out of the cell or organelle.

What property describes membranes in terms of what can pass through them?

Membranes are selectively permeable.

What types of substances typically move across cell membranes?

Dissolved gases, ions, and small organic molecules.

What is simple diffusion?

The direct unaided movement of molecules dictated by differences in concentration on the two sides of the membrane.

What is the primary function of transport proteins?

They assist most solutes across membranes and are integral membrane proteins with great specificity for the substances they transport.

What is the difference between facilitated diffusion (passive transport) and active transport?

Facilitated diffusion moves solutes down a concentration gradient without energy, while active transport moves solutes against a concentration gradient and requires energy.

How is the movement of a molecule with no net charge determined across a membrane?

By its concentration gradient.

What determines the movement of an ion across a membrane?

Its electrochemical potential, which is the combination of its concentration gradient and charge gradient.

What effect does active transport of ions have on the cell?

It creates an asymmetric distribution of ions inside and outside the cell and establishes a membrane potential.

In which direction do solutes always move during diffusion?

Towards equilibrium, moving from regions of higher concentration to lower concentration until concentrations are equal.

What factors affect simple diffusion?

Size, polarity, and charge of the solute.

Are lipid bilayers more permeable to polar or nonpolar substances?

Lipid bilayers are more permeable to nonpolar substances.

What type of relationship exists between the rate of simple diffusion and the concentration gradient?

A linear relationship; the net rate of transport is directly proportional to the concentration difference across the membrane.

What is facilitated diffusion?

Protein-mediated movement of substances down their concentration gradient, requiring no energy.

What are the two main types of proteins that facilitate diffusion, and how do they differ?

Carrier proteins bind solutes and undergo conformational changes, while channel proteins form pores for solutes to pass through.

How do carrier proteins function according to the alternating conformation model?

They alternate between two conformational states, making the solute-binding site accessible on one side of the membrane, then shifting to the other side to release the solute.

What are uniport and coupled transport?

Uniport is the transport of a single solute, while coupled transport involves the simultaneous transport of two solutes.

What is the difference between symport and antiport?

Symport moves two solutes in the same direction, while antiport moves two solutes in opposite directions.

How does phosphorylation of glucose upon entry keep its internal concentration low?

Once phosphorylated, glucose cannot bind the carrier protein again, effectively locking it inside the cell.

Name three types of gated ion channels and what triggers them.

Voltage-gated channels respond to changes in membrane potential, ligand-gated channels respond to substance binding, and mechanosensitive channels respond to mechanical forces.

What are aquaporins?

Transmembrane channels that allow rapid passage of water through membranes.

What are the three main cellular functions of active transport?

Uptake of essential nutrients, removal of wastes, and maintenance of nonequilibrium concentrations of certain ions.

What is the key difference in directionality between diffusion and active transport?

Diffusion is nondirectional with respect to the membrane (directed by concentration), while active transport has an intrinsic directionality.

What is the difference between direct and indirect active transport?

Direct active transport couples solute movement directly to ATP hydrolysis, while indirect active transport uses the favorable movement of one solute to drive the unfavorable movement of another.

What is an example of direct active transport in animal cells?

The Na+/K+ ATPase (or pump).

What is an example of indirect active transport, specifically a symporter?

The Na+/glucose symporter.