A cell must exchange its materials with its surroundings a process controlled by

The plasma membrane

Plasma membranes are selectively permeable regulating what?

the cell's molecular traffic

What can dissolve in the lipid bilayer and pass through the membrane rapidly

hydrophobic (non polar) molecules, such as hydro carbons

What does not pass through the membrane easily?

Polar molecules such as sugars

transport proteins allow passage of hydrophilic substances where?

Across the membrane

What are aquaporins?

Channel proteins that facilitate the passage of water

These transport proteins bind to molecules and change shape to shuttle them across the membrane

Carrier proteins

a transport protein is specific for?

the substance it moves

what is diffusion?

The tendency for molecules to spread out evenly in the available space.

Transport proteins allow what?

passage of hydrophilic substances across the membrane

Although each molecule moves randomly, diffusion of a population of molecules may be what?

directional

as many molecules cross the membrane in one direction as in the other

dynamic equilibrium

The region along which the density of a chemical substance increases or decreases?

concentration gradient, where substances diffuse

How much work must be down to move substances down there concentration gradient?

none