Membrane transport

Key terms

Term

Meaning

Passive transport

Type of transport that does not require energy to occur

Concentration gradient

A region of space over which the concentration of a substance changes

Permeability

The quality of a membrane that allows substances to pass through it

Equilibrium

The state at which a substance is equally distributed throughout a space


Diffusion: substances move from an area of high concentration to an area of low concentration until the concentration becomes equal throughout a space.

Image showing the process of diffusion across the plasma membrane. Initially, the concentration of molecules is higher on the outside. There is net movement of molecules from the outside to the inside of the cell until the concentrations are equal on both sides.

Facilitated Diffusion

Membrane proteins can either be channel or carrier proteins.

Left side: Image of a channel protein, which forms a tunnel allowing a specific molecule to cross the membrane (down its concentration gradient). Right side: Diagram showing how a carrier protein can bind a target molecule on one side of the membrane, undergo a shape change, and release the target molecule on the other side of the membrane.

Key terms

Term

Meaning

Active transport

Type of transport that requires an input of energy to occur

Concentration gradient

A region of space over which the concentration of a substance changes

ATP

Adenosine triphosphate, the primary energy carrier in living things


Active Transport

During active transport, substances move against the concentration gradient.

  • moves from an area of low concentration to an area of high concentration.

  • process is “active” because it requires the use of energy

  • (usually in the form of ATP)

  • opposite of passive transport

Figure showing the transport cycle of the sodium-potassium pump.

  • requires the assistance of carrier proteins, which change conformation when ATP hydrolysis occurs.


Active vs. Facilitated Diffusion

  • both use proteins to assist in transport.

  • active transport works against the concentration gradient, moving substances from low to high

  • in addition the both use different types of proteins

  • active transport uses carrier proteins, NOT channel proteins.

  • they need ATP in order to change conformation

  • Channel proteins are not used because substances can only move through them along the concentration gradient



Bulk Transport