Type of Transport

Type of Protein

Direction X Moves

Source of Energy to transport X

Example(s)

Name

Function

1

Simple Diffusion

None

Down its gradient

Gradient of X

Restricted to very small molecules and hydrophobics

How C0₂ enters RBC

2

(Facilitated) diffusion through a Channel

Transmembrane   channel

Down its
gradient

Gradient of X

Water channels in kidney & RBC; many types of ion channels

control vol. of urine & RBC; flux of ions 

3

Facilitated  Diffusion using a carrier protein; also called 'carrier mediated transport'

Carrier or Permease

Down its
gradient

Gradient of X

Glucose transporter in many plasma  membranes
(GLUT 1, 2, etc.)

How Glucose exits epithelial cells to body; enters adipose tissue

RBC anion Exchanger*

Maximizes CO₂ transport by blood

4

Primary (Direct) Active Transport

Pump

Up its gradient

ATP

Na⁺/K⁺ pump

Maintain high [K⁺], low [Na⁺] in cells

5

Secondary  (Indirect) Active Transport

Pump or co-transporter

Up its gradient

Not ATP (directly): usually a gradient of some substance other than X

Glucose/Na⁺ Co-transport

How Glucose enters epithelial cells from lumen

* (1) In some older books band 3 protein is called an anion channel. It transports ions but is not a channel.
   (2) An exchanger can be considered facilitated diffusion, since the concentrations of the substances themselves drive the reaction . Alternatively,  it can be considered secondary active transport, because movement of one of the substances down its gradient can drive transport of the other substance up its gradient.