Passive Transport

Section 4.1

Difference between active and passive transport Cells maintain homeostasis by controlling

the movement of substances across their cell membrane.

No energy needed = passive transport Energy needed = active transport

Passive Transport

Differences in concentration = concentration gradient

Concentration is the same all through = Equilibrium

Passive transport can happen when there is a concentration gradient

Molecules in equilibrium continue to move but the movement is equal in all directions

Diffusion – the most common form of passive transport

Molecules move from an area of high concentration to an area of lower concentration

Many substances enter or leave cells by diffusing across the cell membrane.

Diffusion

Osmosis – a special type of diffusion The diffusion of water through a

selectively permeable membrane = osmosis.

Water goes into or out of a cell by osmosis

Osmosis, continuedThere are three possibilities for the

direction of water movement:

1. Water moves out - the cell shrinks. A solution that causes a cell to shrink is a hypertonic solution

2. Water moves in - the cell swells. A solution that causes a cell to swell is a hypotonic solution

3. Equal water movement – the cell stays the same. A solution that produces no change in cell volume is an isotonic solution

Hypertonic, Hypotonic, and Isotonic Solutions

Other forms of passive transport

1. Diffusion Through Ion Channels

A transport protein with a pore through which ions can pass.

An ion can cross the cell membrane without touching the interior of the lipid bilayer.

Ion Channels

Other forms of passive transport, continued

2. Facilitated Diffusion

Carrier proteins - can bind to a substance on one side of the cell membrane, carry the substance across the cell membrane, and release it on the other side.

Facilitated Diffusion