Diffusion and Osmosis: How does stuff get into

and out of a cell?

The Cell in its EnvironmentCh 7, Section 2

L3 Biology

Molecules are always moving• Molecules move randomly and bump into

each other and other barriers

Diffusion• Movement of molecules from an area of

high concentration to an area of lower concentration

• Continues until equilibrium is reached.

Diffusion

Diffusion• Occurs without control or energy use – it

is called Passive Transport• Affected by:

• Size of molecules• Size of pores in a membrane• Temperature• Stirring

Simple Diffusion

Cell Membranes are Semipermeable• Let some molecules pass through and not

others.– Small molecules pass through – ex: water– Large molecules can’t pass through – ex:

proteins and complex carbohydrates

Which molecules will diffuse?1 2 3 4

5 6

How long does diffusion continue?• Until equilibrium is reached:

– This means there will be an equal distribution of molecules throughout the space – this is why odors smell strong at first, then seem to disappear

Osmosis – A Special kind of Diffusion

• Diffusion of water across a selectively permeable membrane

• Cytoplasm is a solution of water and solutes (stuff dissolved in the water).

• Water moves into and out of cells because of the different concentrations of the solutes.

• Different kinds of cells react differently to osmosis, depending on the solution they are in:

Blood in different solutions:

Active Transport – uses carrier molecules to move molecules from an area of lower

concentration to an area of higher concentration

Endocytosis

• Very large molecules or substances must enter the cell by the membrane pinching inward:

Pinocytosis• The cell takes in liquid droplets and

forms a vacuole

Phagocytosis

• The cell takes in large particles

Exocytosis• The cell gets rid of secretions or wastes

The End

Phagocytosis in action – phagocyte (white blood cell)

going after a bacteria cell