Diffusion and Osmosis:
How does stuff get into
and out of a cell?
The Cell in its Environment
Ch 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
5
3
6
4
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
Phagocytosis in action –
phagocyte (white blood cell)
going after a bacteria cell
The End