Diffusion and Osmosis
Cells need a way to move water molecule, food particles, and
other materials through their membranes. Some things, like
water, pass through freely. Others have to be carried through
channels.
Terms to know and understand
Solute: substance that is dissolved in a solvent to make a
solution
Solvent: substance in which a solute is dissolved to form a
solution
e.g. saltwater – salt is the solute, water is the solvent
I.
Diffusion: mixing of 2 substances by the random motion
of molecules
A. molecules move from an area of high concentration to
an area of low concentration
B. When the molecules are spread out evenly, diffusion
stops because there is no longer a concentration
gradient
concentration gradient: the difference between the
concentration of a particular molecule in one area and
the concentration of the same molecule in an adjacent
area (adjacent = beside)
 When the concentration of the solute is the same throughout
a system, the system has reached equilibrium.
II. Osmosis: diffusion of water across a semipermeable
membrane (semipermeable = only certain things get
through)
 Water moves across a membrane from a region of high
concentration of water to an area of low concentration of
water
III. Facilitated Diffusion
A. facilitated diffusion: another type of movement of
particles where particles diffuse across cell
membranes with the help of proteins in the
membranes
1. particles always move DOWN the concentration
gradient going from high concentration to low
concentration
 facilitated diffusion increases the rate that
some particles cross the cell membrane
Passive Transport vs. Active Transport
 The processes of diffusion, osmosis, and facilitated
diffusion DO NOT require any energy to be used by the
cell.
 For this reason, these 3 processes can be called passive
transport.
 When a cell uses energy to move particles across the
membrane, those processes can be described as active
transport.
IV. Active Transport
A. Sometimes the cell has to move things against (or up)
the concentration gradient, going from low
concentration to high concentration. This requires
energy, so it’s called active transport.
1. Like facilitated diffusion, active transport uses
proteins to move particles, but now it takes
energy.
2. example: sodium – potassium ion pump
 uses active transport to keep the right
balance of sodium and potassium ions in
and out of the cell
 This balance is crucial for muscle
contraction, nutrient absorption, and nerve
pulse transmission.
V. Bulk Transport
A. Bulk transport is used to move large particles in and
out of the cell. During bulk transport, large particles
move across the cell membrane by being packaged in
membrane-bound sacs.
B. There are 2 types of bulk transport: exocytosis and
endocytosis
1. Exocytosis: material is moved from inside the cell
to outside the cell ( “exo” = exit )
a. wastes and cell products are packaged by the
Golgi body in sacs called Golgi vesicles
b. the vesicles then fuse with the cell membrane
and the materials are secreted outside of the
cell
(outside the cell)
#1
#2
#3
Vesicle
2. Endocytosis: material is brought into the cell
a. a portion of the cell membrane surrounds a
particle that is outside of the cell
b. the cell pinches off a saclike portion of its outer
membrane to form a new vesicle
 once inside the cell, the vesicle can fuse with
other organelles or release its contents into
the cytoplasm
(outside the cell)
#1
#2
inside the cell (cytoplasm)
#3
3. Two types of endocytosis:
a. pinocytosis: cell membrane encloses a droplet
of fluid to bring into the cell
b. phagocytosis: cell engulfs a solid substance to
bring into the cell
 human white blood cells use phagocytosis to
engulf and destroy bacteria and other invaders
of the body
Hypertonic, Hypotonic , and Isotonic Solutions
1. Hypertonic solution: the concentration of solutes (in the
solution) is HIGHER than the concentration of solutes
inside the cell
 water diffuses OUT of the cell
example: potatoes in salt water – water left the cells and
the potatoes became very flexible (no water to make the
cells rigid)
2. Hypotonic Solution: concentration of solutes (in the
solution) is LOWER than the concentration of solutes inside
the cell
 water diffuses INTO the cell
example: potatoes in distilled water – water came into the
cells, causing the cells to swell and making the potato rigid
3. Isotonic Solution: concentration of solutes (in the
solution) EQUALS the concentration of solutes inside the
cell