Diffusion and Osmosis
Movement of particles and water
along concentration gradients
Diffusion: motion of particles in a liquid
• Particles will move along a concentration gradient from
an area of high concentration to low concentration.
• Once equilibrium is reached there is no longer a
concentration gradient so diffusion stops.
• How does temperature affect the rate of diffusion?
Diffusion across a membrane
• Most biological membranes (cell membrane,
nuclear membrane, membranes around
organelles) are selectively permeable or
semipermeable.
• Some substances can pass through easily, some
pass slower, others cannot pass at all
Substances such as water,
oxygen, carbon dioxide
and glucose pass easily
through cell membranes.
Proteins, carbohydrates
and lipids pass more slowly
or not at all.
What will happen to the concentrations
of O2 and CO2 assuming both are
permeable to the cell membrane?
Osmosis: the diffusion of water across
a semipermeable membrane
• Osmosis is the movement of water molecules
from high concentration to low concentration
• What will the water levels and concentration
of water look like after equilibrium is reached?
Thistle Tube Experiment
• Thistle tube is filled
with a concentrated
solution of sugar
water covered with a
semipermeable
membrane which
sugar cannot pass
through. What will
happen if it is placed
in a beaker of water?