OSMOSIS
DIFFUSION
ACTIVE TRANSPORT



The net movement of particles from an area
of high concentration to an area of low
concentration until the particles are evenly
distributed.
Particles move down a concentration
gradient.
Diffusion seeks to establish equilibrium
How cells behave in different solutions!

Osmosis is the net movement of water molecules from a
region of higher water concentration (low solute
concentration) to a region of low water concentration (high
solute concentration), through a semi-permeable or
partially permeable membrane.

Osmosis is a form of diffusion.

Osmosis seeks to establish balance or equilibrium.

A solution is formed when a solute dissolves
in a solvent.

A dilute solution contains a high
concentration of water molecules.

A concentrated solution contains a low
concentration of water molecules.

Cells behave differently when exposed to
these solutions. Let’s explore!

Tonicity refers to the ability of extracellular solution to make water
move into or out of a cell by osmosis! Tonicity is related to
osmolarity. What’s that?

Osmolarity is the total concentration of all solutes in the solution.

A solution with a low osmolarity has fewer solute particles (per litre
of solution). This solution is more dilute.

A solution with high osmolarity has more solute particles (per litre
of solution. This solution is more concentrated.

Isotonic solutions
have equal concentration of solutes and water
across a semi-permeable membrane. There is no
net movement of water into or out of the cell.

Hypotonic solutions
have a lower osmolarity than inside of the
cell. (Remember low osmolarity means less
solutes which means more water molecules).

Water therefore moves into the cell (high to
low concentration)

Hypertonic solutions
have a higher osmolarity than inside of the
cell. (Remember high osmolarity means more
solutes which means less water molecules).

Water therefore moves out of the cell (high to
low concentration).
ANIMAL CELLS (RED BLOOD CELL) IN DIFFERENT TYPES OF SOLUTIONS
Image credit: Mariana Ruiz Villareal
PLANT CELLS IN DIFFERENT TYPES OF SOLUTIONS
Image credit: OpenStax Biology, modification of work by Mariana Ruiz Villareal



Plant cells, unlike animal cells, have a strong
freely permeable cell wall. This wall provides
support to the cell.
If a plant cell is placed in water or a solution
more dilute solution than its cytoplasm, the
cell contents push against the cell wall
(swells) and becomes turgid.
The cell does not burst or lyse like an animal
cell because of the rigid cell wall.

A plant cell in a solution that is more
concentrated than its cytoplasm loses water
and shrinks, becoming flaccid, pulling the cell
membrane away from the cell wall. When this
happens the cell has plasmolysed.

Passive method of transport of substances from a low to a
high concentration through cell membranes (against a
concentration gradient) that requires energy released in
respiration.
For example:

uptake of mineral ions by root hairs from the soil (plants)

movement of glucose molecules across the gut wall into
the blood.
PROCESS
DESCRIPTION
SUBSTANCES
TRANSPORTED
ENERGY
REQUIRED
DIFFUSION
Substances move from high to
low concentration down a
concentration gradient
Oxygen
Carbon dioxide
Nutrients
Wastes
Water
NO
Water molecules move from
high to low concentration
across a semi-permeable
membrane down a
concentration gradient
Water molecules
No
Substances move against a
concentration against
Mineral ions into plant
roots
Glucose from intestinal
cells into the blood
YES
OSMOSIS
ACTIVE
TRANSPORT

Gadd, Phil. 2009. Human and Social Biology for CSEC Examinations.
Oxford: Macmillan, 2009.

Tindale, Anne, and Shaun DeSouza. Human and Social Biology: A
Concise Revision Course for CSEC. London: Collins, 2018.

Khan Academy “Osmosis and Tonicity,” accessed 30th September,
2020. https://www.khanacademy.org/science/ap-biology/cellstructure-and-function/mechanisms-of-transport-tonicity-andosmoregulation/a/osmosis