7:3 Movement Through the
Membrane
Cell Membrane
Every cell has a cell membrane
 Regulates what enters and leaves the cell.
 Provides protection & support
 The middle section is called a lipid bi-layer, a
double layer lipid sheet.

Lipid Bi-layer
Proteins
Carbohydrate
chains
Cell
membrane
Protein
channel
Inside
of cell
(cytoplasm)
Lipid bilayer
Diffusion
Liquids are present both in and out of the
cell
 The membrane monitors what enters or exits
the cell
 The cytoplasm has various solutes within it
 The concentration of a solution is the mass
of solute in a given volume (mass/volume)
 Diffusion is when molecules move from one
area of concentration to another (usually
through a membrane)

Diffusion causes many substances to move
across a cell membrane but does not require
the cell to use energy!
 Equilibrium is reached when the
concentration of the solute is the same
throughout a solution of a system.

High
Concentration
Cell
Membrane
Low
Concentration
Protein
channel
Osmosis
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Osmosis is the diffusion of water through a selectively
permeable membrane
Selectively permeable means the membrane allows certain
substances to pass by and others not.
Osmosis allows water molecules to pass through a
biological membrane easily.
Osmosis is the movement of water from an area of high
concentration of water molecules to an area of low
concentration of water molecules.
When a cell membrane does not allow substances in, it is
said to be impermeable.
Water will move across a membrane until there is
equilibrium.
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Biological membranes are permeable to water, but
impermeable to larger molecules such as sugar.
Membranes may be selectively permeable,
allowing only substances of their choice into the
cell.
When a cell is at equilibrium with the surrounding
solution, the cell is isotonic.
Cell membranes are completely permeable to
water, therefore, the environment the cell is
exposed to can have a dramatic effect on the cell.
3 types of solutions:

Isotonic, Hypotonic and Hypertonic
Let’s take a
look!
Osmosis in cells
Facilitated Diffusion
During facilitated diffusion, molecules such
as glucose that cannot cross the cell
membrane’s lipid bilayer directly move
through protein channels instead.
 Hundreds of different protein channels have
been found that allow ions, sugars and salts
to cross various membranes.

Active Transport
When materials move across a membrane AGAINST
concentration differences.
Requires the input of energy
Some types of active transport move materials
through the cell membrane.
Endocytosis – taking in through pockets of cell
membrane (process is called phagocytosis)
Exocytosis – exits the cells via the membrane
7-4 The Diversity of
Cellular Life
UNICELLULAR – one celled
 MULTICELLULAR – many cells
 Multicellular organisms have cell specialization.
They are specialized to perform specific functions
within the organisms.
 LEVELS OF ORGANIZATION IN AN ORGANISM:
Cells  Tissues  Organs  Organ Systems 
Organism
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