
Homeostasis – Process by which organisms
maintain a relatively stable internal environment;
All organisms have ranges that are tolerated (i.e.
pH and temperature)

Example - Paramecium
contain contractile
vacuoles that collect
and remove excess
water, thereby helping
to achieve homeostasis

Composed of a double-layered sheet called the
lipid bilayer which includes:
Two layers of lipids (phosphate group + two fatty
acids)
1.
 Creates a strong barrier that is flexible
Proteins that are embedded in the bilayer
2.
 Some form channels and pumps that help to move
material across the cell membrane
Carbohydrate molecules that are attached to the
proteins
3.

Allows individual cells to identify one another




Many substances can diffuse across biological
membranes, but some are too large or too
strongly charged to cross the lipid bilayer.
If a substance is able to diffuse across a
membrane, the membrane is said to be
permeable to it.
A membrane is impermeable to substances
that cannot pass across it.
Most biological membranes are selectively
permeable, meaning that some substances can
pass across them and others cannot.


Concentration - the mass of solute in a given
volume of solution, or mass/volume
Example:

12 grams of salt in 3 liters of water
 Concentration = 12 g/3 L (4 g/L)

If you had 12 grams of salt in 6 liters of water
 concentration = 12 g/6 L (2 g/L)
***The 1st solution is
2X as concentrated
as the 2nd.
Weeee!!!


Passive transport –
Transport of materials
from a higher to lower
concentration. Energy
is not required for this
to occur.
Active transport –
Transport of materials
from a lower to higher
concentration. Energy
is needed for this to
occur.
high
low
This is
gonna
be hard
work!!
high
low
Three Types of Passive Transport:
1. Diffusion - Transport of substances
2. Osmosis – Transport of water
3. Facilitative Diffusion – Transport with the
help of carrier proteins (for molecules too large
to pass through the membrane without help)
Diffusion
• Diffusion - Movement of
molecules from an area of
HIGH concentration to an
area of LOW concentration
• when the concentration of a
solute is the same
throughout a solution it has
reached equilibrium
(particles never stop once
this is reached – ALWAYS in
MOTION!)
Diffusion (Continued)
•DOES NOT require the cell to
use energy.
Examples…..
View Diffusion
• http://www.indiana.edu/~phys215/lecture/le
cnotes/diff.html
OSMOSIS
 Water passes easily across most membranes,
but most solute does not.
 Osmosis - Diffusion of water through a
selectively permeable membrane
 High to low concentration
 NO ENERGY required by the cell
 There are more sugar molecules on the right. That
means that the concentration of water is lower on the
right than the left.
 The membrane is permeable to water but not to
sugar. As a result, there is a net movement of water
from the area of high concentration to the area of low
concentration.
OSMOSIS TERMS
 Water will tend to move across the membrane to
the left until equilibrium is reached.
O
 Is
tonic - when the concentration of two
solutions is the same
 Hypotonic - when comparing two solutions, the
solution with the lesser concentration of solutes
 Hypertonic - when comparing two solutions, the
solution with the greater concentration of
solutes
Facilitated Diffusion
•Facilitated Diffusion -
Movement of specific
molecules across cell
membranes through protein
channels
• Each protein channel only allows a
particular substances to cross.
• The protein facilitates (helps) the
diffusion across the membrane
Facilitated Diffusion and Energy
•
Occurs with the following:
Charged molecules (ions)
2. When molecules are too large
1.
•
HIGH concentration → LOW concentration.
•
DOES NOT require energy.
Weeee!!!


Passive transport –
Transport of materials
from a higher to lower
concentration. Energy
is not required for this
to occur.
Active transport –
Transport of materials
from a lower to higher
concentration. Energy
is needed for this to
occur.
high
low
This is
gonna
be hard
work!!
high
low

Active Transport - Energy-requiring process
that moves material across a cell membrane
against a concentration difference.


Move materials in the opposite direction—
against a concentration difference.
Requires:
1.
2.
Energy
Transport proteins
1.
2.
Endocytosis
Exocytosis
Endocytosis - Process by which a cell takes
material into the cell by infoldings or pockets of
the cell membrane;
Low → high concentration


Transports:
1. Larger molecules
2. Solid clumps of material
1. Pinocytosis –
For fluids, cell
“drinking”
2. Phagocytosis –
For solids, cell
“eating”
3. Using vesicles
Macrophage (white blood cell)
engulfing bacteria
Amoeba engulfing food.

Exocytosis - Process by which a cell releases
large amounts of material; Low → high
concentration

Vacuole fuses with
cell membrane

Transports:
1. Larger molecules
2. Solid clumps of
material


For organisms to survive, they must have a
way to balance the intake and loss of water.
Osmosis exerts a pressure known as osmotic
pressure on the hypertonic side of a selectively
permeable membrane.

Osmotic Pressure - The force exerted by osmosis
that “pushes” water through a selectively permeable
membrane to make the concentration of solutes the
same on both sides.

Turgor Pressure - The
force behind the cell
walls of plants, which
allows them to keep
their shape.


Very similar to osmotic
pressure, but occurs in
plants.
When the cells contents
press the plasma membrane against the cell wall and
the cell wall in turn presses against the membrane

The cell is filled with salts, sugars, proteins, and
other molecules.

If placed in a hypertonic solution.
 Cell will shrink or become shriveled
(plasmolysis)
 Water moves out of cell.

If placed in a hypotonic solution
 Cell will be swell or become enlarged.
 Cytolysis occurs when a cell ruptures
or lyses.
 Water moves into the cell.
Plasmolysis occurs
as the cell shrinks.
Cytolysis occurs when
the cell bursts
ISOTONIC ENVIRONMENT
HYPOTONIC ENVIRONMENT
HYPERTONIC ENVIRONMENT