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 All cells have a cell membrane
 Functions:
a. Controls what enters and
exits the cell (cell transport)
to maintain an internal
balance called homeostasis
b. Provides protection and
support for the cell
TEM picture of a real
cell membrane.
 Homeostasis (maintaining a constant
environment) depends upon appropriate
movement of materials across the cell
membrane.
 Required materials must pass into the cells so they can
be utilized.
 Ex. Oxygen and glucose for cellular respiration
 Waste materials must pass out of the cells as they are
produced
 Ex. The CO2 produced as a waste product of cellular
respiration
Membrane
movement
animation
 Each individual cell exists in a fluid environment,
and the cytoplasm within the cell also has a fluid
environment. The presence of a liquid makes it
possible for substances (such as nutrients,
oxygen, and waste products) to move into and
out of the cell.
 Materials can enter or exit through the cell
membrane by passive transport or active
transport.

1.
2.
3.

Weeee!!!
Passive Transport
cell doesn’t use energy
Diffusion
Facilitated Diffusion
Osmosis
high
low
Active Transport
cell does use energy
1.
2.
3.
Protein Pumps
Endocytosis
Exocytosis
This is
gonna be
hard
work!!
high
low



cell uses no energy
molecules move randomly
The random motion of molecules occurs along the
concentration gradient meaning molecules
spread out from an area of high concentration to
an area of low concentration.
 Movement from
High [ ]Low [ ]
1. Diffusion – is the spreading out of molecules across a
cell membrane until they are equally concentrated.
2. Facilitative Diffusion – diffusion with the help of
transport proteins
3. Osmosis – diffusion of water specifically

Diffusion: random movement of particles from an
area of high concentration to an area of low
concentration.

Diffusion continues until all molecules are evenly
spaced (equilibrium is reached)-Note:
molecules will still move around but stay spread
out.
2. Facilitated Diffusion
A
B
Facilitated
diffusion
(with Channel
Protein)
Diffusion
 Facilitated diffusion: diffusion
of specific particles along the
concentration gradient (High 
Low) with the help of transport
proteins found in the membrane
a. Transport Proteins are specific
– they “select” only certain
molecules to cross the
membrane
b. Transports larger or charged
molecules that cannot pass
through the membrane on
their own
c. Glucose is an example of a
molecule that passes into the
cell through facilitated
diffusion
Carrier Protein
 Osmosis: diffusion of water through a selectively
permeable membrane
 Water moves from a high concentration of water
to a low concentration of water
 Because water is so small and in such
abundance, the cell cannot control its movement
through the cell membrane.
 Water moves freely
through pores.
 Solute (green) too large
to move across.
 Reminder:
 Solute: what is being dissolved
 Solvent: what dissolves the solute
 In salt water, the solute is the salt and the solvent is the
water
Hypotonic: The solution has a lower concentration of
solutes and a higher concentration of water than inside the
cell. (Low solute; High water)
Result: Water moves from the solution to inside the cell): Cell
Swells and bursts open (lyse)!
Hypertonic: The solution has a higher concentration of
solutes and a lower concentration of water than inside the
cell. (High solute; Low water)
shrinks
Result: Water moves out of the cell into the solution:
Cell shrivels!
Isotonic: The concentration of solutes in the solution is
equal to the concentration of solutes inside the cell.
Result: Water moves equally in both directions and the cell
remains same size! (Dynamic Equilibrium)
In what type of solution are these cells?
A
B
Hypertonic
Isotonic
C
Hypotonic
 Bacteria and plants have cell walls that prevent
them from over-expanding. In plants the pressure
exerted on the cell wall is called turgor pressure.
 A protist like the paramecium has contractile
vacuoles that collect water flowing in and pump it
out to prevent them from over-expanding.
 Salt water fish pump salt out of their specialized
gills so they do not dehydrate.
 Animal cells are bathed in blood. Kidneys keep
the blood isotonic by remove excess salt and water.
 Requires the cell to use energy
 Actively moves molecules to where they are needed
 Molecules move against the concentration gradient -
from an area of low concentration to an area of
high concentration
Movement from
Low [ ]  High [ ]
1. Protein Pumps
2. Endocytosis
3. Exocytosis
Sodium Potassium
Pumps (Active
Transport using
proteins)
1. Protein Pumps transport proteins that
require energy to do work
• Example: Sodium /
Potassium Pumps are
important in nerve
responses.
Protein changes
shape to move
molecules: this
requires energy!

2. Endocytosis: taking
bulky material into a cell by
forming a vesicle
•
•
•
•
•
Uses energy
Cell membrane in-folds
around food particle
“cell eating”
forms food vacuole &
digests food
This is how white blood cells
eat bacteria!
3.
Exocytosis: Forces
material out of cell in bulk
• Membrane surrounding the
material fuses with cell
membrane
• Cell changes shape – requires
energy
• Ex: Hormones or wastes
released from cell
Endocytosis &
Exocytosis
animations