Movement through the Cell Membrane
I.
REVIEW
A. Cell Membrane
Structure
 Two layers(Bilayer) of phospholipid molecules
 Head = Phosphate group
Polar-Hydrophilic (water-loving)
 Tail = Glycerol and two Fatty Acid chains
Nonpolar- Hydrophobic (water-fearing)
 Proteins/Carbohydrates/Cholesterol interspersed throughout
B. Structure insures that molecules that are soluble(are able to dissolve)
in water do not automatically pass through the membrane
C. The layer of lipids separates the two aquatic environments, inside the
cell and outside the cell.
II.
Homeostasis
A. The maintenance of a stable environment.
 In cells this happens by monitoring what goes in and out of the cell.
How do cells maintain homeostasis?
1.
Passive Transport
2.
Active Transport
III.
PASSIVE TRANSPORT
1) Movement of any substance across a membrane without the use of
chemical energy (ATP).
2) Movement of molecules will go with the concentration gradient
(High to Low)
C.Four Types
a.
Diffusion
b.
Osmosis
c.
Facilitated Diffusion
d.
Diffusion with Ion Channels
IV.
1)
ACTIVE TRANSPORT
Movement of any substance across the membrane using chemical
energy - ATP
Movement of the molecule will go across/against the concentration
gradient (opposite-low to high)
2)
PASSIVE TRANSPORT
I.
Simple Diffusion
A. Definition:
The process that molecules or ions spread from an area of high
concentration to an area of low concentration
Examples: Air freshener, food coloring in water
B. What causes diffusion?
Diffusion is driven by the molecules’ kinetic energy. Particles are
constantly moving in a random motion. If they are concentrated in a
small area, they will keep rebounding until they spread out.
C. Which way will the molecules move?
Particles will move down the CONCENTRATION GRADIENT
(Difference in concentration of molecules across a distance)
D. In the absence of other influences, the molecules will reach
EQUILIBRIUM
(When all the molecules are spread out equally in a given area.)
E. Diffusion across the Cell Membranes
 Semi-permeable- Will let some molecules through
 Diffusion of a molecule across a membrane depends on the size
of the molecule and the solubility
 Small molecules like water are small enough to get through the
pores in the membrane
Example: Carbon Dioxide and Oxygen enter by diffusion
Starch molecules need to enter by a different route
Dialysis also uses diffusion to clean the blood
Chemistry Review
What is a solution?
Solute - Item that is dissolved
ex:Sugar
Solvent - Item that does the dissolving
Ex:Water
In a cell, Water is the solvent and Organic and Inorganic molecules are the
solutes.
II.
Osmosis
A. The process by which WATER molecules diffuse through a cell
membrane from an area of high concentration to an area of lesser
concentration
B.
Direction of OSMOSIS
Depends on relative concentration of solute molecules on both
sides of the cell membrane
1)Hypotonic Solution –When the concentration of solute
molecules outside the cell is lower than the concentration of the
solute molecules inside the cytosol of the cell.
The outside solution is said to be hypotonic compared to the
cytosol inside the cell
Direction of Water: Into the cell
Condition: Cytolysis
2)Hypertonic Solution- When the concentration of solute
molecules outside the cell is Higher than the concentration of
the solute molecules inside the cytosol of the cell.
The outside solution is said to be hypertonic compared to the
cytosol inside of the cell
Direction of Water: Out of the cell
Condition: Plasmolysis
Water moves from the hypotonic solution to the hypertonic solution
C. Osmotic Pressure – The ability of osmosis to generate enough pressure
to increase a volume of water
D. Turgor Pressure - Water pressure within a cell
E. Plasmolysis- When water leaves the cell and turgor pressure is lost
and the cell shrinks
F. Cytolysis – When the cell takes in so much water it bursts caused by
an increase in turgor pressure
G. Contractile Vacuoles – Organelles that remove water
Direction of Osmosis
Conditions
Environment
Solution is
If solute concentration in the
environment is lower than in the
cell
Hypotonic
If solute concentration in the
environment is higher than in the
cell
Hypertonic
If Solute concentration in the
environment is equal to that in the
cell
Isotonic
Cell
Solution is
Water will
Move
Hypertonic
Into the
cell
Hypotonic
Out
of
the cell
Isotonic
Water will
not move
What type of solutions would a cell have to be in for cytolysis or plasmolysis
to occur?
If you were to place celery into a glass of water, would you have cytolysis,
plasmolysis or neither?
III. FACILITATED DIFFUSION
 Used when molecules are too large to pass through membrane
or are not soluble in the membrane
 These molecules moves with the concentration gradient
 Do not use cellular energy
A.
B.
C.
D.
E.
IV.
Carrier Proteins transport solute molecules through the
membrane without expending energy.
Sugars (Glucose) are insoluble in the lipids and are too large to
pass through membrane pores
Glucose molecule will bind with a carrier protein on the surface
of the membrane and the carrier protein will change shape
The carrier molecule moves glucose to the other side and then
returns back to its normal shape
Are specific for one type of molecule
Ion Channels/Gated Channels
Proteins in the membrane form a gated channel/Ion channel across
the lipid membrane.
 Molecules that cannot diffuse through the membrane can move
through these channels
 Move molecules from high concentration to low concentration w/o use
of energy
A. Moves ions such as sodium, potassium, calcium and chloride.
B. They are specific to one ion.

C. Some are permanently open and some open according to external
stimuli such as stretching of the cell membrane, electrical signals, and
chemicals.
ACTIVE TRANSPORT
A. Moves solutes against the concentration gradient (from low to high)
B. Particles are moved from areas of LOW concentration to areas of
HIGH concentration with the use of energy(opposite of passive
transport)
C. Uses energy in the form of ATP
D. May use a Carrier Protein
Examples of substances that are transported this way: Sugar, amino
acids, Sodium, Potassium, Calcium, and Hydrogen ion
Cell Membrane Pumps
Example: Sodium-Potassium Pump
Moves Sodium and Potassium against the concentration gradient
-Moves Sodium (Na +) ions out of the cell ( 3 at a time)
-Moves Potassium (K +) in the cell ( 2 at a time )
-Carrier protein must change shape
-An ATP molecule is required to change the shape of the protein carrier
molecule
-ATP is a high energy molecule
ENDOCYTOSIS (enter)
The process by which cells engulf substances that are too large to enter
the cell by passing through the membrane (using ATP)
1. The particle is enclosed by a portion of the cell, called a Vesicle
2. This vesicle then pinches off from the cell membrane and enters the
cytoplasm
3. Now the contents of the Cell are digested by cellular enzymes
Examples: Food Particles, waste materials, proteins
Two Types of endocytosis
1. Pinocytosis – the movements of solutes or fluids into the cell
2. Phagocytosis – the movement of large food particles or whole
microorganisms into the cell
Example: White blood cells engulf and digest bacteria
EXOCYTOSIS (exit)
The passage of large molecules outside the cell
Process is the same as endocytosis but moves the molecules out of the
cell (uses ATP)
Example:The Golgi Bodies put the proteins in vesicles to be transported
out of the cell.