The Cell (or plasma) membrane:
• Separates cell from surrounding
environment
• Controls movement of materials in and
out of cell
• Makes it possible for cell to be
chemically different from environment
• Maintains HOMEOSTASIS
Cell Membrane
Structure of cell membrane:
• Two layers made of lipid (fat) molecules
• Lipid molecules are arranged so that
heads point outward or inward
• The head of the lipid molecule is polar or
hydrophilic (water loving)
• The tail is nonpolar or hydrophobic water
fearing)
• Called a lipid bilayer
• Cholesterol provides stability
Structure of cell membrane:
• Proteins embedded in membrane, inside,
outside and through
• Transport control movement of substances
through membrane
• Receptors docking sites for chemical
messages
• Enzymes for chemical reactions
• Bind to neighboring cell or organelles
Structure of cell membrane:
• Carbohydrates branch from the surface
• “recognize” molecules like hormones,
antibodies, and viruses
• How HIV “tricks” cell into accepting it
• Why organ transplants are rejected
Fluid Mosaic model:
Fluid Mosaic model:
• lipid and protein molecules move and flow
like a fluid
• Rearrange to form different patterns
Selectively permeable
• Some substances pass in and out freely
• Some substances pass to slight extent or
at certain times
• Others cannot pass through at all
This allows cell to regulate
chemical composition of the cell
depending on chemical or electrical
properties of the membrane and
materials inside and outside of the
cell.
Maintaining a Constant Cell
Environment (Homeostasis)
•
Remember from previous chapters that
living organisms try to maintain a constant
environment, regardless of the external
environment. Cells also do this. If
homeostasis is disrupted, a cell may die.
Cells are able to maintain homeostasis
because of the special properties of the
cell membrane.
Selective permeability of cell
membrane:
• Some substances pass through cell membrane
easily. Lipids and molecules that dissolve in
lipids, such as alcohol and chloroform, pass
through cell membrane easily. Small molecules
like water and oxygen pass through easily.
Large molecules like starches and proteins do
NOT. Charged ions do NOT.
• Some cells are more permeable to some
substances than others. Some are more
permeable at special times.
Diffusion:
• Diffusion is the driving force behind the
movement of many substances into or
out of a cell.
• Diffusion is the process by which
molecules of a substance move from
an area of greater concentration to an
area of lesser concentration
Diffusion
Why does diffusion occur?
• Molecules are always in motion
• In gases and liquids they move in all directions
until they run into (collide) with another
molecule.
• Collisions send them off in another direction
• The resulting pattern of zig zag movements
causes molecules to spread from areas of
greater concentration to areas of lesser
concentration until equilibrium is reached
What does “concentration gradient”
mean?
• The difference between a greater
concentration and a lesser concentration.
• Like water flowing downhill
Great concentration
Less concentration
Facilitated Diffusion:
• A process by which some chemicals
diffuse across the cell membrane faster
transport of glucose from your
blood into your cells
Example:
Osmosis:
• Water is the most important substance that
passes through the cell membrane.
• (How much of a cell is made of water?)
~60%
• Water passes through the cell membrane
by a special type of diffusion called
osmosis
Osmosis is the diffusion of
water across a selectively
permeable membrane from a
region of higher concentration to
a region of low water
concentration
Remember, in osmosis
WATER moves.
Suppose you put a cell into a glass of
salt water.
• 20% salt (NaCl) and _______ % water in glass
1% NaCl
The cell has a higher concentration of water
than the glass of salt water
Water will leave the cell
cell will shrink
A solution that has more solute than
a normal cell is called hypertonic
What happens when a cell is put into a
solution with less salt than a normal cell?
The cell has less water than the solution
Water will move into cell
Cell fills with water and could burst
A solution that has less solute
than a normal cell is called
hypotonic
How about a cell in water with the
same amount of solute as a cell?
The cell and the solution have the same
percent of water and solute
No net movement; cell stays the same
A solution that has the same
concentration of solute as the
cell is called
isotonic
When you receive an IV in the hospital,
the solution is often labeled isotonic, or normal saline,
meaning it is the same concentration of salts normally
found in the cells.
Normal saline is 0.9% NaCl
Osmostic pressure: the pressure
caused by the weight of the water
as it moves from one area to
another through a semipermeable
membrane
Turgor pressure: the pressure
exerted by water in the
vacuole against the cell wall.
Turgor pressure accounts for
the crispness of vegetables.
Active Transport
• the cell membrane can “select” or let
certain substances diffuse based on size,
concentration, and charge.
• Diffusion and osmosis are sometimes
called passive transport because they
require no expenditure of energy by the
cell.
• Sometimes a cell must obtain or excrete
products when concentrations do not
permit diffusion or osmosis.
• a cell must use energy to actively transport
materials.
Types of Active transport:
Transport by binding to a protein:
• specific protein for each transported
substance
• molecule binds to the protein
• “carries” the molecule inside the cell.
• Requires the expenditure of energy in the
form of ATP
Transport by a vesicle:
• part of the cell membrane forms a
“pocket”
• “pinches” off the pouch to form a
vesicle or sac
Endocytosis:
• transporting material into the cell
Endocytosis:
• Pinocytosis is for
liquids
• Phagocytosis is for
solids
Exocytosis:
• excreting materials
Sodium-potassium pump:
• Cells must keep the concentration of
sodium ions (Na+) lower inside the cell
than the outside environment.
• They must keep the potassium ion (K+)
concentration higher than what is
outside the cell.
It does this by using a special
type of active transport called
the sodium-potassium pump.
• This difference is needed for nerve
impulses to travel.
• Cells use much of their energy
maintaining this difference.