Download Cell Membrane Structure and Transport

The Living Cell Membrane
The Living Cell Membrane
• Flexible envelope that holds cytoplasm and
organelles in place
• Regulates what enters & leaves the cell
• Only found in living cells
Membrane Structure
• Made up of two layers of phospholipids (called a bilayer)
• What is a phospholipid?
– A type of amphipatic lipid (both water-hating and
water-loving)
– Head is made up of a nitrogen and a phosphate group
along with glycerol
Polar hydrophilic
“head”
Water
Non-polar
hydrophobic “tail”
Water
Membrane Structure contd.
• Mosaic of various components scattered
throughout (i.e. proteins, cholesterol, and
carbohydrates)
Hydrophilic
Hydrophobic
Hydrophilic
Structure contd.
• 2 major categories of
membrane proteins:
– Integral proteins –
inserted into the
membrane and span the
entire membrane
– Peripheral proteins –
attached to the surface
of membrane (not
embedded)
Structure contd.
• Proteins embedded in the CM vary in
structure (i.e. blood cell different from others)
– With chains of sugar  glycoproteins
– With lipid molecules  glycolipids
Structure contd.
• “The Fluid-mosaic membrane model”
describes the structure of the CM
– Phospholipids can drift/move sideways or rotate in
place
– Vertical flip-flop of phospholipids is unfavourable
(rarely occurs)
– Watch this!
What gives fluidity?
• Saturated vs. Unsaturated fatty acid ‘tails’
Adding Fluidity
• Why cholesterol?
• The presence of cholesterol molecules decreases
permeability and allows the CM to function in a
wide range of temperatures
– At high temperatures, it maintains its rigidity
– At low temperatures, it keeps the CM fluid
Structure relates to Function!
• What are the functions of the cell membrane?
– Provide a barrier – hydrophobic environment
prevents most substances from entering the cell
– Gatekeeping – opens and closes paths through
the cell membrane (i.e. nerve cell  K goes in at
rest, Na goes in when excited)
– Receptor sites – provides a binding site for
chemical messengers (like hormones) to induce
cell activity (i.e. insulin hormone binds to receptor
to open pores to allow sugar in)
Structure relates to Function!
• What are the functions of the cell membrane?
– Transport – transport proteins aid the movement
of particles across the membrane
• also controls what enters/exits
– Structural Support – proteins are attached as
microfilaments to maintain cell shape
– Catalyze biological reactions – proteins
present on the surface of the membrane assist in
chemical reactions
– Check it out!
Putting it all together!
• Label your diagram of the cell membrane
Movement Across the Cell
Membrane
How do particles go in and out of
the cell if the membrane is
selective?
• Chemicals in our body must be kept in
balance in order for us to survive
• Maintenance of constant conditions is called
homeostasis
• To achieve homeostasis, materials must be
exchanged from the inside of the cell to the
extracellular environment
• There are two types of transport
mechanisms across the cell membrane:
– Passive transport
– Active transport
• The extracellular fluid is a mixture of water
and dissolved materials that surrounds the cell
cell
ECF
(extracellular
fluid)
Passive Transport
• Movement of materials across the cell
membrane without the expenditure of cell
energy
• 3 types of passive transport:
– Diffusion
– Osmosis
– Facilitated Diffusion
• Many small and uncharged molecules can
move easily through the membrane (i.e.
oxygen and water)
• Brownian motion - molecules are constantly
moving randomly and colliding with each
other
– This is the force behind diffusion
Diffusion
• The movement of particles (solute) from a higher
concentration to a lower concentration until they are
spread out evenly
• Concentration = # particles/unit volume
• Movement is “down” the concentration gradient
since molecules are in constant and random motion
(Brownian motion)
• Will occur until equilibrium is met (equal conc.’s on
both sides)
• Can you think of any examples?
– Someone sprays cologne in one end of the hallway but
eventually can be smelled at the other end
Diffusion contd.
• Examples:
– Oxygen diffuses from blood (high conc.) to the cell
(low conc.)
– Carbon dioxide diffuses from the cell (high conc.) into
the blood (low conc.)
What would happen if I added food
colouring to a beaker of water?
Why are cells so small?
• Copy the following table for a sphere into your
notes and fill it out:
Diameter
(cm)
2
4
6
8
10
Radius (cm) Surface Area
(cm2)
=4r2
Volume (mL)
=4/3r3
Surface
Area ÷
Volume
What can you conclude?
• How does surface area change as the radius
increases?
• As the size of an object increases, does the
surface area or volume increase more
rapidly?
• What does this have to do with cells?
– The amount of nutrients that a cell can take in and
the amount of waste that can be expelled
depends on the amount of surface area
– Thus, as the cell size increases, at a certain point,
the cell will not have enough surface area to
support its volume
Osmosis
• Diffusion of water from an area of high
concentration to an area of low concentration
across a semi-permeable membrane *passive
• If a solution has a high solute concentration
then it must have a low water concentration
(less space for water molecules)
• A low solute concentration means high water
concentration (more space for water
molecules)
• Note: the cell membrane is permeable to
water
Which way will water move?
• HYPOTONIC conditions: when the water
concentration outside the cell is greater than
inside the cell…water moves INTO the cell
H2O
Outside
Inside
Hypotonic conditions contd.
• If an animal cell is placed in a hypotonic
solution, it could burst through a process called
lysis
• Plant cells become turgid (normal)
Which way will water move?
• HYPERTONIC conditions: when the water
concentration inside the cell is greater than
outside the cell…water moves OUT of the cell
H2O
Outside
Inside
Hypertonic conditions contd.
• Cells placed in hypertonic
solutions will shrink/shrivel
(water exits)
• Plasmolysis is a condition in
plants where the cell
membrane shrinks away from
the cell wall
– Plants will wilt because of a loss
of turgor pressure (the pressure
of the cell contents on the cell
wall)
– Salt on roads – good thing?
Which way will water move?
• ISOTONIC conditions: When water
concentrations outside and inside the cell are
equal, equal amounts of water move in and
out of the cell
H2O
H2O
Outside
Inside
Isotonic conditions contd.
Isotonic conditions contd.
• Animal cells in isotonic
conditions are normal
• Plant cells in isotonic conditions
are flaccid (lacks stiffness)
A
B
A
B
*Solutes
cannot pass
through the
membrane
• Side A is hypotonic to side A – why?
– Side A has more water molecules than B
• Water will always diffuse from a hypotonic
solution to a hypertonic solution until both
sides have equal concentrations
hypotonic
hypertonic
hypertonic
isotonic
isotonic
hypotonic
Osmosis in action!
• Watch this!
Summary – Where is the net
movement of water?
• Isotonic solution 
– no net movement of water
• Hypotonic solution 
– net movement of water into the cell
• Hypertonic solution 
– net movement of water out of the cell
Homework
• Read the case study (Part I and II) and
answer the questions