Download 1. Describe the function of the plasma membrane

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Chapter 8 RQ
What is the term for how the cell
membrane “chooses” what enters and leaves
the cell?
What kind of microscopes are used to study
the cell membrane?
What is the term for the movement of a
substance from a high to a low
concentration?
What happens to an animal cell in a
hypotonic solution?
What osmotic solution are most of your
cells in (you hope)?
1. Describe the function of the plasma membrane and explain
how scientists used early experimental evidence to make
deductions about its structure and function.
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Boundary that separates
the living cell from its
nonliving surroundings 
can discriminate in its
chemical exchanges with
the environment
Lipids can enter rapidly
 membranes are made
of lipids
Phospholipids form a
membrane in water
Phospholipid content is
enough to cover cells 2X,
creating a ‘bilayer’
Membranes contain
protein and lipid 
protein in membrane 
2. Describe the Davson-Danielli membrane
model and explain how it contributed to our
current understanding of fluid-mosaic
membrane structure.
• Cell membrane is a phospholipid bilayer between 2
layers of globular proteins
• Polar heads oriented toward the protein layers
(philic zone)
• Nonpolar tails are oriented in between heads
(phobic zone)
• Membrane is about 8um thick
Other contributions:
• Fluid mosaic model  
• Proteins individually
embedded 
3. Describe the fluid properties of the cell membrane and
explain how membrane fluidity is influenced by membrane
composition. Also, explain how hydrophobic interactions
determine membrane structure and function.
• Most membrane lipids
and protein drift
laterally
• Solidification would
result in permeability
changes
• Hydrocarbon tails kink
and hinder the close
packing of
phospholipids
• Carbohydrates act as
cell markers (to
identify cells from one
another) 
4. Describe how proteins are spatially
arranged in the cell membrane and how they
contribute to membrane function.
• Proteins are individually embedded in the
phospholipid layer
• Hydrophilic portions are maximally exposed
to water to promote stability
• Membrane is a mosaic of proteins bobbing
in fluid bilayer
• Proteins drift move slowly than lipids 
5. Describe the factors that affect selective
permeability of membranes.
• Regulation of the type and rate of traffic
• Membrane solubility characteristics of
phospholipid bilayer
• Presence of specific integral transport proteins
• Nonpolar will dissolve easily (O2, CO2, and
hydrocarbons)
• Polar  uncharged: (H2O, ethanol) & small will pass
 large (glucose) & ions will not pass easily
6. Define diffusion, concentration gradient,
and passive transport. Also, explain what
regulates the rate of active transport.
Diffusion – the net movement of a substance down a
concentration gradient
- results from random movement
- decreases free energy, increases
entropy(random)
Concentration gradient – the potential to move
stuff across the membrane
Passive transport – diffusion of a substance across
a biological membrane
- regulated by the permeability of the membrane

7. Explain why a concentration gradient
across a membrane represents potential
energy.
• The concentration gradient is the
POTENTIAL to move stuff across a
membrane, therefore representing
potential energy, until movement.
• KINETIC energy is present when
movement of the particles occurs
across the membrane with the
concentration gradient. 
8. Define osmosis, hypertonic, hypotonic, and
isotonic and predict the direction of water
movement based upon differences in solute
concentrations.
Osmosis – the diffusion of WATER across a
selectively permeable membrane (diffusion
down a concentration gradient)
Isotonic solution –
equal concentrations
of solutes inside and
out of the cell 
Continued…
Hypertonic solution – will
shrivel a cell due to a
higher concentration
of solutes outside cell
(water leaves cell)
Hypotonic solution – will
lyse a cell due to a
lower concentration of
solutes outside (water
rushes into cell and
bursts)
9. Describe how living cells with and without
walls regulate water balance.
Animal cells  not tolerant of excessive
uptake or loss of water
- prefer isotonic solutions
-can osmoregulate – pump in & out water
Plant cells  must be hypoosmotic with the
environment; allows cell to be ‘turgid’
- provides mechanical support to cells 
10. Describe one model for facilitated
diffusion.
Example: Transport proteins
• Transport protein most likely remains in
place within the plasma membrane,
alternating between 2 conformations
• In 1, the transport protein binds to the
solute and deposits it on the cell-side
• Solute binding may trigger the
conformational change
• Diffusion of ions and polar molecules that
is assisted by membrane proteins 
11. Describe how transport proteins are like
enzymes.
• Transport proteins can be inhibited by
molecules that resemble the solute
normally carried by the protein (similar to
competitive inhibitors in enzymes)
• They are specific for the solutes they
transport  specific binding site analogous
to an enzyme’s active site 
12. Explain how active transport differs from
diffusion.
Active transport – an
energy-requiring
process during which a
transport protein
pumps a molecule
across a membrane
AGAINST the
concentration gradient
• Different: 1. Requires
energy
2. Is against gradient

13. Explain what mechanisms can generate a
membrane potential or electrochemical
gradient.
Membrane potential = voltage
across membranes
Electrochemical gradient = the
chemical force (the ion’s
concentration gradient) and
the electrical force
(membrane potential) on the
ion’s movement across the
cell membrane
1.
The concentration gradient
of an ion
2.
Negatively charged proteins
inside the cell
3.
Plasma membrane’s
selective permeability to
various ions
4.
The Na – K pump 
14. Explain how large molecules are
transported across the cell membrane.
Exocytosis
• A transport vesicle buds
from the Golgi apparatus
and is moved by the
cytoskeleton to the plasma
membrane, where it fuses
and the contents spill out
Endocytosis
• The cell takes in
macromolecules and
particles by forming new
vesicles from the plasma
membrane
• Used to incorporate
extracellular substances 
15. Give an example of receptor-mediated
endocytosis.
• The process of importing specific
macromolecules into the cell by the inward
budding of vesicles formed from coated
pits; occurs in response to the binding of
specific ligands to receptors on the cell’s
surface
• Ligand  a molecule that binds to a
specific receptor site of another
• This is how cholesterol enters the cell
- it enables cells to acquire bulk quantities
of specific substances even if in low
concentrations in the extracellular fluid 