Chapter 7

Composed of lipids and
proteins
◦ Phospholipid bilayer
◦ each protein type has
a specific function
◦ in animal cells
cholesterol acts as a
buffer for membranes
to resist changes in
temperature

membrane carbohydrates
(extracellular surface) aid
in recognition of other
cells
◦ glycolipids (lipid +
carb)
◦ glycoproteins (protein
+ carb)


Selectively Permeable
◦ allows some substances to cross more easily than others
◦ regulates transport across cell boundaries
◦ non-polar molecules cross easily (CO2, O2, hydrocarbons)
 hydrophobic
◦ polar molecules can’t cross easily (hydrophilic)
 ions, sugars, water--pass slowly
Fluid Mosaic Model
◦ membrane is a fluid structure that has a variety of
proteins scattered throughout
◦ Membranes float in or on phospholipid bilayer

Proteins determine the membrane’s function
◦ different cell types contain different membrane proteins
 Transport
 Enzymes
 Signal Transduction
 Cell-Cell Recognition
 Intercellular joining
 Attachments to ECM (extra-cellular matrix)


Integral Proteins: into the membrane
Peripheral Proteins: not embedded in the
membrane; loosely bound to the surface

proteins that allow the passage of specific ions and
hydrophilic substances
◦ carrier proteins--hold onto substances and
change shape which allows them to pass through
membrane
◦ channel proteins--hydrophilic channel that
molecules/ions can use to tunnel through the
membrane
 water passes through proteins called
aquaporins

Primary role is importing resources and
exporting wastes from cell

Diffusion: movement of molecules from an area
of high to low concentration
◦ any substance will diffuse down a
concentration gradient (high to low)
◦ eventually movement of molecules will reach
equilibrium (moving at the same rate)

no energy is required to move molecules across
membrane

Non-polar molecules diffuse easiest

Osmosis: diffusion of water across a selectively permeable membrane
◦ osmoregulation--ability for cells/organisms to control solute
concentrations and water balance
 most important in cells without walls
 Freshwater vs. saltwater fish

tonicity: ability of a solution to gain or lose water (depends on
concentration of solutes that can’t pass through membrane)
◦ isotonic--no net movement of water
◦ hypertonic--solution with high concentration of solutes unable to cross
membrane
 Ex. increased salinity
◦ hypotonic--solution with low concentration of solutes unable to cross
membrane
 Ex. Distilled water

Facilitated Diffusion: transport proteins help certain
polar molecules and ions pass through the cell
membrane
◦ molecules move down a concentration gradient, so no
energy required! (high to low concentration)
◦ provides efficient passage of a solute through the
membrane
◦ channel and carrier proteins
 ion channels (gated channels)--open and close in
response to a stimulus
 glucose channels

membrane proteins move solutes against a
concentration gradient (all carrier proteins)
◦ requires energy, provided by ATP

allow cells to maintain different internal
concentrations of smaller molecules than
their environment (create concentration
gradients)
◦ sodium/potassium pumps: high internal K+
concentration and low Na+ concentration

Large molecules (polysaccharides and
proteins) move in vesicles
◦ Exocytosis: vesicle membrane and cell membrane
fuse, releasing material OUT of the cell
◦ Endocytosis: vesicles form from cell membrane to
take IN materials
 Phagocytosis “eating”
 Pinocytosis “drinking”
http://www.bozemanscience.com/water-potential/

Used to predict the direction in which water will diffuse through living
plant tissues
◦ solute potential (solute concentration)
◦ pressure potential (pressure +/- on a solution).
◦ In an open system, pressure potential will be the same as atmospheric
pressure and can be ignored (so water potential = solute potential)

Water will move from an area of higher water potential to an area of
lower water potential
◦ Water potential of solutions at equilibrium will be zero
◦ Higher M concentration = lower water potential (hypertonic)
 Increase solute = decrease in water
 Water will diffuse in the direction of the highest M at atmospheric pressure
◦ Pure water (WP= 0)

Turgor pressure (cell wall exerts pressure back to prevent excessive
uptake of water and bursting of cell)