Membrane Structure
and Transport
Section 3-2 Chapter 7
Phospholipid Bilayer
• The membrane is composed of two layers
of phospholipids
• Phospholipids are ampipathic
• Hydrophobic tail
• Hydrophyllic head
Selectively Permeable
Selectively= only allows specific substances
Permeable= to cross the membrane
Fluid Mosaic
Fluidity
• Phospholipids can change places rapidly
• Proteins can also move fluidly throughout the
membrane
• Slower rate because they are larger
• More structured movement, likely guided by
the cytoskeleton in some way
Maintaining Fluidity
• Decrease in temperature can cause the
phospholipid bilayer to solidify
• Unsaturated hydrocarbons help maintain
fluidity because they cause kinks in the tails
and keep the phospholipids from packing too
close
Maintaining FluidityCholesterol
• At normal temperatures, animal membranes
are less fluid because of the inability of the
phospholipids to move
• Advantage as the temperature drops however,
because the cholesterol helps the membrane
keep from solidifying
Maintaining Fluidity
Mosaic
• Refers to the various proteins that are
found in/on the membrane
• Integral Proteins/Transmembrane
Proteins- span the entire membrane
• Peripheral Proteins- occur on one side or
another of the membrane
General Function
General Function
Carbohydrates and Cell to Cell
Recognition
• Membrane carbohydrates are usually
short, branched chains
• Function in cell-to-cell recognition
• Distinguish one cell type from another
• Help sort cells during development
• Help the organism to recognize foreign
cells
Carbohydrates and Cell to Cell
Recognition
• Glycolipid- lipid with an attached
carbohydrate
• Glycoprotein- protein with an attached
carbohydrate
The Rules of Membrane
Passage
• The plasma membrane is hydrophobic and
phospholipids are tightly packed- only
small nonpolar molecules can travel
across the membrane quickly
• Oxygen
• Carbon dioxide
• Hydrocarbons
The Rules of Membrane
Passage
• If a chemical is “too charged or too large”
it must have another way to get across the
membrane
Channel Proteins
• Provide a hydrophyllic channel for polar
molecules and ions to pass through
• Aquaporins- specific channel proteins
allow the passage of water
Carrier Proteins
• Capable of physically changing their shape
to carry large molecules across the
membrane
Passive Transport
• movement of a substance across a
membrane without the cell expending
energy to make it happen
Diffusion
• The tendency for molecules to spread
out evenly into the available space
• Diffusion is a spontaneous process
• It can occur for small, nonpolar
substances directly across a membrane
Diffusion
Facilitated Diffusion
The diffusion of a substance with the
assistance of a transport protein
Channel Proteins
• provided channels through which charged or
polar substances may pass
• Aquaporins
• Ion Channels
• Gated Channels
Carrier Proteins
Carrier proteins- undergo subtle changes
that help larger solutes move across the
membrane
Osmosis
• Diffusion of water across a selectively
permeable membrane
Tonicity
• The ability of a solution to cause a cell to
gain or lose water
• Dependent on the concentration of
solutes that cannot cross the membrane
relative to the cell itself
Hypotonic Solution
• Contains less dissolved solute than the cell
itself
• Will cause water to rush into the cell to
equalize the solute concentration
• Animal cell will burst
• Plant cell will turgid or firm as the cell will
push out against the cell wall- this is the
favorable condition for a plant
Hypotonic Solution
Isotonic Solution
• Contains the same amount of dissolved
solute as the cell
• Will cause water to move in and out of the
cell at equal rates
• Animal cell will stay the same- this is the
favorable condition for an animal cell
• A plant cell will become saggy and the plant
will be droopy
Isotonic Solution
Hypertonic solution
• Contains more dissolved solute than the
cell
• Will cause water to rush out of the cell to
equalize the solute concentrations
• Animal cell will shrivel
• Plant cell will undergo plasmolysis- the
process of the plasma membrane pulling
inward away from the cell wall
Hypertonic Solution
Active Transport
• movement of a substance against a
concentration gradient that requires the
cell to expend energy
Electrogenic Pumps
• Membrane protein that generates voltage
across a membranes by actively pumping ions
across a membrane to contribute to
membrane potential
• Membrane Potential
• Voltage (potential energy caused by the
difference in charges across a membrane)
• Cytoplasm is in general more negatively
charged
Sodium-Potassium Pump
• Found in animal cells
• Pumps out 3 sodium ions for every two
potassium ions that enter the cell
• Helps to keep the inside of the cell at a net
negative charge
Proton Pump
• Found in plants
• Pumps hydrogen ions (protons) out of the
cell
Cotransport
• Coupling the “downhill” diffusion of a
substance with the “uphill” diffusion of
another substance against its concentration
gradient
Exocytosis
Endocytosis- Phagocytosis
Endocytosis- Pinocytosis
Receptor-Mediated
Endocytosis
Water Potential
The Problem with Cell Walls
• Osmosis can easily be predicted when there is not a physical
barrier to stand in the way of the movement of water into our
out of a cell
• Plant cells have cell walls so osmosis into our out of a plant
cell depends on two factors:
• The concentration of solutes
• Physical pressure on both sides of the cell wall
Water Potential
• Water potential (Ψ) is the measurement of the combined
effects of solute concentration and physical pressure on
osmosis into our out of a plant cell
• Measured in units of megapascals (MPa)
• When water is bound to a solute it does not have as much
freedom to move; this can also be viewed as the inability of
water to perform work (little potential energy)
• Water will ALWAYS move from a region of high water potential
to regions of lower water potential
The Formula
• Ψ= Ψs + Ψp
• Ψs= solute potential or osmotic potential
• Solute potential of pure water is 0MPa
• Adding a solute ALWAYS lowers solute potential so it can be
negative, but never positive
• Ψp= pressure potential
• Can be positive or negative relative to atmospheric potential
• Turgor Pressure results when the cell is under positive pressure
relative to the atmosphere
Another Wacky Formula
• Solute Potential Ψs= -iCRT
• i= ionization constant
• always 1 for a molecular compound because they do not ionize
• unique to ionic compounds
• in chemistry represented by Keq
• C= molar concentration of the solute (molarity)
• R= Pressure constant 0.831 L*atm/mol*K
• T= Temperature in Kelvin (°C+ 273)