Membrane Structure
and
Cellular Transportation
Ch. 7
AP Biology
Ms. Haut
amphipathic
Membrane Structure
Made of phospholipid
bilayer
Polar (hydrophilic) heads of
phospholipids oriented
towards protein layers
• Nonpolar (hydrophobic)
tails of phospholipids
are oriented
between polar
heads
Membrane Structure
Proteins are individually
embedded in the bilayer


Hydrophilic portions
exposed to water
Hydrophobic portions in
nonaqueous environment
inside the bilayer
Phospholipid
bilayer
Fluid-Mosaic Model
Membranes held together by weak
hydrophobic interactions
Lipids and some proteins can drift
laterally within the membrane
Mosaic of Different Molecules
Integral proteins—transmembrane
proteins that span the hydrophobic
interior of the membrane

Transport proteins
Mosaic of Different Molecules
Integral proteins—transmembrane
proteins that span the hydrophobic
interior of the membrane
Peripheral proteins—attached t the
membrane’s surface

Transport proteins
Carbohydrates—function in cell-to-cell
recognition (cell markers)

Glycolipids, glycoproteins
Traffic across Membranes
Membrane’s molecular organization
results in selective permeability


Permits exchange of nutrients, waste
products, oxygen, and inorganic ions.
Allows some substances to cross more
easily than others:
Hydrophobic molecules—hydrocarbons, CO2,
and O2 dissolve in and cross membrane
Very small polar molecules, including H2O can
cross easily
Passive Transport: Diffusion
Diffusion—movement of a substance
down its concentration gradient due to
random thermal motion

Spontaneous process that decreases free
energy and increases entropy
⁂ NO ENERGY EXPENDED
Diffusion of a gas➞
Free energy
Less stable
Free energy
More stable
A substance will diffuse from where it is more
concentrated to where it is less concentrated
←Water
Selectively Permeable
Membrane
Solute Molecules
Passive Transport: Osmosis
Osmosis—diffusion of water across a
selectively permeable membrane

Water diffuses down its own concentration
gradient (from hypotonic solution to hypertonic
solution)
Hypotonic—lower
concentration of solutes
Hypertonic—higher
concentration of solutes
Isotonic—equal solute
concentration
Alternative Methods of
Cellular Transportation
Ch. 8
AP Biology
Facilitated Diffusion
Diffusion of solutes across a membrane, with
the help of transport proteins
Is passive transport because solute is
transported down its concentration gradient
Aides transport of many polar molecules
and ions that are inhibited by phospholipid
bilayer
Facilitated Diffusion
Transport proteins share similar
properties with enzymes:
They are specific for the solutes they
transport
They can be saturated with solute—
maximum rate occurs when all binding
sites are occupied
They can be inhibited by molecules that
resemble the solute (similar to competitive
inhibition)
Active Transport
Energy-requiring process during which a
transport protein pumps a molecule across a
membrane, against its concentration gradient
Is energetically uphill (+G) and requires
the cell to expend energy
Helps cells maintain steep ionic gradients
across cell membrane (e.g., Na+, K+, Mg 2+,
Ca 2+ , and Cl-)
Transport proteins involved get energy
from ATP to pump molecules against their
concentration gradients
Sodium/Potassium Pump
Membrane Potential
Gererated by some ion pumps
Membrane potential—voltage across
membranes



Ranges from –50 to –200 mv (the inside of
the cell is negatively charged relative to
outside
Affects traffic of charged substances
across membrane
Favors diffusion of cations into cell; anions
out of cell (due to electrostatic
attractions—cytoplasm is negatively
charged)
Electrochemical Gradient
Two forces drive the diffusion of ions
across a membrane
Chemical force—concentration gradient
Electrical force—effect of membrane
potential
Electrochemical Gradient
Electrogenic pump—transport protein
that generates voltage across
membranes
–Example is Na+/K+ Pump
•3 Na+ ions out/ 2 K+ ions in equals a net
transfer of one positive charge from the
cytoplasm to the extracellular fluid (a net
loss of one positive charge), a process that
stores energy in the form of voltage
–Stored energy can be trapped for
cellular work
Cotransport
Process where a single ATP-powered pump
actively transport one solute and indirectly
drives the transport of other solutes against
their concentration gradient
1. ATP-powered pump actively transports one
solute and creates potential energy in the
gradient it creates
2. Another transport protein couples the
solute’s downhill diffusion as it leaks back
across the membrane with a second solute’s
uphill transport against its concentration
gradient
Exocytosis
Endocytosis
Process of exporting
macromolecules from a cell
by fusion of vesicles with the
cell membrane
Process of importing
macromolecules into a cell by
forming vesicles derived from
the cell membrane
Vesicle usually budded from
the ER or Golgi and migrates
to cell membrane
Vesicle forms from a
localized region of cell
membrane that sinks inward;
pinches off into cytoplasm
Used by secretory cells to
export products (insulin in
pancreas; neurotransmitter
from neuron)
Used by cells to incorporate
extracellular substances
Phagocytosis—endocytosis of solid
particles

Forms food vacuoles that fuse with
lysozome to be digested
• Pinocytosis—endocytosis of fluid droplets
•Takes in solutes dissolved in the droplet
• Receptor-mediated endocytosis—process
of importing specific macromolecules into
the cell by inward budding of vesicles
formed from coated pits
•Occurs in response to binding specific ligands
to receptors on cell’s surface