5.1-5.9 Study Guide
● 5.1 - Membranes are a fluid mosaic of phospholipids and proteins
○ Membrane are composed of phospholipids and proteins and it is sometimes
referred to as a fluid mosaic.
○
most phospholipids
are made from fatty acids
with kinks in their tails (on
the right)
○
integrin- gives the
membrane a stronger
framework
○
cell-cell recognitionglycoproteins serve as
identification tags
○
membrane proteins
function as enzymes, signal
transduction and transport
○ selectively permeability- membranes only allow
some substances to cross or be transported more
easily than others
■ non polar crosses easily
■ polar does not cross easily
● 5.2- Membranes form
spontaneously, a critical step in the
origin of life
○ phospholipids
spontaneously assemble into
simple membranes
○ a critical step of evolution is formation of a membrane that encloses collections of
molecules necessary for life
● 5.3 - Passive transport is diffusion across a membrane with no energy investment
○ Diffusion - tendency for particles of any kind to spread out evenly in an available
space, from higher concentrations to lower concentrations
○ Concentration gradient - the concentration of a solute in its solvent
■ Concentration between two areas separated by a semipermeable
membrane will want to reach equilibrium of concentration
○ Passive Transport - the diffusion of molecules across a cell’s membrane does not
take energy, therefore it is spontaneous or “passive”
● 5.4 - Osmosis is the diffusion of water across a membrane
○ Osmosis - diffusion of water across a membrane
○ If the membrane is permeable to water but not to a solute in water, the water will
have a net movement until both sides have an equal concentration gradient.
■ Ex: 90% water & 10% salt solution and 80% water and 20% salt solution
are separated by a membrane that allows water through but not salt. The
water from the solution with 90% water will diffuse across the membrane
to balance both solution’s concentration.
● 5.5 - Water balance between cells and their surroundings is crucial to organisms
Tonicity- the ability of a solution to cause a cell to gain or lose water.
Isotonic- when the solute concentration of a cell and its environment are essentially
equal.
Hypotonic- a solution with a solute content lower than that of a cell immersed in it. the
cell gains water
causing it to swell,
and possibly burst.
Hypertonic- a solution
with a higher solute
concentration than
that of a cell
immersed in it.
Causes the cell to lose water, and shrivel and die.
● 5.6 - Transport proteins may facilitate diffusion across membranes
○ Non-Polar, hydrophobic molecules can cross the lipid bilayer membrane easily,
but many substances cannot, due to their polarity or charge.
○ Facilitated
Diffusion- a
process
when
transport
proteins
make it
possible for
a substance
to move
down its
concentration gradient; requires no energy.
○ Some proteins form a hydrophilic channel that crosses a membrane.
○ Some bind to the passenger on one side, change shape, and then release it on the
other side.
● 5.7 - Peter Agre talks about aquaporins, water-channel proteins found in some cells
○ hourglass-shaped proteins responsible for entry and exit
■ discovered by Dr. Peter Agre of Johns Hopkins
■ called aquaporins
● 5.8 - Cells expand energy in the active transport of a solute against its concentration
gradient
○ expends ATP for energy
○ mechanism for moving alters shape of membrane using ATP
○ transport proteins change shape through phosphorylation
● 5.9 - Exocytosis and endocytosis transport large molecules across membranes
○ Exocytosis - process to export bulky materials
■ Transport vesicle filled with macromolecules moves to plasma membrane
and fuses with it. The contents spill out of the cell, and the vesicle
becomes part of the plasma membrane.
● Tears are moved out of our tear glands using this method.
● The pancreas manufactures insulin and secretes it into the
bloodstream
○ Endocytosis - process to intake substances
■ Depression in plasma membrane pinches in and forms a vesicle that
encloses materials.
■ Different types of endocytosis exist
● Phagocytosis - cell engulfs the particles with pseudopodia.
Packages it in a membrane large enough to be a vacuole.
● Pinocytosis - “cellular drinking”, cell takes droplets of fluid into
tiny vesicles. Takes any solute in the droplets.
● Receptor-mediated endocytosis - Highly specific, receptor proteins
for specific molecules are embedded in a membrane that are lined
by coat proteins. The coated pit pinches to form a vesicle and
carries the molecules into the cytoplasm.
○ Used to take in cholesterol for membranes and as a
precursor for other steroids.