BABS1201 Lecture Notes - Lecture 7: Facilitated Diffusion, Passive Transport, Cell Membrane
17 May 2018
School
Department
Course
Professor
BABS1201
23RD MARCH 2018
NUTRIENT AND ION TRANSPORT
EXPLAIN THE MECHANISMS BY WHICH SMALL MOLECULES MAY BE SELECTIVELY TRANSPORTED INTO AND
OUT OF CELLS
× The phospholipid bilayer is impermeable to most molecules
- But the cell needs energy, building blocks for its components, cofactors, etc. which
are outside of the cell
× A concentration gradient exists when there is a difference in the number of molecules on
either side of the plasma membrane
× Other molecules can cross the phospholipid bilayer, if a channel is created for them- cells
can create a ‘corridor’ using proteins, which the molecules then travel through if there is a
concentration gradient- this is still a form of passive transport
× Active transport is required when molecules need to be transported against the
concentration gradient (e.g. the green squares in image above)- in this case, ATP (energy)
is needed for cells
× Facilitated diffusion- passive transport aided by proteins: in facilitated diffusion, transport
proteins (of two different classes, which differ in transport mechanisms) assist movement
of molecules down a concentration gradient; requires no energy input
- Channels- conduits allow direct passage from one side of the membrane to the
other side (essentially creating a ‘hole’ which only allows specific molecules or ions
to cross through)
× Integral membrane proteins, which allow diffusion across the membrane
down a concentration gradient
× Channel proteins provide a corridor for specific molecules or ions to cross
the membrane
× Water can diffuse across the membrane- but only very slowly as it is polar
× For example, aquaporins are the specific channel proteins for water
× These proteins allow the cell to take up and retain the molecules it needs
and exclude what is unwanted
× An important property of channels- usually they can exist in several
different conformations- most of the time they exist in a “closed”
conformation
× Channels (and transporters) may be ‘gated’- they may require another
type of molecule to be bound to a specific site before they function, e.g.
neurotransmitter receptors in the brain are neurotransmitter-gated ion
channels
- Carriers- binding of solute on one side of membrane produces conformational
change in protein moving solute through
× Transporters- integral membrane proteins which help molecules to cross
the membrane
× Alternates between two shapes- the changes in the shape of the protein is
what drives the transport
× Moves the solute across the membrane during the shape change
× Transports solute in either direction depending on the concentration
gradient
× Shows specificity- transporters have binding sites equivalent to the
substrate binding sites of enzymes. Transport through transporters is
slower than via channels, e.g. glucose transporters in mammalian cells
× Concentration gradients- differences in ion concentrations inside and outside of cells- these
will disappear with time if they are not maintained. They are maintained by active
transport against the gradient
× Ion concentrations inside and outside of a typical mammalian cell
× Types of active transport- active transport (uniport- one type of molecule in one direction),
active cotransport (symport- two types of molecules in one direction), active exchange
transport (antiport-two types of molecules in opposite directions)
× Transport is directional
× The energy released by ATP hydrolysis allows transport against a concentration gradient
Document Summary
Explain the mechanisms by which small molecules may be selectively transported into and. The phospholipid bilayer is impermeable to most molecules. But the cell needs energy, building blocks for its components, cofactors, etc. which are outside of the cell. A concentration gradient exists when there is a difference in the number of molecules on either side of the plasma membrane. Active transport is required when molecules need to be transported against the concentration gradient (e. g. the green squares in image above)- in this case, atp (energy) is needed for cells. Facilitated diffusion- passive transport aided by proteins: in facilitated diffusion, transport proteins (of two different classes, which differ in transport mechanisms) assist movement of molecules down a concentration gradient; requires no energy input. Channels- conduits allow direct passage from one side of the membrane to the other side (essentially creating a hole" which only allows specific molecules or ions to cross through)
