BIOPHYSICS N°10 (FILL)

Transport of amino acids or sugars into a cell requires the partecipation of membrane

proteins known as ..........

Transporters or carrier proteins

Facilitated diffusion catalyzes the movement of a substrate through a membrane .........

a concentration gradient and does not require ........ energy.

down, external

the electrochemical potential gradient across a membrane includes a contribution from

the ........... gradient for the uncharged solutes, and for charged solutes - a

contribution from ......... that exist between the two compartments.

concentration, electric gradient

facilitated diffusion occurs when transported molecule needs .......... to cross

the membrane.

carrier proteins

Saturability and ........... are important characteristics of the membrane facilitated

transport systems.

Specificity

In contrast to ........... diffusion, in which the rate of transport is directly proportional to

the substrate concentration, facilitated diffusion is a saturable process, characterized by a

maximum transport rate.

simple

The red blood cell GLUT-1 transporter has a high affinity for ....... glucose, but 10-20 times

lower affinity for the related sugars, D-mannose and D-galactose.

D-

The speed of facilitated transport is limited by the number of ...........

available, whereas the speed of diffusion is dependent only on the concentration gradient.

Carrier proteins

The b-helical gramicidin A molecule, a linear peptide with 15 amino acid residues,

forms a ..........

pore

Voltage changes and ligand binding induce conformational changes in the channel

structure that have the effect of opening or closing the channels-processes known as

voltage or ligand .........

gating

"pore" is used to describe more open, somewhat ........... structures that

discriminate between substrates, e.g. peptides or proteins on basis of size.

non-selective

Ionophores often exert ............ effect by discharging the vital ion concentration

............... maintained by cells.

antibiotic, gradient

There are two types of ionophores: .......... and .......... forming.

carrier and channels

Facilitated transport can be carried by proteins or can use other carrier molecules,

such as ........... for potassium ions.

channels proteins

............. ionophores form transmembrane channels through which their

selected ions can diffuse.

Channel forming

Valinomycin is a typical example of a ........... ion carrier.

Mobile

The flux of a lipid-soluble solute will be grater if it has a higher diffusion coefficient

and if the distance that it must traverse is ...........

short

ionophores are organic molecules of diverse types, often of .......... origin, that

increase the permeability of membranes to ions.

bacterial

If the concentration of a substance is higher in the extracellular compartment than the

intracellular compartment, and assuming no electrical potential difference, the

concentration gradient will act as ............. to bring about the net movement of the

substance toward the inside of the cell.

driving force

Diffusion of water through a semipermeable membrane is called .........

osmosis

Diffusion of substances across the membrane using a transport protein is called ............

facilitated diffusion

Movement of a substance from a high concentration area to a low concentration area

is referred to as movement "in the direction of ..............

concentration gradient" or "downhill"

the membrane permeability is defined by the following formula .......... where

P = Dk/d

D = diffusion coefficient

K = Partition coefficient

d = membrane thickness

Hydrostatic pressure difference between the solution on the two sides of the

membrane is given by the formula ..........

delta P = ro x g x delta H

Movement of water and solute molecules across a cell membrane due to the

hydrostatic pressure generated by the cardiovascular system is called .........

filtration

The flux of a substrate in facilitated diffusion can be described by the Michaelis-

Menten equation ...........

J = Jmax x C / Km + C

The laws of thermodynamics predict that when the concentration gradient for a

neutral substance is zero (i.e., when ci/ce = 1 ), the system is in equilibrium and

therefore the net flux must be ........

Zero

Km is the solute concentration at which the substrate flux is .........

half maximal

An example of a membrane protein that mediates ........... diffusion is the glucose

transporter GLUT-1

facilitated

Biological membranes are not equally permeable for all substances. They are

............., i.e. the membranes can be permeable for one substance but not

for another substance.

selectively permeable

According to Fick's law, high diffusion rate means: ....... distance, .......... surface area,

.......... concentration difference.

short, high, high

Gap junction are channels that interconnect the cytosols of ......... cells.

adjacent

In facilitated diffusion, the maximum flux of a substance occurs when the carriers are

...........

saturated