Chapitre11 Flashcards
qu’est-ce qui influence la vitesse de diffusion passive des ions?
In general, the smaller the molecule and the
more hydrophobic, or nonpolar, it is, the more easily it will diffuse across a lipid
bilayer.By contrast, lipid bilayers are essentially impermeable
to charged molecules (ions), no matter how small:
Comment fonction les protéines de transport en général?
All membrane transport proteins that have been studied in detail are multipass
transmembrane proteins—that is, their polypeptide chains traverse the lipid
bilayer multiple times. By forming a protein-lined pathway across the membrane,
these proteins enable specific hydrophilic solutes to cross the membrane without
coming into direct contact with the hydrophobic interior of the lipid bilayer.
Qu’est-ce qu’un transporteur?
Transporters (also called carriers, or permeases) bind the
specific solute to be transported and undergo a series of conformational changes
that alternately expose solute-binding sites on one side of the membrane and
then on the other to transfer the solute across it
Qu’est-ce qu’un canal?
Channels, by contrast, interact
with the solute to be transported much more weakly. They form continuous pores
that extend across the lipid bilayer. When open, these pores allow specific solutes
(such as inorganic ions of appropriate size and charge and in some cases small
molecules, including water, glycerol, and ammonia) to pass through them and
thereby cross the membrane
+++ rapide
Qu’influence le gradient de concentration et le potentiel membranaire?
In the case of transport
of a single uncharged molecule, the difference in the concentration on the two
sides of the membrane—its concentration gradient—drives passive transport and
determines its direction (Figure 11–4A). If the solute carries a net charge, however,
both its concentration gradient and the electrical potential difference across
the membrane, the membrane potential, influence its transport. The concentration
gradient and the electrical gradient combine to form a net driving force, the
electrochemical gradient, for each charged solute
Comment sont les charges de chaque côté de la membrane plasmique?
In fact, almost all
plasma membranes have an electrical potential (i.e., a voltage) across them, with
the inside usually negative with respect to the outside.This potential favors the
entry of positively charged ions into the cell but opposes the entry of negatively
charged ions (see Figure 11–4B); it also opposes the efflux of positively charged
ions.
Qu’est-ce que le transport actif?
addition to passive transport, cells need to be
able to actively pump certain solutes across the membrane “uphill,” against their
electrochemical gradients. ATP
Quels sont les trois étape du transport par transporteur?
transfers the solute across the lipid bilayer by undergoing reversible conformational changes that alternately expose the solute-binding site first on
one side of the membrane and then on the other—but never on both sides at the
same time. The transition occurs through an intermediate state in which the solute
is inaccessible, or occluded, from either side of the membrane
Qu’est-ce que la vitesse maximale d’un transporteur?
When the transporter is saturated (that is, when all solute-binding sites are occupied),
the rate of transport is maximal. This rate, referred to as Vmax (V for velocity),
is characteristic of the specific carrier.
Qu’est-ce qui peut bloquer les transporteur?
As with enzymes, the binding of solute can be blocked by
either competitive inhibitors (which compete for the same binding site and may
or may not be transported) or noncompetitive inhibitors (which bind elsewhere
and alter the structure of the transporter).
Nommer les trois type de transporteur actif?
- Coupled transporters harness the energy stored in concentration gradients
to couple the uphill transport of one solute across the membrane to the
downhill transport of another. - ATP-driven pumps couple uphill transport to the hydrolysis of ATP.
- Light- or redox-driven pumps, which are known in bacteria, archaea, mitochondria,
and chloroplasts, couple uphill transport to an input of energy
from light, as with bacteriorhodopsin (discussed in Chapter 10), or from a
redox reaction, as with cytochrome c oxidase (discussed in Chapter 14).
Amino acid sequence and three-dimensional structure comparisons suggest
Qu’est-ce qu’un transporteur couplé?
Others function as coupled transporters, in which
the transfer of one solute strictly depends on the transport of a second. Coupled
transport involves either the simultaneous transfer of a second solute in the same
direction, performed by symporters (also called co-transporters), or the transfer
of a second solute in the opposite direction, performed by antiporters (also called
exchangers)
Quel est le soluté souvent utilisé dans la cellule animale pour le transport couplé?
In the plasma
membrane of animal cells, Na+ is the usual co-transported ion because its electrochemical
gradient provides a large driving force for the active transport of a
second molecule. The Na+ that enters the cell during coupled transport is subsequently
pumped out by an ATP-driven Na+-K+ pump in the plasma membrane
(as we discuss later), which, by maintaining the Na+ gradient, indirectly drives
the coupled transport
Comment est souvent appellé ce transport couplé?
secondary active transport
Comment son typiquement construit les transporteurs?
Transporters are typically
built from bundles of 10 or more α helices that span the membrane. Solute- and
ion-binding sites are located midway through the membrane, where some helices
are broken or distorted and amino acid side chains and polypeptide backbone
atoms form ion- and solute-binding sites. Some other types of important membrane transport proteins are also built from
inverted repeats.
