Ch 8: Transport Across Membranes: Overcoming the Permeability Barrier

Three different mechanisms
are involved in moving solutes
across membranes:

Simple diffusion, facilitated diffusion, active transport

Simple diffusion

• Moves down the concentration/electrochemical gradient

• No energy needed

• No membrane protein needed

• Transports:

  • Small polar molecules: H₂O, glycerol

  • Small nonpolar molecules: O₂, CO₂

  • Large nonpolar molecules: oils, steroids

• No saturation

• No competitive inhibition

Facilitated diffusion

• Moves down the concentration/electrochemical gradient

• No energy needed

• Requires a membrane protein

• Transports:

  • Small polar molecules: H₂O, glycerol

  • Large polar molecules: glucose

  • Ions: Na⁺, K⁺, Ca²⁺

• Can become saturated

• Can have competitive inhibition

Active Transport

• Moves up/against the electrochemical gradient

• Requires energy

• Requires a membrane protein

• Transports:

  • Large polar molecules: glucose

  • Ions: Na⁺, K⁺, Ca²⁺

• Can become saturated

• Can have competitive inhibition

• Has intrinsic directionality

______ = no protein, no energy
______= protein, no energy
_______= protein + energy

simple, facilitated, active

The Movement of a Solute Across a Membrane Is
Determined by Its Concentration Gradient or Its
Electrochemical Potential: The movement of a molecule that has no net charge is
determined by its ________ _____

concentration gradient

The Movement of a Solute Across a Membrane Is
Determined by Its Concentration Gradient or Its
Electrochemical Potential: The movement of an ion is determined by its _______ ______, the combined effect of its concentration gradient and the charge gradient (membrane potential) across the membrane.

electrochemical potential

Simple Diffusion: Unassisted
Movement down the Gradient: Simple diffusion always moves solutes toward _______
to create a uniform solution in which the concentration is the same everywhere

equilibrium

Simple Diffusion: Unassisted
Movement down the Gradient: diffusion tends towards ______ free energy

minimum

______ occurs when the membrane between the two chambers is not permeable to the dissolved solute.

Osmosis

Simple Diffusion Is Typically Limited to
Small, Nonpolar Molecules: The three main factors affecting diffusion of solutes are _____, ____, _____

size, polarity, charge.

Simple Diffusion Is Typically Limited to
Small, Nonpolar Molecules: Polarity of a solute can be measured by the ratio of its
solubility in an organic solvent to its solubility in water,
called the ______ _______

partition coefficient.

Simple Diffusion Is Typically Limited to
Small, Nonpolar Molecules: In general, the more _____ (hydrophobic) a substance
is, the higher the partition coefficient.

nonpolar

The Rate of Simple Diffusion Is Directly
Proportional to the Concentration Gradient:
Thermodynamically, simple diffusion is always an
_____ process, requiring no input of energy.

exergonic

Kinetically, the net rate of transport for a substance is
_____ to its concentration difference across the
membrane.

proportional

Simple diffusion has a ______ relationship between inward flux of solute and the concentration gradient of the solute

linear

The process that the solute is transported down the
gradient without energy input is called _____
diffusion.

facilitated

Carrier Proteins and Channel
Proteins Facilitate Diffusion by
Different Mechanisms: ______ _____ (transporters or permeases) bind solute molecules on one side of a membrane, undergo a conformation change, and release the solute on the other side of the membrane.

Carrier Proteins

Carrier Proteins and Channel
Proteins Facilitate Diffusion by
Different Mechanisms: ______ ____form hydrophilic channels through the membrane to provide a passage route for solutes.

Channel Proteins

Carrier proteins are _____ to enzymes:
– Carrier proteins share the property of high specificity
with enzymes.
– Carrier-facilitated transport (like enzyme catalysis)
exhibits saturation kinetics.
– Competitive inhibition of carrier proteins can occur

analogous

_____ is a process that a carrier protein (uniporter)
transports a single solute across the membrane.

Uniport

______ is a process that the two solutes are moved
across a membrane in the same direction

Symport

_____ is a process that the two solutes are moved in
opposite directions.

Antiport

The Erythrocyte Glucose Transporter and
Anion Exchange Protein Are Examples of
______ Proteins

Carrier

The _____ ____ is a uniport carrier for glucose.

glucose transporter

The _____ _____ _____ is an antiport anion carrier
for Cl– and HCO3–.

anion exchange protein

glucose transporter and anion exchange protein are found where?

Plasma membrane of erythrocytes

Channel Proteins Facilitate Diffusion by
Forming ______ Transmembrane
Channels

hydrophilic

Channel proteins form hydrophilic transmembrane
channels that allow specific solutes to cross the
membrane _____

directly

what are the three types of channels

ion channels, porins, and aquaporins

Ion Channels: Transmembrane Proteins
That Allow rapid Passage of _______ Ions

specific

_____ _____, tiny pores lined with hydrophilic atoms, are remarkably selective

ion channels

Most ion channels are ____, meaning that they open and close in response to some stimulus

gated

Ion channels play roles in what types of cellular
communication?

muscle contraction and electrical signaling of nerve cells

Porins: Transmembrane Proteins That
Allow Rapid Passage of ______ Solutes

various

Pores on outer membranes of bacteria, mitochondria, and
chloroplasts are larger and _____ specific than ion channels

less

The pores are formed by multipass transmembrane
proteins called ____.

porins

Aquaporins: Transmembrane Channels
That Allow Rapid Passage of _____

water

_____ allow rapid passage of water through
membranes of erythrocytes and kidney cells in animals,
and root cells and vacuolar membranes in plants

aquaporins

All ______ are tetrameric integral membrane proteins.

aquaporins

Active Transport: Protein-Mediated
Movement ___ the Gradient

up

____ ____ is used to move solutes up a
concentration gradient, requiring a protein transporter and
an input of energy.

active transport

Active transport performs three important cellular
functions:

— Uptake of essential nutrients
– Removal of wastes
– Maintenance of nonequilibrium concentrations of certain ions.

The Coupling of Active Transport to an
Energy Source May Be ____ or ____

direct or indirct

____ ____ ____ (primary active transport)is commonly powered by ATP hydrolysis. Transport proteins driven by ATP hydrolysis are called transport ATPases or ATPase pumps.

Direct active transport

_____ _____ ____ depends on the simultaneous transport of
two solutes.

Indirect active transport

Direct Active Transport Depends on Four
Types of Transport ATPases: Four types of transport ATPases have been identified (They differ in structure, mechanism, location, and roles
(Table 8-2):

– P-type
– V-type
– F-type
– ABC-type

Uptake of ____ via Sodium
Symport Requires Energy

glucose

The Na+/glucose symporter uses _____ active transport to move
glucose into the cell, along with Na+

indirect

A _____ Na+ gradient that
is maintained across the
plasma membrane (via
the Na+/K+ pump) is used
to provide the energy
needed.

steep

The Bacteriorhodopsin Proton Pump Uses
______ Energy to Transport Protons

light

_________ is a small
integral membrane protein in the
plasma membrane of archaea of
the genus Halobacterium. it uses energy from photons of light
to drive active transport of protons out of the cells. It creates an electrochemical proton gradient that powers synthesis of
ATP by an ATP synthase.

Bacteriorhodopsin