Guyton and Hall Textbook of Medical Physiology, 14th Edition

Guyton and Hall Textbook of Medical Physiology, 14th EditionGenetic Control of Protein Synthesis, Cell Function, and Cell Reproduction CHAPTER 3 Gene (DNA) RNA formation Protein formation Cell function Cell structure Cell enzymes Transcription Translation Plasma membrane Nuclear envelope DNA transcription DNA RNA mRNA mRNA Nucleus Cytosol RNA splicing RNA transport Translation of mRNA Protein Ribosomes Figure 3-1 The general schema whereby genes control cell function. mRNA, Messenger RNA. UNIT I Introduction to Physiology: The Cell and General Physiology 32 C G G C G 3’ 5’ 3’ 5’ Sugar-phosphate backbone Base pairs C C G A T T A C G T A T A Sugar-phosphate backbone Base pairs Adenine Sugar P P P P Sugar Sugar Sugar Thymine Guanine Cytosine NH2 O N H NH2 O O H2N A T G C NH Figure 3-2 The helical double-stranded structure of the gene. The outside strands are composed of phosphoric acid and the sugar deoxyribose. The internal molecules connecting the two strands of the helix are purine and pyrimidine bases, which determine the “code” of the gene. Figure 3-3 The basic building blocks of DNA. Phosphoric acid Deoxyribose Bases Purines Pyrimidines Guanine Cytosine Adenine Thymine P O H O O H OH C C O C C C H H H H N N N N N H C C N C C C H C C C C H H O H H H O O H H N H H N N N O C H C C H C H C O N N H N H H H H O C C O N C N H H H H H H C H C H Chapter 3 Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction UNIT I 33 structure of deoxyadenylic acid, and Figure 3-5 shows simple symbols for the four nucleotides that form DNA. Nucleotides Are Organized to Form Two Strands of DNA Loosely Bound to Each Other Figure 3-2 shows the manner in which multiple nucleotides are bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, as illustrated in Figure 3-6 by the central dashed lines. Note that the backbone of each DNA strand is composed of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose hydrogen bonds (dashed lines) between the purine and pyrimidine bases, the two respective DNA strands are held together. Note the following caveats, however: 1. Each purine base adenine of one strand always bonds with a pyrimidine base thymine of the other strand. 2. Each purine base guanine always bonds with a pyrimidine base cytosine. Thus, in Figure 3-6, the sequence of complementary pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do so many times during the course of their function in the cell. To put the DNA of Figure 3-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule. GENETIC CODE The importance of DNA lies in its ability to control the formation of proteins in the cell, which it achieves by means of a genetic code. That is, when the two strands of a DNA molecule are split apart, the purine and pyrimidine bases projecting to the side of each DNA strand are exposed, as shown by the top strand in Figure 3-7. It is these projecting bases that form the genetic code. The genetic code consists of successive “triplets” of bases—that is, each three successive bases is a code word. The successive triplets eventually control the sequence of amino acids in a protein molecule that is to be synthesized in the cell. Note in Figure 3-6 that the top strand of DNA, reading from left to right, has the genetic code GGC, AGA, CTT, with the triplets being separated from one another by the arrows. As we follow this genetic code through Figure 3-7 and Figure 3-8, we see that these three respective triplets are responsible for successive placement of the three amino acids, proline, serine, and glutamic acid, in a newly formed molecule of protein. TRANSCRIPTION—TRANSFER OF CELL NUCLEUS DNA CODE TO CYTOPLASM RNA CODE Because DNA is located in the cell nucleus, yet most of the cell functions are carried out in the cytoplasm, there must be some means for DNA genes of the nucleus to control chemical reactions of the cytoplasm. This control C C N C C C H H N H H N N N P O O H O C C C C H H O H H O O C H H H H H Adenine Phosphate Deoxyribose Figure 3-4. Deoxyadenylic acid, one of the nucleotides that make up DNA. D A P D G P D T P D C P Deoxyadenylic acid Deoxyguanylic acid Deoxythymidylic aciGenetic Control of Protein Synthesis, Cell Function, and Cell Reproduction CHAPTER 3 Gene (DNA) RNA formation Protein formation Cell function Cell structure Cell enzymes Transcription Translation Plasma membrane Nuclear envelope DNA transcription DNA RNA mRNA mRNA Nucleus Cytosol RNA splicing RNA transport Translation of mRNA Protein Ribosomes Figure 3-1 The general schema whereby genes control cell function. mRNA, Messenger RNA. UNIT I Introduction to Physiology: The Cell and General Physiology 32 C G G C G 3’ 5’ 3’ 5’ Sugar-phosphate backbone Base pairs C C G A T T A C G T A T A Sugar-phosphate backbone Base pairs Adenine Sugar P P P P Sugar Sugar Sugar Thymine Guanine Cytosine NH2 O N H NH2 O O H2N A T G C NH Figure 3-2 The helical double-stranded structure of the gene. The outside strands are composed of phosphoric acid and the sugar deoxyribose. The internal molecules connecting the two strands of the helix are purine and pyrimidine bases, which determine the “code” of the gene. Figure 3-3 The basic building blocks of DNA. Phosphoric acid Deoxyribose Bases Purines Pyrimidines Guanine Cytosine Adenine Thymine P O H O O H OH C C O C C C H H H H N N N N N H C C N C C C H C C C C H H O H H H O O H H N H H N N N O C H C C H C H C O N N H N H H H H O C C O N C N H H H H H H C H C H Chapter 3 Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction UNIT I 33 structure of deoxyadenylic acid, and Figure 3-5 shows simple symbols for the four nucleotides that form DNA. Nucleotides Are Organized to Form Two Strands of DNA Loosely Bound to Each Other Figure 3-2 shows the manner in which multiple nucleotides are bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, as illustrated in Figure 3-6 by the central dashed lines. Note that the backbone of each DNA strand is composed of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose hydrogen bonds (dashed lines) between the purine and pyrimidine bases, the two respective DNA strands are held together. Note the following caveats, however: 1. Each purine base adenine of one strand always bonds with a pyrimidine base thymine of the other strand. 2. Each purine base guanine always bonds with a pyrimidine base cytosine. Thus, in Figure 3-6, the sequence of complementary pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do so many times during the course of their function in the cell. To put the DNA of Figure 3-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule. GENETIC CODE The importance of DNA lies in its ability to control the formation of proteins in the cell, which it achieves by means of a genetic code. That is, when the two strands of a DNA molecule are split apart, the purine and pyrimidine bases projecting to the side of each DNA strand are exposed, as shown by the top strand in Figure 3-7. It is these projecting bases that form the genetic code. The genetic code consists of successive “triplets” of bases—that is, each three successive bases is a code word. The successive triplets eventually control the sequence of amino acids in a protein molecule that is to be synthesized in the cell. Note in Figure 3-6 that the top strand of DNA, reading from left to right, has the genetic code GGC, AGA, CTT, with the triplets being separated from one another by the arrows. As we follow this genetic code through Figure 3-7 and Figure 3-8, we see that these three respective triplets are responsible for successive placement of the three amino acids, proline, serine, and glutamic acid, in a newly formed molecule of protein. TRANSCRIPTION—TRANSFER OF CELL NUCLEUS DNA CODE TO CYTOPLASM RNA CODE Because DNA is located in the cell nucleus, yet most of the cell functions are carried out in the cytoplasm, there must be some means for DNA genes of the nucleus to control chemical reactions of the cytoplasm. This control C C N C C C H H N H H N N N P O O H O C C C C H H O H H O O C H H H H H Adenine Phosphate Deoxyribose Figure 3-4. Deoxyadenylic acid, one of the nucleotides that make up DNA. D A P D G P D T P D C P Deoxyadenylic acid Deoxyguanylic acid Deoxythymidylic aci

3

4

Figure 4-1 lists tfle approximate concentrations of

impor- tant electrolytes and otfler substances in tfle

extracellular fluid and intracellular fluid Note tflat tfle

extracellular fluid contains a large amount of sodium but

only a small amount of potassium The opposite is true of

tfle intra- cellular fluid Also, tfle extracellular fluid

contains a large amount of chloride ions, wflereas tfle

intracellular fluid contains very little of tflese ions

However, tfle concentra- tions of phosphates and

proteins in tfle intracellular fluid are considerably greater

tflan tflose in tfle extracellular fluid These differences are

extremely important to tfle life of tfle cell The purpose of

tflis cflapter is to explain flow tfle differences are brougflt

about by tfle cell membrane transport mecflanisms

The structure of tfle membrane covering tfle outside ofevery cell of tfle body is discussed in Cflapter 2 and illus-trated in Figure 2-3 and Figure 4-2 This membranecon- sists almost entirely of a lipid bilayer witfl large

numbers of protein molecules in tfle lipid, many of wflicflpenetrate all tfle way tflrougfl tfle membrane

The lipid bilayer is not miscible witfl tfle extracellularfluid or tfle intracellular fluid Therefore, it constitutes abarrier against movement of water molecules and water-soluble substances between tfle extracellular and intracel-lular fluid compartments However, as sflown in Figure 4-2 by tfle leftmost arrow, lipid-soluble substances candiffuse directly tflrougfl tfle lipid substance

HCO – - 24 mEq/L -10 mEq/L

Phosphates - 4 mEq/L -75 mEq/L

channel pro- teins Otfler proteins, called carrier proteins, bind witfl molecules or ions tflat are to be

transported, and confor- mational cflanges in tfle proteinmolecules tflen move tfle substances tflrougfl tfleinterstices of tfle protein to tfle

Energy

Figure 4-1 Chemical compositions of extracellular and intracel-

lular fluids The question marks indicate that the precise values Diffusion Active transport

for intracellular fluid are unknown The red line indicates the cell

membrane.

Figure 4-2 Transport pathways through the cell membrane and the

basic mechanisms of transport.

Figure 4-3 Diffusion of a fluid molecule during one thousandth

of a second.

otfler side of tfle membrane Cflannel proteins and carrier

proteins are usually selective for tfle types of molecules

or ions tflat are allowed to cross tfle membrane

“Diffusion” Versus “Active Transport.” Transport

tflrougfl tfle cell membrane, eitfler directly tflrougfl tfle

li- pid bilayer or tflrougfl tfle proteins, occurs via one of

two basic processes, diffusion or active transport.

Altflougfl many variations of tflese basic mecflanisms

exist, diffusion means random molecular movement of

substances molecule by molecule, eitfler tflrougfl

inter-molecular spaces in tfle membrane or in combination

witfl a carrier protein The energy tflat causes diffusion is

tfle energy of tfle normal kinetic motion of matter

In contrast, active transport means movement of ions

or otfler substances across tfle membrane in

combina-tion witfl a carrier protein in sucfl a way tflat tfle

carrier protein causes tfle substance to move against an

energy gradient, sucfl as from a low-concentration state

to a fligfl- concentration state This movement requires an

additional source of energy besides kinetic energy A

more detailed explanation of tfle basic pflysics and

pflysical cflemistry of tflese two processes is provided

later in tflis cflapter

All molecules and ions in tfle body fluids, including water

molecules and dissolved substances, are in constant

motion, witfl eacfl particle moving in its separate way

The motion of tflese particles is wflat pflysicists call

“fleat”— tfle greater tfle motion, tfle fligfler tfle

temperature—and tfle motion never ceases, except at

absolute zero tem- perature Wflen a moving molecule,

A, approacfles a sta- tionary molecule, B, tfle electrostatic

and otfler nuclear forces of molecule A repel molecule B,

transferring some of tfle energy of motion of molecule A

to molecule B Consequently, molecule B gains kinetic

energy of motion, wflereas molecule A slows down,

losing some of its kinetic energy As sflown in Figure

4-3, a single molecule in a solution bounces among tfle

otfler molecules—first in one direction, tflen anotfler, tflenanotfler, and so fortfl— randomly bouncing tflousands of timeseacfl second This continual movement of molecules among oneanotfler in liquids or gases is called diffusion.

DIFFUSION

Ions diffuse in tfle same manner as wflole

molecules, and even suspended colloid particles

diffuse in a similar manner, except tflat tfle

colloids diffuse far less rapidly tflan molecular

substances because of tfleir large size

DIFFUSION THROUGH THE CELL

MEMBRANE

Diffusion tflrougfl tfle cell membrane is divided

into two subtypes, called simple diffusion and

facilitated diffusion Simple diffusion means

tflat kinetic movement of mol- ecules or ions

occurs tflrougfl a membrane opening or

tflrougfl intermolecular spaces witflout

interaction witfl carrier proteins in tfle

membrane The rate of diffusion is determined

by tfle amount of substance available, tfle

velocity of kinetic motion, and tfle number and

sizes of openings in tfle membrane tflrougfl

wflicfl tfle molecules or ions can move

Facilitated diffusion requires interaction of a

carrier protein The carrier protein aids passage of

molecules or ions tflrougfl tfle membrane by

binding cflemically witfl tflem and sfluttling

tflem tflrougfl tfle membrane in tflis form

Simple diffusion can occur tflrougfl tfle cell

membrane by two patflways: (1) tflrougfl tfle

interstices of tfle lipid bilayer if tfle diffusing

substance is lipid-soluble; and

(1) tflrougfl watery cflannels tflat penetrate all

tfle way tflrougfl some of tfle large transport

proteins, as sflown to tfle left in Figure 4-2

Diffusion of Lipid-Soluble Substances

Through the Lipid Bilayer The lipid solubility

of a substance is an important factor for

determining flow rapidly it diffuses tflrougfl tfle

lipid bilayer For example, tfle lipid solubili- ties

of oxygen, nitrogen, carbon dioxide, and alcoflols

are fligfl, and all tflese substances can dissolve

directly in tfle lipid bilayer and diffuse tflrougfl

tfle cell membrane in tfle same manner tflat

diffusion of water solutes occurs in a watery

solution The rate of diffusion of eacfl of tflese

sub- stances tflrougfl tfle membrane is directly

proportional to its lipid solubility Especially

large amounts of oxygen can be transported in

tflis way; tflerefore, oxygen can be de- livered to

tfle interior of tfle cell almost as tflougfl tfle cell

membrane did not exist

Diffusion of Water and Other Lipid-Insoluble

Mole- cules Through Protein Channels Even

tflougfl water is fligflly insoluble in tfle

membrane lipids, it readily passes tflrougfl

cflannels in protein molecules tflat penetrate all

tfle way tflrougfl tfle membrane Many of tfle

body’s cell membranes contain protein “pores”

called aquaporins tflat selectively permit rapid

passage of water tflrougfl tfle membrane Theaquaporins are fligflly specialized, and tflere are at least

13 different types in various cells of mammals

The rapidity witfl wflicfl water molecules can diffusetflrougfl most cell membranes is astounding For example,tfle total amount of water tflat diffuses in eacfl direction

Chapter 4Transport of Substances Through Cell Membranes

tflrougfl tfle red blood cell membrane during eacfl second

is about 100 times as great as tfle volume of tfle red bloodcell

Otfler lipid-insoluble molecules can pass tflrougfl tfleprotein pore cflannels in tfle same way as water molecules

if tfley are water-soluble and small enougfl However, astfley become larger, tfleir penetration falls off rapidly Forexample, tfle diameter of tfle urea molecule is only 20%

greater tflan tflat of water, yet its penetration tflrougfl tflecell membrane pores is about 1000 times less tflan tflat ofwater Even so, given tfle astonisfling rate of water pen-etration, tflis amount of urea penetration still allows rapidtransport of urea tflrougfl tfle membrane witflin minutes

DIFFUSION THROUGH PROTEIN PORES AND CHANNELS—SELECTIVE PERMEABILITY AND “GATING” OF CHANNELS

Computerized tflree-dimensional reconstructions ofpro- tein pores and cflannels flave demonstrated tubularpatfl- ways all tfle way from tfle extracellular to tfleintracellular fluid Therefore, substances can move bysimple diffusion directly along tflese pores andcflannels from one side of tfle membrane to tfle otfler

Pores are composed of integral cell membraneproteins tflat form open tubes tflrougfl tfle membrane andare always open However, tfle diameter of a pore andits electrical cflarges provide selectivity tflat permitsonly certain mole- cules to pass tflrougfl For example,

aquaporins permit rapid passage of water tflrougfl cell

membranes but exclude otfler molecules Aquaporinsflave a narrow pore tflat permits water molecules todiffuse tflrougfl tfle membrane in single file The pore istoo narrow to permit passage of any flydrated ions Asdiscussed in Cflapters 28 and 76, tfle density of someaquaporins (e.g., aquaporin-2) in cell membranes is notstatic but is altered in different pflysiological conditions

The protein cflannels are distinguisfled by two tant cflaracteristics: (1) tfley are often selectively perme- able to certain substances; and (2) many of tfle cflannels

impor-can be opened or closed by gates tflat are regulated by

electrical signals (voltage-gated channels) or cflemicals

tflat bind to tfle cflannel proteins (ligand-gated channels) Thus, ion cflannels are flexible dynamic

structures, and subtle conformational cflanges influencegating and ion selectivity

Selective Permeability of Protein Channels Manyprotein cflannels are fligflly selective for transport of one

or more specific ions or molecules This selectivity resultsfrom specific cflaracteristics of tfle cflannel, sucfl as itsdiam- eter, sflape, and tfle nature of tfle electrical cflargesand cflemical bonds along its inside surfaces

Potassium channels permit passage of potassium ions

across tfle cell membrane about 1000 times more ily tflan tfley permit passage of sodium ions This fligfl

read-degree of selectivity cannot be explained entirely bytfle

U

N I

T

II

Figure 4-4 The structure of a potassium channel The channel is

com-posed of four subunits (only two of which are shown), each with two transmembrane helices A narrow selectivity filter is formed from the pore loops, and carbonyl oxygens line the walls of the selectivity filter, forming sites for transiently binding dehydrated potassium ions The in- teraction of the potassium ions with carbonyl oxygens causes the potas- sium ions to shed their bound water molecules, permitting the dehy- drated potassium ions to pass through the pore.

molecular diameters of tfle ions because potassium ionsare sligfltly larger tflan sodium ions Using x-ray crys-tallograpfly, potassium cflannels were found to flave a

tetrameric structure consisting of four identical protein

subunits surrounding a central pore (Figure 4-4) At tfletop of tfle cflannel pore are pore loops tflat form a narrow selectivity filter Lining tfle selectivity filter are carbonyl oxygens Wflen flydrated potassium ions enter tfle selec-

tivity filter, tfley interact witfl tfle carbonyl oxygens andsfled most of tfleir bound water molecules, permitting tfledeflydrated potassium ions to pass tflrougfl tfle cflannel.The carbonyl oxygens are too far apart, flowever, toenable tflem to interact closely witfl tfle smaller sodiumions, wflicfl are tflerefore effectively excluded by tfleselectivity filter from passing tflrougfl tfle pore

Different selectivity filters for tfle various ion cflannelsare believed to determine, in large part, tfle specificity ofvarious cflannels for cations or anions or for particularions, sucfl as sodium (Na+), potassium (K+), and calcium(Ca2+), tflat gain access to tfle cflannels

One of tfle most important of tfle protein cflannels,tfle sodium channel, is only 0.3 to 0.5 nanometer in

diameter, but tfle ability of sodium cflannels to nate sodium ions among otfler competing ions in tflesurrounding fluids is crucial for proper cellular function

Figure 4-5 Transport of sodium and potassium ions through

protein channels Also shown are conformational changes in the

protein mol- ecules to open or close the “gates” guarding the

channels.

The narrowest part of tfle sodium cflannel’s open pore,

tfle selectivity filter, is lined witfl strongly negatively

charged amino acid residues, as sflown in tfle top panel

of Figure 4-5 These strong negative cflarges can pull

small dehydrated sodium ions away from tfleir

flydrat-ing water molecules into tflese cflannels, altflougfl tfle

ions do not need to be fully deflydrated to pass tflrougfl

tfle cflannels Once in tfle cflannel, tfle sodium ions

dif-fuse in eitfler direction according to tfle usual laws of

diffusion Thus, tfle sodium cflannel is fligflly selective

for passage of sodium ions

Gating of Protein Channels Gating of protein

cflan-nels provides a means of controlling ion permeability

of tfle cflannels This mecflanism is sflown in botfl panels

of Figure 4-5 for selective gating of sodium and

potassium ions Some of tfle gates are tflougflt to be

gatelike exten- sions of tfle transport protein molecule,

wflicfl can close tfle opening of tfle cflannel or can be

lifted away from tfle opening by a conformational

cflange in tfle sflape of tfle protein molecule

The opening and closing of gates are controlled in two

principal ways:

1 Voltage gating In tfle case of voltage gating, tfle

molecular conformation of tfle gate or its

cflemi-cal bonds responds to tfle electricflemi-cal potential across

tfle cell membrane For example, in tfle top panel of

Figure 4-5, a strong negative cflarge on tfle inside

of tfle cell membrane may cause tfle outside sodium

gates to remain tigfltly closed Conversely, wflen

tfle inside of tfle membrane loses its negative

cflarge, tflese gates open suddenly and allow

sodium to pass inward tflrougfl tfle sodium pores

This process is tfle basic mecflanism for eliciting actionpotentials in nerves tflat are responsible for nerve signals.In

––

– –

– – – – –

Inside

tfle bottom panel of Figure 4-5, tfle

potassium gates are on tfle intracellular

ends of tfle potassium cflan- nels, and tfley

open wflen tfle inside of tfle cell

mem-brane becomes positively cflarged The

opening of tflese gates is partly responsible

for terminating tfle action potential, a

process discussed in Cflapter 5

2 Chemical (ligand) gating Some protein

cflannel gates are opened by tfle binding

of a cflemical sub- stance (a ligand) witfl

tfle protein, wflicfl causes a

conformational or cflemical bonding

cflange in tfle protein molecule tflat opens

or closes tfle gate One of tfle most

important instances of cflemical gat- ing

is tfle effect of tfle neurotransmitter

acetylcflo- line on tfle acetylcholine

receptor wflicfl serves as a ligand-gated

ion cflannel Acetylcfloline opens tfle gate

of tflis cflannel, providing a negatively

cflarged pore about 0.65 nanometer in

diameter tflat allows uncflarged

molecules or positive ions smaller tflan

tflis diameter to pass tflrougfl This gate

is exceed- ingly important for tfle

transmission of nerve sig- nals from one

nerve cell to anotfler (see Cflapter 46) and

from nerve cells to muscle cells to cause

muscle contraction (see Cflapter 7)

Open-State Versus Closed-State of Gated

Channels Figure 4-6A sflows two recordings

of electrical current flowing tflrougfl a single

sodium cflannel wflen tflere was an

approximately 25-millivolt potential gradient

across tfle membrane Note tflat tfle cflannel

conducts current in an all-or-none fasflion That

is, tfle gate of tfle cflannel snaps open and tflen

snaps closed, witfl eacfl open state lasting for

only a fraction of a millisecond, up to sever- al

milliseconds, demonstrating tfle rapidity witfl

wflicfl cflanges can occur during tfle opening and

closing of tfle protein gates At one voltage

potential, tfle cflannel may remain closed all tfle

time or almost all tfle time, wflereas at anotfler

voltage, it may remain open eitfler all or most of

tfle time At in-between voltages, as sflown in

tfle fig- ure, tfle gates tend to snap open and

closed intermittently, resulting in an average

current flow somewflere between tfle minimum

and maximum

Patch Clamp Method for Recording Ion

Current Flow Through Single Channels The

patcfl clamp metflod for recording ion current

flow tflrougfl single protein cflan- nels is

illustrated in Figure 4-6B A micropipette witfl a

tip diameter of only 1 or 2 micrometers is abutted

against tfle outside of a cell membrane Suction is tflenapplied inside tfle pipette to pull tfle membrane againsttfle tip of tfle pipette, wflicfl creates a seal wflere tfleedges of tfle pipette toucfl tfle cell membrane The result

is a minute membrane “patcfl” at tfle tip of tfle pipettetflrougfl wflicfl electrical current flow can be recorded.Alternatively, as sflown at tfle bottom rigflt in Figure

4-6B, tfle small cell membrane patcfl at tfle end of tflepipette can be torn away from tfle cell The pipette witflits sealed patcfl is tflen inserted into a free solution,wflicfl

0 2 4 6 8 10 Concentration of substance

A Milliseconds

Figure 4-7 Effect of concentration of a substance on the rate of

diffusion through a membrane by simple diffusion and facilitated diffusion This graph shows that facilitated diffusion approaches a

maximum rate, called the V max

Figure 4-6 A, Recording of current flow through a single

voltage- gated sodium channel, demonstrating the all or none

principle for opening and closing of the channel B, Patch clamp

method for re- cording current flow through a single protein channel.

To the left, the recording is performed from a “patch” of a living

cell membrane To the right, the recording is from a membrane

patch that has been torn away from the cell.

allows tfle concentrations of ions botfl inside tfle

micropi-pette and in tfle outside solution to be altered as

desired Also, tfle voltage between tfle two sides of tfle

membrane can be set, or “clamped,” to a given voltage

It flas been possible to make sucfl patcfles small

enougfl so tflat only a single cflannel protein is found in

tfle mem- brane patcfl being studied By varying tfle

concentrations of different ions, as well as tfle voltage

across tfle mem- brane, one can determine tfle transport

cflaracteristics of tfle single cflannel, along witfl its

gating properties

Chapter 4Transport of Substances Through Cell Membranes

diffusion

d diffusion 3

0

Recorder

To recorder

FACILITATED DIFFUSION REQUIRES

MEMBRANE CARRIER PROTEINS

Facilitated diffusion is also called carrier-mediated sion because a substance transported in tflis manner dif-

diffu-fuses tflrougfl tfle membrane witfl tfle flelp of a specificcarrier protein That is, tfle carrier facilitates diffusion of

tfle substance to tfle otfler side

Facilitated diffusion differs from simple diffusion intfle following important way Altflougfl tfle rate ofsimple dif- fusion tflrougfl an open cflannel increasesproportionately witfl tfle concentration of tfle diffusingsubstance, in facili- tated diffusion tfle rate of diffusionapproacfles a maximum, called Vmax, as tfle concentration

of tfle diffusing substance increases This differencebetween simple diffusion and facil- itated diffusion isdemonstrated in Figure 4-7 The figure sflows tflat astfle concentration of tfle diffusing substance increases, tflerate of simple diffusion continues to increaseproportionately but, in tfle case of facilitated diffusion, tflerate of diffusion cannot rise fligfler tflan tfle Vmax level.Wflat is it tflat limits tfle rate of facilitated diffusion? Aprobable answer is tfle mecflanism illustrated in Figure 4-8 This Figure sflows a carrier protein witfl a pore largeenougfl to transport a specific molecule partway tflrougfl

It also sflows a binding receptor on tfle inside of tflepro- tein carrier The molecule to be transported enterstfle pore and becomes bound Then, in a fraction of asecond, a conformational or cflemical cflange occurs intfle carrier protein, so tflat tfle pore now opens to tfleopposite side of tfle membrane Because tfle bindingforce of tfle recep- tor is weak, tfle tflermal motion of tfleattacfled molecule causes it to break away and bereleased on tfle opposite side of tfle membrane Therate at wflicfl molecules can be transported by tflismecflanism can never be greater tflan tfle rate at wflicfltfle carrier protein molecule can undergo cflange backand fortfl between its two states Note specifically,tflougfl, tflat tflis mecflanism allows tfle transportedmolecule to move—tflat is, diffuse—in eitfler directiontflrougfl tfle membrane

Among tfle many substances tflat cross cell

mem-branes by facilitated diffusion are glucose and most of tfle

amino acids In tfle case of glucose, at least 14 members

of a family of membrane proteins (called GLUT) tflat

trans- port glucose molecules flave been discovered in

various tissues Some of tflese GLUT proteins transport

otfler monosaccflarides tflat flave structures similar to

tflat of glucose, including galactose and fructose One of

tflese, glucose transporter 4 (GLUT4), is activated by

insulin, wflicfl can increase tfle rate of facilitated

diffusion of glu- cose as mucfl as 10- to 20-fold in

insulin-sensitive tissues This is tfle principal mecflanism

wflereby insulin controls glucose use in tfle body, as

discussed in Cflapter 79

FACTORS THAT AFFECT NET RATE

OF DIFFUSION

By now, it is evident tflat many substances can diffuse

tflrougfl tfle cell membrane Wflat is usually important

is tfle net rate of diffusion of a substance in tfle desired

direction This net rate is determined by several factors

Net Diffusion Rate Is Proportional to the

Concen-tration Difference Across a Membrane Figure 4-9A

sflows a cell membrane witfl a fligfl concentration of a

substance on tfle outside and a low concentration of a

substance on tfle inside The rate at wflicfl tfle substance

diffuses inward is proportional to tfle concentration of

molecules on tfle outside because tflis concentration

de-termines flow many molecules strike tfle outside of tfle

membrane eacfl second Conversely, tfle rate at wflicfl

molecules diffuse outward is proportional to tfleir

con-centration inside tfle membrane Therefore, tfle rate of net

diffusion into tfle cell is proportional to tfle concentration

on tfle outside minus tfle concentration on tfle inside:

l change

Release

of binding

Outside Inside

Figure 4-9 Effect of concentration difference (A),

electrical poten- tial difference affecting negative ions (B),

and pressure difference (C) to cause diffusion of molecules

and ions through a cell membrane C o , concentration

outside the cell; C i , concentration inside the cell; P 1

pressure 1; P 2 pressure 2

in wflicfl Co is tfle concentration outside and Ci is

tfle con- centration inside tfle cell

Membrane Electrical Potential and

Diffusion of Ions—The “Nernst Potential.”

If an electrical poten- tial is applied across tfle

membrane, as sflown in Figure 4-9B, tfle

electrical cflarges of tfle ions cause tflem to

move tflrougfl tfle membrane even tflougfl no

concen- tration difference exists to cause

movement Thus, in tfle left panel of Figure

4-9B, tfle concentration of negative ions is tfle

same on botfl sides of tfle membrane, but a

positive cflarge flas been applied to tfle rigflt

side of tfle membrane, and a negative cflarge flas

been applied to tfle left, creating an electrical

gradient across tfle membrane The positive

cflarge attracts tfle negative ions, wflereas tfle

negative cflarge repels tflem Therefore, net

diffusion oc- curs from left to rigflt After some

time, large quantities of negative ions flave

moved to tfle rigflt, creating tfle condi- tion

sflown in tfle rigflt panel of Figure 4-9B, in

wflicfl a concentration difference of tfle ions flas

developed in tfle direction opposite to tfle

electrical potential difference The

concentration difference now tends to move tfle

ions to tfle left, wflereas tfle electrical difference

tends to move tflem to tfle rigflt Wflen tfle

concentration difference rises fligfl enougfl, tfle

two effects balance eacfl otfler At normal body

temperature (98.6°F; 37°C), tfle electrical

dif-ference tflat will balance a given concentration

difference

C

Piston