Chapter 11: Biological Membranes and Transport

Question 1

Membranes are impermeable to most ____ or ______ solutes, but permeable to ______ compounds. They are ___ to ___ nm (____ to ___ Å) thick when proteins protruding on both sides are included

Answer 1

• polar
• charged
• nonpolar
• 5 to 8
• 50 to 80

Question 2

• Phospholipids form a bilayer in which the ______ regions of the lipid molecules in each layer face the core of the bilayer and their _____ head groups face outward, interacting with the aqueous phase on either side.
• Proteins are embedded in this bilayer sheet, held by ______ ______ between the membrane lipids and _____ ______ in the proteins. Some proteins protrude from only one side, others through both.
• Orientation of proteins in the bilayer is asymmetric, giving the membrane “sidedness” meaning ….

Answer 2

• nonpolar, polar
• hydrophobic interaction, hydrophobic domains
• the protein domains exposed on one side of the bilayer are different from those exposed on the other side, reflecting functional asymmetry

Question 3

fluid mosaic model for

Answer 3

• The fatty acyl chains in the interior of the membrane form a fluid, hydrophobic region
• is fluid because most of the interactions among its components are noncovalent, leaving individual lipid and protein molecules free to move laterally in the plane of the membrane, but movement of either from one leaflet of the bilayer to the other is restricted
• Integral proteins float in this sea of lipid, held by hydrophobic interactions with their nonpolar amino acid side chains
• The carbohydrate moieties attached to some proteins and lipids of the plasma membrane are exposed on the extracellular surface

Question 4

Glycerophospholipids, sphingolipids, and sterols are ______ in water. When mixed with water, they spontaneously form microscopic lipid aggregates, clustering together resulting in an increase in ______. ______ interactions among lipid molecules provide the _______ driving force for the formation and maintenance of these clusters.

Answer 4

• insoluble
• entropy
• Hydrophobic
• thermodynamic

Question 5

three types of lipid aggregate can form when amphipathic lipids are mixed with water

Answer 5

• Micelles
  
  • spherical structures that contain anywhere from a few dozen to a few thousand amphipathic molecules
  • hydrophobic chains of the fatty acids are sequestered at the core of the sphere
  • virtually no water in the hydrophobic interior
  • hydrophilic head groups at the surface
  • favored when the cross-sectional area of the head group is greater than that of the acyl side chain(s), as in free fatty acids, lysophospholipids (phospholipids lacking one fatty acid), and detergents
• bilayer
  
  • two lipid monolayers (leaflets) form a two-dimensional sheet
  • favored if the cross-sectional areas of the head group and acyl side chain(s) are similar, as in glycerophospholipids and sphingolipids
  • hydrophobic portions in each monolayer, excluded from water, interact with each other
  • hydrophilic head groups interact with water at each surface of the bilayer
  • Because the hydrophobic regions at its edges are in contact with water, the bilayer sheet is relatively unstable and spontaneously folds back on itself to form a hollow sphere, a vesicle
• vesicle, liposome
  
  • a hollow sphere
  • continuous surface of vesicles eliminates exposed hydrophobic regions, allowing bilayers to achieve maximal stability in their aqueous environment
  • creates a separate aqueous compartment

Question 6

The lipid bilayer is ___ nm (___ Å) thick. The hydrocarbon core, made up of the —CH2— and —CH3 of the fatty acyl groups, is about as nonpolar as ______.

Answer 6

• 3
• 30
• decane

Question 7

The flow of membrane components from the endoplasmic reticulum through the Golgi apparatus and to the plasma membrane via transport vesicles is accompanied by changes in lipid ______ and _____ across the bilayer. Phosphatidylcholine is the principal phospholipid in the lumenal monolayer of the Golgi membrane, but in transport vesicles phosphatidylcholine has been largely replaced by ______ and ______, which, on fusion of transport vesicles with the plasma membrane, make up the majority of the lipids in the outer monolayer of the plasma membrane

Answer 7

• composition
• disposition
• sphingolipids
• cholesterol

Question 8

Three Types of Membrane Proteins

Answer 8

• Integral membrane proteins
  
  • very firmly associated with the lipid bilayer
  • covalently attached to a membrane lipid
  • removable only by agents that interfere with hydrophobic interactions such as detergents, organic solvents, or denaturants
  • these agents form micelle-like clusters around individual protein molecules
• Peripheral membrane proteins
  
  • associate with the membrane through electrostatic interactions and hydrogen bonding with the hydrophilic domains of integral proteins and with the polar head groups of membrane lipids.
  • can be released by relatively mild treatments that interfere with electrostatic interactions or break hydrogen bonds
    
    • by changes in pH (carbonate at high pH) or ionic strength,
    • removal of Ca²⁺ by a chelating agent
    • addition of urea or carbonate
  • a commonly used agent is carbonate at high pH
• Amphitropic proteins
  
  • found both in the cytosol and in association with membranes
  • affinity for membranes results
    
    • in some cases from the protein’s noncovalent interaction with a membrane protein or lipid
    • in other cases from the presence of one or more lipids covalently attached to the amphitropic protein
  • the reversible association of amphitropic proteins with the membrane is regulated
    
    • i.e. phosphorylation or ligand binding can force a conformational change in the protein, exposing a membrane binding site that was inaccessible

Question 9

Integral Proteins

Membrane protein topology (the localization of protein domains relative to the lipid bilayer) can be determined with reagents that react with _____ _____ ______ but cannot cross membranes. ______ _____ ______that react with primary amines of Lys residues, for example, or enzymes such as ______ that cleave proteins but cannot cross the membrane but does not affect domains buried within the bilayer or exposed on the inner surface only.

Answer 9

• protein side chains
• Polar chemical reagents
• trypsin

Question 10

Integral Proteins

glycophorin

Answer 10

• erythrocyte glycoprotein
• spans the plasma membrane
• Has 2 hydrophilic and 1 hydrophobic domain
  
  • hydrophilic
    
    • Its amino-terminal domain (bearing the carbohydrate chains) is on the outer surface and is cleaved by trypsin
    • The carboxyl terminus protrudes on the inside of the cell
  • hydrophobic
    
    • a segment in the center of the protein (residues 75 to 93) contains mainly hydrophobic amino acid residues
• orientation of glycophorin in the membrane is asymmetric: its amino-terminal segment is always on the outside

Question 11

Integral Proteins

6 types

Answer 11

• Types I and II
  
  • have a single transmembrane helix
  • type I: the amino-terminal domain is outside
  • type II: the amino-terminal domain is inside
• Type III
  
  • have multiple transmembrane helices in a single polypeptide
• Type IV
  
  • transmembrane domains of several different polypeptides assemble to form a channel through the membrane
• Type V
  
  • held to the bilayer primarily by covalently linked lipids
• Type VI
  
  • have both transmembrane helices and lipid anchors

Question 12

Hydrophobic interactions between the nonpolar _____ _____ and the _____ _____ ____ of the membrane lipids firmly anchor the protein in the membrane

Answer 12

• amino acids
• fatty acyl groups

Question 13

Annular lipids

aka shell lipids or boundary lipids

Answer 13

• set of lipids or lipidic molecules which preferentially bind or stick to the surface of membrane proteins in biological cells
• they constitute a layer, or an annulus/shell, of lipids around the protein
• partially immobilized due to the existence of lipid-protein interactions
• they form a “grease seal.”

Question 14

Annular lipids

sheep aquaporin (PDB ID 2B6O)

Answer 14

• transmembrane water channel
• includes a shell of phospholipids with their head groups (blue) on the inner and outer membrane surfaces
• hydrophobic acyl chains (gold) intimately associated with the surface of the protein exposed to the bilayer
• lipid forms a “grease seal” around the protein, which is depicted as a dark blue surface representation.

Question 15

The presence of unbroken sequences of more than ___ ______ residues in a membrane protein is commonly taken as evidence that these sequences traverse the lipid bilayer, acting as _____ ______ or forming _____ _____. Virtually all _____ _____ have at least one such sequence. 20% to 30% of all proteins are integral membrane proteins.

Answer 15

• 20 hydrophobic
• hydrophobic anchors
• transmembrane channels
• integral proteins

Question 16

What can we predict about the secondary structure of the membrane-spanning portions of integral proteins?

Answer 16

• An α-helical sequence of 20 to 25 residues is just long enough to span the thickness (30 Å) of the lipid bilayer
  
  • an α-helix is 1.5 Å (0.15 nm) per amino acid residue
• polypeptide chain surrounded by lipids, having no water molecules with which to hydrogen-bond, will tend to form α helices or β sheets where intrachain hydrogen bonding is maximized
• If the side chains of all amino acids in a helix are nonpolar, hydrophobic interactions with the surrounding lipids further stabilize the helix

Question 17

hydropathy index (hydrophobicity)

Answer 17

• free energy of transfer
• relative polarity of each amino acid has been determined experimentally by measuring the free-energy change accompanying the movement of that amino acid side chain from a hydrophobic solvent into water
• ranges from very exergonic for charged or polar residues to very endergonic for amino acids with aromatic or aliphatic hydrocarbon side chains
• overall hydropathy index (hydrophobicity) of a sequence of amino acids is estimated by summing the free energies of transfer for the residues in the sequence
• To scan a polypeptide sequence for potential membrane-spanning segments
  
  • hydropathy index for successive segments (windows) of a given size, from 7 to 20 residues is calculated
    
    • For a window of seven residues, for example, the average indices for residues 1 to 7, 2 to 8, 3 to 9
  • calculations for the middle residue in each window is plotted
    
    • residue 4 for residues 1 to 7
  • A region with more than 20 residues of high hydropathy index is presumed to be a transmembrane segment

Hydropathy plots example

• horizontal axis shows the residue number in the middle of the window
• (a) Glycophorin from human erythrocytes
  
  • has a single hydrophobic sequence between residues 75 and 93 (yellow)
• (b) Bacteriorhodopsin
  
  • has seven transmembrane helices
  • has seven hydrophobic regions
  • hydropathy plot is ambiguous in the region of segments 6 and 7
    
    • region has two transmembrane segments.

Question 18

two remarkable feature of many transmembrane proteins of known structure

Answer 18

• presence of Tyr and Trp residues at the interface between lipid and water
  
  • their side chains serve as membrane interface anchors, able to interact simultaneously with the central lipid phase and the aqueous phases on either side of the membrane
• positive-inside rule
  
  • positively charged Lys, His, and Arg residues of membrane proteins occur more commonly on the cytoplasmic face of membranes.

positive-inside rule

• Tyr and Trp are found predominantly where the nonpolar region of acyl chains meets the polar head group region
• Charged residues (Lys, Arg, Glu, Asp) are found almost exclusively in the aqueous phases.

Question 19

β barrel transmembranes

Answer 19

• 20+ transmembrane segments form ␤ sheets that line a cylinder
• when no water molecules are available to hydrogen-bond with the carbonyl oxygen and nitrogen of the peptide bond, maximal intrachain hydrogen bonding gives the most stable conformation
• Planar β sheets do not maximize these interactions
• β barrels allow all possible hydrogen bondsand are common common among membrane proteins
• porins have many-stranded β barrels lining the polar transmembrane passage
• A polypeptide is more extended in the β conformation than in an β helix; just seven to nine residues of β conformation are needed to span a membrane
• in the β conformation, alternating side chains project above and below the sheet
• In β strands of membrane proteins, every second residue in the membrane-spanning segment is hydrophobic and interacts with the lipid bilayer; aromatic side chains are commonly found at the lipid-protein interface
• The other residues may or may not be hydrophilic
• hydropathy plot is not useful in predicting transmembrane segments for proteins with β barrel motifs,

Question 21

• Some membrane proteins contain one or more covalently linked lipids, which may be of several types:
• The attached lipid provides a hydrophobic anchor that inserts into the_____ _____ and holds the protein at the membrane surface
• The strength of the hydrophobic interaction protein is barely enough to anchor the protein securely, but many proteins have more than one attached _____ _____
• Other interactions, such as _____ _____ between positively charged Lys residues in the protein and negatively charged lipid head groups, probably contribute to the stability of the attachment.
• The association of these lipid-linked proteins with the membrane is ______ than that for integral membrane proteins and is sometimes ______
• Beyond merely anchoring a protein to the membrane, the attached lipid may have a more specific role like in ______

Answer 21

• longchain fatty acids, isoprenoids, sterols, or glycosylated derivatives of phosphatidylinositol (GPIs)
• lipid bilayer
• lipid moiety
• ionic attractions
• weaker
• reversible
• signaling

Question 22

liquid-ordered (L_o) state,

Answer 22

• Below normal physiological temperatures, the lipids in a bilayer form a semi solid form
• all types of motion of individual lipid molecules are strongly constrained
• the bilayer is paracrystalline

Question 23

the liquid disordered (L_d) state

Answer 23

• Above physiological temperatures
• individual hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon–carbon bonds of the long acyl side chains and by lateral diffusion of individual lipid molecules in the plane of the bilayer

Question 24

In the transition from the L_o state to the L_d state, the general shape and dimensions of the bilayer are maintained; what changes is the _____ of ______ allowed to individual lipid molecules.

Answer 24

degree of motion (lateral and rotational)

Question 25

At temperatures in the physiological range for a mammal (about 20 to 40 8C), long-chain saturated fatty acids tend to pack into an ______ gel phase, but the kinks in unsaturated fatty acids interfere with packing, favoring the ______ state. _____-_____fatty acyl groups have the same effect.

Answer 25

• L_o
• L_d​
• Shorter-chain

Question 26

• sterol content of a membrane is another important determinant of lipid state. Sterols (such as cholesterol) have paradoxical effects on bilayer fluidity. Give 2 examples.
• In biological membranes composed of a variety of phospholipids and sphingolipids, cholesterol tends to associate with ______ and to form regions in the L_o state surrounded by cholesterol-poor regions in the L_d state

Answer 26

• they interact with phospholipids containing unsaturated fatty acyl chains, compacting them and constraining their motion in bilayers.
• association with sphingolipids and phospholipids with long, saturated fatty acyl chains tends, rather, to fluidize the bilayer which w/out cholestorl would adopt the L_o state.
• sphingolipids

Question 27

At physiological temperatures, ______—or “flipflop”—diffusion of a lipid molecule from one leaflet of the bilayer to the other occurs very slowly if at all in most membranes, although _____ _____ in the plane of the bilayer is very rapid. Transbilayer movement is a process with a _____, _____ _____-_____ change. To get from their site of synthesis to their eventual point of deposition, these lipids must undergo _____-_____ diffusion

Answer 27

• transbilayer
• lateral diffusion
• large, positive free-energy
• flip-flop

Question 28

_____, ______, and ______ which facilitate the transbilayer movement of lipids, providing a path that is energetically more favorable and much faster than the uncatalyzed movement.

Answer 28

• flippases
• floppases
• scramblases

Question 29

Besides contributing to ______ of composition, the energy-dependent transport of lipids to one bilayer leaflet may, by creating a larger surface on one side of the bilayer, be important in generating the _____ _____ essential in the budding of vesicles.

Answer 29

• asymmetry
• membrane curvature

Question 30

Flippases

Answer 30

• catalyze translocation of
  
  • aminophospholipids phosphatidylethanolamine and phosphatidylserine from the extracellular to the cytosolic leaflet
    
    • Keeping phosphatidylserine out of the extracellular leaflet is important because its exposure on outer surface triggers apoptosis by engulfment of macrophages that carry phosphatidylserine receptors
  • sphingolipids and phosphatidylcholine in the outer leaflet
• contributes to the asymmetric distribution of phospholipids
• also act in the ER
  
  • moves newly synthesized phospholipids from their site of synthesis in the cytosolic leaflet to the lumenal leaflet
• consume about one ATP per molecule of phospholipid translocated
• structurally/functionally related to the P-type ATPases
• ABC transporter

Question 31

Floppases

Answer 31

• move plasma membrane phospholipids from the cytosolic to the extracellular leaflet
• ATP-dependent
• members of the ABC transporter family
• actively transport hydrophobic substrates outward across the plasma membrane

Question 32

Scramblases

Answer 32

• move any membrane phospholipid across the bilayer down its concentration gradient
  
  • from the leaflet where it has a higher concentration to the leaflet where it has a lower concentration
• not dependent on ATP
• leads to controlled randomization of the head-group composition on the two faces of the bilayer
• activity rises sharply with an increase in cytosolic Ca²⁺ concentration, which may result from cell activation, cell injury, or apoptosis (by exposure of phosphatidylserine on the outer surface)

Question 33

phosphatidylinositol transfer proteins

Answer 33

• primarily moves phosphatidylinositol lipids across lipid bilayers
• believed to have important roles in lipid signaling and membrane trafficking

Question 34

Individual lipid molecules can move laterally in the plane of the membrane by changing places with neighboring lipid molecules; that is, they undergo ______ movement within the bilayer which can be rapid

Answer 34

• Brownian

Question 35

FRAP

Answer 35

• rate of fluorescence recovery after photobleaching
• a measure of the rate of lateral diffusion of the lipids

Question 36

There is rapid lateral diffusion within small, discrete regions of the cell surface and movement from one such region to a nearby region (“_____ ______”) is inhibited; membrane lipids behave as though corralled by fences that they can occasionally cross by hop diffusion

Answer 36

• hop diffusion

Question 37

Many membrane proteins move as if afloat in a sea of lipids. Some membrane proteins associate to form large ______ on the surface of a cell or organelle. (i.e. acetylcholine receptors form dense, nearcrystalline patches on neuronal plasma membranes at synapses). Other membrane proteins are anchored to ______ ______ that prevent their free diffusion.

Answer 37

• aggregates (“patches”)
• internal structures

Question 38

Sphingolipids and Cholesterol Cluster Together in _____ ______

Answer 38

• Membrane Rafts

Question 39

Caveolae

Answer 39

• small invaginations in the plasma membrane
• unusual rafts
• involve both leaflets of the bilayer the cytoplasmic and extracellular leaflet
• caveolin globular domains project from the cytoplasmic leaflet
• a typical sphingolipid/cholesterol raft with associated GPI-anchored proteins

Question 40

Caveolin

Answer 40

• integral membrane protein
• caveolin monomers have a central hydrophobic domain and three long-chain acyl groups (red), which hold the molecule to the inside of the plasma membrane
• they forms dimers
• When several caveolin dimers are concentrated in a small region (a raft), they force a curvature in the lipid bilayer, forming a caveola
• associates with cholesterol-rich regions in the membrane

Question 41

Exocytosis, endocytosis, cell division, fusion of egg and sperm cells, and entry of a membrane-enveloped virus mostly begin with a local increase in _____ _____.

Answer 41

membrane curvature

Question 42

Three models for protein-induced curvature of membranes

Answer 42

Question 43

BAR domains

Answer 43

• can assemble into a crescent-shaped scaffold that binds to the membrane surface, forcing or favoring membrane curvature
• consist of coiled coils that form long, thin, curved dimers with a positively charged concave surface that tends to form ionic interactions with the negatively charged head groups of membrane phospholipids
• some have a helical region that inserts into one leaflet of the bilayer, expanding its area relative to the other leaflet and thereby forcing curvature

Question 44

fusion proteins

Answer 44

• mediate events needed for membranes to fuse
• brings about specific recognition and a transient local distortion of the bilayer structure that favors membrane fusion

Question 45

Membrane fusion during neurotransmitter release at a synapse

Answer 45

• Proteins involved
  
  • v-SNAREs: SNAREs in the cytoplasmic face of the intracellular vesicle
  • t-SNAREs: those in the target membrane where vesicle fuses (the plasma membrane during exocytosis)
  • SNAP25 and NSF proteins are involved
• v-SNARE and t-SNARE bind to each other
• they undergo a structural change
  
  • SNARES produce a bundle of long, thin rods
  • SNAP25 produces two helices
• SNAREs initially interact at their ends, then zip up into the bundle of helices
• two membranes come into contact and fusion is initiated

Question 46

Integrins

Answer 46

• surface adhesion proteins that mediate a cell’s interaction with the extracellular matrix and with other cells, including some pathogens.
• carry signals in both directions across the plasma membrane
• serve as transporters, ion channels, receptors for hormones, neurotransmitters, and growth factors
• central to oxidative phosphorylation and photophosphorylation, cell-cell and cell-antigen recognition in the immune system
• important players in the membrane fusion that accompanies exocytosis, endocytosis, and the entry of many types of viruses into host cells
• heterodimeric proteins composed α and β subunits
  
  • each subunit is anchored to the plasma membrane by a single transmembrane helix
  • large extracellular domains of the α and β subunits combine to form a specific binding site for extracellular proteins

Question 47

transporter types

Answer 47

Question 48

simple diffusion

Answer 48

• solute moves from the region of higher concentration, through the membrane, to the region of lower concentration, until the two compartments have equal solute concentrations
• removal of the hydration shell is highly endergonic, and the energy of activation (ΔG^‡) for diffusion through the bilayer is very high

Question 49

membrane potential, V_m

Answer 49

• transmembrane electrical gradient
• when ions of opposite charge are separated by a permeable membrane
• expressed in millivolts
• produces a force opposing ion movements that increase V_m and driving ion movements that reduce V_m
• electrochemical gradient or electrochemical potential
  
  • direction ithat charged solute tends to move across a membrane depends on the chemical gradient and the electrical gradient (V_m)

Question 50

• To pass through a lipid bilayer, a polar or charged solute must first give up its interactions with the water molecules in its _____ _____, then diffuse about 3 nm (30 Å) through a substance (lipid) in which it is poorly soluble.
• The energy used to strip away the hydration shell and to move the polar compound from water into lipid, then through the lipid bilayer, is ______ as the compound leaves the membrane on the other side and is ______
• The intermediate stage of transmembrane passage is a high-energy state comparable to the _____ ______ in an enzyme-catalyzed chemical reaction.
• In both cases, an _____ ______ must be overcome to reach the intermediate stage

Answer 50

• hydration shell
• regained, rehydrated
• transition state
• activation barrier

Question 51

The energy of activation (ΔG‡) for translocation of a polar solute across the bilayer is so large that pure lipid bilayers are virtually impermeable to polar and charged species over periods relevant to cell _____ and _____

Answer 51

• growth
• division

Question 52

facilitated diffusion

Answer 52

• Membrane proteins lower the activation energy for transport of polar compounds and ions by providing an alternative path across the membrane for specific solutes
• their “substrates” are moved from one compartment to another but are not chemically altered

Question 53

transporters or permeases

Answer 53

• Membrane proteins that speed the movement of a solute across a membrane by facilitating diffusion
• they form a transmembrane pathway lined with hydrophilic amino acid side chains
• Like enzymes, they bind their substrates with stereochemical specificity through multiple weak, noncovalent interactions, to replace the hydrogen bonding with water
• negative free energy change associated with these weak interactions, ΔG_binding, counterbalances the positive free-energy change that accompanies loss of the water of hydration from the substrate, ΔG_dehydration, lowering ΔG^‡ for transmembrane passage

Question 54

Transporters

Answer 54

• for molecules and ions
• bind their substrates with high specificity
• catalyze transport at rates well below the limits of free diffusion
• saturable in the same sense as are enzymes: there is some substrate concentration above which further increases will not produce a greater rate of transport
• can move a substrate against a concentration gradient
• Pumps
  
  • have two gates, and they are never both open
  • Movement limited by the time needed for one gate to open and close and for the second gate to open
• contransport : simultaneously carry two solutes across a membrane
• antiport: moves the substrates in opposite directions
• symport: two substrates are moved simultaneously in the same direction
• uniport: Transporters that carry only one substrate

Question 55

passive transporters

Answer 55

simply facilitate diffusion down a concentration gradient

Question 56

Channels

Answer 56

• allow transmembrane movement of ions at rates that are orders of magnitude greater than those typical of transporters, approaching the limit of unhindered diffusion
• show some specificity for an ion, but are not saturable with the ion substrate
• direction of ion movement through an ion channel is dictated by the ion’s charge and the electrochemical gradient
• a transmembrane pore is either open or closed, depending on the position of the single gate

Question 57

Active transporters

Answer 57

• can drive substrates across the membrane against a concentration gradient
• some use energy provided directly by a chemical reaction (primary active transporters)
• some couple uphill transport of one substrate with downhill transport of another (secondary active transporters).

Question 58

Model of glucose transport into erythrocytes by GLUT1.

Answer 58

• with GLUT1 glucose always moves down its concentration gradient, into the cell
• Glucose that enters a cell is generally metabolized immediately, and the intracellular glucose concentration is thereby kept low relative to its concentration in the blood
• GLUT1 is specific for D-glucose, with a measured K_t of about 6 mM (glucose concentration is maintained at about 5 mM)
• passive transport, has high rates of diffusion down a concentration gradient, saturability, and specificity
• spans the membrane at least 12 times

Question 59

rate equations for glucose transport

Answer 59

• analogous to the Michaelis-Menten equation
• V₀: initial velocity of accumulation of glucose inside the cell
• [S]_out: glucose concentration outside of cell
• K_{t ,} K_transport: constant analogous to the Michaelis constant
• equation describes the initial velocity, the rate observed when [S]_in = 0
• slope-intercept form of the equation describes a linear plot of 1/V₀ against 1/[S]_out, from which we can obtain values of K_t and V_max
• When [S]_out = Kt, the rate of uptake is 1/2 V_max
• the transport process is half-saturated. The concentration of glucose in blood is 4.5 to 5 mM, close to the Kt, which ensures that GLUT1 is nearly saturated with substrate and operates near Vmax

Question 60

• an _____ _____ that is essential in CO₂ transport to the lungs from tissues
• Waste CO₂ released from respiring tissues into the blood plasma enters the erythrocyte, where it is converted to ______ (the primary buffer of blood pH) by carbonic anhydrase
• HCO₃^- reenters the blood plasma for transport to the lungs, and there reenters the erythrocyte and is converted to _____

Answer 60

• anion exchanger
• bicarbonate (HCO₃^-)
• CO₂

Question 61

chloride-bicarbonate exchanger, anion exchange (AE) protein

Answer 61

• increases the rate of HCO₃^- transport across the erythrocyge membrane
• spans the membrane at least 12 times
• mediates the simultaneous movement of two anions: for each HCO₃^-
  that moves in one direction, one Cl^- ion moves in the opposite direction with no net transfer of charge (electroneutral)
• it is a contransport system, in the absence of chloride, bicarbonate transport stops
• it is an antiport, it moves the substrates in opposite directions

Question 62

Active transport

Answer 62

• results in the accumulation of a solute above the equilibrium point
• thermodynamically unfavorable (endergonic)
• takes place only when coupled to an exergonic process such as the absorption of sunlight, an oxidation reaction, the breakdown of ATP, or the concomitant flow of some other chemical species down its electrochemical gradient
• Most cells maintain more than a 10-fold difference in ion concentrations across their plasma therefore a major energy-consuming process.

Question 63

primary active transport

Answer 63

• solute accumulation is coupled directly to an exergonic chemical reaction, such as conversion of ATP to ADP 1 Pi

Question 64

Secondary active transport

Answer 64

occurs when endergonic (uphill) transport of one solute is coupled to the exergonic (downhill) flow of a different solute that was originally pumped uphill by primary active transport

Question 65

equation to calculate amount of energy needed for the transport of a solute

Answer 65

• against a gradient
  
  • between a substrate/product
  • can be calculated from the initial concentration gradient
  • ΔG = ΔG’° + RT ln ([P]/[S])
  • ΔG’°: standard free-energy change
  • R: gas constant, 8.315 J/mol • K
  • T: absolute temperature
• If transport is from a region where its concentration is C₁ to a region where its concentration is C₂, no bonds are made or broken and ΔG’° is zero
  
  • between substrate/substrate or uncharged solute
  • ΔG = RT ln (C₂/C₁)
• When the solute is an ion
  
  • electrogenic: an ion’s movement w/out a counterion results in the endergonic separation of positive and negative charges, producing an electrical potential
  • energetic cost of moving an ion depends on the electrochemical potential
  • ΔG = RT ln (C₂/C₁) + Z ℱ ΔΨ
  • Z: charge on the ion
  • ℱ: Faraday constant 96,480 J/V • mol
  • ΔΨ: transmembrane electrical potential (in volts)
  • Eukaryotic cells typically have plasma membrane potentials of about 0.05 V
• ΔG for flow down an electrochemical gradient has a negative value
• ΔG for transport of ions against an electrochemical gradient has a positive value
• 1.0 mM = 1.0 • 10^-3

Question 66

P-type ATPases

Answer 66

• active transporter
• cation transporters
• reversibly phosphorylated by ATP as part of the transport cycle
• Phosphorylation forces a conformational change that is central to movement of the cation across the membrane
• Asp residue in the P domain undergoes phosphorylation
• integral proteins with 8 or 10 membrane-spanning regions
• sensitive to inhibition by the phosphate analog vanadate
• set the transmembrane electrochemical potential in cells by establishing ion gradients across the plasma membrane
• These gradients provide the driving force for secondary active transport and are the basis for electrical signaling in neurons

Question 67

P-type pumps have similar structures and mechanisms

Answer 67

• have three cytoplasmic domains A, N, and P and two transmembrane domains T and S
• N (nucleotide) domain
  
  • binds ATP and Mg2⁺
  • has protein kinase to phosphorylates Asp residue found in the P (phosphorylated) domain
• A (actuator) domain
  
  • protein phosphatase activity
  • removes the phosphoryl group from the Asp residue with each catalytic cycle of the pump
  • communicates movements of the N and P domains to the ion-binding sites
• Transport (T) domain
  
  • has six transmembrane helices
  • includes ion-transporting structure
  • Ca²⁺ ions to be transported bind to the T domain
• Support (S) domain
  
  • has four transmembrane helices
  • provides physical support to the transport domain and may have other specialized functions
• binding sites for the ions are near the middle of the membrane, 40 to 50 Å from the phosphorylated Asp residue
  
  • so Asp phosphorylation-dephosphorylation does not directly affect ion binding

Question 68

P-type pumps postulated mechanism

SERCA pump

Answer 68

the energy released by hydrolysis of ATP during one phosphorylation-dephosphorylation cycle drives Ca²⁺ across the membrane against a large electrochemical gradient

Question 69

Na⁺K⁺ ATPase

Answer 69

• movement of both Na⁺ and K⁺ against their electrochemical gradients
• responsible for maintaining low Na⁺ and high K⁺ concentrations in the cell relative to the extracellular fluid
• transporter moves two K⁺ ions inward and three Na⁺ ions outward across the plasma membrane
• Cotransport is electrogenic—it creates a net separation of charge across the membrane, producing a membrane potential of -50 (inside) to -70 mV (outside)
• about 25% of the total energy consumption of a human at rest

Question 71

V-type ATPases

Answer 71

• class of proton-transporting ATPases
• responsible for acidifying intracellular compartments in many organisms: lysosomes, endosomes, the Golgi complex, and secretory vesicles

Question 72

F-type ATPase

Answer 72

• active transporters
• catalyze the uphill transmembrane passage of protons driven by ATP hydrolysis
• catalyze their reactions in both directions
• a sufficiently large proton gradient can supply the energy to drive the reverse reaction, ATP synthesis, making them ATP synthases
  
  • central to ATP production in mitochondria during oxidative phosphorylation and in chloroplasts during photophosphorylation
  • proton gradient needed is produced by other types of proton pumps powered by substrate oxidation or sunlight

Question 73

ABC transporters

Answer 73

• a large family of ATP-dependent transporters
• Some have very high specificity for a single substrate
• others are more promiscuous
• pump amino acids, peptides, proteins, metal ions, various lipids, bile salts, and many hydrophobic compounds, including drugs, out of cells against a concentration gradient
• flippases that move membrane lipids from one leaflet of the bilayer to the other are ABC transporters

Question 74

MDR1; also called P glycoprotein

Answer 74

• ABC transporter in humans
• multidrug transporter
• responsible for the resistance of certain tumors to some generally effective antitumor drugs
• has broad substrate specificity for hydrophobic compounds
• pumps drugs out of the cell preventing their accumulation within a tumor blocking their therapeutic effects
• in the placental membrane and in the blood-brain barrier keep out toxic compounds that would damage the fetus or the brain

Question 75

ABC transporters structure

Answer 75

• has two cytoplasmic nucleotide-binding domains (NBDs)
• has two transmembrane domains (TMDs) containing multiple transmembrane helices
• Process
  
  • substrate is brought to the transporter on the periplasmic side by a substrate-specific binding protein
  • ATP bibnds to the NBD sites
  • transporter opens to the outside (periplasm)
  • once substrate binds and ATP is hydrolyzed to ADP, a conformational change exposes the substrate to the inside surface, and it diffuses away from the transporter and into the cytosol
• Many are in the plasma membrane,
• some are found in the endoplasmic reticulum and in the membranes of mitochondria and lysosomes
• can be coupled to a wide variety of pumps and channels
  
  • when coupled with a pump it moves solutes against a concentration gradient
  • approximately one ATP hydrolyzed per molecule of substrate transported

Question 76

ion gradients formed by primary transport of ___ or ____ can in turn provide the driving force for cotransport of other solutes. Many cell types contain transport systems that couple the spontaneous, downhill flow of these ions to the simultaneous uphill pumping of another ion, sugar, or amino acid

Answer 76

• Na+
• H+

Question 77

Glucose transport in intestinal epithelial cells

Answer 77

• Glucose is cotransported with Na+ across the apical plasma membrane into the epithelial cell
• Glucose moves through the cell to the basal surface, where it passes into the blood via GLUT2, a passive glucose uniporter
• The Na+ K+ ATPase continues to pump Na+ outward to maintain the Na+ gradient that drives glucose uptake