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Transport: Breaking the Barrier Flashcards

Lecture 17

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1
Q

What is relative permeability of the phospholipid bilayer based on?

A

concentration
size
hydrophobicity
charge

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2
Q

Are gases (like CO2, N2, and O2) permeable, slightly permeable, or impermeable?

A

Permeable

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3
Q

Are small uncharged polar molecules (like ethanol, urea, and water) permeable, slightly permeable, or impermeable?

A

Some are permeable, others aren’t. Ethanol is permeable, but urea and water are only slightly permeable.

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4
Q

Are large uncharged polar molecules (like glucose and fructose) permeable, slightly permeable, or impermeable?

A

Impermeable

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5
Q

Are ions permeable, slightly permeable, or impermeable?

A

Impermeable

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6
Q

Are charged polar molecules (like amino acids, ATP, glucose 6-phosphate, proteins, and nucleic acids) permeable, slightly permeable, or impermeable?

A

Impermeable

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7
Q

What substances can be transported via simple diffusion? Why?

A

Nonpolar molecules and very small polar molecules (water, glycerol, and ethanol)
Due to the hydrophobic interior of the membrane

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8
Q

What are the three passive transport mechanisms?

A

Simple Diffusion
Facilitated Diffusion with pores and channels
Facilitated Diffusion with transporters (carriers)

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9
Q

Describe the rate of influx of simple diffusion.

A

Linear, solely based on going down a concentration gradient

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10
Q

Describe the rate of influx of facilitated diffusion.

A

Linear towards the beginning, but begins to plateau towards the end as the extracellular concentration decreases

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11
Q

Describe the rate of influx of facilitated diffusion using carriers.

A

Linear towards the beginning, but plateaus at Vmax, when all carrier molecules are occupied

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12
Q

Does passive transport require an energy source?

A

No

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13
Q

Does active transport require an energy source?

A

Yes

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14
Q

Does active transport go up or down a concentration gradient?

A

Up

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15
Q

Does passive transport go up or down a concentration gradient?

A

Down

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16
Q

What are the two ways active transport can find the energy to proceed?

A

Directly, via use of ATP, redox reactions, or light
Indirectly by coupling to concentration gradient

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17
Q

How many ions can move through an ion channel per second?

A

10^7-10^8 ions per second

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18
Q

What are the 4 types of ion channels in mammal cells?

A

Voltage-gated
Ligand-gated w/extracellular ligand
Ligand-gated w/intracellular ligand
Mechanically-gated

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19
Q

How many molecules move through transporters (carriers) per second?

A

100-10,000 (10^2-10^4) molecules per second

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20
Q

What is a uniporter?

A

a uniporter is a transporter that only transports one solute across a membrane

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21
Q

What is a symporter?

A

a symporter is a transporter that transports two things in the same direction simultaneously; the transport of either is reliant on the presence of the other; needed for indirect active transport

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22
Q

What is an antiporter?

A

an antiporter is a transporter that transports two things in the opposite directions simultaneously; the transport of either is reliant on the presence of the other; needed for indirect active transport

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23
Q

What are the the two kinds of transporters used for coupled transport? Which is the only transporter not used for coupled transport?

A

Symporters and antiporters are used for coupled transport.
Uniporters are not.

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24
Q

What is a coupled transport?

A

When two solutes are transported simultaneously and their transport is coupled such that transport of either stops if the other is absent

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25
Q

What is an example of a uniporter?

A

GLUT1 transports glucose down its concentration gradient, from the exterior of the cell into the cytosol.

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26
Q

Describe how glucose enters the cell.

A
  1. Glucose arrives at a transport site in GLUT 1.
  2. An in-ward facing conformation occurs on the cytosolic side of a cell.
  3. The glucose enters the cell.
  4. There’s an outward-facing conformation that opens the transport site to more glucose.
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27
Q

Describe transepithelial transport of sodium and glucose.

A

Uses a 2 Na+/glucose symporter to transport from the apical membrane into the membrane.
Uses GLUT2 to transport glucose further into the basolateral membrane and a Na+/K+ ATPase to transport sodium into the basolateral membrane and potassium out of it.

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28
Q

How many ions are transported via ATP-powered pumps per second?

A

1-1000 (10^0 to 10^3) ions per second

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29
Q

What is the general concentration state of a cell’s glucose and sodium levels?

A

There is more sodium outside of the cell.
There is more glucose inside of the cell.

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30
Q

What are the four classes of pumps?

A

P-Class
V-Class
F-Class
ABC Class

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31
Q

What are ATP-powered pumps?

A

Pumps that use the energy of ATP hydrolysis to transport molecules against their concentration gradients (active transport)

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32
Q

Which pumps can run backwards?

A

F-Class pumps

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33
Q

What is osmosis? When does it occur?

A

simple diffusion of water toward equilibrium, from low solute concentration to high solute concentration; occurs when a membrane is not permeable to the dissolved solute

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34
Q

Why does osmosis occur?

A

When water has few solutes to interact with, it forms very highly-ordered structures with itself and is in a state of high free energy. The presence of a solute interrupts the interactions of water molecules, leading to an increase in entropy and a state of lower free energy (an equilibrium point). Water equalizes the concentration of solutes on both sides of a selectively-permeable membrane.

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35
Q

What happens to a cell in a hypertonic solution?

A

It shrivels (becomes crenated) and water leaves the cell.

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36
Q

What is a hypertonic solution?

A

One in which there is a higher concentration of solute in the solvent than in the cell

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37
Q

What is an isotonic solution?

A

a solution in which there is an equal concentration of solute in the solvent and in the cell

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38
Q

What happens to a cell in an isotonic solution?

A

Water moves in and out, but there is no net movement in either direction

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39
Q

What happens to a cell in a hypotonic solution?

A

Water moves into the cell, causing it to lyse (burst).

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40
Q

What is a hypotonic solution?

A

A solution in which there is a lower solute concentration in the solvent and a higher concentration inside of a cell

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41
Q

Which forms of transport requires a membrane protein?

A

Facilitated Diffusion
Active Transport

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42
Q

What determines the movement of a solute without a net charge?

A

concentration gradient across the membrane

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43
Q

What is a concentration gradient?

A

magnitude of difference in concentration of a substance on opposite sides of a membrane; larger concentration difference = larger gradient = larger driving force

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44
Q

What determines the movement of a charged solute?

A

electrochemical potential

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45
Q

What is the electrochemical potential?

A

the sum of the chemical and electrical driving forces; combined effect of the concentration gradient and the net difference in charge across the membrane

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46
Q

Can simple diffusion occur down an electrochemical potential?

A

No, ions cannot simply diffuse. Must use facilitated diffusion or active transport methods.

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47
Q

Explain how electrochemical gradients work with sodium ions. What impact does the charge gradient have? What impact does the concentration gradient have?

A

Many cells have mostly negative charges inside. If Na+ were in equal concentration inside and outside the cell, the charge gradient would drive the Na+ inside (where the negative charge is), causing the concentration inside the cell to become higher than outside. The concentration gradient would begin favoring the outward movement of Na+. These two forces together would balance the charge gradient, favoring inward movement.

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48
Q

What is a membrane potential (Vm)? What creates it?

A

the charge gradient across a membrane created by ion gradients
created by active transport

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49
Q

What is the inside of a cell’s charge like, relative to the outside?

A

The inside of a cell is generally negatively charged compared to the outside.

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50
Q

What is the Vm of a resting nerve cell? What does it do to the movement of cations and anions?

A

-60 mV, causing cations to move inside and anions to move outwards; also opposes the outward movement of cations and the inward movement of anions

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51
Q

What do pores/channels do?

A

allow water and ions to enter and leave the cell rapidly in response to cell needs

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52
Q

How does simple diffusion work?

A

O2, CO2, and H20 diffuse directly across the plasma membrane in response to relative concentrations inside and outside the cell without using transport proteins or extra energy.

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53
Q

What is simple diffusion?

A

unassisted net movement of a solute from a region where its concentration is higher to a region where its concentration is lower

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54
Q

How does facilitated diffusion using carrier proteins work? What are two examples?

A

A carrier protein (like GLUT1) transports a molecule inside the cell, where the concentration of the molecule is lower. An anion exchange protein transports Cl- and HCO3- in opposite directions.

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55
Q

How does facilitated diffusion using channel proteins work? Give an example.

A

Aquaporin channel proteins can facilitate the rapid inward or outward movement of water depending on the relative solute concentration on opposite sides of the membrane.

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56
Q

How does active transport using ATP-requiring pumps work? Give an example.

A

Driven by the hydrolysis of ATP, the sodium-potassium pump moves sodium ions outward and potassium ions inward, establishing an electro-chemical potential for both ions.

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57
Q

What is the result of simple diffusion when the concentration of solutes differs across a membrane? Why?

A

creation of a uniform solution where the concentration of all solutes is the same everywhere; moves towards equilibrium, is a spontaneous process requiring no additional energy

58
Q

What is the driving force for simple diffusion?

A

entropy, the randomization of molecules as concentrations equalize on both sides of the membrane

59
Q

Why does the unregulated movement of water often cause cells without cell walls to swell?

A

There is usually a higher concentration of solutes inside a cell, causing water to move inward.

60
Q

What is the driving force of osmosis?

A

osmolarity, the total solute concentration of the cytoplasm relative to the extracellular solution

61
Q

How do cells without cell walls address osmolarity? How do animal cells, specifically, do this?

A

By continuously and actively pumping out ions, reducing the intracellular osmolarity and minimizing the difference in solute concentration between the cell and its surroundings
Animal cells continuously remove sodium ions via the sodium-potassium pump

62
Q

Are lipid bilayers generally more permeable to small or large molecules?

A

Small, especially water, oxygen, and carbon dioxide

63
Q

Are lipid bilayers generally more permeable to nonpolar or polar molecules?

A

Nonpolar molecules

64
Q

Which amino acids are more likely to be found in transmembrane regions of a membrane protein?

A

amino acids with nonpolar side chains, like tryptophan, leucine, and valine

65
Q

Why does the solute charge influence permeability of polar substances (like ions)?

A

Due to water molecules forming a shell of hydration around the ion.
To move into the membrane, the associated water molecules must be stripped off and energy must be input to eliminate the bonds between the ions and the water molecules

66
Q

What is the mathematical expression describing the net rate of transport for the inward diffusion of solute S?

A

velocity inward = Permeability coefficient * Change in solute concentration

67
Q

How to increase the rate of inward simple diffusion?

A

increase the permeability or the concentration gradient

68
Q

How does saturation impact simple diffusion?

A

It doesn’t. Simple diffusion is not reliant on proteins.

69
Q

How does the relationship between velocity inward, the rate of diffusion, and solute concentration gradient influence facilitated diffusion?

A

The relationship is linear when the concentration gradient is small but exhibits saturation kinetics and is hyperbolic, reaching a maximum value at a very high solute concentration gradient.

70
Q

Is simple diffusion exergonic or endergonic?

A

exergonic

71
Q

Is facilitated diffusion exergonic or endergonic?

A

exergonic; the solute still diffuses in the direction dictated by the concentration gradient (for uncharged molecules) or by the electrochemical gradient (for ions) without input of additional energy

72
Q

Is active transport exergonic or endergonic?

A

endergonic

73
Q

What does a transport protein do during facilitated diffusion?

A

provides a path through the lipid bilayer, facilitating the downhill diffusion of a large, polar, or charged solute across an otherwise impermeable barrier

74
Q

Why does facilitated diffusion become saturated at high solute concentrations (unlike simple diffusion)?

A

Because there are a limited number of transport proteins

75
Q

Why does glucose utilize facilitated diffusion?

A

The concentration of glucose is higher in the blood than in the cell, so the inward transport is exergonic (doesn’t require additional energy), but glucose is too large and polar to diffuse without aid.

76
Q

Describe transport proteins. What are the two kinds?

A

Integral membrane proteins that contain several transmembrane segments and cross the membrane multiple times
1. Carrier proteins
2. Channel proteins

77
Q

What is a carrier protein?

A

AKA transporter/permease
membrane protein that transports solutes across the membrane by binding to the solute on one side of the membrane and then undergoing a conformational change that transfers the solute to the other side of the membrane; binds in such a way that the polar or charged groups of the solute are shielded from the membrane’s nonpolar interior

78
Q

What are channel proteins?

A

proteins that form hydrophilic channels through the membrane, allowing the passage of solutes without a major conformational change in the protein

79
Q

What is a big difference between carrier and channel proteins?

A

When channel proteins are not blocked, they are open to the inner and outer surface of the membrane simultaneously. In contrast, carrier proteins undergo conformational changes that ensure the outer and inner sides of a membrane are never accessed simultaneously.

80
Q

Describe ion channels.

A

pores that are small and highly selective; transport ions rather than molecules (usually only one kind)

81
Q

What two factors contribute to the specificity of ion channels?

A

Ion-specific binding sites involving specific amino acid side chains and polypeptide backbone atoms inside the channel
Constricted center of the channel that filters for size

82
Q

Which is faster, movement through a carrier protein or a channel protein?

A

Channel proteins allow solutes to move faster because they do not require conformational changes.

83
Q

Describe the alternating conformation model.

A

A carrier protein is an allosteric membrane-spanning protein that alternates between two conformational states. In one state, the solute-binding site of the protein is open or accessible on one side of the membrane. Following solute binding, the protein changes to an alternate conformation in which the solute-binding site is on the other side of the membrane, triggering its release.

84
Q

What are the five similarities between enzymes and carrier proteins?

A
  1. Carrier-facilitated diffusion involves an initial binding of the “substrate” (the solute to be transported) to a specific site on a protein surface (the solute’s binding site on the carrier protein) and the eventual release of the “product” (the transported solute), with an “enzyme-substrate” complex (solute bound to carrier protein) as an intermediate.
  2. Like enzymes, carrier proteins can be regulated by external factors that bind and modulate their activity.
  3. Specificity, often for one compound or a small group of closely related compounds; can be stereospecific
  4. Capacity to be saturated at high substrate/solute concentrations - hyperbolic plot
  5. Competitive inhibition by molecules or ions that are structurally similar to the intended substrate
85
Q

The carrier protein for glucose is very specific. What does it allow in? Why?

A

It recognizes glucose, galactose, and mannose, closely related monosaccharides.
The protein is stereospecific, only accepts the D-isomer (not the L-isomer), probably due to a specific stereochemical fit between the solute and its binding site on the carrier protein.

86
Q

Describe the glucose transporter that transfers glucose into an erythrocyte.

A

GLUT1 is a uniport carrier that uses an alternating conformation mechanism.

Has 12 hydrophobic transmembrane segments that fold in the membrane to form a cavity lined with hydrophilic side chains that form H-bonds with glucose molecules while they move through the membrane.
Specific for glucose
Transport proceeds down a concentration gradient without energy input
Saturation kinetics
Susceptible to competitive inhibition (galactose, mannose)

87
Q

Describe the process of glucose diffusing into an erythrocyte.

A
  1. Glucose binds to a GLUT1 transporter protein that has its binding site open to the outside of the cell.
  2. Glucose binding causes the GLUT1 transporter to undergo a conformational change, causing the binding site to open to the inside of the cell.
  3. Glucose is released to the interior of the cell, initiating a second conformational change in GLUT1.
  4. Loss of bound glucose causes GLUT1 to return to its original conformation.
88
Q

Which direction do carrier proteins function?

A

They function equally well in either direction. Transport direction depends on the relative concentrations of the solute on either side of the membrane.

89
Q

What is the average range of intracellular glucose concentration for mammalian cells?

A

0.5-1.0 mM (about 15-20% of the glucose level in the blood outside the cell)

90
Q

Why does hexokinase phosphorylate glucose after GLUT1 allows it into the cell?

A

Ensures the concentration of free glucose within the cell is kept low, maintaining the concentration gradient across the cell membrane
Locks glucose into the cell because there is no transport protein for glucose-6-phosphate (GLUT1 does not recognize the phosphorylated form).
Phosphorylation keeps the level of free glucose low in the cell, ensuring equilibrium is never reached, allowing the cell to continue to import glucose.

91
Q

What does hexokinase do?

A

phosphorylates free glucose in an erythrocyte; glucose -> glucose-6-phosphate

92
Q

What does GLUT2 do?

A

GLUT2 is the glucose transporter in liver cells, which break down glycogen to produce glucose for the blood. GLUT2 facilitates glucose transport out of liver cells to keep blood glucose constant.

93
Q

What is the Na+/glucose symporter? How is it different from GLUT1? Why is it needed in addition to GLUT1?

A

GLUT1 only carries glucose. The Na+/glucose symporter simultaneously transports sodium and glucose molecules across a membrane in the same direction.
The facilitated diffusion of sodium down its electrochemical gradient provides energy to transport glucose into a cell that already has a higher concentration of glucose than its environment (up the concentration gradient).

94
Q

What are three kinds of channel proteins?

A

ion channels
porins
aquaporins

95
Q

What is an aquaporin?

A

a channel protein that rapidly transports water down a concentration gradient into or out of cells; found in various tissues (ex. proximal tubules of the kidneys, in erythrocytes)

96
Q

Why do kidneys have aquaporins?

A

So they don’t have to excrete huge amounts of water per day. 180 L of blood is filtered per day and 178.5 L of water is reabsorbed, resulting in only 1.5 L of urine per day.

97
Q

Describe aquaporins.

A

Tetrameric integral membrane proteins formed of 4 identical monomers; each monomer contains 6 helical transmembrane segments
Monomers form 4 identical water channels lined with amino acid residues with hydrophilic side chains; space in the center is blocked by a lipid molecule to prevent the passage of gas or ions

98
Q

Name 4 ways aquaporins ensure only water is allowed in or out of a cell.

A
  1. The space at the center of the tetramer is blocked by a lipid molecule, which prevents the passage of gas or ions.
  2. Each water channel is about 0.3 nm, only large enough for water molecules to pass through in a single file, one at a time.
  3. Amino acid residues lining the channel only allow H2O molecules through, repelling OH- and H3O+ ions.
  4. The positively charged arginine residues inside the channels repel the passage of H+ ions.
99
Q

What’s the rate of water flow through aquaporin channels?

A

Several billion per second

100
Q

What is vasopressin?

A

a hormone that increases the transcription and insertion of aquaporins into the apical plasma membrane of the epithelial cells in the collecting tubule and collecting duct of the kidney, increasing water reabsorption

101
Q

Why does alcohol act as a dieuretic?

A

It inhibits vasopressin. Rather than being collected by aquaporins in the plasma membrane of kidney epithelial cells, water must be excreted as urine.

102
Q

What is the Donnan effect?

A

Although it could be predicted that there would be an equal solute concentration inside and outside of the cell, we observe there is a significantly higher concentration inside the cell.

103
Q

What explains the Donnan effect?

A

The inside of the cell has many negatively charged molecules (ex. DNA, proteins) that must be electrically counterbalanced by positive ions.

104
Q

Why is the Donnan effect relevant for osmosis?

A

Large negatively charged macromolecules (DNA, RNA, protein) don’t contribute much to the osmolarity of the cell, but their counter ions do. The high ion concentration inside a cell encourages water to flow in, potentially bursting the cell.

105
Q

How do cells prevent themselves from bursting due to osmosis and the Donnan effect?

A
  1. Can use ATP-powered ion pumps to remove the ions (especially for mammalian cells)
  2. Can get rid of the water (ex. protozoan using contractile vacuoles)
  3. Cell walls provide a rigid structure that prevents bursting in fungal and plant cells
106
Q

What are the 3 major functions of active transport?

A
  1. Allows uptake of essential nutrients from environment, even when intracellular concentrations are already higher
  2. Allows substances to be removed from the cell or organelle even when the concentration in the environment is greater than the concentration inside
  3. Enables the cell to maintain constant, nonequilibrium intracellular concentrations of specific ions (K+, Na+, Ca2+, and H+)
107
Q

Describe the directionality of passive and active transport.

A

Simple and facilitated diffusion are nondirectional - solutes can move in either direction depending on the concentration or electrochemical gradient.
Active transport is (usually) inherently unidirectional - a system that moves a solute in one direction usually does not move the same solute in the opposite direction, regardless of gradients.

108
Q

What is direct active transport?

A

the accumulation of solute molecules or ions on one side of a membrane is directly coupled to an exergonic chemical reaction (usually ATP hydrolysis)

109
Q

What are transport ATPases?

A

transport proteins driven directly by ATP hydrolysis

110
Q

What are P-type ATPases?

A

pumps that are reversibly phosphorylated at a specific aspartic acid residue by ATP as part of an active transport mechanism

111
Q

Describe the structure of P-type ATPases.

A

8-10 transmembrane segments in a single polypeptide - crosses the membrane multiple times

112
Q

How do researchers identify P-type ATPases?

A

Competitive inhibition via the vanadate ion, which resembles the phosphate ion, competes with phosphorylation.

113
Q

Where are most P-type ATPases located?

A

plasma membrane

114
Q

What kind of solutes do P-type ATPases pump?

A

often ions, sometimes hydrophobic phospholipids

115
Q

What are V-type ATPases?

A

pumps protons into vacuoles, vesicles, lysosomes, endosomes, and the Golgi apparatus

116
Q

Why aren’t V-type ATPases inhibited by vanadate?

A

They do not undergo phosphorylation as part of the transport process.

117
Q

Describe the structure of a V-type ATPase.

A

2 multisubunit components
Integral subunit embedded within the membrane
Peripheral subunit that juts from membrane surface and contains the
ATP-binding site

118
Q

What are F-type ATPases?

A

pumps found in bacteria, mitochondria, and chloroplasts; involved in proton transport

119
Q

Describe the structure of F-type ATPases.

A

2 multisubunit complexes
Fo is the integral membrane component - transmembrane pore for protons
F1 is the peripheral membrane component, which includes the ATP binding site.

120
Q

What happens when F pumps are reversed?

A

The exergonic flow of protons down their gradient can drive ATP synthesis. Pumps are called ATP synthases.

121
Q

What do ATP synthases need to work?

A

a transmembrane proton gradient produced due to sugar oxidation (aerobic respiration) or solar radiation (photosynthesis)

122
Q

What is the ABC-type ATPase/ABC transporter?

A

ATP-driven pump with a catalytic domain that binds ATP; present in all organisms; often function as importers and exporters of VARIETY of solutes (as a class; individual transporters are specific to one solute or class of closely-related solutes)

123
Q

Describe ABC transporters.

A

four protein domains that are separate polypeptides (can be part of a large, single polypeptide); two are highly hydrophobic and embedded in the membrane; two are peripheral on the cytoplasmic side of the membrane
Both of 2 embedded domains has 6 membrane-spanning segments that form a channel for solutes to pass through.

124
Q

What role do ABC transporters play in medicine?

A

Some pump drugs out of the cell, making the cell resistant to the dug. Some tumors, with high concentrations of multidrug resistance transport proteins, are resistant to drugs that normally work to address growth.

125
Q

What do multidrug resistance transport proteins do?

A

MDR transport proteins use energy from ATP hydrolysis to pump hydrophobic drugs out of cells, reducing the cytoplasmic concentration of the drugs and their effectiveness. Very broad, so can provide resistance to multiple drugs.

126
Q

What drives direct active transport?

A

energy released from a chemical reaction, like ATP hydrolysis

127
Q

What drives indirect active transport?

A

movement of an ion (sodium ions in animals, protons in plants, fungi, and bacteria) down its electrochemical gradient

128
Q

Which part of the membrane does the sodium potassium pump operate at?

A

basolateral membrane

129
Q

Which part of the membrane does the 2 Na+/glucose symporter operate at?

A

apical membrane

130
Q

What is the ratio of potassium inside a mammalian neuron to potassium outside of a mammalian neuron?

A

30 inside:1 outside

131
Q

What is the ratio of sodium inside a mammalian neuron to sodium outside of a mammalian neuron?

A

0.08 inside :1 outside

132
Q

Which solute is moving up their electrochemical gradients with the sodium potassium pump?

A

Both sodium and potassium move up their electrochemical gradients.

133
Q

Which class of ATPase is the sodium potassium pump?

A

P type ATPase

134
Q

What direction does the sodium potassium pump pump potassium?

A

inside the cell, up the concentration gradient

135
Q

What direction does the sodium potassium pump pump sodium?

A

outside the cell, up the concentration gradient

136
Q

How many sodium and potassium ions are moved per molecule of ATP hydrolyzed?

A

3 Na+ moved out per 2 K+ moved in per 1 ATP molecule hydrolyzed

137
Q

Describe the Na+/K+ pump’s structure.

A

Trimeric transmembrane protein with an alpha, beta, and gamma subunit.
Alpha subunit forms the ion channel and contains binding sites for ATP and sodium ions inside the membrane and for potassium ions on the external side.
Beta subunit is glycosolated.

138
Q

Describe the transport mechanism of the sodium potassium pump.

A
  1. Three Na+ from inside the cell bind to E1 on the inner side of the membrane.
  2. Step 1 triggers autophosphorylation of the alpha subunit of the pump using bound ATP as the phosphate donor, causing a conformational change from E1 to E2.
  3. Bound Na+ ions are moved through the membrane to the external surface and released to the environment.
  4. K+ ions from the outside bind to the alpha subunit.
  5. Step 4 triggers dephosphorylation and a return to the E1 conformation while K+ ions are moved to the inner surface.
  6. ATP binding results in the K+ exit from the pump, providing the opportunity for the carrier to accept more Na+ ions.
139
Q

Why is the uptake of sodium ions by the Na+/glucose symporter exergonic?

A

Because a steep electrochemical gradient is maintained across the plasma membrane by the Na+/K+ pump

140
Q

Describe the mechanism for the Na+/glucose symporter.

A
  1. Two external Na+ ions bind to sites on the symporter.
  2. A molecule of glucose binds.
  3. A conformational change in the protein exposes the Na+ and glucose to the inner surface of the membrane.
  4. 2 Na+ ions dissociate due to the low intracellular Na+ ion concentration.
  5. Step 4 causes the transporter to be locked in the inward-facing conformation until the glucose dissociates.
  6. The empty transporter returns to the outward-facing conformation.

cellular biology (22 decks)

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