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BME 501 > Section 4 - Membrane Structure and Transport > Flashcards

Section 4 - Membrane Structure and Transport Flashcards

1
Q

True or false: a cell can exist without a membrane

A

False: it needs a boundary to distinguish itself

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2
Q

True or false: for proper functioning, the membrane must be immpermeable

A

False: it must be permeable, although this can be controlled

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3
Q

What is the purpose of the cell membrane?

A

Maintain the difference between inside and outside, and facilitate transport

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4
Q

What kinds of transport does the cell membrane facilitate?

A

Transfer of material, information, and energy

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5
Q

What is the structure of the cell membrane?

A

A think film of lipids and proteins

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6
Q

What holds together the cell membrane?

A

Noncovalent interactions

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7
Q

True or false: the cell membrane is static

A

False: it is a very dynamic structure

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8
Q

What do the proteins on the cell membrane do?

A

Function in signaling, transport, and connection to the cytosol

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9
Q

What is the average thickness of a cell membrane?

A

~5 nm

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10
Q

What is the cell membrane impermeable to?

A

Water-soluble molecules

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11
Q

What is the typical structure of a transmembrane protein?

A

Tend to have a large section on one end of the membrane, and barely poking out on the other side

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12
Q

How much lipid (by mass) composes the cell membrane?

A

~50%

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13
Q

True or false: all lipids in the cell membrane are amphiphilic

A

True: they all have hydrophobic regions and hydrophilic regions

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14
Q

What does amphiphilic mean?

A

A molecule has both hydrophobic regions and hydrophilic regions

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15
Q

What are the majority of the lipids in the cell membrane?

A

Phospholipids

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16
Q

What is the structure of a phospholipid?

A

Two hydrocarbon tails and a polar head group are connected via glycerol

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17
Q

What are the majority of the phospholipids (in terms of their tails)?

A

Have one saturated tail and one unsaturated tail

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18
Q

What does an unsaturated lipid tail look like?

A

Kinked

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19
Q

What does a saturated lipid tail look like?

A

Straight

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20
Q

How long is the average carbon chain in a lipid tail?

A

16-18 carbons long

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21
Q

What happens if a cell membrane has only saturated phospholipids?

A

It increases packing density, thus decreasing fluidity

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22
Q

What is the significance of having some unsaturated lipid tails in the cell membrane?

A

Helps reduce packing density, and thus increases fluidity

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23
Q

What is the most common head group on phospholipids?

A

Choline

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24
Q

What percentage of phospholipids in the cell membrane are phosphatidylcholine?

A

60-70%

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25
Q

What is the net charge on a choline side chain?

A

Positive

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26
Q

Where does the positive net charge on a choline come from?

A

A nitrogen with three methyl groups and another bond

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27
Q

What is the net charge of phosphatidylcholine?

A

Neutral

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28
Q

What is the second most common head group on phospholipids?

A

Serine

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29
Q

What percentage of phospholipids in the cell membrane are phosphatidylserine?

A

30%

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30
Q

What is the net charge on a serine side chain?

A

Neutral

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31
Q

Where does the neutral net charge on a serine come from?

A

A negative carboxyl group, and a positive amino group

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32
Q

What s the net charge of phosphatidylserine?

A

Negative

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33
Q

What is the third most common head group on phospholipids?

A

Ethanolamine

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34
Q

What is the net charge of an ethanolamine side chain?

A

Positive

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35
Q

Where does the net positive charge on an ethanolamine come from?

A

A positive amino group

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36
Q

What is the net charge of phosphatidylethanolamine?

A

Neutral

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37
Q

What is a membrane potential?

A

A separation of charge across a cell membrane

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38
Q

What is a typical value of a membrane potential?

A

-70 mV

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39
Q

Which leaflet is PS found?

A

The inner leaflet

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40
Q

Why is PS found on the inner leaflet?

A

It contributes to the negative membrane potential

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41
Q

What is a marker of cell death?

A

PS found on the outer leaflet of the cell membrane

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42
Q

What does cholesterol do?

A

Helps to locally increase the rigidity of the membrane

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43
Q

Where is cholesterol found in the cell membrane?

A

Between the phospholipids

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44
Q

What happens if there is no cholesterol in the cell membrane?

A

The membrane bursts

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45
Q

Why does a cell membrane burst if there is no cholesterol?

A

The membrane is too flimsy without cholesterol providing local rigidity

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46
Q

What phospholipid motions are possible in the cell membrane?

A

Rotate (spin), flex (open/close tails), lateral diffusion (side to side), and flip-flop (switch leaflets)

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47
Q

Which phospholipid motion is the most rare?

A

Flip-flopping

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48
Q

Why is flip-flopping of phospholipids rare?

A

Need to bring the hydrophilic head group through a hydrophobic cell membrane

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49
Q

What is needed to help phospholipids flip-flop?

A

Proteins

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50
Q

What does scramblase do?

A

Moves a phospholipid from the inner leaflet to the outer leaflet

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51
Q

What does flippase do?

A

Moves a phospholipid from the outer leaflet to the inner leaflet

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52
Q

Which enzyme moves a phospholipid from the inner leaflet to the outer leaflet?

A

Scramblase

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53
Q

Which enzyme moves a phospholipid from the outer leaflet to the inner leaflet?

A

Flippase

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54
Q

What is the overall fluidity of the cell membrane influenced by?

A

Composition (cholesterol), types of side chains (saturated vs. unsaturated), and temperature

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55
Q

If the membrane is thicker, what can you say about the lipid tails?

A

They are saturated

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56
Q

What is a lipid raft?

A

A specialized structure that are fairly rigid

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57
Q

What is contained in a lipid raft?

A

Cholesterol, transmembrane proteins, and anchoring proteins

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58
Q

True or false: lipid rafts are rigid structures

A

True: they have many proteins and cholesterol which makes them rigid

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59
Q

Why are many anchoring proteins found on a lipid raft?

A

This is a rigid structure within the more fluid cell membrane

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60
Q

True or false: proteins are only found in lipid rafts

A

False: they can be in other areas of the cell membrane too

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61
Q

True or false: the leaflets of the cell membrane are symmetrical

A

False: they are asymmetrical

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62
Q

What comprises the majority of the outer leaflet?

A

Phosphatidylcholine

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63
Q

What comprises the majority of the inner leaflet?

A

Phosphatidylserine

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64
Q

What is the purpose of sugar linked phospholipids?

A

Self recognition

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65
Q

Where are sugar linked phospholipids found?

A

The outer leaflet

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66
Q

Which phospholipid is important in cell signaling?

A

Phosphatidylinositol

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67
Q

What does PI3K do?

A

Adds phosphate groups to PI to make PIP3

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68
Q

What does phospholipase do?

A

Cleave PIP2 (head group) to create other second messengers

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69
Q

What is the importance of phosphorylating PI?

A

This can lead to protein docking sites or creating of second messengers, both for cell signaling

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70
Q

What are glycolipids?

A

Lipids that contain sugar groups

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71
Q

What is the function of glycolipids?

A

Alter the charge distribution, protect from harsh environments (pH, shear), and self recognition

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72
Q

True or false: membrane bound proteins are highly regular among different cell types

A

False: they are highly variable in type and quantity among different cell types

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73
Q

True or false: membrane proteins are highly regulated

A

True: this is similar to gene expression and protein synthesis

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74
Q

What is a transmembrane protein?

A

A protein that passes through the membrane

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75
Q

What is a single-pass protein?

A

A protein that passes through the cell membrane once

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76
Q

What is a multi-pass protein?

A

A protein that passes through the cell membrane multiple times

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77
Q

True or false: transmembrane proteins must be amphipathic

A

True: they must have hydrophilic regions and hydrophobic regions

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78
Q

Why do transmembrane proteins need to be amphipathic?

A

They need to interact with both the aqueous environment and the cell membrane environment

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79
Q

True or false: multi-pass proteins can be parallel or antiparallel

A

False: they can only be antiparallel

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80
Q

What is a beta barrel?

A

A protein channel composed of criss-crossing beta sheets

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81
Q

True or false: not all membrane proteins pass through the membrane

A

True: some are found only in the interior or exterior of the cell

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82
Q

How are proteins that are found only in the interior or exterior of the cell linked to the cell membrane?

A

Through covalent linkages, or their amphipathic nature

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83
Q

True or false: peripheral proteins directly interact with the cell membrane

A

False: they are attached through linker proteins

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84
Q

True or false: peripheral proteins can be released from the membrane

A

True: this can be through chemical or mechanical stimulation

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85
Q

True or false: integral proteins directly interact with the cell membrane

A

True: they typically modify the bilayer

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86
Q

True or false: integral proteins can be released from the membrane

A

False: integral proteins stay associated with the cell membrane

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87
Q

What structures are commonly seen in transmembrane proteins?

A

Alpha helices and beta sheets

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88
Q

What are hydropathy plots?

A

Plots that show regions of hydrophobicity and hydrophilicity in a protein

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89
Q

What is the purpose of a hydropathy plot?

A

Determine the number of passes a transmembrane protein makes

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90
Q

True or false: hydropathy plots are step functions

A

False: there are changes in polarity close to the membrane pass

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91
Q

True or false: multiple alpha helices can dictate the function of a protein

A

True: they can come together to dictate this function

92
Q

How can multiple alpha helices come together to dictate the function of a protein?

A

They can form a channel to regulate movement, or set up charge distributions

93
Q

What is the function of beta barrels?

A

Act as transport proteins

94
Q

True or false: beta barrels are not active

A

True: they are fairly rigid

95
Q

How come beta barrels are not active?

A

Their rigid structure prevents conformational changes

96
Q

True or false: alpha helices are not active

A

False: they can move into active and inactive conformations

97
Q

How come alpha helices are active?

A

They can move about each other for functional changes

98
Q

Where are beta barrels localized?

A

Bacteria, mitochondria, and chloroplasts

99
Q

What are the characteristics of the inside of the pore?

A

Hydrophilic

100
Q

Why are the insides of pores hydrophilic?

A

They need to interact with water to let molecules pass through

101
Q

What are the characteristics of the exterior of the beta barrel?

A

Hydrophobic

102
Q

Why are the exteriors of beta barrels hydrophobic?

A

They have to interact with the lipophilic cell membrane

103
Q

How are amino acids arranged in alpha helices (in terms of transmembrane proteins)?

A

One side of the turn has hydrophobic amino acids, and the other side of the turn has hydrophilic amino acids

104
Q

How are amino acids arranged in beta sheets (in terms of transmembrane proteins)?

A

One zig has hydrophobic amino acids, while the zag has hydrophilic amino acids

105
Q

True or false: proteins that face the external environment are rarely glycosylated

A

False: they are commonly glycosylated with oligosaccharides

106
Q

What is a common example of a glycoprotein?

A

The glycocalyx

107
Q

What is the function of the glycocalyx?

A

Used for cell signaling and cell recognition

108
Q

What is the structure of the sugars on glycoproteins?

A

Highly branched with many sugar molecules

109
Q

True or false: most membrane proteins function as a single unit

A

False: most membrane proteins function as a complex

110
Q

What comprises the membrane protein complexes?

A

Transmembrane proteins, cytosolic proteins, and extracellular proteins

111
Q

True or false: membrane protein complexes are static structures

A

False: they can have movement within the membrane

112
Q

What type of movement is possible for membrane protein complexes?

A

Rotation and lateral translation

113
Q

What type of movement is not possible for membrane protein complexes?

A

Flip-flopping

114
Q

True or false: proteins that are found in lipid rafts stay in lipid rafts

A

False: proteins can enter and leave lipid rafts

115
Q

Why is flip-flopping not possible for membrane proteins?

A

Need to move large hydrophilic regions through the cell membrane

116
Q

True or false: if a membrane protein could flip-flop, it would function similarly

A

False: it would probably not function

117
Q

Why would flipping a membrane protein not result in a similar function?

A

It may only work when certain molecules are on one particular side of the membrane (and attaches to a specific domain of the protein)

118
Q

True or false: membrane proteins are uniformly distributed along the cell membrane

A

False: proteins can be localized in specific portions of the cell membrane

119
Q

What is the consequence of membrane proteins being localized to a specific region of the cell membrane?

A

This can lead to the cell membrane performing different functions

120
Q

What is an example of a cell that localizes its cell membrane proteins to achieve a unique function?

A

Epithelial cells

121
Q

What is the role of an epithelial cell?

A

Act as a barrier

122
Q

What is the function of the tight junctions in epithelial cells?

A

Prevent movement of fluids and cell membrane proteins between the apical and basolateral domains

123
Q

What is the purpose of the cytoskeleton?

A

Give membrane its strength

124
Q

What gives the cell membrane its strength?

A

The cytoskeleton

125
Q

How does the cytoskeleton give the membrane its strength?

A

Providing a site for membrane components to anchor to

126
Q

What is an example of the cytoskeleton giving the membrane its shape?

A

Red blood cells have a unique shape (concave disc), mediated by the cytoskeleton

127
Q

What molecules are excluded due to the cell membrane?

A

Most polar molecules

128
Q

Why are transport proteins needed?

A

Many polar molecules that cannot cross the cell membrane are necessary to survive

129
Q

True or false: some cells can spend 2/3 of their metabolic energy on transport

A

True: these cells are usually excitable

130
Q

Why are there no “oxygen transporters” in the cell membrane?

A

Oxygen is nonpolar, and thus can cross the cell membrane without the need of a transport protein

131
Q

What is the consequence of recreational drugs being small and nonpolar?

A

They can diffuse through the membrane easily

132
Q

What kinds of cells spend a lot of energy on transport?

A

Excitable cells

133
Q

Why do excitable cells spend a lot of energy on transport?

A

Need proper ion concentration gradients function properly when excited

134
Q

When happens if ion gradients are not maintained through transport?

A

The cell will die

135
Q

True or false: given enough time, almost any molecule can cross the bilayer down its concentration gradient

A

True: however, this can be a very slow and inefficient process

136
Q

What are the fastest molecules that can diffuse through the cell membrane?

A

Small, lipophilic molecules

137
Q

What types of proteins do membrane transport proteins transport?

A

Polar molecules

138
Q

True or false: all membrane transport proteins are multi-pass proteins

A

True: this structure is needed for proper transport

139
Q

Why are all membrane transport proteins multi-pass proteins?

A

They need regions of both hydrophobicity and hydrophilicity in order to function

140
Q

What is another name for a carrier?

A

A permease

141
Q

How do carriers work?

A

A solute binds, causing a conformational change to transport the solute

142
Q

How do channels work?

A

Solutes go through the channel down their concentration gradient

143
Q

Which membrane proteins are easiest to control?

A

Carriers and pumps

144
Q

What is facilitated diffusion?

A

Molecules move through a channel protein down its concentration gradient

145
Q

What is active transport?

A

Energy is required to move a molecule up its concentration gradient

146
Q

What is the electrochemical gradient?

A

The electrical potential difference also determines how solutes will move across a membrane

147
Q

What types of proteins utilize passive transport?

A

Channels

148
Q

What types of proteins utilize active transport?

A

Pumps

149
Q

What is the energy source for active transport?

A

ATP, or an electrochemical gradient

150
Q

True or false: the solute is changed when it moves through a transporter

A

False: the solute does not change shape, the protein does

151
Q

True or false: the conformational changes of the transporter are reversible

A

True: this allows for the protein to transport solutes multiple times

152
Q

What are the types of active transporters?

A

Coupled transporters, ATP-driven pumps, and light-driven pumps

153
Q

How do coupled transporters work?

A

The uphill transfer of one solute is coupled to the downhill transfer of another solute

154
Q

How do ATP-driven pumps work?

A

ATP hydrolysis is coupled with a solute moving up its concentration gradient

155
Q

How do light-driven pumps work?

A

Input of energy from light is coupled with a solute moving up its concentration gradient

156
Q

Why are coupled transporters used a lot in the cell?

A

They are an indirect use of energy (concentration gradient)

157
Q

What is a uniport transporter?

A

Moves one solute up its concentration gradient

158
Q

What is a symport transporter?

A

Two molecules move together across the cell membrane

159
Q

What is an antiport transporter?

A

Two molecules move to opposite sides of the cell membrane

160
Q

Which cells use a lot of symports and antiports?

A

Epithelial cells

161
Q

What is an example of a cotransporter?

A

The glucose transporter

162
Q

How does the glucose transporter work?

A

Sodium moving down its concentration into the cell is coupled to glucose moving up its gradient into the cell

163
Q

What kind of coupled transporter is the glucose transporter?

A

Symport

164
Q

How do epithelial cells achieve transcellular transport?

A

Through distributing transporters non-uniformly

165
Q

What transporter is seen at the apical domain of the epithelial cell?

A

A glucose-sodium symport, bringing both of these solutes into the cell

166
Q

What transporter is seen at the basolateral domain of the epithelial cell?

A

A glucose channel, and a sodium-potassium pump

167
Q

How does transcellular transport in epithelial cells work?

A
  1. A glucose-sodium symport brings sodium and glucose into the cell (down sodium’s concentration gradient, and up glucose’s concentration gradient) at the apical domain
  2. Glucose is released through a glucose channel at the basolateral domain (down its concentration gradient)
  3. Sodium is pumped back out of the cell through a sodium/potassium pump (up its concentration gradient)
168
Q

At the apical domain, how is glucose moving (in terms of its concentration gradient and direction)?

A

Up its concentration gradient into the cell

169
Q

At the apical domain, how is sodium moving (in terms of its concentration gradient and direction)?

A

Down its concentration gradient into the cell

170
Q

At the basolateral domain, how is glucose moving (in terms of its concentration gradient and direction)?

A

Down its concentration gradient out of the cell

171
Q

At the basolateral domain, how is sodium moving (in terms of its concentration gradient and direction)?

A

Up its concentration gradient out of the cell

172
Q

What drives glucose into the epithelial cell?

A

The movement of sodium down its concentration gradient

173
Q

What drives sodium into the epithelial cell?

A

Its concentration gradient

174
Q

What drives glucose out of the epithelial cell?

A

Its concentration gradient

175
Q

What drives sodium out of the epithelial cell?

A

An ATP-driven pump

176
Q

How come the glucose-sodium symport in epithelial cells cannot be flipped?

A

There is little sodium inside the cell to drive the transport

177
Q

How come the glucose channel cannot be at the apical domain?

A

This will cause the glucose to leave the cell, reversing the work of the glucose-sodium symport

178
Q

What is another name for ATP pumps?

A

Transport ATPases

179
Q

How do transport ATPases get their energy to function?

A

Through the hydrolysis of ATP into ADP and Pi

180
Q

What are the three classes of ATP pumps?

A

P-type, F-type, and ABC transporter

181
Q

What are P-type pumps?

A

ATP pumps that can phosphorylate themselves

182
Q

What are P-type pumps used for?

A

Maintaining ion gradients across the cell membrane

183
Q

What are F-type pumps?

A

ATP pumps that use a proton gradient to produce ATP

184
Q

Where are F-type pumps found?

A

In the inner matrix of the mitochondria

185
Q

What are ABC transporters?

A

ATP pumps that transport small molecules across the cell membrane

186
Q

What is a V-type pump?

A

An ATP pump that uses ATP to pump protons against their concentration gradient

187
Q

What type of pump is a V-type pump similar to?

A

An F-type pump

188
Q

What is the difference between a V-type pump and an F-type pump?

A

The direction that they work (use ATP or make ATP)

189
Q

Which pumps can phosphorylate themselves?

A

P-type pumps

190
Q

Which pumps act like a turbine to produce ATP?

A

F-type pumps

191
Q

Which pumps transport small molecules across a membrane?

A

ABC transporters

192
Q

Which pumps act like a turbine that uses ATP?

A

V-type pumps

193
Q

What engineering structure resembles the F-type pump?

A

A turbine

194
Q

What type of pump is the calcium pump?

A

A P-type pump

195
Q

When the calcium pump is unphosphorylated, what is its structure?

A

Two alpha helices are not in contact (closed state), allowing for calcium to bind

196
Q

When the calcium pump is phosphorylated, what is its structure?

A

A conformational change pushes calcium out of the cell, against its gradient

197
Q

What does the sodium-potassium ATPase do?

A

Moves 3 sodium out of the cell, and moves 2 potassium into the cell

198
Q

True or false: the sodium-potassium ATPase uses ATP to drive both sodium and potassium up their concentration gradients

A

True: this resets the ion gradient, which is important for excitable cells

199
Q

What kinds of cells are sodium-potassium ATPases critical for?

A

Excitable cells

200
Q

Why are sodium-potassium ATPases critical for excitable cells?

A

Need to be able to reset concentration gradients that drive action potentials

201
Q

What determines whether sodium or potassium will bind to the sodium-potassium ATPase?

A

The conformation of the pump (which changes throughout the cycle)

202
Q

What is the rate limiting step for the calcium pump?

A

Presence of calcium

203
Q

Why is the presence of calcium the rate limiting step for the calcium pump?

A

The cytosolic concentration of calcium is so low, so the pump is always ready

204
Q

What is the rate limiting step for the sodium-potassium ATPase?

A

The binding of sodium

205
Q

Why is the binding of sodium the rate limiting step for the sodium-potassium ATPase?

A

Need to bind three ions close together in the pocket of the pump

206
Q

How much ATP is needed to drive an ABC transporter?

A

2 ATP molecules

207
Q

What is the typical structure of an ABC transporter?

A

A dimer

208
Q

What is an example of a general ion channel?

A

Cation / anion channels, etc.

209
Q

What is an example of a specific ion channel?

A

Sodium / potassium channels, etc.

210
Q

Which types of transporters are the fastest?

A

Ion channels

211
Q

Why are ion channels the fastest transporter?

A

No conformational change is needed to function

212
Q

Which is larger: sodium or potassium?

A

Potassium

213
Q

What question can be raised about potassium channels letting sodium through?

A

How can the potassium ion channel exclude sodium if potassium is bigger than sodium?

214
Q

What is a selectivity filter?

A

Interactions between the channel proteins to allow only certain ions to pass through

215
Q

Compared to the ion, what size should the channel protein be?

A

Enough for force interactions between the ion and the channel protein (selectivity filter)

216
Q

What is the state of ions in a biological environment?

A

Hydrated with water

217
Q

What is the key to understanding the solution to the problem of sodium passing through the potassium channel?

A

Ions must shed all of their hydrating ions

218
Q

What does it mean for an ion channel to be gated?

A

It can be turned on or off

219
Q

How can a gated ion channel be turned on or off?

A

Through voltage, chemical, or mechanical stimulation

220
Q

Why do ions associate with water?

A

It lowers the energetics of the ions (more stable)

221
Q

How do voltage gated ion channels work?

A

They have highly charged regions, which can restrict or open the ion channel

222
Q

What is the structure of the selectivity filter for potassium?

A
  1. It has negative amino acids to attract the potassium cation
  2. The carbonyl oxygens are spaced enough to bind to potassium when it sheds its water molecules
223
Q

What is used to match the hydration of the ions?

A

The carbonyl oxygens on the amino acids

224
Q

Why are carbonyl oxygens used in the selectivity filter?

A

They resemble the oxygens on water (hydration)

225
Q

What happens if a hydrated sodium tries to enter a potassium channel?

A

It is too big to fit through the pores

226
Q

What happens if a nonhydrated sodium tries to enter a potassium channel?

A

It is too small to interact with all of the carbonyl oxygens, and thus is energetically unfavorable (will not enter the pore)

227
Q

What are the differences in structure between a cation channel and an anion channel?

A
  1. The cation channel has four subunits, while the anion channel has two
  2. The cation selectivity filter is at the edge, while the anion selectivity filter is in the middle

BME 501 (5 decks)

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