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BIOL4002 - Cell biology > Membrane Structure, Synthesis and Transport > Flashcards

Membrane Structure, Synthesis and Transport Flashcards

1
Q

What is membrane structure?

A

Refers to the arrangement and composition of the lipid bilayer that forms the outer boundary of cells and separates them from their environment.

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2
Q

What is the main component of membranes?

A

phospholipid bilayer

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3
Q

What is the structure of the phospholipid molecule?

A

Generally consists of:
- 2 fatty acid “tails”
- a “head” consisting of a phosphate group
(slide 3)

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4
Q

What are phospholipids?

A

Phospholipids are ‘amphipathic’ molecules that contain:
- a hydrophobic, water-avoiding, non-polar region (the tails)
- a hydrophilic, water-seeking, polar region (the head)

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5
Q

What is the fluid-mosaic model?

A

A model that describes the structure of the cell membrane as a mosaic of lipid, protein, and carbohydrate molecules in a fluid-like arrangement.

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6
Q

What do membranes contain besides lipids?

A

Membranes also contain proteins and carbohydrates

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7
Q

Why is a membrane considered a mosaic?

A

A membrane is considered a mosaic because it is composed of a variety of different types of lipid, protein, and carbohydrate molecules.

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8
Q

Why is a membrane described as fluid?

A

Because the lipids and proteins can move relative to each other within it

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9
Q

Are the two leaflets of the lipid bilayer symmetrical?

A

No, the two leaflets (inner and outer faces of the bilayer) are asymmetrical, with different types of each component.

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10
Q

What are the proteins associated with membranes?

A
  • integral or intrinsic membrane proteins
  • peripheral or extrinsic membrane proteins
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11
Q

What are integral or intrinsic membrane proteins?

A

Proteins that are physically embedded within the membrane and span the entire phospholipid bilayer

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12
Q

What are the 2 types of integral or intrinsic membrane proteins?

A
  • transmembrane proteins
  • lipid-anchored proteins
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13
Q

What are transmembrane proteins?

A

Type of integral membrane protein that:
- spans the entire phospholipid bilayer
- has regions that face both the extracellular & intracellular environments

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14
Q

What are lipid-anchored proteins?

A

Type of integral membrane protein that:
- have an amino acid covalently attached to a lipid in a membrane

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15
Q

What are peripheral or extrinsic membrane proteins?

A

Proteins that are bound to projecting regions of integral membrane proteins that project out from the membrane, or to polar head groups of phospholipids.

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16
Q

How can computer programs predict when a protein will be a transmembrane protein?

A

Computer programs can use algorithms to:
- analyse the protein sequence
- predict whether it has the characteristics necessary to span a lipid bilayer (e.g. hydrophobic & hydrophilic regions)

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17
Q

What percentage of all genes may encode transmembrane proteins?

A

20-30% of all genes may encode transmembrane proteins

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18
Q

In which domains of life are transmembrane proteins found?

A

Transmembrane proteins are found throughout all domains of life, including archaea, bacteria, and eukaryotes.

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19
Q

What is the function of many transmembrane proteins?

A

The function of many transmembrane proteins remains unknown

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20
Q

What is Transmission Electron Microscopy (TEM) used for in visualising membranes?

A

TEM is used to visualise membranes in biological samples by thin sectioning and staining them with heavy-metal dyes

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21
Q

What is the process of staining membranes in TEM?

A
  • biological sample is stained with heavy-metal dyes
  • which bind tightly to the polar head groups of phospholipids (but not to the fatty acyl chains)
  • results in the membranes appearing as dark lines resembling railroad tracks
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22
Q

What part of the phospholipids do the heavy-metal dyes bind to?

A

The heavy-metal dyes bind tightly to the polar head groups of phospholipids.

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23
Q

What is Freeze Fracture Electron Microscopy?

A

Freeze Fracture Electron Microscopy is a specialised form of TEM used to analyse the interior of the phospholipid bilayer.

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24
Q

What is the process of Freeze Fracture Electron Microscopy?

A
  • sample is frozen in liquid nitrogen & fractured with a knife
  • due to weakness of the central membrane, leaflets separate into the P face (Protoplasmic face next to the cytosol) and the E face (Extracellular face)
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25
Q

What information can be obtained through Freeze Fracture Electron Microscopy?

A

This technique can provide significant detail about the form and organisation of membrane proteins, including their orientation and distribution within the membrane.

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26
Q

What is the P face in Freeze Fracture Electron Microscopy?

A

Protoplasmic face, which is the face next to the cytosol

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27
Q

What is the E face in Freeze Fracture Electron Microscopy?

A

Extracellular face, which is the face facing the external environment

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28
Q

Are membranes fluid or rigid?

A

Membranes are semifluid

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29
Q

What does it mean for a membrane to be semifluid?

A

A semifluid membrane means that:
- most lipids can rotate freely around their long axes
- move laterally within the membrane leaflet

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30
Q

Does “flip-flop” of lipids occur spontaneously in a membrane?

A

“Flip-flop” of lipids from one leaflet to the opposite leaflet does not occur spontaneously

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31
Q

What is the enzyme responsible for transporting lipids between leaflets in a membrane?

A

flippase

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32
Q

Does flippase require ATP to transport lipids between leaflets in a membrane?

A

Yes, flippase requires ATP to transport lipids between leaflets in a membrane

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33
Q

What are lipid rafts?

A

Lipid rafts are
- groups of lipids
- that associate strongly with each other
- to form a distinct unit
- within the larger sea of lipids in the membrane

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34
Q

How do lipids in a lipid raft differ from the rest of the membrane?

A

The lipid composition of a raft is different than within the rest of a membrane.
- e.g. a lipid raft might have a higher conc of cholesterol and a unique set of membrane proteins

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35
Q

Do all membranes contain lipid rafts?

A

No, not all membranes contain lipid rafts

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36
Q

What are some factors that affect membrane fluidity?

A
  • length of fatty acyl tails
  • presence of double bonds
  • presence of cholesterol
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37
Q

How does the length of fatty acyl tails affect membrane fluidity?

A

Shorter acyl tails are less likely to interact, which makes the membrane more fluid

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38
Q

How does the presence of double bonds affect membrane fluidity?

A

The presence of double bonds:
- creates a kink in the fatty acyl tail
- making it more difficult for neighboring tails to interact
- making the bilayer more fluid

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39
Q

How does cholesterol affect membrane fluidity?

A

Cholesterol tends to
- stabilise membranes & reduce their fluidity at high temperatures
- while increasing their fluidity at low temperatures

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40
Q

What did the Larry Frye and Michael Edidin experiment demonstrate?

A

The Larry Frye and Michael Edidin experiment demonstrated the lateral movement of membrane proteins

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41
Q

What was the setup of the Frye and Edidin experiment?

A
  • mouse and human cells were fused
  • temperature was either lowered to 0°C or raised to 37°C
  • mouse membrane protein ‘H-2’ was fluorescently labeled
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42
Q

What was the result of the Frye and Edidin experiment at 0°C?

A

At 0°C, the fluorescent label stayed on the mouse side of the fused cell

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43
Q

Q: What was the result of the Frye and Edidin experiment at 37°C?

A

At 37°C, the fluorescent label moved over the entire fused cell

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44
Q

Can all integral membrane proteins move laterally within the membrane?

A

No
- depending on cell type, 10–70% of membrane proteins may be restricted in their movement
- due to being bound to components of the cytoskeleton
- or being attached to molecules outside the cell

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45
Q

How do eukaryotes synthesise lipids?

A
  • eukaryotes synthesise lipids by working together with the cytosol and endomembrane system
  • fatty acid building blocks are made by enzymes in the cytosol or taken into cells from food
  • this process occurs at the cytosolic leaflet of the smooth ER
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46
Q

What do the cytosol and endomembrane system synthesise in eukaryotes?

A

The cytosol and endomembrane system work together to synthesize lipids in eukaryotes.

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47
Q

Where are the fatty acid building blocks for lipid synthesis made?

A

The fatty acid building blocks for lipid synthesis are made by enzymes in the cytosol or taken into cells from food

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48
Q

Where does the process of synthesizing lipids occur in the Endoplasmic Reticulum (ER)?

A

The process of synthesizing lipids occurs at the cytosolic leaflet of the smooth ER

49
Q

How do lipids in the ER membrane diffuse into other membranes?

A

Lipids in the ER membrane can diffuse laterally into other membranes, such as the nuclear envelope

50
Q

How are lipids transported to different organelles?

A

Lipids are transported, via vesicles, to different organelles, such as:
- the Golgi apparatus
- lysosomes
- vacuoles
- plasma membrane

51
Q

What are lipid exchange proteins?

A

Proteins that
- extract lipids from one membrane
- insert them into another
- allowing for the transfer of lipids between different membranes

52
Q

How are transmembrane proteins synthesized?

A

Most transmembrane proteins are directed to the ER membrane first, via co-translational insertion

53
Q

What happens to transmembrane proteins after they are synthesized in the ER?

A

From the ER, transmembrane proteins can be transferred via vesicles to other membranes of the cell

54
Q

Where does the synthesis of transmembrane proteins occur?

A

The majority of transmembrane proteins are synthesized in the ER membrane

55
Q

What is glycosylation?

A

Glycosylation is the process of covalently attaching a carbohydrate to a protein or lipid

56
Q

What is a glycolipid?

A

A glycolipid is a carbohydrate attached to a lipid

57
Q

What is a glycoprotein?

A

A glycoprotein is a carbohydrate attached to a protein

58
Q

What is the function of attached carbohydrates in glycosylation?

A

Attached carbohydrates can:
- serve as recognition signals for other cellular proteins
- often playing a role in cell surface recognition
- also help protect proteins from damage

59
Q

How can carbohydrates be added to proteins?

A

2 different ways:
- N-linked glycosylation
- O-linked glycosylation

60
Q

What is N-linked glycosylation?

A

N-linked glycosylation is the attachment of a carbohydrate to a N atom in the side chain of the amino acid asparagine.
- occurs in the endoplasmic reticulum

61
Q

What is O-linked glycosylation?

A

O-linked glycosylation is the addition of carbohydrates to an O atom in the side chain of the amino acids serine or threonine.
- occurs only in the Golgi apparatus

62
Q

What is the selective permeability of the plasma membrane?

A

The plasma membrane is selectively permeable, which means it allows some ions and molecules to pass through but not others.

63
Q

What is the purpose of selective permeability?

A

Selective permeability ensures that:
- essential molecules can enter the cell
- metabolic intermediates remain within the cell
- waste products can exit the cell

64
Q

What are the ways in which molecules move across membranes?

A

There are two main ways in which molecules move across membranes:
- passive transport
- active transport

65
Q

What is passive transport?

A

Passive transport is
- the movement of molecules across a membrane without the input of energy
- occurs down a gradient, from areas of higher concentration to lower concentration of the solute

66
Q

What are the two types of passive transport?

A

The two types of passive transport are:
- passive diffusion
- facilitated diffusion

67
Q

What is passive diffusion?

A

Passive diffusion is the diffusion of a solute through a membrane without interaction with a transport protein

68
Q

What is facilitated diffusion?

A

Facilitated diffusion is the diffusion of a solute through a membrane with the aid of a transport protein

69
Q

What is active transport?

A

Active transport is the movement of molecules:
- across a membrane that requires energy
- occurs up a gradient, from areas of lower concentration to higher concentration of the solute

70
Q

What is the phospholipid bilayer barrier?

A

The phospholipid bilayer barrier refers to
- the two layers of phospholipids that make up the plasma membrane of cells
- which act as a barrier to hydrophilic molecules and ions due to their water-repelling interior

71
Q

What factors affect the rate of diffusion of solute molecules through the phospholipid bilayer barrier?

A

The rate of diffusion of solute molecules through the phospholipid bilayer barrier depends on the chemistry of the solute and its concentration

72
Q

What molecules have high permeability through the phospholipid bilayer barrier?

A

Gases and small uncharged molecules

73
Q

What molecules have moderate permeability through the phospholipid bilayer barrier?

A

Water and urea

74
Q

What molecules have low permeability through the phospholipid bilayer barrier?

A

Polar organic molecules

75
Q

What molecules have very low permeability through the phospholipid bilayer barrier?

A

Ions, charged polar molecules, and large molecules

76
Q

Sum up the range of permeability of molecules through the phospholipid bilayer barrier

A

high permeability = gases and small uncharged molecules
moderate permeability = water and urea
low permeability = polar organic molecules
very low permeability = ions, charged polar molecules, and large molecules

77
Q

Provide an example of how the phospholipid bilayer barrier selectively allows molecules to pass through

A
  • diethylurea diffuses 50 times faster through the bilayer than urea, despite being larger
  • due to its nonpolar ethyl groups
  • this demonstrates how the bilayer selectively allows nonpolar molecules to pass through more easily than polar molecules
78
Q

What is the concept of maintaining gradients in cells?

A

Cells maintain gradients by keeping a relatively constant internal environment that differs from their external environment

79
Q

What is a transmembrane gradient?

A

A transmembrane gradient is
- a concentration difference of a solute across a membrane
- where the concentration is higher on one side of the membrane than the other

80
Q

What is an ion electrochemical gradient?

A

An ion electrochemical gradient is a gradient that combines:
- an electrical gradient, which is due to the ionic charge
- a chemical gradient, which is due to differences in solute concentration

81
Q

What is an isotonic solution?

A

An isotonic solution is one in which
- the concentration of solutes is equal on both sides of a membrane
- resulting in equal water concentration

82
Q

What is a hypertonic solution?

A

A hypertonic solution is one in which the concentration of solutes is
- higher (and the water concentration lower) on one side of a membrane
- resulting in water moving from an area of higher to lower concentration

83
Q

What is a hypotonic solution?

A

A hypotonic solution is one in which the concentration of solutes is
- lower (and the water concentration higher) on one side of a membrane
- resulting in water moving from an area of higher to lower concentration

84
Q

What is osmosis in relation to membrane transport?

A

Osmosis is the movement of water through
- a selectively permeable membrane
from an area of higher water concentration to an area of lower water concentration

85
Q

How can water movement through osmosis affect the size of a cell?

A

Water movement through osmosis can cause the cell
- shrink as water leaves the cell
- swell as or enters the cell

86
Q

What is osmotic pressure?

A

Osmotic pressure is the tendency for water to move into a cell
- due to differences in solute concentration between the cell and its surrounding environment
- it is the pressure required to stop the net movement of water
- through a selectively permeable membrane

87
Q

How does osmosis in animal cells affect their size and shape?

A

Animal cells must maintain a balance between extracellular and intracellular solute concentrations to maintain their size and shape.

88
Q

What is crenation?

A

Crenation is the shrinkage of a cell when it is placed in a hypertonic solution

89
Q

What is osmotic lysis?

A

Osmotic lysis is the swelling and bursting of a cell when it is placed in a hypotonic solution

90
Q

How does osmosis in plant cells affect their size and shape?

A

The plant cell wall prevents major changes in cell size

91
Q

What is turgor pressure?

A

The pressure that is created by the influx of water into a plant cell
- pressure pushes the plasma membrane against the cell wall and maintains the cell’s shape and size

92
Q

What is plasmolysis?

A

Plasmolysis is when the volume of the plant cell shrinks when water exits the cell
- as water leaves, the plasma membrane pulls away from the cell wall
- resulting in the cell losing its turgor pressure and wilting

93
Q

How does osmosis occur more quickly in some cells?

A

Osmosis occurs more quickly in cells with transport proteins that allow the facilitated diffusion of water

94
Q

What was the protein called, that was abundant in red blood cells, bladder, and kidney cells, when it was first identified by Peter Agre and colleagues?

A

‘Channel-forming Integral Membrane Protein, 28kDa’ (CHIP28)

95
Q

What is the current name of the protein that Peter Agre and colleagues identified?

A

Aquaporin, since it forms a channel that allows water to pass through the membrane

96
Q

What are transport proteins?

A

Transport proteins are transmembrane proteins that provide a passageway for the movement of ions and hydrophilic molecules across membranes

97
Q

How many classes of membrane transport proteins are there and what are they based on?

A
  • two classes
  • based on the way in which they move molecules across membranes
98
Q

What are the two classes of membrane transport proteins?

A
  • channels
  • transporters
99
Q

What are channels in transport proteins?

A

Transmembrane proteins that form an open passageway for the direct diffusion of ions or molecules across the membrane.

100
Q

Are most channels gated?

A

Yes, most channels are gated
e.g. Aquaporins

101
Q

What are transporters (aka carriers) in transport proteins?

A

Transmembrane proteins that transport solutes across the membrane via conformational (shape) change

102
Q

What is the principal pathway for uptake of organic molecules in transporters?

A

Through transporters in transport proteins

103
Q

What are the three types of transporters in transport proteins?

Answer: The three types of transporters in transport proteins are uniporter, symporter or cotransporter, and antiporter

A
  • uniporter
  • symporter or cotransporter
  • antiporter
104
Q

What does a uniporter transporter do?

A

Uniporter transporter: transports a single molecule or ion type

105
Q

What does a symporter or cotransporter transporter do?

A

Symporter or cotransporter transporter: transports two or more ion or molecule types in the same direction

106
Q

What does an antiporter transporter do?

A

Antiporter transporter: transports two or more ion or molecule types in opposite directions

107
Q

What is active transport in transport proteins?

A

Movement of a solute across a membrane
- against its gradient
- from a region of low to higher concentration
- energetically unfavorable
- requires the input of energy

108
Q

What is primary active transport?

A

Primary active transport is a type of active transport that uses - a ‘pump’
- & directly uses energy to transport solute

109
Q

What is secondary active transport?

A

Secondary active transport is a type of active transport that uses
- a pre-existing gradient of another molecule to drive transport

110
Q

What is the role of ATP-driven ion pumps in transport proteins?

A

Generate ion electrochemical gradients

111
Q

What is Na+/K+-ATPase?

A

Na+/K+-ATPase is a type of ATP-driven ion pump that
- actively transports Na+ & K+ against their gradients
- using the energy from ATP hydrolysis

112
Q

How many Na+ ions are exported for every 2 K+ ions imported into the cell by Na+/K+-ATPase?

A

3 Na+ ions are exported for every 2 K+ ions imported into the cell

113
Q

What type of pump is Na+/K+-ATPase?

A

Na+/K+-ATPase is an antiporter, which means ions move in opposite directions

114
Q

What is an electrogenic pump?

A

An electrogenic pump is a type of pump that exports one net positive (+) charge.
- e.g. Na+/K+-ATPase

115
Q

What is the role of electrochemical gradients in transport proteins?

A

Electrochemical gradients:
- aid in the function of ion & molecule transport
- production of energy intermediates
- osmotic regulation
neuronal signaling
muscle contraction
- bacterial swimming

116
Q

What are exocytosis and endocytosis?

A

The bulk movement of material across membranes
- used to transport large molecules
- such as proteins & polysaccharides which are not free to diffuse across the membrane

117
Q

What is exocytosis?

A

Exocytosis is the process of moving material out of the cell (think ‘exit’) using vescicles

118
Q

What is endocytosis?

A

Endocytosis is the process of moving material into the cell (think ‘in’)
- invaginates (folds inwards) to form a vesicle

BIOL4002 - Cell biology (7 decks)

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