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Molecular and Cellular Biology: > Transport Across Cell Membranes: > Flashcards

Transport Across Cell Membranes: Flashcards

1
Q

Why don’t cells exist in isolation in our bodies?

A

They need to communicate with our environment in order to survive, grow and maintain the organism’s function

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2
Q

What is an example of cells communicating with their environment?

A
  • Exchanging molecules
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3
Q

Why do cells exchange substances?

A
  • Cells cannot synthesis all the nutrients they need by themselves, so they import them
  • Unwanted products are exported from the cell
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4
Q

What are examples of substances that cells import?

A
  • Sugars

- Amino acids

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5
Q

The intracellular concentration of what must the cell have control over?

A

Certain ions

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6
Q

What in the cell plasma membrane facilitates transport of substances across the membrane?

A

Membrane transport proteins

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7
Q

What are membrane transport proteins?

A

A protein embedded in the lipid bilayer which allows for the transport of specific molecules

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8
Q

Why do we need specialised membrane transport systems?

A
  • Hydrophobic molecules are prevented from entering cells through the plasma membrane due to the hydrophobic interior in the lipid bilayer
  • Cells and organelles need to exchange many hydrophobic, water soluble substances
  • The concentration of many of these solutes have to be controlled within the cell
  • This requires specialised transport systems
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9
Q

Why what process do the specialised transport systems transport substances across the membrane?

A

Facilitated diffusion

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10
Q

The concentration of solutes in the cell must be the same throughout all parts of the cell.
True or false? Why?

A

False
The concentration of solutes varies across different parts of the cell- from the cytoplasm to fluid within organelles. This is because different organelles require different substances at different concentrations in order to carry out their function

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11
Q

What types of molecules need facilitated transport to enter cells?

A
  • Charged, water-soluble molecules

- Large molecules

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12
Q

List the order of what types of substances the lipid bilayer is most to least permeable to:

A
  • Small, non-polar molecules
  • Small, uncharged polar molecules
  • Larger, uncharged polar molecules
  • Charged molecules
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13
Q

What are examples of small, non-polar molecules?

A
  • Oxygen molecule
  • Carbon dioxide molecule
  • Nitrogen molecule
  • Steroid hormones
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14
Q

How easy is it for small, non-polar molecules to pass through the lipid bilayer?

A
  • They dissolves readily into the bilayer

- Require no help to diffuse across

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15
Q

What are examples of small, uncharged, polar molecules?

A
  • Water
  • Ethanol
  • Glycerol
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16
Q

How easily do small, uncharged, polar molecules diffuse across the lipid bilayer?

A

They diffuse across the bilayer without any assistance

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17
Q

What is an example of a larger, uncharged polar molecule?

A
  • Glucose
  • Amino acids
  • Nucleosides
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18
Q

How easily do large, uncharged polar molecules get across the lipid bilayer?

A

Hardly diffuse by themselves across the bilayer

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19
Q

What are examples of charged molecules?

A
  • All ions
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20
Q

How easily do charged molecules diffuse across the lipid bilayer?

A

Charged molecules cannot cross the lipid bilayer, no matter how small they are

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21
Q

What effect would having no proteins have on the permeability if a lipid bilayer?

A

This lipid bilayer would be impermeable to most water soluble substabces

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22
Q

What are transmembrane proteins?

A

A type of membrane transport protein whose polypeptide chain transverse the lipid bilayer

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23
Q

How are transmembrane proteins able to extend the full way across the lipid bilayer of a membrane?

A

They cross the membrane with the hydrophobic segments of their polypeptide chains

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24
Q

What is the name given to the hydrophobic segments of a transmembrane protein polypeptide chain?

A

Transmembrane domains

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25
Q

In what structures do transmembrane proteins cross the lipid bilayer?

A
  • A single alpha helix
  • Multiple alpha helices (where it loops back into the membrane again to form the other helices)
  • Rolled up Beta sheets ( known as Beta barrels)
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26
Q

How do some transmembrane proteins function as channels?

A

By forming pores

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27
Q

List the main classes of membrane transport proteins:

A
  • Channel proteins

- Transporters

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28
Q

What do channel proteins form in the phospholipid bilayer?

A

Aqueous (water filled) pores across the lipid bilayer

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29
Q

What do aqueous pores help transport across the membrane?

A

Small, water soluble molecules

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30
Q

Describe the structure and arrangement of the transmembrane proteins when forming channels in the lipid bilayer:

A
  • The channels are formed by transmembrane proteins with several alpha helices that cross the bilayer a number of times
  • The alpha helices are arranged so that the central, hydrophilic pore is formed
  • The hydrophilic amino acid side chains form the aqueous pore in the centre
  • The hydrophobic side chains of amino acids on one side of the alpha helices pack side by side in a ring that faces the lipids in the lipid bilayer
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31
Q

Where are membrane transport proteins present?

A

In all cellular membranes (the plasma membrane and the membranes of intracellular organelle)

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32
Q

What does it mean that membrane transport proteins provide a private portal across the membrane?

A

They allow the entry and exit of selective substances

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33
Q

What are the similarities between transporters and channel proteins?

A
  • Both are transmembrane proteins

- The polypeptide chains of both transverse the lipid bilayer several times

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34
Q

How do aqueous channels allow water soluble (hydrophilic) substances to pass across the membrane?

A

It prevents these hydrophilic substances from coming into direct contact with the hydrophobic interior of the lipid bilayer

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35
Q

What substances do transporters transport across the lipid bilayer?

A

Molecules which fit into a specific binding site on the transporter

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36
Q

Describe how transporters work:

A
  • Molecules bind are specific and complementary to transporter binding site
  • Transporter undergoes conformational changes, transferring small molecules across the lipid bilayer
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37
Q

What are conformational changes to a protein?

A

A change in the shape of the protein

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38
Q

Using what factors do channel proteins discriminate the molecules it transports?

A
  • Size

- Charge

39
Q

How does discriminating according to size and charge affect the substances a channel protein transports when it opens?

A

When open, any ion or molecule that is small enough and carries the appropriate charge will pass through

40
Q

What water-soluble substances can be transported across the lipid bilayer via channel proteins?

A
  • Specific inorganic ions

- Small polar organic molecules (in some cases)

41
Q

How do ion channels exist?

A

In open and closed conformations

42
Q

What types of molecules do channel and transporter proteins transports across the membrane?

A

Small hydrophobic molecules

43
Q

What determines whether the small, hydrophilic molecules move in or out of the cell, via transmembrane proteins?

A
  • Direction of transport often depends on relative concentration of molecules on either side of the membrane
  • Molecules flow from a region of high to low concentration spontaneously
  • This is a passive process as no additional energy is needed
44
Q

What is active transport?

A

The transport of a substance from low to high concentration with the addition of energy

45
Q

What does “down the concentration gradient” mean?

A

From area of high to low concentration

46
Q

What type of transport can channel proteins mediate?

A

Passive transport

47
Q

What type of transport do transporters mediate?

A
  • Passive transport

- Active transport

48
Q

When are transporters able to carry out active transport?

A

When coupled with another process that provides an input of energy

49
Q

What is the name given to transporters that carry out active transport?

A

Pumps

50
Q

What is the name given to passive transport mediated by transport proteins?

A

Facilitated diffusion

51
Q

What is the name given to transporters that mediate passive transport?

A

Facilitated transporters

52
Q

Channel proteins do not have binding sites. How do they achieve specificity?

A

Using the size of the internal pore and the charged regions at the entrance of the pore that attract and repel ions of the appropriate charge

53
Q

The plasma membrane is highly impermeable to all charged molecules.
True or False? Why?

A

False.
Unlike pure lipid bilayer, the plasma membrane contains transport proteins for the selective transfer of some charged molecules

54
Q

What affect does the impermeability of the lipid bilayer towards inorganic ions have on the concentration of inorganic ions on either side of the bilayer?

A

The ion concentration on either side of the lipid bilayer is different- this is because the ions can’t just diffuse through the bilayer, so an equilibrium is not reached

55
Q

Why is it important that the concentration of inorganic ions on either side of the lipid bilayer is different?

A

The differences are important for the survival of the cell

56
Q

List important inorganic ions for a cell:

A
  • Sodium ion (Na+)
  • Potassium ion (K+)
  • Calcium ion (Ca2+)
  • Chloride ion (Cl-)
  • Hydrogen ion (H+)
57
Q

Why is it important for certain inorganic ions to be transported into a cell?

A

They are crucial for biological processes that take place inside the cell

58
Q

What is created from small differences in electrical imbalances across the membrane?

A

Membrane potential

59
Q

What is membrane potential?

A

The difference in voltage, caused by small changes in inorganic ion concentration, across a cellular membrane

60
Q

What is the most common cation to be found outside of the cell?

A

Sodium ions

61
Q

What is the most common cation to be found inside the cell?

A

Potassium ion

62
Q

What needs to be carefully balanced in cells and their surrounding fluids?

A

Positive and negative charges

63
Q

The quantity of positive charges is balanced by an almost equal quantity if negative charge inside cells and the surrounding fluid. Give examples of this:

A
  • High sodium ion concentration outside cell is balanced mainly by chloride ions
  • High concentration of potassium ions inside cell is balanced by other inorganic and organic anions with different negative charges
64
Q

How is the potential difference across a cellular membrane created?

A
  • Electrical charges inside and outside of cells are generally balanced
  • Tiny excesses of positive or negative charge occur near the plasma membrane
  • Thus electrical imbalance creates a voltage difference
65
Q

What are positive and negative charges in and around a cell concentrated around?

A

The membrane

66
Q

Describe the charge imbalance that takes place in most cells:

A
  • Majority cells are more negative charge inside the cell (cytoplasm) than outside
  • Creating a negative membrane potential
67
Q

What affects the direction of net, passive movement of uncharged molecules?

A

The concentration gradient

68
Q

What affects the direction of the passive movement of charged molecules?

A
  • Concentration gradient

- membrane potential

69
Q

What is produced from having a membrane potential?

A

A voltage

70
Q

What are the effects of the cytoplasm having a more negative charge than outside the cell?

A
  • Molecules that are charged electrically are influenced by the membrane potential
  • Positively charged substances are attracted so move into the cell
  • Negatively charged substances are repelled and exit the cell
71
Q

The direction of flow of charged substances is affected by concentration gradient and membrane potential. What is the combined effect of these?

A

The electrochemical gradient

72
Q

Why are sodium ions pulled into the cell?

A

Sodium has a big electrochemical gradient

73
Q

Explain how sodium having a large electrochemical gradient causes it to be taken into the cell?

A
  • There is a higher concentration of sodium ions outside of the cell
  • Sodium is positively charged so is attracted to the negative charge in the cytoplasm
  • Therefore, sodium is taken into the cell
74
Q

What is the effect on the electrochemical gradient of sodium and potassium ions when the membrane potential and the concentration gradient act in the same way?

A

There is a higher concentration of sodium ions outside than inside the cell

75
Q

What is the effect on the electrochemical gradient of sodium and potassium ions when the membrane potential and the concentration gradient act in two different ways?

A

There is a higher concentration of sodium ions on the inside than outside- as they rarely move out

76
Q

Describe the electrochemical gradient of potassium ions:

A

Shallow

77
Q

Describe the effects of potassium ions having a small electrochemical gradient:

A
  • Lower concentration of potassium ions outside than inside the cell
  • Potassium is attracted to the negative charge inside the cell
  • As membrane potential and concentration gradient influence flow of potassium ions in different directions, there is barely movement of potassium ions across the membrane
78
Q

What does it mean that transporters are very selective?

A

They transport only one type of a substance

79
Q

Where are transporters present?

A
  • Plasma membrane

- Organelle membranes

80
Q

Do all membranes (plasma or organelle) have every type of transporter? Explain why?

A
  • No
  • Every membrane has the necessary transporters needed to transport the substances needed in and out of the cell/organelle
81
Q

What kind of things do the plasma membrane transporters transfer across?

A
  • Amino acids
  • Sugars
  • Nucleotides
82
Q

What kind of transporters are in the inner mitochondrial membrane?

A
  • Pyruvate (in)

- ATP (out- after being made)

83
Q

What kind of transporters are contained in the lysosomal transporters?

A
  • Transporters that move hydrogen ions into lysosome

- Transporters that transport products of digestion out of the lysosome and into the cytosol

84
Q

Why do transporters in the lysosomal membrane transport hydrogen ions into the lysosome?

A

So the inside is more acidic

85
Q

What are co-transporters?

A

Transporters that transport more than one kind of solutes

86
Q

What are co-transporters also known as?

A
  • Coupled transporters

- Symports

87
Q

What transporters are referred to as coupled transporters?

A

The transporters that move one solute down the electrochemical gradient while transporting the other against it’s electrochemical gradient

88
Q

What are examples of co-transporters?

A

Pumps

89
Q

List the types of transporters:

A
  • Uniport

- Co-transporter

90
Q

What is a uniport transformer?

A

A transformer that transports only one type of molecule across the cell membrane

91
Q

How do symport co-transporters work?

A
  • One solute is transported down concentration gradient
  • Energy is released when the first solute moves to fit into the co-transporter
  • This energy is used to fuel the transport of another molecule against it’s concentration gradient
  • Moves two different molecules the same direction across membrane
92
Q

What is an antiport transporter?

A

A coupled-transporter that moves two substrates in different directions across the membrane

93
Q

What are the types of co-transporters?

A
  • Symport

- Antiport

94
Q

How do antiport co-transporters work?

A
  • One substance is moved along electrochemical gradient
  • This movement releases energy
  • This fuels the movement of a molecule against electrochemical gradient
  • Transport does not happen at the same time

Molecular and Cellular Biology: (4 decks)

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