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CELL1001 > Cell biology 3: membranes and cytoskeleton (Dr. Whitmore) > Flashcards

Cell biology 3: membranes and cytoskeleton (Dr. Whitmore) Flashcards

1
Q

Do prokaryotic and eukaryotic cells contain equal amount of membranes ?

A

No, eukaryotes have internal membranes surrounding organelles or vesicles etc.

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2
Q

What is the main function of the cell membrane ?

A

Cell communication, import and export of molecules, cell growth and motility.

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3
Q

How thick is the lipid bilayer ?

A

About 5nm, 3nm lipid and 2nm of polar heads (1nm on each side).

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4
Q

What is the most common lipid in cell membranes ?

What is it made of ?

A

Phosphatidylcholine.
Made of:
- choline + phosphate = polar
- glycerol + double hydrocarbon tail = non-polar

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5
Q

How do phospholipid bilayers assemble ?

A

Phospholipid bilayers spontaneously close in on themselves to form sealed compartments. These conformation are much more NRGetically favourable.

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6
Q

What do the properties of the lipid bilayer allow ?

A

The properties of the lipid bilayer ensure that the plasma membrane is extremely flexible and able to self repair.

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7
Q

During a transfer between compartments (e.g. Golgi apparatus to plasma membrane) via vesicles, what can we say about the orientation of the proteins transfered ?

A

Membrane and proteins retain their orientation during transfer between cell compartments.

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8
Q

What types of proteins are found in membranes ?

A

Pumps/transporters, carriers, ion channels, anchors (integrins), receptors (PDGF receptor) and enzymes.

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9
Q

What are the 4 main ways in which proteins can associate w/ the lipid bilayer ?

A

Proteins can be:

  • transmembrane
  • monolayer-associated alpha-helices
  • lipid-linked
  • protein attached
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10
Q

What 2ary structure do transmembrane polypeptide chains often adopt ?

A

Because the backbone of a polypeptide chain is hydrophilic, a transmembrane polypeptide chain usually crosses the lipid bilayer as an α helix. The amino group H-bonds the caroboxyl group n+4 AAs down the helix.

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11
Q

What do single-pass transmembrane proteins often serve as ?

A

Receptors for extracellular signals.

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12
Q

How are water or other hydrophilic molecules or ions capable of crossing the membrane ?

A

By going through transmembrane hydrophilic pores which can be formed by multiple amphipathic α helices or β sheets.

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13
Q

Give an exemple where the lateral mobility of plasma membrane proteins is restricted.

A

In the epithelial cells in the gut, membrane proteins are restricted to particular domains of the plasma membrane (apical, lateral or basal).

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14
Q

What is the fct of the cytoskeleton ?

A

It gives a cell its shape and allows it to organize its internal components and to move = “bones and muscles” of the cell

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15
Q

What are the 3 types of protein filaments that form the cytoskeleton ?

A

Intermediate filaments, microtubules and actin filaments.

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16
Q

What are the main properties of intermediate filaments (IFs) ?

A
  • rope-like fiber
  • 10nm diameter
  • made of fibrous intermediate filamentous proteins
  • can form the meshwork = the nuclear lamina just beneath the inner nuclear membrane
  • can extend across cytoplasm –> give cell mechanical strength + distribute the mechanical stress in an epithelial tissue by spanning the cytoplasm from 1 cell-cell jct to another
  • v flexible
  • great tensile strength
  • deform under stress but don’t rupture
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17
Q

What are the main properties of microtubules ?

A
  • hollow cylinders made of tubulin protein
  • long and straight
  • connected to a microtubule-organizing center = centrosome
  • 25nm diameter
  • v rigid
  • rupture when stretched
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18
Q

What are the main properties of actin filaments/microfilaments ?

A
  • helical polymers of actin protein
  • flexible
  • 7nm diameter
  • organized into a variety of linear bundles, 2D networks and 3D gels
  • dispersed throughout the cell
  • highest concentration in cortex = layer of cytoplasm just beneath plasma membrane
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19
Q

How many alpha-helical monomers are necessary to make 1 IF ?

A

32 –> 8 tetramers

1 tetramer = 2 coil-coil dimers

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20
Q

What are the 4 major classes of IFs ?

A 
1 cytoplasmic:
- keratin filaments --> epithelial cells
- vimentin + vimentin-related filaments --> connective tissue, muscle cells, glial cells
- neurofilaments --> neurons
1 nuclear:
- nuclear lamins --> all animal cells
21
Q

What is the mitotic spindle and why are microtubules important for these ?

A

Mitotic spindle = the macromolecular machine that segregates chromosomes to two daughter cells during mitosis. The major structural elements of the spindle are microtubule polymers, whose intrinsic polarity and dynamic properties are critical for bipolar spindle organization and function.

22
Q

Microtubules can also be found in the _____ of ciliated cells.

A

Cilium

23
Q

How many genes encode the microtubule proteins ?

What does this imply ?

A

2 = alpha + beta –> 2 proteins = tubulin heterodimer (=microtubule subunit)

24
Q

How many protofilaments make 1 microtubule ?

A

13

25
Q

How are microtubules structurally different from IFs ?

A

They are polar (1 + and a - end).

26
Q

Where do tubulin proteins polymerize from ?

A

From nucleation sites on a centrosome = gamma-tubulin ring complexes

27
Q

What are centrioles made of ?

A
  • of microtubules
  • they are not needed for tubulin polymerization (gamma tubulin is crucial and plant cells don’t even have them) but in cilia and flagella (where they are called basal bodies) they act as organizing center for MTs
28
Q

What is the role of microtubules in cilia and flagella (where they are also called basal bodies) ?

A

They act as organizing centres for microtubules.

29
Q

What drives the dynamic instability of MTs ?

A

GTP hydrolysis

30
Q

When growing in the cell, what must MTs meet to become stable ?
What does this confer to the cell ?

A
  • MT capping proteins stabilize MTs
  • MTs that don’t contact these proteins become unstable
  • This can confer polarity to the cell
31
Q

By which process do MTs grow ?

A
  1. GTP-tubulin add to growing end of molecule

2. Addition proceeds faster than GTP hydrolysis by the dimer = dynamic instability

32
Q

By which process do MTs shrink ?

A
  1. Protofilaments containing GTP-tubulin peel away from the microtubule wall
  2. GDP-tubulin is released in the cytosol
33
Q

What 2 major proteins move along MTs ?

How do these proteins do this, and to what effect ?

A

Kinesins and dyneins move along MTs using their globular heads (have ATPase activity) and transport different kinds of cargo on their tails.

34
Q

Which way do dyneins and kinesins ?

A

Kinesins –> + end

Dyneins –> - end

35
Q

What is the fct of the motor proteins on the MTs ?

A

Helps position mainly the ER (but also other organelles) in the cell.

36
Q

Where are MTs situated in the respiratory tract ?

A

In the many hair-like cilia that project from the surface of the epithelial cells (that line the human respiratory tract).

37
Q

How are MT arranged in a flegellum or cilium ?

A

In a “9+2” array = 9 pairs of MTs on the outside, 2 MTs on the inside

38
Q

What different actions are mediated by dynein in:

  • isolated doublet MTs ?
  • a normal flagellum ?
A
  • isolated doublet MTs –> dynein produces MT sliding (ATP needed)
  • normal flagellum –> dynein causes MT bending (w/ linking proteins)
39
Q

How are actin filaments (AFs) similar to MTs ?

A

They have a polarised structure.

40
Q

How does ATP regulate the length of actin polymers ?

A

ATP hydrolysis decreases the stability of the actin polymer, and leads to “treadmilling” a the -ve end.
ATP binding extends the actin polymer at the +ve end.

41
Q

How are actin treadmilling and tubulin dynamic instability similar ?

A

Both rely on hydrolysis of a bound nucleoside triphosphate to regulate polymer length

42
Q

What is treadmilling ?

A

A phenomenon that occurs when one end of filament grows in length while the other end shrinks resulting in a section of filament seemingly “moving” across a stratum or the cytosol.

43
Q

How does actin help move cells forward ?

A

Forces generated in the AF rich cortex help move cells (e.g., carnivorous amoebae) forward.

44
Q

What are the 3 steps through which actin help cells move forward ?

A
  1. Actin polymerization at + end protrudes lamellipodium
  2. Integrins adhere to molecules in ECM or on substratum and capture intracellular actin filaments
  3. Cell uses the anchorage to drag itself forward
45
Q

How can actin form contractile structures ?

A

By associating w/ motor proteins e.g. myosin-I (simplest myosin). All myosin (except IV) move towards + end.

46
Q

Muscle contraction depends on interacting filaments of actin and _____ _____.

A

Myosin II dimers (which are bipolar!)

47
Q

How long is a sarcomere ?

A

2.2μm

48
Q

Towards which structure do the negatively charged myosin II heads move during muscle contraction ?

A

The positively charged Z disc.

49
Q

What triggers muscle contraction ?

A

A sudden rise in cytosolic [Ca2+].

CELL1001 (16 decks)

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