Lecture 20 (11b) - Cell Communication Part 2 Flashcards Preview

Cell Bio & Developmental Genetics > Lecture 20 (11b) - Cell Communication Part 2 > Flashcards

Flashcards in Lecture 20 (11b) - Cell Communication Part 2 Deck (66):
1

Record jumps

Frog - Rosie (6.5m)
Human - Jackie Joyner Kersee (7.m)

2

three types of vertebrate muscle

• skeletal
• cardiac
• smooth

3

Skeletal muscle

• voluntary movement, breathing, maintaining posture
• appear striped bc of regular arrangement of sarcomeres

4

Cardiac muscle

• beating of heart
• cells interdigitate, forming a mesh

5

Smooth muscle

• involuntary, movement of internal organs
• do NO have stripes of regularly arranged actin and myosin

6

One muscle is composed of

hundreds of thousands of muscle cells (fibers) bundle together with connective tissue

7

One cell is called a

muscle fiber - large and multinucleate
• comes from the fusion of many myoblasts during development)

8

Each muscle cell (one fiber) consists of

several myofibrils

9

1 muscle fiber = 1 cell

actin + myosin -->
sarcomere -->
myofibril -->
muscle fiber + CT -->
1 muscle

10

Muscle cells need lots of

mitochondria to make ATP

11

Muscle cell membrane

sarcolema

12

Muscle cell cytoplasm

sarcoplasm

13

Each myofibril consists of repeating units called

sarcomeres

14

Each sarcomere is made of

overlapping actin and myosin filaments

15

1 sarcomere is from

Z line to Z line

16

I band

actin

17

A band

myosin with overlapping actin
• darker because myosin is thinner

18

H zone

only myosin

19

Each myofibril =

plenty of sarcomeres repeating horizontally

20

Each sarcomere is bound by

Z lines which anchor the actin

21

A band

in the middle of the sarcomere
• all the myosin filaments

22

H zone

non overlapping part of actin

23

I band

non overlapping part of myosin

24

H zone and I band

non overlapping part of actin and myosin

25

Titin

holds the myosin to the Z
• "bungee cord" = resistance to stretch in a relaxed muscle

26

Sliding filaments cause the muscle to contract

actin slides past myosin = contraction

27

When a muscle contracts

• sarcomeres shorten and band pattern changes
• non overlapping parts - H and I - diminish because there is more overlapping between actin and myosin filaments

28

2 main actors of the muscle contraction

myosin and actin

29

Myosin

2 polypeptide chains coiled together, ending in a globular head
• a myosin filament is made of many molecules in parallel with heads projecting sideways

30

Actin filament

actin monomers in a long twisted molecule

31

Tropomyosin is twisted around

actin
• troponin binds to tropomyosin, which binds to actin

32

Troponin has 3 subunits and each of them binds to

• tropomyosin
• actin
• Ca2+

33

Calcium binds to

troponin
• 1 site changes conformation and
changes conformation of tropomyosin and
twist tropomyosin around actin
--> reveals hidden site
--> binds to myosin

34

Actin and myosin can interact

there is a binding site on actin for myosin
BUT ONLY WHEN THERE IS CA2+

35

By default, the actin sites for myosin

are hidden,
but when there is Ca2+ in the sarcoplasm,
it binds to troponin,
troponin changes conformation,
which twists the tropomyosin around the actin to
expose actin sites for myosin

• myosin head binds -->
changes conformation, head bends
= actin moves relative to myosin
(5-10nm)

36

The role of the Ca2+

• when actin sites are exposed, the myosin head can bind the specific sites on actin to form cross-bridges between myosin and actin
• when myosin head binds the actin, the head conformation changes, the head bends and causes a tiny force that causes the actin filament to move 5-10 nm relative to the myosin filament

37

The role of ATP

• when a myosin head is bound to actin, it can bind a hydrolyze ATP
• the energy released when it binds ATP changes the conformation of the head again and causes it to release the actin and return to its extended position
• the head is now ready to start a new binding with actin (new cycle)
• ATP contributes to myosin and actin dissociation

myosin head bound, binds and hydrolyzes ATP, changes head conformation --> releases actin

38

Rigor mortis (right after death)

• ATP production stops, and so myosin can't release actin filaments and muscles stay contracted
• eventually the proteins lose their integrity and the muscles soften
• timing of these events help medical examiners examine the time of death

39

Repetition of a cycle

• contraction of the sarcomere involves many consecutive cycles of interaction between actin and myosin molecules
• when a cycle is over, a single myosin head breaks its contact with actin
• the actin filaments do not slip backward because there are many other myosin heads attached to the actin filament
• as long as Ca2+ and ATP are available, the cycle of actin and myosin interactions continues and the filaments slide past each other

1 binds, another releases
• actin doesn't slip back bc myosin is still grabbing

40

End of the contraction

• when Ca2+ pumps (active pumps requiring ATP) remove Ca2+ from sarcoplasm, contraction stops...
• when absence of Ca2+, tropomyosin twists back to its original conformation and is hiding the actin site for myosin

- Ca2+ always pumped inside
- need Ca2+
(usually low concentration in sarcoplasm bc of Ca pump)

41

The sliding of actin and myosin is regulated by

the sarcoplasmic concentration of calcium ions

42

Muscle cells are

excitable - the plasma membrane can conduct action potentials

43

Contraction is initiated by

action potentials from a motor neuron at the neuromuscular junction

44

Motor unit

one motor neuron and all the muscle fibers it synapses with (3-1,000)

45

The neuromuscular junction

the fibers in a motor unit contract simultaneously when the neuron fires

46

Chemical synaptic transmission begins with

the arrival of an action potential

47

The action potentials in muscle fiber travel

deep within the cell

48

The plasma membrane is continuous with

T tubules that run through the sarcoplasm

49

T tubules run close to the

sarcoplasmic reticulum (muscle fiber ER) that surrounds every myofibril

50

T tubule

fold (in) of sarcolema around myofibril
• action potential through membrane, depolarize through T tubule
• meets sarcoplasmic reticulum that's full of Ca2+

51

The sarcoplasmic reticulum is full of

Ca2+

52

The T tubule is connected to the sarcoplasmic reticulum by

receptor proteins

53

Channel opens in sarcoplasmic reticulum with action potential -->

Ca2+ into sarcoplasm and available for myofibrin

54

Ca2+ binds to

tropomyosin

55

In the resting muscle, the Ca2+ is highly concentrated in

the sarcoplasmic reticulum

56

Ca2+ has a low concentration in the sarcoplasm thanks to

the active pumps of Ca2+ in the sarcoplasmic reticulum membrane that pumps it back from the cytoplasm to the sarcoplasmic reticulum

57

An action potential spreads through the T tubules...

when it reaches the receptor proteins, they change conformation
• this opens Ca2+ channels and
Ca2+ flows out of the sarcoplasmic reticulum

58

Summary of contraction

1. release of Ca2+ from the sarcoplasmic reticulum triggers muscle contraction
2. when Ca2+ binds to troponin, troponin changes conformation which causes the tropomyosin attached to troponin to rotate and expose the binding sites for myosin -->
exposes myosin-binding sites of the actin
3. Myosin heads can bind to actin
4. when myosin head binds the actin, the head changes conformation.
The head bends and and causes a tiny force that causes the actin filament to move 5-10 nm relative to the myosin filament
5. when a myosin head is bound to actin, it can bind and hydrolyze ATP. The energy released when it binds changes the conformation of the head again and causes it to release the actin and return to its extended position. The head is now ready to a bind with a new actin. ATP helps myosin and actin to dissociate.
6. as long as Ca2+ and ATP are available, the cycle of actin and myosin interactions continues and the filaments slide past each other. A Ca2+ active pump (ATP) helps the Ca2+ reuptake in the sarcoplamsic

59

Contraction steps

1. Ca2+ is released from the sarcoplasmic reticulum
2. Ca2+ in the sarcoplasm binds troponin and exposes myosin-binding sites on the actin filaments
3. myosin heads bind to actin - release of Pi initiates a power stroke
4. In the power stroke, the myosin head changes conformation. filaments slide past each other
5. ADP is released - ATP binds to myosin, causing it to release actin
6. ATP is hydrolyzed. the myosin head returns to its extended conformation
7. if Ca2+ is returned to the sarcoplasmic reticulum, the muscle relaxes
8. if Ca2+ remains available, the cycle repeats and muscle contraction continues

60

Ca2+ is released from the

sarcoplasmic reticulum

61

Ca2+ in the sarcoplasm binds

troponin and exposes myosin-binding sites on the actin filaments

62

Myosin heads bind to

actin - release of Pi initiates a power stroke

63

In the power stroke, themyosin head

changes conformation - filaments slide past each other

64

ADP is release - ATP binds to

myosin, causing it to release actin

65

If Ca2+ is returned to the

sarcoplasmic reticulum, the muscle relaxes

66

If Ca2+ remains available

the cycle repeats and muscle contraction continues