17

Question 1

Living organisms can undergo controlled and target-oriented movement:

Answer 1

Subcellular level
- shape transformation,
- phagocytosis
Cellular level
- cell migration in the environment: sperms, macrophages
-displacement of fluid around the cell, endothelial cells: microvilli
Tissue level and organism as a whole
- movements of organs: heart, lungs, eyes etc.
- walking, swimming, jumping, speaking etc.

Question 2

Target-oriented movement

Answer 2

Triggered and controlled by signals from the CNS, resulting in action potentials and nerve impulses being distributed as synaptic transduction, leading to muscle contraction and hence movement. All types of these movements in living organisms are based on similar mechanisms at the molecular level – focussing on motor proteins which are able to transform chemical energy (ATP hydrolysis) into mechanical work.

Question 3

Classification of muscle contractions:

Answer 3

Isotonic – at constant tension/load
Concentric – shortening
Eccentric – elongating
Isometric – at a constant length
Isokinetic – with constant velocity

Question 4

Muscles

Answer 4

Composed of muscle fibers (myocytes). The fibers are long cylindric cells that consist of bundles of myofibrils that again are composed of contractile units, the sacromeres ~ 2.2 µм. These sacromeres are positioned adjacent to one another and contain alternating thick and thin protein filaments. The thick filaments are named Myosin and the thin filaments Actin. Myosin is anchored to a middle line in the sacomere called the M-Line. Actin filaments are connected to the outer line of the sacromere called the Z-Line.

Question 5

Structure of myofibrils

Answer 5

Have a hexagonal arrangement with the thick and thin filaments positioned in the vertexes of equilateral hexagons. Each thick filament is surrounded by six thin filaments. A sarcomere is defined as the segment between 2 neighbouring Z-lines.
Surrounding the Z-line is the region of the I-band which is the zone of thin filaments not superimposed by thick filaments. Within the A-band is a region known as the H-zone which is the zone of thick filaments not superimposed by thin filaments. Within the H-zone is a thin M-line formed of cross-connecting elements of the cytoskeleton.

Question 6

Contraction

Answer 6

Each sarcomere contracts proportionally to the contraction of the muscle fiber:
E.g. for 20 % muscle contraction each sarcomere shortens by 20 %
The dimension of А-bands remain constant but I-bands shrink
The length of the thin and thick filaments remains constant
This forms the Sliding filament theory which is a molecular mechanism of contraction.

Question 7

Myosin

Answer 7

Has a long, fibrous tail and a globular head which binds to actin and also to ATP (ATPase activity is located in the S1 segments). The myosin tails are arranged to point toward the centre of the sarcomere and the heads point to the sides of the myofilament band. The heads can move and the necks of the filaments serve as a lever allowing easy bending – S1 fragment. Myosin is an actin dependent ATPase – in the presence of ATP alone myosin performs conformation changes.

Question 8

Actin

Answer 8

Molecules are bound to the Z-line. Actin can be present either as a free monomer called G-actin or as part of a linear polymer called F-actin. The F-actin filament also contains a tropomyosin molecule that is wrapped around the F-actin helix. In the resting phase, it covers the actins active site. Composition of Thin Filaments:
Tropomyosin – alpha-helical double stranded coil-coil protein
Troponin – globular protein, 3 subunits:
TnT- Tropomyosin binding unit
TnC – Calcium-binding unit
TnI – Actin-binding unit

Question 9

Molecular mechanism of sliding filaments:

Answer 9

1. Each thick filament is surrounded by 6 thin filaments
2. The filaments are connected via myosin bridges (heads)
3. ATP and actin binding sites in the heads
4. The head domains of myosin can perform specific conformational changes

Question 10

Excitation Contraction Coupling

Answer 10

Skeletal muscles act under the control of the CNS by nerve impulses. The nerve impulse triggers a series of events in the highly-specialised membranes of the muscle cell leading to and controlling the process of contraction. The nerve impulse is transduced to the sarcolemma as an action potential that propagates to the T tubule near the Z lines. Depolarisation spreads to the T tubule and the SR – Ca2+ channels (voltage gated) in the SR membrane open and Ca2+ released into the sarcoplasm. An electrical impulse is finally transferred into a mechanical process (contraction).