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Nutrient cycle in environment

  BIO-GEO CHEMICAL CYCLE OR NUTRIENT CYCLE       Bio  -    Living organism       Geo  -    Rock, Soil, Air, Water Chemical   -  Material or Nutrients        Cycle   -   Path All the types of material required by ecosystem in addition of energy, are available continuously to system through recycling. Thus there is a constant exchange of materials between the living organism and their abiotic environment through the recyling of materials. This phenomenon is called Bio-geo chemical cycle. Note: Environment factors,e.g soil , moisture, ph, temperature etc, regulate the release of nutrients into the atmosphere. Biochemical cycle The following types of cycle are found in an ecosystem. (i) Gaseous Cycle - C, H, N. O cycles. Reservoir is in the atmosphere (air) or in Hydrosphere(water). Sedimentary cycle - P. S, Ca cycles reservoirs are in earth's crust (lithosphere). Note : ...
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MECHANISM OF MUSCLE CONTRACTION

  •  MECHANISM OF MUSCLE CONTRACTION


  1. Mechanism of muscle contraction is best explained by the sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
  2. Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron. A motor neuron alongwith the muscle fibres connected to it constitute a motor unit. The junction between a motor neuron and the sarcolemma of the muscle fibre is called the neuromuscular junction or motor-end plate.

  3. A neural signal reaching this junction releases a neurotransmitter (Acetyl choline) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm.
  4. Increase in Ca++ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. This pulls the attached actin filaments towards
  5. the centre of 'A' band. The Z' line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. During shortening of the muscle (contraction), the T bands get reduced, whereas the 'A' bands retain the length.
  6. The myosin, releasing the ADP and P1, goes back to its relaxed state. A new ATP binds and the cross-bridge is broken.
  7. The ATP is again hydrolysed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding. The process continues till the Ca++ ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments. This causes the return of Z' lines back to their original position i.e., relaxation.
  8. Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue.     
  9. Role of ATP :
  10. (i) The 'back & forth' movement of myosin head with in the groove.
  11. (ii) Deatatachment of myosin head from the actini


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