The Cell

THE CELL 1

TheCell

InstitutionName

Topic 1: Cytoskeleton

  1. Actin filaments

The F-actin arelinear polymers of G-actin and they form the building proteins foractin filaments. They comprise of a rich 43-kd protein that isresponsible for polymerization to form cytoskeletal filaments. Thefilaments are bound by actin to ATP which serves as the energycarrier for the actin filaments. ATP is required in treadling withATP-actin polymerizing the filaments at the end points.

  1. Microtubules

Tubulins (alphatubulin and beta tubulin) form the building blocks for microtubules.Heterodimers are auxiliary to the end of a microtubule during thegrowth and reconstruction of the microtubules. The tubulins come offduring the time of shrinkage as intact subunits. Attachment of thetwo tubulins (alpha tubulin and beta tubulin) form an heterodimerthat comes out and goes back on since the micro tubular structureexperience intrinsic instability. However, they are described to bein a dynamic equilibrium.

TopicTwo: Skeletal Muscle Contraction

Explain themechanism of skeletal muscle contraction

During skeletalmuscle contraction, the following events occur. Calcium is attachedto troponin, Complaisant conformational alterations occurs in thesystem of troponin-myosin. Actin and myosin interaction inhibitor aredeactivated and actin and myosin filament forms a cross bridge. Thefilaments creates a sliding mechanism and tension is exerted thatleads to shortening of the muscle that is referred to as contraction.

  1. The action potential propagates through axon and reaches to the nerve terminal of motor neuron. It opens sodium, channels.

  2. Potassium ions moves (into/out) of the nerve terminal, and this triggers vesicular fusion (you will learn this mechanism more in detail in Ch. 15!).

  3. Neurotransmitter, molecules, will be released from the nerve terminal, and it will bind to postsynaptic receptor on skeletal muscle cells (at neuromuscular junction).

  4. This receptor is a voltage-gated sodium channel. The movement of K+&nbspions (into / out) of the cytosol will occur when the channel opens, and the membrane potential around the channel will be repolarized.

  5. This change in the membrane potential opens neighboring volgated sodium channels and action potential will propagate along with the plasma membrane.

  6. When it reaches to deformation, which are invagination of plasma membrane into myofibrils, it opens sodium, channels.

  7. This leads to the movement of potassium ion (into / out) of the cytosol. In the cytosol, this ion will bind to protein channels on sodium binding site to release more potassium ions to the cytosol. This wave of Ca2+ ions released from sarcoplasmic reticulum throughout the myofibrils will initiate muscle contraction by the following processes.

  8. Calcium ion will bind to troponin, which then causes a conformational change of troponin-tropomyosin, a filamentous structure associated with actin filaments. This change exposes Crossbridges of actin filaments.

  9. Crossbridges of myosin now can bind to actin filament, and muscle contraction occurs.

  10. Calcium ion will be quickly taken up by troponin (Ch. 12!) expressed on plasma membrane to terminate skeletal muscle contraction.

Mechanismof power stroke during muscle contraction (watch the movie 17.10!)

  1. When skeletal muscle is at rest (relaxed), active sites of actin filament is covered by tropomysin . Thus, actin filament and myosin head are dissociated. At this stage, ATP and actin are likely bound to myosin head (and the myosin head is waiting loosely and ready to attach to actin filament when the signal comes).

  2. When the muscle contraction signal (increased concentration of calcium ion) comes, Tropomyosin moves away from the myosin-binding sites of actin filament, and the myosin head is allowed to bind to it.

  3. When the actin is released from the myosin head, it strengthens the binding between myosin head and actin filament and generates power stroke that moves actin filaments (myosin moves from negative -end to positive -end of the actin filament).

  4. ATP molecule will bind to the myosin head, causing a release of myosin head from actin filament.

Topic3: Bioenergetics and Energy Transformation

  1. Complete glucose reaction in aerobic respiration of mammalian cells.

    1. The carbon atoms are oxidized each by four electrons hence making a total of 24 electrons.

    2. The oxygen atoms require 2 e- making a total of 4electrons. Four protons are added to balance the reaction.

    3. Six water molecules are needed to add enough oxygen molecules to the reaction to complete the equation. The products of the reaction will also include 24 protons.

    4. There are 6 NADH`s are generated in the citric cycle with 3 for every Acetyl CoA that enters. 2 FADH2&nbspare produced with 1 for every Acetyl CoA entering the cycle. 2 ATP molecules for every single Acetyl CoA that enters and 4 CO2 molecules are released from the cycle with 2 per every Acetyl CoA.

    5. Typically, the electron Transport Phosphorylation gives 32 ATP`s whereby each ATP is produced as hydrogen ions that moves down their concentration gradient by means of an enzyme called&nbspATP synthase.

  2. Dark reaction

    1. During the dark reaction in the chloroplasts, oxygen is produced from the CO2.

    2. Biosynthesis of sucrose occurs in the chloroplasts.

    3. During the frost conditions, the threat of the system water freezing to a solid state triggers production of more sugar by the plant organs. The high levels of sugar act as impurities and therefore prevent the water component from changing into a solid state.

References

Swanson, C. P.(1964).&nbspThe cell. Englewood Cliffs, N.J: Prentice-Hall.

Alberts, B.(2002).&nbspMolecular biology of the cell. New York: GarlandScience.