Explain the relation between muscles, ATP energy, and the calcium ion.
The relationship between muscles, ATP, and calcium exists during the process of muscle contraction.
Muscle cells are made up of structures called sarcomeres, and these consist of two different kinds of filaments (or myofilaments): thick and thin. The thin filaments contain actin, and the thick filaments contain myosin and myosin heads.
When muscle cells are at rest, there are bunches of calcium ions located within the sarcoplasmic reticulums of the muscle cells (the SR is a special type of endoplasmic reticulum, which is the cellular organelle that produces and transports proteins).
When a nerve signal reaches a muscle cell, the action potential opens up the calcium channels, or "gates" of the sarcoplasmic reticulums, and this causes all of the calcium ions inside the sarcoplasmic reticulum to spill out. These calcium ions then spread out among the thick and thin filaments that make up muscle cells, and bind to the troponin C complex located on the actin-containing thin filaments. When this happens, the troponin changes its shape, or conformational change, and one of the proteins involved (tropomyosin) then moves out of the way so that certain binding sites for the myosin heads (on the thick filaments) are now unblocked on the thin filaments. Ordinarily, the tropomyosin protein blocks these binding sites, and prevents the binding of the myosin heads of thick filaments, but when an action potential releases calcium, the two different filaments are able to interact and bind.
After the two types of filaments bind, the myosin heads engage in something called a "power stroke," by releasing ADP and an inorganic phosphate that was originally bound to the myosin heads. This power stroke allows the myosin heads to sort of "push" the filaments together; resulting in a muscle contraction. This results in a contraction because it causes the actin filaments to be pulled closer together and shortens the overall muscle cell.
After the contraction, ATP then binds to the myosin heads, causing it to "let go" of the actin filaments, and the thick and thin filaments then unbind and the muscle relaxes. The bound ATP is converted into ADP and an inorganic phosphate again, so that it can be used again for the next muscle contraction (in which it would be released and result in another power stroke). While all of this is going on, the calcium is pumped back into the sarcoplasmic reticulum so it can be re-released during the next action potential. This is accomplished by calcium ATPase, and ATP is produced as this happens, so it can be used during the contraction process when needed.
Even though muscle fibers store some oxygen, that oxygen is quickly used up, especially during strenuous exercise. In order to convert glucose into ATP so they can continue working, muscles must receive more oxygen via the blood. That is why respiration or breathing rate increases during physical exertion. In times where work or play activities are exhausting; muscle fibers may literally run out of oxygen. If not enough oxygen is present in muscle fibers, the fibers convert glucose into lactic acid, a chemical waste product.
When lactic acid builds up in muscle fibers, it increases the acidity in the fibers. Key enzymes in the fibers are then deactivated, and the fibers can no longer function properly. As a result, muscles are not as effective, contracting less and less. This condition is known as tetany or muscle fatigue. In a state of fatigue, muscle contractions may be painful. Finally, muscles may simply stop working. Lactic acid is normally carried away from muscles by the blood. It is then transported to the liver, where it is changed back into glucose. In order to do this, however, the liver needs ATP. To produce ATP in the liver, oxygen is once again needed. This is why breathing rate remains high even after vigorous physical activity is stopped. Only after the liver produces the necessary ATP does breathing gradually return to normal.
When muscles need to create ATP, their only energy source, they combine glucose with oxygen. This reaction also creates heat as a by-product. The body uses this heat to maintain normal body temperature. When the temperature of the body drops below normal, the brain signals the muscles to contract rapidly—what we perceive as shivering. The heat generated by these rapid muscle contractions helps to raise or at least stabilize body temperature.
In death, all reactions tend to stabilize. Among the first of these processes is that of ion equilibration across all compartments of the body as ion pumps lose their energy supplies. In the case of muscle, this results in the receptacle holding fluid and extracellular calcium leaking into the sarcoplasm, raising calcium concentrations to high levels. The calcium induces conformational changes in the troponin-tropomyosin complex, exposing myosin binding sites on thin filaments. The resulting uncontrolled contractile activity hastens the total exhaustion of ATP supplies and ends with all or nearly all myosin molecules in cross-linked actomyosin complexes. The rigid state of muscles that develops shortly after death is due to this highly cross-linked state of thin and thick filaments and is known as rigor mortis.
Muscles, ATP energy, and the calcium ion are all intricately linked in the process of muscle contraction. Let me explain how they relate to each other.
Muscles are made up of specialized cells called muscle fibers. These fibers contain small structures called sarcomeres, which are the basic units of muscle contraction. Sarcomeres consist of two types of protein filaments: actin and myosin.
To contract a muscle, ATP (adenosine triphosphate) energy is required. ATP is the energy currency of cells and is found in abundance in muscle cells. When a muscle needs to contract, ATP molecules are broken down into ADP (adenosine diphosphate) and inorganic phosphate. This breakdown releases energy that powers the muscle contraction process.
The role of calcium ions comes into play to initiate muscle contraction. Under resting conditions, calcium ions are stored within the muscle cell's sarcoplasmic reticulum, a network of tubules. When a muscle is stimulated to contract, signals from the nervous system travel to the muscle and cause the release of calcium ions from the sarcoplasmic reticulum into the surrounding cytoplasm of the muscle fiber.
Once released, the calcium ions bind to a protein called troponin, which is located on the actin filaments within the sarcomere. This binding causes a conformational change in the structure of the actin filaments, exposing binding sites for myosin. Myosin, which has heads that can attach to actin, forms cross-bridges with actin by binding to these exposed sites.
The interaction between myosin and actin initiates the sliding filament mechanism, where the myosin heads pull the actin filaments inward toward the center of the sarcomere. This shortens the sarcomere, resulting in muscle contraction.
Now, to summarize the relation between muscles, ATP energy, and calcium ions: ATP energy is required for muscle contraction, as it provides the energy needed for the myosin-actin cross-bridge formation and sliding filament mechanism. Calcium ions, on the other hand, trigger muscle contraction by binding to troponin and exposing binding sites on actin. Without ATP energy and calcium ions, muscles would be unable to contract effectively.
The relationship between muscles, ATP energy, and the calcium ion is essential for muscle function. Here is a step-by-step explanation:
1. Muscles: Muscles are composed of muscle fibers that contract and relax to create movement in our bodies. They enable us to perform voluntary actions like walking, running, and lifting objects.
2. ATP Energy: ATP (Adenosine Triphosphate) is a molecule that stores and provides energy for cellular processes. When ATP is broken down, it releases energy that can be used by the body for various functions, including muscle contraction.
3. Muscle Contraction: The process of muscle contraction begins when a nerve impulse signals the muscle fibers to contract. This signal triggers the release of calcium ions from the muscle's sarcoplasmic reticulum, a specialized structure within the muscle cell.
4. Calcium Ions: Calcium ions play a crucial role in muscle contraction. When released into the muscle cell, they bind to a protein called troponin, located on the actin filaments within the muscle fiber.
5. Troponin-Tropomyosin Complex: The binding of calcium ions to troponin causes a conformational change in the troponin-tropomyosin complex. This complex normally covers the binding sites on actin, preventing the formation of cross-bridges with the myosin heads.
6. Cross-Bridges Formation: The conformational change in the troponin-tropomyosin complex exposes the binding sites on the actin filaments, allowing the myosin heads to bind to actin. This forms cross-bridges between the actin and myosin filaments.
7. ATP Energy for Muscle Contraction: To initiate muscle contraction, ATP binds to the myosin heads, causing them to detach from actin. As ATP is hydrolyzed into ADP (Adenosine Diphosphate) and inorganic phosphate (Pi), energy is released, which is used to reposition the myosin heads and generate the force for muscle contraction.
8. Role of Calcium in ATP Energy: Calcium ions further facilitate muscle contraction by promoting the release of ATP from myosin heads. This ATP can then be used to re-energize the myosin heads, allowing them to continue the process of cross-bridge formation and muscle contraction.
In summary, muscles require ATP energy for contraction, and calcium ions play a crucial role in regulating the interaction between actin and myosin filaments, allowing the muscles to contract and relax.