Like red blood cells, platelets are derived from myeloid stem cells. Some of these stem cells develop into megakaryoblasts, which give rise to cells called megakaryocytes in the bone marrow.
After a megakaryocyte has matured, pieces of its cytoplasm break away into cell fragments called platelets. A single megakaryocyte can produce — platelets. However, they do contain numerous granules or vesicles. The hormone thrombopoietin, produced by the liver and kidneys, regulates the production of megakaryocytes and platelets. Platelets have different appearances in their inactivated and activated states.
When inactivated , platelets are irregularly shaped discs. Activated platelets are spherical, with protrusions that allow them to stick to wound tissue and to other platelets to form a plug at the site of a blood vessel tear. Activated platelets also release chemicals from their granules to initiate clotting. The life span of a platelet is about 10 days. Like red blood cells, old platelets are phagocytosed. Reserve platelets are stored in the spleen.
Only mammals have anucleated red blood cells; however, some mammals camels, for instance have nucleated red blood cells. The advantage of nucleated red blood cells is that these cells can undergo mitosis.
Anucleated red blood cells metabolize anaerobically without oxygen , making use of a primitive metabolic pathway to produce ATP and increase the efficiency of oxygen transport. Not all organisms use hemoglobin as the method of oxygen transport. Invertebrates that utilize hemolymph rather than blood use different pigments containing copper or iron to bind to the oxygen.
Hemocyanin, a blue-green, copper-containing protein is found in mollusks, crustaceans, and some of the arthropods b. Chlorocruorin, a green-colored, iron-containing pigment, is found in four families of polychaete tubeworms. Hemerythrin, a red, iron-containing protein, is found in some polychaete worms and annelids c. Despite the name, hemerythrin does not contain a heme group; its oxygen-carrying capacity is poor compared to hemoglobin.
The first mechanistic study of how a red blood cell loses its nucleus, the research sheds light on one of the most essential steps in mammalian evolution. The genes and signaling pathways that drive the pinching-off process, however, were a mystery. His cell-culture system began with red blood cell precursors drawn from an embryonic mouse liver in mammalian embryos, the liver is the main producer of such cells, rather than bone marrow as in adults. The cultured cells, synchronized to develop together, divided four or five times before losing their nuclei and becoming immature red blood cells.
The researchers used fluorescence-based assays that enabled them to probe the changes in the red blood cells through the different stages leading up to the loss of the nucleus. The researchers plan to further investigate the entire process of red blood cell formation, which may lead to insights about genetic alterations that underlie certain red blood cell disorders.
Until now, scientists were unable to study these cells because they were unable to see them. Harvey Lodish's primary affiliation is with Whitehead Institute for Biomedical Research, where his laboratory is located and all his research is conducted. Red blood cells RBCs perform a number of human respiratory and cardiovascular system functions.
Most of these functions are attributed to hemoglobin content. The main RBC functions are facilitating gas exchange and regulating blood pH. Heme : This is a diagram of the molecular structure of heme. RBCs facilitate gas exchange through a protein called hemoglobin.
Hemoglobin is a quaternary structure protein consisting of many smaller tertiary structure proteins composed of amino acid polypeptide chains. Each hemoglobin molecule contains four iron-binding heme groups, which are the site of oxygen O 2 binding. Oxygen bound hemoglobin is called oxyhemoglobin.
The binding of oxygen is a cooperative process. Hemoglobin bound oxygen causes a gradual increase in oxygen-binding affinity until all binding sites on the hemoglobin molecule are filled. As a result, the oxygen-binding curve of hemoglobin also called the oxygen saturation or dissociation curve is sigmoidal, or S-shaped, as opposed to the normal hyperbolic curve associated with noncooperative binding.
This curve shows the saturation of oxygen bound to hemoglobin compared to the partial pressure of oxygen concentration in blood. Oxygen saturation curve : Due to cooperative binding, the oxygen saturation curve is S-shaped.
RBCs control blood pH by changing the form of carbon dioxide within the blood. Carbon dioxide is associated with blood acidity. RBCs alter blood pH in a few different ways. Quaternary structure: hemoglobin : Hemoglobin is a globular protein composed of four polypeptide subunits two alpha chains, in blue, and two beta pleated sheets, in red.
The heme groups are the green structures nestled among the alpha and beta. RBCs secrete the enzyme carbonic anhydrase, which catalyzes the conversion of carbon dioxide and water to carbonic acid. This dissociates in solution into bicarbonate and hydrogen ions, the driving force of pH in the blood.
This reaction is reversible by the same enzyme. Carbonic anhydrase also removes water from carbonic acid to turn it back into carbon dioxide and water. This process is essential so carbon dioxide can exist as a gas during gas exchange in the alveolar capillaries. As carbon dioxide is converted from its dissolved acid form and exhaled through the lungs, blood pH becomes less acidic. This reaction can occur without the presence of RBCs or carbonic anhydrase, but at a much slower rate.
With the catalyst activity of carbonic anhydrase, this reaction is one of the fastest in the human body. Hemoglobin can also bind to carbon dioxide, which creates carbamino-hemoglobin. However, because of allosteric effects on the hemoglobin molecule, the binding of carbon dioxide decreases the amount of oxygen bound for a given partial pressure of oxygen. Conversely, when the carbon dioxide levels in the blood decrease i.
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