Structure And Functions Of Erythrocytes
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Red blood cells are the main type of blood cell in the blood plasma, they are also called erythrocytes and have oxygen carrying properties. They pick up oxygen and release carbon dioxide at the lungs where the capillaries and alveoli's are in very close contact, the carbon dioxide diffuses from the capillary membrane into the alveoli membrane while the oxygen diffuses in the opposite direction.
The erythrocytes carrying the oxygen in the haemoglobin molecules (which are responsible for the red colour of the blood) inside the cells are then carried inside the circulatory system throughout the whole body.
Erythrocytes are hollow shaped disks with a diameter of 7 micrometer and thickness of 3 micrometer and they are generally much smaller then normal cells in the human body. These cells roughly have a volume of 90 femto litre and a surface area of 136 μm2, they can swell up and can carry 150 femto litre at any given time.
A healthy human being can have 25 trillion of erythrocytes at a time, which counts for one fourth of the total cells in the body.
People who live at high altitude can have more red blood cells due to low oxygen tension in the air. Erythrocytes are the majority of cells in the blood plasma, other cells which can also be found in the blood plasma include platelet cells and white blood cells.
Erythrocytes do not contain nucleus and all of the other organelles which are present in majority of cell types having said that there is a recent study revealing that red blood cells posses all the necessary bio machinery for protein biosynthesis.
Immature erythrocytes discard their nucleus early on in order to facilitate more space for its oxygen carrying activities during maturation, therefore supporting the notion that erythrocytes are incapable to self maintain and repair by synthesising the required proteins. It was shown that washed erythrocytes contained RNA after they have been washed from components that might have slipped through the permeable cell membrane of the red blood cells. Kabanova et al have shown using 2D electrophoresis for the first time in scientific history that there are genes inside the erythrocytes encoding activation, transcription, initiation, regulation and translation. The proteins found were cysteinyl, poly A binding protein, RNA polymerases, beclin 1, reticulon 4, bcl2 and IAP which are anti apoptotic proteins.
The molecule responsible for the red colour of the blood is the haemoglobin protein, every erythrocyte carries approximately about 300 million of haemoglobin proteins.
Each haemoglobin protein has four haeme groups, and haemoglobin makes up a third of the erythrocyte volume and transports almost all of the oxygen (the rest is transported dissolved in the blood plasma).
Erythrocytes are made through erythropoiesis in which a stem cell from the bone marrow specializes in to a nucleated erythrocyte then loses its nucleus to become a mature erythrocyte in a time span of just seven days. Mature erythrocytes have a life span of 120 days after which they are recognised by macrophages and are phagocytosed and useful components are then recycled.
The membrane of the erythrocyte is very crucial for its flexibility, adhesion, formability and immune recognition.
The erythrocyte is made out of the different layers: the exterior is made out of glycocalyx which is high in carbohydrates, the middle lipid bi layer which has a lot of trans membrane proteins, and the membrane skeleton which is network of structural proteins based on the inner surface of the lipid bi layer.
The red blood cell membrane is made out of a lipid bi layer which is similar to those lipid bi layers of normal cells. The lipid bi layer is made out of cholesterol and phopholipids in equal proportions by weight. The lipid is crucial as it gives the membrane the opportunity to be permeable.
The red blood cell membrane has outer mono layer which is made out of phophatidlcholine and sphingomyelin, and inner mono layer which has phosphatidylethanolamine, phosphatidylethanolamine and phosphatidylserine.
Mainly energy-dependent and energy-independent phospholipid transport proteins and cause the distribution of phospholipid in the bi layer of the red blood cell.
Flippases are enzymes that flip phopholipids from extra cellular space into the intra cellular space, the enzymes responsible for the opposite movement of phospholipids are called floppases. Scramblase is another enzyme that is responsible of both in and out movements of phospholipids of the permeable red blood cell membrane.
These asymmetric assembly f phospholipids are very crucial for the cell structure and function because:
Macrophages destroy red blood cells that have phosphatidylserine displayed on their surfaces, so it is very crucial for the red blood cell to keep phosphatidylserine in the inner mono layer.
Exposed phosphatidylserine can cause adhesion of red cells to vascular endothelial cells, mostly obstructing normal blood flow through the micro vasculature, and this is very well demonstrated in sickle cell anemia where the phosphatidylserine are exposed on the sickle red blood cell membrane, and result in the agglutination of sickle red blood cells and adhesion to the capillary cell wall creating a blockage of blood flow. Macrophages can also easily spot these mutated cells and destroy them in advance leaving the body far less erythrocytes to transport oxygen around the body.
It is absolutely vital for the red blood cell's survival that the phosphatidylserine are kept in the inner sheet of the lipid bi layer, and as equal importance of maintaining adequate blood flow in the micro circulation of the circularly system.
PIP2 (phosphatidylinositol-4, 5-bisphosphate) and phosphatidylserine both posses regulation of membrane mechanical function, because they can associate with spetrin and protein 4.1R which are defined as skeletal protein. Phosphatidylinositol -4 ,5- bisphosphate proliferates the attachment of glycophorin C to protein band 4.1 R, phosphatidylserine also interacts with spectrin and enhances mechanical stabilty of the red cell membrane skeleton.
Newly identified red cell membrane structures called lipid rafts are known to contain sphingolipids in addition to the usual cholestrol molecules. These novel sphingolipids have close interactions with flotillins, G - proteins, stomatins and beta adrenergic receptors which all membrane proteins with different roles in cell signalling.
In the membrane of a red blood cell are proteins which enable the red blood cell to enter and cross an entry half of its own size and still return its original hollow disc shape when the space is big enough, these proteins give the red blood cell its agility and durability.
Band 3 (an anion transporter), Aquaporin 1 (water transporting protein), Glut 1 (protein responsible for the transportation of glucose and L - dehydroascorbic acid) and Kidd antigen protein (urea transporting protein) are the main transporter proteins in the membrane of the red cells. Other proteins that have adhesive activities in the red cell membrane include ICAM (binds with integrins) and BCAM (interacts with major proteins in the basal lamina).
Abnormalities in the erythrocytes are called anemia in a wider term consisting of different forms and types of diseases. Sometimes it is the lack of haemoglobin or its inability to carry oxygen, but all of these variable defects in the red blood cell lead to hypoxia (the lack of oxygen) in most organs.
It is now clear that the red blood cell has its own bio machinery to self maintain and have similar life span as other cells, however the cells do not use these facilities.
Maybe the cell membrane of a red blood cell has different and multiple proteins which all are involved in different pathways that erythropoiesis is easier then maintaining an already existing red blood cell. This cell is so unique that it so sophisticated without having nucleus and organelles in place it can still provide the whole body vital necessities such as oxygen, taking away carbon dioxide, maintaining pH levels and assisting the immune system. But still there is yet more to be learned and investigated about this cell.
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