Introduction
A cell is a combined plasma membrane structure, nucleus, cytoplasm, and cell organelle suspended. It is a structural and functional unit of life. Initially, it is present in monomeric form, and its structure is microscopic. Previously we have discussed how Robert Hooke discovered the first dead cells under the microscope from the cork slice of bark. Anton van Leeuwenhoek discovered the first living cell under the microscope and discovered free-living cells of spirogyra algae in water in the pond in 1674. The first organism found was chemoheterotrophic in nature. The cell is the smallest living unit made up of Carbon (C), Oxygen(O), Hydrogen(H), and Nitrogen(N).
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Cell Organelle : Composition of plasma membrane
The plasma membrane or cell membrane is the outermost covering of the cell. That acts as a barrier to separating the cytoplasm from the external environment and defining cells’ chemical and physical boundaries. Some organelles inside the cytoplasm also have their plasma membrane. Both plasma and organelle membranes consist of PHOSPHOLIPID. The phospholipid is a combination of Lipid and a specific type of protein. As it is selectively permeable, it acts as a gatekeeper of the cell.
This plasma membrane is electrically charged, inside negatively charged, and outside positively charged. This charge difference is due to the membrane potential difference, which is -20 mv. This difference is due to the high amount of potassium (K+) ions inside the cell compared to outside the cell. The various models are discovered for the structure of this plasma membrane.
Models of the plasma membrane by various scientists
Overton model – according to this model, the plasma membrane is made up of a single layer of Lipid. It can pass only lipid-soluble components across it. Still, this model fails to explain the transport of ions across the membrane.
Gorion and Grendel model – says that the plasma membrane is bilayer. The experiment was performed on mature RBCs but failed to explain ion transport.
Daniell and Devson model – plasma membrane is a combination of Lipid and protein. Theory suggests a trilaminar model like a sandwich, but this fails to explain the function of the plasma membrane.
Robertson model – supports Daniell and devson’s model and says plasms membrane is of bilayer lipids, including proteins in it. They provide dimensions of the plasma membrane, which is 75A0.
Singers and Nicolson model – proposed in 1971. According to this plasma membrane is a FLUID MOSAIC MODEL you can imagine icebergs like in a titanic movie. Means proteins are embedded in Lipid bilayer, through which ions can travel in-out and float inside and outside of the cell.
The Overton, Gorion, and Grendel model was known as the lipid and lipid bilayer model. The model proposed by Devon, Daniell, and Robertson was known as the protein-lipid bilayer model or unit membrane model. The universally acceptable model was the Fluid mosaic model by Singer and Nicolson.
Lipid
50% of the plasma membrane is composed of lipids. Lipids are hydrocarbons that include oils, fats, vitamins (such as A, D, K, E). It of amphiphilic nature, which contains both polar and non-polar portions. Lipids hold polar hydrophilic heads and non-polar hydrophobic tails. Phosphoglyceride or Glycerophospholipids are the most common phospholipid present in the animal cells, which are glycerol-based. Different types of phosphoglycerides are present, viz.
Types of Phosphoglycerides present in Cell Organelle
Phosphatidylethanolamine
Phosphatidylserine
Phosphatidylcholine
Sphingolipids
Three major classes of lipids are present in plasms membrane are Phospholipids (Phosphate group + Lipid),
Glycolipids (Carbohydrate + Lipids), and Sterols. The transport across the plasma membrane depends on the size of the molecule.
According to the organelle, this lipid layer can be present in single or doubled and linear or circular layer format. If the single layer is present in circular form, it forms a structure called MICELLE. These micelles are polar head outside and non-polar tail inside. They did not get used for transport. If the double layer of Lipid is present in linear form is known as lipid monolayer. In a circular shape, it forms a structure called LIPOSOME.
These liposomes are further used to transport the DNA and drug molecules to the target site. It has a polar head outside and inside both; it can easily fuse with the plasma membrane.
The stability of the plasma membrane depends on its fluidity. This fluidity depends on the temperature and the nature of the lipids present. Temperature and fluidity are directly proportional to each other. Some enzymes also help in the movement of the lipids inside the formed structure, whether it is present in linear or circular form.
Protein
Similarly, like lipids, proteins also covered 50% of the plasma membrane. These proteins are also amphiphilic. The ribosomes synthesize proteins, and these are the chain of many amino acids. These amino acids are assembled in the ribosome and formed a specific protein sequence of a particular size. In the plasma membrane, some proteins are present in an integrated form that is transmembrane proteins. Some are present in the peripheral area that is extrinsic proteins.
Peripheral proteins
These are present in the peripheral region. These are cytosolic protein means present in cytosol in interaction with the surface of the lipid bilayer. Peripheral proteins do not enter into the hydrophobic region that is the non-polar region.
Integral or Intrinsic Proteins
These proteins are embedded in the lipid bilayer. It includes all the linkers, channels, receptors, enzymes, etc. These proteins can be monotopic (single pass, which crosses plasma membrane only once) or polytopic (multipass, which crosses plasma membrane multiple times).
Lipid linked membrane proteins
These are the type of proteins which are linked to the lipid bilayer.
Carbohydrates
A very small portion of the plasma membrane is composed of carbohydrates, around 5 –10%. These carbohydrates always present in combination with lipids and proteins in glycoproteins, glycolipids, etc. They can be monosaccharides or oligosaccharides. Though present in small amounts, carbohydrates functions for antigen determination in the case of blood group determination. They also help in molecular recognition and cell adhesion.
Today we have covered the detailed composition of the plasma membrane. In the next blog, ill built some basics of biochemistry about lipids, proteins, and carbohydrates. Till the time, stay connected.
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