Plasma membrane is the outermost covering of a human (animal) cell that not only protects the internal organelles but also separates them form the external environment. However, it is more than just a barrier which separates cell from their neighbors. It performs quite a number of other tasks with the help of some proteins, glycolipids and glycoproteins anchored in the membrane.
Structure of Plasma membrane:
In 1972, S. J. Singer and Garth L. Nicolson had put forward a model for plasma membrane termed as Fluid mosaic model which still holds true to explain the structure and function of Plasma membrane. According to fluid mosaic model, Plasma membrane is two layered membrane made up two phospholipids and other molecules such as, cholesterol, proteins, and carbohydrates. These molecules provide stability to the membrane and also helps it to perform its normal functions. Plasma membrane is not a rigid wall but rather a membrane where molecules can move laterally and change positions without affecting the integrity of the membrane. This fluidity of the membrane is essential for proper functioning of some enzymes and for transportation of certain cells such as red blood cells and white blood cells, as it allows them to alter their shape little bit as they pass through narrow spaces capillaries.
Arrangement of bilayer
The bilayer is composed of glycerophospholipids, which is made up of a glycerol molecule, a phosphate group and two fatty acid chains. A single glycerophospholipid molecule can be divided into two major regions: a hydrophilic ( water-loving) head – comprising of a choline (variable head groups can are found), phosphate molecule and hydrophobic (water-fearing) tails – made of two fatty acids chains.
These glycerophospholipids are water insoluble like all lipids however their unique structural arrangement allows them to align side-by-side to form broad sheets. These molecules take advantage of their affinity towards water in arranging themselves to form bilayers efficiently without use of energy. In water, these molecules spontaneously align with their water loving heads facing outward and the hydrophobic tails of two molecules face each other to line up towards interior side of bilayer. In this manner, the hydrophilic heads of the glycerophospholipids (which are hydrophilic) in a cell’s plasma membrane ends up facing water-based cytoplasm and the exterior of the cell
As we mentioned earlier that plasma membrane is more than just a barrier separating cell organelles from the cell’s exterior. Apart from giving rigidity to the cell, the other important functions of the Plasma membrane performs can be broadly categorized as following:
Gatekeepers: Controls what goes in and what comes out
Tags: They are adorned with some tags which helps the immune cells to identify between self and non-self
Messengers: Allows communications between cells.
To perform these above function they need proteins – cross-membrane transport and cell communication, lipids- to make membrane semi-permeable and carbohydrates – when bound to lipids and proteins as in glycolipids, glycoproteins helps the cells to recognize each other and also to differentiate between self and non-self.
Proteins are the second major component of the plasma membrane. They are stacked inside the plasma membrane and can contribute to many functions of plasma membrane like cellular transport and as communicators with other cells. These proteins can be present attached to lipids and proteins on internal and external surfaces of the membrane or they can extend all the way through the plasma membrane. Such proteins that go all the way through the membrane are called transmembrane proteins.
Based on their presence on plasma membrane and also the associated molecule with the protein decides the function played by them.
Transport proteins : Being a semi-permeable membrane, some molecules such as small hydrophobic molecules and gases like oxygen and carbon dioxide can cross membranes rapidly. While small polar molecules, such as water and ethanol, can also pass through membranes, but at a slower pace. However large molecules such as sugars, amino acids and charged ions (due to the hydrophobic nature of the fatty acid tails) need the help of transport proteins to cross plasma membrane.
Transmembrane proteins will function as channels or pumps to move molecules in or out of the cell and are hence crucial for cellular transport. Membrane transport proteins are specific towards particular molecule and sometimes require energy (needs ATP / energy in case of active transport and not for passive, facilitated diffusion of molecules) for the transport. For example, if glucose molecule is to be imported into the cell, then – first glucose binds to the glucose transporter protein (Glut) present in the plasma membrane and this causes a conformational change in the transporter protein. This allows the glucose transporter protein to open inside of the cell to deliver the glucose molecule into the cytoplasm. This process is called as facilitated diffusion, which enables bigger molecules such as glucose to cross the plasma membrane..
Receptor proteins : Membrane or transmembrane proteins in the plasma membrane acts as a receptor protein ( this work as receivers of extracellular inputs), which when interacted by a ligand, induces a chemical reaction inside the cell. Ligands are very specific to the receptors they bind and act as activators of intracellular processes. Hormones, neurotransmitters, cytokines, growth factors, cell adhesion molecules, or nutrients can act as ligands to the receptor proteins for activating different processes within the interior of the cell. For example, ligand binding might activate enzymes involved in a particular metabolic pathways or binding of hormones to plasma membrane receptors might transmit a signal into the cell to some of the intra-cellular molecules to activate or repress their function.
Aquaporins: These are water channels is a transmembrane protein that has a selective preference for transporting only water molecules in and out of the cell, while preventing the passage of ions and other solutes.
Carbohydrates: Carbohydrates are the third major component of plasma membranes after phospholipids and proteins. They are found on the outside surface of plasma membrane and usually attached by either to proteins (to form glycoproteins) or to lipids (to form glycolipids). These carbohydrate molecules attached to proteins provide unique cellular identity or tags. These tags allows our immune system to differentiate between self and non-self. Encountering the tags helps our immune cells to not initiate a immune response against them and cells lacking the cellular tags are immediately recognized as foreign cells and destroyed quickly.
The carbohydrates that make up glycoproteins are responsible for determining human A, B, O blood types.
Cholesterol : Adding to the membrane fluidity of plasma membrane is cholesterol. It lies alongside the phospholipids in the membrane and minimize the effects of temperature on the membrane.
Plasma membrane – Take-home message
Image Credits : All images are from OpenStax except – Lipid bilayer arrangement is modified from smartservier
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