Plasma Membrane, Definition, Discovery of cell membrane, structure, Function,

Plasma Membrane

The plasma membrane, also called the cell membrane, is the membrane found in all cells that separates the interior of the cell from the outside environment. In bacterial and plant cells, a cell wall is attached to the plasma membrane on its outside surface. The cell membrane consists of a lipid bilayer that is semipermeable. The cell membrane regulates the transport of materials entering and exiting the cell.

Discovery of cell membrane

In 1837, Schwann and Schleiden started to discuss the idea of a cell membrane. But the idea wasn't at all populaR. In 1895, Ernest Overton proposed that cell membranes were made of lipids. The lipid bilayer hypothesis, proposed in 1925 by Gorter and Grendel, created speculation to the description of the cell membrane bilayer structure based on crystallographic studies and soap bubble observations.

Definition

“A microscopic membrane of lipids and proteins which forms the external boundary of the cytoplasm of a cell or encloses a vacuole, and regulates the passage of molecules in and out of the cytoplasm”.

Explain

The plasma membrane of a cell is a network of lipids and proteins that is  forms the boundary between a cell’s contents and the outside of the cell. It is also simply called the cell membrane. The main function of the plasma membrane is to protect the cell from its surrounding environment. It is semi-permeable membrane and regulates the materials that enter and exit from the cell. The cells of all living things have plasma membranes. plasma membrane is found in both plant and animal cells.

Structure of the Plasma Membrane

Like all other cellular membranes, the plasma membrane consists of both lipids20-40% and proteins 60-80%. The fundamental structure of the membrane is the phospholipid bilayer, which forms a stable barrier between two aqueous compartments. In the case of the plasma membrane, these compartments are the inside and the outside of the cell. Proteins embedded within the phospholipid bilayer carry out the specific functions of the plasma membrane, including selective transport of molecules and cell-cell recognition. While lipids are the fundamental structural elements of membranes, proteins are responsible for carrying out specific membrane functions. Most plasma membranes consist of approximately 50% lipid and 50% protein by weight, with the carbohydrate portions of glycolipids and glycoproteins constituting 5 to 10% of the membrane mass. Since proteins are much larger than lipids, this percentage corresponds to about one protein molecule per every 50 to 100 molecules of lipid. In 1972, Jonathan Singer and Garth Nicolson proposed the fluid mosaic model of membrane structure, which is now generally accepted as the basic paradigm for the organization of all biological membranes. In this model, membranes are viewed as two-dimensional fluids in which proteins are inserted into lipid bilayers .

Functions of the Plasma Membrane

1.      Act as a Physical Barrier

2.      Selective Permeability

3.      Endocytosis and Exocytosis

4.      Cell Signaling

5.      Phospholipids

6.      Proteins

7.      Carbohydrates

8.      Fluid Mosaic Model.

 

1.      Act as a Physical Barrier

The plasma membrane protects all the components of the cell from the outside environment and allows separate activities to occur inside and outside the cell. The plasma membrane provides structural support to the cell. It leads near the cytoskeleton, which is a network of protein filaments inside the cell that hold all the parts of the cell in the fix place. This gives the cell its shape. Certain organisms such as plants and fungi have a cell wall in addition to the membrane. The cell wall is composed of molecules such as cellulose. It provides additional support to the cell, and it is why plant cells do not burst like animal cells do if too much water diffuses into them.

2.      Selective Permeability

Plasma membranes are selectively permeable or semi-permeable, means that only certain molecules can pass through them. Water, oxygen, and carbon dioxide can easily travel through the membrane. Generally, ions (e.g. sodium, potassium) and polar molecules cannot pass through the membrane; they must go through specific channels or pores in the membrane instead of freely diffusing through. This way, the membrane can control the rate at which certain molecules can enter and exit the cell.

3.      Endocytosis and Exocytosis

Endocytosis is when a cell ingests relatively larger contents than the single ions or molecules that pass through channels. Through endocytosis, a cell can take in large quantities of molecules or even whole bacteria from the extracellular fluid. Exocytosis is when the cell releases these materials. The cell membrane plays an important role in both of these processes. The shape of the membrane itself changes to allow molecules to enter or exit the cell. It also forms vacuoles, small bubbles of membrane that can transport many molecules at once, in order to transport materials to different places in the cell.

4.      Cell Signaling

Another important function of the membrane is to facilitate communication and signaling between cells. It does so through the use of various proteins and carbohydrates in the membrane. Proteins on the cell “mark” that cell so that other cells can identify it. The membrane also has receptors that allow it to carry out certain tasks when molecules such as hormones bind to those receptors.

5.      Phospholipids

The membrane is partially made up of molecules called phospholipids, which spontaneously arrange themselves into a double layer with hydrophilic (“water loving”) heads on the outside and hydrophobic (“water hating”) tails on the inside. These interactions with water are what allow plasma membranes to form. . In 1925, two Dutch scientists (E. Gorter and R. Grendel) extracted the membrane lipids from a known number of red blood cells, corresponding to a known surface area of plasma membrane. They then determined the surface area occupied by a monolayer of the extracted lipid spread out at an air-water interface. The surface area of the lipid monolayer turned out to be twice that occupied by the erythrocyte plasma membranes, leading to the conclusion that the membranes consisted of lipid bilayers rather than monolayers.

The bilayer structure of the erythrocyte plasma membrane is clearly evident in high-magnification electron micrographs

6.      Proteins

Proteins are wedged between the lipids that make up the membrane, and these transmembrane proteins allow molecules that couldn’t enter the cell otherwise to pass through by forming channels, pores or gates. In this way, the cell controls the flow of these molecules as they enter and exit. Proteins in the cell membrane play a role in many other functions, such as cell signaling, cell recognition, and enzyme activity.

7.      Carbohydrates

Carbohydrates are also found in the plasma membrane; specifically, most carbohydrates in the membrane are part of glycoproteins, which are formed when a carbohydrate attaches to a protein. Glycoproteins play a role in the interactions between cells, including cell adhesion, the process by which cells attach to each other.

Fluid Mosaic Model

A model that describes the composition of the plasma membrane and how phospholipids, proteins, and carbohydrates freely move within it.

Technically, the cell membrane is a liquid. At room temperature, it has about the same consistency as vegetable oil. Lipids, proteins, and carbohydrates in the plasma membrane can diffuse freely throughout the cell membrane; they are essentially floating across its surface. This is known as the fluid mosaic model, which was coined by S.J. Singer and G.L. Nicolson in 1972.

Factors Affecting Fluidity of Plasma Membrane

The fluidity of the cell membrane is influenced by three factors:

1.      Temperature

Phospholipids are found close together when it is cold. When it’s hot, they move apart.

2.      Cholesterol

The cholesterol molecules are randomly distributed along the phospholipid bilayer and hold it preventing it from separating too far, or compact too tightly.

3.      Saturated and Unsaturated Fatty Acids

Fatty acids make up the phospholipid tails. Saturated fatty acid chains have a single bond between the carbon atoms whereas, unsaturated fatty acid chains have double bonds between the carbon atoms.

Double bonds make it harder for the chain to pack tightly by creating kinks. These kinks increase the fluidity of the membrane.

Restriction to Fluidity of Plasma Membrane

Lipid Rafts

These are the lipid domains found on the external leaflet of the plasma membrane. Cholesterol, glycosphingolipids, glycosylphosphatidylinositol are the building blocks of lipid rafts.

Protein Complexes

Proteins and glycoproteins are diffused within the plasma membrane. These help in the transport of ions and metabolites, cell signalling, adhesion, and migration.

Key Points on Fluid Mosaic Model

• The plasma membrane comprises amphiphilic, phospholipid molecules.
• The second important component of the plasma membrane is integral proteins that are integrated completely into the membrane.
• Carbohydrates are found on the external surface of the membrane where they are bound to proteins or lipids.