Cell membrane class12

 Cell membrane

Let's talk about cell membranes- 

The cell membrane also referred to as the plasma membrane, exists in all cells and acts as a barrier, separating the cell's interior from the external environment. The discovery of cells by Robert Hooke in 1665 laid the foundation for the Cell Theory. However, Hooke's observations led to a misconception regarding the cell membrane, as he mistakenly believed that all cells had a rigid cell wall, based on his limited observations of plant cells at that time.

For more than 150 years, microscopists focused on the cell wall until advancements in microscopy techniques were achieved. In the early 19th century, it was realized that cells were distinct entities, each with its own individual cell wall, and were not interconnected as previously thought. This revelation came after the discovery that plant cells could be separated from each other.

In 1895, Ernest Overton proposed the theory that cell membranes were composed of lipids, providing a significant advancement in our understanding of cell structure and function.

In 1925, Gorter and Grendel put forward the lipid bilayer hypothesis, which sparked speculation regarding the structure of the cell membrane. Their proposal drew upon crystallographic studies and observations of soap bubbles to describe the bilayer arrangement.

What is a cell membrane?

The semipermeable membrane surrounds the cytoplasm of a cell.

Cell membrane

Plasma membrane

The thin membrane surrounds each of the living cells and delimits the cell from the atmosphere around it. enveloped by this plasma membrane (also referred to as the plasma membrane) are the cell’s constituents, usually giant, soluble, extremely charged molecules like proteins, nucleic acids, carbohydrates, and substances concerned in cellular metabolism. Outside the cell, within the close water-based atmosphere, are ions, acids, and alkalis that are noxious to the cell, additionally, as nutrients that the cell should absorb to measure and grow. The plasma membrane, therefore, has 2 functions: initial, to be a barrier keeping the constituents of the cell in and unwanted substances out and, second, to be a gate permitting transport into the cell of essential nutrients and movement from the cell of waste merchandise.

Cell membranes primarily consist of lipids based on fatty acids and proteins. Membrane lipids are in the main 2 varieties, phospholipids and sterols (generally cholesterol). each variety shares the shaping characteristic of lipids—they dissolve promptly in organic solvents—but additionally, they each have a section that's drawn to and soluble in water. This “amphiphilic” property (having a twin attraction; i.e., containing each a lipid-soluble and a soluble region) is basic to the role of lipids as building blocks of cellular membranes. Membrane proteins also are of 2 general varieties. One type, referred to as the adscititious proteins, is loosely hooked up by ionic bonds or metallic element bridges to the electrically charged phosphoryl surface of the bilayer. they'll conjointly attach to the second kind of macromolecule, referred to as the intrinsic proteins. The intrinsic proteins, as their name implies, are firmly embedded inside the lipide bilayer. In general, membranes actively concerned with metabolism contain a better proportion of macromolecules.

Functions of cell membrane- Physical Barrier, Selectively permeable, Transporting Solutes, Transporting Macromolecules, Responding to External Signals, Intercellular Interaction, Energy Transduction. 

1. Physical Barrier

 The cell membrane (plasma membrane) is surrounded by all the cells and separates the cells into the cytoplasm which is the material by which the cell is made.  This 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 the structure and shape of the cell. 

2. Selectively permeable

The cell membrane is selectively permeable or semi-permeable. This implies that the membrane selectively allows certain molecules to pass through, while others are restricted. Water, oxygen, and carbon dioxide are among the molecules that can easily traverse the membrane. On the other hand, ions like sodium and potassium, as well as polar molecules, cannot pass through freely. Instead, they must rely on specialized channels or pores in the membrane for transport. This controlled passage of molecules enables the membrane to regulate the rate at which certain substances can enter and leave the cell.

3. Transporting Solutes 
4. Transporting Macromolecules
5. Responding to External Signals 
6. Intercellular Interaction 
7. Energy Transduction. 
Etc...

Transportation by the cell membrane (plasma membrane)

(i) Passive Transport:

It is a type of diffu­sion in which an ion or molecule crossing a membrane moves down its electrochemical or concentration gra­dient.

It is of the following types:

(a) Osmosis:

The process by which the water molecules pass through a semipermeable membrane from a region of higher water concentration to a region of lower water concen­tration is known as osmosis.

(b) Simple Diffusion:

In simple diffu­sion, transport across the membrane takes place without the help of any permease and occurs only in the direction of a concentration gradient. During simple diffusion, a small molecule in an aqueous solution dis­solves into the phospholipid bilayer crosses it, and then dissolves into the aqueous solution on the opposite side. There is little specificity to the process.

(c) Facilitated Diffusion:

In this unique form of passive transport, ions or molecules traverse the membrane swiftly, aided by permeases within the membrane. This process exclusively follows the concentration gradient and exhibits remarkable specificity. Each facilitated diffusion protein is designed to transport only a particular type of ion or molecule.

At present, the prevailing belief is that transport proteins create channels in the membrane, allowing specific ions or molecules to traverse across it. For example, Ca++ channels occur in axonal mem­branes for the entrance of Ca++ in the cell and glucose permease in the mammalian RBC facilitates the diffu­sion of glucose into the cell.

(ii) Active Transport:

During this process, ions or molecules traverse the membrane against the concentration gradient by utilizing metabolic energy. This is achieved through specific transport proteins known as pumps. For example, Na+-K+-ATPase – is an ion pump or cation exchange pump that is driven by the energy of one ATP molecule to export three Na+ outside the cell in exchange for the import of two K+ inside the cell.