Plasma Membrane – Definition, Structure, Components, Functions

The cell is the basic and functional unit of the cell. The components of the cell are the nucleus, cell membrane, mitochondrion, lysosomes, ribosomes, cytoplasm, etc. the shape of a cell can be flexible or rigid. Some cells are visible from the naked eyes and some are only visible through a microscope. the number of cells in a multicellular organism is indefinite.

The unit membrane which acts as an outer cover for all the other cell organelles such as lysosomes, mitochondria, ribosomes, chromosomes, and nucleus, is known as the cell membrane or plasma membrane. It is a selectively permeable membrane that allows selective particles to pass through it. It is present in animals, plants, and microorganisms. 

Structure

• A plasma membrane or cell membrane is a living, thin, flexible, and delicate membrane that is about 7 nm thick. 
• When seen under a microscope, it entirely appears as a thin sheet.
• Analysis shows that the membrane is 75 percent phospholipids, which form the main element in its structure. 
• It also contains proteins, cholesterol, and polysaccharides.

Components of Plasma membrane

It is composed of the following components:

• Phospholipids: these form 75 percent of the ultimate fabric of the membrane.
• Peripheral proteins: they are present in the inner and outer layer of the phospholipid bilayer but are not implanted in the hydrophobic core.
• Integral proteins: are found to be implanted in the phospholipid bilayer. 
• Cholesterol: it is folded between the hydrophobic parts of phospholipids.
• Carbohydrate: they are found to be attached to lipids or proteins on the extracellular sides of the membrane which leads to the formation of glycolipids and glycoproteins:

Significance Of Plasma Membrane

Since it is a selectively permeable membrane, therefore :

• The useful molecules(food, water, salts, and oxygen) enter the cell
• All the waste products and secretions ( nitrogenous waste and carbon dioxide) leaves the cell 
• The metabolic intermediate remains in the cell.

Functions Of Plasma Membrane

Some biological activities performed by the membrane are:

Provide protection

The primary function of the cell membrane is to protect the cell from its surrounding by allowing only certain particles to pass through it. This ability of the cell membrane is called semi permeability or selective permeability. it regulates the flow of ions and organic molecules into the cell.

Diffusion

This process is useful in the transportation of carbon dioxide, oxygen, etc. The molecules of these substances are very small thus they can easily diffuse through the membrane. The concentration of carbon dioxide in the cell’s outer environment is more than inside the cell. Therefore, due to this concentration difference inside and outside the cell, carbon dioxide moves out of the cell and thus diffusion occurs.

Osmosis

This process is useful in the spontaneous movement of water through the membrane. There can be three cases when the cell is in water:

• If the water concentration is more in the surroundings than in the cell then the water molecules will move in both directions but more water molecules will enter the cell than its leaves and thus cell swells up.
• If the water concentration in the surrounding is exactly the same as inside the cell then an equal amount of water will move in both direction and thus the size of the cell remain the same.
• If the water concentration in the surroundings is less than the cell again water will move in both directions but more water will leave the cell than it enters and thus cell shrinks.

A physical barrier

It physically separates the cell cytoplasm from the fluid present inside the cell. it protects all the components from the outside environment and allows separate activities to occur inside and outside the cell. it also provides structural support to the cell.

Cell signaling

The other important function of the cell membrane is to communicate and send signals among the cells. it does so by the use of proteins and carbohydrates.

Conceptual Questions

Question 1: What difference is between the cell walls and cell membranes?

Answer:

Cell wall	Cell membrane
It is present only in plants and some fungi, bacteria, and algae.	It is present in all types of cells- plants, animals, bacteria, fungi, and algae.
It is the outermost covering of plants.	It is the outermost covering of animals.
It is thick and rigid structure with a fixed shape.	It is thin and delicate in structure. It is flexible to change its shape when needed.
It is metabolically inactive.	It is metabolically active.

Question 2: What are the main functions of the cell membrane?

Answer:

The cell membrane is present in all types of cells- plants, animals, bacteria, fungi, and algae. The main function of the cell membrane is to protect the cell from its surroundings, regulation of solute exchange through it, and isolate cytoplasm from its surroundings.

Question 3: What is a plasma membrane made up of?

Answer: 

The plasma membrane is mainly made of proteins and lipids. it is made up of phospholipids bilayer .each phospholipid molecule has a head and a tail where the head is hydrophilic and the tail is hydrophobic. it also contains carbohydrates, cholesterol, etc.

Question 4: What is unique about cell membrane?

Answer:

The uniqueness of cell membrane is its ability of semi permeability which means it allows only certain materials to pass through it. it allows only ions and small organic molecules into the cell.

Question 5: What would happen without a cell membrane?

Answer:

There would be no cell without a cell membrane. if there would be no cell membrane the cell will not be protected and all the substances will pass through it. 

Question 6: Why do proteins not pass through the cell membrane?

Answer:

Small molecules penetrate the cell membrane by binding the membrane and then diffusing it. but due to the very big size of some proteins, they are unable to pass through it.

Question 7: Why it is called plasma membrane?

Answer:

Cell is made up of protoplasm(or simply plasma) which is semi-fluid living matter .thus this fluid is contained inside a membrane called the plasma membrane. 

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