Cell Membrane - Protector of the Cell

Simple diagram showing
lipid bilayer and a protein

Cell membrane is the outermost covering of the cell. The thickness varies from 0.1uM to several microns. Almost all the membranes have a typical fluid mosaic model. According to this model, the membrane is composed primarily of lipids and proteins. The lipids vary from 20 to 80 percent with the remainder being proteins. Why proteins and lipids? The lipids give membrane its flexibility while proteins are responsible for maintaining the environment of the cell and also helps in transportation of molecules.

Structure:
A. Lipid bilayer - The cell membrane has two layers (hence called bilayer). Various components of lipids are: 

i. Phospholipids - It is the major component of cell membrane. Forms a lipid bilayer in which the hydrophilic (water-loving) heads are arranged to face the extracellular fluid while the hydrophobic (water-repelling) tails face the cytosolic fluid. This layer is semi-permeable i.e.; it allows only certain molecules to pass through

ii. Glycolipids: Are present on cell membrane surfaces and they have a carbohydrate sugar chain attached to them. This helps in recognizing other cells of the body. 

iii. Cholesterol: Another lipid component of cell membrane. Helps to stiffen the membrane. However, cholesterol is not found in the cell membrane of the plants.

Fluid Mosaic Model of Cell Membrane

 
B. Proteins - There are several types of proteins present in the cell membrane. Some of them are: 

i. Structural proteins - These gives support and shape to the cell.
ii. Receptor proteins - They help cells communicate with their external environment through the use of hormones, neurotransmitters and other signaling molecules.
iii. Transport proteins (globular proteins) - These proteins transport molecules across cell membranes through facilitated diffusion.
iv. Glycoproteins - They have a carbohydrate chain attached to them. They are embedded in the cell membrane and help in cell to cell communications and also helps transport of molecule across the membrane.
v. Channel Proteins: These proteins allow molecules of certain size to pass through the membrane.

Functions:
a. Maintains cell shape - The cell membrane encloses the cell and defines it. Also, it maintains physical integrity of the cell. It forms a barrier between the interior an exterior of the cell thereby protecting the organelles from the outside environment.

 
b. Selectively permeable - The cell membrane has selective permeability meaning it allows certain molecules to pass through thereby regulating entry and exit of molecules. It just acts like a guard saying “Hey Dude! You are not permitted inside unless you have a license!”.

Diagram showing Passive Transport - Simple and Facilitated Diffusion

 
c. Passive transport - It involves moving molecules through membranes without the expenditure of energy. the movement is down the gradient. It involves the process of diffusion.

i. Simple Diffusion - It is a form of passive transport where only some substances like small ions     and molecules like carbon dioxide, oxygen and water can easily move across the plasma membrane. 

ii. Facilitated Diffusion - In this process, the molecules move across the membrane but with the help of membrane transport proteins which temporarily binds the molecule which is to be moved. Again, as no energy (ATP) is required, it is a passive process.

Diagram showing Active Transport

 
d. Active Transport - It involves moving the substances across the membrane against concentration gradient. Energy (ATP) is required for this process. Generally requires two carrier proteins - one to recognize the substance to be carried and one to release ATP to provide energy for protein carriers (pumps).

 
e. Exocytosis - Certain substances can be transported out of the cell by fusing vesicles with the membrane by the process called exocytosis.

Diagram showing the process of Exocytosis

f. Endocytosis - This is exactly opposite of exocytosis where the substances are moved into the cell via fusing with the cell membrane. Endocytosis involves pinocytosis (internalizing liquid substances), phagocytosis (internalizing solid particles) and receptor-mediated endocytosis

 
g. Markers and Signaling: Surface protein markers are embedded in the cell membrane that identify the cell, enabling nearby cells to communicate with each other.