Plasma Membrane – Structure And Function

Key Concepts

• Cell wall
• Cell membrane or plasma membrane

Introduction:

We know that all organisms consist of basic unit of life called cell. The body of lower organisms like bacteria, some amoeba is made up of single cell, whereas the body of higher organisms like plant and animals is made up of many cells. Hence, they are multicellular organisms. 

Cells vary in size and shape. Based on their function, the size and shape changes. Cells are very tiny and can be observed by using microscope. Let us discuss about the structure and functions of cell organelles. 

Cell components: 

The cell consists of various organelles such as cell membrane, ribosomes, mitochondria, Golgi apparatus, nucleus, vacuole, lysosomes. In this topic we will discuss about cell membrane structure and functions. 

Cell Membrane: 

It is sometimes also called as plasma membrane. It is the outermost layer of animal cells. In plant cells this layer is inside the cell wall. It is a very flexible membrane. It can be folded in and out. Plant and animal cells contain cell membrane. 

It is composed of proteins, lipids and carbohydrates. 

Phospholipid: 

Cholesterol: This is another lipid component present in cell membrane. These are selectively distributed between membrane phospholipids. It is present only in animal cell membrane, absent in plant cell membrane. 

Glycolipids: These are situated on the surface of cell membrane. They help the cell to recognize the other cells. 

Proteins: 

There are two types of proteins present in the cell membrane. These are peripheral membrane proteins and integral membrane proteins. 

Peripheral membrane proteins: 

These are outside and connected to the plasma membrane by connections with other proteins. 

Integral membrane proteins: 

These proteins are incorporated into the membrane, and they pass through the membrane. These proteins can be seen from both sides of the membrane. 

Structural proteins:

Help to give shape and support to the cell. 

Receptor proteins:

Help in the communication of cells with their outside environment with the help of neurotransmitters and other signaling molecules. These proteins have binding sites for substances such as hormones that can cause changes in the action of cells. 

Transport proteins:

Globular proteins are transport proteins that transport molecules through the plasma membrane by facilitated diffusion. So, transport protein allows water soluble substances to pass through the membrane which are open from both the sides. 

Recognition proteins:

These proteins recognize cell types and helps in cell to cell identification and coordination between them. 

Adhesion proteins:

These proteins are glycoproteins. It helps cells to remain connected with one another in a tissue. 

Glycoproteins:

It is attached with glycoprotein chain. These are enclosed in the plasma membrane and helps in the communication from one cell to another cell and in transport of molecules through the membrane.:  

Cell Membrane 

Cholesterol: 

It is also present in the plasma membrane. It prevents the effect of lower temperatures on the fluidity of the membrane. It also checks the effect of high temperatures from increasing the fluidity of the membrane. 

Carbohydrates: 

They are either attached to proteins or lipids. The proteins which are attached to carbohydrate are called glycoproteins and the carbohydrates that are attached to lipids are called glycolipids.  The carbohydrate form locations on the surface of plasma membrane that help the cells to identify each other. This identification is important as it helps the immune system to identify whether the cell is foreign body or part of the body cell.  

Functions of cell membrane (plasma membrane): 

• All the cell organelles are enclosed inside the plasma membrane. 
• Plasma membrane gives proper shape to the cell.  
• Selectively permeable membrane: It allows transport of selective substances inside the cell and outside the cell. Hence plasma membrane is called as selectively permeable membrane. It maintains the entry of required substances into the cytoplasm from external area and rejects the unwanted substances to enter in the cytoplasm.  
• Plasma membrane helps to keep adjacent cell in contact. 
• Plasma membrane reacts to external stimuli. The process of response to external stimuli is known as signal transduction. Plasma membrane consists of receptors that combine with ligand or some specific molecules. The interaction between ligand and membrane receptor may lead to the generation of a new signal that either stimulates or inhibits internal activities. Plasma membrane is involved in energy transduction. It is a process in which energy gets converted from one form to another. For example: In the process of photosynthesis, light energy is absorbed by pigments present in the cell membrane and converted to chemical energy. 

Summary:

• Cell is the basic unit of life.
• The cell consists of various organelles such as cell membrane, ribosomes, mitochondria, Golgi apparatus, nucleus, vacuole, lysosomes.
• Plasma membrane composed of proteins, lipids and carbohydrates.
• The plasma membrane has a phospholipid bilayer, which has many types of phospholipid molecules, with different sizes of head and tail molecules.
• The head and tail part of phospholipid join by glycerol molecule.
• There are two types of proteins present in the cell membrane. These are peripheral membrane proteins and integral membrane proteins.
• Carbohydrates are either attached to proteins or lipids.
• Plasma membrane is selectively permeable membrane.