Chemical Constituents of Plasma membrane

 Chemical Constituents of Plasma membrane

All plasma membranes, regardless of source contains proteins and lipids. The ratio of proteins to lipids varies depending on the cell types.



Lipid

The basic structure of the plasma membrane is the lipid bilayer.This bilayer is composed of 2 leaflets of amphipathic liquid molecules, whose poar head groups are in contact with the intra or extracellular aqueous phase where as their non polar tails face each other, constituting the hydrophobic interior of the membrane.The primary physical forces for organising lipid bilayer are hydrophobic force of interactions. Apart from hydrophobic force, Van dar waals forces of interaction between the hydrocarbon tails favor close packing of tails.Finally there are electrostatic interaction and hydrogen bonding beteen the polar head groups and water molecules.All lipid molecules present in the lipid bilayer can be broadly classified into 3 classes

l Phospholipids

l Glycolipids

l Sterol

Phospholipids

Phospholipid are made up of 4 components:an alcohol(glycerol or sphingosine), fatty acids, phosphate, and an alcohol attached to the phosphate.The fatty components are hydrophobic where as the remainder of the molecule has hydrophilic.There are two types of phospholipids: glycerophospholipids and sphingophospholipids.

Phosphlipids derived from glycerol are called glycerophospholipids. Glycerophospholipids contain glycerol, fatty acids, phosphate and an alcohol. Phosphoglycerides are the most numerous phospholipid molecules found in plasma membrane. Phosphoglyceride molecules are classified according to the types of alcohol linked to the phosphate group.

Eg: If the alcohol is linked to choline, the molecule is called phosphatidylcholine.

If the alcohol is linked to serine, the molecule is called phosphatidylserine.

Sphingophospholipids contain an amino alcohol called sphingosine instead of glycerol,fatty acid, phosphate and an alcohol attached to the phosphate.In Sphingophospholipid, the amino group of sphingosine backbone is linked to a fatty acid by an amide bond. Sphingomyelin is the most abundant sphingophospholipid.The plasma membrane of animals contain 4 major phospholipids such as phosphatidylcholine (the most abundant glycerophospholipids in th plasma membrane) phosphatidylserine, phosphatidylethanolamine and sphingomyelin. At neutral

 

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Carbohydrates

 

 

Proteins

 Peripheral membrane proteins

 Peripheral membrane proteins are proteins that are associated with the cell membrane but do not span its hydrophobic core. They can be found on the cytoplasmic or extracellular face of the membrane, and are typically attached through interactions with integral membrane proteins or by binding to the polar head groups of membrane lipids. Many of the proteins associated with the membrane can be released from the membrane by relatively gentle extraction procedures such as by changing pH or ionic strength. These procedures interfere with weak non covalent interactions between protein-protein or protein membrane lipid but leave the lipid bilayer intact, thus these proteins are referred to as peripheral proteins.(extrinsic proteins)These proteins are bound to membranes  primarily by electrostatic interactions and hydrogen bonds and do not interact with hydrophobic core of the phospholipid bilayer. Most peripheral proteins are soluble in aqueous solution.

Types of Peripheral Membrane Proteins:

Proteins that bind to integral membrane proteins:

These proteins associate with other membrane proteins, often through electrostatic or hydrophobic interactions. Examples include some proteins involved in electron transport chains (like cytochrome c) and proteins that form the membrane skeleton (like spectrin and ankyrin in red blood cells). Spectrin acts as a structural protein, determines the shape of the RBC.

 

Proteins that bind to membrane lipids:

These proteins interact with the polar head groups of membrane lipids, often through specific binding domains. Examples include phospholipases and lipases, which are involved in lipid metabolism. 

 

Amphitropic proteins:

 

These proteins can exist both in the cytoplasm(cytosol) and associated with the membrane. They can reversibly bind to the membrane via interactions with lipids or proteins. 

 

Key Characteristics of Peripheral Membrane Proteins:

 

Easily removed from the membrane:

Unlike integral membrane proteins, peripheral proteins can be detached without disrupting the lipid bilayer, often by using high salt concentrations or pH changes. 

 

Often involved in signaling and cellular processes:

Many peripheral proteins play crucial roles in signal transduction, cellular communication, and cytoskeletal interactions. 

Examples of Peripheral Membrane Proteins:

· Cytochrome c: Involved in electron transport and ATP synthesis in mitochondria. 

· Spectrin and ankyrin: Important for maintaining the structure of red blood cell membranes. 

· Phospholipases and lipases: Enzymes involved in lipid metabolism. 

· Annexins: A family of proteins involved in calcium signaling and membrane trafficking. 

· Cytoskeletal proteins: Proteins that interact with the cytoskeleton and provide structural support to the cell membrane.

 

 

 

Integral membrane proteins

Proteins that are held in the lipid bilayer very tightly through hydrophobic interactions and can be seperated from lipid bilayer only by treatment with detergents that disrupt bilayer are called integral proteins.Integral membrane proteins are permanently embedded within the lipid bilayer of cell membranes. These proteins may be monotopic (interacting with just one leaflet of the bilayer ) or polytopic (transverses the lipid bilayer once or several times)  They are crucial for various cellular functions, including transport, signal transduction, and cell adhesion. These proteins can span the entire membrane (transmembrane proteins) or be anchored to one side of the membrane (integral monotopic). Integral membrane proteins are mostly polytopic transmembrane proteins that have one or more segments embedded in the phospholipid bilayer.Transmembrane proteins contain non polar  amino acids residues with hydrophobic side chains that interact with fatty acyl groups of the membrane phospholipids, thus anchoring the protein to the membrane. All transmembrane are amphipathic.Transmembrane proteins may be single pass or multipass.Glycophorin is a major single pass membrane of RBC.It is a small glycoprotein of 131 amino acid residues. Band 3 protein(chloride-bicarbonate exchanger) is a 95 KDa multipass trans membrane protein.It is responsible for mediating the electroneutral exchange of chloride for bicarbonate.

Key characteristics of integral membrane proteins:

 

Permanent Attachment:

Unlike peripheral membrane proteins, integral proteins are not easily removed from the membrane. 

 

Hydrophobic Interactions:

They are embedded in the hydrophobic core of the lipid bilayer due to hydrophobic amino acid side chains. 

 

Transmembrane or Anchored:

They can either span the entire membrane (transmembrane) or be associated with only one side (integral monotopic). 

 

Diverse Functions:

They play vital roles in:

· Transport: Moving molecules across the membrane (e.g., channels, transporters). 

· Signal Transduction: Transmitting signals from the outside to the inside of the cell. 

· Cell Adhesion: Connecting cells to each other or to the extracellular matrix. 

· Energy Generation: Participating in processes like photosynthesis and cellular respiration. 

Examples of integral membrane proteins:

· Transmembrane Receptors: Proteins that bind to signaling molecules outside the cell and trigger a response inside the cell. 

· Ion Channels: Proteins that create pores in the membrane, allowing ions to pass through. 

· Transport Proteins: Proteins that carry specific molecules across the membrane. 

· Enzymes: Proteins that catalyze reactions within or at the membrane. 

Importance of integral membrane proteins:

Cellular Processes:

They are essential for various cellular processes, making them vital for life. 

 

Drug Targets:

Many integral membrane proteins are targets for drug development due to their functional importance. 

 

Disease Mechanisms:

Understanding their structure and function is crucial for understanding and treating diseases. 

 

References:

Pranav Kumar, Usha Mina, Life Sciences Fundamentals and practice I  Eight Edition, Pathfinder Publication, New Delhi.

Dr. Veer Bala Rastogi, Introduction to Cytology,Tenth Edition

PKG Nair, K Prabhakar Achar, A Textbook of Cell Biology, Konark Publishers Pvt Ltd.

https://microbenotes.com/sandwich-davson-danielli-model-of-cell-membrane/

 





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