Components of cell membrane

Introduction

Cells are the structural and functional units of the organism. They carry metabolic activity and hereditary information and hence an organism consists of one or more cells. A cell is so small that it is not visible to the naked eye and thus requires a microscope. At present small parts of the cell can be better observed by an electron microscope. The size of the cell varies and also has a specific structure depending upon the cell type. All cells have a cell membrane and other important organelles. the following article is all about the different components of cell membranes and their detailed discussion (1) & (5).

What is a cell membrane?

The word “cell membrane” was first used in 1855 by C. Nageli and C. Cramer.  A cell membrane is the outside part (protoplasm) of a cell that is covered by a fine, elastic, and semipermeable membrane. Hence it is a delicate, living, and microscopic membrane outside the cytoplasm that is made up of proteins and lipids. The cell membrane also has a role in interconnection, growth, and movement.

The cell membrane is also called the plasma membrane or plasmalemma. In 1913, the term plasmalemma was first used by scientist J. Q. Plowe. The cell membrane provides the distinction between intracellular and extracellular environments. The cell membrane preserves the cell organs and regulates the movement of various objects (3) & (2).

Components of cell membrane

Cell membranes are made up of about 40% lipids, about 60% of proteins, and 5 to 10% of carbohydrates. The lipids in the cell membrane consist of phospholipids and cholesterol molecules. And 60% of proteins include lipoproteins, glycoproteins, enzymatic proteins, carrier proteins, structural proteins, etc. Thus, the components of cell membrane components are discussed below (4).

1. Lipids

• One of the major components of cell membrane is lipid.
• The cell membrane is made up of about 40% lipids. Hence is the primary and significant component.
• Lipids are mainly in the form of phospholipids, cholesterol, and glycolipids.
• Cholesterol molecules are located between phospholipid molecules.
• The lipids of cell membranes have two ends, the head is called the hydrophilic end and the tail is called the hydrophobic end.
• Glycerol serves as a bridge between these two ends. That is why the lipid is called an amphipathic molecule.
• The head part of the phospholipid layer consists of one molecule of phosphate. These hydrophilic ends are also called polar ends and are electrically charged.
• Tail part of the phospholipid layer consists of two molecules of fatty acids. The other name of hydrophobic ends is non-polar ends thus uncharged.
• The tails or hydrophobic ends of the two-layer phospholipids are water-resistant and are located face to face.
• However, the hydrophilic ends or polar ends are water-absorbing and are located in two rows inside and outside the cell membrane.

Most of the lipids present in the cell membrane are phospholipid type. The phospholipid molecules are vibrated and arranged together. The feature of phospholipid molecules is that they are active all the time (1).

There are many different kinds of lipids located in the cell membrane. The following table discusses the lipids present in the cell membrane.

Function of lipids

1. It is the structural base of the cell membrane.
2. Lipids control membrane permeability. Thus lipid bilayer acts as a barrier between the cells.
3. It prevents some molecules and ions from moving in and out of the cell.
4. Lipids actively participate in the reaction of biosynthesis (1) & (3).

2. Proteins

Proteins are the second most abundant components of cell membrane after lipids and are composed of about 60% of the membrane. It is the structural block of cell membranes. There are many protein molecules floating between phospholipid and cholesterol molecules. Proteins are attached to the phospholipid layer. There are three types of protein found in the cell membrane (2).

1. Integral or intrinsic protein

• Integral or intrinsic proteins are located on the inner side of the phospholipid layer. These proteins are spread across those two phospholipid layers.
• About 70% of the total protein in the cell membrane is formed by integral proteins.
• These proteins are permanently attached to the cell membrane.
• The integral proteins that completely enter the lipid bi-layer are called transmembrane proteins.
• Integral proteins have both hydrophilic and hydrophobic parts.
• There are three types of Integral or intrinsic proteins. These are Integral monotopic proteins, integral bitopic proteins, and Integral Polytopic proteins.
• The monotopic proteins are attached to the membrane from one side.
• Integral bitopic proteins are projected both ways across the phospholipid layer.
• The integral polytopic proteins form channels in the cell membrane (1) & (5).

2. Peripheral or extrinsic protein

• The Peripheral protein molecules are partially water-soluble and partially water-resistant.
• They contain only hydrophilic parts. These proteins are located on the inner or outer surface of the phospholipid bilayer.
• About 30% of the total protein in the cell membrane is formed by the Peripheral proteins.
• These proteins are the outer or inner adjacent proteins of the phospholipid layer (2) & (4).

3. Lipid-anchored protein

• Lipid-anchored proteins are located on the surface of the cell or plasma membrane also called linked proteins.
• The names of the three types of lipid-anchored proteins located in the cell membrane are glycosylphosphatidylinositol-linked proteins, fatty acylated proteins, and prenylated proteins (2).

Besides the above proteins, some other types of proteins are found on the cell membrane.

Glycoprotein

The Glycoproteins are the receptors of the cell. It contains oligosaccharide chains attached to amino acids by covalent bonds. Glycoproteins are located on the surface of the lipid bi-layer of cell membranes. There are also some tiny carbohydrate chains attached to the protein molecules. This is called glycoprotein (1) & (3).

Lipoprotein

Lipoproteins also act as receptors. The function of lipoproteins is to transport hydrophobic lipid molecules in water. These proteins carry cholesterol. There are two types of lipoprotein in the cell membrane. These are low-density lipoprotein and high-density lipoprotein (3).

Structural protein

The primary function of these proteins is skeleton formation in the cell membrane (1).

Enzymatic protein

Enzymatic proteins are a type of catalytic protein. There are about 30 types of enzymatic proteins located in the cell membrane. These are Acetyl phosphate, lactase, acid phosphate, adenylate cyclase, NADH Oxidising enzymes, cholesterol esterase, alkaline phosphodiesterase, etc. (1).

Carrier protein

Carrier proteins help in transporting various types of ions inside the cell. These proteins form a channel in the cell membrane. Channel protein is another name for a carrier protein (1).

The function of the protein

1. Membrane proteins help to regulate metabolic processes in a specific way.
2. The glycoproteins and lipoproteins which are present in the cell membrane act as receptors.
3. Peripheral proteins are involved in the transfer of small molecules or electrons. The primary function of peripheral proteins is to manage and maintain both the intracellular cytoskeleton and extracellular matrix substances.
4. The integral proteins act as membrane transport proteins such as carrier proteins and channel proteins.
5. Proteins are helpful in the transportation of substances across the cell membrane in a variety of ways (3) & (2).

3. Carbohydrates

• There are about 5 to 10% carbohydrates located in the cell membrane.
• Carbohydrate chains are always located outside the membrane.
• Mainly 2 types of carbohydrates are found in the cell membrane.
• Many phospholipid molecules have small carbohydrate chains. It is called glycolipids.
• Glycolipids are not attached to the phospholipid layers by covalent bonds.
• The other carbohydrate chain is attached to the cell membrane protein. This is called glycoprotein.
• Glycoproteins and glycolipids are collectively called glycocalyxes. The glycocalyx can be observed with an electron microscope (2) & (5).

Function of carbohydrates

• Cell membrane helps in the identification of one cell from another cell.
• Carbohydrates participate in the structure of cell membranes.
• Cellulose, fibrous carbohydrates by which the cell wall of plant cells is formed.
• Carbohydrates protect the intestinal microvilli.
• Glycocalyx blocks the flow of fluid to the cells inside the blood vessels in the human body.
• Blood groups (A, B, AB, and O) are determined by the cell surface carbohydrates (1) & (2).

References

1. B Agarwal and V. K. Agarwal. Unified Botany, B.Sc. Third Year. Shiva Lal Agarwal & Company Publications, Indore. Chapter: plasma membrane: bilayer lipid structure and functions. Page no 222-223.

2. Chandrasekhar Chakrabarti.  A modern approach to a textbook of core Zoology, General & Honours. Nirmala Library, A Publishing House under the Prestigious International Standard Book Number (ISBN) System. Kolkata, (India). Part – II, chapter- cell and cellular organelles. Page: 2^nd -10- 2 to 2^nd – 10- 4.