Introduction

Cells are the basic units of life that are capable of carrying out all the processes of life. They are the building blocks of all organisms. Some living organisms are made up of only one cell and are called unicellular, while the rest are made up of many cells called multicellular organisms. In both unicellular and multicellular organisms, the cells are kept separated from each other and surroundings by a barrier called plasma membrane or cell membrane. 

Cell membrane, also called the plasma membrane, is a physical barrier between a cell and the surrounding environment. It is the outermost part of the cell in animals. However, in plants, bacteria, and fungi, it is surrounded by a thick cell wall.  Everything that is contained within the cell is not allowed to leave it unless allowed by the plasma membrane. Similarly, nothing can enter the cell unless it is permeable through the cell membrane. 

In this article, we will discuss the structure and composition of the cell membrane as well as its functions. 

Fluid Mosaic Model

The structure of the cell membrane is described by the fluid mosaic model, a universally accepted model of the plasma membrane. According to this model, the cell membrane is considered as a liquid having two surfaces. It is composed of a lipid bilayer with proteins embedded in it. Let us study the detailed composition of this lipid bilayer and other substances found in the cell membrane.

Lipid Bilayer

The lipid bilayer is made up of two layers of phospholipids that are amphipathic compounds. Recall that a phospholipid molecule has a polar head and two nonpolar tails. The phospholipid molecules are arranged into two layers form a lipid bilayer. They are organized in such a way that the tails of the molecules in two layers face each other, while their heads are directed opposite. The heads are in contact with the water in the cytoplasm as well as in the extracellular fluid. 

Certain hydrophobic interactions develop among the tails of phospholipids, forming a spherical hydrophobic barrier around the cell. As a result, water and other dissolved substances in the cytoplasm or the extracellular fluid cannot enter or leave the cell. However, lipid-soluble compounds can easily cross this lipid bilayer. It is also impermeable to polar as well as charged compounds. 

Other lipids

In addition to the phospholipids, two other types of lipids are also present in the cell membrane. These are cholesterol and glycolipids. The percentage of these lipids in the cell membrane varies from cell to cell. 

Cholesterol

It is abundantly present in the cell membranes of animal cells. Cholesterol molecules are present in the spaces among the hydrophobic tails of phospholipids in the lipid bilayer.

The function of cholesterol is to regulate the fluidity of the cell membrane. According to the fluid mosaic model, the cell membrane is just like a fluid in which the individual components can move freely. Cholesterol molecules in the lipid bilayer check this mobility. The role of cholesterol is temperature-dependent. At high temperatures, it decreases the mobility of phospholipid molecules while at a lower temperature, it promotes their mobility.

Glycolipids

The amount of glycolipids also varies from cell to cell. They are mostly seen in the membranes of nerve cells. The hydrophobic chains of such lipids have an even number of fatty acids. The fatty acid chains are buried inside the lipid bilayer while the carbohydrate component is present either on the cellular or extracellular face of the membrane. 

The glycolipids serve to stabilize the cell membrane. They are also responsible for cell to cell interactions. Some glycolipids in the membrane also serve as cell surface receptors. The blood antigens are also glycolipids in nature, present on the cell membrane of red blood cells. The carbohydrate part of glycolipid is antigenic. 

Proteins

Proteins are the second-largest component of cell membranes. They make around 50% of the total cell membrane. They can be divided into two types;

• Integral proteins
• Lipid anchored proteins
• Peripheral proteins

Integral Proteins

These are the integral components of the cell membrane. Integral proteins are those that span throughout the width of the cell membrane. Therefore, they are also called transmembrane proteins. 

A transmembrane protein has the following structures;

• A hydrophilic domain that is present on the cytosolic side of the membrane. This domain interacts with the molecules present in the cytoplasm of the cells. 
• A hydrophobic domain that spans the width of the lipid bilayer. It is responsible for anchoring the protein in the cell membrane.
• Another hydrophilic domain that is present on the extracellular side of the membrane. It interacts with the molecules in the surrounding environment of the cell.

Examples of integral proteins are transport channel proteins like glucose transporter, potassium channels, etc. 

Anchored Proteins

Such proteins are not in contact with the cell membrane. They are attached to a lipid residue that is inserted into the lipid bilayer of the cell membrane. The protein is present either in the cytoplasm or in the extracellular fluid. 

Examples of anchored proteins include alkaline phosphatase enzyme on the extracellular surface and G-proteins coupled to receptors on the cytoplasmic side of the membrane. 

Peripheral Proteins

These are the proteins that are present only on one side of the cell membrane. They are present in association with the integral proteins or with the peripheral parts of the lipid bilayer.

These are the temporary proteins and lose their association with the cell membrane as soon as they have performed their function. Examples of such proteins include hormones attached to receptor proteins, etc. 

Carbohydrates

Carbohydrates are also present in the cell membranes. However, they are always present in conjugation with lipids or proteins. They are involved in cell to cell recognition and other inter-cellular interactions. 

Glycocalyx is formed by glycoproteins and glycolipids around some eukaryotic and prokaryotic cells. It is responsible for cell to cell adhesions. It is mostly seen in the case of the epithelial cells and bacterial cells. 

Membrane Polarity

An important concept that must be understood in the context of cell membranes is membrane polarity. It means that the charges on the two sides of the cell membrane are not equal.

In a resting cell, the cytoplasm has more negative charges as compared to the extracellular fluid. It is due to the presence of organic anions in the cytoplasm. Also, potassium ions keep diffusing outside the cell down the concentration gradient. This further makes the cytoplasm negative as compared to the extracellular fluid. 

On the other hand, extracellular fluid is rich in cations, the major cations being the sodium ions. Due to this difference of charges on both sides, the cell membrane is never neutral. Rather, it shows electrical polarity. The cytoplasmic side carries a negative charge while the extracellular side carries a positive charge with respect to each other. 

Functions

In this section, we will discuss the several functions performed by cell membrane concerning a cell. 

Cell membrane acts as Barrier

This is the most important function performed by the cell membrane. It acts as a barrier that controls the movements of substances across the cell. It is a selectively permeable membrane that allows only limited substances to pass through it. 

Different substances can cross the cell membrane via one of the following processes. 

Passive Diffusion

It is the movement of substances down their concentration gradient without using energy or any carrier protein. Cell membrane allows simple diffusion of only lipid-soluble substances. The hydrophobic compounds like steroid hormones can easily cross the cell membranes via simple diffusion. Besides, gases like CO₂ and O₂  can also freely diffuse across the cell membranes. 

However, polar substances like water as well as charged compounds cannot simply diffuse through the cell membranes. They need some type of channel proteins to cross them. 

Facilitated Diffusion

This type of diffusion is facilitated by some channel proteins. It is the process by which substances move down the concentration gradient by passing through a channel proteins. Polar and charged compounds use specialized transmembrane proteins called channel proteins to cross the cell membranes. 

Examples of such protein channels are potassium channels that allow the diffusion of potassium ions, and aquaporins that allow the diffusion of water molecules across the membranes. 

Nutrients such as sugars and amino acids also enter or leave the cell via specialized transport channels. 

Active Transport

It’s the movements of substances against the concentration gradient using energy in the form of ATP.  It takes place via specialized transmembrane proteins that have an integral ATPase activity so the they can breakdown the ATP and use the energy that is released. 

Cell membrane is necessary for Cell Signalling

Cell signalling is the process by which various metabolic processes taking place in the cell are controlled and regulated to maintain homeostasis. It is necessary for the overall coordination of the body. 

Cells respond to the signals when a ligand binds to the cell surface receptors. These receptors are mostly proteins present in the cell membrane. This is true for all types of signalling molecules except the steroid hormones that are lipid-soluble. 

Cell membrane anchors the Cytoskeleton

The cytoskeletal framework of the cell is necessary for the maintenance of its shape and structure. It provides structural and mechanical support to the cell. Cytoskeleton is anchored to the cell membrane via linker proteins such as integrin. The microfilaments and other components of the cytoskeleton are kept anchored to the cell membrane via these anchoring proteins. 

The anchoring of the cytoskeleton is necessary for its function. If the framework is not anchored, it will not be able to provide mechanical support to the cell. 

Cell membrane is Involved in Cell to Cell recognition

This function is performed by glycolipids and glycoproteins present on the outer surface of cell membranes. Cells belonging to the same tissue have a particular arrangement of glycolipid and glycoproteins on the membrane that can be recognized by partner cells but not by the cells of the other tissues.

Endocytosis and Exocytosis

These are the methods of transporting bulk substances across the cell membranes. In endocytoses, the membrane of the cell extends around the large substance to form an endocytic vesicle. This vesicle is later taken in by the cell as it breaks from the cell membrane. During exocytosis, the preformed vesicles fuse the cytoplasmic surface of the cell membrane. The vesicle becomes a part of the membrane while the substance is dumped into the extracellular fluid. 

Summary

Cell membrane is the barrier that separates a cell from the surrounding cells and the extracellular environment. 

It is the outermost part of the cell except in the case of cells having a cell wall. 

The structure of the cell membrane is described by the fluid mosaic model which states that the cell membrane is a fluid-structure made up of a lipid bilayer along with the proteins floating in it. 

The lipid bilayer is made of phospholipids that are arranged in two layers with their tails facing each other. 

Other lipids present in the membrane are;

• Cholesterol, it regulates the fluidity of membrane
• Glycolipids, they stabilize the membrane

Three types of proteins are present in the cell membrane;

• Integral proteins, that span the entire width of membranes
• Anchored proteins, that are kept anchored to the membrane via lipids
• Peripheral proteins, they are temporarily attached to the membrane on either side

 The cell membrane has a polarity with a net negative charge on the inner surface of the membrane in the resting state. 

Cell membrane performs the following functions;

• It acts as a filter, only allowing selective substances to pass through it
• It is necessary for cell signaling
• It is required for anchoring the cytoskeletal framework of the cell
• It is involved in cell to cell recognition
• Endocytosis and exocytosis takes place through the cell membrane

Frequently Asked Questions

What is the cell membrane made of?

Cell membranes are made up of a lipid bilayer and proteins. The lipid bilayer is made up of phospholipids. Glycoproteins, glycolipids and cellulose are also present in cell membranes. 

What are the functions of the cell membrane?

The cell membrane makes the boundary of a cell and keeps its contents in place. It acts as a barrier controlling the movement of molecules into and out of the cell. It also plays an important role in cell signalling. 

Why cell membrane is important?

The cell membrane is important to maintain the integrity of the cell as well as the composition of the cytoplasm. It is needed for communication between the cells. It is also essential for regulating the substances entering or leaving the cell. 

Which substances can easily cross cell membranes?

Small, non-ionic particles can easily cross cell membranes. However, large particles and polar or charged substances are difficult to pass through the cell membrane unless a channel protein is present. 

References

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2. Alberts B, Johnson A, Lewis J, et al. (2002). Molecular Biology of the Cell(4th ed.). New York: Garland Science. ISBN 978-0-8153-3218-3. Archived from the original on 2017-12-20.
3. Gorter E, Grendel F (March 1925). “On Bimolecular Layers of Lipoids on the Chromocytes of the Blood”. The Journal of Experimental Medicine. 41 (4): 439–43. doi:10.1084/jem.41.4.439. PMC 2130960. PMID 19868999.
4. S J Singer and G L Nicolson.”The fluid mosaic model of the structure of cell membranes.” Science. (1972) 175. 720-731.
5. Sharp, L. W. (1921). Introduction To Cytology. New York: McGraw Hill, p. 42.
6. Kleinzeller, A. 1999. Charles Ernest Overton’s concept of a cell membrane. In: Membrane permeability: 100 years since Ernest Overton (ed. Deamer D.W., Kleinzeller A., Fambrough D.M.), pp. 1–18, Academic Press, San Diego,.