Cell Membrane | Plasma Membrane

What is Cell Membrane | Plasma membrane

The cell membrane, which is also known as the plasma membrane, is a thin, semi-permeable membrane or barrier that separates the inside of the cell (nucleus + cytoplasm) from the outside extracellular environment. It maintains the integrity of the interior organization of the cell as it allows only selected substances to move into the cell and keeps unwanted substances outside. The cell membrane is the outermost covering or layer of animal cells. Whereas, in plants, the cell wall is the outermost layer of the plant cells.

Apart from cells, some cell organelles that are located in the cytoplasm are also surrounded by membranes. For example, endoplasmic reticulum, lysosomes, vacuoles, Golgi apparatus, and nucleus. Chloroplasts and mitochondria are surrounded by a double membrane. The membrane of different organelles varies in composition as they are suited for the specific functions performed by organelles. The membrane of cell organelles plays a vital role in various cell functions like protein synthesis, cellular respiration and lipid production.

Structure of Cell Membrane:

Structure of the cell membrane can be understood from the fluid mosaic model. The Fluid mosaic model for the structure of the plasma membrane was proposed by S.J. Singer and Garth L. Nicolson in 1972. They called their model as proteins icebergs in the sea of lipids. This model was widely accepted across the world and has evolved somewhat over time and still provides an accurate basic description of the structure and function of the plasma membrane.

According to the fluid mosaic model, the plasma membrane is a mosaic of different components and the major components of the cell membrane include phospholipids and proteins. Besides this, the carbohydrates are present on the outer surface of the cell membrane. They may be attached to proteins to form glycoproteins or may be attached to lipids to form glycolipids. So, a glycolipid, which is made of carbohydrate and lipid, and glycoprotein, which is a combination of carbohydrate and protein, is also present in the cell membrane.

The protein present in the cell membrane is of two types; intrinsic and extrinsic. Intrinsic proteins can be partially or completely embedded in the phospholipid bilayer and when completely embedded they extend from one side of the bilayer to the other side. Whereas, extrinsic proteins are present externally on the surface of the lipid bilayer.

So, the cell membrane mainly consists of a mix of proteins and lipids. Lipids in the form of phospholipids that make membrane flexible and proteins control and maintain the chemical environment of the cell and help in the transfer of molecules across the membrane. Due to the flexibility of the cell membrane, the cells like red blood cells and white blood cells are able to change their shape as they pass through narrow blood capillaries.

Components of cell membrane:

The cell membrane consists of lipids, proteins and carbohydrates. The proteins can be up to 60 to 80 %. The lipids, which is present in the form of phospholipid and cholesterol, can be up to 20 to 60 %. The carbohydrates are 2 to 10% and are present in the form of glycolipids and glycoproteins. The arrangement or distribution of these components is not uniform.

The arrangement and proportion of cell membrane components depend on the location of the cell and its functions in the body. So, the proportions of these components vary from cell to cell, for example, a normal human cell membrane generally has around 50 percent of protein by mass and around 40 percent of lipids, the remaining 10 percent is occupied by carbohydrates. For example, in the cell membrane of human red blood cells, lipids are around 30%. The inner membrane of mitochondria contains 76 % proteins and 24 percent lipid.

The major cell membrane components are described below;

1) Cell Membrane Lipids: It can be of the following types:

i) Phospholipid: It is the major component of the cell membrane. There are two layers of phospholipids in the membrane, which is called a phospholipid bilayer. A phospholipid molecule has two parts, one is phosphate head as it is attached to a phosphate group and is hydrophilic (water-loving or attraction for water) in nature as it is a polar group it attracts other polar molecules like water, glucose, etc.

The second part of phospholipid is two tails, which are made up of fatty acids so it does not form hydrogen bonds with water and hence it is non-polar and hydrophobic in nature. So, the phospholipid is a lipid that is made of a phosphate linked head group and two fatty acid tails. Besides this, the phospholipid molecule is amphipathic which means both hydrophilic and hydrophobic in nature. This nature of the membrane helps it maintain its semi permeability.

The arrangement of two layers of phospholipids in the phospholipid bilayer is such that the hydrophilic heads of the phospholipids face outward and their tails face inward towards each other. This arrangement of phospholipids forms the phospholipid bilayer.

Furthermore, the phospholipid molecules show two types of movement. The first movement is the transition movement in which this phospholipid molecule moves in the same layer. The other is the flip-flop movement in which the phospholipid molecule moves from one to other layer of the phospholipid bilayer.

ii) Cholesterol: It is another lipid part of the cell membrane. The cholesterol molecules are present in the cell membrane embedded in the hydrophobic region (tail part) of the phospholipid bilayer. This arrangement prevents the cell membrane from becoming stiff as it does not allow phospholipids to closely pack together. Thus, cholesterol provides the flexibility and rigidity to the cell. It is absent in the cell membrane of plants.

iii) Glycolipids: This lipid is present on the surface of the cell membrane. A carbohydrate chain is attached to it so it becomes a glycolipid. It enables cells to recognize other cells.

2) Cell Membrane Proteins: There are two types of proteins in the cell membrane.

i) Peripheral proteins: They are also called extrinsic proteins as they are located on the exterior portion or outer surface of the membrane and are attached to the membrane through interactions with other membrane proteins.

They do not extend from one side to the other side of the membrane. They may be attached to the phosphate heads of phospholipids or integral proteins. They are loosely attached as they do not form strong bonds with membrane like integral proteins. As they are loosely attached, they can detach and attach to the membrane while transporting substance in or out of the cell.

ii) Integral proteins: They are also called intrinsic proteins. They are deeply inserted or embedded into the phospholipid bilayer of the membrane. Most of them that pass through the membrane, stretch from one side of the membrane to its other side, are known as transmembrane proteins.

Intrinsic proteins provide a channel to the polar molecules like glucose, water, ions to move in or out of the cell membrane. The polar molecules cannot move without this passage provided by the intrinsic proteins as the non-polar hydrophobic (water-repelling) part, which is made of tails, does not allow their movement. This transport of molecules through intrinsic proteins is called facilitated transport or passive transport as it does not require energy. It occurs from higher concentration to lower concentration or along the concentration gradient. For example, if the concentration of glucose is higher on the outside than the inside of the cell, the glucose can move along the concentration gradient from outside to the inside (cytoplasm) of the cell through intrinsic proteins.

Functions of the Cell Membrane:

The cell membrane is a vital component of the cell that enables cells to perform various important functions. Some of the major functions of the cell membrane are as follows:

• Physical Barrier: It acts as a physical barrier that separates the cytoplasm of the cell from the extracellular fluid and thus protects the cell contents from the outside environment.
• Shape and structure: It gives the cell its shape and also provides structural support to the cell.
• Semi-permeable: It is a semi-permeable membrane which means it allows only certain molecules to pass through it.
• Endocytosis and Exocytosis: It allows endocytosis, a process in which the cell ingests contents that is usually larger than ions and molecules. It also helps in exocytosis, a process in which cell releases the materials such as waste products. The membrane changes its shape to allow molecules to enter or exit the cell. It can also form vacuoles and bubbles to facilitate the transport of molecules.
• Regulates movement: It regulates the movement or transport of molecules and ions in and out of the cell. Thus, it also controls cell metabolism through the selective movement of substances across the membrane.
• Cell Signalling: It also carries out communication and signalling between cells with the help of proteins and carbohydrates in the membrane.
• Protect organelles: It also helps the membrane-bound organelles in maintaining their shape and internal environment and also protects them from the external environment.
• Compartmentalization: It helps create distinct compartments inside the cell like the membrane-bound nucleus, membrane-bound cell-organelles, and keep everything intact and well-organized.
• Ability to differentiate: It can distinguish different substances with the help of glycolipids and glycoproteins. It can differentiate between foreign substances and cells' own material and thus can form tissues and fight against microbes.
• Antigens: It contains antigens that identify blood groups, and helps in checking immune response, acceptance or rejection of an organ transplant, etc.
• Microvilli: They are finger-like projections on the plasm membrane. It is found on some cells such as epithelial cells. They increase surface area for absorption, secretion and cellular adhesion, and more.
• Impulse transmission: The nerve impulse is transmitted along the neurons due to the electrical changes in the plasma membrane of the neurons.
• Osmosis: Osmosis also occurs through the cell membrane. For example, root cells absorb water from the soil through osmosis.
• Enzymes: It also contains various enzymes that perform various catalytic activities. For example, the membrane of human red blood cells contains various enzymes such as ATPase, anion transport protein, adenylate cyclase, protein kinase, etc.
• Receptors: It also contains various receptors for hormones, neurotransmitters, antibodies and other biochemical. These receptors perform signal transduction, which converts an extracellular signal to an intracellular signal.
• Movement: Some cell membranes have temporary cytoplasmic projections that help in movement. For example, amoeba, macrophages, and white blood cells move by using pseudopodia.

Transport across the plasma membrane:

There are two methods of transport across the plasma membrane, which are passive transport and active transport. Let us see what exactly happens in these two types of transports;

Passive transport: As the name suggests, it occurs without the expenditure of energy such as ATP is not required. The movement of substances is slow as it does not involve energy. The movement is based on concentration gradient so the substances move from higher concentration to lower concentration. For example, 100 molecules are outside the cell and 20 inside then there will be a movement of molecules from outside to the inside of the cell across the membrane until 60 molecules are present on each side of the membrane. Passive transport can occur without a semi-permeable membrane and it is not affected by low temperature, absence of oxygen, metabolic inhibitors such as cyanide. Examples of passive transport include osmosis, diffusion, facilitated diffusion and filtration.

Active transport: As the name suggests, it is energy-dependent which means it requires energy, which may be in the form of ATP, to transport molecules across the cell membrane. The movement is fast as it uses energy. In the active transport, the movement can be against the concentration gradient which means from low concentration to higher concentration. Active transport occurs only through the semipermeable membrane as it needs special proteins that are present in the semipermeable membrane. It is also affected by certain conditions such as low temperature, absence of oxygen, and metabolic inhibitors like cyanide. The examples of active transport include Sodium-Potassium (Na/K) pump, Calcium (Ca) pump and endocytosis and exocytosis. Na⁺ ions are moved from low concentration (inside the cell) to a high concentration of Na⁺ ions (outside the cell) in interstitial fluid. Three sodium ions move out and two potassium ions move into the cell through active transport across the membrane.

Difference between Active Transport and Passive Transport: