Plasma Membrane: Definition, Types, Examples, Diagram, Questions

What is a Plasma Membrane?

The plasma membrane is also referred to as the cell membrane, and it is one of the vital structures of the cell, acting as a barrier between the inside and outside environment. It is made up of phospholipids with proteins as well as other particles forming the layer of the cell membrane. The plasma membrane is also characterized as a selectively permeable membrane acting as a filter to allow substances like ions, nutrition, and other products formed in cells to penetrate in or out of the cell.

The idea of the cell membrane was formulated in the 19th century when plant and animal cells were observed through the microscope. The plasma membrane is a living membrane that is complex in composition and serves an essential role in cell survival and function. Due to its ability to be selectively permeable and participate in several cellular activities, it plays a crucial role in cell biology.

Structure of the Plasma Membrane

The following describes the plasma membrane structure:

Phospholipid Bilayer

Each phospholipid molecule has one active part, known as its hydrophilic or water-loving interface, while the other two portions of the molecule are hydrophobic or water-hating. One fact resulting from this arrangement is that there is a practical divide between the internal and external parameters of the cell.

Amphipathic Nature

Essentially, phospholipids are molecules that are biphasic. They have both polar and nonpolar regions, the hydrophilic and hydrophobic regions, respectively. This property enables them to self-assemble into bilayers in an aqueous solution.

Membrane Proteins

Types and functions:

• Transport: The peripheral proteins are involved in the transport of ions and molecules through the cell membrane.

• Receptors: They interact with specific signalling molecules where receptor proteins will bind to the molecules so that the cellular actions can begin.

• Enzymatic: There are membrane proteins that, in addition, perform the function of enzymes as catalysts for biochemical processes.

• Structural: Some of the proteins in the plasma membrane are involved in holding the shape of the plasma membrane.

Carbohydrates

Cell Recognition and Adhesion: Carbohydrates are involved in the cell recognition and adhesion of other cells as well as any interaction with the components of the confining matrix.

Structure and types: Carbohydrates also differ in terms of their structure and hence their compositions which add to the differences in cell surface markers and signaling molecules.

Cholesterol

Cholesterol is embedded within the phospholipid layer of animal cells. They regulate the general fluidity and rigidity of membranes by decreasing permeability to water-soluble, small molecules, and avoiding increased fluidity at temperatures.

Cholesterol contributes to the maintenance of membrane fluidity under different environmental conditions since it assists in enhancing the organization of the structure of the membrane.

Functions of the Plasma Membrane

The functions of plasma membrane are listed below:

Selective Permeability

Mechanism and importance: As a selective barrier, the plasma membrane regulates the entry and exit of ions, nutrients, and waste products that assist in sustaining the cell's internal environment.

Passive and active transport: A selective membrane is another characteristic of all cells, which allows the regulation of the composition inside a cell and the occurrence of crucial biochemical reactions.

Transport Mechanisms

• Diffusion: Diffusion is the process of the movement of molecules from a place of high concentration to an area of low concentration with or without involving transport proteins.

• Osmosis: Osmosis is the passage of water molecules through the selectively permeable membrane with the movement from the dilute solution to a more concentrated solution.

• Active transport: Uses energy and is specific for certain transport proteins to transport molecules from areas of low concentration to ones of high concentration.

• Endocytosis and exocytosis: Transport mechanisms that involve the ingression of large molecules or particles into the cell or the expression of the same from the cell with the assistance of vesicles made from the cell membrane.

Cell Communication

Signal transduction pathways: Serve as conversion mechanisms for transmitting signals from the outside of the cell to the inside of the cell so that cells can react to their surroundings and synchronise events inside them.

Receptor-mediated signalling: Signalling molecules attach themselves to receptors on the surface of the cell. This triggers intracellular signalling that produces a cellular reaction.

Cell Adhesion

• Role in tissue formation: CAMs play an important role during cell-to-cell and cell-to-matrix contacts for the formation of tissues and organ architecture.

• Types of cell junctions: Tight junctions, desmosomes, and gap junctions are three types of cell junctions: each has its function of cell adhesion and communication.

Maintenance of Homeostasis

Role in maintaining the internal environment: Due to its selective, allowed-pass capabilities, it plays a role in the control of substances within the cell.

Involvement in cell signalling and transport: In its selective nature and through its functions in cell signalling, the plasma membrane is involved in aspects of cellular operation that maintain homeostasis.

Fluid Mosaic Model of Plasma Membrane

It is illustrated as a flexible and fluid organisation that is made up of lipids, proteins, and carbohydrates arranged in a mosaic pattern. In this case, phospholipid molecules form a fluid bilayer in which protein molecules are inserted or embedded in an arrangement similar to tiles in a mosaic.

Factors Affecting Membrane Fluidity

The factors that affect plasma membrane fluidity are mentioned below-

Temperature: The higher membrane fluidity caused by increased movement of lipids and proteins at high temperatures. With a decrease in the temperature of the cell, the fluidity of the cell membrane is reduced, resulting in a more rigid cell membrane.

Lipid Composition: The kind of lipids present in the membrane plays a role in determining its fluidity. Saturated fatty acids lead to the compact arrangement of molecules in the cell membrane since the fatty acid tails of these molecules are straight, while those of unsaturated fatty acids become ‘kinked’ because of the double bonds they contain and therefore make the membrane more fluid.

Cholesterol Content: Cholesterol is the regulator of membrane fluidity. At high temperatures, cholesterol decreases the fluidity of the membrane by solidifying it to some extent. At low temperatures, it avoids the stiffening of the membrane by cracking the simulation of lipid molecules.

Membrane Asymmetry

The kind of membrane asymmetry seen in plasma membrane is discussed below-

Distribution of lipids and proteins: Various lipids and proteins are also unequally partitioned between the inner and outer surfaces of the membrane. As an example, phosphatidylserine is commonly in the internal monolayer, while glycolipids are often in the external monolayer.

Functional implications: Asymmetry of membranes provides significant significance to the following cellular functions such as signalling, transport, and interactions. For instance, the lipid rafts which are affected in an asymmetric manner function as signals for incidents such as cell death and coagulation.

Facts about Plasma Membrane

The terms cell membrane and plasma membrane are therefore used haphazardly and this creates confusion. Here are some facts about the plasma membrane to help clarify their usage:

1. Definition: In fact, the plasma membrane and cell membrane are two names for the same thing: a phospholipid bilayer that surrounds a cell, thrusting the internal environment from the outside world.

2. Composition: The plasma membrane consists of a phospholipid bilayer in which there are some proteins and other molecules. It controls the transport of molecules across the cell membrane and is involved with cell signalling and reception.

3. Cellular Boundary: The plasma membrane acts as a sort of gatekeeper that regulates the movements of ions, nutrients, and waste products into and out of the cell. It is involved in the regulation of cellular housekeeping and shields the cell from the environment.

4. Fluid Mosaic Model: The constituents of the plasma membrane are lipids, protein and carbohydrates arranged in a way that refers to as the fluid mosaic model. This model focuses on the flexibility of the membrane as well as the distribution of various components of the mosaic model.

5. Functions: Some of the functions of the plasma membrane include understanding signal transmission, selective permeability, adhesion, and regulation of the composition of the cell and its internal environment. Cell Respiration means that it is essential to the survival and normal functioning of cells of all living organisms.

Comparison with Other Biological Membranes

Comparison of plasma membrane and other biological membrane:

Plasma Membrane NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Structure of Plasma Membrane

• Functions of plasma membrane

• Fluid Mosaic Model

• Comparison with other membranes

Practice Questions for NEET

Q1. Microvilli increases absorptive surface area of the cell which is formed due to

1. Evagination of cell wall

2. Evagination of plasma membrane

3. Invagination of cell wall

4. Invagination of plasma membrane

Correct answer: 2) Evagination of plasma membrane

Explanation:

Evagination is the process where the cell membrane creates out folds to form structures projecting from the cell surface. Microvilli are small projections that increase the absorption surface area and allow for close-fitting interactions between cells. The cell membrane also continues as outfolds over cilia and flagella, which are important for cell movement. These help in improving the functions of cells, such as nutrient uptake and motility.

Hence, the correct answer is option 2) Evagination of plasma membrane.

Q2. Which of the following is not the function of protoplasm

1. Reproduction

2. Excretion

3. Growth

4. Cell migration

Correct answer: 4) Cell migration

Explanation:

Cell migration refers to the movement of cells from one location to another, often within tissues or across surfaces. Protoplasm facilitates the synthesis of proteins and other molecules, transports nutrients and waste, and regulates genetic functions through the nucleoplasm.

Hence, the correct answer is option 4) Cell migration.

Q3. Proteins attached to the exterior hydrophilic region of the plasma membrane are

1. Intrinsic proteins

2. Peripheral proteins

3. Transmembrane proteins

4. Globular proteins

Correct answer: 2) Peripheral proteins

Explanation:

Peripheral proteins are loosely attached to the surface of the cell membrane, often interacting with the hydrophilic regions of integral transmembrane proteins or the membrane's polar head groups. Unlike integral proteins, they do not penetrate the lipid bilayer. These proteins are more easily detached from the membrane by altering the ionic environment or using mild detergents. Peripheral proteins play various roles, including signal transduction, enzyme activity, and maintaining the cell's shape by anchoring to the cytoskeleton.

Hence the correct answer is option 2) Peripheral proteins.

Also Read:

• MCQs on Plasma Membrane

• Cytoplasm

• Cell wall

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