What are the different functions of plasma membrane?

The plasma membrane, also known as the cell membrane, is a thin, flexible barrier that surrounds the cell, separating its interior from the external environment. It is a fundamental component of all living cells, whether they belong to prokaryotes (like bacteria) or eukaryotes (like plants, animals, and fungi). The plasma membrane plays a crucial role in maintaining the integrity of the cell and regulating the movement of substances in and out of the cell.

Function of Plasma Membrane

The key functions of the plasma membrane include selective permeability, protection, communication, cell recognition, structural support, transport, cell adhesion, and enzymatic activity.

01. Selective Permeability

It has selective permeability as it selectively allows the flow of molecules. Molecules can be transported across the plasma membrane by any one of the following three methods:

(a). Diffusion:

This process does not require energy. Molecules move from their regions of higher to lower concentration. Gases like carbon dioxide and oxygen pass through diffusion.

(b). Osmosis:

Osmosis is the diffusion of water through a semipermeable membrane. Osmosis occurs due to the imbalance of solutes between cell exterior and interior without expenditure of energy.

Solutions can be of three types:
  1. Isotonic: Two solutions that have a similar solute (salt or sugar) and water concentration are said to be isotonic (equal tension). In isotonic solutions, cells maintain their normal shape and function.
  2. Hypertonic: A solution that has a higher concentration of solutes or salts (less water concentration) than a cell is said to be hypertonic. A cell in a hypertonic solution shrinks because water molecules tend to diffuse into a hypertonic solution, resulting in the disruption of the plasma membrane.
  3. Hypotonic: In contrast, a solution that contains lesser salt (higher water concentration) in the medium than the cell is said to be hypotonic. Cells kept in a hypotonic solution take in excess water as water molecules enter inside the cells resulting in swelling and bursting of cells.

(c). Active Transport:

During active transport, energy or ATP is required for the movement of certain molecules across the membrane from the region of their lower concentration towards the region of their higher concentration with the help of protein carriers called transporters.

Apart from active transport and passive diffusion across the membrane, other mechanisms also allow certain substances in traversing the cell membrane:
  1. Endocytosis (taking the content in) – It is the process through which cells ingest extracellular substances. Endocytosis is of two types :
    1. Phagocytosis (cell eating) is the endocytosis of large particles. Immune cells are engaged in the phagocytosis of invading pathogens. The membrane folds out around the particle, generating a cavity, thus engulfing the particle.
    2. Pinocytosis (cell drinking) brings fluid-containing solutes into the cell through membrane vesicles.
  2. Exocytosis (passing the content out) – This process is the reverse of endocytosis and involves the fusion of the vacuoles, containing large quantities of materials, with the plasma membrane, as materials are discharged out of the cell.

Mechanisms of Selective Permeability

Lipid Bilayer Permeability:

  • The lipid bilayer itself is permeable to small, nonpolar molecules such as oxygen (O₂), carbon dioxide (CO₂), and lipophilic (fat-soluble) molecules. These molecules can diffuse directly across the membrane because they are not repelled by the hydrophobic core of the bilayer.
  • However, the bilayer is largely impermeable to polar molecules (e.g., water) and ions (e.g., sodium, potassium) due to the hydrophobic interior of the membrane. This prevents many substances from simply diffusing across the membrane.

Membrane Proteins:

  • Channel Proteins: These proteins form pores or channels that allow specific ions or molecules to pass through the membrane. These channels are often selective, meaning they only allow a particular type of ion or molecule to pass. For example, ion channels allow ions like sodium (Na⁺) and potassium (K⁺) to move across the membrane.
  • Carrier Proteins: These proteins bind to specific molecules on one side of the membrane, undergo a conformational change, and then release the molecule on the other side. Carrier proteins are essential for transporting large polar molecules, such as glucose and amino acids, across the membrane.
  • Aquaporins: These are specialized channel proteins that facilitate the rapid transport of water molecules across the membrane, addressing the challenge of water's polarity.

Active Transport:

  • In cases where substances need to be moved against their concentration gradient (from low to high concentration), the cell uses active transport. This process requires energy, usually in the form of ATP (adenosine triphosphate).
  • Pumps: Active transport is carried out by proteins known as pumps. A well-known example is the sodium-potassium pump (Na⁺/K⁺-ATPase), which actively transports sodium ions out of the cell and potassium ions into the cell, maintaining the necessary ion gradients.

Endocytosis and Exocytosis:

Larger molecules or particles that cannot pass through the membrane via channels or carriers are transported by endocytosis and exocytosis. In endocytosis, the plasma membrane engulfs the substance to bring it into the cell, while exocytosis involves vesicles fusing with the membrane to expel substances out of the cell.

02. Protection

The plasma membrane acts as a protective barrier, safeguarding the cell's internal components from external threats such as toxins and pathogens. It helps maintain the integrity of the cell by preventing the entry of harmful substances and controlling the internal conditions necessary for cellular function. In multicellular organisms, the plasma membrane also plays a role in immune responses by presenting specific markers that allow immune cells to recognize and protect the body's own cells from attack.

03. Communication

Cellular communication is another critical function of the plasma membrane. It is equipped with receptor proteins that detect and respond to signals from the environment and other cells. These receptors bind to signalling molecules such as hormones and neurotransmitters, initiating a cascade of intracellular events that result in specific cellular responses. This signalling is vital for processes such as growth, immune responses, and homeostasis.

04. Cell Recognition

The plasma membrane facilitates cell recognition, which is essential for immune responses, tissue formation, and cellular interactions. The membrane's surface contains glycoproteins and glycolipids that serve as identification tags, allowing cells to recognize and interact with each other. This function is particularly important in the immune system, where cells need to distinguish between "self" and "non-self" to protect the body from pathogens.

05. Structural Support

The plasma membrane provides structural support to the cell by anchoring the cytoskeleton, a network of protein filaments that maintains cell shape and enables movement. This interaction between the membrane and the cytoskeleton is crucial for maintaining the structural integrity of the cell, especially in tissues subject to mechanical stress.

06. Transport

Transport across the plasma membrane is essential for the cell's survival. Passive transport mechanisms like diffusion and osmosis allow substances to move along their concentration gradients, while active transport mechanisms require energy to move substances against their gradients. This controlled transport ensures that the cell maintains the necessary balance of ions, nutrients, and waste products.

07. Cell Adhesion

Cell adhesion, mediated by adhesion molecules in the plasma membrane, allows cells to stick together and form tissues. This is critical for processes such as tissue formation, wound healing, and embryonic development.

08. Enzymatic Activity

Lastly, the plasma membrane hosts enzymes that catalyse essential biochemical reactions, contributing to metabolic processes, signal transduction, and the synthesis of molecules necessary for cell function.



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SAQ 1

  1. What is a cell? What are the essential characteristics of cells?
  2. Explain the fluid mosaic model of the plasma membrane
  3. Which organelles are involved in photosynthesis?
  4. Why the mitochondria is called the powerhouse of the cell?
  5. Which organelle contains enzymes for cellular respiration?
  6. Why mitochondria and chloroplast are called semi-autonomous?
  7. Mention any two advantages of the extensive network of the endoplasmic reticulum
  8. What is the function of peroxisomes in plant cells?
  9. Explain the following terms: (a) chromatin network (b) chromosomes (c) Nucleosome (d) Solenoid Model
  10. What is the function of the nucleolus in the cell?

SAQ 2



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