What is the primary function of the cell membrane?

The primary function of the cell membrane is to regulate the passage of substances into and out of the cell, acting as a selectively permeable barrier. This control is crucial for maintaining the cell's internal environment (homeostasis), ensuring it receives necessary nutrients, expels waste products, and maintains appropriate ion concentrations. Beyond transport, it also plays vital roles in cell-to-cell communication, recognition, and providing structural integrity to the cell.

How does the Fluid Mosaic Model describe the cell membrane?

The Fluid Mosaic Model describes the cell membrane as a dynamic, flexible structure rather than a rigid one. It proposes that the membrane is a 'mosaic' of various components—phospholipids, proteins, cholesterol, and carbohydrates—that are not static but are constantly moving and shifting within the lipid bilayer. The 'fluid' aspect refers to the ability of these components to move laterally, rotate, and flex, giving the membrane its adaptability and functional versatility.

What is the role of cholesterol in the cell membrane?

Cholesterol molecules are embedded within the phospholipid bilayer of animal cell membranes and act as a 'fluidity buffer'. At moderate temperatures, cholesterol reduces membrane fluidity by restricting the movement of phospholipids, preventing the membrane from becoming too fluid. At low temperatures, it prevents phospholipids from packing too tightly together, thus maintaining fluidity and preventing the membrane from becoming overly rigid or solidifying. This ensures the membrane remains functional across a range of temperatures.

Differentiate between integral and peripheral proteins in the cell membrane.

Integral proteins are deeply embedded within the lipid bilayer, often spanning the entire membrane (transmembrane proteins) or penetrating partially. They are tightly bound and difficult to remove without disrupting the membrane structure. Peripheral proteins, on the other hand, are loosely associated with the surface of the membrane, either on the cytoplasmic or extracellular side. They are not embedded in the hydrophobic core and can be easily detached without damaging the membrane's integrity. Both types contribute to various membrane functions.

What is selective permeability and why is it important?

Selective permeability refers to the cell membrane's ability to allow certain substances to pass through while blocking others. This property is crucial because it enables the cell to maintain a stable internal environment (homeostasis) that is distinct from its surroundings. It ensures that essential nutrients are taken in, metabolic waste products are expelled, and specific ion concentrations are maintained, all of which are vital for the cell's survival and proper functioning. Without it, the cell would be unable to regulate its composition.

What is the glycocalyx and what is its function?

The glycocalyx is a fuzzy, carbohydrate-rich layer found on the outer surface of the plasma membrane, particularly in animal cells. It is formed by the carbohydrate portions of glycolipids and glycoproteins. Its primary functions include cell-cell recognition, allowing cells to identify each other (crucial for immune responses and tissue formation), cell adhesion, and acting as receptor sites for various signaling molecules. It also provides some protection to the cell surface.

Cell Membrane

Biology

NEET UG

Version 1Updated 21 Mar 2026

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The cell membrane, also known as the plasma membrane, is a dynamic, selectively permeable outer boundary of the cytoplasm in all cells. Composed primarily of a phospholipid bilayer with embedded proteins, it regulates the passage of substances into and out of the cell, maintains cellular homeostasis, and facilitates cell-to-cell communication and recognition. Its fluid mosaic nature, as described …

Quick Summary

The cell membrane, or plasma membrane, is the outer boundary of all cells, separating the cell's interior from its external environment. It is primarily composed of a phospholipid bilayer, which forms the basic structural framework.

Embedded within and associated with this bilayer are various proteins, cholesterol (in animal cells), and carbohydrates. The most accepted model describing its structure is the Fluid Mosaic Model, which highlights its dynamic and flexible nature, where components can move laterally.

The membrane is selectively permeable, meaning it controls which substances enter and exit the cell, a critical function for maintaining cellular homeostasis. Transport across the membrane occurs via passive processes (diffusion, facilitated diffusion, osmosis) that do not require energy, and active processes (active transport, bulk transport like endocytosis and exocytosis) that do require energy.

Beyond transport, the cell membrane is vital for cell-cell recognition, signal transduction, and providing structural support.

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Key Concepts

Types of Membrane Proteins

Membrane proteins are crucial for the diverse functions of the cell membrane. They are categorized based on…

Passive vs. Active Transport

Transport across the cell membrane is broadly divided into passive and active processes based on energy…

Membrane Fluidity and Cholesterol

The fluidity of the cell membrane is a critical property that allows for membrane movement, fusion, and…

Structure: — Phospholipid bilayer (hydrophilic heads, hydrophobic tails).

Model: — Fluid Mosaic Model (Singer & Nicolson, 1972) - dynamic, fluid, mosaic of components.

Components: — Phospholipids, Proteins (Integral, Peripheral), Cholesterol (animal cells), Carbohydrates (Glycolipids, Glycoproteins).

Cholesterol: — Regulates fluidity (decreases at high temp, increases at low temp).

Glycocalyx: — Carbohydrate layer on outer surface; cell recognition, adhesion.

Permeability: — Selectively permeable.

Passive Transport (No ATP):

- Simple Diffusion: Small, nonpolar molecules (O $_2$ , CO $_2$ ) directly through bilayer. - Facilitated Diffusion: Larger polar molecules/ions via channel/carrier proteins (e.g., glucose, ions). - Osmosis: Water diffusion across membrane (aquaporins facilitate).