Cell Membrane — Revision Notes

⚡ 30-Second Revision

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).

Active Transport (ATP required): — Moves against concentration gradient via pumps (e.g., Na$^+$/K$^+$ pump).
Bulk Transport: — Endocytosis (phagocytosis, pinocytosis, receptor-mediated), Exocytosis.

2-Minute Revision

The cell membrane, or plasma membrane, is the vital outer boundary of every cell, separating its internal environment from the external. Its structure is best described by the Fluid Mosaic Model, which posits a dynamic phospholipid bilayer with embedded and associated proteins, cholesterol (in animal cells), and carbohydrates.

Phospholipids form the basic framework, with their hydrophilic heads facing outwards and hydrophobic tails forming the core. Proteins act as channels, carriers, receptors, and enzymes. Cholesterol modulates membrane fluidity, preventing it from becoming too rigid or too fluid.

Carbohydrates, forming the glycocalyx, are crucial for cell-cell recognition and adhesion.

The membrane's most critical function is selective permeability, controlling what enters and leaves the cell. Transport mechanisms are categorized into passive and active. Passive transport (simple diffusion, facilitated diffusion, osmosis) occurs down a concentration gradient without energy.

Active transport moves substances against their gradient, requiring ATP and specific carrier proteins. Bulk transport (endocytosis, exocytosis) handles large molecules. Understanding these components and transport mechanisms is key for NEET.

5-Minute Revision

The cell membrane is a dynamic, selectively permeable barrier essential for all cellular life. Its structure is accurately depicted by the Fluid Mosaic Model, where a fluid phospholipid bilayer serves as the foundation.

This bilayer consists of amphipathic phospholipids, orienting with hydrophilic heads towards the aqueous environments and hydrophobic tails forming the membrane's core. Embedded within this bilayer are integral proteins, which can span the membrane or be partially inserted, performing roles like transport, signaling, and enzymatic activity.

Peripheral proteins are loosely attached to the membrane surface, often involved in signaling or structural support.

Cholesterol, present in animal cell membranes, acts as a crucial regulator of fluidity. It reduces fluidity at higher temperatures by restricting phospholipid movement, while increasing it at lower temperatures by preventing tight packing. Carbohydrates, linked to lipids (glycolipids) or proteins (glycoproteins), form the glycocalyx on the outer surface, vital for cell recognition, adhesion, and as receptor sites.

The membrane's selective permeability dictates substance movement. Passive transport does not require ATP and occurs down a concentration gradient: simple diffusion for small, nonpolar molecules (e.

g., O$_2$); facilitated diffusion for larger polar molecules or ions via specific channel or carrier proteins (e.g., glucose via GLUT transporters); and osmosis for water movement, often aided by aquaporins.

Active transport requires ATP to move substances against their concentration gradient, mediated by protein pumps (e.g., the Na$^+$/K$^+$ pump). For very large molecules, bulk transport mechanisms like endocytosis (ingestion) and exocytosis (secretion) are employed.

Mastery of these structural components and transport mechanisms is fundamental for NEET success.

Prelims Revision Notes

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Cell Membrane (Plasma Membrane): — Outer living boundary of cytoplasm in all cells.
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Fluid Mosaic Model (Singer & Nicolson, 1972): — Most accepted model. Membrane is a fluid, dynamic mosaic of components.
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Composition:

* Phospholipid Bilayer: Basic structure. Amphipathic molecules (hydrophilic heads, hydrophobic tails). Forms a stable barrier. * Proteins: Functional components. * Integral (Intrinsic): Embedded, often transmembrane.

Difficult to remove. E.g., channels, carriers, receptors. * Peripheral (Extrinsic): Loosely associated with surface. Easy to remove. E.g., enzymes, cytoskeleton anchors. * Cholesterol (Animal Cells): Interspersed among phospholipids.

Regulates fluidity (decreases at high temp, increases at low temp). * Carbohydrates: On outer surface, linked to lipids (glycolipids) or proteins (glycoproteins). Forms Glycocalyx. Important for cell-cell recognition, adhesion, receptors.

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Functions:

* Selective Permeability: Controls passage of substances. * Transport: Passive, Active, Bulk. * Cell-Cell Recognition & Adhesion: Via glycocalyx. * Signal Transduction: Receptor proteins bind ligands. * Enzymatic Activity: Some membrane proteins. * Structural Support: Attachment to cytoskeleton/ECM.

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Membrane Transport:

* Passive Transport (No ATP): Down concentration gradient. * Simple Diffusion: Small, nonpolar (O$_2$, CO$_2$) directly through bilayer. * Facilitated Diffusion: Larger polar/ions via specific proteins (channels/carriers).

E.g., glucose, ions. Still down gradient. * Osmosis: Diffusion of water across selectively permeable membrane. Aided by aquaporins. * Active Transport (ATP Required): Against concentration gradient.

Via specific carrier proteins (pumps). E.g., Na$^+$/K$^+$ pump. * Bulk Transport: For large molecules/particles. * Endocytosis: Ingestion (Phagocytosis - solids, Pinocytosis - liquids, Receptor-mediated).

* Exocytosis: Secretion/expulsion.

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Key Terms: — Amphipathic, Hydrophilic, Hydrophobic, Glycocalyx, Aquaporins, Pumps, Channels, Carriers.

Vyyuha Quick Recall

To remember the components of the Fluid Mosaic Model: Proteins Like Chocolate Cookies.

Proteins (Integral & Peripheral)
Lipids (Phospholipid Bilayer)
Cholesterol
Carbohydrates (Glycolipids & Glycoproteins)