Transport Across Membrane — Revision Notes

⚡ 30-Second Revision

Passive Transport — No ATP, down gradient.

- Simple Diffusion: Small, nonpolar, lipid-soluble (O$_2$, CO$_2$). - Facilitated Diffusion: Protein-mediated (channels/carriers), saturable, specific (Glucose, Ions). - Osmosis: Water movement (high water potential to low), via aquaporins.

Active Transport — Requires ATP, against gradient.

- Primary Active: Direct ATP use (Na$^+$/K$^+$ pump: 3Na$^+$ out, 2K$^+$ in). - Secondary Active: Uses existing ion gradient (Na$^+$-glucose symporter).

Bulk Transport — For large molecules, vesicles.

- Endocytosis: Ingestion (Phagocytosis: large particles; Pinocytosis: fluids; Receptor-mediated: specific ligands). - Exocytosis: Secretion (hormones, neurotransmitters).

Key Terms — Selectively permeable, concentration gradient, isotonic, hypotonic, hypertonic, plasmolysis, turgor.

2-Minute Revision

Transport across the cell membrane is vital for cellular life, regulating what enters and exits. It's broadly divided into passive and active processes. Passive transport doesn't use cellular energy and moves substances down their concentration or electrochemical gradient.

This includes simple diffusion for small, nonpolar molecules like O$_2$ and CO$_2$ directly through the lipid bilayer. Facilitated diffusion uses specific membrane proteins (channels or carriers) for larger or charged molecules (e.

g., glucose, ions), exhibiting specificity and saturation. Osmosis is the passive movement of water across the membrane, often via aquaporins, from high to low water potential. Active transport, conversely, requires metabolic energy (ATP) to move substances *against* their concentration gradient.

Primary active transport directly uses ATP, like the Na$^+$/K$^+$ pump which maintains crucial ion gradients. Secondary active transport (co-transport) uses the energy from an existing ion gradient (established by primary active transport) to move another substance (e.

g., Na$^+$-glucose symporter). For very large substances, bulk transport involves vesicle formation: endocytosis for intake (phagocytosis, pinocytosis, receptor-mediated) and exocytosis for release.

5-Minute Revision

The cell membrane, a selectively permeable barrier, orchestrates the movement of all substances essential for cellular function. This 'transport across membrane' is categorized into passive and active mechanisms.

Passive transport is energy-independent, driven by concentration or electrochemical gradients. Simple diffusion allows small, lipid-soluble molecules (like O$_2$, CO$_2$) to pass directly through the lipid bilayer, with its rate proportional to the gradient.

Facilitated diffusion aids larger or charged molecules (e.g., glucose, ions) via specific membrane proteins (channels or carriers). While still passive, it's faster, specific, and saturable due to limited protein availability.

Osmosis is the specialized diffusion of water, moving from higher to lower water potential, often through aquaporins, crucial for cell volume regulation. For instance, a plant cell in a hypertonic solution undergoes plasmolysis as water exits.

Active transport is energy-dependent, moving substances *against* their concentration gradient. Primary active transport directly uses ATP hydrolysis, exemplified by the **Na$^+$/K$^+$ pump**, which expels 3 Na$^+$ ions and imports 2 K$^+$ ions per ATP, vital for nerve impulses and maintaining cell volume.

Secondary active transport (co-transport) indirectly uses ATP; it harnesses the energy of an existing ion gradient (often created by primary active transport) to move another solute. Symporters move both substances in the same direction (e.

g., Na$^+$-glucose symporter), while antiporters move them in opposite directions (e.g., Na$^+$-Ca$^{2+}$ exchanger).

Finally, bulk transport handles very large molecules or particles via membrane-bound vesicles. Endocytosis is the process of cellular intake: phagocytosis ('cell eating' for large particles), pinocytosis ('cell drinking' for fluids), and receptor-mediated endocytosis for specific ligands.

Exocytosis is the process of cellular release, secreting substances like hormones or neurotransmitters. Understanding these mechanisms, their energy requirements, specificity, and examples is paramount for NEET.

Prelims Revision Notes

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Cell Membrane Structure — Phospholipid bilayer, integral and peripheral proteins. Selectively permeable.
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Passive Transport — No ATP. Down concentration/electrochemical gradient.

* Simple Diffusion: Direct passage through lipid bilayer. For small, nonpolar, lipid-soluble molecules (O$_2$, CO$_2$, N$_2$, ethanol, urea). Rate $propto$ gradient, lipid solubility, surface area; $propto 1/\text{size}$, $1/\text{thickness}$.

No saturation. * Facilitated Diffusion: Requires membrane proteins (channels/carriers). For larger, polar, or charged molecules (glucose, amino acids, ions). Down gradient. Specific, saturable, competitive.

No ATP. * Channel Proteins: Form pores (e.g., ion channels, aquaporins). Can be gated (ligand, voltage, mechanical). * Carrier Proteins: Bind, conformational change, release (e.g., GLUT transporters).

* Osmosis: Water movement across selectively permeable membrane. From high water potential (low solute) to low water potential (high solute). Via aquaporins or lipid bilayer. Important for cell volume.

* Isotonic: No net water movement. * Hypotonic: Water enters cell (animal cell lyses, plant cell becomes turgid). * Hypertonic: Water leaves cell (animal cell crenates, plant cell undergoes plasmolysis).

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Active Transport — Requires ATP. Against concentration/electrochemical gradient.

* Primary Active Transport: Directly uses ATP hydrolysis. Carrier protein is an ATPase. E.g., Na$^+$/K$^+$ pump (3Na$^+$ out, 2K$^+$ in), Ca$^{2+}$ pump, H$^+$ pump. * Secondary Active Transport (Co-transport): Uses energy from an existing ion gradient (established by primary active transport).

No direct ATP hydrolysis. * Symporters: Transport two substances in the same direction (e.g., Na$^+$-glucose symporter). * Antiporters: Transport two substances in opposite directions (e.g., Na$^+$-Ca$^{2+}$ exchanger).

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Bulk Transport — For large molecules/particles. Requires ATP.

* Endocytosis: Intake via vesicle formation. * Phagocytosis: 'Cell eating' (large particles, bacteria). * Pinocytosis: 'Cell drinking' (fluids, dissolved solutes). * Receptor-mediated Endocytosis: Specific uptake of ligands via coated pits. * Exocytosis: Release via vesicle fusion with membrane (hormones, neurotransmitters, waste).

Vyyuha Quick Recall

To remember the types of transport and their energy needs: Passive Always Doesn't Need Energy; Active Always Demands Energy. For passive types: Simple Facilitated Osmosis. For active types: Primary Secondary Bulk.