Transport Across Cell Membranes

Overview of Membrane Transport

The movement of substances across the cell membrane is essential for maintaining cellular homeostasis. Various mechanisms allow molecules and ions to enter or exit the cell, each with specific terminology and characteristics.

• Cell membrane (plasma membrane): A selectively permeable barrier that regulates the passage of substances into and out of the cell.

• Transport processes: Classified as either passive (not requiring energy) or active (requiring energy).

Passive Transport

Passive transport involves the movement of substances down their concentration gradient without the use of cellular energy (ATP).

• Simple diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

• Facilitated diffusion: Movement of larger or polar molecules (e.g., glucose, ions) via specific transmembrane proteins (channels or carriers).

• Osmosis: Diffusion of water molecules across a selectively permeable membrane, typically through aquaporins.

Key vocabulary: concentration gradient, equilibrium, selectively permeable, channel protein, carrier protein.

Active Transport

Active transport requires energy (usually from ATP) to move substances against their concentration gradient.

• Primary active transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary active transport (cotransport): Uses the energy from the movement of one substance down its gradient to drive another substance against its gradient (e.g., symporters and antiporters).

Key vocabulary: ATP, pump, symport, antiport, cotransport.

Vesicular Transport

Large particles and macromolecules are transported via vesicles in processes that require energy.

• Endocytosis: Uptake of materials into the cell by engulfing them in vesicles. Includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.

• Exocytosis: Release of substances from the cell by fusion of vesicles with the plasma membrane.

Key vocabulary: vesicle, phagocytosis, pinocytosis, receptor-mediated endocytosis, exocytosis.

Comparison of Membrane Transport Mechanisms

Key Equations

• Fick's Law of Diffusion:

• Where J is the rate of diffusion, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

Summary

• Substances cross the cell membrane via passive or active mechanisms, each with specific vocabulary and characteristics.

• Understanding these processes is essential for comprehending cellular function and physiology.