Cell Membrane Transport Mechanisms

Selective Permeability

A property of the cell membrane that allows certain molecules to pass more easily than others. Small, nonpolar molecules (e.g., O₂, CO₂) can move through freely, while larger, polar, or charged molecules (e.g., ions, water) require assistance.

Passive Transport

The movement of molecules across the cell membrane without energy input, as molecules move from areas of high concentration to low concentration (down their gradient). Examples include diffusion, osmosis, and facilitated diffusion.

Diffusion

A type of passive transport where molecules move from an area of high concentration to low concentration due to random motion. It requires no energy and continues until equilibrium is reached.

Osmosis

The diffusion of water across a selectively permeable membrane. Water moves from a region of low solute concentration to high solute concentration until balance is achieved.

Facilitated Diffusion

A passive transport process in which molecules move down their concentration gradient through specialized proteins in the membrane. Transport proteins like channels or carriers assist substances such as ions, water, and glucose.

Channel Proteins

Proteins that form a hydrophilic channel through the membrane, allowing certain ions or molecules to pass. Some channels, like aquaporins, are specific to water, and many are gated, only opening in response to specific stimuli.

Carrier Proteins

Proteins that undergo a change in shape to transport molecules across the membrane. These proteins are selective, allowing only certain substances to pass by binding and changing conformation.

Active Transport

The movement of molecules against their concentration gradient (from low to high concentration) that requires energy, usually in the form of ATP. Active transport maintains concentrations of ions and molecules necessary for cell function.

Sodium-Potassium Pump

A type of active transport that moves 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, creating a net positive charge outside the cell. This pump is essential for maintaining the cell's electrochemical gradient.

Membrane Potential

The electrical charge difference across a cell membrane due to unequal distribution of ions. The inside of the cell is usually more negative compared to the outside, creating an electrochemical gradient that stores energy.

Proton Pump

An active transport protein that moves hydrogen ions (H⁺) across the membrane, generating a proton gradient. This gradient can then be used for other cellular processes, such as ATP production in plants, fungi, and bacteria.

Cotransport

A type of active transport where the movement of one substance down its concentration gradient drives the movement of another substance against its gradient. For example, plants use H⁺ gradients to import sugars like sucrose.

Exocytosis

A form of bulk transport where vesicles within the cell fuse with the membrane and release their contents outside the cell. This process is crucial for secretion of substances like hormones and neurotransmitters.

Endocytosis

The process by which cells take in large molecules by engulfing them in vesicles formed from the cell membrane. Types of endocytosis include phagocytosis (cell 'eating') and pinocytosis (cell 'drinking').

Phagocytosis

A type of endocytosis where a cell engulfs large particles, such as bacteria or debris, into a vacuole, which then fuses with a lysosome to digest the contents.

Pinocytosis

A type of endocytosis where the cell engulfs extracellular fluid containing dissolved molecules into small vesicles. This process is nonspecific and helps the cell absorb fluids and nutrients.

Receptor-Mediated Endocytosis

A specific type of endocytosis where receptor proteins on the cell membrane bind to specific molecules, causing them to cluster into coated vesicles for uptake into the cell.