Cell Membrane Structure and Transport Mechanisms

Comprehensive vocabulary flashcards covering membrane structure, passive and active transport mechanisms, osmosis, tonicity, cotransport, and bulk vesicular transport.

Amphipathic molecule

A molecule that possesses both hydrophobic (non-polar) and hydrophilic (polar) regions, such as phospholipids and integral membrane proteins.

Peripheral proteins

Membrane proteins that are not embedded in the lipid bilayer but are loosely bound to the surface of the membrane.

Integral proteins

Membrane proteins that penetrate the hydrophobic interior core of the phospholipid bilayer.

Transmembrane proteins

A major class of integral membrane proteins that span entirely across the membrane from one side to the other.

Passive transport

The movement of substances across a biological membrane down their concentration gradient without any expenditure of cellular energy ($ATP$).

Simple diffusion

A form of passive transport where lipid-soluble (hydrophobic) molecules or small, uncharged molecules (such as $O_2$ and $CO_2$) pass directly through the phospholipid bilayer on their own.

Concentration gradient

The region along which the density of a chemical substance increases or decreases; each solute diffuses down its own gradient unaffected by the concentrations of other solutes.

Facilitated diffusion

The passive transport of hydrophilic substances across a membrane with the assistance of specific transmembrane transport proteins (channels or carriers).

Channel proteins

Transport proteins that provide a hydrophilic tunnel or pore across the membrane, allowing specific molecules or ions to diffuse through.

Aquaporins

Channel proteins that specifically facilitate the rapid passage of water molecules across cell membranes.

Carrier proteins

Transport proteins that bind to specific solute molecules and undergo a conformational shape change to shuttle them across the membrane.

Osmosis

The diffusion of free water across a selectively permeable membrane toward the region with a higher solute concentration.

Hypotonic solution

An external solution with a lower solute concentration than the inside of the cell, causing net water entry that can lyse animal cells or make plant cells turgid.

Isotonic solution

An environment having an equal solute concentration compared to the cell interior, resulting in no net movement of water (animal cells remain normal; plant cells become flaccid).

Hypertonic solution

An external solution with a higher solute concentration than the cytoplasm, causing net water loss that shrivels animal cells and plasmolyzes plant cells.

Plasmolysis

The phenomenon in plant cells placed in a hypertonic environment where excessive water loss causes the plasma membrane to pull away from the cell wall.

Active transport

The energy-requiring movement of a substance across a membrane against its concentration gradient (from low to high concentration), mediated by carrier proteins called pumps.

Sodium-potassium pump

An active transport pump that consumes $ATP$ to export $3\,Na^+$ ions out of the cell and import $2\,K^+$ ions into the cell, establishing cellular electrochemical gradients.

Cotransport

A mechanism where the 'downhill' diffusion of a solute down its electrochemical gradient drives the 'uphill' transport of a second substance against its own concentration gradient.

Bulk transport

The transport of large molecules, such as proteins and polysaccharides, across the plasma membrane enclosed within membrane-bound vesicles.

Exocytosis

A form of bulk transport where intracellular transport vesicles migrate to the plasma membrane, fuse with it, and discharge their contents outside the cell.

Endocytosis

A form of bulk transport where the plasma membrane invaginates and pinches inward to engulf external materials, internalizing them within vesicles.

Phagocytosis

A specialized type of endocytosis ('cellular eating') in which a cell engulfs large particles or cellular debris, such as macrophages engulfing and destroying bacteria.