Cell Membrane Basics and Transport Across Membranes

Vocabulary flashcards covering membrane structure, lipids, proteins, and transport mechanisms based on Lecture 2.

Amphipathic

Describing molecules that possess both hydrophilic (water-loving) and hydrophobic (water-fearing) regions, such as membrane phospholipids.

Phospholipid

The most abundant lipid in cell membranes, composed of a hydrophilic polar head group (containing choline, phosphate, and glycerol) and nonpolar hydrophobic hydrocarbon tails.

Sterols

Amphipathic membrane lipids containing a rigid planar steroid ring structure; cholesterol is the most abundant sterol in humans.

Glycolipids

Amphipathic membrane lipids that feature sugars as part of their hydrophilic head group, such as galactocerebroside.

Lipid Bilayer

A self-assembling double layer of amphipathic lipid molecules roughly $5\,\text{nm}$ thick, featuring a central hydrophobic core flanked by hydrophilic heads facing the aqueous environment.

Unsaturated Hydrocarbon Tails

Fatty acid chains containing one or more double bonds that create kinks, preventing tight molecular packing and increasing membrane fluidity.

Cholesterol Effect on Fluidity

At normal body temperatures, cholesterol stiffens the lipid bilayer and decreases membrane fluidity, whereas at lower temperatures it hinders lipid packing to maintain fluidity.

Low-Density Lipoproteins (LDLs)

Protein complexes that transport cholesterol through the bloodstream; elevated levels lead to cholesterol accumulation and plaque formation in arterial walls.

Integral Membrane Proteins

Proteins directly embedded in the lipid bilayer having hydrophobic amino acids that interact directly with the hydrophobic tails of lipids.

Peripheral Membrane Proteins

Proteins bound indirectly to the cell membrane through interactions with integral membrane proteins or with lipid hydrophilic head groups.

FRAP (Fluorescence Recovery After Photobleaching)

A technique used to measure protein lateral mobility in membranes by photobleaching GFP-tagged proteins with a laser and recording recovery time as unbleached proteins diffuse into the region.

Channel Protein

A transport protein that forms an aqueous pore discriminating solutes mainly by size and charge, existing in open or closed conformations.

Transporter Protein

A highly selective transport protein that binds specific solutes matching its binding site and transfers them across the bilayer via conformational changes.

Passive Transport

Energetically favorable movement of a solute across a membrane along its concentration or electrochemical gradient, requiring no energy input.

Active Transport

Energetically unfavorable movement of a solute across a membrane against its concentration or electrochemical gradient, requiring energy input.

Pump

A membrane transporter protein that uses an energy source (such as ATP, light, or an ion gradient) to transport solutes against their concentration gradient.

Gradient-Driven Active Transport

Active transport driven by coupling the downhill, favorable movement of one molecule to the uphill, unfavorable movement of another molecule.

Symport

A gradient-driven coupled pump mechanism that transfers two distinct molecules or ions across the membrane in the same direction.

Antiport

A gradient-driven coupled pump mechanism that transfers two distinct molecules or ions across the membrane in opposite directions.

Selectivity Filter

A narrow region inside an ion channel lined with specific amino acids that forces ions to shed bound water molecules, allowing only specific ions (e.g., $K^+$) to pass.

Membrane Permeability

The relative ease with which molecules pass across an artificial lipid bilayer: highly permeable to small nonpolar molecules, moderately permeable to small uncharged polar molecules, and impermeable to ions and large polar molecules.

Transcellular Glucose Transport

An epithelial cell transport system where glucose is actively taken up from the gut lumen at the apical domain via $Na^+$ symport and passively released at the basal domain via uniport.