Water, Macromolecules, Biological Membranes, Energetics, and Transport

Comprehensive vocabulary review flashcards covering water properties, amino acid and protein structures, biological membrane fluidity and architecture, thermodynamics, ATP energetics, and cellular transport mechanisms.

Polar Molecule

A molecule with an uneven charge distribution, featuring partial positive and negative regions due to differences in electronegativity, such as between hydrogen and oxygen in water.

Hydrogen Bond

A weak bond formed between a partially positive hydrogen atom and a partially negative atom (such as oxygen or nitrogen) in another molecule; each water molecule can form up to four of these bonds.

Hydrophilic

"Water-loving" substances that interact favorably with water, typically composed of polar or charged molecules that dissolve readily in aqueous solutions.

Hydrophobic

"Water-fearing" substances that lack charges or partial charges; these nonpolar molecules, such as lipids and oils, fail to dissolve and aggregate in aqueous environments.

pH

A scale ranging from $0$ to $14$ measuring the concentration of hydrogen ions ($\text{H}^+$) in a solution, where $7$ is neutral, below $7$ is acidic, and above $7$ is basic (alkaline).

Physiological pH Range

The tightly regulated normal pH range of human blood ($7.35\text{ to }7.45$) that is essential for cellular homeostasis, enzyme function, and metabolism.

Amino Acids

The monomer building blocks of proteins, each composed of a central alpha ($\alpha$) carbon bonded to an amino group ($-\text{NH}_2$), a carboxyl group ($-\text{COOH}$), a hydrogen atom, and a variable R-group (side chain).

Peptide Bond

A covalent linkage formed through a condensation reaction connecting the carboxyl group of one amino acid to the amino group of another.

Primary Structure

The linear sequence of amino acids in a polypeptide chain, strictly determined by the genetic information encoding the protein.

Secondary Structure

Regular, repeating local folding patterns in a polypeptide backbone, primarily including coiled $\alpha$-helices and pleated $\beta$-sheets stabilized by hydrogen bonds.

Tertiary Structure

The overall three-dimensional shape of a single polypeptide, stabilized by interactions among amino acid R-groups including hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges.

Quaternary Structure

The arrangement and functional association of multiple individual polypeptide subunits into a single multi-subunit protein complex, such as hemoglobin.

Disulfide Bridge

A strong covalent bond formed between the sulfhydryl ($-\text{SH}$) groups of two cysteine residues that reinforces protein tertiary and quaternary structures.

Sickle Cell Mutation

A specific genetic mutation in the $\beta$-globin gene that substitutes valine for glutamic acid at position $6$, producing abnormal hemoglobin S (HbS) that aggregates under low oxygen and deforms red blood cells into rigid sickles.

Hydrophobic Effect

The physical phenomenon whereby nonpolar molecules or fatty acid tails aggregate together in an aqueous medium to minimize water exposure, driving membrane bilayer assembly and protein folding.

Plasma Membrane

The selectively permeable external lipid bilayer of a cell, measuring approximately $5\text{ to }7\,\text{nm}$ thick, composed predominantly of phospholipids, proteins, and cholesterol (in animal cells).

Open System

A thermodynamic system that exchanges both matter and energy with its surroundings, exemplified by living cells exchanging nutrients, wastes, and heat.

Phospholipid

An amphipathic membrane lipid consisting of a hydrophilic head (composed of glycerol, a phosphate group, and a polar group) and two nonpolar hydrophobic fatty acid tails.

Fluid Mosaic Model

The model proposed in $1972$ by Singer and Nicolson characterizing the plasma membrane as a dynamic, fluid phospholipid bilayer embedded with a mosaic of mobile proteins.

Unsaturated Fatty-Acid Tails

Hydrocarbon chains containing one or more carbon-carbon double bonds that introduce bends or "kinks," preventing tight packing and thereby increasing membrane fluidity while lowering the melting point.

Cholesterol (Membrane Buffer)

A steroid lipid constituting up to $25\%$ of animal cell membranes that buffers fluidity by restraining phospholipid motion at higher temperatures and preventing rigid packing at colder temperatures.

Integral Membrane Protein

A membrane protein containing both hydrophobic and hydrophilic domains that penetrates or completely spans the hydrophobic core of the lipid bilayer.

Peripheral Membrane Protein

A membrane protein lacking exposed hydrophobic groups that does not enter the bilayer core, attaching loosely to the exterior or interior membrane surface.

First Law of Thermodynamics

The principle of energy conservation stating that energy cannot be created or destroyed, only transferred or transformed from one form to another.

Second Law of Thermodynamics

The principle stating that natural processes always increase the entropy (disorder or randomness) of a system and its surroundings over time.

Adenosine Triphosphate (ATP)

The primary energy currency of the cell, composed of the nitrogenous base adenine, a ribose sugar, and three negatively charged phosphate groups whose terminal high-energy bond powers cellular work.

Phosphorylation

The enzymatic transfer of a high-energy phosphate group from ATP to another molecule, which induces conformational activation to drive mechanical, transport, or chemical work.

Cell Transport

The organized systems of cellular movement categorized into passive transport (simple diffusion, osmosis, facilitated diffusion) and active transport (carrier pumps and bulk vesicle transport).

Selective Permeability

The property of the plasma membrane allowing nonpolar substances ($\text{O}_2$, $\text{CO}_2$) to diffuse easily while restricting large polar compounds (glucose) and charged ions ($\text{Na}^+$, $\text{Cl}^-$).

Simple Diffusion

The passive transport of solute particles directly through the lipid bilayer down their concentration gradient (from high to low concentration) until dynamic equilibrium is reached.

Osmosis

The passive net movement of water molecules across a selectively permeable membrane from an area of lower solute concentration (higher free water) to an area of higher solute concentration (lower free water).

Facilitated Diffusion

Passive transport down a concentration gradient aided by transmembrane channel or carrier proteins that shield polar or charged solutes from the membrane's hydrophobic core without consuming ATP.

Primary Active Transport

Transport that directly consumes ATP to phosphorylate a carrier protein, driving molecules across the membrane against their established concentration gradient.

Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+$ Pump)

An electrogenic primary active transport pump that uses ATP to export $3\,\text{Na}^+$ ions and import $2\,\text{K}^+$ ions against their concentration gradients in animal cells.

Secondary Active Transport

A transport process in which the movement of a substance against its gradient is powered by the potential energy stored in an electrochemical ion gradient created by primary active transport.

Endocytosis

The active, vesicle-mediated bulk transport process by which cells invaginate their plasma membrane to engulf external fluids, solutes, or particles into the cytoplasm.

Exocytosis

The active bulk transport mechanism where membrane-bound secretory vesicles fuse with the plasma membrane to release hormones, neurotransmitters, or cellular waste into the extracellular fluid.

Familial Hypercholesterolemia

A genetic disorder resulting from defective cell surface LDL receptors that impairs receptor-mediated endocytosis of cholesterol, producing elevated low-density lipoprotein levels in the blood.