Comprehensive Study Guide to Molecular Biology, Bioenergetics, and Cellular Physiology

Comprehensive vocabulary flashcards covering transcription and RNA processing, membrane transport, cell signaling mechanisms, thermodynamic principles, photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation.

Transcription

The enzymatic synthesis of an RNA molecule from a DNA template strand, catalyzed by RNA polymerase reading the DNA template in the $3' \rightarrow 5'$ direction and synthesizing RNA in the $5' \rightarrow 3'$ direction.

Reverse Complement

The relationship between two polynucleotide strands in which 'reverse' indicates antiparallel orientation ($5' \rightarrow 3'$ paired with $3' \rightarrow 5'$) and 'complementary' indicates base pairing (such as adenine with uracil/thymine and guanine with cytosine).

Promoter

A specific DNA sequence that designates where transcription begins and serves as the initial binding site for transcription machinery, such as general transcription factors like the TATA-binding protein.

Polyadenylation Signal Sequence

A conserved eukaryotic DNA and pre-mRNA sequence ($\text{AAUAAA}$) that is transcribed and subsequently recognized by an endonuclease to cleave the nascent RNA transcript prior to poly-A tail addition.

Transcription Bubble

The unwound region of DNA where RNA polymerase binds, separating the non-coding template strand from the coding strand to synthesize a growing complementary RNA chain.

Template Strand

The non-coding DNA strand that is directly read by RNA polymerase in the $3' \rightarrow 5'$ direction to generate a complementary RNA transcript.

Coding Strand

The DNA strand whose base sequence directly matches the sequence of the newly synthesized RNA transcript (with uracil replacing thymine) and is complementary to the template strand.

Spliceosome

A large ribonucleoprotein complex that removes non-coding introns from pre-mRNA as a lariat loop and ligates coding exons together to prepare mRNA for nuclear export.

snRNP (Small Nuclear Ribonucleoprotein)

A component of the spliceosome whose RNA moiety is responsible for identifying and base-pairing with specific intron-exon boundaries during RNA splicing.

Paracrine Signaling

A local cell-signaling mechanism in which a signaling cell secretes regulatory molecules into the extracellular fluid to act on nearby target cells located within $4\text{ to }5$ cell diameters.

Endocrine Signaling

Long-distance hormonal signaling in which specialized cells or endocrine organs (such as the pituitary gland or pancreas) secrete hormones into the bloodstream to reach target cells throughout the body.

G Protein-Coupled Receptor (GPCR)

A class of seven-transmembrane cell-surface receptors that, upon ligand binding, stimulate an associated intracellular G protein to exchange bound GDP for GTP.

G Protein

A molecular switch protein located on the cytoplasmic face of the plasma membrane that is active when bound to GTP and inactive when bound to GDP, possessing intrinsic GTPase activity to shut itself off.

Receptor Tyrosine Kinase (RTK)

A transmembrane enzyme-linked receptor that dimerizes upon extracellular ligand binding, activating its intrinsic cytoplasmic kinase domain to autophosphorylate specific tyrosine residues.

Ligand-Gated Ion Channel

A transmembrane pore-forming receptor that undergoes a conformational change to open or close upon binding a specific extracellular chemical signal, allowing passive ion flux across the membrane.

Protein Kinase

An enzyme that transfers a terminal phosphate group from ATP to a target protein substrate, typically altering the target protein's conformation and catalytic activity.

Protein Phosphatase

An enzyme that removes phosphate groups from phosphorylated proteins via hydrolysis, reversing the action of protein kinases and terminating signal transduction cascades.

Cyclic AMP (cAMP)

A ubiquitous second messenger derived from ATP by the enzyme adenylyl cyclase that relays intracellular signals by activating downstream effectors such as protein kinase A.

Phospholipase C (PLC)

A membrane-associated effector enzyme activated by G proteins that cleaves phosphatidylinositol 4,5-bisphosphate ($\text{PIP}_2$) into diacylglycerol and inositol trisphosphate ($\text{IP}_3$).

Inositol Trisphosphate (IP3)

A soluble second messenger that diffuses through the cytosol and binds to ligand-gated calcium channels on the smooth endoplasmic reticulum, inducing calcium release into the cytoplasm.

Cholera Toxin

A bacterial enterotoxin that chemically modifies G proteins to inhibit their GTPase activity, locking adenylyl cyclase in an active state and causing excessive cAMP production, salt secretion, and watery diarrhea.

Fluid Mosaic Model

A structural model depicting the biological membrane as a dynamic, fluid phospholipid bilayer embedded with diverse proteins that are free to diffuse laterally in the plane of the membrane.

Amphipathic

The chemical property of having both hydrophilic (polar) and hydrophobic (nonpolar) regions within the same molecule, which drives the spontaneous formation of phospholipid bilayers in aqueous environments.

Fluidity Buffer

A stabilizing property of cholesterol that dampens changes in membrane fluidity by restricting phospholipid acyl chain movement at elevated temperatures and preventing tight packing at low temperatures.

CCR5 Receptor

A chemokine G protein-coupled receptor found on the surface of T cells and macrophages that serves as an essential co-receptor exploited by HIV to bind and enter host cells.

Facilitated Diffusion

Passive transport of polar or charged molecules across a biological membrane down their concentration gradient, mediated by specific transmembrane channel or carrier proteins without direct energy expenditure.

Electrochemical Gradient

The combined dual driving force governing the passive movement of an ion across a membrane, comprising its chemical concentration gradient and the electrical potential difference across the bilayer.

Sodium-Potassium Pump

An electrogenic primary active transport protein that hydrolyzes one ATP molecule to export $3\,\text{Na}^+$ ions and import $2\,\text{K}^+$ ions against their respective electrochemical gradients.

Secondary Active Transport

Transport mechanism in which a cotransporter moves a solute uphill against its concentration gradient by coupling its movement to the downhill flow of an ion whose gradient was established by primary ATP-driven pumps.

Phagocytosis

A form of endocytosis ('cell eating') in which specialized immune cells, such as macrophages and neutrophils, engulf large particulate matter, bacteria, or cellular debris into large vacuoles called phagosomes.

Pinocytosis

A constitutive, nonspecific endocytic process ('cell drinking') occurring in nearly all eukaryotic cells, whereby droplets of extracellular fluid and dissolved solutes are internalized in small vesicles.

Receptor-Mediated Endocytosis

A highly selective endocytic pathway wherein specific extracellular ligands (such as LDL or transferrin) bind cell-surface receptors concentrated in clathrin-coated pits, triggering vesicle formation.

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 during physical and chemical processes.

Second Law of Thermodynamics

The thermodynamic law stating that every spontaneous energy transfer or transformation increases the entropy of the universe, with some usable energy unavoidably dissipated as heat.

Gibbs Free Energy Change ($\Delta G$)

The thermodynamic quantity defined as $G_{\text{products}} - G_{\text{reactants}}$ that determines whether a chemical process is spontaneous ($\Delta G < 0$) or non-spontaneous ($\Delta G > 0$) at constant temperature and pressure.

Exergonic Reaction

A spontaneous chemical reaction where the free energy of the reactants exceeds that of the products, resulting in a negative free energy change ($\Delta G < 0$) and the release of free energy.

Endergonic Reaction

A non-spontaneous chemical reaction where the free energy of the products is greater than that of the reactants ($\Delta G > 0$), requiring an external input of free energy to proceed forward.

Dynamic Equilibrium

The chemical state where the forward and reverse reaction rates are identical, the free energy change equals zero ($\Delta G = 0$), and no net change in reactant or product concentrations occurs.

Reaction Coupling

The bioenergetic strategy of driving an endergonic reaction forward by linking it with an exergonic reaction (such as ATP hydrolysis) to yield a net negative free energy change ($\Delta G < 0$).

Activation Energy

The initial kinetic energy barrier that reactants must absorb to reach the unstable transition state, preventing exergonic macromolecules such as DNA and proteins from breaking down spontaneously.

Photosystem II (PSII)

A multi-protein pigment complex in the thylakoid membrane that absorbs light energy to catalyze the photolysis of water into molecular oxygen, protons, and electrons, reducing plastoquinone ($\text{Pq}$).

Cytochrome Complex (Photosynthesis)

A thylakoid transmembrane complex that accepts electrons from reduced plastoquinone ($\text{Pq}_{\text{red}}$) and passes them to plastocyanin ($\text{Pc}$) while translocating protons from the stroma into the thylakoid lumen.

Photosystem I (PSI)

A thylakoid light-absorbing complex that re-energizes electrons from plastocyanin ($\text{Pc}$) using photons and transfers them to the mobile electron carrier ferredoxin ($\text{Fd}$).

NADP+ Reductase

The stromal-facing terminal enzyme of the photosynthetic light reactions that catalyzes the reduction of $\text{NADP}^+$ to $\text{NADPH}$ by transferring electrons from reduced ferredoxin ($\text{Fd}_{\text{red}}$).

Proton Motive Force (Chloroplast)

The electrochemical proton gradient generated across the thylakoid membrane (high proton concentration inside the thylakoid space, low in the stroma) that provides energy for ATP synthase.

Rubisco

Ribulose-1,5-bisphosphate carboxylase-oxygenase, the primary enzyme of the Calvin cycle that catalyzes the initial carbon-fixation reaction by attaching $\text{CO}_2$ to ribulose bisphosphate ($\text{RuBP}$).

Calvin Cycle

The cyclical light-independent metabolic pathway of photosynthesis in the stroma consisting of three phases: carbon fixation, reduction of $3\text{-phosphoglycerate}$ to $\text{G3P}$, and regeneration of $\text{RuBP}$.

Glyceraldehyde 3-Phosphate (G3P)

A three-carbon phosphorylated sugar produced as the net output of the Calvin cycle, functioning as the primary building block for the cellular synthesis of glucose and other carbohydrates.

Complex I (NADH-Q Oxidoreductase)

The first multi-subunit enzyme complex of the mitochondrial electron transport chain that oxidizes $\text{NADH}$ to $\text{NAD}^+$, reduces ubiquinone ($\text{Q}$) to $\text{QH}_2$, and pumps $4\,\text{H}^+$ into the intermembrane space.

Complex II (Succinate Dehydrogenase)

An integral inner mitochondrial membrane enzyme that functions concurrently in the citric acid cycle and the electron transport chain, oxidizing succinate to fumarate and reducing $\text{Q}$ without pumping protons.

Complex III (Q-Cytochrome c Oxidoreductase)

A respiratory chain complex that catalyzes the transfer of electrons from reduced ubiquinone ($\text{QH}_2$) to oxidized cytochrome $c$ ($\text{Cyt } c$) while translocating $4\,\text{H}^+$ into the intermembrane space.

Complex IV (Cytochrome c Oxidase)

The terminal complex of the mitochondrial electron transport chain that transfers electrons from reduced cytochrome $c$ to molecular oxygen ($\text{O}_2$) to form water, translocating $2\,\text{H}^+$ across the inner membrane.

Oxidative Phosphorylation

The overall ATP-generating process comprising the mitochondrial electron transport chain (generating a proton gradient) coupled to chemiosmosis via ATP synthase.

Chemiosmosis

The biological mechanism whereby ATP synthase harnesses the potential energy stored in an electrochemical proton gradient across an energy-transducing membrane to phosphorylate ADP into ATP.

2,4-Dinitrophenol (DNP)

A chemical uncoupling agent that shuttles protons across the inner mitochondrial membrane independent of ATP synthase, abolishing the proton gradient and uncoupling oxygen consumption from ATP production.

Oligomycin

An antibiotic inhibitor that directly blocks proton flow through the channel subunit of ATP synthase, shutting down both ATP synthesis and subsequent respiratory electron transport.

Substrate-Level Phosphorylation

The enzymatic transfer of a high-energy phosphate group directly from a phosphorylated organic metabolic intermediate to ADP (or GDP), producing ATP (or GTP) without an electron transport chain.

Glycolysis

A ubiquitous cytosolic catabolic pathway that breaks down one molecule of glucose into two molecules of pyruvate, generating a net of $2\,\text{ATP}$ and $2\,\text{NADH}$ under anaerobic or aerobic conditions.

Pyruvate Dehydrogenase Complex

A large mitochondrial matrix enzyme complex that catalyzes the oxidative decarboxylation of pyruvate into acetyl-CoA, producing one molecule of $\text{CO}_2$ and one molecule of $\text{NADH}$ per pyruvate.

Citric Acid Cycle

A central metabolic hub located in the mitochondrial matrix that oxidizes acetyl-CoA into $\text{CO}_2$, yielding $3\,\text{NADH}$, $1\,\text{FADH}_2$, and $1\,\text{ATP}$ (or GTP) per acetyl group.

Reduced Carbon

A carbon atom bonded to more hydrogen atoms and fewer oxygen atoms; it holds high-energy electrons that release substantial free energy upon oxidative catabolism (as seen in hydrocarbon-rich fatty acids).

Phosphofructokinase-1 (PFK-1)

The primary committed regulatory enzyme of glycolysis that is allosterically inhibited by high cellular levels of ATP and citrate and allosterically stimulated by AMP.