General Biology:lesson6/9.2 Cellular Energetics, Cell Communication, and Photosynthesis

Comprehensive vocabulary flashcards covering receptor types and signal transduction, thermodynamics and enzymes, metabolism, cellular respiration, and photosynthesis from General Biology I.

Cell Surface Receptor (Membrane Receptor)

A transmembrane protein located on the plasma membrane in contact with both the cytoplasm and the extracellular environment that binds a signaling ligand outside the cell.

Chemically Gated Ion Channel

A channel-linked cell surface receptor that opens to allow a specific ion to pass across the plasma membrane in response to binding a ligand.

Enzymatic Receptor

A cell surface receptor that acts as an enzyme and is activated by ligand binding; almost all enzymatic receptors are protein kinases.

Receptor Tyrosine Kinase (RTK)

A membrane receptor characterized by an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular kinase domain that phosphorylates target proteins.

Autophosphorylation

The activation process of receptor tyrosine kinases ($RTKs$) where ligand binding causes two receptors to dimerize and phosphorylate each other on tyrosine residues.

G Protein-Coupled Receptor (GPCR)

The largest category of cell surface receptors in animal cells that transmits signals to an effector enzyme by coupling with a G protein bound to $GTP$.

Adenylyl Cyclase

An effector enzyme in $GPCR$ signaling pathways that converts $ATP$ into the second messenger cyclic $AMP$ ($cAMP$), which subsequently activates protein kinase A ($PKA$).

Phospholipase C

An effector enzyme activated by a G protein that cleaves the membrane phospholipid phosphatidylinositol 4,5-bisphosphate ($PIP_2$) into inositol trisphosphate ($IP_3$) and diacylglycerol ($DAG$).

Inositol Trisphosphate (IP3)

A second messenger produced by phospholipase C that binds to receptors on the endoplasmic reticulum to stimulate the release of calcium ions ($Ca^{2+}$).

Intracellular Receptor

A receptor located in the cytoplasm or nucleus that binds small, hydrophobic ligands (such as steroid hormones) and regulates gene expression by translocating to the nucleus.

Direct Contact Signaling

A form of cell communication where signaling molecules pass directly between adjacent plasma membranes via gap junctions or surface molecules.

Paracrine Signaling

A type of intercellular signaling where a secretory cell releases signaling molecules into the local extracellular environment to signal adjacent target cells.

Endocrine Signaling

Long-distance intercellular communication involving hormones secreted by endocrine glands into blood vessels to reach distant target cells.

Synaptic Signaling

A rapid form of local communication in which a nerve cell releases neurotransmitter molecules across a synaptic gap to bind receptors on a target cell.

Signal Amplification

The geometric multiplication of a signal through a kinase cascade, where each activated enzyme can phosphorylate and activate multiple downstream substrate proteins.

Scaffold Protein

A structural protein that physically binds and spatially organizes specific kinases in a cascade to increase signaling efficiency while decreasing signal amplification.

Ras Protein

A small monomeric GTP-binding protein that acts as a molecular switch linking receptor tyrosine kinases ($RTKs$) to the $MAP\text{ kinase}$ cascade; active when bound to $GTP$ and inactive when bound to $GDP$.

First Law of Thermodynamics

The physical principle stating that energy cannot be created or destroyed, only transformed from one form to another, maintaining a constant total energy in the universe.

Second Law of Thermodynamics

The physical principle stating that entropy is continuously increasing in the universe, causing spontaneous energy transformations to proceed toward less ordered and more stable states.

Enthalpy ($H$)

The total energy contained within the chemical bonds of a molecule.

Entropy ($S$)

A thermodynamic measure of the disorder or unavailable energy within a system.

Free Energy ($G$)

The energy available to do work in a system under constant temperature and pressure, defined by the formula $G = H - TS$, with changes expressed as $\Delta G = \Delta H - T\Delta S$.

Exergonic Reaction

A spontaneous chemical reaction with a negative change in free energy ($\Delta G < 0$) in which the reactants possess more free energy than the products.

Endergonic Reaction

A nonspontaneous chemical reaction with a positive change in free energy ($\Delta G > 0$) that requires an input of energy because the products possess more free energy than the reactants.

Activation Energy

The extra energy required to destabilize existing chemical bonds and initiate a chemical reaction.

Adenosine Triphosphate (ATP)

The cellular energy currency composed of ribose, adenine, and a chain of three phosphate groups with high-energy, unstable phosphate bonds.

Active Site

The pocket or cleft on an enzyme's surface where substrate molecules bind to form a temporary enzyme-substrate complex.

Ribozyme

A nonprotein $RNA$ molecule that acts as a biological catalyst to accelerate specific chemical reactions in cells.

Competitive Inhibitor

An inhibitor that competes directly with the substrate by binding to the active site of an enzyme, preventing the substrate from binding.

Noncompetitive Inhibitor

An inhibitor that binds to an enzyme at a site other than the active site, inducing a conformational change that prevents substrate binding or catalytic turnover.

Allosteric Site

A specific regulatory pocket on an enzyme distinct from the active site that acts as a chemical switch, binding allosteric activators or inhibitors to stabilize active or inactive conformations.

Feedback Inhibition

A metabolic control mechanism where the end product of a pathway accumulates and binds allosterically to an early enzyme in the pathway, shutting down synthesis to prevent waste of energy and raw materials.

Anabolism

Metabolic reactions that expend energy to synthesize complex organic molecules from simpler precursors.

Catabolism

Metabolic reactions that harvest energy by breaking down complex organic molecules into simpler molecules.

Autotroph

An organism capable of producing its own organic molecules from inorganic materials through photosynthesis.

Heterotroph

An organism that extracts energy by feeding on organic compounds produced by other living organisms.

Dehydrogenation

A redox reaction in which lost electrons are accompanied by protons, effectively meaning a complete hydrogen atom ($1\text{ electron}, 1\text{ proton}$) is transferred.

Nicotinamide Adenine Dinucleotide (NAD+)

An electron carrier and enzymatic cofactor that accepts $2\text{ electrons}$ and $1\text{ proton}$ during metabolic oxidation reactions to become reduced $NADH$.

Substrate-Level Phosphorylation

A mechanism of $ATP$ synthesis wherein an enzyme transfers a high-energy phosphate group directly from a phosphorylated substrate molecule to $ADP$.

Oxidative Phosphorylation

The generation of $ATP$ using energy from an electrochemical proton ($H^+$) gradient produced by the electron transport chain to power $ATP\text{ synthase}$.

Glycolysis

A 10-step metabolic pathway in the cytoplasm that splits one $6\text{-carbon}$ glucose into two $3\text{-carbon}$ pyruvates, yielding a net of $2\text{ ATP}$ and $2\text{ NADH}$.

Pyruvate Dehydrogenase

A multienzyme complex in the mitochondrial matrix that oxidizes pyruvate to $acetyl\text{-}CoA$, generating $1\text{ CO}_2$ and $1\text{ NADH}$ per pyruvate molecule.

Citric Acid Cycle

A 9-step cyclic pathway in the mitochondrial matrix that oxidizes acetyl groups from $acetyl\text{-}CoA$, producing $2\text{ CO}_2$, $3\text{ NADH}$, $1\text{ FADH}_2$, and $1\text{ ATP}$ per turn.

Proton-Motive Force

The electrochemical concentration gradient formed by protons ($H^+$) pumped into the mitochondrial intermembrane space or thylakoid lumen that drives $ATP\text{ synthase}$.

Chemiosmosis

The movement of protons down their concentration gradient through $ATP\text{ synthase}$ across a membrane to drive the phosphorylation of $ADP$ into $ATP$.

Phosphofructokinase

The key regulatory enzyme of step 3 in glycolysis that is allosterically inhibited by high levels of $ATP$ and citrate, and activated by $ADP$.

Ethanol Fermentation

An anaerobic pathway in yeast where pyruvate is decarboxylated into acetaldehyde and then reduced to ethanol, oxidizing $NADH$ back to $NAD^+$ to sustain glycolysis.

Lactic Acid Fermentation

An anaerobic process occurring in animal muscle cells and certain microbes where pyruvate is directly reduced by $NADH$ to lactate, regenerating $NAD^+$ without producing $CO_2$.

Chloroplast Anatomy

The specialized photosynthetic organelle composed of an outer membrane, an inner membrane, a semiliquid stroma, and disc-shaped thylakoid membranes stacked into grana.

Chlorophyll a

The primary photosynthetic pigment in plants and cyanobacteria that absorbs violet-blue and red light and is the only pigment that can directly convert light energy into chemical energy.

Carotenoid

An accessory pigment that absorbs light over a wide range of wavelengths, scavenges harmful free radicals as an antioxidant, and is visible in fall foliage.

Absorption Spectrum

A graphical representation showing the specific wavelengths and relative efficiency of photons that a photosynthetic pigment is capable of absorbing.

Antenna Complex (Light-Harvesting Complex)

A photosystem component consisting of hundreds of accessory pigment molecules that captures photons and transfers the excitation energy to the reaction center.

Reaction Center

A transmembrane protein complex within a photosystem containing chlorophyll a molecules that releases high-energy electrons to a primary electron acceptor upon excitation.

Photosystem II (P680)

The photosystem functioning first in linear electron flow that absorbs light at $680\,nm$, oxidizes water to generate $O_2$ and replace lost electrons, and passes electrons to the $b_6\text{-}f\text{ complex}$.

Photosystem I (P700)

The photosystem that absorbs light at $700\,nm$, receives replacement electrons from plastocyanin, and passes excited electrons via ferredoxin to reduce $NADP^+$ into $NADPH$.

b6-f Complex

A transmembrane protein complex that accepts electrons from plastoquinone ($PQ$) and transfers them to plastocyanin ($PC$), using released energy to pump protons into the thylakoid space.

Calvin Cycle (C3 Photosynthesis)

The light-independent metabolic pathway in the chloroplast stroma consisting of carbon fixation, reduction, and $RuBP$ regeneration that synthesizes $G3P$ using $ATP$ and $NADPH$.

Rubisco

The enzyme that catalyzes the initial carbon fixation step of the Calvin cycle by joining carbon dioxide ($CO_2$) to ribulose 1,5-bisphosphate ($RuBP$) to form 3-phosphoglycerate ($PGA$).

Glyceraldehyde 3-Phosphate (G3P)

A $3\text{-carbon}$ sugar produced as the direct carbohydrate output of the Calvin cycle, utilized by plant cells to synthesize glucose, sucrose, and starch.