Metabolism, Cellular Respiration & Photosynthesis

Vocabulary flashcards covering metabolism, thermodynamic laws, enzyme regulation, cellular respiration pathways, and photosynthesis.

Metabolism

All of the chemical reactions that occur inside cells.

Metabolic pathway

A series of biochemical reactions that converts a starting substrate into a final product.

Anabolic pathway

A metabolic pathway that builds complex molecules from simpler ones, requiring an energy input and commonly using ATP (for example, joining nucleotides to build DNA).

Catabolic pathway

A metabolic pathway that breaks down complex molecules into simpler ones, releasing energy that can be captured in molecules such as ATP (for example, breaking down proteins into amino acids).

Bioenergetics

The study of energy flow through living systems, including cells.

Chemical energy

Potential energy associated with the arrangement of atoms and electrons in chemical substances.

First Law of Thermodynamics

The principle of conservation of energy stating that energy cannot be created or destroyed; it can only be transferred from place to place or transformed from one form into another.

Second Law of Thermodynamics

The principle stating that every energy transfer makes some energy less available for useful work (often dispersing it as heat), meaning no energy transfer is completely efficient and total entropy tends to increase.

Entropy ($S$)

A measure of the dispersal of energy and the number of possible arrangements of a system; in introductory biology, it is often described as randomness or disorder.

Gibbs free energy ($G$)

A thermodynamic quantity that measures the energy available to do useful work after accounting for entropy, expressed through the relationship $\Delta G = \Delta H - T\Delta S$.

Exergonic reaction

A reaction with a negative $\Delta G$ where net free energy is released, the products have less free energy than the reactants, and the process is thermodynamically spontaneous.

Endergonic reaction

A reaction with a positive $\Delta G$ where a net input of free energy is required, the products have more free energy than the reactants, and the process is nonspontaneous on its own.

ATP (adenosine triphosphate)

The primary energy currency used by cells, composed of adenine, a five-carbon pentose sugar (ribose), and three phosphate groups.

Energy coupling

The process by which an exergonic process releasing free energy directly drives an endergonic process that requires free energy.

Phosphorylation

The addition of a phosphate group to a molecule (such as a protein), which can alter its shape or activity.

Activation energy ($E_A$)

The initial energy barrier that must be overcome to start a chemical reaction by reaching the transition state.

Transition state

The brief, high-energy, unstable structural arrangement in a chemical reaction during which chemical bonds are in the process of breaking and/or forming.

Active site

The specific region of an enzyme where substrate binding and catalysis take place.

Denaturation

The structural alteration of an enzyme outside its suitable range of temperature or pH, which alters its native properties and reduces or halts catalytic activity.

Allosteric regulation

An enzyme regulatory mechanism in which an effector molecule binds somewhere other than the active site, inducing a conformational change that modifies catalytic activity.

Competitive inhibition

A mode of enzyme regulation where an inhibitor molecule directly binds to the active site and competes with the substrate, physically blocking substrate binding.

Feedback inhibition

A regulatory mechanism where the downstream product of a metabolic pathway binds to and inhibits an earlier enzyme in that pathway, preventing overproduction.

Cofactors

Inorganic helper substances required for enzyme activity, frequently consisting of metal ions such as $Fe^{3+}$ or $Mg^{2+}$.

Coenzymes

Organic helper molecules with a carbon-and-hydrogen framework (often derived from vitamins) necessary for enzyme function.

Oxidation

The loss or removal of electrons from a chemical substance.

Reduction

The gain or addition of electrons to a chemical substance.

Reducing agent

An electron donor that reduces another substance while becoming oxidized itself.

Oxidizing agent

An electron acceptor that oxidizes another substance while becoming reduced itself.

Cellular respiration

The process by which cells extract usable energy from organic molecules to synthesize ATP, characterized overall by $C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O + \text{energy}$.

Glycolysis

An anaerobic 10-step pathway occurring in the cytoplasm that splits one glucose molecule into two pyruvate molecules, yielding a net $2\,\text{ATP}$ and $2\,\text{NADH}$.

Pyruvate oxidation

The conversion of pyruvate into acetyl-CoA in the mitochondrial matrix, producing $2\,\text{acetyl-CoA}$, $2\,CO_2$, and $2\,\text{NADH}$ per glucose molecule.

Citric acid cycle (Krebs cycle)

An eight-step closed loop in the mitochondrial matrix that oxidizes acetyl-CoA, yielding $4\,CO_2$, $2\,\text{ATP}$, $6\,\text{NADH}$, and $2\,\text{FADH}_2$ per glucose molecule.

Oxidative phosphorylation

The final stage of cellular respiration consisting of the electron transport chain and chemiosmosis, generating approximately $26\text{--}28\,\text{ATP}$ per glucose.

Chemiosmosis

The movement of $H^+$ ions down their electrochemical gradient across a membrane through ATP synthase to drive ATP formation.

Fermentation

An anaerobic process in which an organic molecule serves as the terminal electron acceptor to regenerate $NAD^+$ from $NADH$, allowing glycolysis to continue generating ATP.

Photoautotrophs

Organisms that utilize solar light energy to manufacture their own food (literally 'self-feeders using light').

Heterotrophs

Organisms that cannot make all of their required organic food from inorganic sources and must consume other organisms or their products (literally 'other-feeders').

Chemoautotrophs

Microorganisms that synthesize organic compounds from $CO_2$ using chemical energy extracted from inorganic substances rather than light.

Stomata

Microscopic openings on leaf surfaces that facilitate the exchange of carbon dioxide and oxygen between the plant and the external environment.

Light-dependent reactions

The first stage of photosynthesis, occurring in thylakoid membranes, that converts solar energy into chemical energy stored in ATP and NADPH while releasing $O_2$.

Calvin cycle

The light-independent stage of photosynthesis occurring in the chloroplast stroma that consumes $CO_2$, ATP, and NADPH across three phases (fixation, reduction, and regeneration) to synthesize G3P.

RuBisCO

The chloroplast enzyme that catalyzes the initial fixation of $CO_2$ onto ribulose bisphosphate (RuBP) during the Calvin cycle.