Cellular Energetics

Bioenergetics

the study of how energy flows through and is transformed within living organisms.

First Law of Thermodynamics

energy cannot be created or destroyed, only converted from one form to another.

Second Law of Thermodynamics

energy transformations increase the entropy (disorder) of the universe; no energy conversion is 100% efficient.

Energy

the capacity to do work.

Exergonic reaction

a chemical reaction that releases energy (products have less energy than reactants).

Energy diagram

a graph showing the energy changes during a reaction, including activation energy and net energy change.

Endergonic reaction

a chemical reaction that requires an input of energy (products have more energy than reactants).

Transition state

the high-energy, unstable arrangement of atoms at the peak of a reaction's energy diagram.

Activation energy

the minimum energy needed to start a chemical reaction.

Enzymes

Enzymes

proteins that act as biological catalysts, speeding up reactions without being consumed.

Enzyme specificity

the property of an enzyme acting on only one or a few particular substrates.

Substrates

the molecule(s) an enzyme acts upon in a reaction.

Active site

the region of an enzyme where the substrate binds and the reaction occurs.

Enzyme-substrate complex

the temporary structure formed when an enzyme binds to its substrate.

Induced-fit

the model describing how an enzyme's active site slightly changes shape to better fit its substrate upon binding.

Cofactors

non-protein inorganic ions or molecules that help enzymes function properly.

Coenzymes

organic (often vitamin-derived) cofactors that assist enzyme function.

Denatured

describes an enzyme/protein that has lost its normal shape (and function) due to heat, pH, or other factors.

Q10

a measure of how much a reaction rate (often enzymatic) increases for every 10°C rise in temperature.

Allosteric sites

locations on an enzyme, other than the active site, where molecules can bind and change enzyme activity.

Competitive inhibition

enzyme inhibition where a molecule competes with the substrate for the active site.

Allosteric inhibitor

a molecule that binds an allosteric site and reduces enzyme activity.

Noncompetitive inhibition

enzyme inhibition where an inhibitor binds somewhere other than the active site, changing the enzyme's shape and reducing activity.

Cellular respiration

the metabolic process that breaks down glucose to produce ATP, using oxygen (aerobic) or not (anaerobic).

Photosynthesis

the process by which plants and other organisms convert light energy into chemical energy (glucose).

Light reactions

the first stage of photosynthesis, occurring in the thylakoid membranes, that convert light energy into ATP and NADPH.

Dark reactions

(Calvin cycle) the second stage of photosynthesis that uses ATP and NADPH to build sugars from CO2; doesn't directly require light.

Photons

discrete packets of light energy.

Stroma

the fluid-filled space surrounding the thylakoids inside a chloroplast, where the Calvin cycle occurs.

Grana

stacks of thylakoid membranes within a chloroplast.

Thylakoids

membrane-bound discs inside chloroplasts where the light reactions occur.

Chlorophyll a and b

the main pigments in plants that absorb light energy for photosynthesis, reflecting green light.

Carotenoids

accessory pigments that absorb light in wavelengths chlorophyll misses, appearing yellow/orange.

Reaction center

the location in a photosystem where light energy is converted into chemical energy by exciting an electron.

Antenna pigments

clusters of pigment molecules that capture light energy and funnel it to the reaction center.

Photosystem II (PS II)

the first protein complex in the light reactions, which splits water and starts the electron transport chain.

P680

the special chlorophyll pair at the reaction center of Photosystem II.

P700

the special chlorophyll pair at the reaction center of Photosystem I.

Photophosphorylation

the production of ATP using light energy during the light reactions.

Absorption spectrum

a graph showing how much light a pigment absorbs across different wavelengths.

Emission spectrum

the spectrum of wavelengths of light given off by an excited molecule.

Photolysis

the light-driven splitting of water molecules in Photosystem II, releasing electrons, protons, and oxygen.

NADPH

an electron carrier molecule that stores energy from the light reactions to be used in the Calvin cycle.

Carbon fixation

the process of incorporating CO2 into an organic molecule, the first step of the Calvin cycle.

Calvin-Benson Cycle

the set of light-independent reactions that use ATP and NADPH to convert CO2 into glucose.

Photorespiration

a wasteful process where the enzyme RuBisCO binds oxygen instead of CO2, reducing photosynthetic efficiency.

CAM plants

plants that fix CO2 at night and store it as an acid, minimizing water loss in dry climates (Crassulacean Acid Metabolism).

C4 plants

plants that use a special pathway to concentrate CO2 before the Calvin cycle, reducing photorespiration.

Aerobic respiration

cellular respiration that uses oxygen as the final electron acceptor, yielding more ATP.

Anaerobic respiration

respiration that occurs without oxygen, yielding less ATP and often producing fermentation byproducts.

NADH

an electron carrier molecule that transports electrons to the electron transport chain.

FADH2

another electron carrier molecule that transports electrons to the electron transport chain.

Glycolysis

the breakdown of glucose into two pyruvic acid molecules, occurring in the cytoplasm; produces a small amount of ATP and NADH.

Pyruvic acid

the 3-carbon molecule produced at the end of glycolysis.

Acetyl-coenzyme A (acetyl-CoA)

the 2-carbon molecule formed from pyruvate that enters the Krebs cycle.

Pyruvate dehydrogenase complex (PDC)

the enzyme complex that converts pyruvate into acetyl-CoA, releasing CO2.

Krebs cycle (citric acid cycle)

the series of reactions in the mitochondrial matrix that break down acetyl-CoA, producing ATP, NADH, FADH2, and CO2.

Matrix

the innermost compartment of the mitochondria, where the Krebs cycle takes place.

Oxaloacetate

the 4-carbon molecule that combines with acetyl-CoA to start the Krebs cycle.

Citric acid

the 6-carbon molecule formed at the start of the Krebs cycle (from oxaloacetate + acetyl-CoA).

Electron transport chain

a series of protein complexes in the mitochondrial (or thylakoid) membrane that pass electrons along, pumping protons to create an energy gradient.

Cytochrome C

a protein in the electron transport chain that shuttles electrons between complexes.

pH gradient (proton gradient)

the difference in proton (H+) concentration across a membrane, created by the electron transport chain.

Chemiosmosis

the process of ATP production driven by the flow of protons down their gradient through ATP synthase.

ATP synthase

the enzyme that uses the proton gradient to produce ATP from ADP and phosphate.

Oxidative phosphorylation

the production of ATP using energy from the electron transport chain and chemiosmosis, requiring oxygen.

Fermentation

Lactic acid

the byproduct of fermentation in animal cells (and some bacteria) when oxygen is unavailable.

Ethanol (ethyl alcohol)

the byproduct of fermentation in yeast and some plant cells.

Fermentation

the anaerobic process of regenerating NAD+ from NADH without an electron transport chain, allowing glycolysis to continue.