Photosynthesis & Cellular Respiration

metabolic pathways

series of biochemical reactions that convert nutrients into energy and build cellular components.

catabolic pathways

break down molecules to release energy.

anabolic pathways

build molecules by consuming energy.

bioenergetics

the study of energy flow in biological systems.

kinetic energy

the energy possessed by an object due to its motion.

thermal energy

the energy that comes from the heat of an object, often related to the temperature and motion of its particles.

potential energy

the stored energy in an object due to its position or configuration, which can be converted into kinetic energy.

chemical energy

the energy stored in the bonds of chemical compounds, which can be released during a chemical reaction.

thermodynamics

the branch of physics that deals with the relationships between heat and other forms of energy.

first law of thermodynamics

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

entropy

a measure of the disorder or randomness in a system, often associated with the second law of thermodynamics, which states that the total entropy of an isolated system can never decrease over time.

second law of thermodynamics

every energy transfer or transformation increases the entropy (disorder/randomness) of the universe, and no energy conversion is 100% efficient.

free energy

the portion of a system's energy available to do work at constant temperature and pressure.

exergonic reaction

a spontaneous chemical process that releases free energy, characterized by a negative change in Gibbs free energy.

endergonic reaction

a non-spontaneous chemical process that requires an input of energy to proceed, resulting in products with higher free energy than the reactants.

energy coupling

the mechanism where an energetically favorable, exergonic reaction (releasing energy, usually ATP hydrolysis) is directly linked to an unfavorable, endergonic reaction.

hydrolysis of ATP

exergonic biological process of breaking the terminal phosphoanhydride bond in adenosine triphosphate (ATP) using water (H₂O) to produce adenosine diphosphate (ADP) and inorganic phosphate (P_i)

phosphorylation

the crucial biochemical process of adding a phosphate group to an organic molecule (usually a protein, sugar, or ADP) to alter its structure, activate/deactivate it, or store energy.

fermentation

an anaerobic (oxygen-absent) metabolic process that regenerates (NAD⁺) from (NADH) by transferring electrons to an organic molecule, allowing glycolysis to continue producing ATP.

aerobic respiration

the highly efficient metabolic process in which cells use oxygen to break down glucose, converting it into ATP (energy), carbon dioxide, and water.

redox reaction

a chemical process involving the transfer of electrons between species.

oxidation

the loss of electrons from an atom, ion, or molecule during a chemical reaction.

reduction

the gain of electrons by a molecule, atom, or ion, usually accompanied by a decrease in its oxidation state.

NAD+

a vital coenzyme found in all living cells that acts as an electron carrier in redox reactions during cellular respiration.

electron transport chain (ETC)

a series of protein complexes and electron carrier molecules embedded in the inner mitochondrial membrane (or prokaryotic plasma membrane) that transfers electrons from donors like NADH and FADH₂ to a final electron acceptor, typically oxygen.

glycolysis

the anaerobic, metabolic pathway occurring in the cytoplasm that splits one molecule of glucose (6C) into two molecules of pyruvate (3C).

citric acid cycle/kreb’s cycle

the second stage of aerobic cellular respiration, occurring in the mitochondrial matrix of eukaryotes. It oxidizes acetyl-CoA, derived from carbohydrates, fats, or proteins, into carbon dioxide (CO₂), producing 2 ATP, 6 NADH, and 2 FADH₂ per glucose molecule to fuel electron transport.

cristae

extensive infoldings of the inner mitochondrial membrane that significantly increase surface area.

oxidative phophorylation

the final stage of cellular respiration occurring in the inner mitochondrial membrane, where ATP is synthesized using energy derived from redox reactions in an electron transport chain (ETC) and chemiosmosis.`

chemiosmosis

the mechanism of ATP synthesis where hydrogen ions (H⁺) flow down their electrochemical gradient across a selectively permeable membrane (inner mitochondrial or thylakoid) through the enzyme ATP synthase.

alcohol fermentation

an anaerobic metabolic pathway in yeast and some bacteria that converts pyruvate (from glycolysis) into ethanol and carbon dioxide, regenerating NAD⁺ to allow glycolysis to continue producing 2 ATP. It occurs in the cytosol without oxygen, reducing acetaldehyde to ethanol while releasing CO₂.

lactic acid fermentation

an anaerobic metabolic pathway that converts pyruvate (derived from glycolysis) into lactic acid (lactate) to regenerate NAD⁺, allowing ATP production to continue in the absence of oxygen. It occurs in muscle cells during intense exercise and in certain bacteria.

photosynthesis

the essential metabolic process in photoautotrophs (plants, algae, cyanobacteria) that captures light energy using chlorophyll and converts it into chemical energy, creating glucose (C₆H₁₂O₆) from carbon dioxide (CO₂) and water (H₂O) while releasing oxygen (O₂). It occurs in chloroplasts via light-dependent reactions and the Calvin cycle.

autotroph

an organism that produces its own organic food molecules (usually glucose) from inorganic raw materials (CO₂ and sunlight or chemical energy), serving as the primary producer at the base of ecosystems. They are "self-feeders" that do not consume other living things, forming the foundation of food webs.

heterotroph

an organism that cannot produce its own food and must obtain energy and carbon by consuming other organisms.

stomata

microscopic pores, primarily located on the epidermis of leaves and stems, flanked by two specialized guard cells that control their opening and closing.

stroma

the dense, alkaline, fluid-filled space within a chloroplast that surrounds the thylakoid membranes.

thylakoid

flattened, disc-like membrane sacs stacked into grana inside chloroplasts, serving as the site for the light-dependent reactions of photosynthesis. Embedded with chlorophyll and photosystems, they capture light energy, split water, and use an electron transport chain to generate ATP and NADPH.

chlorophyll

a green pigment located within the thylakoid membranes of chloroplasts in plants, algae, and cyanobacteria. It acts as the primary molecule for photosynthesis by absorbing solar energy—specifically blue and red wavelengths—and converting it into chemical energy, primarily reflecting green light.

light reaction

the first stage of photosynthesis, occurring in the thylakoid membranes of chloroplasts. They capture solar energy using photosystems (PSI, PSII) and convert it into chemical energy (ATP and NADPH) while splitting water (photolysis) to release oxygen as a byproduct.

calvin cycle

the set of light-independent, anabolic chemical reactions occurring in the chloroplast stroma that fixes atmospheric \(CO_{2}\) into organic sugar (G3P) using ATP and NADPH generated by light-dependent reactions. It comprises three main stages: carbon fixation (via RuBisCO), reduction, and regeneration of RuBP.

carbon fixation

the initial stage of the Calvin cycle in photosynthesis where inorganic carbon dioxide (\(CO₂) is captured from the atmosphere and incorporated into organic compounds. It is catalyzed by the enzyme rubisco in the chloroplast stroma, converting CO₂ and ribulose bisphosphate (RuBP) into 3-phosphoglycerate (3-PGA).

photons

massless, fundamental particles representing a quantum of light or electromagnetic radiation. They travel at the speed of light and possess energy proportional to their frequency, acting as discrete packets of energy. They are crucial in photosynthesis, where they are absorbed by chlorophyll to excite electrons.

photosystem 1

a light-harvesting protein-pigment complex in the thylakoid membrane of chloroplasts that absorbs light (optimal wavelength ~700 nm, P700) to excite electrons. It acts as the second photosystem in the light-dependent reactions, using light energy to reduce NADP+ to NADPH for the Calvin cycle.

photosystem 2

the first, light-dependent protein complex in the thylakoid membrane, initiating photosynthesis by capturing photons and using that energy to split water molecules into electrons, protons, and oxygen. It boosts electrons to a high energy level to drive the electron transport chain, contributing to ATP synthesis.

C3 plants

organisms that perform photosynthesis by directly fixing CO₂ via the Calvin cycle in mesophyll cells, producing a three-carbon compound (3-PGA) as the first stable product. Using the enzyme RuBisCO, they are the most common plants but are less efficient in hot, dry conditions due to high rates of photorespiration. Example: rice, wheat.

C4 plants

specialized plants adapted to hot, sunny environments that minimize photorespiration by spatially separating initial CO₂ fixation (in mesophyll cells) and the Calvin cycle (in bundle sheath cells). Example: corn.

CAM plants

specialized, drought-adapted plants that conserve water by opening their stomata at night to fix CO₂ into organic acids (like malate). During the day, stomata close to minimize water loss while stored CO₂ is released for the Calvin cycle. Example: cacti.

bromothymel blue

a pH indicator used in AP Biology to detect changes in carbon dioxide concentration, specifically for monitoring photosynthesis and cellular respiration. It turns yellow in acidic conditions (lower pH, high CO₂) and blue in basic/neutral conditions (higher pH, low CO₂).

indicator

an organism—often a plant, microorganism, or animal—whose presence, absence, or population health provides qualitative, real-time data about specific environmental conditions, such as pollution levels, water quality, or ecosystem health.