Photosynthesis: Capturing Light Energy

Comprehensive vocabulary flashcards covering the photosynthetic mechanisms, chloroplast structures, electron transport pathways, photophosphorylation, Calvin cycle reactions, $$C_4$$ and CAM adaptations, and metabolic diversity based on Chapter 9.

Photosynthesis

The biological redox process by which light energy is converted into chemical energy, summarized by the overall equation: $6\,CO_2 + 12\,H_2O \rightarrow C_6H_{12}O_6 + 6\,O_2 + 6\,H_2O$.

Photon

A small particle or packet of light energy whose energy content is inversely proportional to its wavelength, meaning shorter wavelengths possess higher energy per photon.

Chloroplast

The specialized double-membrane organelle found predominantly in the mesophyll layer of leaves that contains chlorophyll pigments and serves as the site of photosynthesis.

Stroma

The fluid-filled interior space of a chloroplast surrounding the thylakoids, where the carbon fixation reactions (Calvin cycle) take place.

Thylakoid

An interconnected system of flattened, saclike membranous discs within a chloroplast containing photosynthetic pigments, electron transport chains, and ATP synthase.

Granum

A stacked column of individual thylakoid discs located within the stroma of a chloroplast.

Chlorophyll a

The primary photosynthetic pigment, bright green in appearance, that initiates the light-dependent reactions of photosynthesis.

Chlorophyll b

An accessory photosynthetic pigment, yellow-green in appearance, that differs from chlorophyll a by having a terminal formyl group ($-CHO$) in place of a methyl group ($-CH_3$).

Carotenoids

Yellow and orange accessory pigments that absorb light energy and transfer absorbed energy to chlorophyll.

Porphyrin ring

The light-absorbing resonant head of a chlorophyll molecule, characterized by alternating conjugated double bonds and a central coordinated $Mg^{2+}$ ion that delocalizes electrons.

Hydrocarbon side chain

The long, nonpolar lipid tail of a chlorophyll molecule that anchors the pigment into the hydrophobic core of the thylakoid membrane.

T. W. Engelmann's Experiment

An experiment measuring the action spectrum of photosynthesis by projecting light through a prism onto an algal filament and tracking the accumulation of oxygen-seeking aerobic bacteria in the blue and red wavelengths.

Antenna complex

A cluster of approximately $200$ chlorophyll and accessory pigment molecules organized with pigment-binding proteins that absorbs light and funnels excitation energy toward a reaction center.

Reaction center

A complex within a photosystem comprising a specialized pair of chlorophyll a molecules that releases energized electrons to a primary electron acceptor.

P680

The reaction center chlorophyll a complex of photosystem II, which has an absorption spectrum maximum at $680\,nm$.

P700

The reaction center chlorophyll a complex of photosystem I, which has an absorption spectrum maximum at $700\,nm$.

Noncyclic electron transport

The linear pathway of electron flow from $H_2O$ through photosystem II, an electron transport chain, photosystem I, and ferredoxin to reduce $NADP^+$ into NADPH while generating ATP by chemiosmosis and releasing $O_2$.

Cyclic electron transport

A light-driven pathway involving photosystem I only, in which energized electrons cycle back through ferredoxin, plastoquinone, cytochrome complex, and plastocyanin to P700 to generate ATP via chemiosmosis without generating NADPH or $O_2$.

Photolysis

The light-dependent enzymatic splitting of water molecules at photosystem II, producing protons ($H^+$), electrons to replace those lost by P680, and molecular oxygen ($O_2$).

Photophosphorylation

The synthesis of ATP from ADP and inorganic phosphate coupled to the transfer of electrons energized by light, driven by a proton gradient across the thylakoid membrane.

ATP synthase

A thylakoid membrane enzyme complex that utilizes the electrochemical proton gradient generated across the thylakoid lumen to phosphorylate ADP into ATP as protons diffuse into the stroma.

Calvin cycle

A cyclical series of carbon fixation reactions in the chloroplast stroma that utilizes ATP and NADPH to convert atmospheric $CO_2$ into carbohydrates.

Rubisco

Ribulose bisphosphate carboxylase/oxygenase; the stroma enzyme that catalyzes the initial fixation step of $CO_2$ onto ribulose bisphosphate (RuBP).

Ribulose bisphosphate (RuBP)

A five-carbon sugar phosphate that reacts with $CO_2$ during the $CO_2$ uptake phase of the Calvin cycle, forming an unstable intermediate that splits into two molecules of 3-phosphoglycerate (PGA).

Glyceraldehyde-3-phosphate (G3P)

The three-carbon sugar phosphate formed during the carbon reduction phase of the Calvin cycle, two molecules of which leave the cycle to produce one glucose molecule.

Photorespiration

A wasteful process in $C_3$ plants occurring on hot, dry days when stomata close and rubisco fixes $O_2$ instead of $CO_2$, degrading organic molecules to $CO_2$ and $H_2O$ without producing ATP.

$C_4$ pathway

A photosynthetic adaptation that spatially separates initial carbon fixation (forming four-carbon oxaloacetate in mesophyll cells) from the Calvin cycle (in bundle sheath cells) to minimize photorespiration.

Crassulacean acid metabolism (CAM) pathway

A photosynthetic adaptation in succulent plants that temporally separates carbon fixation by opening stomata at night to fix $CO_2$ into malate, and releasing $CO_2$ for the Calvin cycle during the day while stomata remain closed.

Photoautotrophs

Organisms that utilize light energy to generate ATP and NADPH, synthesizing complex organic compounds from inorganic raw materials such as $CO_2$ and $H_2O$.

Chemoheterotrophs

Organisms that obtain energy from chemical redox reactions and acquire carbon by consuming organic compounds produced by other organisms.

Photoheterotrophs

Organisms that utilize light energy to synthesize ATP but cannot fix carbon dioxide, relying instead on organic compounds for their carbon source.

Chemoautotrophs

Organisms that obtain energy by oxidizing reduced inorganic molecules such as hydrogen sulfide ($H_2S$), nitrite ($NO_2^-$), or ammonia ($NH_3$) and use that energy to carry out carbon fixation.