Photosynthesis: Light Reactions and the Calvin Cycle

A comprehensive vocabulary review set covering the light reactions, photophosphorylation, Calvin cycle phases, photorespiration, and C4/CAM photosynthetic adaptations.

Photoautotrophs

Organisms that capture light energy and use it to synthesize organic molecules from $CO_2$ and $H_2O$, serving as the primary producers of the biosphere.

Heterotrophs

Organisms that obtain energy and organic matter by consuming other organisms or organic products; known as the consumers of the biosphere.

Phytoplankton

Microscopic photosynthetic aquatic organisms responsible for producing between $50\%$ and $80\%$ of global $O_2$.

Stroma

The dense fluid compartment within the chloroplast, enclosed by the inner membrane, where the Calvin cycle takes place.

Thylakoid

A flattened, membrane-bound sac inside a chloroplast whose membrane contains photosynthetic pigments and hosts the light reactions.

Thylakoid Lumen

The interior space within a thylakoid where protons ($H^+$) accumulate during the light reactions to generate a proton-motive force.

Photon

A discrete quantum or packet of electromagnetic energy that travels as a wave, behaves as a particle, and has no mass.

Absorption Spectrum

A graph that depicts the degree of light absorption by a pigment across different wavelengths of electromagnetic radiation.

Action Spectrum

A graph illustrating the relative effectiveness of different wavelengths of radiation in driving photosynthesis, typically measured by $O_2$ release.

Chlorophyll a

The primary photosynthetic pigment in plants that absorbs violet-blue and red light, characterized chemically by a methyl group ($-\text{CH}_3$) on its porphyrin ring.

Chlorophyll b

An accessory photosynthetic pigment that expands the range of usable light wavelengths, characterized chemically by a carbonyl/formyl group ($-\text{CHO}$) on its porphyrin ring.

Carotenoids

Accessory pigments that absorb light in the violet and blue-green ranges, functioning in photoprotection by dissipating excessive light energy that would otherwise damage chlorophyll.

Chlorophyll Molecule Structure

A structure composed of a light-absorbing porphyrin ring containing a central magnesium atom ($Mg$) and a hydrophobic hydrocarbon tail that anchors it into the thylakoid membrane.

Fluorescence

The emission of a photon with lower energy and a longer wavelength when an excited electron in an isolated pigment molecule drops back down to its ground state.

Photosystem

A light-capturing unit located in the thylakoid membrane, consisting of a reaction-centre complex surrounded by multiple light-harvesting complexes.

Reaction-Centre Complex

A complex of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor, executing the initial redox reaction of photosynthesis.

Primary Electron Acceptor

A specialized molecule in the reaction-centre complex that accepts photoexcited electrons transferred from the special pair of chlorophyll a molecules.

P680

The special pair of chlorophyll a molecules in the reaction centre of Photosystem II (PS II) that has an absorption peak at $680\,\text{nm}$.

$$\text{P680}^+$$

The oxidized form of P680 produced after electron excitation and transfer; recognized as the strongest known biological oxidizing agent, capable of oxidizing water ($H_2O$).

P700

The special pair of chlorophyll a molecules in the reaction centre of Photosystem I (PS I) that absorbs light most effectively at a wavelength of $700\,\text{nm}$.

Plastoquinone (Pq)

A mobile hydrophobic electron carrier that receives electrons from the primary acceptor of Photosystem II and passes them to the cytochrome complex.

Cytochrome Complex

An electron transport chain complex in the thylakoid membrane that transfers electrons from Pq to Pc while pumping $4\,H^+$ from the stroma into the thylakoid space.

Plastocyanin (Pc)

A water-soluble, copper-containing mobile protein that shuttles electrons from the cytochrome complex to the oxidized $\text{P700}^+$ in Photosystem I.

Ferredoxin (Fd)

An iron-sulfur protein that receives photoexcited electrons from Photosystem I's primary electron acceptor during the light reactions.

$$\text{NADP}^+\text{ Reductase}$$

A stromal thylakoid enzyme that transfers two electrons from ferredoxin to $\text{NADP}^+$, consuming a stromal proton ($H^+$) to yield $\text{NADPH}$.

Linear Electron Flow

The primary pathway of electron flow during light reactions, moving electrons from water through PS II, an ETC, and PS I to $\text{NADP}^+$, producing $\text{ATP}$, $\text{NADPH}$, and $O_2$.

Cyclic Electron Flow

An alternative electron pathway involving only Photosystem I, where electrons cycle from ferredoxin back to the cytochrome complex and P700, generating extra $\text{ATP}$ without producing $\text{NADPH}$ or $O_2$.

Chemiosmosis

An energy-coupling mechanism that uses an $H^+$ electrochemical gradient across a membrane to drive the phosphorylation of $\text{ADP}$ into $\text{ATP}$ via $\text{ATP synthase}$.

Calvin Cycle

An anabolic cyclic metabolic pathway consisting of 11 enzymes located in the stroma that uses $\text{ATP}$ and $\text{NADPH}$ to reduce $CO_2$ into the 3-carbon sugar G3P.

Rubisco

Ribulose bisphosphate carboxylase-oxygenase; the abundant stromal enzyme that catalyzes the primary fixation of $CO_2$ to RuBP, but also mistakenly binds $O_2$.

Ribulose Bisphosphate (RuBP)

A five-carbon sugar that acts as the initial carbon dioxide acceptor in the Calvin cycle and is regenerated in the final phase using $\text{ATP}$.

3-Phosphoglycerate (3PG)

The three-carbon intermediate produced when the unstable six-carbon compound created by Rubisco splits during Phase 1 of the Calvin cycle.

Glyceraldehyde 3-Phosphate (G3P)

The three-carbon carbohydrate directly produced as the net output of the Calvin cycle, serving as the precursor for synthesizing glucose and other organic molecules.

Photorespiration

A wasteful metabolic pathway that occurs when Rubisco binds $O_2$ instead of $CO_2$, forming 2-phosphoglycolate, which consumes $\text{ATP}$ and releases $CO_2$ without producing sugar.

2-Phosphoglycolate

The two-carbon waste product formed when Rubisco binds $O_2$; its energetic salvage pathway requires transport between the chloroplast, peroxisome, and mitochondrion.

$$C_3\text{ Plants}$$

Plants that fix carbon directly through Rubisco to form 3-phosphoglycerate, making them prone to significant photorespiration on hot, dry days.

$$C_4\text{ Leaf Anatomy}$$

A specialized arrangement where photosynthetic cells are partitioned into outer mesophyll cells and inner, vein-surrounding bundle-sheath cells to concentrate $CO_2$.

Bundle-Sheath Cells

A layer of tightly packed photosynthetic cells surrounding vascular veins in $C_4$ plant leaves, where the Calvin cycle takes place in a high-$CO_2$ environment.

PEP Carboxylase

An enzyme in $C_4$ and CAM mesophyll cells that adds $CO_2$ to phosphoenolpyruvate (PEP) to form oxaloacetate; it exhibits high affinity for $CO_2$ and zero affinity for $O_2$.

Spatial vs. Temporal Carbon Fixation

The organizational difference where $C_4$ plants separate initial carbon capture and the Calvin cycle between two cell types, whereas CAM plants separate them between night and day within the same cell.

CAM Plants

Plants that open stomata at night to fix $CO_2$ into organic acids via PEP carboxylase, then close stomata during the day to release $CO_2$ internally to the Calvin cycle.