Photosynthesis: Light Reactions, Calvin Cycle, and Alternative Carbon Fixation

Vocabulary practice flashcards covering chloroplast structures, non-cyclic and cyclic light reactions, phases of the Calvin-Benson cycle, photorespiration, and evolutionary adaptations in C3, C4, and CAM plants.

Chloroplast

A double-membrane photosynthetic organelle containing an outer membrane, an inner membrane, a fluid-filled space called the stroma, and disc-shaped membranous sacs called thylakoids.

Stroma

The fluid-filled internal space of the chloroplast surrounding the thylakoids where the Calvin-Benson cycle takes place.

Thylakoid Membrane

The internal membrane structure within chloroplasts where light reactions take place, absorbing light to split $\text{H}_2\text{O}$, produce $\text{O}_2$, and generate $\text{ATP}$ and $\text{NADPH}$.

Non-Cyclic Photophosphorylation

A light-dependent electron flow from Photosystem II ($\text{P680}$) through an electron transport chain to Photosystem I ($\text{P700}$), producing $\text{ATP}$ via proton-motive force, reducing $\text{NADP}^+$ to $\text{NADPH}$, and releasing $\text{O}_2$ from water splitting.

Cyclic Photophosphorylation

A light-driven pathway where excited electrons from chlorophyll pass through an electron transport system (ETS) and return to chlorophyll, generating $\text{ATP}$ without producing $\text{NADPH}$ or oxygen.

Photosystem II (P680)

A photosystem in the thylakoid membrane that absorbs photons, excites electrons transferred to pheophytin, and oxidizes water according to the reaction $2\,\text{H}_2\text{O} \rightarrow 4\,\text{H}^+ + \text{O}_2 + 4e^-$.

Photosystem I (P700)

A photosystem that absorbs light to boost electrons received from plastocyanin (PC) and transfers them through ferredoxin to reduce $2\,\text{NADP}^+ + 2\,\text{H}^+$ into $2\,\text{NADPH}$.

Calvin-Benson Cycle

An anabolic, cyclic pathway in the stroma that consumes $\text{ATP}$ and $\text{NADPH}$ to reduce $\text{CO}_2$ into sugars, regenerating its starting material.

Rubisco

The primary carbon-fixing enzyme that catalyzes the initial step of the Calvin cycle by attaching $\text{CO}_2$ to ribulose bisphosphate (RuBP).

Glyceraldehyde 3-Phosphate (G3P)

A three-carbon sugar output of the Calvin cycle produced after fixing three $\text{CO}_2$ molecules across three cycles, utilized by the plant to synthesize glucose, sucrose, and starch.

Carbon Fixation Phase

The first phase of the Calvin cycle where $3\,\text{RuBP} + 3\,\text{CO}_2 \rightarrow 6\,\text{3-phosphoglycerate}$, catalyzed by the enzyme Rubisco.

Reduction Phase

The second phase of the Calvin cycle where $6\,\text{3-phosphoglycerate} + 6\,\text{ATP} + 6\,\text{NADPH}$ are converted into $6\,\text{G3P}$, consuming energy and reducing equivalents.

Regeneration Phase

The third phase of the Calvin cycle where $5\,\text{G3P} + 3\,\text{ATP} \rightarrow 3\,\text{RuBP}$, restoring the starting acceptor molecule to allow the cycle to continue.

Photorespiration

A wasteful metabolic process (also called $\text{C}_2$ photosynthesis) where Rubisco fixes $\text{O}_2$ instead of $\text{CO}_2$ to RuBP, consuming $\text{ATP}$, $\text{O}_2$, and Calvin cycle fuel while releasing $\text{CO}_2$ without generating sugar or ATP.

C3 Plants

Plants that fix $\text{CO}_2$ directly in mesophyll cells via Rubisco to produce a three-carbon compound (3-phosphoglycerate), lacking specialized structural adaptations against photorespiration.

C4 Plants

Plants, such as corn and sugarcane, that minimize photorespiration by spatially separating carbon fixation (in mesophyll cells) and the Calvin cycle (in bundle-sheath cells).

Bundle-Sheath Cells

Tightly packed photosynthetic cells surrounding leaf veins in $\text{C}_4$ plants where high concentrations of $\text{CO}_2$ are released from four-carbon acids to run the Calvin cycle without photorespiration.

Phosphoenolpyruvate (PEP)

A three-carbon organic compound that acts as the primary $\text{CO}_2$ acceptor in the mesophyll cytoplasm of $\text{C}_4$ plants.

Oxaloacetic Acid

The four-carbon compound that serves as the first stable product of carbon fixation in the $\text{C}_4$ pathway.

PEP-ase

An enzyme in $\text{C}_4$ and CAM plants that fixes $\text{CO}_2$ onto phosphoenolpyruvate (PEP), capable of operating at low $\text{CO}_2$ concentrations without oxygenase activity.

Crassulacean Acid Metabolism (CAM)

A water-conserving carbon-fixation pathway in succulents, pineapples, and cacti that temporally separates processes by opening stomata at night to store $\text{CO}_2$ in organic acids, releasing it to the Calvin cycle during the day.