Photosynthesis: Light Reactions, Calvin Cycle, and Adaptations

Comprehensive vocabulary flashcards covering chloroplast anatomy, light-dependent and light-independent reactions, photophosphorylation mechanisms, photorespiration, and C4/CAM adaptations.

Autotroph

An organism capable of producing organic, energy-rich nutrients from inorganic compounds, including blue-green algae, unicellular and multicellular eukaryotic algae, and green land plants.

Cuticle

A waxy outer protective layer made of cutin that coats the single-cell-thick epidermis of a plant leaf.

Stomata

Specialized microscopic pores situated in the lower epidermis of leaves, flanked by guard cells, that regulate gas exchange and water transpiration.

Palisade Layer

A sub-epidermal mesophyll tissue consisting of vertically oriented cylindrical cells, $1$ to $2$ cell layers thick, packed with numerous chloroplasts for light harvesting.

Thylakoid

A flattened, membranous sac inside the chloroplast containing embedded pigments, electron transport chain complexes, and ATP synthase, separating the stroma from an inner lumen where a $H^+$ concentration gradient is generated.

Granum

A compact stack of thylakoids providing a large membrane surface area within a small internal volume to optimize proton accumulation and chemiosmosis.

Stroma

The protein-rich fluid matrix of the chloroplast containing ribosomes, circular prokaryote-like DNA, and the soluble enzymes required for carbon fixation.

Chlorophyll a

The primary light-trapping pigment in cyanobacteria and photosynthetic eukaryotes, consisting of a light-absorbing porphyrin head with a central $Mg^{2+}$ ion anchored to the thylakoid membrane via a non-polar hydrocarbon tail.

Carotenoids

Accessory pigments composed of long hydrocarbon chains flanked by rings at both ends; they absorb violet, blue, and green light to pass excitation energy to chlorophyll, protect cells from damaging oxygen free radicals, and produce fall foliage colors.

Engelmann's Experiment

An experiment that demonstrated the action spectrum of photosynthesis by illuminating a filamentous alga with light split by a prism and observing aerobic bacteria clustering where oxygen release was highest in the blue and red wavelengths.

Photosystem

A functional light-trapping complex organized within the thylakoid membrane, composed of approximately $300$ pigment molecules arranged into antenna complexes surrounding a central reaction center chlorophyll.

Antenna Molecules

Pigment molecules in a photosystem that absorb photons of specific wavelengths ($\lambda$) and transfer resonance excitation energy inward toward the reaction center.

Reaction Center

A specialized pair of chlorophyll molecules situated at the core of a photosystem with the lowest free energy state, which transfers high-energy excited electrons to a primary electron acceptor.

Photosystem II (P680)

A photosynthetic unit containing roughly $200$ pigment molecules whose reaction center has peak light absorption at $680\,nm$; it initiates linear electron flow upon being excited by photons.

Photosystem I (P700)

A photosynthetic unit whose reaction center chlorophyll exhibits peak absorption at $700\,nm$; it accepts electrons from the plastocyanin ($Pc$) chain and can participate in cyclic or non-cyclic electron transport.

Enzyme Complex Z

An enzyme associated with Photosystem II that photolyzes water according to the reaction $2\,H_2O \rightarrow 4\,e^- + 4\,H^+ + O_2$, supplying electrons to replenish oxidized $P680$.

Cyclic Photophosphorylation

A primitive, inefficient pathway where excited electrons cycle from $P700$ through ferredoxin ($Fd$), plastoquinone ($PQ$), and cytochromes to pump $2\,H^+$ per pair of electrons, generating only ATP before returning to $P700$.

Non-Cyclic Photophosphorylation

The linear pathway of electron flow from $H_2O$ through PSII, the cytochrome intermediate chain, and PSI to $NADP^+$ reductase, creating a proton gradient for ATP production and yielding $NADPH + H^+$ for carbon fixation.

Light-Independent Reactions (Dark Reactions)

The chemical reactions occurring in the chloroplast stroma that consume the ATP and $NADPH + H^+$ generated by light reactions to reduce $CO_2$ into stable carbohydrates ($(CH_2O)_n$).

Calvin-Bensen Cycle (C3 Cycle)

The enzymatic pathway in the stroma that fixes $3\,CO_2$ using $9\text{ ATP}$ and $6\text{ NADPH}_2$ to yield a net output of one $3\text{-carbon}$ sugar molecule ($PGAL$).

RuBisCO (RuBP Carboxylase)

The primary stroma enzyme responsible for fixing $CO_2$ onto ribulose bisphosphate ($RuBP$); it possesses moderate affinity for $CO_2$ and can competitively bind $O_2$ when $CO_2$ levels are depleted.

Ribulose Bisphosphate (RuBP)

A 5-carbon ($C_5$) compound that combines with $CO_2$ to produce an unstable 6-carbon ($C_6$) intermediate that rapidly splits into two molecules of 3-phosphoglycerate ($PGA$).

3-Phosphoglycerate (PGA)

The first stable 3-carbon ($C_3$) intermediate produced during carbon fixation in the Calvin cycle before being phosphorylated by ATP and reduced by $NADPH_2$.

Phosphoglyceraldehyde (PGAL / G3P)

The direct 3-carbon ($C_3$) carbohydrate end-product of the Calvin cycle, which plants assemble into glucose, starch, cellulose, lipids, amino acids, or nucleotides.

Hygroscopic

A property describing a substance with an exceptionally high attraction for water; free glucose exhibits this trait, which would disrupt intracellular osmotic balance if stored in high concentrations rather than converted into starch.

Photorespiration

A wasteful process occurring under high heat, high oxygen, and low $CO_2$ where RuBisCO fixes $O_2$ to $RuBP$, breaking it down to $PGAL$ and a 2-carbon compound released as $CO_2$ without producing ATP.

Kranz Anatomy

The specialized internal arrangement of photosynthetic cells in $C_4$ plant leaves, featuring tightly packed bundle-sheath cells encircled by mesophyll cells surrounding the vascular bundle.

Phosphoenolpyruvate (PEP)

A 3-carbon ($C_3$) substrate in mesophyll cells of $C_4$ plants that binds $CO_2$ to produce a 4-carbon organic acid intermediate.

PEP Carboxylase

An enzyme in $C_4$ plant mesophyll cells that fixes $CO_2$ onto PEP; it lacks oxygenase activity, does not bind $O_2$, and functions efficiently even when stomata are closed and $CO_2$ levels are low.

Temporal Separation (CAM Pathway)

A carbon-fixation strategy found in arid-adapted plants like pineapple, where stomata open at night to fix $CO_2$ into organic acids, which are subsequently decarboxylated during the day to supply the Calvin cycle while stomata remain closed.