Chapter 08: Photosynthesis

Vocabulary flashcards covering photosynthetic structures, light-dependent reactions, cyclic and noncyclic electron pathways, the Calvin cycle, and C3, C4, and CAM adaptations based on Chapter 08 of Inquiry into Life.

Photosynthesis

The biological process that converts solar energy into the chemical energy of carbohydrates, represented by the overall chemical equation $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$.

Autotroph

An organism that produces its own food by capturing solar energy through photosynthesis, including plants, algae, and cyanobacteria.

Heterotroph

An organism that cannot synthesize its own food and must feed on other organisms, depending on organic molecules produced by autotrophs.

Chlorophyll

The primary photosynthetic pigment in plants and algae that absorbs solar energy (primarily violet, indigo, blue, and red light) and reflects green light, making plants appear green.

Carotenoids

An accessory pigment group in photosynthetic organisms that absorbs light strongly in the blue-to-green spectrum ($440\,\text{nm}$ to $520\,\text{nm}$) and becomes noticeable in autumn when chlorophyll breaks down.

Mesophyll

The internal leaf tissue specialized for photosynthesis containing chloroplasts into which water and carbon dioxide diffuse.

Stomata

Small openings in the leaf epidermis (singular: stoma) through which carbon dioxide enters and oxygen exits the leaf.

Stroma

The fluid-filled interior space of a chloroplast enclosed by a double membrane, containing the enzymes that reduce carbon dioxide to carbohydrates during the Calvin cycle.

Thylakoids

Flattened membranous sacs inside the stroma of a chloroplast that contain chlorophyll and other light-absorbing pigments within their membranes.

Granum

A column or stack of interconnected thylakoids located inside the chloroplast (plural: grana).

Thylakoid Space

The continuous internal compartment enclosed by thylakoid membranes that functions as a reservoir for hydrogen ions ($\text{H}^+$) to establish an electrochemical gradient.

Light Reactions

The light-dependent stage of photosynthesis taking place in the thylakoid membranes; uses solar energy and $\text{H}_2\text{O}$ to produce $\text{O}_2$, $\text{ATP}$, and $\text{NADPH}$.

Calvin Cycle

The light-independent stage of photosynthesis occurring in the stroma; uses $\text{CO}_2$, $\text{ATP}$, and $\text{NADPH}$ to synthesize carbohydrates ($\text{CH}_2\text{O}$), returning $\text{ADP} + \text{P}$ and $\text{NADP}^+$ to the light reactions.

Visible Light

The portion of the electromagnetic spectrum between $380\,\text{nm}$ and $750\,\text{nm}$ that drives photosynthesis, ranging from high-energy violet light to lower-energy red light.

Photosystem

A light-gathering unit embedded in the thylakoid membrane consisting of a pigment complex antenna and an electron acceptor.

Reaction Center

A specialized pair of chlorophyll a molecules within a photosystem that concentrates gathered solar energy and releases energized electrons to a primary electron acceptor.

Noncyclic Electron Pathway

A light reaction pathway using both $\text{PSII}$ and $\text{PSI}$; electrons flow from water to $\text{NADP}^+$, producing both $\text{ATP}$ and $\text{NADPH}$ while releasing $\text{O}_2$.

Cyclic Electron Pathway

An alternate light reaction pathway active when $\text{NADPH}$ is high that uses only $\text{PSI}$; energized electrons cycle through an electron transport chain back to $\text{PSI}$, producing only $\text{ATP}$.

Chemiosmosis

The synthesis of $\text{ATP}$ driven by the flow of hydrogen ions ($\text{H}^+$) down their concentration and electrical gradient through $\text{ATP}$ synthase complexes.

Plastoquinone (Pq)

A molecular carrier at the beginning of the thylakoid electron transport chain that pumps hydrogen ions ($\text{H}^+$) from the stroma into the thylakoid space as electrons pass through it.

NADP+ Reductase

An enzyme located in the thylakoid membrane adjacent to $\text{PSI}$ that catalyzes the transfer of electrons and a proton to $\text{NADP}^+$, reducing it to $\text{NADPH}$.

ATP Synthase Complex

A membrane protein complex containing a channel for $\text{H}^+$ flow that uses electrochemical energy to bind $\text{ADP}$ and phosphate into $\text{ATP}$.

Carbon Dioxide Fixation

The initial step of the Calvin cycle where three molecules of atmospheric $\text{CO}_2$ are attached to three molecules of 5-carbon $\text{RuBP}$, yielding six molecules of 3-phosphoglycerate ($\text{3PG}$).

RuBP Carboxylase (Rubisco)

A slow, highly abundant chloroplast enzyme that catalyzes the attachment of $\text{CO}_2$ to $\text{RuBP}$ during the Calvin cycle, but can also bind oxygen.

Carbon Dioxide Reduction

The second phase of the Calvin cycle in which $\text{3PG}$ is converted into $\text{BPG}$ using $\text{ATP}$, and $\text{BPG}$ is subsequently reduced to $\text{G3P}$ using electrons and energy from $\text{NADPH}$.

RuBP Regeneration

The third phase of the Calvin cycle where five molecules of 3-carbon $\text{G3P}$ are re-formed into three molecules of 5-carbon $\text{RuBP}$ using three molecules of $\text{ATP}$.

Glyceraldehyde-3-Phosphate (G3P)

The 3-carbon carbohydrate end-product of the Calvin cycle used by plants to synthesize glucose, sucrose, starch, cellulose, fatty acids, and amino acids.

Photorespiration

A wasteful reaction occurring when Rubisco binds $\text{O}_2$ instead of $\text{CO}_2$, consuming oxygen and releasing carbon dioxide while decreasing Calvin cycle productivity.

C3 Photosynthesis

The standard photosynthetic pathway where initial $\text{CO}_2$ fixation and the Calvin cycle both occur in leaf mesophyll cells, forming 3-carbon $\text{3PG}$.

C4 Photosynthesis

An adaptation for hot, dry environments where initial $\text{CO}_2$ fixation into a 4-carbon molecule occurs in mesophyll cells, while the Calvin cycle is segregated into bundle sheath cells to prevent photorespiration.

CAM Photosynthesis

Crassulacean acid metabolism; an adaptation in desert succulents where stomata open only at night to fix $\text{CO}_2$ into $\text{C}_4$ molecules in vacuoles, which release $\text{CO}_2$ to the Calvin cycle during the day.

PEP Carboxylase

An enzyme used in $\text{C}_4$ and CAM plants that fixes $\text{CO}_2$ (as $\text{HCO}_3^-$) to phosphoenolpyruvate ($\text{PEP}$) to form a 4-carbon molecule without binding oxygen.