Biol 1081 Study Sheet | Ch 8 - Photosynthesis

Photoautotroph

An organism that harnesses light energy to synthesize organic molecules from inorganic raw materials.

Chemoautotroph

An organism that derives energy from the oxidation of inorganic chemical compounds rather than sunlight to produce food.

Photon

A discrete packet or quantum of electromagnetic radiation that behaves as both a particle and a wave.

Ground state (electron)

The lowest, most stable energetic arrangement of an orbital electron before absorbing light energy.

Excited state (electron)

The elevated, unstable energy level an electron reaches after absorbing a photon.

Chlorophyll a

The primary photosynthetic photopigment directly involved in light reactions, absorbing blue-violet and red light while reflecting green.

Chlorophyll b

An accessory photosynthetic pigment that absorbs blue and red-orange light, transferring captured photon energy to the primary pigment.

Carotenoid

An accessory photopigment that absorbs excessive blue-green light for photoprotection and reflects yellow, orange, or red hues.

Stomata

Microscopic pores in the epidermis of plant leaves that regulate gas exchange ($CO<em>2$ and $O</em>2$) and transpiration.

Guard cells

Specialized paired epidermal cells flanking each stoma that swell or shrink to regulate the opening and closing of the pore.

Mesophyll

The interior leaf ground tissue between the upper and lower epidermis, housing high concentrations of chloroplasts for photosynthesis.

Thylakoid

A flattened, membranous sac inside chloroplasts whose membrane houses photosystems, pigments, and electron transport machinery.

Granum

A column-like stack of interconnected thylakoid discs inside a chloroplast.

Stroma

The dense fluid-filled space surrounding the thylakoid membranes within a chloroplast, where carbohydrate synthesis occurs.

Photosynthesis chemical equation

6CO2 + 6H2O + Light Energy → C,H1206 + 602

Metabolic classification of photosynthesis

Anabolic and endergonic, because it consumes solar energy to assemble complex organic sugars from smaller inorganic molecules.

Light-dependent reactions

The initial stage of photosynthesis in the thylakoid membrane that absorbs solar energy to generate $ATP$, $NADPH$, and $O_2$.

Calvin cycle (light-independent reactions)

The enzymatic pathway located in the stroma that utilizes $ATP$ and $NADPH$ to assimilate carbon dioxide into carbohydrate intermediates like $G3P$.

Photosystem

A multi-protein light-harvesting complex in the thylakoid membrane consisting of antenna pigment molecules surrounding a reaction center.

Source of replacement electrons for Photosystem II

Water ($H<em>2O$), which undergoes photolysis to yield electrons, protons ($H^+$), and oxygen gas ($O</em>2$).

Source of replacement electrons for Photosystem I

Electrons transported down the thylakoid electron transport chain originating from Photosystem II.

RuBisCO

The key stromal enzyme that catalyzes the primary carbon fixation reaction by joining $CO_2$ to ribulose-1,5-bisphosphate ($RuBP$).

Carbon fixation

The enzymatic conversion of inorganic carbon ($CO_2$) into an organic molecule.

Photorespiration

A counterproductive pathway occurring when RuBisCO oxygenates $RuBP$ instead of carboxylating it, consuming ATP without producing sugar.

C4 pathway

An anatomical and biochemical adaptation that spatially isolates carbon fixation in mesophyll cells from the Calvin cycle in bundle-sheath cells.

CAM pathway

A photosynthetic adaptation that temporally separates gas exchange by fixing $CO_2$ into organic acids at night and running the Calvin cycle during the day.

PEP carboxylase

An enzyme with high affinity for $CO<em>2$ and no affinity for $O</em>2$, fixing carbon into a four-carbon intermediate in C4 and CAM plants.

NADP+ and NADPH

A coenzyme electron carrier that alternates between an oxidized state and a reduced state to transport high-energy electrons and hydrogen from the light reactions to the Calvin cycle.

Special pair of chlorophyll a

A designated dimer of pigment molecules situated in the reaction center complex of a photosystem that donates photo-excited electrons directly to the primary electron acceptor.

Bundle-sheath cells

Tightly packed photosynthetic cells arranged around leaf veins in C4 plants that isolate the Calvin cycle from ambient oxygen, thereby preventing photorespiration.

Three stages of the Calvin cycle (in order)

1. Carbon fixation, 2. Reduction of 3-PGA to form $G3P$, and 3. Regeneration of the $CO_2$ acceptor ($RuBP$).

Three environmental factors that limit the rate of photosynthesis

Light intensity, carbon dioxide ($CO_2$) concentration, and ambient temperature.

Effect of elevated temperature on RuBisCO activity

It promotes photorespiration over carbon fixation because thermal energy lowers the relative solubility and affinity of the enzyme for $CO<em>2$ compared to $O</em>2$.

Relationship between photosynthesis and cellular respiration

A complementary metabolic cycle where the outputs of one pathway ($C<em>6H</em>{12}O<em>6$ and $6O</em>2$) serve as the inputs for the other, while respiratory byproducts ($6CO<em>2$ and $6H</em>2O$) drive the former.

C3 plants

Vegetation that directly fixes inorganic carbon into three-carbon molecules via RuBisCO in mesophyll cells without specialized anatomical adaptations against photorespiration (e.g., wheat, rice, and soybeans).

C4 plant adaptations and examples

Species that physically isolate carbon fixation into a four-carbon intermediate in mesophyll cells before running the Calvin cycle in bundle-sheath cells, thriving in hot, sunny regions (e.g., corn and sugarcane).

CAM plant adaptations and examples

Species that temporally separate carbon assimilation by opening stomata at night to store malic acid and closing them by day to conserve water (e.g., cacti, pineapples, and succulents).

Two stages of photosynthesis (in sequential order)

1. Light-dependent reactions (occurring in the thylakoid membranes), followed by 2. The Calvin cycle or light-independent reactions (occurring in the stroma).

Role of chemiosmosis in light-dependent reactions

The process by which a proton gradient across the thylakoid membrane drives the catalytic synthesis of $ATP$ through $ATP$ synthase into the stroma.