Photosynthesis: Plant Anatomy, Light Reactions, Carbon Fixation, and Environmental Physiology

Comprehensive vocabulary flashcards covering plant tissues and floral morphology, the light reactions of photosynthesis, the Calvin-Benson cycle stoichiometry, photorespiration, C4 and CAM metabolic adaptations, phloem translocation, and photosynthetic environmental physiology.

Xylem

The vascular plant tissue responsible for conducting water ($H_2O$) and dissolved inorganic ions upward from the roots to the shoot system.

Phloem

The vascular tissue responsible for translocating organic photosynthetic sugars from source leaves to non-green living plant tissues to fuel cellular respiration.

Stoma

A microscopic pore in the epidermis flanked by two guard cells that regulates gas exchange, allowing $CO_2$ ($\sim 0.04\%$ in atmospheric air) to enter while excess $O_2$ and $H_2O$ vapour exit.

Palisade Parenchyma

The densely packed, column-shaped upper mesophyll cells of a dicot leaf containing abundant chloroplasts, specialized for maximum light absorption.

Spongy Parenchyma

The loosely arranged lower mesophyll tissue containing abundant intercellular air spaces that facilitate the diffusion of gases ($CO_2$, $O_2$, and water vapour) within the leaf.

Cutin

The waxy, hydrophobic lipid material forming the protective cuticle layer on leaf epidermal surfaces to prevent transpirational water loss.

Pistil / Carpel

The female reproductive organ of a flower, consisting of the pollen-receptive stigma, the elongated style, and the basal ovary containing egg-bearing ovules.

Stamen

The male reproductive organ of a flower, consisting of a pollen-bearing anther supported by a slender filament.

Fruit

The ripened ovary of a flower that encloses and protects developing seeds, which contain an embryo and nutritional reserves.

Phytol Chain

The non-polar, hydrophobic hydrocarbon tail of a chlorophyll molecule that embeds and anchors the molecule firmly within the thylakoid lipid bilayer.

Antenna Complex

An array of light-harvesting pigment molecules that absorbs photons and transmits resonance excitation energy to a central reaction center chlorophyll.

Photolysis

The light-driven oxidation of water at Photosystem II ($2H_2O \rightarrow 4H^+ + O_2 + 4e^-$) catalyzed by a manganese ($Mn^{2+}$) complex, generating electrons for the photosystems and releasing $O_2$ as waste.

Noncyclic Photophosphorylation

The unidirectional flow of electrons from $H_2O$ through Photosystem II (P680), plastoquinone, the cytochrome $b_6f$ complex, plastocyanin, Photosystem I (P700), and ferredoxin to $NADP^+$ reductase, synthesizing both $ATP$ and $NADPH$.

Cyclic Photophosphorylation

An alternative electron pathway where electrons excited from Photosystem I (P700) pass to ferredoxin, return to plastoquinone and the cytochrome $b_6f$ complex, and return via plastocyanin to PSI, pumping extra $H^+$ to generate additional $ATP$ without yielding $NADPH$ or $O_2$.

Rubisco

Ribulose-1,5-bisphosphate carboxylase/oxygenase; the stromal enzyme that catalyzes primary carbon fixation in the Calvin-Benson cycle by combining $CO_2$ with RuBP, but which can also bind $O_2$ at high temperatures to initiate photorespiration.

Ribulose Bisphosphate (RuBP)

A 5-carbon compound containing 2 phosphate groups ($5C, 2P_i$) that acts as the initial carbon dioxide acceptor in the Calvin-Benson cycle.

3-Phosphoglycerate (3PG)

A 3-carbon ($3C$) intermediate formed when the unstable 6-carbon intermediate produced by Rubisco-catalyzed carbon fixation immediately cleaves; six molecules of 3PG are formed per $3\,CO_2$ fixed.

Glyceraldehyde 3-Phosphate (G3P)

The primary 3-carbon phosphorylated sugar output of the Calvin-Benson cycle; one G3P exits the cycle per $3\,CO_2$ fixed (costing $9\,ATP$ and $6\,NADPH$), and two G3P molecules unite to form one glucose ($C_6H_{12}O_6$).

Photorespiration

A wasteful metabolic process occurring in C3 plants under high $O_2$, low $CO_2$, or elevated temperatures, in which Rubisco binds $O_2$ instead of $CO_2$, producing one molecule of $3PG$ ($3C$) and one toxic molecule of 2-phosphoglycolate ($2PG$, $2C$).

2-Phosphoglycolate (2PG)

A toxic 2-carbon ($2C$) metabolic byproduct formed during photorespiration when Rubisco acts as an oxygenase, which must be dephosphorylated to glycolate and salvaged across peroxisomes and mitochondria at the cost of $ATP$ and $NADPH$.

Kranz Anatomy

The specialized leaf anatomy characteristic of C4 plants, featuring an inner wreath-like ring of chloroplast-containing bundle sheath cells surrounding the vascular bundle, enclosed by outer mesophyll cells.

PEP Carboxylase

The primary carbon-fixing enzyme in C4 and CAM plants that carboxylates phosphoenolpyruvate ($PEP$) with $CO_2$ ($HCO_3^-$) to form oxaloacetate ($OAA$, $4C$); it has high affinity for $CO_2$ at low concentrations and zero affinity for $O_2$.

C4 Photosynthesis

A photosynthetic adaptation to heat and drought that achieves spatial separation of light reactions (in mesophyll cells) from the Calvin-Benson cycle (in bundle sheath cells), concentrating $CO_2$ around Rubisco to eliminate photorespiration.

Crassulacean Acid Metabolism (CAM)

A photosynthetic adaptation to arid environments using temporal separation within the same mesophyll cell; stomata open at night to fix $CO_2$ via PEP carboxylase into malate stored in the vacuole, which is decarboxylated by day to feed the Calvin cycle while stomata stay closed.

Light Saturation Point

The irradiance threshold on a photosynthetic light response curve beyond which any further increase in light intensity results in no additional increase in the rate of photosynthesis.

Law of Limiting Factors

The physiological principle dictating that the overall rate of photosynthesis is governed and constrained by whichever essential environmental factor (such as irradiance, temperature, or $[CO_2]$) is in shortest supply.

Pressure Flow Hypothesis

The physiological mechanism of translocation whereby active loading of sugar into sieve tube elements at a source drives osmotic water influx from xylem, creating high hydrostatic pressure that drives bulk sap flow to a sink where sugar is unloaded.

Compensation Point

The specific light intensity or $[CO_2]$ level at which gross photosynthetic carbon fixation exactly equals the rate of respiratory carbon loss, resulting in zero net gas exchange ($\text{Net PS} = 0$).

Gross Photosynthesis

The total quantitative amount of light-driven carbon fixation and oxygen generation accomplished by a plant before deducting the energetic and metabolic costs of respiration.

Net Photosynthesis

The net rate of carbon gain available for plant biomass growth and reproduction, calculated as gross photosynthesis minus the combined metabolic costs of cellular respiration and photorespiration.

Optimum Temperature

The temperature at which net photosynthesis reaches its peak value, located below the peak of gross photosynthesis because respiratory and photorespiratory metabolic costs accelerate sharply as temperature rises.