Photosynthesis Adaptations: C3, C4, CAM, and Climate Change

Comprehensive vocabulary flashcards reviewing the biochemical pathways of C3, C4, and CAM photosynthesis, photorespiration, anatomical specializations, and the greenhouse effect.

Photorespiration

A metabolic pathway occurring when Rubisco oxygenates RuBP instead of fixing $CO_2$, producing 2-phosphoglycolate; it consumes $O_2$, ATP, and NADPH while releasing $CO_2$ without producing sugar, reducing photosynthetic efficiency by up to $50\%$ on hot, dry days.

2-Phosphoglycolate

A two-carbon product formed when Rubisco reacts with $O_2$ during photorespiration that must be broken down by the plant using $O_2$, ATP, and NADPH.

C3 Plants

Plant species comprising approximately $85\%$ of all plant species ($>330{,}000$ species) whose initial carbon fixation product is the 3-carbon molecule 3-phosphoglycerate (3PG); they are prone to photorespiration in hot, dry environments.

3-Phosphoglycerate (3PG)

A 3-carbon ($3\text{C}$) molecule formed in C3 plants when Rubisco combines $CO_2$ with ribulose 1,5-bisphosphate (RuBP).

C4 Plants

A group of over $8{,}000$ plant species (including corn, sugar cane, and some grasses) that evolved independently at least $61$ times and utilize anatomical and biochemical adaptations to fix $CO_2$ into a 4-carbon compound, minimizing photorespiration.

Phosphoenolpyruvate (PEP)

A 3-carbon ($3\text{C}$) molecule that acts as the initial carbon acceptor in C4 and CAM mesophyll cells, reacting with $CO_2$ to produce oxaloacetate.

PEP Carboxylase

An enzyme in C4 and CAM plants that catalyzes the addition of $CO_2$ to phosphoenolpyruvate (PEP); it possesses a higher affinity for $CO_2$ than Rubisco and does not bind $O_2$.

Oxaloacetate

The initial 4-carbon ($4\text{C}$) product of carbon fixation synthesized by PEP carboxylase in C4 and CAM photosynthetic pathways.

Malate

A 4-carbon ($4\text{C}$) organic acid formed from oxaloacetate that functions as a "$CO_2$ pump" by transporting carbon to bundle-sheath cells in C4 plants or into vacuoles in CAM plants.

Bundle-Sheath Cells

Specialized cells encircling the vascular bundles of C4 leaves where malate releases concentrated $CO_2$ to Rubisco for the Calvin cycle, away from atmospheric oxygen.

Mesophyll Cells (in C4 Plants)

Photosynthetic leaf cells where initial $CO_2$ fixation by PEP carboxylase takes place to produce oxaloacetate and malate, physically separated from the Calvin cycle.

Spatial Separation of Steps

The structural mechanism in C4 photosynthesis where preliminary carbon fixation by PEP carboxylase occurs in mesophyll cells, whereas decarboxylation and the Calvin cycle occur separately in bundle-sheath cells.

Pyruvate Regeneration (in C4 Pathway)

The transport of 3-carbon ($3\text{C}$) pyruvate from bundle-sheath cells back to mesophyll cells, where it is converted back into PEP using ATP.

Water-Use Efficiency (C4 vs. C3 Plants)

The physiological benefit allowing C4 plants to keep stomata closed more frequently to conserve water in hot/dry environments, requiring $\frac{1}{3}$ less water than C3 plants.

Crassulacean Acid Metabolism (CAM)

An alternate carbon fixation pathway found in approximately $16{,}000$ plant species (mostly succulents and cacti) that uses temporal separation to conserve water, consuming roughly $\frac{1}{20}$ the water of C3 plants.

Temporal Separation of Steps

The timing-based adaptation in CAM plants where stomata open at night to fix $CO_2$ into organic acids, and close during the day to prevent water loss while $CO_2$ is released to the Calvin cycle.

Malic Acid Vacuolar Storage

The nighttime accumulation of malic acid inside cell vacuoles of CAM plants, which is exported during daylight to supply $CO_2$ to the Calvin cycle when ATP and NADPH are available.

Greenhouse Effect

The planetary warming process where the atmosphere allows short-wavelength infrared and visible light to pass through, but greenhouse gases absorb and re-emit trapped long-wavelength infrared radiation back to Earth.

Greenhouse Gases

Atmospheric gases—primarily carbon dioxide ($CO_2$), methane ($CH_4$), and nitrous oxide ($N_2O$)—that enhance heat retention by re-radiating thermal energy toward Earth's surface.

Mauna Loa Observatory

A research station in Hawaii whose continuous monitoring through the Scripps $CO_2$ Program documented atmospheric $CO_2$ concentrations rising from around $315\,\text{ppm}$ in the late 1950s to over $420\,\text{ppm}$ by May 2024.