Chapter 25: Photosynthetic Pathways Notes

Chapter 25: Photosynthetic Pathways

Learning Objectives

• Contrast C3, C4, and CAM photosynthesis with regard to:
  • Timing of stomatal opening
  • Efficiency of water use
  • Use of stored energy
  • Location of primary carbon capture and the Calvin cycle

Introduction

• Different plant species have adaptations for variations of light-independent reactions, called photosynthetic pathways.
• Most plants use one of three pathways: C3, C4, or CAM.
• Some plants can switch pathways based on environmental conditions.
• The light-independent reactions described previously correspond to the C3 pathway, where the first compound formed (3-PGA) has three carbon atoms.
• C4 and CAM pathways evolved in response to heat and drought stress.

Photorespiration

• RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes two reactions:
  • Carboxylase activity: adds $CO_2$ to RuBP (ribulose-1,5-bisphosphate)
  • Oxygenase activity: adds $O_2$ to RuBP
• Oxygenase activity:
  • Forms 3-phosphoglycerate (3-PGA) and glycolate.
  • Glycolate enters peroxisomes, uses $O_2$ to form intermediates.
  • Intermediates enter mitochondria, broken down to $CO_2$.
  • Consumes $O<em>2$ and releases $CO</em>2$, similar to aerobic cellular respiration.
• Photorespiration undoes the work of photosynthesis by breaking down sugars.
• RuBisCO activity depends on relative concentrations of $O<em>2$ and $CO</em>2$. High $CO<em>2$ favors carboxylase activity; high $O</em>2$ favors oxygenase activity.
• Light reactions liberate oxygen; higher temperatures increase oxygen dissolution in the cytosol.
• High light intensities and temperatures (above ~$30°C$) lead to high oxygen concentrations and photorespiration.
• Plants open stomata to release excess oxygen and take in carbon dioxide.
• Open stomata also allow water vapor exchange, leading to water loss in dry environments.
• Desert plants have evolved adaptations to conserve water, such as temporary carbon fixation and CAM photosynthesis.
• CAM photosynthesis allows plants like cacti to carry out low levels of photosynthesis without opening stomata at all, which is especially useful during extremely dry periods

C3 Photosynthesis

• "Normal" plants without adaptations to reduce photorespiration are called C3 plants.
• The first step of the Calvin cycle is the fixation of carbon dioxide by rubisco.
• C3 plants produce a three-carbon compound (3-PGA).
• About 85% of plant species are C3 plants, including rice, wheat, soybeans, and trees.
• Photorespiration is more likely to occur for a C3 plant under hot, dry conditions.

C4 Photosynthetic Pathway

• C4 and CAM pathways concentrate $CO_2$ around RuBisCO to increase efficiency.
• CAM concentrates $CO_2$ temporally (providing it during the day, not at night).
• C4 plants concentrate $CO<em>2$ spatially, with RuBisCO in bundle sheath cells inundated with $CO</em>2$.
• Light-dependent reactions occur in mesophyll cells; the Calvin cycle occurs in bundle sheath cells.
• Atmospheric $CO_2$ is fixed in mesophyll cells to form a 4-carbon organic acid (oxaloacetate) by PEP carboxylase.
• PEP carboxylase does not bind $O_2$.
• Oxaloacetate is converted to malate, transported into bundle-sheath cells; malate releases $CO_2$.
• The $CO_2$ is fixed by rubisco and enters the Calvin cycle.
• Mesophyll cells pump $CO<em>2$ into bundle-sheath cells, maintaining high $CO</em>2$ concentration around rubisco to minimize photorespiration.
• Requires ATP to return the three-carbon "ferry" molecule to pick up more $CO_2$.
• C4 plants produce more sugar than C3 plants in high light and temperature.
• Important crop plants are C4 plants (maize, sorghum, sugarcane, millet).
• C4 pathway is used in about 3% of all vascular plants; common in hot habitats.
• The benefits of reduced photorespiration outweigh the ATP cost in hot conditions.
• C4 plants conduct the Calvin Cycle in bundle sheath cells.

CAM Photosynthetic Pathway

• CAM (crassulacean acid metabolism) plants minimize photorespiration in dry environments (cacti, pineapples).
• CAM separates light-dependent reactions and $CO_2$ use in the Calvin cycle in time, not space.
• At night, CAM plants open stomata, allowing $CO_2$ to diffuse into leaves.
• $CO_2$ is fixed into oxaloacetate by PEP carboxylase, then converted to malate or other organic acids and stored in vacuoles.
• During the day, stomata remain closed, but photosynthesis continues.
• Organic acids are transported out of the vacuole and broken down to release $CO_2$, which enters the Calvin cycle.
• This maintains a high $CO_2$ concentration around rubisco.
• CAM pathway requires ATP at multiple steps.
• CAM plants avoid photorespiration and are water-efficient because stomata open at night, reducing water loss.
• CAM plants are dominant in hot, dry areas, like deserts.
• CAM plants tend to open their stomata and fix carbon at night.

Comparisons of C3, C4, and CAM plants

• C3, C4, and CAM plants use the Calvin cycle to make sugars from $CO_2$.
• Pathways have different advantages and disadvantages, suiting plants for different habitats.
• C3 mechanism works well in cool environments.
• C4 and CAM plants are adapted to hot, dry areas.
• C4 spatially separates light-dependent reactions from the Calvin cycle, whereas C CAM temporally separates them.
• C4 and CAM pathways evolved independently over two dozen times, suggesting they provide a significant evolutionary advantage in hot climates.