How C3, C4 and CAM Plants Do Photosynthesis (Old version!)

Photosynthesis: Understanding C3, C4, and CAM Plants

Introduction to Photosynthesis

• The discussion focuses on three classes of plants involved in photosynthesis: C3, C4, and CAM plants.

• Carbon fixation is a key concept; it refers to the process of converting atmospheric carbon into organic molecules like glucose.

The Process of Carbon Fixation

• In photosynthesis, carbon dioxide combines with RUBP to produce a 3-carbon compound called PGA, leading to glucose formation.

• This process is efficient when there is a constant supply of carbon dioxide from the atmosphere.

Stomata and Water Loss

• Stomata allow gas exchange but can lead to water loss through evaporation.

• In hot conditions, C3 plants may close their stomata to prevent water loss but risk depleting their carbon dioxide supply.

Challenges Faced by C3 Plants

• Closing stomata limits carbon dioxide intake, which can lead to photorespiration if oxygen binds with RUBP instead.

• Without sufficient carbon dioxide, the Calvin cycle cannot function effectively for C3 plants.

Transitioning to C4 Pathway

• The lecture introduces the C4 pathway as an adaptation for plants in hot environments that face challenges similar to those experienced by C3 plants.

• A comparison between typical structures of C3 and C4 plants highlights differences in cell types and arrangements crucial for their respective processes.

Structural Differences Between Plant Types

• Key differences include more palisade mesophyll cells and larger bundle sheath cells in C4 plants compared to those in C3 plants.

• The arrangement of these cells allows for more efficient processing of carbon dioxide under high-temperature conditions.

C4 and CAM Photosynthesis Explained

Overview of C4 Photosynthesis

• C4 plants utilize bundle sheath cells alongside palisade mesophyll cells, differing from C3 reactions by starting with phosphoenol pyruvate (PEP) instead of ribulose bisphosphate (RUBP).

• PEP is more selective than RUBP, combining only with available carbon dioxide in the palisade mesophyll cells while ignoring oxygen. It has three carbons and forms oxaloacetate when combined with CO2.

• The formation of oxaloacetate marks the first step in the C4 pathway, which is characterized by its four-carbon compound.

• Oxaloacetate converts to malate, which exits the palisade mesophyll cell and enters bundle sheath cells where it splits into pyruvate and CO2.

• In bundle sheath cells, RUBP captures the released CO2 to proceed with the Calvin cycle for glucose production.

Recycling Pyruvate

• Pyruvate can be converted back into PEP within the palisade mesophyll cell, a process that requires ATP.

• This highlights that carbon fixation occurs twice: once when CO2 is captured by PEP and again when CO2 from malate is added to RUBP.

Importance of C4 Pathway

• The necessity of this complex pathway arises because palisade mesophyll cells contain both desired CO2 and unwanted oxygen; thus, it prevents RUBP from interacting with oxygen.

• Even if there's limited CO2 available, C4 plants can efficiently use this pathway while also being capable of performing standard C3 photosynthesis under favorable conditions.

Introduction to CAM Plants

• CAM (Crassulacean Acid Metabolism) plants thrive in hot, dry environments by keeping their stomata closed during daytime to minimize water loss but open them at night for gas exchange.

• At night, CAM plants fix carbon dioxide into malate similarly to C4 plants; this malate is stored until daylight returns for photosynthesis.

Daytime Processes in CAM Plants

• During the day, stored malate splits into pyruvate and CO2; now light allows for normal Calvin cycle operations leading to glucose production.

• Unlike C4 plants that utilize bundle sheath cells, all processes in CAM plants occur within mesophyll cells due to lower oxygen levels present.

Conclusion on Photosynthetic Pathways