C4 Photosynthesis

C4 Photosynthesis AP Biology Unit 4

Review: C3 Photosynthesis • During “regular” photosynthesis, CO2 is trapped into a 3-carbon compound by Rubisco  C3 Photosynthesis • This 3 carbon compound then goes through the calvin cycle to produce glucose (eventually)

C4 Photosynthesis • Certain plants go through a slightly modified photosynthesis process (C4 Photosynthesis) • C4 Photosynthesis is an adaptation that evolved due to the environment these plants are in.

Rubisco • ALL plants have it and use it for the Calvin Cycle. • Rubisco usually reacts with CO2, but it can also react with O2 – 2 competing reactions.

Competing Reactions • Rubisco can react with CO2 (Carboxylase Reaction) – good for glucose output 

Competing Reactions • Rubisco can also react with O2 (Oxygenase Reaction) • not good for glucose output • Even though CO2 is eventually regenerated, it wastes time and energy (occupies Rubisco)

Photorespiration • When Rubisco reacts with O2 instead of CO2 • Occurs under the following conditions: • High O2 concentrations • High heat • Photorespiration is estimated to reduce photosynthetic efficiency by 25%

Why high heat? • When it is hot, plants close their stomata to conserve water • They continue to do photosynthesis  use up CO2 and produce O2  creates high O2 concentrations inside the plant  photorespiration occurs

C4 Photosynthesis • Certain plants have developed ways to limit the amount of photorespiration • C4 Pathway* • CAM Pathway* * Both convert CO2 into a 4 carbon intermediate  C4 Photosynthesis

Mesophyll cells Bundle sheath cells (no photosynthesis Leaf Anatomy • In C3 plants (those that do C3 photosynthesis), all photosynthesis processes occur in the mesophyll cells. Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C4 Pathway • In C4 pathway plants photosynthesis occurs in both the mesophyll and the bundle sheath cells. • Light reactions in mesophyll • Calvin Cycle in Bundle sheath Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C4 Pathway • CO2 is fixed into a 4-carbon intermediate first • Has an extra enzyme– PEP Carboxylase that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate

C3 Pathway C4 Pathway • The 4 carbon intermediate “smuggles” CO2 into the bundle sheath cell • The bundle sheath cell is not very permeable to CO2 • CO2 is released from the 4C molecule  goes through the Calvin Cycle

How does the C4 Pathway limit photorespiration? • Bundle sheath cells are far from the surface– less O2 access • PEP Carboxylase doesn’t have an affinity for O2  allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is)

CAM Pathway • Fix CO2 at night and store as a 4 carbon molecule • Keep stomates closed during day to prevent water loss • Same general process as C4 Pathway • Has the same leaf anatomy as C3 plants

How does the CAM Pathway limit photorespiration? • Collects CO2 at night so that it can be more concentrated during the day • Plant can still do the calvin cycle during the day without losing water

Summary of C4 Photosynthesis • C4 Pathway • Separates by space (different locations) • CAM Pathway • Separates reactions by time (night versus day)

Comparison • Light intensity is directly related to temperature • C4 Plants (CAM and C4 Pathway) are able to do more photosynthesis at high temperatures

C4 Photosynthesis