C4 Photosynthesis AP Biology Unit 4

Review: C3 Photosynthesis Regular photosynthesis = CO2 trapped into a 3-c molecule by Rubisco This 3C enters calvin cycle to produce glucose …. eventually

C4 Photosynthesis Some plants used slightly modified photosynthesis (C4 Photosynthesis) C4 Photosynthesis = adaptation that evolved due to environment. Dry/Arid and Protein “issues”

Rubisco ALL plants have it and use. Rubisco reacts with CO2 also react with O2 – 2 competing reactions. Carboxylase Reaction Good for glucose Oxygenase Reaction No good Uses ATP to burn carbon compound No sugar

Photorespiration Rubisco reacts with O2 instead of CO2 Occurs when: High O2 concentrations High heat (closed stomata) Photorespiration cuts sugar by 25%

Why high heat? Hot temp = closed stomata conserve H2O Photosynthesis continues  CO2 lvl drops and O2 increase photorespiration occurs

C4 Photosynthesis Certain plants limit photorespiration C4 Pathway* CAM Pathway* Both convert CO2 into a 4c molecule  C4 Photosynthesis

Leaf Anatomy C3 plants all photosynthesis occurs in mesophyll cells. Bundle sheath cells (no photosynthesis Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C4 Pathway Photosynthesis occurs in mesophyll and the bundle sheath cells. Light reactions in mesophyll Calvin Cycle in Bundle sheath Affinity of PEP Carboxylase for CO2 is much higher than its affinity for O2. Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C4 Pathway CO2 fixed into 4c molecule by enzyme (PEP Carboxylase) in mesophyll Rubico not present to make mistake

C4 Pathway 4c molecule “smuggles” CO2 into the bundle sheath cell CO2 released Enters Calvin Cycle C3 Pathway

How? Bundle sheath (BS) cells far from surface less O2 access PEP Carboxylase has no affinity for O2 Allows plant to concentrate CO2 in the bundle sheath (where Rubisco is)

CAM Pathway Fix CO2 at night Calvin Cycle happens during day Stomates open store as 4c molecule Calvin Cycle happens during day Stomates closed prevent H2O loss

How? 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 CAM Pathway Separates reactions by time

Comparison Light intensity is directly related to temp C4 Plants (CAM and C4) do more photosynthesis at high temp