C4 Photosynthesis

Review: C3 PhotosynthesisDuring regular photosynthesis, CO2 is trapped into a 3-carbon compound by Rubisco C3 PhotosynthesisThis 3 carbon compound then goes through the calvin cycle to produce glucose (eventually)

C4 PhotosynthesisCertain 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.

RubiscoALL 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 ReactionsRubisco can react with CO2 (Carboxylase Reaction) good for glucose output

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

PhotorespirationWhen Rubisco reacts with O2 instead of CO2Occurs under the following conditions:High O2 concentrationsHigh heatPhotorespiration is estimated to reduce photosynthetic efficiency by 25%

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

C4 PhotosynthesisCertain plants have developed ways to limit the amount of photorespiration C4 Pathway*CAM Pathway*

* Both convert CO2 into a 4 carbon intermediate C4 Photosynthesis

Leaf AnatomyIn 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 PathwayIn C4 pathway plants photosynthesis occurs in both the mesophyll and the bundle sheath cells.Light reactions in mesophyllCalvin Cycle in Bundle sheath

Image taken without permission from http://bcs.whfreeman.com/thelifewire|

*Affinity of PEP Carboxylase for CO2 is much higher than its affinity for O2.

C4 PathwayCO2 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

C4 PathwayThe 4 carbon intermediate smuggles CO2 into the bundle sheath cellThe bundle sheath cell is not very permeable to CO2CO2 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 accessPEP Carboxylase doesnt 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 PathwayFix CO2 at night and store as a 4 carbon molecule Keep stomates closed during day to prevent water loss Same general process as C4 PathwayHas 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 dayPlant can still do the calvin cycle during the day without losing water

Summary of C4 PhotosynthesisC4 PathwaySeparates by space (different locations)CAM PathwaySeparates reactions by time (night versus day)

ComparisonLight intensity is directly related to temperatureC4 Plants (CAM and C4 Pathway) are able to do more photosynthesis at high temperatures

*Affinity of PEP Carboxylase for CO2 is much higher than its affinity for O2.