C4 Photosynthesis AP Biology Unit 4

Review: C3 Photosynthesis During “regular” photosynthesis, CO 2 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 CO 2, but it can also react with O 2 – 2 competing reactions.

Competing Reactions Rubisco can react with CO 2 (Carboxylase Reaction) – good for glucose output

Competing Reactions Rubisco can also react with O 2 (Oxygenase Reaction) –not good for glucose output –Even though CO 2 is eventually regenerated, it wastes time and energy (occupies Rubisco)

Photorespiration When Rubisco reacts with O 2 instead of CO 2 Occurs under the following conditions: –High O 2 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 CO 2 and produce O 2  creates high O 2 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 CO 2 into a 4 carbon intermediate  C4 Photosynthesis

Leaf Anatomy In C3 plants (those that do C3 photosynthesis), all photosynthesis processes occur in the mesophyll cells. Image taken without permission from Mesophyll cells Bundle sheath cells (no photosynthesis

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

C4 Pathway CO 2 is fixed into a 4- carbon intermediate first Has an extra enzyme– PEP Carboxylase that initially traps CO 2 instead of Rubisco– makes a 4 carbon intermediate

C4 Pathway The 4 carbon intermediate “smuggles” CO2 into the bundle sheath cell The bundle sheath cell is not very permeable to CO 2 CO 2 is released from the 4C molecule  goes through the Calvin Cycle C3 Pathway

How does the C4 Pathway limit photorespiration? Bundle sheath cells are far from the surface– less O 2 access PEP Carboxylase doesn’t have an affinity for O 2  allows plant to collect a lot of CO 2 and concentrate it in the bundle sheath cells (where Rubisco is)

CAM Pathway Fix CO 2 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 CO 2 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