C4 PhotosynthesisC4 Photosynthesis

AP BiologyUnit 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

CH2OPO32-

C O

C OHH

C OHH

CH2OPO32-

Ribulose 1,5-bisphosphate

CO2

CO O-

C OHH

CH2OPO32-

3-Phosphoglycerate

CO O-

C OHH

CH2OPO32-

3-Phosphoglycerate

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)

CH2OPO32-

C O

C OHH

C OHH

CH2OPO32-

Ribulose 1,5-bisphosphate

O2

CO O-

C OHH

CH2OPO32-

3-Phosphoglycerate

O O-C

CH2OPO32-

Phosphoglycolate

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

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|

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 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

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

C3 Pathway

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

Light Intensity (cal/cm2/min)

Ne

t Ph

oto

syn

the

sis

(mg

CO

2 /d

m2

/hr)