Alternative Pathways in Photosynthesis

Figure 10.20 A review of photosynthesis

“C3” Plants

Guard cells Pore Stoma

18 µm

Leaf surfaces contain stomata

Photosynthesis relies on CO2 entering the leaf and O2 leaving the leaf

Leaf cross-section

CO2

Carbon dioxide diffuses into leaves through stomata

H2O

BUT: water also escapes through

the stomata

CO2 stored in mesophyll cell

CO2 used inadjacent bundle sheath cell

Organicacid

Calvin cycle

CO2

G3P

CO2

C4 plants sequester CO2 in certain cells

C4 pathway

Examples of C4 plants: corn, sugarcane, many grasses

Figure 10.18 C4 leaf anatomy and the C4 pathway

C4 Plants

CO2 usedduring the day

CO2 storedat night

CO2

CO2

G3P

Calvincycle

Organicacid

CAM plants sequester CO2 at night

C4 pathway

Examples of CAM plants: pineapples, cacti, and most succulents.

CAM Plants

Welwitschia

Figure 10.19 C4 and CAM photosynthesis compared