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Plant Physiology 2Photosynthesis

• Photo means ‘light’ and synthesis means ‘to
• Process in which plants convert carbon
dioxide and water into sugars using solar
• Occurs in chloroplast


6 CO2 + 6 H2O

C6 H12 O6 + 6 O2

carbon dioxide + water =

sugar + oxygen

products often
stored as starch
•Starch = glucose
Tracking atoms


Fig. 10.1

Fig. 10.2a

Fig. 10.2b

Fig. 10.2c

Fig. 10.4

•Absorbs red & blue light
•Reflects green light

Fig. 10.6

Fig. 10.8

Fig. 10.20

Fig. 10.17

• Ribulose bisphosphate
carboxylase oxygenase
• (fixes CO2 & O2)
• Enzyme in Calvin
Cycle (1st step)
• Most abundant protein
on Earth
– Ca. 25% total leaf


When rubisco “fixes” O2, not CO2
Lose 1/2 C as CO2; costs 2.5 extra ATP
Take up O2
Only occurs in light
Occurs 1 out of 4 reactions under today’s
atmospheric [CO2]
• Rate increases with temperature

Types of photosynthesis
• C3
– The majority of plants

• C4
– CO2 temporarily stored as 4-C organic acids resulting in more
more efficient C exchange rate
– Advantage in high light, high temperature, low CO 2
– Many grasses and crops (e.g., corn, sorghum, millet, sugar cane)

– Stomata open during night
– Advantage in arid climates
– Many succulents (e.g., cacti, euphorbs, bromeliades, agaves)

Fig. 10.21

Fig. 10.22

Global Environmental Change &
C3 vs. C4 vs. CAM
• Increasing CO2
• Increasing chronic and acute temperatures
• Increasing N (vs. decreasing C:N from
increasing CO2)
• Changes in water

CO2 effects on photosynthesis
• C4 > C3 at low CO2
• But, C3 > C4 at high CO2

*At high CO2, C3 more efficient than C4 at all temps.
(photosynthesis only, not other processes)

Photosynthetic N-use efficiency
• C4 plants need (have) less leaf N than C3
• Photosynthesis higher per unit N in C4
• Humans are increasing global N, which
benefits C3 more than C4
• Increasing CO2 decreases leaf N content,
more in C3 than C4

Photosynthetic water-use
• C4 plants use less water than C3
• (cause stomates open less)
• Water availability may increase or decrease
in the future.

Predicting the future for plants
• How will increases in CO2, N, and chronic
and acute heat stress affect photosynthesis?
• Who will win or lose? C3? C4?
• How will pollution (eg, ozone) interact?
• Current research in my lab an example.

Elevated CO2

leaf C:N...
Plant Physiology 2-
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