1. chlorophyll in chloroplasts
Figure 6.3
Calvin-Benson or C3 Cycle
in mesophyll cell

2. Fig. 6.1 (EFB) Rubisco Photorespiration with O2 to 2-C to CO2
‘fixes’ C
Photorespiration
with O2 to 2-C to CO2
sugar +
rubisco
3-C molecules
(18)
Calvin-Benson Cycle
or C3 cycle
Fig. 6.1 (EFB)

3. Efficiency of this process is relatively low:
Of all light striking a plant, only ~38% is absorbed
More energy lost in conversion processes during
photosynthesis
Amount of chemical energy ultimately stored by plant is
only 0.01 to 3.0% of light energy absorbed

4. Respiration: releasing energy stored in glucose
to complete cellular functions
Aerobic respiration
C6H12O6 + 6O CO2 + 6H2O + ATP
Uses O2, many steps, but net gain is ATP
per glucose molecule, efficiency %
2. Anaerobic respiration (glycolysis)
C6H12O6 + 2ATP C2H6O + 2CO2 + 4ATP
Net gain of only 2 ATP, efficiency only 2.4%

5. Limiting Factors in Ecology
H2O
Light
CO2 and other gases
Nutrients
Temperature
Law of the Minimum
At least one factor, or a combination of factors,
ultimately will limit how much a plant or animal will grow

6. Figure 6.6 (EFB)

7. Figure 7.12 (EFB) Role of Water in Plants: Photosynthesis
Turgor Pressure
Transport nutrients
Figure 7.12 (EFB)

8. The higher the concentration of water on one side
Osmosis: movement of water across a concentration gradient
The higher the concentration of water on one side
of the membrane, the higher the osmotic potential

9. The force or pull of osmosis = permanent wilting point
Water Potential:
The force or pull of osmosis
-15 bars maximum in most
environments
= permanent wilting point

10. C4 Pathway Uses PEP (phosphoenolpyruvate) to fix carbon from CO2
PEP not lost by photorespiration
like Rubisco
Plant saves water in warm
climates by closing stomates
temporarily during the day
Still uses Calvin-Benson Cycle,
but in different cell
PEP: phosphoenolpyruvate, a 3-C compound

11. C3 C4
Figure 6.3 (EFB)

12. If C4 plants are more efficient than C3,
then why aren’t all plants C4?
Ability of C4 plants to concentrate CO2 in bundle
sheath cells saves water and increases
efficiency of Rubisco and C3 cycle in fixing CO2
Production of PEP requires more energy
and is less efficient under certain conditions

13. C4 plants most efficient at 30-45 °C,
water-stressed environments, and high
light intensity
C3 plants most efficient at °C,
wetter and more temperate regions

15. Under ideal conditions, C3 plants range from 0.01 to 3.0%
efficiency in converting light energy to chemical energy
For C4 plants, that level goes up to 8%, mainly
due to Rubisco working more efficiently at higher
temperatures

16. e.g., trees, shrubs, tropical to tundra species
Most plants (~95%) are C3
e.g., trees, shrubs, tropical to
tundra species
C4 plants include many grasses,
plants in water-stressed environments
It is rare to find a species that can use both
pathways, but tobacco and a marine diatom
have been found to do so

17. CAM Pathway

18. Quiz What are the light and dark reactions in photosynthesis?
2. What is photorespiration?
3. When is the C-4 pathway more efficient
than the C-3?
4. What is the Law of the Minimum?
5. What is meant by light saturation in plants?