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

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)

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

Respiration: releasing energy stored in glucose to complete cellular functions Aerobic respiration C6H12O6 + 6O2 ---------- 6CO2 + 6H2O + ATP Uses O2, many steps, but net gain is 21-36 ATP per glucose molecule, efficiency 25 - 43% 2. Anaerobic respiration (glycolysis) C6H12O6 + 2ATP ----------- 2C2H6O + 2CO2 + 4ATP Net gain of only 2 ATP, efficiency only 2.4%

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

Figure 6.6 (EFB)

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

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

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

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

C3 C4 Figure 6.3 (EFB)

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

C4 plants most efficient at 30-45 °C, water-stressed environments, and high light intensity C3 plants most efficient at 10-25 °C, wetter and more temperate regions

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

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

CAM Pathway

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?