Photosynthesis LIKE CELLULAR RESPIRATION BUT IN REVERSE

Photosynthesis

• Energy within light is captured and used to synthesize carbohydrates

• CO2 is reduced

• H2O is oxidized

• Energy from light drives this endergonic reaction

Two stages of photosynthesis

Light reactions

◦ Use light energy

◦ Take place in thylakoid membranes

◦ Produce ATP, NADPH and O2

Calvin cycle

◦Occurs in stroma

◦Uses ATP and NADPH to incorporate CO2 into carbohydrate

Absorption versus action spectrum

Absorption spectrum

◦ Wavelengths that are absorbed by different pigments

Action spectrum

◦ Rate of photosynthesis by whole plant at specific wavelengths

a) Absorption spectra

b) Action spectrum

Photosystems I and II 1

Thylakoid membranes of chloroplast contain two distinct complexes of molecules

◦ Photosystem I (PSI) – discovered first

◦ Photosystem II (PSII) – first step in photosynthesis

Light excites pigment molecules in both PSII and PSI

Formation of ATP in chloroplasts

ATP synthesis in chloroplasts

◦ Achieved by chemiosmotic mechanism called photophosphorylation

◦ Driven by flow of from thylakoid lumen into stroma via ATP synthase

gradient generated three ways:

• in thylakoid lumen by splitting of water

• by ETC pumping into lumen

• in stroma from formation of NADPH

Three chemical products

Oxygen, O2

◦ Produced in thylakoid lumen by oxidation of H2O by PSII

◦ Two electrons transferred to molecules

NADPH

◦Produced in the stroma from high-energy electrons that start in PSII and are boosted in PSI

ATP

◦ Produced in stroma by ATP synthase using the electrochemical gradient

Synthesizing Carbohydrates via the Calvin Cycle

CO2 incorporated into carbohydrates

◦ Precursors to other organic molecules

◦ Energy storage

Requires massive input of energy

Calvin cycle

Phase 1 – Carbon fixation

• C02 → RuBP

• Rubisco → enzyme

Phase 2 – Reduction and carbohydrate production

• Stabilizing

Phase 3 – Regeneration of RuBP

• Continues the cycle

Variations in Photosynthesis

Environmental conditions can influence both the efficiency and way the Calvin cycle works

◦ Light intensity

◦ Temperature

◦ Water availability

Photorespiration

• More likely in hot and dry environments

• Favored when CO2 low and O2 high

C4 plants

• Evolved a mechanism to minimize respiration

• Leaves have two-cell layer organization

◦ Mesophyll cells

◦ Bundle-sheath cells

Which is better – C3 or C4?

• It depends on the environment

• In warm dry climates plants conserve water and prevent photorespiration

• In cooler climates, plants use less energy to fix CO2

• 90% of plants are C3

CAM plants

• Some plants separate processes using time

• Crassulacean Acid Metabolism

• CAM plants open their stomata at night