Light Energy, Photosynthetic Pigments, and Photosynthetic Research

Electromagnetic Radiation As photons pass through a prism they are separated according to their energies. Forms Electromagnetic Spectrum. Visible Spectrum: A narrow range that humans see (380 nm to 750 nm).

Electromagnetic Radiation

Why Are Plants Green? Chlorophyll a & b: Chlorophyll a: Chlorophyll b: Absorbs energy from blue-violet and red regions. Reflects green light. Chlorophyll a: Transfers light energy to the carbon fixing reactions of photosynthesis. Chlorophyll b: An accessory pigment that absorbs photons that chlorophyll a absorbs poorly.

Other Accessory Pigments Carotenoids (e.g.,β-carotene): Responsible for yellow-orange coloured leaves in fall. Precursor of vitamin A  helpful for low light vision. Xanthophylls: Responsible for yellow-coloured leaves in fall. Anthocyanins: Responsible for red-coloured leaves in fall. With onset of cooler temperatuers, plants stop producing chlorophyll and begin breaking down chlorophyll…causes colours reflected by other pigments to become visible.

Photosynthetically Active Radiation (PAR) Light is absorbed from the entire visible spectrum when all pigments are combined.

Photosynthetic Research

J.B. Van Helmont (1600s) Set out to study truth in common belief: Plants obtained all food from soil. Found that after 5 years the plant increased its mass more than the mass of soil lost. Incorrect conclusion: Increase in water due to absorption of water.

Joseph Priestley (1771) Conducted experiment with bell jar, candle and mint plant. Discovered how gases play a role in photosynthesis. Plants release a gas necessary for combustion.

Jan Ingenhousz (1796) Confirmed Priestley’s findings. Discovered that plants release O2. First to discover: Sunlight essential for photosynthesis. CO2 used by plants. Incorrect conclusion: Production of O2 from CO2.

Light

C.B. Van Niel (1930s) Experimented with purple sulfur bacteria. Bacteria absorb H2S and release sulfur gas. Conclusion: Plants absorb and split H2O to release O2.

S.M. Ruben and M. Kamen (1938) Confirmed Van Niel’s findings. Placed Chlorella in “heavy” water. Used mass spectrometer to detect heavy oxygen released by Chlorella. Placed Chlorella in “normal” water containing “heavy” carbon dioxide. Normal oxygen was released by Chlorella. Chlorella is a type of green algae (photosynthetic). Heavy water is highly enriched in the hydrogen isotope deuterium (D2O).

F.F. Blackman (1905) Determined the following: At low light intensities, rate of photosynthesis increases as light intensity increases (not temperature). At high light intensities, rate of photosynthesis increases by increasing the temperature (not light intensity).

F.F. Blackman (1905) Conclusion: Occurs in 2 stages: Light-dependant (photochemical). Light-independent (biochemical)

F.F. Blackman (1905) Also determined the following: Rate of photosynthesis decreases when the availability of CO2 is limited.

Light Reactions Photons excite electrons in the chlorophyll (in the thylakoid membrane). Chemiosmotic ATP synthesis and formation of NADPH.

The Calvin Cycle Incorporation of CO2 into organic compounds such as glucose. Endergonic: Requires ATP and reducing power of NADPH. Occurs in the stroma via Calvin Cycle.