Light Reactions & Photosynthetic Pigments

1. Light Reactions & Photosynthetic Pigments

2. LIGHT! • Of all the light energy that reaches the earth’s surface, ~5% is transferred to carbohydrates by a leaf • Light = a form electromagnetic radiation that travels at 3x108 m/s as photons ( • That’s 300,000,000m in ONE SECOND!

3. LIGHT! • Light from the sun is a mixture of photons with different energies • When light passes through a spectroscope which contains a prism, photons are separated by energy • Most of the photons are invisible to humans but we can see wavelengths of light that range from 380-750nm • nm = nanometer = 1 x 10-9 m (it’s really small) **The Vatican’s chief astronomer invented the spectroscope (1860s)

4. A Short and Painless History of Photosynthesis • Like many people, it was initially thought that plants obtained their ‘food’ from the soil • They do? Don’t they? • In the early 1600s, J.B. Van Helmont decided to test this theory • Planted a willow tree in soil and ONLY added water for 5 years • After the 5 years, the tree’s massincreased by 75kg • The soil decreased by only 60g!! • Thus, the soil was NOT a contributing factor • Thought that it must be water!

5. A Short and Painless History of Photosynthesis • In 1770, Joseph Priestly discovered, by accident, that gases in the air played a role in photosynthesis • Placed a burning candle in a closed container • When it went out, he placed a plant in the container with the candle • After 10 days, he was able to reignite the candle • Thus, proving that plants release a gas into the atmosphere • This gas was confirmed asoxygen in 1796

6. A Short and Painless History of Photosynthesis • Jan Ingenhousz, 1796, was the first to realize that sunlight is essential to photosynthesis • Also noted that CO2 was the source of carbon in plants • But, mistakenly said that CO2 broke down to produce oxygen • “Sunshine splits apart the carbon dioxide molecule that a plant has absorbed in the air; the plant throws out at that time the oxygen alone, and keeps the carbon to itself as nourishment” • In 1930, C.B. Van Niel proved that oxygen was produced by water splitting (i.e. Not from CO2) • This was confirmed using isotopes (18O) in 1938

7. A Short and Painless History of Photosynthesis Light and Photosynthesis • In 1905, F.F. Blackman measured the effect in changes in light intensity, [CO2], and temperature on photosynthesis • Two main results: • At low light intensities, the rate of photosynthesis increased by increasing light intensity but nottemperature • At high light intensities, the rate of photosynthesis increased by increasing temperature but notlight intensity • From this study, Blackman concludedthat there must be: • a light-dependent reaction (photochemical) • a light-independent reaction (biochemical)

8. A Short and Painless History of Photosynthesis • Blackman then showed that the rate of photosynthesis is sensitive to [CO2] • Controlling for temperature, he subjected plants to different [CO2] • He found that photosynthesis decreased with a decreased [CO2] • CONCLUSIONS: • Light reactions occur in the presence oflight, independent of temperature*use light & water to produce NADPHand ATP • Carbon fixation reactions rely on the energy of light reactions *dependent on temperature not light intensity

9. LIGHT and PLANTS • Plants have photosynthetic pigments called photosystems • Found in the thylakoid membrane • Photosystems absorb photons of specific wavelengths • Then, use light reactions to form ATP and NADPH in the stroma

10. LIGHT and PLANTS • Action spectrum • illustrates the effectiveness with which different wavelengths of light promote photosynthesis • Absorption spectrum • Illustrates how much light, at a specific wavelength, is absorbed by a specific molecule/substance (ex. chlorophyll a or b)

11. Chlorophyll & Accessory Pigments • Chlorophyll a & b absorb photons with energies in the blue-violet and red regions • They reflect energies between 500nm – 600nm • Hence, why we see photosynthesizing organisms as green • Chlorophyll a transfers light energy to carbon fixation reactions • Chlorophyll b acts as an accessory pigment • Absorbs photons that chlorophyll a cannot

12. Chlorophyll & Accessory Pigments • Carotenoids = accessory pigments • Ex. ß-Carotene • Enzymes split ß-Carotene into vitamin A • Vitamin A is used to help vertebrae vision indim light environments • Carrots help vision as they have ß-Carotene • Carotenoids absorb light energy from 400nm - 500nm (blue-violet) • Reflect yellow-red appear yellow-orange Functions: • Absorb energy that would damage chlorophyll, then give off energy as heat • Protects human eye from excessive exposure to light

13. Why do leaves change colors in the fall? • Leaves have accessory pigments that reflect multiple colors of light energy (red, yellow, orange) • In spring/summer leafs appear green because they have [chlorophyll] in their chloroplasts of leaf cells • Remember: chlorophyll reflect green light! • Most likely temperature dependent • In the fall, plants stop producing chlorophyll and dissemble any chlorophyll that is left in the leaves • Occurs due to the cooler temperatures • This allos the reflected light of the accessorypigments to become visible!

14. PHOTOSYNTHETIC REACTION STAGES • All photosynthetic reactions can be broken down into3 distinct, but connected stages: • Capturing light energy • Using captured light energy to make ATP and reduced NADP+**NADP+ is the photosynthetic equivalent of NAD+** • Using the free energy of ATP and the reducing power of NADPH to synthesize organic compounds(ex. creating glucose from CO2)