Light Reactions & Photosynthetic Pigments

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!

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)

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 mass

increased by 75kg• The soil decreased by only 60g!!

– Thus, the soil was NOT a contributing factor

– Thought that it must be water!

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

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

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:

1. At low light intensities, the rate of photosynthesis increased by increasing light intensity but not temperature

2. At high light intensities, the rate of photosynthesis increased by increasing temperature but not light intensity

• From this study, Blackman concludedthat there must be:– a light-dependent reaction (photochemical)– a light-independent reaction (biochemical)

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:1. Light reactions occur in the presence of

light, independent of temperature*use light & water to produce NADPHand ATP

2. Carbon fixation reactions rely on the energy of light reactions *dependent on temperature not light intensity

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

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)

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

Chlorophyll & Accessory Pigments• Carotenoids = accessory pigments– Ex. ß-Carotene– Enzymes split ß-Carotene into vitamin A– Vitamin A is used to help vertebrae vision in

dim 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

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!

PHOTOSYNTHETIC REACTION STAGES

• All photosynthetic reactions can be broken down into3 distinct, but connected stages:

1. Capturing light energy2. Using captured light energy to make ATP and

reduced NADP+**NADP+ is the photosynthetic equivalent of NAD+**

3. Using the free energy of ATP and the reducing power of NADPH to synthesize organic compounds(ex. creating glucose from CO2)