3.2

PHOTOSYNTHESIS 2

Photosynthesis

Can be broken down into three stages.

LIGHT REACTIONS (in thylakoids)

1) Capturing light energy.2) Using captured light

energy to make ATP and reduce NADP+ (nicotinamide adenine dinucleotide phosphate) to NADPH

CARBON FIXATION (in stroma)

3) Uses the energy of ATP and the reducing power of NADPH to drive Calvin Cycle, which incorporates CO2 into carbon compounds such as glucose.

Properties of Light

Of the total solar energy that reaches the earth, 60% is lost to the atmosphere. Of the 40% which reaches plants, only 5% is used in photosynthesis.

Light is also known as electromagnetic (EM) radiation that travels at 300,000,000m/s

Properties of Light

That light comes in packets known as photons (or quanta). Photons of light energy come in different wavelengths of light energy. The longer the wavelength, the lower the energy. The shorter the wavelength, the higher its energy. Visible light is between 380 nm (violet) and 750nm (red).

Photosystems

Are clusters of photosynthetic pigments embedded in the thylakoid membranes which absorb photons of particular wavelengths.

transfers energy to ADP, P, and NADP+, forming ATP and NADPH.

The electrons that reduce NADP+ to NADPH are supplied by water molecules that enter the thylakoids through the stroma.

ATP and NADPH are synthesized in the stroma, where carbon fixation reactions occur.

History of Photosynthesis

In ancient times, people thought all food for the plant was from the soil.

In early 1600’s, J.B. Van Helmont planted s a willow tree after predetermining the weight of the soil and the willow tree.

After five years, the tree’s mass had increased by 74.4 kg, but the soil’s mass only decreased by 60 g. Therefore, most of the added weight of the plant could be attributed to water absorption (incorrect)

History of Photosynthesis

1771, Joseph Priestley discovers by accident that gases play a role in photosynthesis.

Put a candle in a glass container, and it eventually burns out.

Put a candle in a glass jar with a plant, and in ten days the candle was able to combust again.

There must be a gas released by the plant that supports combustion.

History of Photosynthesis

In 1776, Dutch medical doctor Jan Ingenhousz identified the gas as oxygen, and was the first to realize light was necessary for photosynthesis.

He was wrong in thinking that O2 was produced by the sun splitting CO2 and leaving the carbon atom for the plant to use.

History of Photosynthesis

In the 1930’s, C.B. Van Niel was able to show that water provided the oxygen gas by way of purple sulfur bacteria.

CO2 + 2H2S + light [CH2O](aq) +H2O(l) )+ 2S(s)

(hydrogen sulfide) (carbohydrate)

History of Photosynthesis

In 1938, S.M. Ruben and M. Kamen confirm Niel’s findings.

Use Chlorella algae - grow it in heavy water (uses an oxygen-18 isotope)

Used mass spectrometer to track the isotope. The spectrometer separates and detects molecules by mass.

The isotope was found in the oxygen gas.

Rate of Photosynthesis

In 1905, Blackman measured the effect in changes in light intensity, CO2 and temperature and temperature. Concluded that,

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

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

Rate of Photosynthesis

Concluded that photosynthesis takes place in two stages, an initial light-dependent stage, and a second, light-independent stage that is affected by heat, not light.

He later showed that the level of carbon dioxide affects the rate of photosynthesis.

The light reactions use light and water to produce NADPH and ATP.

The carbon fixation reactions then use that NADPH and ATP.

Colours and Photosynthesis

In 1882 T.W. Engelmann placed a triangular glass prism between a light source and a microscope stage. On the stage was the algae Spirogyra, long and filamentous.

He placed aerobic bacteria along the length of the alga, to see if photosynthesis was equal across all colour wavelengths. Aerobic bacteria would grow where oxygen is

produced.

Colours and Photosynthesis

Very little aerobic bacteria grew near the green spectrum.

Why is chlorophyll green?

Colours are determined by the wavelengths of light that reflect back to us (not absorbed)

Absorption Spectrum

The wavelengths of light absorbed by a pigment.

Chlorophylls a and b absorb blue-violet and red and transmit green 500-600 nm.

Chlorophyll a is the pigment that transfers energy from light to the carbon fixation reactions.

Action Spectrum

Graph illustrating the effectiveness with which different wavelengths of light produce photosynthesis.

Accessory Pigments Carotenoids: Absorb in the blue-violet range (400-500nm).

Contain two hydrocarbon rings connected by an alternating single and double-bond hydrocarbon chain.

Reflect red and yellow. Dispersed in the thylakoid membranes. Some don’t participate in photosynthesis, but rather absorb

energy that can damage chlorophyll release that energy as heat.

Accessory Pigments

Carotenoids: (yellow-red)Xanthophylls – pigments in chloroplast

(thylakoid) membranes that give rise to yellow colour in leaves

Anthocyanins – pigments in vacuoles that give rise to the red colour in autumn leaves.

So, why do plants appear to be green in the spring/summer?

Why do leaves turn colour in the fall?

Photosynthetically Active Radiation (PAR)

The wavelengths from 400 nm to 700 nm that support photosynthesis.

Chlorophylls a and b combined with other pigments pretty much absorb the entire visible spectrum.

Summary

Seatwork/Homework

Reread section 3.2Answer PPs on page 154:

#1,2,3, 5, 7 (read text), 8.