7.1 Photosynthetic Organisms

A. Photosynthesis Transforms Solar Energy
B. Organic molecules built by photosynthesis provide both the building blocks and energy for cells.
C. Plants use the raw materials: carbon dioxide and water
D. Chloroplasts carry out photosynthesis
E. Chlorophylls and other pigments involved in absorption of solar energy reside within thylakoid membranes of chloroplasts

7.2 Plants as Solar Energy Converters

A. Solar Radiation - Only 42% of solar radiation that hits the earth’s atmosphere reaches surface; most is visible light.
B. Photosynthetic Pigments - Pigments found in chlorophyll absorb various portions of visible light; absorption spectrum.

1. Two major photosynthetic pigments are chlorophyll a and chlorophyll b.
2. Both chlorophylls absorb violet, blue, and red wavelengths best.
3. Very little green light is absorbed; most is reflected back; this is why leaves appear green.
4. Carotenoids are yellow-orange pigments which absorb light in violet, blue, and green regions.
5. When chlorophyll breaks down in fall, the yellow-orange pigments in leaves show through.

C. Absorption and action spectrum - A spectrophotometer measures the amount of light that passes through a sample of pigments.

1) As different wavelengths are passed through, some are absorbed.
2) Graph of percent of light absorbed at each wavelength is absorption spectrum.
3) Photosynthesis produces oxygen; production of oxygen is used to measure the rate of photosynthesis.
4) Oxygen production and, therefore, photosynthetic activity is measured for plants under each specific wavelength; plotted on a graph, this produces an action spectrum.
5) Since the action spectrum resembles absorption spectrum, this indicates that chlorophylls contribute to photosynthesis.

D. Photosynthetic Reaction
1. In 1930 C. B. van Niel showed that O2 given off by photosynthesis comes from water and not from CO2.
2. The net equation reads:

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E. Two Sets of Reactions in Photosynthesis

1. Light reactions cannot take place unless light is present. They are the energy-capturing reactions.
b. Chlorophyl within thylakoid membranes absorbs solar energy and energizes electrons.
c. Energized electrons move down the electron transport system; energy is captures and used for ATP production.
d. Energized electrons are also taken up by NADP+, becoming NADPH.

2. Calvin Cycle Reactions
a. These reactions take place in the stroma; can occur in either the light or the dark.
b. These are synthesis reactions that use NADPH and ATP to reduce CO2.