Page 121 - Demo 1

reduced electron carrier NADPH. In plants, the light reacons take place in the
thylakoid membranes of organelles called chloroplasts.
The first key event of photosynthesis happens when a beam of sunlight
strikes the surface membrane of a thylakoid. Scaered across the thylakoid
membrane are clusters of light-absorbing molecules called pigments. Pigments
are molecules that give color to plants, algae, and bacteria.

There are three main pigment groups: chlorophylls, carotenoids, and
phycobilins. Chlorophylls are green-colored pigments that are capable of
trapping blue and red light. Carotenoids are red, orange, or yellow-colored
pigments that absorb bluish-green light. Examples of carotenoids are
xanthophyll (yellow) and carotene (orange) from which carrots get their color.
Phycobilins are red or blue pigments that absorb wavelengths of light that are
not as well absorbed by chlorophylls and carotenoids. They are seen in
cyanobacteria and red algae.

The primary pigment molecule responsible for photosynthesis is
chlorophyll. Although it absorbs blue and red light, chlorophyll does not absorb
green wavelengths. Instead, green light is reflected, giving the thylakoid and
the chloroplast that contains it an intense green color. Plants are green
because they are rich in green chloroplasts. Chlorophyll molecules are
specifically arranged in and around pigment-protein complexes called
photosystems, which are embedded in the thylakoid membranes of
chloroplasts and are opmized to harvest light. The light reacons use two
photosystems, called photosystem I (PS I) and photosystem II (PS II). Both
photosystems contain many pigments that help collect light energy, as well as
a special pair of chlorophyll molecules found at the core (reacon center) of the
photosystem. The special pair of photosystem I is called P700, while the
special pair of photosystem II is called P680.

1. Both photosystems receive photons. The light-absorbing chlorophyll
molecules of each photosystem act together as an antenna to
capture photons (units of light energy). The energy is passed from
one pigment to the other unl it is concentrated in a parcular pair of
chlorophyll molecules, called the reacon center.

2. PS II splits water. The excitaon energy causes the transfer of an
energized electron (e-) from the reacon center of PS II to another
molecule that is an electron acceptor. The reacon center replaces
this lost electron with an electron from the breakdown of a water
molecule, which releases oxygen and two hydrogen ions (H+).

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