Biology (Holt)

Production of Oxygen
As shown in Figure 7,pigments involved in plant photosynthesis are
located in the chloroplasts of leaf cells. Clusters of pigments are
embedded in the membranes of disk-shaped structures called
(THIE luh koydz).When light strikes a thylakoid in a
chloroplast, energy is transferred to electrons in chlorophyll. This
energy transfer causes the electrons to jump to a higher energy level.
Electrons with extra energy are said to be “excited.” This is how
plants first capture energy from sunlight.
Excited electrons jump from chlorophyll molecules to other
nearby molecules in the thylakoid membrane, where the electrons
are used to power the second stage of photosynthesis. The excited
electrons that leave chlorophyll molecules must be replaced by
other electrons. Plants get these replacement electrons from water
molecules, H 2 O. Water molecules are split by an enzyme inside the
thylakoid. When water molecules are split, chlorophyll molecules
take the electrons from the hydrogen atoms, H, leaving hydrogen
ions, H+. The remaining oxygen atoms, O, from the disassembled
water molecules combine to form oxygen gas, O 2.

thylakoids

SECTION 2 Photosynthesis 99

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Topic:Light Absorption

Keyword:HX4116

Thylakoid

space

Thylakoid

Thylakoid

membrane

Inner

membrane

Cluster of

pigments

Water-splitting

enzyme

Thylakoid

space

Outside of

thylakoid

Outer

membrane Chloroplast

Plant cell

Leaf

e–

4H+

2H 2 O O

2

Thylakoid

membrane

Pigment molecules are embedded in thylakoid membranes, as are other

molecules that participate in photosynthesis.

Figure 7 Chloroplast