Tip:
Highlight text to annotate it
X
In plant cells, photosynthesis takes place in organelles called chloroplasts. Inside the
chloroplast, folds of membrane form compartments called thylakoids. The thylakoid
membrane encloses the thylakoid space and separates it from the surrounding stroma.
Light energy is captured by photosystems inserted into the thylakoid membrane. Let’s take a
closer look at a photosystem.
Photosystems are clusters of 200-300 pigment molecules held together by a protein matrix and
sitting in the thylakoid membrane. Most of these pigment molecules work as the antennae
complex. One specialized chlorophyll molecule, called the reaction center, acts as an
electron donor. The antennae complex acts like an antennae because it collects energy and
funnels it to the reaction center
Let’s look at how light energy is captured by a photosystem. Each pigment molecule has one
or more electrons that can absorb photons of visible light. When sunlight hits a
photosystem, some of the photons have exactly the right amount of energy to be absorbed
by the pigment molecules.
When one of these photons is absorbed, it boosts an electron in the pigment molecule to a
higher energy state. The excited electron is not stable, so it quickly returns to its
original state. The energy that it absorbed is passed on to a neighboring pigment molecule.
Notice that the electron itself is not passed along. Only the energy is passed from molecule
to molecule like a hot potato. Eventually, by random chance, the energy is passed to the
reaction center. Here the excited electron is transferred to an electron acceptor. At his
point, light energy from the sun has been converted into chemical energy. Once enough
high energy electrons have been collected in this way, their energy can be used to make
sugar.