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Chapter 6
Photosynthesis
Chapter 6, Section 1
Photosynthesis converts light
energy into chemical energy
through complex series of
reactions known as biochemical
pathways. Autotrophs use
photosynthesis to make organic
compounds from carbon dioxide
and water.
In plants and algae,
photosynthesis occurs inside the
chloroplasts.
White light from the sun is
composed of an array of colors
called the visible spectrum have
different wavelengths.
Pigments absorb certain colors of
light and reflect or transmit the
other colors.
The light reactions of
photosynthesis begin with the
absorption of light by chlorophyll a
and accessory pigments in the
thylakoids.
Accessory pigments absorb colors
of light that aren’t absorbed by
chlorophyll a, and they transfer
some of the energy in this light to
chlorophyll a.
Excited electrons that leave
chlorophyll a travel along two
electron transport chains,
resulting in the production of
NADPH. The electrons are
replaced when water is split into
electrons, protons, and oxygen in
the thylakoid,. Oxygen is released
as a byproduct of photosynthesis.
As electrons travel along the
electron transport chains, a
concentration gradient of
protons builds up across the
thylakoid membrane. The
movement of protons down this
gradient results in the synthesis
of ATP through chemiosmosis.
Chapter 6, Section 2
The ATP and NADPH produced in
the light reactions drive the
second part of photosynthesis, the
Calvin cycle. In the Calvin cycle,
CO2 is incorporated into organic
compounds, a process referred to
as carbon fixation.
The Calvin cycle produces a
compound called PGAL. Three turns
of the Calvin cycle are needed to
produce one PGAL molecule.
Most PGAL molecules are converted
into another molecule that keeps the
Calvin cycle operating. However,
some PGAL molecules are used to
make other organic compounds
including amino acids, lipids, and
carbohydrates.
In the overall equation for
photosynthesis, CO2 and water
are the reactants, and
carbohydrate and O2 are the
products.
Some plants living in hot, dry
climates supplement the Calvin
cycle with the C4 or CAM
pathways. These plants carry out
carbon fixation and the Calvin
cycle either in different cells or at
different times.
The rate of photosynthesis
increases and then reaches a
plateau as light intensity or CO2
concentration increases. Below a
certain temperature, the rate of
photosynthesis increases as
temperature increases. Above that
temperature, the rate of
photosynthesis decreases as
temperature increases.