Download Overview of Photosynthesis

Overview of Photosynthesis
Electrons taken from water molecules are boosted
"uphill" by the energy from sunlight. The
chloroplast uses these "excited" electrons, along
with carbon dioxide and hydrogen ions, to produce
sugar molecules. The reaction steps add up to the
overall chemical equation for photosynthesis
shown in Figure 1
Figure 1 - the chemical equation for photosynthesis
summarizes many reaction steps.
Photosynthesis occurs in two main stages, each
with many steps: the light reactions and the Calvin
cycle (light independent reactions) (Figure 2).
Figure 2
This "road map" shows the two main stages of
photosynthesis: the light reactions, which
occur in the thylakoids, and the Calvin cycle,
which occurs in the stroma.
The light reactions convert the energy in sunlight
(light energy) to chemical energy. These reactions
depend on molecules built into the membranes of
the thylakoids. First, the green pigment chlorophyll
found in the membranes captures light energy.
Then the chloroplasts use the captured energy to
remove electrons from water. This splits the water
into oxygen and hydrogen ions. The oxygen
formed is a "waste product" of photosynthesis eg it
is not used in the process. It escapes to the
atmosphere through the stomata of leaves.
So what becomes of the water's electrons and
hydrogen ions? Chloroplasts use them to make an
energy-rich molecule called NADPH. (NADPH is
an electron carrier that delivers the energy rich
electrons to the Calvin cycle so sugar can be
made). The chloroplasts also use the captured
light energy to generate ATP. The overall result of
the light reactions is the conversion of light energy
to chemical energy stored in two compounds:
NADPH and ATP.
The light reactions convert light energy to the
chemical energy of ATP and NADPH. But recall
that photosynthesis also produces sugar. So far
no sugar has been produced. That is the job of the
Calvin cycle, which uses the ATP and NADPH
produced by the light reactions.
The Calvin Cycle has chemical inputs and
outputs. The inputs are carbon dioxide from the air
and the ATP and NADPH produced by the light
reactions. The Calvin cycle uses carbon from the
carbon dioxide, energy from the ATP, and highenergy electrons and hydrogen ions from the
NADPH. The cycle's output is an energy-rich
sugar molecule. That sugar is not yet glucose, but
a smaller sugar named G3P. The plant cell uses
G3P as the raw material to make glucose and
other organic molecules it needs.
The Calvin cycle is sometimes referred to as the
"light-independent reactions" because, unlike the
light reactions, it does not directly require light to
begin. However, this doesn't mean that the Calvin
cycle can continue running in a plant kept in the
dark. The Calvin cycle requires two inputs
supplied by the light reactions, ATP and NADPH.
Concept Check
1. Explain why a leaf appears green.
2. Besides oxygen, what two molecules are
produced by the light reactions?
3. Where in the chloroplast do the light reactions
take place?
Summary of Photosynthesis
Lets take a look at the overall process again.
Recall that the overall equation for photosynthesis
is:
6 CO2 + 6 H2O
C6H12O6 + 6 O2
The light reactions, which take place in the
thylakoid membranes, convert light energy to the
chemical energy of ATP and NADPH. The light
reactions use the reactant water from the equation
and release the product oxygen. The Calvin cycle,
which takes place in the stroma, uses ATP and
NADPH to convert carbon dioxide to sugar (Figure
7).
Figure 7
The light reactions and the
Calvin cycle together convert
light energy to the stored
chemical energy of sugar. The
plant can use the sugar to
build other organic molecules.
By converting light energy to chemical energy,
photosynthesis is the first step in the flow of
energy through an ecosystem. Some of that
chemical energy then passes from producers to
consumers. Even when people eat meat, you can
trace its stored energy back to photosynthesis. For
example, the beef in a hamburger came from
cattle that ate plants. Photosynthesis is the
ultimate source of all the food you eat and all the
oxygen you breathe.
Concept Check
1. What are the inputs and outputs of the Calvin
cycle?
2. Which stage of photosynthesis uses each
reactant from the overall photosynthesis equation?
Which stage generates each product from the
overall photosynthesis equation?
3. What molecule is the direct product of
photosynthesis? How is that molecule then used
by plant cells?
Concept Check
1. Explain why a leaf appears green.
2. Besides oxygen, what two molecules are
produced by the light reactions?
3. Where in the chloroplast do the light reactions
take place?
Concept Check
1. What are the inputs and outputs of the Calvin
cycle?
2. Which stage of photosynthesis uses each
reactant from the overall photosynthesis equation?
Which stage generates each product from the
overall photosynthesis equation?
3. What molecule is the direct product of
photosynthesis? How is that molecule then used
by plant cells?