The two stages of photosynthesis explained
Photosynthesis consists of a two-part process: the light-dependent
reactions and the light-independent reactions. These reactions
usually occur simultaneously in plants, because the products of the
light-dependent reactions are needed for the light-independent
reactions.
Light-dependent reactions
Overall, in the light-dependent reactions, water is used to provide a
source of protons and electrons to make ATP and NADPH + H+.
When light hits the chlorophyll pigment in the membranes of the
chloroplasts, electrons in the chlorophyll pigment are excited by the
energy in the light and are accepted by chemicals in the membrane
of the chloroplast. To ‘fill the holes’ that have been left from the
electrons being excited, electrons are taken from water. This releases
protons (hydrogen ions) as well, and finally two oxygen atoms
combine to form an oxygen molecule, which is released from the
thylakoid into the stroma.
The overall word equation for this step is:
The formation of oxygen is not the end of the process. Electrons that
were passed to chemicals in the membrane are passed to molecules
of NADP+, which combines with hydrogen to form NADPH.
The overall word equation for this step is:
The final step in the light-dependent stage involves the movement of
hydrogen ions from a high to a low concentration. There has been a
build-up of hydrogen ions in the thylakoid compartment because
hydrogen ions were released from water. Although hydrogen ions
are small enough to pass through the membrane, there is a quicker
route through the protein pores. The protein pores, in this instance an
enzyme called ATP synthase, are embedded in the membrane. The
movement of hydrogen ions through ATP synthase provides the
energy needed to form ATP.
The overall word equation for this step is:
Photosynthesis summarised —the light-dependent and light-independent
reactions occur simultaneously
Light-independent reactions, or the Calvin cycle
Perhaps the most important aspect of the light-independent stage of
photosynthesis—also known as the Calvin cycle—is that it can
occur all of the time, whether or not light is present. However, the
Calvin cycle requires ATP and NADPH (which are made in the
light-dependent stage) so the light-independent reactions usually
occur at the same time as light-dependent reactions.
The Calvin cycle occurs in the watery stroma of the chloroplast,
where NADPH, ATP and carbon dioxide are used to make glucose
sugar, which can be used elsewhere as a fuel for cells.
The overall word equation for the light-independent reaction is:
Carbon dioxide
Enzymes
Glucose sugar + Water
There are many reactions in the Calvin cycle. Here are the three
main steps in the process to make glucose.
.
1 Carbon dioxide (CO2) is joined to a molecule containing five
carbon atoms (C5), making a new compound with six carbon
atoms (C6).
.
2 The new C6 compound splits into two molecules with three
carbon atoms each (C3).
.
3 The C3 compound reacts with ATP then NADPH. Then some of
these C3 molecules react together to make glucose (C6). Other
C3 molecules react together to make C5 compounds that are
used again for step 1.
.
Sometimes, descriptions of photosynthesis in textbooks—and
perhaps in exam questions—can be confusing. Here are some
expressions that you need to understand:
- ‘Water is split’ means that a water molecule is broken into
two hydrogen ions and an oxygen atom (which can join
another oxygen atom forming oxygen gas).
- ‘Carbon dioxide is fixed’ means that carbon from carbon
dioxide gas is joined to carbon atoms in the stroma, via the
Calvin cycle, to make glucose sugar.
- ‘Light, ATP and NADPH are fuels for carbon fixation’
means that energy from light is transferred by compounds
called ATP and NADPH (formed in the light-dependent
reactions) to the energy-rich glucose sugar (formed in the
light-independent reactions).