Overview of Photosynthesis
Photosynthesis:
In the process of photosynthesis, energy from sunlight is trapped by chlorophyll and used
for the manufacture of carbohydrates from carbon dioxide and water. The process can be
summarised by the following equation:
Light energy
6CO2 + 6H2O → C6H12O6 + 6O2
Light:
White light from the sun is made up of a range of colours, each having different
wavelengths. Chlorophyll is a mixture of various pigments, with each pigment absorbing
a different spectrum of light due to their structure. Blue and red light is generally
absorbed to varying degrees by chlorophyll, when green light is reflected (hence leaf
colour). Chlorophyll a and b absorb both the red and blue/violet parts of the spectrum,
whereas xanthophyll and carotene absorb only the blue/violet part of light.
Light-dependent reaction (making energy):
Non-cyclic Photophosphorylation
In the light dependent reaction, light is absorbed into chlorophyll reaction centres
(photosystems). The light causes the chlorophyll to become energised or ‘excited’, and it
loses electrons to an electron carrier chain. This results in the production of hydrogen
carriers and ATP by chemiosmosis (in a comparable fashion to that achieved by the
electron transport chain in respiration). The lost electrons are replaced by the photolysis
of water (water is split by light). This is called non-cyclic photophosphorylation.
Cyclic Photophosphorylation
If water is not available, energy in the form of ATP can still be produced by an alternate
pathway. In this pathway, electrons flow in a cyclical manner back to the originating
photosystem, without being passed to NADPH (a hydrogen carrier). The cyclical transfer
of electrons still results in the accumulation of hydrogen ions in the thylakoid space
(producing ATP), but no hydrogen carriers are produced. This process is called cyclic
photophosphorylation.
Light-independent reaction (making glucose):
Through a process known as the Calvin cycle, carbon dioxide combines with a 5-carbon
organic compound (RuBP), which then splits into two 3-carbon compounds (glycerate 3phosphate or G3P). ATP and hydrogen from NADPH provide the reducing power and the
energy to convert G3P into glyceraldehyde 3-phosphate (GALP), which can be converted
into glucose and starch. Because NADPH is required for this reaction to occur, glucose
can only be produced from the by-products of non-cyclic photophosphorylation, and thus
indirectly requires water.
C3, C4 and CAM Plants:
The process explained above is common to all C3 plants, as the first recognisable product
after the fixation of CO2 is a 3-carbon compound. The C4 pathway uses phosphoenol
pyruvate (PEP – a 3C compound) to manufacture a 4C compound (oxaloacetate). The
enzyme that catalyses this process (PEP carboxylase) has a much higher affinity for CO2
than RuBP carboxylase, and so can occur at much lower CO2 concentrations. This
process (the Hatch-Slack pathway) is especially advantageous in warmer environments,
where keeping the stomata open for gas absorption would result in significant water loss.
CAM plants utilise an even more effective way of minimising water loss, by breaking the
Hatch-Slack pathway into day and night components, such that stomata are only opened
at night when the temperature is coolest.