Photosynthesis
biology 1
• Photosynthesis plays a key role in
photo-autotrophic existence
• Photosynthesis is a redox process,
occurring in chloroplasts, and involves
two reactions
– Light dependent reaction
– Light independent reaction (Calvin Cycle)
• Pigments in chloroplasts are keyed to
react to specific wavelengths of light
• There are different strategies to
photosynthesis, including the C3, C4 and
CAM pathways
The importance of photosynthesis
• Life on Earth is balanced between
autotrophs (“self”-feeders) and
heterotrophs (“other”-feeders)
• Autotrophs synthesize complex organic
molecules (e.g., sugar), utilizing energy
from
– Light (photo-autotrophs)
– Oxidation of inorganics (chemo-autotrophs
• Autotrophs responsible for ‘producing’
organic molecules that enter ecosystem
Photosynthesis as a redox process
• A general equation for photosynthesis is:
light
6CO2 + 12H2O
C6H12O6 + 6O2 + 6H2O
• Indicating the net consumption of water
simplifies the equation to:
light
6CO2 + 6H2O
C6H12O6 + 6O2
• The simplest form of equation is:
light
CO2 + H2O
CH2O + O2
Thus 6 repetitions of the equation produce a molecule of glucose
Where does the oxygen come from?
• Van Niel demonstrated in chemoautotrophs that:
light
CO2 + 2H2S
CH2O + H2O + 2S
• Therefore, a general form for the
synthesis equation:
light
CO2 + 2H2X
CH2O + H2O + 2X
• Van Niel summarized that oxygen
comes from water (later confirmed with
radio-isotopes)
Photosynthesis as a redox process
• Hydrogen is extracted from water and
incorporated into sugar
• Electrons have higher potential energy
in the sugar molecule
• Light is the energy source that boosts
the potential energy of electrons as they
are moved from water to sugar
• When water is split, electrons are
transferred form the water to carbon
dioxide, reducing it to sugar
The chloroplast
• Photosynthesis occurs in chloroplasts
• Chlorophyll (and other pigments), stored in
thylakoid membranes captures light energy this is where light dependent reactions occur
• The stroma (matrix inside chloroplast)
contains light-independent reactions,
reducing CO2 to CH2O
• Thylakoids and photosynthetic cells are
organized in grana and the mesophyll
respectively to maximize absorption of light
The light-dependent reaction
• Light energy is converted to chemical bond
energy found in ATP and NADPH, occurring
in thylakoid membranes
– NADP+ (nicotinamide adenine dinucleotide
phosphate) is reduced to NADPH, temporarily
storing energized electrons transferred from water,
and an H+
– O2 is a by-product of splitting water
– ATP is produced via photophosphorylation
• ATP and electrons are used in next stage to
fix carbon
The light-independent reaction
• Also known as the Calvin cycle
• Main purpose is to fix carbon (process
of incorporating carbon into organic
molecules
• NADPH provides the reducing power
• ATP provides the chemical energy
How the light reaction works
• The nature of light
– Acts as both a particle and a waveform
– As a wave, is a type of electromagnetic energy:
visible light consists of a spectrum of wavelengths
from 380 nm to 750 nm
– As a particle, light behaves as discrete particles
called photons, each photon having a fixed amount of
energy
– In photosynthesis, the most used (absorbed)
wavelengths are blue and red. Green is transmitted
(hence the green color of chlorophyll)
• A substance that absorbs light is called a pigment
• Each pigment has a characteristic absorption
spectrum
• In the light reaction the most important pigment is
chlorophyll a
• However, the action spectrum for chlorophyll a
does not match that for photosynthesis - therefore,
other pigments are involved
– Carotenoids, e.g., xanthophyll
– Chlorophyll b
• These accessory pigments do not participate
directly in the light reaction, but work with
chlorophyll a to do so
How pigments aid the lightdependent reaction
• Photo-excitation
• Absorbed wavelength photons boost one of
the pigment molecule’s electrons in its lowest
energy state (ground state) to an orbital of
higher energy (excited state)
• The difference in energy is directly equal to
the energy of the photon, and therefore
specific to a specific wavelength
• Conclusion: certain pigments are designed to
absorb particular wavelengths
Photosystems
• Photosystems are organizations of photosynthetic
pigments, consisting of
– An antenna complex (responsible for inductive
resonance, the absorption of energy associated with
photons, and passing that energy between themselves
– A reaction centre chlorophyll. One molecule of
chlorophyll a per photosystem can take that energy use
it to push out an electron, passing it to…
– …the primary electron acceptor (the first step of the
light reaction
• Two types of photosystem: P700 (photosystem I)
and P680 (photosystem II)
Non-cyclic electron flow
• Excited electrons are transferred form
P700 to the primary electron acceptor
for photosystem I
• Primary electron acceptor passes
excited electrons to NADP+ (goes to
NADPH) via ferredoxin
• Oxidized P700 chlorophyll becomes an
oxidizing agent (needs electrons to be
replaced). These electrons come
indirectly from photosystem II
• Electrons ejected from P680 are
trapped by the PS II primary electron
acceptor
• These electrons are transferred to an
electron transport chain (making ATP),
eventually ending in P700 (PS I)
• Electron space vacated in P680 are
filled splitting of water
Light +
H2O enzyme O + 2H+ + 2echemiosmosis
to P680
• Production of ATP via the electron
transport chain is termed photophosphorylation (light energy used to
make ADP into ATP)
• In this case, ATP production is
specifically termed noncyclic photophosphorylation
Cyclic electron flow
• Involves only PS I, P700
• In this cyclic system, ejected electrons
are fed to the ETC to generate ATP
(cyclic photo-phosphorylation)
• P700 acts as the ultimate acceptor of
the shunted electrons
• No NADPH is produced, or O2
• Aim is to produce extra ATP needed for
Calvin cycle
The Calvin cycle
• Powered by ATP and NADPH from light-dependent
reaction
• Carbon enters cycle as CO2 and leaves as triose sugar,
glyceraldehyde 3-phosphate (G3P)
• Three phases:
– Carbon fixation - a molecule of CO2 is attached to a CO2-acceptor,
ribulose biphosphate (RBP)
• Unstable 6-carbon intermediate degrades into 3-carbon molecule
– Reduction - endergonic reaction
• uses ATP (energy) and NADPH (reducing agent) to convert 3phosphoglycerate to glyceraldehyde 6-phosphate
– Regeneration of RBP (requires ATP)
• Calvin cycle needs 18 ATP and 12 NADPH to produce 1
molecule of glucose
Alternative mechanisms
• Photorespiration - a metabolic pathway
that consumes O2, produces CO2,
produces no ATP and decreases
photosynthetic output
• Occurs because Rubisco (enzyme in
Calvin cycle), can accept O2 instead of
CO2
• When O2 conc. higher than CO2 conc. In
leaf, rubisco takes O2 (e.g., when hot,
stomata close, CO2 drops, O2 increases)
Rubisco transfers O2 to RuBP
Resulting 5-C molecule splits into
Two-C molecule
(glycolate)
Three-C molecule
Stays in Calvin cycle
goes to mitochondrion
via peroxisome
Glycolate broken down to CO2
Strategies to prevent photorespiration
• C3 plants produce 3 phosphoglycerate, the first
stable intermediate in the Calvin cycle (eg, rice, soy,
wheat)
• C4 plants (eg, corn, sugar, grasses) have different
morphology. CO2 fixation occurs in different location
• In mesophyll cells, CO2 is added to
phosphoenolpyruvate (PEP) to make oxaloacetate
(4-carbons). Enzyme is PEP carboxylase, which has
a far higher affinity for CO2 than O2
• Oxaloacetate is converted to malate (4C), and then
moved to bundle sheath cells, where remaining
Calvin cycle occurs
Another strategy...
• In succulents adapted to dry, arid environments,
stomata closed during day
• At night, stomata open and CO2 is incorporated into a
number of organic acids (termed crassulacean acid
metabolism: CAM)
• During day, light reactions produce ATP and NADPH
which release CO2 from organic acids
• In summary,
– C4 plants spatially separate carbon fixation from the Calvin
cycle
– CAM plants temporally separate carbon fixation from the
Calvin cycle