Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of ATP and NADPH. It occurs in two stages - the light-dependent reactions where energy from sunlight is captured and converted to chemical energy in ATP and NADPH, and the light-independent reactions where carbon dioxide is incorporated into organic compounds like glucose. There are different pathways for photosynthesis including C3, C4 and CAM which have evolved strategies to prevent the wasteful process of photorespiration.

1. Photosynthesis
biology 1

2. • 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

3. 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

4. Photosynthesis as a redox process
• A general equation for photosynthesis is:
6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O
• Indicating the net consumption of water
simplifies the equation to:
6CO2 + 6H2O C6H12O6 + 6O2
• The simplest form of equation is:
CO2 + H2O CH2O + O2
Thus 6 repetitions of the equation produce a molecule of glucose
light
light
light

5. Where does the oxygen come from?
• Van Niel demonstrated in chemo-
autotrophs that:
CO2 + 2H2S CH2O + H2O + 2S
• Therefore, a general form for the
synthesis equation:
CO2 + 2H2X CH2O + H2O + 2X
• Van Niel summarized that oxygen
comes from water (later confirmed with
radio-isotopes)
light
light

6. 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

7. 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

8. 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

9. 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

10. 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)

11. • 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

12. How pigments aid the light-
dependent 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

13. 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)

14. 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

15. • 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
H2O O + 2H+ + 2e-
Light +
enzyme
to P680
chemiosmosis

16. • Production of ATP via the electron
transport chain is termed photo-
phosphorylation (light energy used to
make ADP into ATP)
• In this case, ATP production is
specifically termed noncyclic photo-
phosphorylation

17. 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

18. 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 3-
phosphoglycerate to glyceraldehyde 6-phosphate
– Regeneration of RBP (requires ATP)
• Calvin cycle needs 18 ATP and 12 NADPH to produce 1
molecule of glucose

19. 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)

20. Rubisco transfers O2 to RuBP
Resulting 5-C molecule splits into
Three-C molecule
Stays in Calvin cycle
Two-C molecule
(glycolate)
goes to mitochondrion
via peroxisome
Glycolate broken down to CO2

21. 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

22. 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