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PHOTOSYNTHESIS
JJA Obico, Insructor
Department of Biology
ENERGY
REDOX REACTION
Energy is transferred from one
molecule to another via redox
reactions.
The reduced form of a molecule thus
has a higher level of energy than the
oxidized form

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Redox in covalently bonded molecules

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Photosynthesis Trivia
On a global scale, photosynthesis makes about
160 billion metric tons of carbohydrate per
year
1 metric ton= 1000 kg
Equivalent to: a stack of 60 trillion copies of a
thick book; 17 stacks of books reaching from
earth to sun!!!
Photosynthesis
TWO STAGES:
1. Light reaction- “photo”
- light energy chemical energy (ATP, NADPH)
Occurs in the thylakoids- Occurs in the thylakoids
2. Dark reaction – Calvin cycle- “synthesis”
- Light independent reactions
- Carbon fixation; Makes sugar
- Occurs in the stroma
Nature of LIGHT
A form of electromagnetic energy/ radiation
Visible light- segment of EM spectrum vital to life
W l h h d b fWavelength- the distance between crests of EM
waves

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Pigments
Pigments- substances that absorb light
Pigments in the chloroplast
Chlorophyll a blue green; primary pigmentChlorophyll a- blue-green; primary pigment
Chlorophyll b- yellow green; acessory pigment
Carotenoids- shades of yellow and orange
Absorption spectrum
Graph of pigment’s light absorption versus
wavelength
The light reaction can perform work with those
wavelengths of light that are absorbed.
In the thylakoid are several pigments that differ in their
absorption spectrum.
Chlorophyll a, the dominant pigment, absorbs best in the
red and blue wavelengths, and least in the green.
Other pigments
with different
structures have
different
absorption
spectra.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.8a

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Collectively, these photosynthetic pigments determine an
overall action spectrum for photosynthesis.
An action spectrum measures changes in some measure of
photosynthetic activity (for example, O2 release) as the
wavelength is varied.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.8b
The action spectrum of photosynthesis was first
demonstrated in 1883 through an elegant experiment by
Thomas Engelmann.
In this experiment, different segments of a filamentous alga
were exposed to different wavelengths of light.
Areas receiving wavelengths favorable to photosynthesis
should produce excess O2.
Engelmann used the
abundance of aerobicabundance of aerobic
bacteria clustered
along the alga as a
measure of O2
production.
Fig. 10.8c
The action spectrum of photosynthesis does not match
exactly the absorption spectrum of any one photosynthetic
pigment, including chlorophyll a.
Only chlorophyll a participates directly in the light
reactions but accessory photosynthetic pigments absorb
light and transfer energy to chlorophyll a.
Chlorophyll b, with a slightly different structure than
chlorophyll a, has a slightly different absorption spectrum and
funnels the energy from these wavelengths to chlorophyll a.gy g p y
Carotenoids can funnel the energy from other wavelengths to
chlorophyll a and also participate in photoprotection against
excessive light.

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LIGHT REACTIONS

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Photosystems
Chlorophyll is organized along with other proteins
and smaller organic molecules into photosystems
PHOTOSYSTEMS
Has light gathering “antenna complex” consisting of a
cluster of chloro a, b and carotenoidmoleculesß
Reaction center
One of the chloro a
Where first light driven chemical reaction of photo
occurs
Includes PRIMARY ELECTRON ACCEPTOR
PHOTOSYSTEM
PHOTOSYSTEM I
P700 (700 nm wavelength)
PHOTOSYSTEM II
P680
Two Routes for electron flow
1. Non cyclic
predominant route; PS I and PS II
ATP and NADPH
2. Cyclic
alternative route; makes use of PS I
ATP only

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Non cyclic electron flow Cyclic electron flow
What is the significance of
cyclic electron flow?
Noncyclic- produces ATP and NADPH
Calvin cycle- consumes more ATP than NADPH
A h l l h hNADPH concentration- helps regulate which
pathway to take (cyclic or noncyclic)
If ATP runs low for Calvin; NADPH accumulates

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Calvin Cycley
Dark Reactions/ Light Independent Reaction
Calvin cycle
Uses ATP and NADPH to convert CO2 to sugar
Direct product: Glyceraldehyde-3-phosphate (G3P)
NOT glucose 3 Carbon sugarg g
Cycle must take place 3X for the net synthesis of
glucose
Carbon fixation – incorporation of CO2 into organic
compounds
Calvin cycle
3 Phases
1. Carbon fixation
CO2 joins ribulose biphosphate (RuBP) by RuBP
carboxylase (Rubisco)
Product is a 6-carbon unstable molecule that breaks into 2
3 b l l d 3 h h l t3-carbon molecule named 3-phosphoglycerate
2. Reduction
3- phosphoglycerate receives a phosphate group 1,3
biphosphoglycerate
NADPH reduces 1,3 biphosphoglycerate
3. Regeneration of CO2 acceptor (RuBP)
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.17.1

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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.17.2
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.17.3
Alternative mechanisms of
carbon fixation
Photorespiration
C3 plants- first organic product of carbon fixation is a 3-
carbon molecule (3 phosphoglycerate)
Some plants close stomata during hot, dry days.
CO2 entry is prevented; Calvin cycle starves.
Inside the leaf, O2 production increases because of
photosynthesisphotosynthesis
Rubisco can also accept O2
Product splits
One 2-carbon compound is exported from the
chloroplast
Mitochondria and peroxisomes break this product
CO2 is released

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Photorespiration is a
wasteful processp
It does not generate ATP and produces no
food
C4 plants
Preface Calvin cycle with 4-carbon compound
Sugarcane and corn
Presence of BUNDLE SHEATH CELLS
Calvin cycle is confined in the chloroplast of bundle
h h llsheath cells
Carbon fixation happens in the MESOPHYLL CELLS
CO2 joins PEP (Phosphoenolpyruvate) by PEP carboxylase
(Pepco) forms oxaloacetate
Pepco- higher affinity with CO2
Mesophyll cells- pump CO2 into bundle sheath so
Rubisco can function well
CAM plants
Crassulacean acid metabolism
Succulent plants
Cacti, pineapple, CrassulaceaeCacti, pineapple, Crassulaceae
Open their stomata during night and close during the
day
Store organic acid into VACUOLES
CO2 is release from the organic acid during the day
when products of light reactions are available (ATP,
NADPH)

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