Photosynthesis
Overview:
 Autotrophs = “self-feeders” – produce organic molecules from
CO2 and other inorganic raw materials from environment.
General equation: Reverse of respiration: energy is stored in
chemical bonds
(Sun) + 6 CO 2 + 6H2O
C6H12O6 + 6 O 2
 light reactions:
- Light energy is converted into chemical energy.
- results in ATP production and reduction of NADP+ to
NADPH
- O2 is also a product
 dark reactions/light independent rxns:
- Chemical energy is converted into food.
- CO2 is incorporated into organic molecules ("fixed") via
the Calvin cycle (C3 cycle)
(also called CalvinBenson cycle)
- may be made more efficient by other plant adaptations:
(more later…)
 Product of dark reactions is glyceraldehyde phosphate (G-3-P)
(Same molecule produced during the energy investment stage
of glycolysis after the “split”
G-3-P can then be:
o used to synthesize macromolecules
o used directly for energy
o converted to more stable glucose/sucrose to be
stored/transported
 Chloroplasts = site of photosynthesis (light and dark
reactions)
- double membrane
- grana are stacks of flattened sacs, each termed a thylakoid
- thylakoid membrane:
 where light energy is converted to chemical energy
 lipid bilayer (forming thylakoids) also contains

chlorophyll and other pigments

cytochromes (protein complexes) and enzymes
necessary for photosynthesis
- stroma: fluid surrounding the grana (singular = granum)
 where CO2 is fixed into organic compounds = sugars
(“dark rxns”/light independent)
The Light Reactions
Chlorophyll: a magnesium containing porphyrin (related to heme
… in blood), capable of absorbing light energy
 Light is a form of energy = electromagnetic energy
(electromagnetic radiation)
- The part that we see is called:
 Visible Light Sectrum (ROYGBIV)
- Photons = particles of light (light waves); fixed quantity
of energy
- Inversely proportional
o The shorter the wavelength = more energy of each
photon of that light wave; Ex. Violet has more E
than red
o Different pigments absorb different wavelengths
of light – that wavelength disappears; we see the
rest.
We see green because violet and red wavelengths are absorbed
- When chlorophyll absorbs sun light,
 electrons move to a higher orbital (excited state) =
• more potential energy = unstable.
 Energy is passed to another chlorophyll molecule
If an individual pigment molecule …electrons cannot stay in
excited state for long (billionth of a sec.); it passes back down
ground state
• stable in ground state
• heat released when it drops back to ground
state.
 chlorophyll a:
- most common plant pigment
- the heart of the photosynthetic process
- reaction center = protein complex, two “special”
chlorophyll a’s, and a primary e- acceptor. (part of
photosystems I and II)
- absorbs light energy at a specific wavelength; 680nm =
chlorophyll a
 accessory pigments: carotenoids, xanthophylls and other
chlorophylls (chlorophyll b = 700 nm)
- form antenna complex along with chlorophyll a
- absorb light at other wavelength
- transfer this energy to chlorophyll a
- enables more of sun’s energy spectrum to be utilized
 Photophosphorylation: light energy is used to generate ATP via
chemiosmosis to phosphorylate ADP.
 non–cyclic: electrons travel one way from water to use in
synthesis of organic molecules = sugars.
A photosystem = in thylakoid membrane, organization of
chlorophyll, proteins, and other organic molecules.

Photosystem I (2nd photosystem in process!)
 Includes:
Reaction center with:
- pigments bound to proteins (enzymes) = P700
- 1 e- acceptor = ferrodoxin
1. light drives e-'s from PS I chlorophyll to electron
acceptor (E )
2. e-'s passed to ferrodoxin, then used to reduce NADP to
NADPH2
- NADPH2 is energy rich
- used in Calvin cycle (dark reactions) to make sugars
…But: The chlorophyll of PS I needs e-'s to recycle, so . . .

Photosystem II (1st photosystem in process!)
3. light drives e-'s from PS II to another acceptor
(pheophytin) ( E)
4. e-'s passed down electron transport chain (cytochromes)
- ATP is generated by chemiosmosis
- ATP to be used in Calvin cycle
- e-'s finally accepted by PS I chlorophyll
…But: the chlorophyll of PS II also needs e-'s to recycle, so . . .
 Photolysis:
5. water is split providing:
- electrons to recycle PS II
- protons for reduction of NADP to NADPH2
- oxygen as a by-product (Thank Goodness!!)
 Chemiosmosis in thylakoids:
- protons cleaved from water by light in thylakoid space
- additional protons pumped by electron transport into
thylakoid space
- pass through ATP Synthase (ATPase) to produce ATP in
stroma
o can be used by dark reactions (w/NADPH2)
Calvin Cycle (Light Indpendent Reactions/Dark Rxns):
- Happens in the Stroma
- ATP made from chemiosmosis (photophosphorylation) &
NADPH2 made from PSI are both used.
 Phase 1: Carbon-Fixation
- Initial incorporation of CO2 into organic material (RuBP)
o Using the enzyme rubisco (most abundant protein on
earth), C of CO2 attaches RuBP --- (3 CO2’s enter at
one time)
o Creates a 6-C very unstable molecule – splits
immediately = 2 X 3-C molecules
 Phase 2: Reduction
o Each 3-C molecule is phosphorylated (using ATP)
o NADPH donates (reduces) these making G3P (where
is respiration did we see this molecule?)
 Remember 3 CO2’s entered Calvin Cycle and a
split happened (Phase 1), so 6 G3P’s were just
made.
 Some G3P’s will be used for phase three
 Some G3P’s will go directly to CR or incorporated
into glucose then starch for storage (in roots)
 Phase 3: Regeneration of RuBP
- Need to make more of that CO2 acceptor RuBP!
o Some of the G3P just made are rearranged back into the
5-C RuBP (Using more ATP)
 Other Plant adaptations
1. C4 photosynthesis – process in sugar cane, corn, grasses.
- Efficient in hot dry regions with intense sunlight.
2. CAM plants – process in succulents, like cacti and pineapple
- arid (dry) regions; water storing plants
Both types = stomata during closed during the day.
C4 and Cam plants are similar, but both have alternate
modes of carbon fixing before entering the Calvin Cycle
(dark rxns)
- Because their stomata are closed during the day, CO2
levels drop.
- Have evolved different modes to deal with low CO2
levels during the day.
- Can’t make sugar without CO2!