1. Photosynthesis

2. provides (either directly or indirectly) the
I. Introduction
All creatures depend on the sun b/c it
provides (either directly or indirectly) the
free NRG that powers life on earth
Autotrophs: organisms that use light
NRG (i.e. sun) & inorganic carbon (CO2)
to live & to produce organic compounds
(i.e. sugars) (e.g. plants)
Heterotrophs: organisms that depend
upon autotrophs for food and NRG b/c
they can’t produce their own
(e.g. animals)

3. green plants, algae, and some bacteria
PHOTOSYNTHESIS – The process in which
green plants, algae, and some bacteria
convert solar NRG into chemical NRG. The
overall process can be summarized according
to the following equation:
6 CO H2O + Light  C6H12O O H2O
raw materials glucose by-products

4. II. Requirements of Photosynthesis
Light
1. Provides NRG for the process to occur
2. Photon – unit of light NRG that travels in
specific wavelengths
Electromagnetic spectrum – a scale that depicts
a range of solar radiation based on wavelength.
--Gamma rays: shortest wavelength
highest NRG
--Radio waves: longest wavelength
lowest NRG
--Photosynthesis utilizes the portion of the
spectrum known as visible light

6. Pigments
1. Chlorophyll: the green pigment in plants
that absorbs or reflects light
 Two main kinds: Chlorophyll a & b
a. absorbs violet, blue, orange, red
b. reflects what?
1. Chlorophyll a: blue-green
2. Chlorophyll b: green
2. Accessory pigments:
 Carotenoids – pigments that absorb &
transfer additional NRG to chlorophyll
(absorb green/blue; reflect orange/red).

7. 3. Spectrophotometer – measures the amt. of
light that passes thru a sample of pigments
a. Absorption spectrum - % of light absorbed
at each wavelength
b. Action spectrum – this will indicate O2
production and thus photosynthetic activity

8. Absorption Spectrum

10. Chloroplast – a double membrane-bound
organelle that contains the chlorophyll
a. Thylakoid – a mem-bound sac within the
inner membrane where chlorophyll is stored
b. Grana – stacks of thylakoids
(used in light rxns)
c. Stroma – the enzyme-rich fluid b/n the
grana & inner membrane
(used in dark rxns)

12. CO2 & H2O
1. CO2 (from atmosphere) supplies C and O
for glucose
2. H2O supplies H for glucose, O is released
as a gas

13. Process of Photosynthesis (Overview)
Light Reactions – “light capture”
1. Also called:
 Light-dependent reactions
 photophosphorylation
2. Takes place in the thylakoid
3. Three pathways:
 Fluorescence
 Cyclic photophosphorylation(includes ETC)
 Non-cyclic photophosphorylation
(i.e. photosystems I & II; includes E.T.C.)
4. Purpose:
 uses light & water to make ATP and O2
 also produces NADPH for dark rxns

15. Dark Reactions
1. Also called:
 Light-independent reactions
 Calvin-Benson Cycle
 “CO2 Fixation”
2. Takes place in the stroma
3. Uses the products of light rxns (i.e. ATP
and NADPH) to reduce CO2 and make
glucose

17. Light Reactions – “Photophosphorylation”
Activation of chlorophyll
1. Light strikes chlorophyll & becomes “excited”
2. e- “jumps” to a higher NRG level
3. If nothing further happens, the e- falls back
to ground state and the NRG that was gained
is emitted as light;
 this is called Fluorescence
 this NRG cannot be used by the plant

22. Keep in Mind:
In order for the plant to use the NRG from the
excited e-, the NRG must be transferred to
another molecule.
The e- can follow 1 of 2 pathways:
a. Cyclic photophosphorylation
b. Non-cyclic photophosphorylation

23. B. Cyclic Photophosphorylation
Chlorophyll absorbs light
2. e- becomes excited & jumps to a higher
NRG level
3. e- returns to chlorophyll by traveling down
E.T.C. (produces ATP)
No oxygen produced b/c no e- deficit in
chlorophyll (thus no need to split water)

25. C. Non-cyclic Photophosphorylation
Results in the synthesis of ATP and NADPH + H+
The production involves two components:
a. Photosystem II
b. Photosystem I
} each has pigments and is
} activated by a diff. wavelength

26. 3. Steps:
a. PS II absorbs light (680 nm)  most reactive
b. 2e- become excited & jump to higher NRG level
 the chlorophyll now has a 2e- deficit
c. The deficit is paid by taking 2e- from H2O, thus
H2O is split (the O is released)
H20  2e- + 2H+ + ½ O2
The excited 2e- travel down E.T.C.
 NRG is released to form ATP
e. The 2e- reach the bottom & enters PS I
light absorbed (700 nm), 2e- excited; jump up
g. excited 2e- react w/ NADP+ and 2 H+ to form:
2e- + NADP H+  NADPH + H+

28.
So why cyclic over non-cyclic?
Sometimes an organism has all the reductive power (NADPH) that it needs to synthesize new carbon skeletons, but still needs ATP to power other
activities in the chloroplast. Many bacteria can shut
off PS2, allowing the production of ATP in the
absence of glucose  this is called cyclic photophosphorylation

29. ATP Production (The Chemiosmotic Model)
-- As e- moves down the E.T.C. b/n PS II and PS I,
H+ ions will be pumped from the stroma into the
interior of the thylakoid. (proton motive force)
They will collect along
the membrane. This creates an electrochemical
gradient (where the interior of the thylakoid is
more acidic than the outside). Thus, H+ ions
return to the stroma by diffusing across the
the thylakoid membrane thru ATP Synthase
“channels”. This action activates the synthase to
catalyze the formation of ATP.

32. Summary of Light Reactions

33. Dark Reactions
Introduction
1. Uses the products of the light rxns
(i.e. ATP and NADPH + H+) to
reduce CO2 & produce glucose

34. 2. Steps:
a. CO2 is “fixed” to a 5-C molecule called
RuBP (Ribulose Biphosphate).
b. This forms a 6-C molecule
c. This 6-C molecule is split into 2, 3-C molecules
called 3-phosphoglyceric acid (3-PGA)
PGA reduced to phosphoglyceraldehyde (PGAL)
 this is where the NADPH and ATP from light
rxns are used
ONE molecule of PGAL leaves to make glucose.
The other 5 molecules are used to regenerate
RuBP through a series of reactions
This occurs 5 more times in order to form a 6-C
compound  glucose

39. C. Modes of Photosynthesis
C3 plants – wheat, oats, rice
--Calvin cycle fixes CO2 directly (takes
place in mesophyll cells)
2. C4 plants – sugarcane, corn
--work best in mild climates
--CO2 is fixed by forming C4 molecule
prior to cycle (takes place in sheath cells)

41. CAM plants – flowering plants
--live in warm, arid regions
--Photosynthesis is minimal b/c of limited
fixation of CO2 at night