Photosynthesis:
Life from Light
AP Biology
Energy needs of life
 All life needs a constant input of energy

Heterotrophs
 get their energy from “eating others”
 consumers of other organisms
 consume organic molecules

Autotrophs
 get their energy from “self”
 get their energy from sunlight
 use light energy to synthesize organic
molecules
AP Biology
How are they connected?
Heterotrophs
making energy & organic molecules from ingesting organic molecules
glucose + oxygen  carbon + water + energy
dioxide
C6H12O6 +
6O2
 6CO2 + 6H2O + ATP
Autotrophs
making energy & organic molecules from light energy
carbon + water + energy  glucose + oxygen
dioxide
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
AP Biology
2005-2006
Energy cycle
sun
Photosynthesis
CO2
H 2O
glucose
Cellular Respiration
The Great Circle
of Life!
AP Biology Where’s Mufasa?
ATP
O2
What does it mean to be a plant
 Need to…

collect light energy
 transform it into chemical energy

store light energy
 in a stable form to be moved around the plant
& also saved for a rainy day

need to get building block atoms from
the environment
 C,H,O,N,P,S

produce all organic molecules needed for
growth
 carbohydrates, proteins, lipids, nucleic acids
AP Biology
Plant structure
 Obtaining raw materials

sunlight
 leaves = solar collectors

CO2
 stomates = gas exchange
regulation
 Found under leaves

H2O
 uptake from roots

nutrients
 uptake from roots
AP Biology
2005-2006
Plant structure
 Chloroplasts
double membrane
 stroma
 thylakoid sacs
 grana stacks

 Chlorophyll & ETC in
thylakoid membrane

H+ gradient built up
within thylakoid sac
H+
AP Biology
+
+ H + H H+
+
H
H
+ H+ H+ H+
+
H
H
Pigments of photosynthesis
 chlorophyll & accessory
Why does this
structure
make sense?
pigments
“photosystem”
 embedded in thylakoid
membrane
 structure  function
AP Biology

2005-2006
Light: absorption spectra
 Photosynthesis performs work only with
absorbed wavelengths of light


AP Biology
chlorophyll a — the dominant pigment —
absorbs best in red & blue wavelengths & least
in green
other pigments with different structures have
different absorption spectra
Photosystems
 Photosystems

collections of chlorophyll molecules
 2 photosystems in thylakoid membrane


act as light-gathering “antenna complex”
Photosystem II
 chlorophyll a
 P680 = absorbs 680nm
wavelength red light

Photosystem I
 chlorophyll b
 P700 = absorbs 700nm
wavelength red light
AP Biology
Transfer of Electrons in PSII & PSI
 In both PSII and PSI, the energy from the


excited e- pumps H+ into the thylakoid as
it moves through the ETC.
Electrons from PSII are transferred to PSI.
After electrons have moved through PSI,
an intermediary molecule (embedded in
the membrane and adjacent to PSI)
transfers the e- to NADP+. A H+ is
attracted to this molecule and NADPH is
formed.
AP Biology
Chemiosmosis in Photosynthesis
**(similar in Cell Respiration)
proton (H+)
gradient across
inner membrane
drive ATP formation
 ATP synthase
enzyme

AP Biology
2005-2006
Summary of the LDR
 PS II absorbs light

excited electron passes from chlorophyll to
“primary electron acceptor” at the REACTION
CENTER.
 splits H2O (Photolysis!!)
 O2 released to atmosphere
 PS I absorbs light

Produces NADPH (stored energy) which will be
used by the Calvin cycle
 Chemiosmosis produces ATP from light
energy
 ATP will be used by the Calvin Cycle
AP Biology
ETC of Photosynthesis
 Chloroplasts transform light
energy into chemical energy
of ATP

AP Biology
split H2O
use electron carrier NADPH
2 Photosystems
 Light reactions
elevate electrons in
2 steps (PS II & PS I)

PS II helps generate
energy as ATP (H+
pumps)

PS I generates
reducing power as
NADPH
This shows Noncyclic
photophosphorylation.
AP Biology
ETC of Photosynthesis
AP Biology
ETC of Photosynthesis
AP Biology
Cyclic photophosphorylation
 If PS I can’t pass
electron to NADP,
it cycles back to
PS II & makes
more ATP, but no
NADPH


AP Biology
coordinates light
reactions to Calvin
cycle
Calvin cycle uses
more ATP than
NADPH
Do Now: Light Reactions Summary Questions
Where did the energy come from?
Where did the H2O come from?
Where did the electrons come from?
Where did the O2 come from?
Where did the H+ come from?
Where did the ATP come from?
Where did the O2 go?
What will the ATP be used for?
What will the NADPH be used for?
AP Biology
Calvin Cycle Overview
 Calvin cycle

uses chemical energy
(NADPH & ATP)
to reduce CO2 to
build C6H12O6 (sugars)
AP Biology
From Light reactions to Calvin cycle
 Calvin cycle

Occurs in the stroma of the chloroplast
 Need products of light reactions to
drive synthesis reactions
ATP
 NADPH

AP Biology
From CO2  C6H12O6
 CO2 has very little chemical energy

fully oxidized
 C6H12O6 contains a lot of chemical energy
reduced
 endergonic

 Reduction of CO2  C6H12O6 proceeds in
many small uphill steps
each catalyzed by specific enzyme
 using energy stored in ATP & NADPH

AP Biology
Calvin cycle
1C
ribulose bisphosphate
3. Regeneration
RuBP
3 ATP
PGAL
to make
glucose
5C
1. Carbon fixation
Rubisco
ribulose
bisphosphate
carboxylase
3 ADP
PGAL
sucrose
cellulose
etc.
CO2
6C
unstable
intermediate
2x 3C
3C x2
PGA
2. Reduction
6 ATP
6 NADPH
6 NADP
AP Biology
2x
3C
6 ADP
Rubisco
 Enzyme which fixes carbon from
atmosphere
ribulose bisphosphate carboxylase
 the most important enzyme in the world!

 it makes life out of air!

AP Biology
definitely the most abundant enzyme
Calvin cycle
 PGAL
end product of Calvin cycle
 energy rich sugar
 3 carbon compound
 “C3 photosynthesis”

 PGAL   important intermediate
PGAL  



AP Biology
glucose   carbohydrates
lipids
amino acids
nucleic acids
Photosynthesis summary
 Light reactions
produced ATP
 produced NADPH
 consumed H2O
 produced O2 as byproduct

 Calvin cycle
consumed CO2
 produced PGAL
 regenerated ADP
 regenerated NADP

AP Biology
Summary of photosynthesis
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy










Where did the CO2 come from?
Where did the CO2 go?
Where did the H2O come from?
Where did the H2O go?
Where did the energy come from?
What’s the energy used for?
What will the C6H12O6 be used for?
Where did the O2 come from?
Where will the O2 go?
What else is involved that is not listed in this
equation?
AP Biology
2005-2006