Photosynthesis 1: The Light Reactions
(Ch. 10)
Energy needs of life
• All life needs a constant input of energy
– Heterotrophs (Animals)
• get their energy from “eating others”
– eat food = other organisms = organic molecules
consumers
• make energy through respiration
– Autotrophs (Plants)
•
•
producers
•
•
produce their own energy (from “self”)
convert energy of sunlight
build organic molecules (CHO) from CO2
make energy & synthesize sugars through
photosynthesis
How are they connected?
Heterotrophs
making energy & organic molecules from ingesting organic molecules
glucose + oxygen  carbon + water + energy
dioxide
C6H12O6 +
6O2
 6CO2 + 6H2O + ATP
oxidation = exergonic
Autotrophs
making energy & organic molecules from light energy
Where’s
the ATP?
carbon + water + energy  glucose + oxygen
dioxide
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
reduction = endergonic
What does it mean to be a plant?
• Need to…
ATP
– collect light energy
• transform it into chemical energy
– store light energy
glucose • in a stable form to be moved around
the plant or stored
– need to get building block atoms
CO2
from the environment
H2O
• C,H,O,N,P,K,S,Mg
N
K P
– produce all organic molecules
…
needed for growth
• carbohydrates, proteins, lipids, nucleic acids
Plant structure
• Obtaining raw materials
– sunlight
• leaves = solar collectors
– CO2
• stomates = gas exchange
– H2O
• uptake from roots
– nutrients
• N, P, K, S, Mg, Fe…
• uptake from roots
Chloroplasts
leaves
cross section
of leaf
absorb
sunlight & CO2
CO2
chloroplasts
in plant cell
chloroplast
chloroplasts
contain
chlorophyll
make
energy & sugar
Plant structure
chloroplast
H+
• Chloroplasts
– double membrane
– stroma
• fluid-filled interior
– thylakoid sacs
– grana stacks
ATP
+
+ H+ H H+
+
H
H
+ H+ H+ H+
+
H
H
thylakoid
outer membrane
inner membrane
stroma
• Thylakoid membrane contains
– chlorophyll molecules
– electron transport chain
– ATP synthase
• H+ gradient built up within
thylakoid sac
thylakoid
granum
Photosynthesis
• Light reactions
– light-dependent reactions
– energy conversion reactions
• convert solar energy to chemical energy
• ATP & NADPH
It’s not the
• Calvin cycle
Dark Reactions!
– light-independent reactions
– sugar building reactions
• uses chemical energy (ATP & NADPH) to
reduce CO2 & synthesize C6H12O6
Light reactions
thylakoid
chloroplast
• Electron Transport Chain
• like in cellular respiration
– proteins in organelle membrane
– electron acceptors
• NADPH
– proton (H+)
gradient across
inner membrane
– ATP synthase
enzyme
+H+ H+ H+
+ + +
H+ H+H
+H+ H H H
H
ATP
+H+ H+ H+
H+ H+H
+ + + +
H+H H H H
ETC of Respiration
Mitochondria transfer chemical energy from food molecules
into chemical energy of ATP

use electron carrier NADH
generates H2O
ETC of Photosynthesis
Chloroplasts transform light energy
into chemical energy of ATP

generates O2
use electron carrier NADPH
The ATP that “Jack” built
photosynthesis
sunlight
respiration
breakdown of C6H12O6
H+
H+
 moves the electrons
H+
H+
H+
H+
H+
H+
 runs the pump
 pumps the protons
 builds the gradient
 drives the flow of protons
ADP + Pi
through ATP synthase
 bonds Pi to ADP
ATP
 generates the ATP
… that evolution built
H+
Pigments of photosynthesis
• Chlorophylls & other pigments
– embedded in thylakoid membrane
– arranged in a “photosystem”
• collection of molecules
– structure-function relationship
How does this
molecular structure
fit its function?
A Look at Light
• The spectrum of color
V
I
B
G
Y
O
R
Light: absorption spectra
• Photosynthesis gets energy by absorbing wavelengths of light
– chlorophyll a
• absorbs best in red & blue wavelengths & least in green
– accessory pigments with different structures absorb light of
different wavelengths
• chlorophyll b, carotenoids, xanthophylls
Why are
plants green?
Photosystems of photosynthesis
• 2 photosystems in thylakoid membrane
– collections of chlorophyll molecules
– act as light-gathering molecules
– Photosystem II
reaction
center
• chlorophyll a
• P680 = absorbs 680nm
wavelength red light
– Photosystem I
• chlorophyll b
• P700 = absorbs 700nm
wavelength red light
antenna
pigments
chlorophyll a
ETC of Photosynthesis
Photosystem II
chlorophyll b
Photosystem I
ETC of Photosynthesis
sun
1
e
e
Photosystem II
P680
chlorophyll a
Inhale, baby!
ETC of Photosynthesis
thylakoid
chloroplast
+H+ H+ H+
+ + +
H+ H+H
+H+ H H H
H
H+
ATP
+H+ H+ H+
+
H
H + + H+H+ H+
HH
Plants SPLIT water!
H H
1
O
H
e-
e e
fill the –e vacancy
Photosystem II
P680
chlorophyll a
H+
e-
+H
OO
e
e
H
2
ETC of Photosynthesis
thylakoid
chloroplast
H+
+H+ H+ H+
+
H
H + + H+H+ H+
HH
+H+ H+ H+
H+ H+H
+ + + +
H+H H H H
ATP
3
2
1
e
e
H+
4
ATP
H+
to Calvin Cycle
H+
H+
H+
Photosystem II
P680
chlorophyll a
H+
H+
+
H+ H
ADP + Pi
ATP
H+
H+
energy to build
carbohydrates
ETC of Photosynthesis
e
e
5
e e
Photosystem II
P680
chlorophyll a
Photosystem I
P700
chlorophyll b
sun
ETC of Photosynthesis
electron carrier
6
e
e
5
sun
Photosystem II
P680
chlorophyll a
Photosystem I
P700
chlorophyll b
$$ in the bank…
reducing power!
ETC of Photosynthesis
sun
sun
+
+
+ H
H
+
+
H+ H +
H H
H+H+ H+ H
+
H
to Calvin Cycle
O
split H2O
ATP
ETC of Photosynthesis
• ETC uses light energy to produce
– ATP & NADPH
• go to Calvin cycle
• PS II absorbs light
– excited electron passes from chlorophyll to “primary
electron acceptor”
– need to replace electron in chlorophyll
– enzyme extracts electrons from H2O & supplies them
to chlorophyll
• splits H2O
• O combines with another O to form O2
• O2 released to atmosphere
• and we breathe easier!
Experimental evidence
• Where did the O2 come from?
– radioactive tracer = O18
Experiment 1
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
Experiment 2
6CO2 + 6H2O + light  C6H12O6 + 6O2
energy
Proved O2 came from H2O not CO2 = plants split H2O!
Noncyclic Photophosphorylation
• Light reactions elevate
electrons in
2 steps (PS II & PS I)
– PS II generates
energy as ATP
– PS I generates
reducing power as NADPH
ATP
Cyclic photophosphorylation
• If PS I can’t pass electron to
NADP…it cycles back to PS II
& makes more ATP, but no
NADPH

– coordinates light reactions to
Calvin cycle
– Calvin cycle uses more ATP
than NADPH
ATP
18 ATP +
12 NADPH
 1 C6H12O6
Photophosphorylation
cyclic
photophosphorylation
NADP
NONcyclic
photophosphorylation
ATP
You can grow if you
Ask Questions!
Review Questions
1. Below is an absorption spectrum for an
unknown pigment molecule. What color would
this pigment appear to you?
A.
B.
C.
D.
E.
violet
blue
green
yellow
red
2. In green plants, most of the ATP for synthesis of
proteins, cytoplasmic streaming, and other
cellular activities comes directly from
A.
B.
C.
D.
E.
photosystem I.
the Calvin cycle.
oxidative phosphorylation.
noncyclic photophosphorylation.
cyclic photophosphorylation.
3. What portion of an illuminated plant cell would
you expect to have the lowest pH?
A.
B.
C.
D.
E.
nucleus
vacuole
chloroplast
stroma of chloroplast
thylakoid space
4. A new flower species has a unique photosynthetic
pigment. The leaves of this plant appear to be
reddish yellow. What wavelengths of visible light
are not being absorbed by this pigment?
A.
B.
C.
D.
E.
red and yellow
blue and violet
green and yellow
blue, green, and red
green, blue, and violet
5. Assume a thylakoid is somehow punctured
so that the interior of the thylakoid is no longer
separated from the stroma. This damage will have
the most direct effect on which of the following
processes?
A. the splitting of water
B. the absorption of light energy by chlorophyll
C. the flow of electrons from photosystem II to
photosystem I
D. the synthesis of ATP
E. the reduction of NADP+