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Chapter 8: Photosynthesis: Energy from the Sun
CHAPTER 8
Photosynthesis: Energy
from the Sun
Chapter 8: Photosynthesis: Energy from the Sun
Photosynthesis
Biochemical process in which light
energy is converted to chemical
energy
 Photos = light
synthesis = to put together
 In plants, photosynthesis takes place
in chloroplasts.
 It involves many enzyme controlled
steps

Chapter 8: Photosynthesis: Energy from the Sun
Photosynthetic Reactants
and Products
6 CO2 + 12 H2O + light  C6H12O6 + 6
O 2 + 6 H 2O
Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.1
Figure 8.1
figure 08-01.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Photosynthesis

Photosynthesis can be
divided into two
pathways:
– The light reaction driven by light energy
captured by chlorophyll.
Consists of Photosystem
I and Photosystem II. It
produces ATP and
NADPH + H+.
– The Calvin–Benson
cycle - does not use
light directly. It uses ATP,
NADPH + H+, and CO2
to produce sugars.
Chapter 8: Photosynthesis: Energy from the Sun
Properties of Light and
Pigments
Light is the source of energy that
drives photosynthesis
 Molecules that absorb light energy in
the visible range are called pigments.

Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.5
Figure 8.5
figure 08-05.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Properties of Light and
Pigments
When light and a pigment molecule meet, one
of 3 things happen
 Reflection – the light bounces off the
molecule
 Transmission – the light passes through the
molecule
 Excitation – the light is absorbed by the
molecule. If absorbed, the molecule goes
from its ground state to and excited state of
higher energy
 An electron is boosted to another orbital
Chapter 8: Photosynthesis: Energy from the Sun
Pigments

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When a beam of white light shines on an
object, and the object appears to be red in
color, it is because it has absorbed all other
colors from the white light except for the
color red.
In the case of chlorophyll, plants look green
because they absorb green light less
effectively than the other colors found in
sunlight and reflect the green light not
absorb
Chapter 8: Photosynthesis: Energy from the Sun
Properties of Light and
Pigments
Different pigment molecules absorb
different wavelengths of light
 The particular set of wavelengths that
a pigment absorbs is called its
absorption spectrum
 Review Figures 8.7

Chapter 8: Photosynthesis: Energy from the Sun
Figure
8.7
Figure 8.7
figure 08-07.jpg
Chapter 8: Photosynthesis: Energy from the Sun
Properties of Light and
Pigments
Chlorophylls are the most important
pigments in photosynthesis
 Chlorophyll a is the primary pigment in
photosynthesis.
 Chlorophylls and accessory pigments
trap light and transfer energy to a
reaction center

Chapter 8: Photosynthesis: Energy from the Sun
Chlorophyll
Chapter 8: Photosynthesis: Energy from the Sun
An excited pigment molecule may
 lose its energy by emitting light of
longer wavelength or
 transfer the absorbed energy to
another pigment molecule as a redox
reaction.
Figure
8.88:Energy
Transfer and
Electron
Transport
Chapter
Photosynthesis:
Energy
from the
Sun
Chapter 8: Photosynthesis: Energy from the Sun

There are two different systems for
transport of electrons in
photosynthesis.
1. Noncyclic electron transport produces
NADPH + H+ and ATP and O2.
2. Cyclic electron transport produces
only ATP.
Chapter 8: Photosynthesis: Energy from the Sun
Noncyclic
In noncyclic electron transport, two
photosystems are required.
 Photosystems consist of many
chlorophyll molecules and accessory
pigments bound to proteins.

Chapter 8: Photosynthesis: Energy from the Sun
Photosystem I
Photosystem I uses light energy to
reduce NADP+ to NADPH + H+.
 The reaction center contains a
chlorophyll a molecule called P700
because it best absorbs light at a
wavelength of 700 nm.

Chapter 8: Photosynthesis: Energy from the Sun
Photosystem II

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Photosystem II uses light energy to split
water, producing electrons, protons, and O2.
The reaction center contains a chlorophyll a
molecule called P680 because it best
absorbs light at a wavelength of 680 nm.
To keep noncyclic electron transport going,
both photosystems must constantly be
absorbing light.
Chapter 8: Photosynthesis: Energy from the Sun
After absorbing light energy:
 an energized electron leaves the Chl*
in the reaction center and participates
in a series of redox reactions.
 the electron flows through a series of
carriers in the thylakoid membrane.
 producing ATP
Figure
8. 98:Noncyclic
Electron
Transport
Uses
Two Photosystems (Part 1)
Chapter
Photosynthesis:
Energy
from the
Sun
Figure
8. 98:Noncyclic
Electron
Transport
Uses
Two Photosystems (Part 2)
Chapter
Photosynthesis:
Energy
from the
Sun
Chapter 8: Photosynthesis: Energy from the Sun
Cyclic Electron Transfer
Cyclic
electron transport produces
only ATP.
The electron passes from an excited
P700 molecule and cycles back to the
same P700 molecule.
No O2 is released.
In cyclic electron flow, photosystem I
acts on its own.
Figure
8.10
Cyclic Electron Transport
Traps
Light Energy as ATP
Chapter
8: Photosynthesis:
Energy from
the Sun
Chapter 8: Photosynthesis: Energy from the Sun
“Z” Scheme
Photosystem I & II (P680 & P700)
work together to generate ATP and
NADPH.
 This pathway is called the “Z” scheme.
 Noncyclic

Chapter 8: Photosynthesis: Energy from the Sun
Noncyclic Electron Flow
or Z Scheme
In Photosystem II chlorophyll a
absorbs light energy to become
energized chloropyll a
 2 electrons are released and caught by
the primary electron acceptor.
 H20  ½ O2 + 2 e- + 2H+

Chapter 8: Photosynthesis: Energy from the Sun

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The electrons pass through a redox chain
for chemiosmotic ATP production.
The electron transport chain pumps protons
across the membrane into the thylakoid
space.
The protons accumulate establishing a
proton concentration gradient
ATP synthases open and the protons diffuse
to generate ATP from ADP.
Chapter 8: Photosynthesis: Energy from the Sun
Z Scheme Cont’d

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The electrons are passed to P700 chlorophyll
P700 loses electrons to Ferredoxin (Fd)
NADP combines with H to form NADPH.
NADPH is the source of H used to make
C6H12O6
Figure
8.118: Photosynthesis:
Chloroplasts Form
ATPfrom
Chemiosmotically
Chapter
Energy
the Sun
Chapter 8: Photosynthesis: Energy from the Sun
The Calvin–Benson Cycle
The Calvin–Benson cycle makes sugar
from CO2
 ATP and NADPH provide the needed
energy
 This pathway was elucidated through
use of radioactive tracers

Chapter 8: Photosynthesis: Energy from the Sun
The Calvin–Benson Cycle
Three phases:
 1. Carbon Fixation – RuBP + CO2  6 carbon
sugar  3 PG (first stable product)
 The reaction is catalyzed by rubisco (ribulose
bisphosphate carboxylase).
 2. Series of reactions to produce G3P
 3. Regeneration of RuBP (7 enzymatic steps)
 RuBP (ribulose biphosphate) is the initial CO2
acceptor
Chapter 8: Photosynthesis: Energy from the Sun
The Calvin–Benson Cycle
The
end product of the cycle is
glyceraldehyde 3-phosphate, G3P.
There are two fates for the G3P:
–One-third ends up as starch, which is
stored in the chloroplast and serves as a
source of glucose.
–Two-thirds is converted to the
disaccharide sucrose, which is
transported to other organs.
Figure
8.13
The Calvin-Benson
Cycle
Chapter
8: Photosynthesis:
Energy
from the Sun
Chapter 8: Photosynthesis: Energy from the Sun
Rubisco
Rubisco
is a carboxylase, adding
CO2 to RuBP. It can also be an
oxygenase, adding O2 to RuBP.
These two reactions compete with
each other.
When RuBP reacts with O2, it cannot
react with CO2, which reduces the
rate of CO2 fixation.
Chapter 8: Photosynthesis: Energy from the Sun
Photorespiration
A specialized metabolic pathway in
which rubisco reacts with O2 instead of
CO2
 Occurs under stress conditions of hot,
dry, bright days when the internal leaf
concentration of O2 is greater than CO2
concentration.
 Glucose production is reduced thereby
limiting plant growth

Chapter 8: Photosynthesis: Energy from the Sun
C3 Plants
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Most common type of plants on earth.
Grow best in temperate zones
Includes rice, wheat, soybeans, bluegrass
On hot days the stomata close, O2 builds up
and photorespiration occurs.
The first product is the 3-C molecule of 3PG
CO2 + RuBP  3 phosophoglycerate (3 C
compound)
Figure
8.16
Leaf Anatomy ofEnergy
C3 andfrom
C4 Plants
Chapter
8: Photosynthesis:
the Sun
Chapter 8: Photosynthesis: Energy from the Sun
C4 Plants
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C4 plants have 2 enzymes (PEP
carboxylase & rubisco) for CO2 fixation in 2
different parts of the leaf.
PEP carboxylase does not have an affinity
for O2 and fixes CO2 even at very low CO2
levels.
What is the significance of this fact?
C4 plants include sugarcane, corn and other
plants that grow in hot, dry climates.
Chapter 8: Photosynthesis: Energy from the Sun
C4 Plants Cont’d
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CO2 + PEP carboxylase  Oxaloacetate (4
C compound).
Occurs in cells near top of leaf
Oxaloacetate diffuses into bundle sheath
cells in the interior of the cells.
Here oxaloacetate loses a C forming CO2
CO2 enters the Calvin-Benson Cycle
Chapter 8: Photosynthesis: Energy from the Sun
Crassulacean Acid
Metabolism (CAM)
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CAM plants are succulents or water storing
plants.
Include cacti and pineapples
CAM plants open their stomata only at night
CO2 enters and forms malic acid which is
stored as an acid in the vacuoles until
morning
In daylight, the CO2 is released from the
acid and enters the Calvin Benson Cycle.
Chapter 8: Photosynthesis: Energy from the Sun
Stomates
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Stomates close when weather is hot & dry.
O2 concentration increases, CO2
concentration decreases.
Why?
Ribulose requires high concentrations of
CO2
If sufficient CO2 is unavailable,
photorespiration occurs.
Chapter 8: Photosynthesis: Energy from the Sun
Metabolic Pathways in
Plants
Both photosynthesis and respiration
occurs in plants.
 Compare photosynthesis and
respiration.
