Photosynthesis
Chapter 8
Adapted by G. Cornwall, Ph.D.
From Raven’s Biology, McGraw Hill Publishing
Photosynthesis Overview
• Energy for all life on Earth ultimately
comes from photosynthesis
6CO2 + 12H2O
C6H12O6 + 6H2O + 6O2
• Oxygenic photosynthesis is carried out by
– Cyanobacteria
– 7 groups of algae
– All land plants – chloroplasts
2
Chloroplast
• Thylakoid membrane –
internal membrane
– Contains chlorophyll and other
photosynthetic pigments
– Pigments clustered into
photosystems
• Grana – stacks of flattened
sacs of thylakoid membrane
• Stroma lamella – connect
grana
• Stroma – semiliquid
surrounding thylakoid
membranes
3
Stages
• Light-dependent reactions
– Require light
1.Capture energy from sunlight
2.Make ATP and reduce NADP+ to NADPH
• Carbon fixation reactions or lightindependent reactions
– Does not require light
3.Use ATP and NADPH to synthesize organic
molecules from CO2
4
5
Pigments
• Molecules that absorb light energy in the visible
range
• Light is a form of energy
• Photon – particle of light
– Acts as a discrete bundle of energy
– Energy content of a photon is inversely proportional to
the wavelength of the light
• Photoelectric effect – removal of an electron from
a molecule by light
6
Absorption spectrum
• When a photon strikes a
molecule, its energy is
either
– Lost as heat
– Absorbed by the electrons
of the molecule
• Boosts electrons into higher
energy level
• Absorption spectrum –
range and efficiency of
photons molecule is
capable of absorbing
7
• Organisms have evolved
a variety of different
pigments
• Only two general types
are used in green plant
photosynthesis
– Chlorophylls
– Carotenoids
• In some organisms, other
molecules also absorb
light energy
8
Chlorophylls
• Chlorophyll a
– Main pigment in plants and cyanobacteria
– Only pigment that can act directly to convert
light energy to chemical energy
– Absorbs violet-blue and red light
• Chlorophyll b
– Accessory pigment or secondary pigment
absorbing light wavelengths that chlorophyll a
does not absorb
9
• Structure of
chlorophyll
• porphyrin ring
– Complex ring structure
with alternating double
and single bonds
– Magnesium ion at the
center of the ring
• Photons excite
electrons in the ring
• Electrons are shuttled
away from the ring
10
• Action spectrum
– Relative effectiveness of different
wavelengths of light in promoting
photosynthesis
– Corresponds to the absorption spectrum for
chlorophylls
11
• Carotenoids
– Carbon rings linked to
chains with alternating
single and double
bonds
– Can absorb photons
with a wide range of
energies
– Also scavenge free
radicals – antioxidant
• Protective role
• Phycobiloproteins
– Important in low-light
ocean areas
12
Photosystem Organization
• Antenna complex
– Hundreds of accessory
pigment molecules
– Gather photons and feed
the captured light energy to
the reaction center
• Reaction center
– 1 or more chlorophyll a
molecules
– Passes excited electrons
out of the photosystem
13
Antenna complex
• Also called light-harvesting
complex
• Captures photons from
sunlight and channels them to
the reaction center chlorophylls
• In chloroplasts, light-harvesting
complexes consist of a web of
chlorophyll molecules linked
together and held tightly in the
thylakoid membrane by a
matrix of proteins
14
Reaction center
• Transmembrane protein–
pigment complex
• When a chlorophyll in the
reaction center absorbs a
photon of light, an electron is
excited to a higher energy
level
• Light-energized electron can
be transferred to the primary
electron acceptor, reducing it
• Oxidized chlorophyll then fills
its electron “hole” by
oxidizing a donor molecule
15
Light-Dependent Reactions
1. Primary photoevent
Capture of light energy
– Photon of light is captured by a pigment molecule
2. Charge separation
– Energy is transferred to the reaction center; an
excited electron is transferred to an acceptor
molecule
3. Electron transport
– Electrons move through carriers to reduce NADP+
4. Chemiosmosis
– Produces ATP
16
Cyclic photophosphorylation
• In sulfur bacteria, only one
photosystem is used
• Generates ATP via
electron transport
• Anoxygenic
photosynthesis
• Excited electron passed to
electron transport chain
• Generates a proton
gradient for ATP synthesis
17
Chloroplasts have two connected
photosystems
• Oxygenic photosynthesis
• Photosystem I (P700)
– Functions like sulfur bacteria
• Photosystem II (P680)
– Can generate an oxidation potential high enough to
oxidize water
• Working together, the two photosystems carry out
a noncyclic transfer of electrons that is used to
generate both ATP and NADPH
18
19
• The Two Photosystems in Plants Work Together
– Photosystem II drives ATP production
• Electron is passed to the reaction center of PS I (Replaces
electron lost through excitation)
• the energy released is used to synthesize ATP
– Photosystem I drives NADPH production.
• Electron is passed to NADP+  NADPH
20
Noncyclic photophosphorylation
• Plants use photosystems II
and I in series to produce
both ATP and NADPH
• Path of electrons not a circle
• Photosystems replenished
with electrons obtained by
splitting water
• Z diagram
21
Chemiosmosis
• Electrochemical gradient used to synthesize ATP
• Chloroplast has ATP synthase enzymes in the
thylakoid membrane
– Allows protons back into stroma
• Stroma also contains enzymes that catalyze the
reactions of carbon fixation – the Calvin cycle
reactions
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• Making more ATP
– It takes more energy to fix Carbon (1 ½ ATP /
NADPH)
– PS I can short circuit to make extra ATP via Cyclic
phosphorylation.
Card Quiz A
What high energy molecule is the final product of
photosynthesis?
 Oxygen
 ATP
 NADPH
 Glucose
Card Quiz A
Where is the chlorophyll located in a plant?
 Cristae
 Outer membrane of the chloroplast
 Thylakoid membrane
 Stroma
Card Quiz A
A particle of light is a ____.
 Wave
 Photon
 Proton
 Newton
Card Quiz A
What happens at the reaction center of a photosystem?
 Light is absorbed
 An electron is energized
 NADP+ is reduced
 ATP is formed
Card Quiz A
Which of the following is made during the light reactions.
 ADP and NADP+
 Glucose and ATP
 ATP and NADPH
 NADPH and Carbon Dioxide
Card Quiz Answers
 Green
 Red
 Blue
 Green
 Green
Carbon Fixation – Calvin Cycle
• To build carbohydrates cells use
• Energy
– ATP from light-dependent reactions
– Cyclic and noncyclic photophosphorylation
– Drives endergonic reaction
• Reduction potential
– NADPH from photosystem I
– Source of protons and energetic electrons
30
Calvin cycle
• Named after Melvin Calvin (1911–1997)
• Also called C3 photosynthesis
• Key step is attachment of CO2 to RuBP to
form PGA
• Uses enzyme ribulose bisphosphate
carboxylase/oxygenase or rubisco
31
3 phases
1. Carbon fixation
– RuBP + CO2 → PGA
2. Reduction
– PGA is reduced to G3P
3. Regeneration of RuBP
– PGA is used to regenerate
RuBP
•
•
3 turns incorporate
enough carbon to produce
a new G3P
6 turns incorporate
enough carbon for 1
glucose
32
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Output of Calvin cycle
• Glucose is not a direct product of the
Calvin cycle
• G3P is a 3 carbon sugar
– Used to form sucrose
• Major transport sugar in plants
• Disaccharide made of fructose and glucose
– Used to make starch
• Insoluble glucose polymer
• Stored for later use
34
Energy cycle
• Photosynthesis uses the products of respiration
as starting substrates
• Respiration uses the products of photosynthesis
as starting substrates
• Production of glucose from G3P even uses part
of the ancient glycolytic pathway, run in reverse
• Principal proteins involved in electron transport
and ATP production in plants are evolutionarily
related to those in mitochondria
35
36
Photorespiration
• Rubisco has 2 enzymatic activities
– Carboxylation
• Addition of CO2 to RuBP
• Favored under normal conditions
– Photorespiration
• Oxidation of RuBP by the addition of O2
• Favored when stoma are closed in hot conditions
• Creates low-CO2 and high-O2
• CO2 and O2 compete for the active site on RuBP
37
Types of photosynthesis
• C3
– Plants that fix carbon using only C3 photosynthesis
(the Calvin cycle)
• C4 and CAM
–
–
–
–
–
Add CO2 to PEP to form 4 carbon molecule
Use PEP carboxylase
Greater affinity for CO2, no oxidase activity
C4 – spatial solution
CAM – temporal solution
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C4 plants
• Corn, sugarcane, sorghum, and a
number of other grasses
• Initially fix carbon using PEP
carboxylase in mesophyll cells
• Produces oxaloacetate, converted to
malate, transported to bundle-sheath
cells
• Within the bundle-sheath cells, malate
is decarboxylated to produce pyruvate
and CO2
• Carbon fixation then by rubisco and
the Calvin cycle
39
• C4 pathway, although it overcomes
the problems of photorespiration,
does have a cost
• To produce a single glucose
requires 12 additional ATP
compared with the Calvin cycle
alone
• C4 photosynthesis is advantageous
in hot dry climates where
photorespiration would remove more
than half of the carbon fixed by the
usual C3 pathway alone
40
CAM plants
• Many succulent (water-storing) plants,
such as cacti, pineapples, and some
members of about two dozen other plant
groups
• Stomata open during the night and close
during the day
– Reverse of that in most plants
• Fix CO2 using PEP carboxylase during the
night and store in vacuole
41
• When stomata closed during the day,
organic acids are decarboxylated to yield
high levels of CO2
• High levels of CO2 drive the Calvin cycle
and minimize photorespiration
42
Compare C4 and CAM
• Both use both C3 and C4 pathways
• C4 – two pathways occur in different cells
• CAM – C4 pathway at night and the C3
pathway during the day
43
Card Quiz B
Which structure is responsible for gas exchange ?
 Matrix
 Stroma
 Stoma
 Thylakoid
Question 4
Making glucose via photosynthesis costs ____ATPs and
___NADPHs. However, 1 molecule of glucose is enough
to make ___ ATPs.
 18, 12, 36
 12, 18, 36
 12, 36, 18
 36, 18, 12
Question 9
What is the function of Rubisco?
 Absorption of photon energy
 Reduction of NADP+
 Chemiosmosis
 Carbon fixation
Card Quiz B
Where could a botanist expect to find C4 plants?
 Canada
 Costa Rica
 Tundra
 Mount Everest
Card Quiz B
The end product of photosynthesis is the starting
material of cellular respiration.
a. This is true
b. This is false
Card Quiz B
Which of the following would prevent the formation
of phosphoglycerate?
 Denaturation of the Rubisco enzyme
 Non-functional b6-f complex
 Anaerobic conditions
 All of the above