Ch. 7
Capturing Solar Energy:
PHOTOSYNTHESIS
http://pbskids.org/sid/videoplayer.html (Sid Science Kid Song)
CASE STUDY: Did the Dinosaurs Die from Lack of Sunlight?
About 65 mya, the Cretaceous-Tertiary (K-T) extinction event brought
the Cretaceous period to a violent end and life on Earth suffered a
catastrophic blow. The fossil record indicates that a devastating mass
extinction eliminated at least 50% of all forms of life known to exist at that
time. The 160-million-year reign of the dinosaurs, including the massive
Triceratops and its predator Tyrannosaurus rex, ended abruptly. It would
be many millions of years before Earth become repopulated with a diversity
of species even approaching that of the late Cretaceous.
In 1980, Luis Alvarez, a Nobel Prize-winning physicist, his geologist son
Walter Avarez, and nuclear chemists Helen Michel and Frank Asaro
published a controversial hypothesis. They proposed than an invader from
outer space-a massive asteroid-had brought the Cretaceous period to an
abrupt and violent end. Their evidence consisted of a thin layer of clay
deposited at the end of the Cretaceous period, found at sites throughout the
world. Known as the “K-T boundary layer,” this clay deposit contains from
30 to 160 times the iridium level typical of Earth’s crust. Iridium is a
silvery-white metal that is extremely rare in Earth’s crust, but abundant in
certain types of asteroids.
How large must an iridium-rich asteroid have been to create the K-T
boundary layer? The Alvarez team calculated this iridium-enriched
space rock must have been at least 6 miles (10km) in diameter. In the
Alvarez scenario, as the asteroid ploughed into Earth at nearly 45,000
mph, its impact released energy greater than 100 trillion tons of TNT,
blasting out a plume of debris that likely extended halfway to the moon.
The asteroid’s fragments, plummeting back into the atmosphere in a
fiery shower, ignited widespread wildfires. A shroud of dust and soot
blocked the sun’s rays, and cooling darkness enveloped Earth.
Although the immediate destruction from fire, earthquakes,
landslides, and tsunamis must have been almost unimaginable, these
paled in contrast to the prolonged effects of the collision. How could
an asteroid impact have eliminated half of all forms of life? Its most
damaging long-term effect would have been disruptrion of the most
important biochemical pathway on Earth: photosynthesis. What
exactly occurs during photosynthesis? What makes this process so
important that interrupting it ended the age of the dinosaurs?
Introduction to Photosynthesis

7.1 What is Photosynthesis?

7.2 The Light Reactions: How Is Light
Energy Converted to Chemical
Energy?
7.3 The Calvin Cycle: How Is
Chemical Energy Stored in Sugar
Molecules?

7.1 What is Photosynthesis
energy required for all life forms
is directly / indirectly derived from
sunlight
 Photosynthesis – process by which light
energy is captured and stored as chemical energy in bonds of organic
molecules (i.e. sugar)
 organisms capable of trapping solar energy
- photosynthetic protists, certain bacteria, land plants
 capture close to 100 billion tons of CARBON / yr
 photosynthetic organisms eventually feed all other forms of
life.

Leaves and Chloroplasts Are Adaptations for Photosynthesis


Leaf structure: flat (increases surface area) & thin (allows sunlight
to penetrate to reach light-trapping chloroplasts)
https://www.youtube.com/watch?v=co0JdqUlycg
 Epidermis (upper and lower layers that consist of transparent
cells)  protect inner parts while light penetrates
 Cuticle (transparent, waxy, waterproof covering of epidermis)
 reduces evaporation of water from leaf
 Stomata (stoma - mouth)  adjustable pores in epidermis that
allow CO2 to pass through
 Mesophyll (middle leaf) – layers of cells where most
chloroplasts are located
 Vascular bundles (veins in leaf)  supply water and minerals
to leaf’s cells and transport sugar product to other parts of pant
 Bundle sheath cells (surround vascular bundle)  lack
chloroplasts in most plants
Chloroplasts
Stomata
Open
Closed
Several Stomata
Showing Guard Cells

Chloroplast (photosynthetic organelle) – found w/in mesophyll

double membrane
Light Reactions Sunlight  chemical energy;
H2O is split and rleases O2
Calvin cycle (Dark Reactions) –
capture C from CO2 to produce sugar
CASE STUDY: Did the Dinosaurs Die from Lack of Sunlight?
More than 2 billion years ago, some bacterial (prokaryotic)
cells, through chance mutations, acquired the ability to harness
the energy of sunlight. Exploiting this abundant energy
source, early photosynthetic cells filled the seas. As their
numbers increased, oxygen began to accumulate in the
atmosphere, radically altering Earth’s environment. Later,
plants evolved, made the transition to land, and eventually
grew in luxuriant profusion. By Cretaceous period, plants had
become sufficiently abundant to provide enough food to
support plant-eating giants, such as th3e 8-ton Triceratops on
which huge meat-eaters, such as the 40-foot-long
Tyrannosaurus, may have preyed or scavenged. Then, as now,
energy harvested through photosynthesis sustained nearly all
forms of life.
Photosynthesis Consists of Light Rxns and the Calvin Cycle

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Involves dozens of rxns each catalyzed by separate enzymes
2 different stages occur in 2 different locations but connected by
energy-carrier molecules
Light Reactions (photo) chlorophyll w/in thylakoid membranes
capture sunlight energy convert it to chemical energy stored in ATP
+ NADPH; water splits releases O2
Calvin Cycle (Dark Reactions) (synthesis)  enzymes in stroma
use CO2 from air and chemical energy in ATP + NADPH to make
3-C sugar required to make glucose
7.2 The Light Reactions:
How Is Light Energy Converted to Chemical Energy
light is electromagnetic radiation composed of packets of energy
called photons
- travels in a vacuum at186,000miles/sec
 other examples include microwaves, radio waves, X-rays, gamma
rays
 each type has a different wavelength and frequency

HIGH
ENERGY
LOW
ENERGY
 visible light  wavelengths with energies able to alter biological
pigments (chlorophyll) but not high enough to damage DNA;
stimulates pigments in our eyes to see
 light that hits an object (leaf) can be absorbed (captured),
transmitted (passed through) or reflected (bounced back).
- light reflected or transmitted seen as color of object
- absorbed light drives biological processes (photosynthesis)
 Chlorophyll a (chloroplast pigment) - absorbs violet, blue, and
red light  reflects GREEN
 Accessory pigments
1. chlorophyll b – absorb add. wavelengths (blue and redorange) missed by chlorophyll a  reflects yellow-green
2. Carotenoids (i.e. beta carotene) – absorb blue and green 
reflect yellow + orange
 beta-carotene  Vit A  makes light capturing pigment in
eyes
Why are leaves green if they contain Carotenoids?
The Light Reactions Occur in Association with the
Thylakoid Membranes
 photosystem II and photosystem I – cluster of chlorophyll and
accessory pigments within thylakoid membranes
 electron transport chain (ETC) – series of electron-carrier
molecules adjacent to photosystems
 PS II  ETC II  PS I  ETC I  NADP
Photosystem II Uses Light Energy to Create a Hydrogen Ion
Gradient and to Split Water
1. Light is absorbed by PS II and energy is passed to an e- in one of
the chlorophyll a molecules w/in rxn ctr
2. Energized e- is ejected from chlorophyll a molecule
3. Energized ejected e- is captured by a primary e- acceptor in rxn ctr
4. High-energy e- is passed through ETC II
5. As ETC II transfers e- along, some released energy is used to
generate ATP; energy depleted e- enters rxn ctr of PS I replacing
ejected e- there
6. Light strikes PS I, and energy is passed to the e- in rxn ctr
chlorophyll molecules
7. Energized e- is ejected from rxn ctr and captured by primary eacceptor
8. E- moves down ETC I
9. NADPH is formed when NADP+ in stroma picks up 2 energized
e-, along with 1 H+
Figure 7-6 Energy transfer and the light reactions of photosynthesis
H2O
CO2
ATP
light
reactions
Calvin
cycle
NADPH
ADP
NADP
sugar
O2
C6H12O6
high
e
e
electron
transport
chain I
e
energy level of electrons
primary
electron
acceptor
NADPH
NADP
e
e
electron
transport
chain II
light
energy
pigment
molecules
e
ATP
reaction center
chlorophyll a molecules
Photosystem I
e
Photosystem II
low
2
H
H2O
½
O2
H
https://www.youtube.com/watch?v=joZ1EsA5_NY
rxns and Calvin cycle
- good leaf structure and light review; details about light
http://www.johnkyrk.com/photosynthesis.html - different view of Light Rxns
 The Hydrogen Ion Gradient Generates ATP by
Chemiosmosis
1. Energy released as e- passes through ETC II is harnessed to pump
H+ across thylakoid membrane and into thylakoid space
2. High concentration gradient of H+ is generated
3. During chemiosmosis, H+ flows down its concentration gradient
through ATP synthase channels, which uses energy from gradient
to generate ATP
1 ATP made for every 3 H+ passed through the channel
https://www.youtube.com/watch?v=LtecIPc30nM – chemiosmosis
Case Study: Did the Dinosaurs Die from Lack of Sunlight?
Scientists have calculated that the force of the asteroid impact blasted
some debris so far into outer space that it took days to rain back down to
Earth. Earth’s rotation while the debris was aloft meant that much of the
material fell on regions far from the point of impact. The plummeting
chunks of rocks would have made a flaming re-entry through Earth’s
atmosphere, setting huge fires on nearly every continent. Oxygen levels
were high in the Cretaceous atmosphere, intensifying the fires. A large
portion of Earth’s vegetation was likely consumed by fire, and many of
the plants that managed to survive the fires must have succumbed during
the cold, dark “global winter” that began as the planet was encompassed
by soot and dust. Plant-eating animals that survived the initial blast
would have soon starved, especially enormous ones like the 12-ton
Triceratops, which needed to consume hundreds of pounds of vegetation
daily. Predators such as the Tyrannosaurus, which relied on plant-eaters
for food, would have died soon afterward. During the Cretaceous, as
now, interrupting the vital flow of solar energy captured by
photosynthetic organisms would be catastrophic.
7.3 The Calvin Cycle: How is Chemical Energy Stored in Sugar Molecules?
https://www.youtube.com/watch?v=0UzMaoaXKaM – good Calvin Cycle review
 our cells produce CO2 as they burn sugar for energy but only
photosynthetic organisms (and chemosynthetic bacteria) can
capture/fix carbon atoms w/in CO2.
Calvin cycle
– discovered by Melvin Calvin, Andrew Benson, James
Bassham in 1950’s
- ATP + NADPH (product of light rxns) dissolve in stroma and
power the synthesis of 3-C sugar glyceraldehyde-3-phosphate
(G3P) from CO2
- ‘cycle’ because it begins and ends with same 5-C sugar,
ribulose biphosphate (RuBP)
- 3 parts: 1) carbon fixation, 2) G3P synthesis, and 3)
regeneration of RuBP
 CALVIN CYCLE or C3 Pathway
1. Carbon fixation
 rubisco enzyme  3 CO2 + 3 RuBP = 3 unstable 6-C
molecules (split into) = 6 3-C PGA (phosphoglyceric acid)
molecules
2. G3P synthesis:
 6 3-C PGA  6 3-C G3P molecules using energy donated by
ATP and NADPH
3. RuBP Regeneration:
 5 G3P  3 RuBP using ATP (restarts cycle)
 1 G3P exists the Calvin cycle
GLUCOSE SYNTHESIS
 2 G3P molecules that exit the Calvin cycle combine to form 1
6-C glucose molecule (outside the chloroplast)
https://www.youtube.com/watch?v=
sQK3Yr4Sc_k – Crash course on
Photosynthesis
Light vs. Dark Rxns

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
Location
Reactants
Products
Summary
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thylakoids
H2O, ADP, NADP+


ATP, NADPH, and O2

energizes e- in PS I + II 
and jump from e
acceptors down ETCs
2 energized e-  1

NADPH

lost energy powers H+
ions through ATP
synthase in chemiosmosis
hydrolysis supplies lost ein PSII and O2
stroma
CO2, RuBP, ATP,
NADPH
C6H12O6 & RuBP
RuBP + CO2  PGA
PGA + ATP + NADPH
 6 G3P
5 G3P  3 RuBP
1 G3P + 1 G3P 
glucose
Light and Dark Phase Compared
unlight
carbon dioxide uptake
water uptake
ATP
LIGHT
DEPENDENT
REACTIONS
ADP + Pi
LIGHT
INDEPENDENT
REACTIONS
NAD+
NADPH
P
oxygen release
glucose
new water
C4 Plants Capture Carbon and synthesize Sugar in Different Cells
 O2 can also bind to active site on rubisco enzyme (ex of comp.
inhibition; usually in dry, hot conditions when stomata close)
 photorespiration –prevents Calvin cycle from making sugar and
wastes energy
 flowering plants evolved 2 different mechanisms to avoid
photorespiration
 C4 pathway
 CAM (crassulacean acid metabolism)
 much more efficient at fixing carbon but require more energy
(advantage only in warm, sunny, dry environments
 Kentucky bluegrass (C3) is taken over by spiky crabgrass (C4) in
summer
C4 Pathway
 C4 plants contain chloroplasts
in mesophyll and bundle sheath
cells (unlike C3 plants)
 use enzyme PEP carboxylase to
fix CO2 (highly selective for CO2
unlike rubisco)
 PEP carboxylase causes CO2 to
react with 3-C PEP to produce a 4-C oxaloacetate
 Oxaloacetate converts to malate which diffuses from mesophyll
cells into bundle sheath cells (shuttles CO2)
 Malate breaks down  3-C pyruvate and releases CO2 creating
high concentration in bundle sheath cells
 rubisco than fixes carbon with little comp. from O2 minimizing
photorespiration
https://www.youtube.com/watch?v=Dq38MpYOb8w – C4 pathway and CAM
CAM plants Capture Carbon and Synthesize Sugar at Different Times
 use C4 pathway but perform carbon fixation and
sugar synthesis at different times (unlike C4 plants
- use different structures)
 stomata of CAM plants open at night (cooler
temps + high humidity) and CO2 is captured by
mesophyll cells using C4 pathway
 malate (product of C4 pathway) shuttled into
central vacuole (stored as malic acid) until daytime
 stomata close during day; malic acid re-enters
cytoplasm as malate
 malate  pyruvate  PEP and CO2 released
into Calvin cycle to produce sugar
Stomata
Open
Closed
Several Stomata
Showing Guard Cells
Case Study: Did the Dinosaurs Die from Lack of Sunlight?
 Did an asteroid end the reign of dinosaurs? The Alvarez hypothesis was
initially met with skepticism. If such a cataclysmic event had occurred, where was the
crater? In 1991, scientists finally located it centered near the coastal town of Chicxulub
on Mexico’s Yucatan Peninsula. The crater, estimated at over 150mi in diameter and 10
mi deep, was filled with debris and sedimentary rock laid down during the 65.5 my
since the impact. Ocean and dense vegetation hid most remaining traces from satellite
images. Some paleontologists argue that the impact may have exacerbated more
gradual changes in climate, to which the dinosaurs could not adapt. Such changes
might have been caused by prolonged intense volcanic activity, such as occurred at a
site in India at about the time of the K-T extinction. Volcanoes spew out soot and ash,
and iridium is found in higher levels in lava from Earth’s molten mantle than in its
crust. So furious volcanism could significantly reduce the amount of sunlight for plant
growth, spew climate-changing gasses into the air, and also contribute to the iridium
rich K-T boundary layer.
In 2010, alternative hypotheses to the asteroid impact were dealt a blow when an
expert group of 41 researchers published a review article in the prestigious journal
Science. This publication analyzed the previous 20yrs of research by paleontologists,
geochemists, geophysicists, climatologists, and sedimentation experts delaying with the
K-T extinction event. The conclusion: Land and ocean ecosystems were destroyed
extremely rapidly, and evidence overwhelmingly supports the asteroid impact
hypothesis first proposed by the Alvarez group 30yrs earlier.
Photosynthesis Problems
1.Click on link below and then select Begin Problem
Set
2.Try and answer the problem OR click on Tutorial
for help and more information.
http://www.biology.arizona.edu/biochemistry/problem_sets/photosynthesis_1/photosynthesis_1.ht
ml