The World of
Plants:
Photosynthesis
and More….
Click here for D.
Attenboroughsecret life of
plants intro
A. Introduction
Life on Earth is solar powered.
 The chloroplasts of plants use a process
called photosynthesis to capture light
energy from the sun and convert it to
chemical energy stored in sugars and
other organic molecules.

B. Plants and other autotrophs are the
producers of the biosphere
Photosynthesis nourishes almost all of
the living world directly or indirectly.
 Autotrophs produce their organic
molecules from CO2 and other inorganic
raw materials obtained from the
environment.

– Autotrophs are the ultimate source of
organic compounds for all nonautotrophic
organisms.
– Autotrophs are the producers of the
biosphere.

Autotrophs can be separated by the
source of energy that drives their
metabolism.
– Photoautotrophs use light as the energy
source.
– Photosynthesis occurs in plants, algae,
some other protists, and some prokaryotes.
– Chemoautotrophs harvest energy from
oxidizing inorganic substances,
including sulfur and
ammonia.
– Chemoautotrophy is
unique to bacteria.
Heterotrophs live on organic compounds
produced by other organisms.
 These organisms are the consumers of
the biosphere.

– The most obvious type of heterotrophs feed
on plants and other animals.
– Other heterotrophs decompose and feed on
dead organisms and on organic litter, like
feces and fallen leaves (saprophytes).
– Almost all heterotrophs are completely
dependent on photoautotrophs for food and
for oxygen, a byproduct of photosynthesis.
C. Chloroplasts are the sites of
photosynthesis in plants
Any green part of a plant has
chloroplasts.
 However, the leaves are the major site of
photosynthesis for most plants.

– There are about half a million chloroplasts
per square millimeter of leaf surface.

The color of a leaf comes from
chlorophyll, the green pigment in the
chloroplasts.
– Chlorophyll plays an important role in the
absorption of light energy during
photosynthesis.
 Chloroplasts
are found mainly in
mesophyll (middle) cells forming the
tissues in the interior of the leaf.
 O2 exits and CO2 enters the leaf
through microscopic pores, stomata,
in the leaf.
 Veins deliver water specifically
through xylem from the roots and
carry off sugar through phloem to
other plant areas.
 We will look more closely at plant
anatomy later…
 Each
chloroplast has two
membranes around a central liquid
space, the stroma.
 Inside each chloroplast are
membranous sacs, the thylakoids.
–Thylakoids may be stacked
into columns called grana.
D. Evidence that chloroplasts split
water molecules enabled researchers
to track atoms through
photosynthesis
Powered by light, the green parts of
plants produce organic compounds and
O2 from CO2 and H2O.
 The net process of photosynthesis is:

– 6CO2 + 12H2O + light -> C6H12O6 + 6O2 +
6H2O
– In reality, water is both a product and a
reactant but usually the equation shows a
net balance.

One of the first clues to the mechanism
of photosynthesis came from the
discovery that the O2 given off by plants
comes from H2O, not CO2.
– Before the 1930s, the prevailing hypothesis
was that water combined with carbon to
form sugar. Therefore, it was thought that
oxygen came from carbon dioxide.
– C.B. van Niel challenged this hypothesis.
– In the bacteria that he was studying,
hydrogen sulfide (H2S), not water, is used in
photosynthesis. Sugar was formed and
sulfur was a waste product.
– Van Niel proposed this reaction:
 CO2 + 2H2S -> CH2O + H2O + 2S
He generalized this idea and applied it to
plants, proposing that oxygen came
from the splitting of water.
 Other scientists confirmed van Niel’s
hypothesis.

– They used 18O, a heavy isotope, as a tracer.
– They could label either CO2 or H2O.
– They found that the 18O label only appeared
if water was the source of the tracer.

Essentially, hydrogen extracted from
water is incorporated into sugar and the
oxygen released to the atmosphere
(where it will be used in respiration).
What do these
results
prove?
O
2
Next, water labeled with radioactive
oxygen was fed to the plants’ soil.
The water traveled up the xylem and
was used by the leaves for
photosynthesis. Radioactive oxygen
came out of the stomates!
CO218
H2O18
First, radioactive carbon dioxide
was introduced into the stomates
of the plant.
Oxygen released from the stomates
as a product of photosynthesis
WAS NOT radioactive!
O218
 Photosynthesis
is a redox reaction.
– It reverses the direction of electron
flow in respiration.
 Water
is split
 Electrons are transferred with H+
from water to CO2, reducing it to
sugar.
E. The light reactions and the Calvin cycle
cooperate in converting light energy to
chemical energy of food: an overview
Photosynthesis is two processes, each
with multiple stages.
 The light reactions convert solar energy
to chemical energy.
 The dark reactions incorporate CO2 from
the atmosphere into an organic molecule
and uses energy from the light reaction to
reduce the new carbon piece to sugar.

F. Light, Leaves and
Photosynthesis
The thylakoids convert light energy into
the chemical energy
 Light, like other forms of electromagnetic
energy, travels in rhythmic waves.
 The distance between crests of
electromagnetic waves is called the
wavelength.

Just in case you thought plants weren’t interesting, watch this…
Click here
 Electromagnetic
spectrum = the
entire range of electromagnetic
radiation

The most important segment for life is a
narrow band between 380 to 750 nm, visible
light.
V
I
B
G
Y
O
R

While light travels as a wave, it travels as
discrete particles, called photons.
– Photons are not tangible objects, but they do have
fixed quantities of energy.

The amount of energy packaged in a photon
is inversely related to its wavelength.
– Photons with shorter wavelengths pack more
energy.
– SO…which wavelength of color is associated with
the most energy but shortest wavelength?
Violet
– Which wavelength of color is associated with the
least energy?
Red
When light meets matter, it may be
reflected, transmitted, or absorbed.
 Therefore, when we see color, it is
because we are looking at the
wavelengths of light that are reflected
back to our eyes.
 The wavelengths that are absorbed are
not seen.
Reflected
 White light is all the colors ___________
(ROYGBIV)
Absorbed
Black light is all the colors ___________.
Fig. 10.6

All the wavelengths of light (ROYGBIV) are
striking the apple.
All but red are absorbed; red is reflected back to
your eyes and interpreted by the brain as “RED”.
A leaf looks green because
chlorophyll, the dominant
pigment, absorbs red and blue
light, while transmitting and
reflecting green light.

In the thylakoid are several pigments
that differ in their absorption spectrum.
– Chlorophyll a, the dominant pigment,
absorbs best in the red and blue
wavelengths, and least in the green.
Draw an absorption
spectrum for chlorophyll
The chlorophyll absorption spectrum
means that photosynthetic
organisms need the wavelengths of
blue and red to perform the most
efficiently.
Ironically, green (which is the color
we mostly think of with respect to
healthy plants) is the wavelength
that plants reflect and therefore,
have no use for.
Application
Questions…
1.
2.
3.
Why do pet stores sell fish tank
lights that illuminate the color
green?
What would happen if you grew
plants only in green light? Only in
red and blue light?
How could you set up an expt. In
the lab to show the effects of
wavelengths of light on plant
photosynthetic efficiency?
Waterweed simulation
This experiment uses an underwater
aquatic plant called Elodea to study
limiting factors that affect the rate of
photosynthesis.
 Independent variable: Wavelength of
light or amount of carbon dioxide, or
amount of light
 Dependent variable: # oxygen bubbles

Click to go to waterweed website
If IV is CO2 level, what
is the control?
Control is no CO2 dissolved in water.
 How would you prepare that?
 Boil the water, so that carbon dioxide
evaporates out, and then cool the
water down
 Or,

Plant Pigments

Only chlorophyll a (blue green pigment)
participates directly in the light reactions but
accessory photosynthetic pigments absorb
light and transfer energy to chlorophyll a. (like
players on a team)
– Chlorophyll b, with a slightly different structure than
chlorophyll a, has a slightly different absorption
spectrum and funnels the energy from these
wavelengths to chlorophyll a. (light green pigment)
– Carotenoids (orange pigments) can funnel the
energy from other wavelengths to chlorophyll a and
also participate in photoprotection against
excessive light.

Collectively, all photosynthetic
pigments absorb the various
wavelengths of light and determine the
action spectrum for photosynthesis.
– An action spectrum measures changes in
some measure of photosynthetic activity
(for example, O2 release) as the wavelength
is varied.
The action spectrum of photosynthesis
was first demonstrated in 1883 through
an elegant experiment by Thomas
Engelmann.
– In this experiment, different segments of a
filamentous alga were exposed to different
wavelengths of light.
– Areas receiving wavelengths favorable to
photosynthesis should produce excess O2.
– Engelmann used the
abundance of aerobic
bacteria clustered
along the alga as a
measure of O2
production.

When a molecule absorbs a photon, one
of that molecule’s electrons is elevated
to a high energy orbital.
– The electron moves from its ground state to
an excited state.
– A particular compound absorbs only
photons corresponding to specific
wavelengths.
– Thus, each pigment has a unique
absorption spectrum.
Photons are absorbed by clusters of
pigment molecules in the thylakoid
membranes.
 The energy of the photon is used to
raise an electron from its ground state
to an excited state.
– In chlorophyll a and b, it is an electron
from magnesium that is excited.

Plants need magnesium and
nitrogen from the soil to
support chlorophyll
production.
They also need phosphorus
to produce ATP
In the thylakoid membrane, chlorophyll is
organized along with proteins and smaller
organic molecules into photosystems.
 A photosystem acts like a light-gathering
“antenna complex” consisting of a few
hundred chlorophyll a, chlorophyll b,
and carotenoid
molecules.

When any antenna molecule absorbs a
photon, it is sent from molecule to
molecule until it reaches a particular
chlorophyll a molecule, the reaction
center.
 At the reaction center, there is a primary
electron acceptor which removes the
excited electron from the reaction center
(chlorophyll a.)

– This starts the light reactions!

This entire light harvesting system is
called a photosystem.
There are two types of photosystems.
 Photosystem I has a reaction center
chlorophyll, the P700 center, that has an
absorption peak at 700nm.
 Photosystem II has a reaction center
with a peak at 680nm.


These two photosystems work
together to use light energy to
generate ATP and NADPH.
2 routes for excited
electrons

• Noncyclic
Cyclic Electron
Electron Flow
Flow
– electrons cycle
•electrons do not
back to
cycle back to
Photosystem I
Photosystem I
(P700)
(P700)
– Most primitive
of the light
• Most evolved
reactions
form of light
reactions
G. Cyclic
Photophosphorylation
Cyclic
Photophosphorylation





What happens?
Electrons in P700 are excited.
As the electrons return back to the
ground state they pass through an
electron transport chain of molecules
through a series of redox reactions.
Energy is derived from this to produce
ATP (chemiosmosis)
The electrons return back to P700 where
they started.
What do you need to
remember about cyclic
photophosphorylation?





It uses P700 (PS I), not P680 (PSII)
No oxygen or NADPH made
No water is split!
ATP is made by chemiosmosis. This
energy will be used to fuel the
production of sugar in the dark reactions
It is the primitive form of light reactions
for photosynthesis. Today, it has
become incorporated into the non-cyclic
(more evolved) form.
H. Noncyclic
Photophosphorylation
(Z-chain)
animation
What happens?
 Incoming
photons from the sun
excite electrons in P700.
 The excited electrons pass
down an electron transport
chain and through
chemiosmosis ATP is made
 (This ATP is needed in the dark
reactions to fuel the production
of sugar.
Noncyclic
Photophosphorylation
(continued)
The
electrons end up “in
the lap” of NADP+, an
electron carrier molecule.
NADP+ becomes
reduced to NADPH which
is later used for the dark
reactions.
What else?
Since the electrons do not return to
P700, there is a void of electrons in
that photosystem.
 To fill that void, photons excite
electrons in P680 which travel down
a transport chain of electron carrier
molecules, which end up in P700.
 ATP is made through chemiosmosis

Finally?




To fill the void created in P680, water is
split.
Oxygen is produced as a by-product
(Some oxygen is used for aerobic
respiration but most is a waste that
escapes into the air).
The electrons from the hydrogen of water
end up in P680, then P700, finally NADPH.
The H+ ions from hydrogen are pumped
into the thylakoid space which helps in
the chemiosmotic synthesis of ATP
animation

The light reactions use the solar power of
photons absorbed by both
photosystem I and
photosystem II to
provide chemical
energy in the form
of ATP and reducing
power in the form
of the electrons
carried by NADPH.
Summary of noncyclic electron
flow- What do you really need to
know??


Low-energy electrons originating from
water end up in high-energy NADPH
ATP is produced
ATP and NADPH are used in Calvin cycle to
reduce CO2 to sugar


Oxygen gas (O2) is produced as byproduct (mostly escapes through stomata)
Two photosystems are involved (2 light
events)
The Great Oxygenation
Event (GOE)





2.3 billion years ago
Cynanobacteria began producing oxygen by
photosynthesis
Most of earth’s anaerobic organisms went
extinct because oxygen was toxic to them
Dissolved oxygen covered ocean surface
Oxygen became incorporated into the
compounds of the earth (ie. FeO2)
DO NOW: Use the following
terms to make a graphic
representing non-cyclic
photophosphorylation
NADP+ +2H+
NADPH + H+
Photosystem I
Photosystem II
ETC:Fd->NADP+ reductase
Primary electron acceptor
ETC: Pq->cytochrome complex->Pc
Energy of electrons
H2O
P680
P700
Light
ATP
O2
If you are stumped try
labeling this:
Check your answer:
I. How does the structure of a
chloroplast aid in its function?
animation
Chemiosmosis in
Chloroplasts
H+
ions are pumped into the
thylakoid space (proton pump)
These will move by facilitated
diffusion into the stroma through the
ATP synthase protein embedded in
the thylakoid membrane.
Using the potential energy gradient,
inorganic phosphate is added to ADP
to form ATP.
The ATP formed in the stroma is
ready to be used for sugar
production…

The proton gradient, or pH gradient,
across the thylakoid membrane is
substantial.
– When illuminated, the pH in the thylakoid
space drops to about 5 and the pH in the
stroma increases to about 8, a
thousandfold different in H+ concentration.

The light-reaction “machinery”
produces ATP and NADPH on the
stroma side of the thylakoid.
animation
Comparing chemiosmosis during
respiration and photosynthesis
What are all of the
names of the “light
reactions”?
The Light Dependent Reactions-why?
Light Is needed to excite electrons and
begin the process.
Photolysis –why?
Water is split (lysed) using photons of light
(only non-cyclic!)
Photophosphorylation –why?
Light is the initiator of reactions which add
phosphate back on to ADP to make ATP
Photochemical reactions
J. What comes next? The Calvin Cycle
Overview
The Calvin cycle regenerates its starting
material after molecules enter and leave
the cycle.
 CO2 enters the cycle and leaves as sugar.
 The cycle spends the energy of ATP and
the reducing power of electrons carried
by NADPH from the light reactions to
make the sugar.
 The actual sugar product of the Calvin
cycle is not glucose, but a three-carbon
sugar, glyceraldehyde-3-phosphate (G3P).

Other names of the
Dark Reactions…



The Calvin Cycle –why?
This cycle was discovered by Melvin
Calvin in 1946 using C-14 as a tracer for
these reactions.
The Light Independent Reactions- why?
Light isn’t needed. The reactions can
occur in the day or the night.
The Carbon Fixing Reactions- why?
Carbon dioxide is “fixed” or made in
sugar.
The Calvin cycle has three phases.
 In the carbon fixation phase, each CO2
molecule is attached to a five-carbon
sugar, ribulose bisphosphate (RuBP).

– This is catalyzed by RuBP carboxylase or
rubisco.
– The six-carbon intermediate splits in half to
form two molecules of 3-phosphoglycerate
per CO2.
Calvin Cycle –How it
begins.
CARBON FIXATION
•Reaction catalyzed by RuBP carboxylase
(rubisco)
•product is an unstable 6C molecule that splits
into two 3C molecules
•for every 3 CO2FIXED, 6 phosphoglycerates
formed
 During
reduction, each 3phosphoglycerate receives
another phosphate group from
ATP to form 1,3
bisphosphoglycerate.
 A pair of electrons from NADPH
reduces each 1,3
bisphosphoglycerate to G3P
(also called PGAL).
Calvin Cycle-cont.
REDUCTION
•uses 6 ATP for every
6 phosphoglycerates
•6 NADPH oxidized to
produce 6 G3P for
every 3 CO2 fixed
•1 G3P (PGAL) exits,
5 recycled
Calvin Cycle-completed
REGENERATION
of CO2 acceptor
(RuBP)
•uses 3 ATP
•five 3C G3P
(PGAL) converted to
three 5C RuBP
ATP/NADPH requirements of
Calvin Cycle
For one G3P
produced (3
turns):
• 3 Carbon dioxide
molecules are fed
in
• 9 ATP used
• 6 NADPH used
For one glucose
produced (6
turns):
• 6 CO2 are used
• 18 ATP* used
• 12 NADPH used
animation
What do you really need
to know about the
animation
Calvin Cycle?
6
carbon dioxide molecules turn the
cycle 6 times
From these 6 turns, two molecules of
G3P (PGAL) come out of the cycle and
can be made into a six carbon glucose
molecule
G3P is the first stable sugar madeTherefore this is called C3 photosynthesis
since G3P is a three carbon compound.
2 G3P  1 glucose eventually starch
and other polysaccharides can form
animation
Melvin Calvin’s
“lollipop” Experiment
• Calvin used a photosynthetic
organism that could reproduce
easily called Chlorella.
• The apparatus he used looked
like a lollipop.
• The algae was exposed to
carbon dioxide that was
radioactively labeled with C14
• At regular time intervals, the
photosynthetic reactions was
stopped by denaturing the
enzymes with alcohol.
• Each intermediate carbon
compound was identified
using chromatography, a
separation technique.
• The research took 10 years
and Calvin won a Noble Prize.
K. Overall Requirements and
Products of Photosynthesis
L. How do plants get
the CO2 they need and
get rid of excess O2?
Through the stomates of the leaves or the lenticels of the
stems
C3 Plant Leaf Structure

See supplemental handouts…
M. Label the Diagram
Check your labels
N. What happens to the end
products of photosynthesis?
Energy (ATP)
RESPIRATION
CO2
PHOTORESPIRATION
G3P
SYNTHESIS
glucose
Fig. 10.20
Amino acids
Vein (phloem)
Non-photosynthetic
tissue
Fatty
acids
Starch
&
cellulose
proteins
lipids
Food for
heterotrophs

Sugar made in the chloroplasts supplies
the entire plant with chemical energy
and carbon skeletons to synthesize all
the major organic molecules of cells.
– About 50% of the organic material is
consumed as fuel for cellular respiration in
plant mitochondria.
– Carbohydrate in the form of the
disaccharide sucrose travels via the veins
to nonphotosynthetic cells.
– There, it provides fuel for respiration and
the raw materials for anabolic pathways
including synthesis of proteins and lipids
and building the extracellular
polysaccharide cellulose.

Plants also store excess sugar by
synthesizing starch.
– Some is stored as starch in chloroplasts or
in storage cells in roots, tubers, seeds, and
fruits.
Heterotrophs, including humans, may
completely or partially consume plants
for fuel and raw materials.
 On a global scale, photosynthesis is the
most important process to the welfare of
life on Earth.

– Each year photosynthesis synthesizes 160
billion metric tons of carbohydrate per year.
O. How Does Photosynthesis
Compare/Contrast to
Respiration?

How do the balanced equations compare?
aerobic respiration
C6H12O6 + 6O2 + 6H2O 
6 CO2 + 12 H20+36ATP
photosynthesis
6 CO2 + 12 H20  C6H12O6 + 6O2 + 6H2O
Are the reactions really opposite each other when
you look at the specifics of each???
How do the organelles
compare and contrast?

Similarities?
– Both the mitochondria and the
chloroplast are double membrane,
with a circular chromosome and
ribosomes similar to prokaryotes
– They have had a similar evolutionary
past. (endosymbiotic theory)
A closer look at inner
membranes…

The mitochondria and cristae and the
chloroplast’s thylakoid membrane have
similar structures and functions.
– They both have a lot of surface area for
reactions to occur.
– On each inner membrane, electron transport
chain molecules conduct redox reactions.
– A proton gradient exists across each to create
ATP chemiosmotically (although for different
purposes.)

Chloroplasts and mitochondria generate ATP
by the same mechanism: chemiosmosis.
– An electron transport chain pumps protons across a
membrane as electrons are passed along a series of
more electronegative carriers.
– This builds the proton-motive force in the form of an
H+ gradient across the membrane.
– ATP synthase molecules harness the proton-motive
force to generate ATP as H+ diffuses back across
the membrane.

Mitochondria transfer chemical energy from
food molecules to ATP and chloroplasts
transform light energy into the chemical
energy of ATP.
Other similarities?
Both processes of respiration and
photosynthesis use electron carrier
molecules that can be reduced or
oxidized.
 NAD+ and FAD (respiration)//
NADP+ (photosynthesis)
 Calvin cycle // Krebs cycle (both
cycle carbon compounds

Differences between
photosynthesis and respiration




Respiration has 5 stages vs.
Photosynthesis with 2 (light and dark)
Respiration uses oxygen as a final
electron acceptor to make water, while
photosyn. splits water to make oxygen.
ATP is made in photosyn. to fuel the dark
rxns. while ATP is made in respiration for
fueling all metabolic activities in the cell.
Photosynthesis must start in the day;
respiration can occur day or night
What else?
 In
the Calvin cycle, ATP is used
vs. Krebs cycle ATP is made.
 In the Calvin cycle, CO2 is a
reactant and in the Krebs cycle,
CO2 is a product.
 In the Calvin cycle, NADPH is
oxidized while in the Krebs
cycle, NAD+ and FAD+ are
reduced to form NADH and
FADH2
What about those strange
carnivorous plants?
CLICK HERE!
P. Global Artificial
Photosynthesis- The future?




BENEFITS:
Limitations
Allows us to mimic natures most
So far the reactions are inefficient /
effective process for creating
unsustainable.
energy rich products from simple
input materials.
The reaction needs the heat generated
Potential for a new fuel that can
from the sunlight to power it, so is not
power vehicles from naturally
suitable for operation worldwide.
occurring input materials, CO2,
water and Sunlight.
Will make Carbon storage more
economically viable as the CO2
can be used to create a saleable
product.
Potential to tap into existing big
producers of CO2, such as power
station exhaust and therefore, the
CO2 is used before it enters the
atmosphere.
http://www.thegreenage.co.uk/tech/artificial-photosynthesis/
Can artificial photosynthesis
save the planet?

Watch the youtube video and decide!

https://video.search.yahoo.com/video/play;_ylt=A2KLqIVV7htWNwYAao0snIlQ;_ylu=X3oDMTByN2Rnb
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