Chapter 10 Photosynthesis

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Chapter 10 Photosynthesis

• Have you thanked a plant today?

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• The ultimate source of power for life on earth is
  the sun

• Plants have the unique ability to convert this
  solar energy into the chemical energy of sugars
  and starches through

photosynthesis
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• Plants are thus said to have autotrophic
  nutrition. This means that they can take
  inorganic substances and manufacture organic
  compounds from them.

• Biologists also refer to autotrophs as

producers
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• Producers may be classified as either

photoautotrophs
or
chemoautotrophs
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• All other living creatures, including animals,
  fungi, and most types of protists and bacteria
  are classified as

heterotrophs

• Heterotrophs must obtain their organic molecules
  by consuming those organisms already possessing
  them

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Therefore, heterotrophs are also known as
consumers
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Chloroplast Location and Structure

• All green parts of the plant contain
  chloroplasts, and are capable of
  photosynthesizing, but the leaves are the major
  food producing sites.

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• There are approximately one-half million
  (500,000) chloroplasts per square millimeter of
  leaf surface

• Most of these are concentrated in the leaf layer
  known as the mesophyll, in the leaf interior.

• Each individual cell will usually contain 30-40
  chloroplasts

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Mesophyll layer
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This organism has a single, ribbon-shaped
chloroplast in each cell
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thylakoids
Each chloroplast consists of a double membrane,
with the chlorophyll concentrated in disk-like
thylakoids
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• The thylakoids are arranged in stacks called

grana

• The fluid portion of the chloroplast is referred
  to as the

stroma
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Chloroplast
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• The chlorophyll itself is found specifically in
  the thylakoid membrane

GRANUM
STROMA
Thylakoid space
Thylakoid membrane
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The Pathways Of Photosynthesis

• Although scientists are still researching the
  details, the overall equation for photosynthesis
  has been known since the 1800s.

• The green parts of plants are capable of taking
  carbon dioxide and water and converting them to
  organic materials in the presence of light.

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To put it another, more scientific way
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Light Energy


6CO2
12H2O
C6H12O6

6O2

6H2O
glucose
oxygen
water
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• Water (H2O) appears on both sides of the equation
  because the 6 molecules formed are new molecules,
  not leftovers

• Water molecules are split in the chloroplast and
  the hydrogen is incorporated into sugar. The
  oxygen is then released into the air.

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• The process of photosynthesis is not as simple as
  the overall equation might indicate

• Photosynthesis actually occurs in two complex
  stages

light reactions
Calvin cycle
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• In the light reactions, solar energy is converted
  into chemical energy in the form of ATP

• The Calvin cycle, which is also known as
  carbon-fixation, uses the ATP formed during the
  light reactions to manufacture glucose from CO2
  and NADPH (an electron carrier nearly identical
  to NADH)

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This diagram summarizes the events of the light
reactions (left) and Calvin cycle
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Melvin Calvin solved the reactions of carbon
fixation during the 1940s. These reactions are
also sometimes called the dark reactions because
they do not require light directly.
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In which parts of the chloroplast do the stages
of photosynthesis occur?
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Calvin Melvin received the Ma Bell Prize in 1999
for his discovery that the gene for baldness is
inherited with the gene for birthday suits
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The Light Reactions And The Nature Of Sunlight

• Light is a form of energy known as
  electromagnetic energy or radiation

• Electromagnetic energy travels in waves

• The distance between the crests of waves is
  called the wavelength

• Wavelengths from about 380 to 750 nm make up what
  is known as visible light

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As you can see, visible light makes up a very
small part of the electromagnetic spectrum
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• The wave nature of light explains many of its
  properties, but at times light behaves as though
  it were particulate in nature

• The term photon is used to describe a packet of
  light that contains a fixed quantity of energy

• The energy content of a photon is inversely
  proportional to its wavelength

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• The shorter the wavelength, the more energy it
  contains

Which of these wavelengths contains more energy?
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• A photon of violet light packs nearly twice the
  energy as a photon of red light

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• Visible light is the radiation that drives
  photosynthesis.

• Substances which can absorb visible light are
  called

pigments
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• When light strikes a pigment, it can be

reflected
transmitted
absorbed
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• Objects under visible light appear to be a
  certain color because that color is either
  transmitted or reflected. All other wavelengths
  are absorbed

• Chlorophyll is a pigment that is extremely
  effective at absorbing both red and blue light

• What wavelengths are reflected or transmitted by
  chlorophyll?

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A 3-dimensional model of the chlorophyll molecule
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• An instrument called a spectrophotometer can
  measure the ability of a pigment to absorb
  specific wavelengths of light

• A graph plotting the light absorption of a
  pigment versus wavelength is called an absorption
  spectrum

absorption
wavelength
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Can you tell what the peaks and valleys of this
absorption spectrum mean?
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• An action spectrum profiles the relative
  performance of different wavelengths

• Usually, the wavelength is plotted against some
  indicator of photosynthetic rate, such as oxygen
  output or carbon dioxide consumption

• This graph then tells us which wavelengths are
  actually driving the photosynthetic process

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• Only chlorophyll a actually is involved in the
  light reactions.

• Other pigments, including chlorophyll b and the
  carotenoids, transfer the energy they absorb to
  chlorophyll a

• Some carotenoids seem to function in
  photoprotection that is they absorb and
  dissipate excessive light energy that might
  otherwise damage chlorophyll

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Photoexcitation Of Chlorophyll

• When photons of specific wavelengths strike
  chlorophyll molecules, one of the molecules
  electrons is elevated to an excited state

e-
light
chlorophyll
e-
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• However, an excited electron cannot remain in
  this state for very long. In less than a
  billionth of a second, the electron will drop
  back down, releasing the excess energy as heat

e-
HEAT
chlorophyll
e-
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This conversion of light to heat energy is the
reason an automobile surface is sooooo!!!!! Hot
on a sunny day
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• Some pigments, and chlorophyll is one of them,
  can emit not only heat but light energy as an
  afterglow called fluorescence

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Photosystems Light-Harvesting Complexes of the
Thylakoid Membrane

• Within the chloroplast, chlorophyll is organized
  along with proteins and other kinds of molecules
  into photosystems

• The photosystem is a light harvesting unit
  consisting of several hundred chlorophyll a
  molecules, chlorophyll b, and carotenoids

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• When a photon is absorbed by a photosystem, the
  energy is passed from pigment molecule to pigment
  molecule until it reaches a special chlorophyll a
  molecule

• This molecule is special because of its position
  in the photosystem referred to as the reaction
  center

• Sharing the reaction center is a molecule which
  will serve as the primary electron acceptor

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• There are two types of photosystems embedded in
  the thylakoid membrane, each with a
  characteristic reaction center

photosystem I
photosystem II
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• Photosystem I has a reaction center that is
  particularly efficient at absorbing the
  wavelength of 700 nm (far-red) and is thus known
  as P700

• Photosystem II has a reaction center that best
  absorbs the wavelength 680 nm (also red) . This
  reaction center is known as P680

• The main difference between the two is in the
  molecule that serves as the primary electron
  acceptor of each reaction center

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The two photosystems at work
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Noncyclic Electron Flow

• When an electron leaves a chlorophyll molecule
  there are two possible routes for its flow

Cyclic electron flow
Non-cyclic electron flow
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• Non-cyclic electron flow involves movement of the
  electron from photosystem II and down the
  electron transport chain to photosystem I.

• The electron then travels the transport chain of
  photosystem I and eventually is stored in a
  molecule called NADPH

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• The electron lost from photosystem II is replaced
  from the splitting of a water molecule, while the
  one lost from photosystem I is replaced by the
  electron coming from photosystem II

• Hydrogen ions from water then are used to
  manufacture ATP as they flow through ATPsynthase
  channels in the thylakoid membrane

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• The ATP synthesis during non-cyclic electron flow
  is called non-cyclic photophosphorylation

• The light reactions use solar energy to generate
  ATP and NADPH which will then provide energy and
  reducing power, respectively, for the Calvin cycle

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Cyclic Electron Flow

• Non-cyclic electron flow produces ATP and NADPH
  in roughly equal amounts

• However, the Calvin cycle uses more ATP than NADPH

• If the chloroplast runs low on ATP, NADPH will
  accumulate as the Calvin cycle slows down

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• The glut of NADPH will cause a shift from
  non-cyclic to cyclic electron flow, using only
  photosystem I

• There is no NADPH produced and no release of
  oxygen in cyclic flow

• ATP is produced and cyclic flow will continue
  until the ATP supply catches up with demand

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In cyclic flow, the electron flow generates only
ATP. Since photosystem II is not involved,
neither is NADPH produced, nor oxygen released
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Chemiosmosis

• As in respiration, photosynthesis generates ATP
  by chemiosmosis

• Built into the thylakoid membrane are the same
  type of ATP synthase complexes that we see in the
  inner mitochondrial membrane

• These complexes couple the flow of hydrogen ions
  to the phosphorylation of ADP in the same manner

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Hydrogen ions
Flow through
ATP SYNTHASE
H
H
H
H
ATP


ADP
P
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• The thylakoid membrane pumps hydrogen ions into
  the thylakoid space, which serves as a reservoir

H
H
H
H
H
H
H
H
H
H
THYLAKOID SPACE
STROMA
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• When the ions then flow through the ATP synthase
  channels in the membrane, they are added to NADP
  to form NADPH in the stroma, where the Calvin
  cycle will take place

H
H
STROMA
H
H
NADP
NADPH
THYLAKOID SPACE
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Light Reaction Summary

• The light reactions use sunlight and water to
  produce ATP and NADPH, both of which will be used
  in the manufacture of glucose

• Oxygen gas is produced from the splitting of the
  water molecule and leaves the leaf

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Sunlight
WATER (H2O)

ATP

NADPH

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OXYGEN
Yes, even blondes need oxygen!!
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The Calvin Cycle Converting ATP, NADPH, and CO2
into Glucose

• The Calvin cycle is similar to Krebs in that a
  starting material is regenerated at the end of
  the cyclic pathway

• CO2, ATP, and NADPH are used in the cycle to make
  glucose

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• The cycle itself can actually be summarized in
  three phases

Phase 1 Carbon Fixation
Phase 2 Reduction
Phase 3 Regeneration of RuBP
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Phase 1 Carbon fixation

• Each molecule of CO2 is attached to a 5-carbon
  molecule known as ribulose bisphosphate (RuBP)

• The enzyme catalyzing this reaction is known as
  rubisco and is probably the most abundant protein
  on earth!!!

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• The 6-carbon intermediate formed is so unstable
  that it almost immediately splits into 2,
  3-carbon molecules of phosphoglycerate

P-C-C-C-C-C-P ribulose bisphosphate (RuBP)

CO2
rubisco
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P-C-C-C-C-C-C-P
For each CO2, there will be 2 phosphoglycerates
formed. When 6 CO2 have been fixed, there are a
total of 12 phosphoglycerates available
P-C-C-C
C-C-C-P
phosphoglycerate
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Phase 2 Reduction

• ATP is now used to add an additional phosphate to
  each phosphoglycerate molecule. A total of 12 ATP
  will be used to phosphorylate all 12
  phosphoglycerates.

P-C-C-C
ATP

ADP
P-C-C-C-P
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• Next, a pair of electrons from NADPH are added
  and a molecule called G3P is formed

• 12 NADPH will be used to reduce the 12 molecules
  to 12 G3P

P-C-C-C-P
NADPH

NADP
C-C-C-P
Also 12 phosphates (P) are removed to be reused
in phosphorylating ADP
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Phase 3 Regeneration of RuBP

• After reduction, the 12 NADP will be reused in
  the formation of NADPH in the light reactions

• 2 G3P molecules will be used to make one glucose
  the other 10 G3P will be recycled into 6 RuBP
  molecules and the cycle starts again

• 6 more ATP are used to regenerate RuBP

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2 G3P (C3) will make glucose (C6)
10 G3P (C3) will be rearranged to form 6 RuBP (C5)
I cant believe everything balances perfectly,
unlike Bombers checkbook!
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Alternative Mechanisms of Carbon Fixation

• Plants that live in hot, arid climates must
  somehow reach a compromise between photosynthesis
  and excessive water loss when water is at a
  premium

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• The CO2 required for photosynthesis enters the
  leaf through the stomata (openings on the leaf
  underside).

• However, the stomata are also the main avenues
  for transpiration (loss of water through
  evaporation)

• On a hot, dry day, plants will generally close
  the stomata partially in order to minimize water
  loss. This process decreases CO2 concentrations
  in the leaf, causing a decrease in output

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• When these conditions occur, a plant resorts to a
  seemingly wasteful process called

Photorespiration
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Photorespiration

• In most plants, CO2 enters the Calvin cycle and
  combines with RuBP and eventually splits to form
  a 3-carbon compound called 3-phosphoglycerate.
  These plants are typically referred to as C3
  plants

• These plants will produce less food on a hot, dry
  day, and include plants such as rice, wheat, and
  soybeans

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Most plants, including nearly all of our garden
plants, are C3 plants
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Even Bombers prize strawberries are C3 plants
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• As CO2 concentrations continue to fall in the
  leaf, rubisco begins accepting oxygen instead.

• This product splits, and one piece, a 2-carbon
  compound is exported out of the chloroplast

• Mitochondria and peroxisomes then break this
  two-carbon compound down into CO2, which can then
  re-enter the Calvin cycle

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Normal Calvin
Rubisco
CO2
6-carbon compound
2 G3P
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Photorespiration (hot, dry day)
Rubisco
O2
5-carbon compound
G3P

2-carbon compound
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2-carbon compound
Leaves chloroplast and goes to mitochondria and
peroxisomes
CO2
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• Photorespirations purpose is to provide CO2 for
  the Calvin cycle

• Photorespiration produces no ATP, as does normal
  cellular respiration

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C4 plants

• C4 plants are so named because they preface the
  Calvin cycle with an alternate form of carbon
  fixation which includes the formation of a
  4-carbon compound

• Several C4 plants you may be familiar with are
  corn, sugar cane, and crabgrass

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• A special enzyme found in the mesophyll cells of
  these plants, PEP carboxylase, adds CO2 to PEP.
  Compared to rubisco, this enzyme has a much
  greater affinity for CO2, and can thus fix
  carbon when rubisco is attracted to oxygen (hot,
  dry days when CO2 levels drop)

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• The mesophyll cells can keep the CO2
  concentration high enough to avoid
  photorespiration

• This is why corn and crabgrass grow fastest when
  other plants are not growing

• Now you know why it takes corn so long to get
  knee high,and then about a month from that
  point to tassel

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You now also know why Mr. Woodard must
continually rid his lawn of crabgrass during July
and August
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CAM Plants

• Other plants adapt to arid conditions using a
  mode of carbon fixation referred to as
  crassulacean acid metabolism or CAM

• These plants include various succulents like
  cacti, pineapple, and others

• These plants open their stomata at night and
  close them during the day, which is the reverse
  of most plants

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• CAM plants take in CO2 during the night and
  incorporate it into a variety of organic acids

• During the daylight hours, when the light
  reactions supply ATP and NADPH for the Calvin
  cycle, the CO2 is released

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CAM Photosynthesis
Organic acids
night
CO2
Day
CO2
Calvin cycle
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• Regardless of the method of carbon fixation used,
  C3, C4, and CAM plants all use the Calvin cycle
  to make sugar from carbon dioxide

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• About 50 of all the organic molecules
  manufactured in photosynthesis are used by the
  plant itself during respiration

• Some are wasted in the process of
  photorespiration

• The remaining products support all other life
  forms on earth

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• Plants manufacture over 160 billion metric tons
  of carbohydrate per year on a global scale

• That is equivalent to 60 trillion copies of this
  textbook, or 17 stacks of books reaching from the
  earth to the sun!!!

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17 times!
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No other chemical process on earth can match the
output of photosynthesis!!!!!!!!!!!!!
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If only I could convert the products of
photosynthesis into American dollars.
The end!!!