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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PHOTOSYNTHESIS
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: The Process That Feeds the Biosphere
• Photosynthesis is the process that converts solar energy (light energy from the sun) into chemical energy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Autotrophs sustain themselves without eating anything derived from other organisms
• Producers
• Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from water and carbon dioxide (inorganic)
Autotrophs – self feeders
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Producers of the biosphere
• Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes
• These organisms feed not only themselves but also the entire living world
Examples of Producers
Plants
Unicellular protist
Multicellular algae Cyanobacteria
Purple sulfurbacteria
10 µm
1.5 µm
40 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Heterotrophs obtain their organic material from other organisms
• Almost all heterotrophs, including humans, depend on photoautotrophs for food and oxygen
Heterotrophs - Consumers
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Basics of Photosynthesis
• Chloroplasts: Sites of Photosynthesis
LE 10-3
Leaf cross sectionVein
Mesophyll
Stomata CO2O2
Mesophyll cellChloroplast
5 µm
Outermembrane
Intermembranespace
Innermembrane
Thylakoidspace
Thylakoid
GranumStroma
1 µm
Photosynthesis takes place in the chloroplasts/found in several organisms
1.Leaves are the major location for photosynthesis
2.Green color is from chlorophyll-the pigment in chloroplasts – absorbs light energy
3.Through microscopic pores (stomata) CO2 enters and O2 leaves (stoma means “mouth”)
4.Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf.
•A typical mesophyll has 30-40 chloroplasts
•6. In the inner membrane encloses a second compartment which is filled with thick fluid called stroma.
•Suspended in the stroma are the thylakoids, which contains the thylakoids space.
•Thylakoids can be concentrated in stacks called grana
•5.The chlorophyll is in the membranes of thylakoids
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Equation of Photosynthesis
• Photosynthesis can be summarized as the following equation:
6 CO2 + 6H2O + Light energy C6H12O6 + 6 O2
What do you notice about this equation compared to respiration?
Requires a lot of energy that comes from sunlight• Chloroplasts split water into hydrogen and oxygen, incorporating the electrons
of hydrogen into sugar molecules, releasing oxygen.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosynthesis is a Redox Process, like cellular respiration
• Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced
• The redox process takes the form of hydrogen transfer from water to CO2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosynthesis Equation
• Water molecules are split apart, yielding O2, and are oxidized. They lose electrons along with hydrogen ions.
• CO2 is reduced to sugar as electrons and hydrogen ions are added to it.
• The food-producing redox reactions of photosynthesis involve an uphill climb
• The light energy captured by chlorophyll molecules in the chloroplast provides the boost for the electrons.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Two Stages of Photosynthesis: A Preview
• Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
LE 10-5_1
H2O
LIGHTREACTIONS
Chloroplast
Light
Light energy is absorbed by chlorophyll and drives the transfer of electrons and hydrogen from water to the electron acceptor….
The Light Reactions – converts solar energy to chemical energy
Take place in the thylakoids
Split water
LE 10-5_2
H2O
LIGHTREACTIONS
Chloroplast
Light
ATP
NADPH
O2
Stores energized electrons
Electron acceptor is NADP+
Reduced to NADPH
Oxygen is released
ATP is formed
NADPH is an electron carrier.
LE 10-5_3
H2O
LIGHTREACTIONS
Chloroplast
Light
ATP
NADPH
O2
NADP+
CO2
ADPP+ i
CALVINCYCLE
[CH2O](sugar)
The Calvin Cycle
•The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
•The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
• Chloroplasts are solar-powered chemical factories
• Their thylakoids transform light energy into the chemical energy of ATP and NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Nature of Sunlight
• Sunlight is a form of electromagnetic energy, also called radiation
• Like other electromagnetic energy, light travels in rhythmic waves
• Wavelength = distance between crests of waves
• Light also behaves as though it consists of discrete particles, called photons
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Visible radiation
• The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
• Visible light consists of colors we can see, including wavelengths that drive photosynthesis
LE 10-6
Visible light
Gammarays
X-rays UV Infrared Micro-waves
Radiowaves
10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m
(109 nm) 103 m
380 450 500 550 600 650 700 750 nm
Longer wavelength
Lower energy
Shorter wavelength
Higher energy
The amount of energy in a photon is inversely related to its wavelength
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosynthetic Pigments: The Light Receptors
• Pigments are substances that absorb visible light
• Different pigments absorb different wavelengths
• Wavelengths that are not absorbed are reflected or transmitted
• Leaves appear green because chlorophyll reflects and transmits green light
Animation: Light and Pigments
LE 10-7
Chloroplast
Light
Reflected light
Absorbed light
Transmitted light
Granum
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Chlorophyll a is the main photosynthetic pigment
– Participates directly in the light reactions
• Accessory pigments, such as chlorophyll b,
– Absorbs mainly blue and orange light (reflects yellow-green)
• Broadens the range of light that a plant can use
• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
– Absorb mainly blue-green light (reflect yellow-orange)
Chloroplast Pigments
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Excitation of Chlorophyll by Light
• Light behaves as discrete packets of energy called photons
– A photon is a fixed quantity of light energy
• When a pigment absorbs a photon, the electron has been raised from a ground state to an excited state, which is very unstable
• When excited electrons fall back to the ground state, photons are given off (electrons lose energy), and emit an afterglow called fluorescence
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes
• A photosystem consists of a reaction center surrounded by light-harvesting complexes
• The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosystem
• A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a
• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions
LE 10-12
Thylakoid
Photon
Light-harvestingcomplexes
Photosystem
Reactioncenter
STROMA
Primary electronacceptor
e–
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
Th
ylak
oid
mem
bra
ne
Pigment molecules include chlorophyll a and b.
Cluster of pigmented molecules functions as a light-gathering antenna.
Photons jump from pigment to pigment and arrive at the reaction center
The reaction center is a chlorophyll a molecule and a primary electron acceptor molecule
Acceptor traps light excited electron from reaction center chlorophyll.
Trapped energy
converted to ATP and NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Two Photosystems in the light reactions
• Water Splitting (photosystem II)
– Uses light energy to extract electrons from water
– Releases O2 as a waste product
• NADPH producing (photosystem I)
– Produces NADPH by transferring light-excited electrons from chlorophyll to NADP+
• An electron transport chain connecting the two systems releases energy that the chloroplast uses to make ATP
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
LE 10-14
ATP
Water splittingsystem
e–
e–
e–e–
MillmakesATP
e–
e–
e–
Ph
oto
n
NADPH producing system
Ph
oto
n
NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Generating ATP in the Light Reactions
Light reactions take place in the thylakoid membrane
• The mechanism of ATP production is similar to ATP production in cellular respiration
• An electron transport chain pumps hydrogen ions (H+) across the thylakoid membrane
• ATP synthases use the energy stored by the H+ gradient to make ATP
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Light Reactions
•
Electrons are stored at a high state of potential energy in NADPH. This and ATP will move on to the Calvin Cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Review of light reactions
http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
• The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
• The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. It requires more ATP than NADPH
• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
• For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Calvin Cycle
• The Calvin cycle has three phases:
– Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor (RuBP)
• Takes place in the stromaPlay
LE 10-18_1
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
LightInput
3
CO2
(Entering oneat a time)
Rubisco
3 P P
Short-livedintermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
CALVINCYCLE
3 P P
Ribulose bisphosphate(RuBP)
CO2 is added to a five-C sugar. The reaction is catalyzed by the enzyme Rubisco
splits into 2 molecules of
LE 10-18_2
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
Light Input
CO2
(Entering oneat a time)
Rubisco
3 P P
Short-livedintermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
3
P P
Ribulose bisphosphate(RuBP)
3
6 NADP+
6
6 NADPH
P i
6 P
1,3-BisphosphoglycerateP
6 P
Glyceraldehyde-3-phosphate(G3P)
P1
G3P(a sugar)Output
Phase 2:Reduction
Glucose andother organiccompounds
Each molecule of 3… is then phosphorylated by ATP to form
2 electrons from NADPH reduce this compound to create
The cycle must turn 3 times to create a net gain of one molecule of G3P - glucose
LE 10-18_3
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
Light Input
CO2
(Entering oneat a time)
Rubisco
3 P P
Short-livedintermediate
Phase 1: Carbon fixation
6 P
3-Phosphoglycerate6 ATP
6 ADP
CALVINCYCLE
3
P P
Ribulose bisphosphate(RuBP)
3
6 NADP+
6
6 NADPH
P i
6 P
1,3-BisphosphoglycerateP
6 P
Glyceraldehyde-3-phosphate(G3P)
P1
G3P(a sugar)Output
Phase 2:Reduction
Glucose andother organiccompounds
3
3 ADP
ATP
Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P5
G3P
The rearrangement of five molecules of G3P into the 3 molecules of RuBP requires 3 more ATP
9 molecules of ATP and 6 of NADPH are required to synthesize one G3P
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Calvin Cycle review
•
http://www.science.smith.edu/departments/Biology/Bio231/calvin.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
C4 and CAM plants
• All photosynthetic plants need CO2 to build sugar
– CO2 binds to RuBP (in Calvin Cycle)
– This is called the C3 pathway because it produces 2 – 3carbon molecules that continue in the cycle
• The enzyme that catalyzes this reaction is RUBISCO
– Rubisco is not efficient at grabbing CO2
– If CO2 levels fall too low, rubisco grabs O2 and sugars are burned up.
– This process is called photorespiration
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photorespiration - problems
• Problem during hot, dry days – or in these climates, plants keep stomates closed to prevent water loss
• Different systems evolved to deal with this problem
– So stomates don’t need to be open too often
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Comparing C4 and CAM plants
C4
• During the day
• Takes place in mesophyll cells
• OAA pumped to bundle sheath (which are other cells)
• CO2 is then released to Calvin Cycle
• Ex. Corn, sugar cane, tropical grass
CAM
• Happens at night
• OAA stored in vacuoles within the cell
• Stomates can be open in evening when cooler avoiding water loss
• CO2 released to Calvin Cycle in day and driven by suns energy
• More common: ex: cacti and succulents
Each use a different enzyme to bind CO2Harvest more CO2 without opening stomates too oftenEnzyme is PEP carboxylase instead of Rubisco (4 –C-molecule, hence the name)Forms OAA (oxaloacetate)CO2 comes from this to use in the Calvin Cycle
LE 10-20
Bundle-sheathcell
Mesophyllcell Organic acid
C4
CO2
CO2
CALVINCYCLE
Sugarcane Pineapple
Organic acidsrelease CO2 toCalvin cycle
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Organic acid
CAM
CO2
CO2
CALVINCYCLE
Sugar
Spatial separation of steps Temporal separation of steps
Sugar
Day
Night
No structural separation like C4 plants: instead, the two processes are separated in time.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Importance of Photosynthesis: A Review
• The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
• Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
• In addition to food production, photosynthesis produces the oxygen in our atmosphere
LE 10-21
Light
CO2H2O
Light reactions Calvin cycle
NADP+
RuBP
G3PATP
Photosystem IIElectron transport
chainPhotosystem I
O2
Chloroplast
NADPH
ADP+ P i
3-Phosphoglycerate
Starch(storage)
Amino acidsFatty acids
Sucrose (export)