photosynthesis
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Photosynthesis. AP Biology Ms. Haut. Introduction. Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, photosynthesis nourishes almost the entire living world - PowerPoint PPT PresentationTRANSCRIPT
AP Biology
Ms. Haut
Light energy
enzymes
Photosynthesis is the process that converts solar energy into chemical energy
Directly or indirectly, photosynthesis nourishes almost the entire living world
Photosynthesis—process in which some of the solar energy is captured by plants (producers) and transformed into glucose molecules used by other organisms (consumers).
6CO2 + 6H2O C6H12O6 + 6O2
Glucose is the main source of energy for all life. The energy is stored in the chemical bonds.
Cellular Respiration— process in which a cell breaks down the glucose so that energy can be released. This energy will enable a cell to carry out its activities.
C6H12O6 + 6O2 6CO2 + 6H2O + energy
enzymes
Autotroph —organisms that synthesize organic molecules from inorganic materials (a.k.a. producers)
–Photoautotrophs —use light as an energy source (plants, algae, some prokaryotes)–Chemoautotrophs —use the oxidation of inorganic substances (some bacteria)
• Heterotroph —organisms that acquire organic molecules from compounds produced by other organisms (a.k.a. consumers)
Sunlight = electromagnetic energy•Wavelike properties•Particlelike properties (photon)
Thylakoids trap sunlight
Light may be reflected, transmitted, or absorbed when it contacts matter
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
Absorb light of varying wavelengths and transfer the energy to chlorophyll a
Chlorophyll b -yellow-green pigment Carotenoids -yellow and orange
pigments
An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process
Chlorophyll a
Chlorophyll b
Carotenoids
Wavelength of light (nm)
Absorption spectra
Ab
sorp
tio
n o
f lig
ht
by
chlo
rop
last
pig
men
ts
400 500 600 700
Endergonic redox process; energy is required to reduce CO2
Light is the energy source that boosts potential energy of electrons (e-) as they are moved from water to CO2
When water is split, e- are transformed from the water to CO2, reducing it to sugar
6CO2 + 6H2O
reduction
oxidation
C6H12O6 + 6O2
Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
Light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH
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
Light reactions —convert light energy to chemical bond energy in ATP and NADPH
Occurs in thylakoids in chloroplasts NADP+ reduced to NADPH—temporary
energy storage (transferred from water) Give off O2 as a by-product Generates ATP by phosphorylating ADP
Calvin Cycle —carbon fixation reactions assimilate CO2 and then reduce it to a carbohydrate
Occur in the stroma of the chloroplast Do not require light directly, but requires products of
the light reactions Incorporates into existing organic molecules and
then reduces fixed carbon into carbohydrate NADPH provides the reducing power ATP provides chemical energy
Light reactions produce: ATP and NADPH that are used by the Calvin cycle; O2 released
Calvin Cycle produces: ADP and NADP+ that are used by the light reactions; glucose produced
Interdependent Reactions
Photosystem: assemblies of several hundred chlorophyll a, chlorophyll b, and carotenoid molecules in the thylakoid membrane
form a light gathering antennae that absorb photons and pass energy from molecule to molecule
Photosystem I —specialized chlorophyll a molecule, P700
Photosystem II —specialized chlorophyll a molecule, P680
• Light drives the light reactions to synthesize NADPH and ATP• Includes cooperation of both photosystems, in which e- pass continuously from water to NADP+
1. When photosystem II absorbs light an e- is excited in the reaction center chlorophyll (P680) and gets captured by the primary e- acceptor.
• This leaves a hole in the P680
2. To fill the hole left in P680, an enzyme extracts e- from water and supplies them to the reaction center
• A water molecule is split into 2 H+ ions and an oxygen atom, which immediately combines with another oxygen to form O2
3. Each photoexcited e- passes from primary e- acceptor to photosystem I via an electron transport chain.
• e- are transferred to plastoquinone (Pq) and plastocyanin (Pc) (e- carriers)
4. As e- cascade down the e- transport chain, energy is released and harnessed by the thylakoid membrane to produce ATP (PHOTOPHOSPHORYLATION)
• This ATP is used to make glucose during Calvin cycle
5. When e- reach the bottom of e- transport chain, it fills the hole in the reaction center P700 of photosystem I.
• Pre-existing hole was left by former e- that was excited
6. When photosystem I absorbs light an e- is excited in the reaction center chlorophyll (P700) and gets captured by the primary e- acceptor.
• e- are transferred to ferredoxin (Fd) (e- carrier)• NADP+ reductase transfers e- from Fd to NADP+,
storing energy in NADPH (reduction reaction)• NADPH provides reducing power for making glucose in
Calvin cycle
Only photosystem I is used Only ATP is produced
ChemiosmosisChemiosmosis
Energy released from e- transport chain is used to pump H+ ions (from the split water) from the stroma across the thylakoid membrane to the interior of the thylakoid. Creates concentration gradient across
thylakoid membrane Process provides energy for chemisomostic
production of ATP
LIGHTREACTOR
NADP+
ADP
ATP
NADPH
CALVINCYCLE
[CH2O] (sugar)STROMA(Low H+ concentration)
Photosystem II
LIGHT
H2O CO2
Cytochromecomplex
O2
H2O O21
1⁄2
2
Photosystem ILight
THYLAKOID SPACE(High H+ concentration)
STROMA(Low H+ concentration)
Thylakoidmembrane
ATPsynthase
PqPc
Fd
NADP+
reductase
NADPH + H+
NADP+ + 2H+
ToCalvincycle
ADP
PATP
3
H+
2 H++2 H+
2 H+
Figure 10.17
The light reactions and chemiosmosis: the organization of the thylakoid membrane
Carbon enters the cycle in the form of CO2 and leaves in the form of sugar (glucose)
The cycle spends ATP as an energy source and consumes NADPH as a reducing agent for adding high energy e- to make sugar
For the net synthesis of this sugar, the cycle must take place 2 times
[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)
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
LightInput
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-Bisphosphoglycerate
P
6 P
Glyceraldehyde-3-phosphate(G3P)
P1
G3P(a sugar)
Output
Phase 2:Reduction
Glucose andother organiccompounds
Calvin CycleCalvin Cycle
[CH2O] (sugar)O2
NADPH
ATP
ADP
NADP+
CO2H2O
LIGHTREACTIONS
CALVINCYCLE
LightInput
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-Bisphosphoglycerate
P
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
1. Carbon Fixation: 3 CO2 molecules bind to 3 5-Carbon sugars, ribulose bisphosphate (RuBP) using enzyme called RuBP carboxylase (rubisco)
• Produces 6 molecules of a 3-carbon sugar, 3-phosphoglycerate
1. Carbon Fixation2. Reduction: 6 ATP molecules transfer
phosphate group to each molecule of 3-phos. to make 1,3-diphosphoglycerate
• 6 molecules of NADPH reduce each molecule of 1,3-diphosph. to make glyceraldehyde 3-phosphate (G3P)
3. One of the G3P exits the cycle to be used by the plant; the other 5 molecules are used to regenerate the CO2 acceptor, RuBP: 3 molecules of ATP are used to convert 5 molecules of G3P into RuBP
• 3 more CO2 molecules enter the cycle, following the same chemical pathway to release another G3P from the cycle.
• 2 G3P molecules can be used to make glucose
Interdependent
Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis
The closing of stomata reduces access to CO2
and causes O2 to build up These conditions favor a seemingly wasteful
process called photorespiration
• In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound
• In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
• Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle
C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle
Corn
Crab Grass
PEP carboxylase-high affinity to CO2 and no affinity for O2, thus no photorespiration possible
C4 Plants
CAM plants open their stomata at night, incorporating CO2 into organic acids
Organic acids stored in vacuoles of mesophyll cells until morning, when stomata close
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
http://ecology.botany.ufl.edu/ecologyf02/graphics/saguaro.GIF
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
Are similar in that CO2 is first incorporated into organic intermediates before it enters the Calvin cycle
Differ in that the initial steps of carbon fixation in C4 plants are structurally separate from the Calvin cycle; in CAM plants, the two steps occur at separate times
Regardless of whether the plant uses C3, C4, or CAM pathway, all plants use the Calvin Cycle to produce sugar from CO2
The CAM and C4 pathways: