AN OVERVIEW OF PHOTOSYNTHESIS
Copyright © 2009 Pearson Education, Inc.
Carbon dioxide
C6H12O6
Photosynthesis
H2OCO2 O2
Water
+ 66
Lightenergy
Oxygen gasGlucose
+ 6
Plants use water and atmospheric carbon dioxide to produce a simple sugar and release oxygen
– Photosynthesis: a process that converts solar energy to chemical energy
Autotrophs Are the Producers of The Biosphere
Autotrophs make their own food without using organic molecules derived from any other living thing
– Photoautotrophs : Autotrophs that use the energy of light to produce organic molecules
– Most plants, algae and other protists, and some prokaryotes are photoautotrophs
– The ability to photosynthesize is directly related to the structure of chloroplasts
Copyright © 2009 Pearson Education, Inc.
CO2 O2Stoma
Vein
Chloroplasts
Chlorophyll light absorbing pigment in
chloroplasts responsible for the green
color of plants located on thylakoid
membrane Stomata tiny pores in the leaf; allow
CO2 to enter and O2 to exit Veins in the leaf deliver
water absorbed by roots
Photosynthesis Occurs in Chloroplasts in Plant Cells
Chloroplast
Outer and innermembranes
IntermembranespaceGranumStroma Thylakoid
space
Thylakoid
Photosynthesis is a Redox Process, as is Cellular Respiration
Photosynthesis is a redox (oxidation-reduction) process
A loss of electrons = oxidation
A gain of electrons = reduction *(OIL RIG)
In photosynthesis: Water is oxidized and CO2 is reduced
The oxidation of water produces oxygen and the reduction of CO2 produces sugar
6 CO2 + 6 H2O C6H12O6 + 6 O2
Reduction
Oxidation
Photosynthesis is a Redox Process, as is Cellular Respiration
H2O
ADP
P
LIGHTREACTIONS
(in thylakoids)
Light
Chloroplast
NADPH
ATP
O2
CALVINCYCLE
(in stroma)
Sugar
CO2
NADP+
Visible Radiation Drives the Light Reactions
Sunlight is a type of electromagnetic energy (radiation)
Visible light is a small part of the electromagnetic spectrum
Light exhibits the properties of both waves and particles
– One wavelength = distance between the crests of two adjacent waves (the shorter the wavelength, the greater the energy)
– Light behaves as discrete packets of energy called photons (fixed quantity of light energy)
Wavelength (nm)
10–5 nm
Increasing energy
Visible light
650nm
10–3 nm 1 nm 103 nm 106 nm 1 m 103 m
380 400 500 600 700 750
Radiowaves
Micro-waves
InfraredX-rays UVGammarays
Pigments are molecules that absorb specific wavelengths of light and transmit others
Many pigments are built into the thylakoid membrane
We see the color of the wavelengths that are transmitted/reflected
– Ex. chlorophyll transmits green
Visible Radiation Drives the Light Reactions
Chloroplasts contain several different pigments and all absorb light of different wavelengths
– Chlorophyll a: absorbs blue violet and red light and reflects green
– Chlorophyll b: absorbs blue and orange and reflects yellow-green
– The carotenoids: absorb mainly blue-green light and reflect yellow and orange
Visible Radiation Drives the Light Reactions
Chlorophyllmolecule
Excited state
Ground state
Heat
PhotonPhoton
(fluorescence)
e–
Reactioncenter complex
e–
Light-harvestingcomplexes
Photosystem
Pigmentmolecules
Pair ofChlorophyll a molecules
Solar energy could be released as heat or light but instead is conserved and passed from one molecule to another
All of the components to accomplish this are organized in thylakoid membranes in clusters called photosystems
Photosystems = light-harvesting complexes surrounding a reaction center complex
Photosystems Capture Solar Power
When light strikes the photosystem energy is passed from molecule to molecule within the photosystem
– Energy from the pigment molecules, passes to the reaction center where it excites an e- of the chl. a molecules
– Excited e- are transferred to the primary electron acceptor (becomes reduced)
– This solar-powered transfer of an electron from the reaction center pigment to the primary electron acceptor is the first step of the light reactions
Photosystems Capture Solar Power
Two types of photosystems exist: photosystem I and photosystem II
– Each type of photosystem has a characteristic reaction center
– Photosystem II: functions first; is called P680 (chl a best absorbs light w/ a wavelength of 680, red)
– Photosystem I: functions next; is called P700 (chl a absorbs wavelength of 700, also red)
Photosystems Capture Solar Power
Two Photosystems Connected by an Electron Transport Chain Generate ATP and NADPH
During the light reactions, light energy is transformed into the chemical energy of ATP and NADPH
– Electrons removed from water pass from photosystem II to photosystem I and are accepted by NADP+
– The bridge between photosystems II and I is an electron transport chain that provides energy for the synthesis of ATP
– NADPH, ATP, and O2 are the products of the light reactions
NADPH
Photosystem II
e–
Millmakes
ATP Ph
oto
n
Photosystem I
ATP
e–
e–
e–
e–
e–
e–
Ph
oto
n
Stroma
O2
H2O 12 H+
NADP+ NADPHPhoton
Photosystem II
Electron transport chainProvides energy forsynthesis of
by chemiosmosis
+ 2
Primaryacceptor
1
Thylakoidmem-brane
P680
2
4
3Thylakoidspace
e–e–
5
Primaryacceptor
P700
6
Photon
Photosystem IATP
H++
Chemiosmosis Powers ATP Synthesis in the Light Reactions
In Chemiosmosis, the potential energy of a H+ gradient is used to synthesize ATP
The H+ gradient is generated by the ETC
ATP synthase couples the flow of H+ to the phosphorylation of ADP
– The chemiosmotic production of ATP in photosynthesis is called photophosphorylation
+
O2
H2O12 H+
NADP+ H+ NADPH
+ 2
H+
H+
H+ H+
H+
H+
H+
H+
H+H+
H+
H+
H+ H+
Photosystem II Photosystem IElectrontransport
chain
ATP synthase
LightLight
Stroma (low H+
concentration)
Thylakoid space(high H+ concentration)
ADP + P ATP
THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS
CO2
ATPNADPH
Input
CALVINCYCLE
G3P
Output:
The Calvin cycle makes sugar within a chloroplast
Atmospheric CO2, ATP, and NADPH are required to produce sugar
Using these three ingredients, an energy-rich, three-carbon sugar called glyceraldehyde-3-phosphate (G3P) is produced
A plant cell may then use G3P to make glucose and other organic molecules
ATP and NADPH Power Sugar Synthesis in the Calvin Cycle
The Calvin Cycle Has Three Phases:
1. Carbon Fixation-Atmospheric carbon in the form of CO2 is incorporated into a molecule of ribulose bisphosphate (RuBP) via rubisco
2. Reduction Phase – NADPH reduces 3PGA to G3P (requires 3 molec’s. of CO2 for 1 G3P)
3. Regeneration of Starting Material –RuBP is regenerated and the cycle starts again
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ATP and NADPH Power Sugar Synthesis in the Calvin Cycle
NADPH
ATP
RuBP
3
P
G3P
P
Input:CO2
1
Rubisco
3 P
Step Carbon fixation
3-PGA6 P
CALVINCYCLE
6
6
6
6
P
Step Reduction
2
2
G3P5 P
3
3
G3P1 P
Glucoseand othercompounds
Output:
Step Release of one
molecule of G3P
1
Step Regeneration of RuBP4
4ATP3
3 ADP
NADP+
6 ADP +
PHOTOSYNTHESIS REVIEWED AND EXTENDED
Copyright © 2009 Pearson Education, Inc.
NADP+
NADPH
ATP
CO2
+
H2O
ADPP
Electrontransport
chainsThylakoidmembranes
LightChloroplast
O2
CALVINCYCLE
(in stroma)
Sugars
Photosystem II
Photosystem I
LIGHT REACTIONS
RuBP
3-PGA
CALVIN CYCLE
Stroma
G3P Cellularrespiration
Cellulose
Starch
Other organiccompounds
EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants
In hot climates, plant stomata close to reduce water loss so oxygen builds up
– Rubisco adds oxygen instead of carbon dioxide to RuBP in a process called photorespiration
– Photorespiration consumes oxygen, produces CO2, and produces no sugar, or ATP
Copyright © 2009 Pearson Education, Inc.
EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants
Some plants have evolved a means of carbon fixation that saves water during photosynthesis
– C4 plants partially shut stomata when hot and dry to conserve water
– This reduces CO2 levels, so contain PEP carboxylase to bind CO2 at low levels (carbon fixation); They’re called C4 plants because they first fix CO2 into a four-carbon compound.
– This allows plant to still make sugar by photosynthesis
Copyright © 2009 Pearson Education, Inc.
EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants
Another adaptation to hot and dry environments has evolved
– CAM open their stomata at night thus admitting CO2 in w/o loss of H2O
– CO2 enters, and is fixed into a four-carbon compound, (carbon fixation)
– It is released into the Calvin cycle during the day
Copyright © 2009 Pearson Education, Inc.
Mesophyllcell
CO2
CALVINCYCLE
CO2
Bundle-sheathcell 3-C sugar
C4 plant
4-C compound
CO2
CALVINCYCLE
CO2
3-C sugar
CAM plant
4-C compound
Night
Day