BIO1PS 2012Plant Science
Lecture 10Photosynthesis Pt. II
Dr. Michael EmmerlingDepartment of BotanyRoom [email protected]
Learning Objectives• Name and describe the function of the
predominant leaf pigments
• Describe the major difference between C3, C4 and CAM photosynthetic pathways
• Describe the difference in leaf structure between C3 and C4 plants
• De"ne photorespiration
Function of Leaves
Generate energy from photosynthesis• this is the primary function• sugars (primarily sucrose) from carbon dioxide
(CO2) and water (H2O)
Photosynthesis• Wavelengths of light• Absorption of light by chlorophylls and accessory
pigments• Photosystems• Fixation of CO2
Photosynthesis
light
e.g. glucosefructose
6 mol + 6 mol
occurs (primarily) in the mesophyll cells
6CO2 + 6H2O C6H12O6 + 6O2
1 mol + 6 mol
Photosynthesis Requires Energy• two unreactive substances CO2 and H2O are
made to react together to give two more reactive substances
• these two more reactive substances are sugar and O2
• this reaction is driven by the energy in sunlight
Why Green?
Knox et al. (2010), 3rd ed., Fig. 3.15 Ladiges et al. (2010), 4th ed., Fig. 4.17
The Electromagnetic Spectrum
Ladiges et al. (2010), 4th ed., Fig. 6.11
Light and Colour
Absorbed and re#ected light determine the colour
Absorption by Leaf Pigments
Knox et al Fig. 5.12Ladiges et al (2010), 4th ed., Fig. 6.12
Chlorophyll a and b
Ladiges et al (2010), 4th ed., Fig. 6.13
Knox et al Fig. 5.13
Carotenoids
"accessory pigment"
Ladiges et al (2010), 4th ed., Fig. 6.13
Absorption by Leaf Pigments
Knox et al Fig. 5.12Ladiges et al (2010), 4th ed., Fig. 6.12
Ladiges et al. (2010), 4th ed., Fig. 4.2
A "Typical" Plant Cell
Chloroplasts
Ladiges et al. (2010), 4th ed., Fig. 6.14
Ladiges et al. (2010), 4th ed., Fig. 4.17
Chloroplasts
Ladiges et al. (2010), 4th ed., Fig. 6.14
Thylakoid Membrane
Ladiges et al. (2010), 4th ed., Fig. 6.14
Ladiges et al. (2010), 4th ed., Fig. 6.15
Thylakoid Membrane
"Interactive" model:http://molvis.sdsc.edu/fgij/fg.htm?mol=http://opm.phar.umich.edu/pdb/1jb0.pdb
Ladiges et al. (2010), 4th ed., Fig. 6.15
Photosystems• Light-absorbing (chlorophyll and accessory
pigments) complexes of many proteins and enzymes
• In eukaryotes, chloroplasts have 2 photosystems (PSI and PSII)
• Cyanobacteria have PSI and PSII
• Purple and green bacteria have only PSI
Photosystem I
http://www.ncbi.nlm.nih.gov/Structure/mmdb/mmdbsrv.cgi?uid=78133
Cn3D Screenshot
Hordeum vulgare
Photosystem II
http://www.ncbi.nlm.nih.gov/Structure/mmdb/mmdbsrv.cgi?uid=72733
Cn3D ScreenshotThermosynechococcus vulcanus
• chlorophyll is “excited” by photons
• transfer of energy to other molecules, and ultimately to a special pair of chlorophylls, “reaction centre” (P680, P700)
• the excited reaction centre expels an electron
• the electron is replaced by the hydrolysis of water, or photolysis (light-induced hydrolysis)
Photosystems
Photosystems
Ladiges et al. (2010), 4th ed., Fig. 6.17
pH Gradient
H+ H+ H+
H+ H+ H+
H+ H+ H+
Ladiges et al. (2010), 4th ed., Fig. 6.14
Ladiges et al. (2010), 4th ed., Fig. 6.17
•chemiosmotic gradient•H+ #ow through an ATP
synthase•synthesis of ATP from
ADP
pH Gradient
Ladiges et al. (2010), 4th ed., Fig. 6.17
H+
H+
Night and Day
Taiz and Zeiger (2010), 5th ed., Fig. 8.1
Carbon Fixation
RuBP
CO2
Calvin - Benson CycleNobel Prize, 1961
Rubisco
StromaCytosol
ATP +NADPHADPNADP+
ATPADP
Refer to Ladiges et al. (2010), 4th ed., Fig. 6.19
Ru5P
6x
3x
3x
3x
6x
6x
5x3x
3x3x
6x3-PGA
GAP
Carbon Fixation
Taiz and Zeiger (2006), 4th ed., Fig. 8.2
Carboxylationfree!
Reduction3-PGA to GAP
Regenerationnew acceptor
Ladiges et al. (2010), 4th ed., Fig. 6.20
C3
Carbon Fixation
"Beyond C3"• Photosynthetic cells look the same
Knox et al. (2005), 3rd ed., Fig. 3.15
• Leaf anatomy can be quite different
• ~5% of species have a very distinctive leaf anatomy called C4 or "Kranz"
C4 Leaf Anatomy“Kranz Anatomy”
Stoma
Bundle Sheathmesophyll
Vascular Bundle
mesophyll cells in contact with bundle sheath cells
Knox et al Fig. 5.22Ladiges et al. (2010), 4th ed., Fig. 6.22
C4 Leaf Anatomy
C4 Carbon Fixation
Occurs in the cytoplasm of the mesophyll cells C4Ladiges et al. (2010), 4th ed., Fig. 6.23
C4 Leaf Anatomy
mesophyllPEP carboxylase
bundle sheathRubisco
"pre-!xation" of CO2 to form C4 product (e.g. malate)
"actual !xation" of CO2 in Calvin-Benson cycle
• is synthesised in cytosol of mesophyll cells
• is transported to bundle sheath cells
• is decarboxylated in bundle sheath cells to give CO2 and pyruvate
Knox et al Fig. 5.24
C4 PhotosynthesisMalate
Ladiges et al. (2010), 4th ed., Fig. 6.24
• the released CO2 is re-"xed by Rubisco
• pyruvate is transported to mesophyll cells
• Ribulose bisphosphate carboxylase oxygenase
• Oxygenase
• Catalyzes the incorporation of O2 into ribulose bisphosphate (then 3-PGA and 2-phosphoglycolate)
Rubisco• Ribulose bisphosphate carboxylase
• Carboxylase
• Catalyzes the incorporation of CO2 into ribulose bisphosphate (then 2x 3-PGA)
Photorespiration• Ribulose bisphosphate carboxylase oxygenase
• Oxygenase
• Catalyzes the incorporation of O2 into ribulose bisphosphate (then 3-PGA and 2-phosphoglycolate)
2-phosphoglycolate• can be "salvaged"• involves chloroplasts, peroxisomes and
mitochondria• consumes O2 and energy• produces CO2
• happens only in the light: photorespirationTaiz and Zeiger (2006), 4th ed., Fig. 8.8
C4 Evolution• C4 photosynthesis may have evolved in response
to reduction in atmospheric CO2 concentration
• 100 MYA CO2 concentration was ~5-10 times higher than today’s levels
• Rubisco performed carboxylase activity rather than oxygenase activity
• From 1500 - 3000 μl CO2 l-1 air down to ~380 μl CO2 l-1
• C4 plants concentrate CO2
• is synthesised in cytosol of mesophyll cells
• is transported to bundle sheath cells
• is decarboxylated in bundle sheath cells to give CO2 and pyruvate
Knox et al Fig. 5.24
C4 PhotosynthesisMalate
Ladiges et al. (2010), 4th ed., Fig. 6.24
• the released CO2 is re-"xed by Rubisco
• pyruvate is transported to mesophyll cells
Rubisco
CO2
C3 and C4 PhotosynthesisFeature C3 C4
!rst CO2-"xing enzyme
Rubisco PEP carboxylase
location mesophyll chloroplast
mesophyll cytoplasm
!rst stable product
phosphoglycerate (C3) malate (C4)
sucrose produced in mesophyll bundle sheath
rate of photosynthesis moderate high
As If That's Not Enough ...
Ladiges et al. (2010), 4th ed., Figs. 6.25 and 6.27
Pyrrosia longifolia
CAMCrassulacean Acid Metabolism • photosynthetic pathway used by many
species of succulent plants (cacti, agave, pineapple)
• "rst discovered in plants of the Crassulaceae family
• ~10% of plant species are CAM plants
CAM MechanismNight• stomata open• CO2 "xed by PEP carboxylase to form
malate (similar to C4)• malate stored in vacuoles
Day• stomata closed• CO2 released from malate• CO2 "xed by Rubisco
Ladiges et al. (2010), 4th ed., Fig. 6.26
CAM Plants and Stomata
Ladiges et al. (2010), 4th ed., Fig. 17.20
CO2 AssimilationC3
• "rst product is 3-phosphoglyceric acid (3-PGA), then glyceraldehyde-3-phosphate (GAP)
• catalysed by Rubisco
C4
• "rst stable product is malate (C4 dicarboxylic acid)• catalysed by phosphoenolpyruvate carboxylase (PEPC)• spacial separation (mesophyll and bundle sheath)
CAM (Crassulacean Acid Metabolism)• "rst stable product is malate (C4 dicarboxylic acid), at night• temporal separation, and storage as malate
Photosynthesis
Taiz and Zeiger (2010), 5th ed., Fig. 8.1