BIO1PS 2012Plant Science

Lecture 10Photosynthesis Pt. II

Dr. Michael EmmerlingDepartment of BotanyRoom 410m.emmerling@latrobe.edu.au

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