Photosynthesis, Respiration

and Assimilationand Assimilation

� Definition:

- a process in which solar energy is converted into chemical energy

6CO2 + 12H2O --> C6H12O6 + 6O2 + 6H2O

� Location: chloroplasts

� Two parts:

1. Light dependent reaction :- to generate NADPH and ATP required for subsequent carbon

reduction

Photosynthesis

reduction

- Phototsystem I – LHC I, P700 (reaction centre chlorophyll a, Amax 700 nm)

- Photosystem II – LHC II, P680 (reaction centre chlorophyll a, Amax 680 nm)

- cytochrome b6f complex

- ATPase

2. Light independent reaction

- Calvin cycle (photosynthetic carbon reduction cycle)

i. carboxylation: fix CO2

ii. reduction: 3-carbon acids triose phosphate (G3P); consume ATP and NADPH

iii. regeneration: triose phosphate (G3P) RuBP; consume ATP

Types of Plastids

Proplastid

leaves

non-photosynthetic

storage tissues

fruit

petals

roots

Chloroplast

AmyloplastChromoplast

leaves

stems

senescing leaves

and stems

leaves

and stems

Adapted from: Hopkins and Huner (2004) Introduction to Plant Physiology. 3rd ed.

Photosynthesis: Light Dependent and Light Independent Reactions

Light Reactions

H2O

Light

CO2

Calvin cycle

NADP+

ADP

Chloroplast

Light Reactions

O2 [CH2O] n(sugars)

cycle

ATP

NADPH

Adapted from: Campbell, Reece & Meyers. 2005. Biology 7th ed.

Cytoplasm

� Definition:

- a process in which solar energy is converted into chemical energy

6CO2 + 12H2O --> C6H12O6 + 6O2 + 6H2O

� Location: chloroplasts

� Two parts:

1. Light dependent reaction :- to generate NADPH and ATP required for subsequent carbon

reduction

Photosynthesis

reduction

- Phototsystem I – LHC I, P700 (reaction centre chlorophyll a, Amax 700 nm)

- Photosystem II – LHC II, P680 (reaction centre chlorophyll a, Amax 680 nm)

- cytochrome b6f complex

- ATPase

2. Light independent reaction

- Calvin cycle (photosynthetic carbon reduction cycle)

i. carboxylation: fix CO2

ii. reduction: 3-carbon acids triose phosphate (G3P); consume ATP and NADPH

iii. regeneration: triose phosphate (G3P) RuBP; consume ATP

Electron Transport System in the Thylakoid Membrane

ADP+Pi ATPH+

stroma

thylakoid outer envelope

inner envelope

granum (a stack of thylakoids)

Chloroplast

Fe-S

PCPC

NADP+ NADPH

FD

2H2O O2 + 4H+

PQ

PQH2

2H+

2H+

PS IIPS I

Cytochrome b6f

complex ATP synthasestroma

thylakoid membrane

lumen

lumen

Adapted from: Hopkins and Huner (2004) Introduction to Plant Physiology. 3rd ed.

� Definition:

- a process in which solar energy is converted into chemical energy

6CO2 + 12H2O --> C6H12O6 + 6O2 + 6H2O

� Location: chloroplasts

� Two parts:

1. Light dependent reaction :- to generate NADPH and ATP required for subsequent carbon

reduction

Photosynthesis

reduction

- Phototsystem I – LHC I, P700 (reaction centre chlorophyll a, Amax 700 nm)

- Photosystem II – LHC II, P680 (reaction centre chlorophyll a, Amax 680 nm)

- cytochrome b6f complex

- ATPase

2. Light independent reaction

- Calvin cycle (photosynthetic carbon reduction cycle)

i. carboxylation: fix CO2

ii. reduction: 3-carbon acids triose phosphate (G3P); consume ATP and NADPH

iii. regeneration: triose phosphate (G3P) RuBP; consume ATP

C3 Photosynthesis

CO2

Ribulose-1,5-bisphosphate Calvin cycle

3-phosphoglycerate (3C)

1, 3-bisphosphoglycerate

Chloroplast ATP

ADP

Ribulose-1,5-bisphosphate (RuBP; 5C)

Sugars

Calvin cycle

Glyceraldehyde-3-phosphate (G3P)

1, 3-bisphosphoglycerate

Cytoplasm

Examples of C3 plants: rice, wheat, soybean, and potato carboxylation

reductionregeneration

NADPH

NADP+

ADP

ATP

C4 Photosynthesis

MC

BSC

MC

Phosphoenolpyruvate(3C)

CO2

Oxaloacetate (4C)

Malate / aspartate (4C)

Mesophyll cell

PEP carboxylase

Cytoplasm

Spatial separation

Edwards et al. 2001. Plant Physiol. 125:46-49

Mesophyll cells (MC)-chloroplasts: stroma – devoid of Rubisco

thylakoids – PS I and PS II highly active

Bundle sheath cells (BSC)-chloroplasts: stroma – replete with Rubisco

thylakoids – lacking PS II

Examples of C4 plants: maize, sorghum and sugarcane

Calvin cycle

sugars

CO2

Malate / aspartate (4C)

Bundle sheath cell

Pyruvate

Chloroplast

Cytoplasm

Anatomy of C3 and C4 Leaves

(Plant Physiology Online; http://3e.plantphys.net/article.php?ch=e&id=290)

Crassulacean Acid Metabolism (CAM)

CO2 is taken up during the night

The prefixed CO2 is stored in the vacuoles

Temporal separation

[Dodd et al. 2002. Journal of Experimental Botany 53(369): 569-580]

Examples of CAM plants: pineapple, cacti, agave and orchids

2

stored in the vacuoles as malate

Malate is transported out of the vacuole during the day for incorporation into carbohydrates via the Calvin cycle

Respiration

� Respiration is a cellular process in which hexose is oxidised to CO2 and water, accompanied by the release of energy

� Stages:

1. Breakdown of starch and sucrose

2. Conversion of hexose sugars to pyruvate – glycolysis

3. Oxidation of pyruvate by the TCA (tricarboxylic acid) cycle or Krebs cycle

4. Transfer of electrons from NADH and FADH to O and the accompanying conversion 4. Transfer of electrons from NADH and FADH2 to O2 and the accompanying conversion of

redox energy to ATP – mitochondria electron transport chain

� Respiration provides carbon skeletons for the biosynthesis of other molecules

Role of Respiration in the Biosynthesis of Cellular Molecules

Nucleic acidsATPNAD

Cytokinins

Shikimic acid

Amino acidsAlanine Protein

Pentose-P Cellulose

GlycerolTriglyceridesPhospholipids

Starch

Glucose-6-P

Triose-P

Phosphoenolpyruvate

PyruvateAmino acids

Auxin

OxaloacetateAspartate

Citrate

α-ketoglutarate

Glutarate

Other amino acids

Protein

Alanine Protein

Isoprenoids

Fatty acids CarotenoidsGibberellinsTerpenes

PorphyrinsChlorophyllCytochromePhytochrome

Pyruvate

AcetylCoA

Other amino acids

Protein

Other amino acidsAlkaloidsProtein

TCA cycle

Adapted from: Hopkins and Huner (2004) Introduction to Plant Physiology. 3rd ed.

Pyruvate Triose-P

Calvin cycle

Triose-P

CO2 O2

Chloroplast

Interaction among Chloroplast, Cytoplasm and Mitochondrion

Pyruvate

FADH2

Mitochondrion

Ethanol

aerobic

anaerobic

Light

CO2

SucroseStarch

Chloroplast

Cytoplasm

TCA cycle

ATP NADH

FADH2

Adapted from: Hopkins and Huner (2004) Introduction to Plant Physiology. 3rd ed.

Photorespiration

• Photorespiration = photosynthetic carbon oxidation cycle = C2 glycolate cycle

• It involves three cellular compartments: - chloroplast- peroxisome - mitochondrion

*

• Light dependent process

• O2 is taken up (oxygenase activity of Rubisco) and CO2 is released

• It is counterproductive to photosynthetic CO2

fixation

• But it protects the plants against abiotic stresscaused by light, drought and salinity

Reumanna and Weber (2006) Biochimica et Biophysica

Acta-Molecular Cell Research 1763 (12): 1496-1510

Respiration, Photorespiration and Photosynthesis

Dark Light

O2 CO2O2

CO2 CO2

O2

PhotosynthesisRespirationRespiration

CO2O2Photorespiration

Adapted from: Hopkins and Huner (2004) Introduction to Plant Physiology. 3rd ed.

Photosynthesis, Respiration and Photorespiration in Higher Plants

Kruse et al. (2005) Photochem. Photobiol. Sci. 4: 957-970

Plant Nutrients

Available FormElement

Hydrogen

Carbon

Oxygen

Nitrogen

Potassium

Calcium

Magnesium

Macronutrients

H2O

CO2

O2 , CO2

NO3 , NH4

K+

Ca2+

Mg2+

Concentration in dry matter (mmol/kg)

60,000

40,000

30,000

1,000

250

125

80Magnesium

Phosphorus

Sulfur

Micronutrients

Chlorine

Boron

Iron

Manganese

Zinc

Copper

Nickel

Molybdenum

Mg

H2PO4 , HPO4

SO4

2 -

2 -

Cl _

BO3

Fe2+

Mn2+

Zn2+

Cu2+

Ni2+

MoO4

3 -

2 -

80

60

30

3.0

2.0

2.0

1.0

0.3

0.1

0.05

0.001

Adapted from: Hopkins and Huner (2004) Introduction to Plant Physiology. 3rd ed.

Modes of Solute Transport across

Membranes

Channel protein

Carrier proteins(transporters)

Membrane

K+

Ca2+

, NO3, H2PO4, SO4- - 2-

O2 , CO2 , NH3

Co

nfo

rmati

on

al

ch

an

ge

Co

nfo

rmati

on

al

ch

an

ge

ATP ADP + Pi

Simple diffusion

Active transport

Passive transport

Facilitated diffusion

Co

nfo

rmati

on

al

ch

an

ge

Co

nfo

rmati

on

al

ch

an

ge

Adapted from: Hopkins and Huner (2004) Introduction to Plant Physiology. 3rd ed.

Nitrogen Assimilation

� Nitrogen� N2 constitutes 78% by volume of the atmosphere

� The 4th most abundant nutrient element in plants

� Essential constituent of proteins, nucleic acids, hormones, chlorophyll etc.

� Higher plants are not able to convert N2 into a biologically useful form� Depend on free-living prokaryotes or nitrogen-fixing

symbionts

� Nitrogen in the form of NO3 or NH4 is absorbed by the roots

Nitrogen Cycle

Atmospheric N2

Biological N2

fixation(60%)

Industrial N2

fixation(30%)

Electrical N2

fixation(10%)

Denitrification

Thiobacillus denitrificansBacteria

Cyanobacteria

Soil Nitrogen Pool

NH3 NO3NO2

Decaying

biomassAnimal biomass

Plant biomass

Adapted from: Hopkins and Huner (2004) Introduction to Plant Physiology. 3rd ed.

UptakeAmmonification

Nitrosomonas, Nitrococcus Nitrobacter

Symbiotic Nitrogen Fixation

MicrosymbiontHost

Leguminous species

AzorhizobiumBradyrhizobium japonicumRhizobium leguminosarum

SesbaniaGlycine max (Soybean)Lens (Lentil)

Nodule-forming

Non-leguminous species

Rhizobium leguminosarumPisum (Garden pea)

ParasponiaAlnus (alder)Myrica (bayberry)Casuarina (pine)

RhizobiaFrankiaFrankiaFrankia

Non-nodule-forming

Azolla (aquatic fern) Anabaena

Hopkins and Huner (2004) Introduction to Plant Physiology. 3rd ed.

End of lectureEnd of lecture