photosynthesis

Photosynthesis

Dinesh D. KhedkarDepartment of Botany

NAAC Reaccredited “A” - “Very Good” Grade

Shri Shivaji Science CollegeAmravati, Maharashtra - 444 603

Phone No.: 0721-2660855 Fax: 0721-2665485e-mail : [email protected]

Web site : www.shivajiscamt.dataone.in

mailto:[email protected]

Life ? ? ? ? ? ?• Living organisms are mostly similar to

nonliving matters.

• Distinguished in growth (Metabolism, Division, Reproduction); and transition of energy

• Energy is being transformed from one form to other

MetabolismThe sum total of all enzymatic reactions occurring in the cell

Highly coordinated, purposeful activity in which many set of reactions exchanging matter

and energy between cells & its environment

Metabolism - Functions

1. To obtain energy from fuel

molecules

2. To convert exogenous

nutrients in to building

blocks

3. To assemble them in

Macromolecules

4. Degrade them in

specialized functions

Metabolism

Anabolism: DivergingConstructive Processes, Viz.

Photosynthesis

Requirements: CO2 & H2O

Catabolism: Converging Degradative Processes, Viz.

Respiration Requirements: Food and Oxygen

Energy Dependent

Energy transformations

Chemical Ele

ctricalKinetic

Light

Energy transformations Plants can harvest light energy animals can’t

Animals consume plants and procure source of energy

Plants: Endowed with great potential to prepare food (Photosynthesis)

Food can not work directly used to carry out routine life processes

It requires Burning of Food (Respiration)

pHOTOSYNTHESIS• Requirements:

CO2

H2O

Green Tissue - Chlorophyll

Light

Enzymes & Coenzymes

photosynthesis

photosynthesis

Photosynthesis converts light energy into the chemical energy of

sugars. Light energy from light drives the reactions.

Oxygen (O2) is a byproduct of

photosynthesis and is released into the

atmosphere:

Overall reaction

CO2 + H2O → (CH2O) + H2O + O2 (Dutch Microbiologist Van Niel, 1941)

It’s a sum of three step reaction –

1. 4 H2O → 4(OH) + 4 H

2. 4 H + CO2 → (CH2O) + H2O

3. 4 (OH) → 2 H2O + O2

Stoicheometric Reaction

6 CO2 + 12 H2O

Chlorophyll

C6H12O6 + 6 H2O + 6 O2

Reaction center• Green Leaves• Chlorenchyma cells• Chloroplast• Granum• Thylacoid• Thylacoid Membrane• Quantasome• Antenna Complex (Pigments)• Chlorophyll

Reaction center

pHOTOSYNTHESIS

Two Step Process (F. F. Blackman)–

1.Light dependent reactions -- Photophosphorylation

Cyclic photophosphorylation

Noncyclic photophosphorylation

2. Light independent reactions -- Carbon fixation--Calvin cycle

Light Reaction

Dark Reaction

Light dependent reaction

• Light

• Pigments

• Chemical Reaction

Light

Quantity

Quality

Light• The distance between the crests of waves is called the wavelength.

The shorter the wavelength, the greater the energy for each unit (photon) of electromagnetic energy.

• Remember, energy cannot be created or destroyed. When light is absorbed by a green plant, a small portion of that energy is converted into chemical energy in the process of photosynthesis.

Schematic diagram of the action spectrum measurements by T. W. Engelmann. Engelmann projected a spectrum of light onto the spiral chloroplast of the filamentous green alga Spirogyra and observed that oxygen-seeking bacteria introduced into the system collected in the region of the spectrum where chlorophyll pigments absorb. This action spectrum gave the first indication of the effectiveness of light absorbed by accessory pigments in driving photosynthesis.

Emerson’s / Red Drop Effect

Emerson, 1940 –

Wavelengths Photosynthesis

Potential

420 – 500 nm Inefficient

500 – 680 nm Efficient

680 – 720 nm Inefficient

PigmentsPhotosynthetic Unit /

Light Harvesting Complex / Antenna complex

-LHC – I & II

-Chlorophylls

-Phycobilins

8 Quanta

Chlorophyll

Electron transfer

Water is oxidized according to the following chemical reaction (Hoganson and Babcock 1997):

2 H2O → O2 + 4 H+ + 4 e–

Electron transfer

Pigments

Carotene

Chlorophyll

Pigments

Pigments

Bacterichl

Chl a

Chl b

Phycoerythrobilin

ß Carotene

ChlorophyLL

Mg +

Pyrrole Rings

Phytyl side Chain

ChlorophyLL Excitation Approx. 200 picoseconds

(1 picosecond = 10–12 s).

Chemical Reaction pHOTOphosphorylation

• Photophosphorylation.

Photo ("light")

phosphorylation (the addition of phosphate to a molecule)

So what plants do is –

use light energy to add phosphate to ADP and phosphorylate it to ATP.

pHOTOphosphorylationLocation – Thylacoid Membrane

pHOTOphosphorylationCyclic Photophosphorylation

An electron excited by light leaves the chlorophyll in photosystem I (PS-I) and cycles back to the

photosystem by traveling down an electron transport system in the membrane of the thylakoid.

Non - Cyclic Photophosphorylation

System involving two photosystems. Here the electrons do not cycle back to the chlorophyll.

pHOTOphosphorylationNon - Cyclic Photophosphorylation


In Out

One light event

ADP ATP

Noncyclic Photophosphorylation

In Out

Two light events

Water Oxygen (as waste)

ADP ATP

NADP NADPH

Light Energy is converted in Chemical Energy

Light Independent reaction• Dark Reaction

• Carbon fixation

• Calvin Cycle (Melvin Calvin, 1950 – 64)

• PCR (Photosynthetic Carbon Reduction) Cycle

Calvin cycle

Calvin Cycle

1. Carboxylation

2. Reduction

3. Regeneration Phase

4. Product Synthesis Melvin Calvin, Nobel - 1961

Regeneration of rubp

Calvin Cycle1. Carboxylation

H2O

RUBISCO – RUBP Carboxylase


2. Reduction 3 – Phosphogycerate (PGA)

Glyceraldehyde 3-P Dehydrogenase

Glyceradehyde 3-Phosphte

Mg+2

3 – Phosphogycerate Kinase1,3 BPGA


Triose Phosphate Isomerase Dihydroxy Acetone

Phosphate

Fructose 1,6 BisPhosphate

Aldolase

Fructose 6- BisPhosphate

Fructose 1,6 BisPhosphatase

Calvin Cycle3. Regeneration Phase



+Fructose 6-

BisPhosphate

Calvin Cycle3. Regeneration Phase

Erythrose – 4 - P

+

Xylulose – 5 – P

Dihydroxyacetone Phosphate

Sedoheptulose 1, 7 – Phosphate

H2O

Sedoheptulose 7 – Phosphate

Transketolase

Aldolase

Sedoheptulose 1, 7 – Phosphatase

Glyceraldehyde 3- Phosphate+

Ribulose 5– Phosphate

Xylulose 5- Phosphate+

Transketolase

Calvin Cycle

Calvin Cycle

1. Carboxylation

2. Reduction

3. Regeneration Phase

4. Product Synthesis

Calvin Cycle

4. Product Synthesis

UTP

Fructose 6 – Phosphate Glucose 6 – Phosphate

Glucose 1 – PhosphateUDP - Glucose

+Pyrophosphate

Calvin Cycle

In Out

Three CO2 One G3P

Nine ATP Nine ADP

Six NADPH Six NADP

To make a molecule of glucose requires 6 turns of the Cycle

Calvin Cycle

pHOTOSYNTHESIS – C4 Cycle• Hatch and Slack Pathway (1965)• Monocots and Few Dicots with Kranz Anatomy

C3 Leaf C4 Leaf

Kranz Anatomy• Kranz, (German for “wreath”) cells

• Vascular Bundles are surrounded by Bundle Sheath of large chlorenchyma cells

• Bundles are surrounded by Mesophyll cell (max 2 – 3 cells away)

• Chloroplasts in Bundle Cell lacks Grana and larger in size; whereas, Mesophyll cell carry it.

• Plasmodesmatal connection.

C4 Cycle

The basic C4 cycle consists of four stages –

1. Fixation of CO2 by the carboxylation of phosphoenolpyruvate in the mesophyll

cells to form a C4 acid (malate / aspartate)

2. Transport of the C4 acids to the bundle sheath cells

3. Decarboxylation of the C4 acids within the bundle sheath cells and generation

of CO2, which is then reduced to carbohydrate via the Calvin cycle

4. Transport of the C3 acid (pyruvate or alanine) that is formed by the

decarboxylation step back to the mesophyll cell and regeneration of the CO2

acceptor phosphoenolpyruvate

pHOTOSYNTHESIS – C4 Cycle

pHOTOSYNTHESIS – C4 Cycle

pyruvate–orthophosphate dikinase

C4 Cycle

• Discovered in the tropical grasses, sugarcane, and maize, the C4 cycle

is now known to occur in 16 families of both monocotyledons and

dicotyledons, and it is particularly prominent in Gramineae (corn,

millet, sorghum, sugarcane), Chenopodiaceae (Atriplex), and

Cyperaceae (sedges).

• About 1% of all known species have C4 metabolism

• Elevated concentration of CO2 at the carboxylation site of RUBISCO

results in suppression of the oxygenation of ribulose-1,5-bisphosphate

and hence of photorespiration

• Light Regulates the Activity of Key C4 Enzymes

C3 plants• Calvin Cycle

• First stable product – 3 PGA

• Diffuse mesophyll, single type of

chloroplast

• Low to High Temp. Photosynthesis

• Photosynthetically less efficient

• Rate of Glucose translocation is low

C4 plants• H & S Pathway

• First stable product – OAA

• Kranz Anatomy, Chloroplast lacks

grana

• High Temp. Photosynthesis

• Photosynthetically more efficient

• Rate of Glucose translocation is high

pHOTOSYNTHESIS - CAM• Crassulacean Acid Metabolism / Dark CO2 Fixation

• The CAM mechanism enables plants to improve water use efficiency

• First discovered in Bryophyllum Succulent raising acidity in halophytes in night.

• Stomata are open in night and closed for day

• CAM plant loses 50 to 100 g of water for every gram of CO2 gained, compared with values of 250 to 300 g and 400 to 500 g for C4 and C3 plants

pHOTOSYNTHESIS - CAM• Opuntia, Kalanchoe are other examples.

• Vacuolar Malic Acid in mesophyll cells contributes acidity.

• CO2 Uptake at night

– Stomata are open at night closed during day

– Malate accumulates by decarboxylation of PEP by PEPcase

• In C4 plants the carboxylase is “switched on,” or active, during the day and in CAM plants during the night.

pHOTOSYNTHESIS - CAM

pHOTOSYNTHESIS - functions• Carbon sinks, removing carbon dioxide from the

atmosphere and oceans by fixing it into organic chemicals.

• Plants also convert energy from light into chemical energy of C-C covalent bonds.

• Animals are carbon dioxide producers that derive their energy from carbohydrates and other chemicals produced by plants by the process of photosynthesis.

Thanks!

photosynthesis

Documents

c4 acids

mesophyll

electron transfer

reaction center

c4 plants

calvin cycle

calvin cycle3

calvin cycle1