17. PHOTOSYNTH

ESISBy: NADIA, SYAFIQA,

SYAFIQAH,AZIRA,AYATI

The process where by light energy is converted to chemical energy that is store in glucose or other organic compounds.

In the presence of light, green plant produce organic compound and oxygen from carbon dioxide and water.

The overall equation for photosynthesis is:

Twelve molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen and six molecules of water.

6CO2 + 12H2O + 18ATP + 12NADPH

C6H12O6 + 18ADP + 12NADP+ + 18Pi

Chlorophyll a The most importance and abundance

pigments Absorbs light mainly in the blue-violet and

red wavelength Blue green colour Has a methyl (-CH3) side group in porphyrin

ring Function to initiate light dependent reaction Found in photosynthetic organisms except

photosynthetic bacteria

Chlorophyll b Yellowish green in colour Absorbs blue and red wavelength Has an aldehyde (-CHO) side group in

porphyrin ring Pass the light energy to chlorophyll a in

the reaction center. Helps to increase the range of light a

plant can utilize for photosynthesis. Found in green plants and green algae

Carotenoids Absorbs light maximally in the blue

violet region Red, orange or yellow pigments Functions as accessory or antenna

pigmentsAssists by capturing energy from light of wavelength that are not absorb by chlorophyl a and b

Protects the chloropyll from oxidation (photoprotection)

Two types : carotenes and xantophyll

Photosystem Network of chlorophyll a, b, accessory pigments

and associated protein embedded in the thylakoid membrane

Functions to channel the excitation energy gathered by any one of its pigments molecule to the reaction centre

Passes the energy out of the photosystem for ATP synthesis

Consists of :Antenna complexReaction centre

Antenna complex Pigments molecule that gathers

photon and channel it to the reaction centre

Reaction centre Consists of chlorophyll a Pass excited electron to the primary

electron acceptor

Two types of photosystem: Photosystem I (PSI) / P700

It is called P700 because reaction centre chlorophyll a of a photosystem I absorbs wavelength of 700 nm

Two types of photosystem: Photosystem II (PSII) / P680

It is called P680 because reaction centre chloropyll a of a photosystem II absorbs wavelength of 680 nm

Photosynthesis occurs in two main stages: the first stage being towards light dependent

reaction the second stage being the light independent

reaction (Calvin Benson Cycle) In short the light reactions capture the light

energy and utilize it to make high-energy molecule (ATP)

ATP are used by the Calvin-Benson Cycle to capture carbon dioxide and make the precursors of carbohydrates.

The light reaction convert solar energy to chemical energy.

In the light reaction, light energy absorbed by chlorophyll in the thylakoid drives the transfer of electrons and hydrogen from water to NADP+ (nicotinemide adenine dinucleotide phosphate), forming NADPH.

The light reaction also generate ATP by photophosphorylation for the Calvin Cycle.

Photophosphorylation Photophosphorylation is the process

of creating ATP using a proton gradient created by the energy gathered from sunlight.

The process of creating the proton gradient resembles that of the electron transport chain of cellular respiration.

But since formation of this proton gradient is light-dependent, the process is called photophosphorylation.

Two types of photophosporylation: Non-cyclic photophosphorylation Cyclic photophosphorylation

Non-cyclic photophosphorylation Light energy is absorbed by accessory (antenna)

pigments of PS II / P680 Then transferred to reaction centre (chlorophyll a) Electron is photoactivated / excited and released This creates an electron deficiency P680 oxidized to P680+ Photolysis of water molecule to form 2 electrons,

2H+ and 1 oxygen atom An enzyme called Z protein helps split up water

molecules

There are 2H+ released into the thylakoid lumen

The oxygen atom immediately combined with an oxygen atom generated by the splitting of another water molecule forming oxygen molecule (O2)

Electron from photolysis of water replace the electrons released from PS II

The P680 /PS II molecule returns to its reduced / stabilized state

The electrons released from PS II are accepted by primary electron acceptor/ pheophytin

And pass along the ETC; consists of plastoquinone/ Pq, cyctochrome complex, plastocyanin/ Pc and then toP700/ PS I

As the electron passed through the cytochrome complex, energy is released through redox reaction.

The energy is used to pump H+ from the stroma (low concentration of H+ ) into the thylakoid lumen (high concentration of H+ )

Creating a proton gradient between the stroma and thylakoid lumen that is used in chemiosmosis

At the same time, high energy electrons in P700 / PS I are ejected and accepted by primary electron acceptor

This creates an electron deficiency P700 oxidized to P700+ PSI functions as electron acceptor,

accepting electron from PSII

The excited electron pass along ferredoxin/ Fd

NADP+ reductase transfers the electron to NADP+

NADP+ receives proton from photolysis of water to form NADPH (which is released into the stroma)

The process also produces oxygen, water and ATP

ATP and NADPH produced will be used in the Calvin cycle

chemiosmosis

Production of ATP is by chemiosmosis High concentration of H+ in the thylakoid

space Low concentration of H+ in the stroma H+ diffuse from the thylakoid space back

into the stroma through ATP synthase The energy release is used to phosphorylate

ADP to form ATP (in the stroma) ATP and NADPH produced by non-cyclic

photophosphorylation will be used in the Calvin cycle

Cyclic photophosphorylation Involves PSI only No production of NADPH, no release

of oxygen Produces ATP Occurs less commonly in plants than

noncyclic photophosphorylation, most likely occurring when there is too little NADP+ available.

Light energy is absorbed by accessory (antenna) pigments of PS I / P700

Then transferred to reaction centre (chlorophyll a)

Electron is photoactivated / excited and released Accepted by primary electron acceptor The electron pass to ferredoxin (Fd), cytochrome

complex, plastocyanin (Pc) and back to chlorophyll a at the reaction centre PS I / P700

ATP produce by chemisosmosis

17.3 LIGHT INDEPENDENT REACTION/CALVIN CYCLE A series of reaction lead to the production

of NADP+ and ADP and sugar.

Occur in stroma of the chloroplast.

Input are NADPH, ATP and CO2.

First step in carbon fixation which is catalyzed by an enzyme name RuBP carboxylase

Carbohydrate produced directly from the Calvin cycle is actually not glucose but a three carbon sugar named glyceraldehyde-3-phosphate (G3P).

The Calvin cycle involves three phases : Carbon fixation Reduction of PGAL/G3P Regeneration of the CO2 acceptor

Ribulose bisphosphate (RuBP)

I) CARBON FIXATION The Calvin cycle incorporates each

CO2 molecule one at the time by attaching it to a five carbon sugar named ribulose biphosphate (RuBP).

The enzyme that catalyze this first step is RuBP carboxylase

The product of the reaction is a six-carbon intermediately split in half forming two molecules of 3-phosphoglycerate (for each CO2)

II) REDUCTION Each molecules of 3-phosphoglycerate

receives an additional phosphate from ATP becoming 1,3-biphosphoglycerate

A pair of electron donated from NADPH reduces 1,3-biphosphoglycerate to G3P , which store more potential energy.

G3P is a sugar ,the same three carbon sugar formed by splitting of glucose (glycolysis)

Every 3 molecules of CO2 that enter the cycle, there are six molecules of G3P formed.

But only one molecule of G3P will exit the cycle become the starting material for metabolic pathway that synthesize other organic compound (lipid , amino acid) including glucose and other carbohydrate.

III) REGENERATION OF THE CARBON DIOXIDE ACCEPTOR (RuBP) In the complex series of reaction, the

carbon skeletons of five molecules of G3P are rearranged by the last step of the Calvin cycle into three molecules of RuBP.

To accomplish this, the cycle spend three more molecules of ATP.

The RuBP is now prepared to receive CO2 again and the cycle continues.

For the net synthesis of one G3P molecule, the Calvin cycle requires a total of nine molecules of ATP, and six molecule of NADPH.

Plants which use only the Calvin cycle for fixing the carbon dioxide from the air are known as C3 plants.

About 85% of plant species are C3 plants. They include the cereal grains: wheat, rice, barley, oats. Peanuts, cotton, sugar beets, tobacco, spinach, soybeans, and most trees are C3 plants.

C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration

17.5 Alternative Mechanism Of Carbon Fixation: Hatch-Slack(C4) And Crassulacean Acid Metabolism (CAM) Pathways

PHOTORESPIRATION Photorespiration occurs when the CO2 levels

inside a leaf become low.

This happens on hot dry days when a plant is forced to close its stomata to prevent excess water loss.

If the plant continues to attempt to fix CO2 when its stomata are closed, the CO2 will get used up and the O2 ratio in the leaf will increase relative to CO2 concentrations.

PG (phosphoglycolate) PGA (glycerate 3 phosphate)

When the CO2 levels inside the leaf drop to around 50 ppm(part per million), RuBP carboxylase starts to combine O2 with RuBP instead of CO2.

Photorespiration uses the ATP and NADPH produced in the light reaction.

The process result in the loss of fixed carbon dioxide from the plant, reducing photosynthetic efficiency and plant growth.

Hatch-Slack(C4) pathwayC4 plant such as sugarcane ,maize and other tropical grasses have evolved a special metabolic adaptation which reduce photorespiration.

The metabolic adaptation to reduce photorespiration is Hatch-Slack pathway.

Comparison between C3 and C4 plant leaves C3 plant leaves C4 plant leaves

C4 plants have Krantz anatomy // mesophyll cells are arranged concentrically around the bundle sheath cells

Plants conduct C4 pathway in the mesophyll cell and Calvin cycle in the bundle sheath cells

Use PEP carboxylase to fix CO2 in the mesophyll cells With phosphoenolpyruvate (PEP) forming

oxaloacetate (OAA) PEP carboxylase has very a high affinity towards CO2 Therefore can fix CO2 even though its level is low

(therefore reduce photorespiration)

OAA will be converted to malate Transported to bundle sheath cells Malate converted to pyruvate Releasing CO2 for normal CO2 fixation

using Calvin cycle Ensure RuBP carboxylase will be exposed

to high CO2 level (reduce photorespiration) Pyruvate transported back to mesophyll

cell Converted to PEP (using energy from ATP)

Crassulacean Acid Metabolism (CAM) Pathway A second strategy to minimize

photorespiration is found in succulent plants, cacti, pineapples and several other plant families.

Open stomata during the night and close them during the day.

During the night, stomata are open. CO2 enters the leaf tissue. CO2 combine with PEP to form

oxaloacetate. Oxaloacetate converted into malate. Malate is transported into the vacuole

for storage. During the day, stomata are close. Malate is moved into the chloroplasts. Malate converted into pyruvate and

released CO2. CO2 enter the Calvin cycle.