CHAPTER 6: PHOTOSYNTHESIS

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CHAPTER 6: PHOTOSYNTHESIS

SUBTOPIC : 6.1 Overview of photosynthesis.

LEARNING OUTCOME : Outline the complete process of photosynthesis: Light dependent and light independent reactions.

MAIN IDEAS /KEY

POINT EXPLANATION NOTES

OVERALL REACTION OF PHOTOSYNTHESIS

The conversion of light energy to chemical energy that is stored in sugar or other organic compound using CO2 and water (occurs in

plants, algae & certain unicellular eukaryotes and some prokaryotes).

FIGURE 1: Overall reaction of photosynthesis

Chemical Equation for photosynthesis process:

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

• Reactants (Begins with):

o Sunlight (Light Energy)

o Water (from the ground)

o Carbon dioxide (from the air)

• Products (Ends with):

o Oxygen (Releases into atmosphere)

o Glucose/ other organic compound (Stores chemical

energy)

o Water

sunlight

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MAIN IDEAS /KEY

• Involves REDOX reaction:

o The reduction (gains electrons and hydrogen)of carbon dioxide into sugars and the oxidation (loses electrons and

hydrogen ions)of water into molecular oxygen.

o Water is split and electrons transferred with H+_{from water}

to CO2 reducing it to sugar.

o An endergonic redox process- energy is required to

reduced carbon dioxide.

TIPS: GER Gain of E lectron-Reduction LEO- Loss of Electron- Oxidation THE STAGES OF PHOTOSYNTHESIS

The anabolic pathway involves TWO STAGES:

FIGURE 2: Stages of photosynthesis a) Light dependent reaction (Light reaction)

- the photo part of photosynthesis - occurs in thylakoids

b)Light independent reaction (Calvin cycle)

- the synthesis part of photosynthesis - occurs in stroma

SUBTOPIC : 6.2 Absorption spectrum of photosyntetic pigments.

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MAIN IDEAS /KEY

POINT EXPLANATION

THE NATURE OF SUNLIGHT

• Light is a form of energy known as electromagnetic energy,

also called as electromagnetic radiation.

• Light travels in rhythmic waves.

• The distance between the crests of electromagnetic waves is

wavelength ().

PHOTOSYNTHETIC PIGMENTS

Photosynthetic pigments

Are "molecules that absorb specific wavelengths (energies) of light and reflect all others."

Can be found in thylakoid membrane and intergranal lamella.

Are the substances that absorb visible light (narrow band between 380 to 750nm)

Each wavelength is different coloured light

TWO types of photosynthetic pigments:

Main pigment ▪ Chlorophyll a

• Most abundant pigment in plants.

• Absorbs light with wavelengths of 430nm (blue) and 662nm

(red).

• Reflects green light strongly

Accessory pigments ▪ Chlorophyll b

• Has a structure similar to that of chlorophyll a

• Absorbs light (red and blue) of 453nm and 642 nm

maximally.

• Appear green olive under visible light.

• Helps increase the range of light a plant can use for energy.

▪ Carotenoid

• Absorb light (violet and blue-green light) maximally

between 460 nm and 550 nm

• Appear red, orange, or yellow

• Most important function: to be protecting the plant from

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MAIN IDEAS /KEY

POINT EXPLANATION

ABSORPTION SPECTRUM OF PHOTOSYNTHETIC

PIGMENTS

Absorption spectrum is a visual representation of how well a particular pigments absorbs different wavelength of visible light.

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SUBTOPIC : 6.3 Light dependent reaction.

LEARNING OUTCOME : Explain the cyclic and non-cyclic photophosphorylation MAIN IDEAS /KEY

LIGHT DEPENDENT REACTION

• Occurs in the thylakoid membrane of grana in presence of light.

• Light energy converted into chemical energy (ATP and NADPH) to be

used during light independent reaction.

FIGURE 3: Light dependent reaction of photosynthesis Photosystem

• The light dependent reaction utilizes two photosystems that are located in

the thylakoid membrane.

• Chlorophyll molecules organized along with other small organic

molecules and proteins into complexes called photosystem. A

photosystem acts like a light-gathering ‘antenna complex’

• 2 types of photosystems • Photosystem I (PSI) Reaction center: P700 Absorption peak: 700 nm • Photosystem II (PSII) Reaction center: P680 Absorption peak: 680 nm

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FIGURE 4: Photosystem

Each photosystem is composed of :

• Reaction centre complex

- organized association of protein

▪ holding special pair of chlorophyll a molecules

- primary electron acceptor

▪ contain molecule capable for accepting electron and become

reduced.

• several light harvesting complexes consists

- various pigment molecules bound to protein

- acts as an antenna for the reaction- center complex.

Excitation of Chlorophyll by Light

• When a chlorophyll molecule absorbs a photon of light

- The molecule’s electron boosts to an orbital of higher energy

→ Pigment molecule in an excited state

→unstable (does not remain long).

Thus, drop back to ground state and release excess energy as heat

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MAIN IDEAS /KEY

Photosystem Harvest Light

• Photon strikes a pigment molecule in light harvesting complex

(antenna molecule).

• Energy passed from one molecule to another until reaches reaction

centre complex.

• An excited electron from reaction centre chlorophyll transferred to

primary electron acceptor

FIGURE 6: How Photosystem Harvest Light Photophosphorylation

• Process of generating ATP from ADP and phosphate by

chemiosmosis

• Using a proton motive force generated across the thylakoid

membrane of chloroplast

• Occurs during light dependent reaction

• 2 types of photophosphorylation:

✓ Non cyclic photophosphorylation

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MAIN IDEAS /KEY

Non-cyclic

Photophosphorylation

FIGURE 7: Non cyclic photophosphorylation

• Pigments in photosystem II (PS II) absorbs light, electron

excited captured by the primary electron acceptor (pheophytin) thus chlorophyll molecules in the reaction centre loses an electron & become oxidized.

• Enzyme catalyze water splitting (water photolysis). Electrons from

hydrogen atom are transferred to reaction center of PS II (P680)

Water Photolysis

FIGURE 8: Water Photolysis

The important of water photolysis

✓ To replace electron in photosystem II

✓ H2O → 2H+ +2e- + ½ O2

• The proton (H+_{) used to}_reduced_NADP+_{into NADPH}

• Oxygen is given off or used in respiration

The electrons from primary electron acceptor of PS II are pass along an electron transport chain, before ending up at an oxidized photosystem I reaction center.

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MAIN IDEAS /KEY

Electron transport chain (ETC)

FIGURE 9: Electron transport chain (ETC)

• A series of carriers that pass the electrons from one to another

• The energy lost when these electrons pass along the electron transport

chain and will used to form ATP from ADP in during the process of chemiosmosis

• At the bottom of this electron transport chain, the electrons fill an electron “hole” in an oxidized P700 center.

• At the same time,P700 absorbs photon and electrons are emitted and

accepted by primary electron acceptor of PS I

• Then electron are passed to the electron transport chain, ferrodoxin (Fd) and then to an enzyme called NADP+ _{reductase (This chain}

doesn’t create a proton gradient, does not produce ATP). Cyclic

photophosphorylation

Photosystem I acts on its own, without photosystem II.

• The P700 molecule which is the reaction centre of photosystem I

absorbs photon and energize the electrons.

• The electrons are emitted and taken up by primary electron acceptor

• The electrons cycle back from ferredoxin (Fd) to the cytochrome complex, then via a plastocyanin molecule (Pc) to a P700 chlorophyll in the PS I reaction-center complex.

• No production of NADPH and

no release of oxygen.

• Formation of ATP by

chemiosmosis.

FIGURE 10:

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MAIN IDEAS /KEY

Summary • Light energy convert into chemical energy, ATP and NADPH.

• ATP and NADPH are used in light independent reaction.

Comparison of cyclic and non-cyclic photophosphorylation Non-cyclic Photophosphorylation Cyclic Photophosphorylation

Involve PS I and PS II Involve only PS I

e

flow is non-cyclic e

-flow is cyclic

Products are ATP, NADPH and O2 Product is ATP.

Last e-_{acceptor is NADP}+ _{Last e}

-acceptor is PS 1 (P700) 1st_e

donor is HO 1st_e-

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SUBTOPIC : 6.4 Light independent reaction/ Calvin cycle

LEARNING OUTCOME : Explain Calvin cycle involving carbon fixation, reduction and regeneration of RuBP

MAIN IDEAS

/KEY POINT EXPLANATION NOTES

LIGHT

INDEPENDENT REACTION/ CALVIN CYCLE

• Use the ATP and NADPH (products from light dependent reaction)to

reduce CO2 to sugar.

• Anabolic pathway

- the cycle builds sugar from smaller molecules (CO2)

- by using ATP as an energy source

- and NADPH as reducing power of electron carrier.

FIGURE 11: Light Dependent Reaction And Light Independent Reaction

Carbohydrate produced directly from the Calvin cycle is three carbon sugar (C3),

Glyceraldehyde 3 - phosphate (G3P) / PGAL.

- 3 phases:

i. Carbon fixation ii. Reduction

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MAIN IDEAS

/KEY POINT EXPLANATION NOTES

Phase 1: Carbon fixation

• CO2 (entering one at a time) attach to 3 molecules of RuBP (5C sugar).

Catalyzed by RuBP carboxylase (rubisco).

• Produce unstable six-carbon compound that immediately split into half,

forming two molecules of 3-phosphoglycerate (3C molecules) for each CO2

fixed.

Phase 2: Reduction

• Each molecule of 3-phosphoglycerate receives an additional phosphate

group from ATP forming a molecules of 1,3-bisphosphoglycerate.

• NADPH reduces 1,3-bisphophoglycerate forming Glyceraldehyde 3-phosphate (G3P) / PGAL.

- 6 NADPH reduces 6 molecules of 1,3-bisphophoglycerate

- Forming 6 molecules of Glyceraldehyde 3-phosphate (G3P) /

PGAL

• From 6 molecules of G3P (3C) formed only 1 molecule of G3P (3C) will be

used to synthesis glucose (6C).

Phase 3: Regeneration of the CO2 acceptor ribulose bisphosphate (RuBP)

• Other 5 molecules (G3P) are rearranged to regenerate 3 molecules of

RuBP (5C).

• Phase 3 spends 3 molecules of ATP.

The cycle repeated.

FIGURE 12: Calvin Cycle

• For the net synthesis of 1 G3P (3C), the Calvin cycle consumed:

o 9 molecules of ATP

o 6 molecules of NADPH

• To form 1 molecule of sugar (glucose), the cycle needs6 molecules of CO2.

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Metabolism (CAM) pathways.

LEARNING OUTCOME : Explain photorespiration and state the alternative mechanism of carbon fixation

(C4 and Crassulacean Acid Metabolism (CAM) pathways)

MAIN IDEAS /KEY

C3 PLANT • The most common and the most efficient at photosynthesis in cool

and wet climate.

• The majority of plants are C3 plant, which have no special features

to combat photorespiration.

PHOTORESPIRATION Photorespiration is a wasteful pathway that occurs when the Calvin cycle enzyme rubisco acts on oxygen rather than carbon dioxide.

FIGURE 13: Photorespiration and Calvin Cycle What is photorespiration?

• Stomata on leaf surface of C3 plant close during hot day to conserve water.

• This cause a drop in CO2 and an increase in O2 (by product of light dependent reaction) in the leaf. • Thus, cause photorespiration..

“A metabolic pathway that consumes oxygen and ATP, releases

carbon dioxide and decreases photosynthetic output.

Generally occur on hot, dry, bright days when stomata close and the O2/

CO2 ratio in the leaves increase favouring the binding of O2 rather than

CO2 by rubisco.”

(Campbell, 10th_edition) Why photorespiration lowers photosynthetic output?

Photorespiration decreases photosynthetic output by fixing oxygen, instead of carbon dioxide to Calvin cycle.

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MAIN IDEAS /KEY

FIGURE 14: Opening and Closing stomata ALTERNATIVE

MECHANISM OF CARBON FIXATION

In some plant species, alternate modes of carbon fixation have evolved

that minimize the photorespiration and optimize the Calvin cycle

even in hot, arid climates.

• There two photosynthetic adaptations:

✓ C4 pathway.

✓ Crassulacean Acid Metabolism (CAM).

✓ HATCH-SLACK

PATHWAYS (C4)

• C4 plant so named because they preface the Calvin cycle with an alternate mode of carbon fixation that forms four-carbon

compound as its first product.

Campbell10th Edition, page 246 Example: corn

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MAIN IDEAS /KEY

LEAF ANATOMY OF C4 PLANT

• In C4 plants, there are two types of photosynthetic cells:

- Bundle sheath cells : arrange tightly packed sheath around the veins of leaf.

- Mesophyll cells : loosely arranged between bundle sheath and leaf surface

• This arrangement are called Kranz anatomy.

• Involves enzyme called PEP carboxylase.

- Present in the mesophyll cells.

• PEP carboxylase has a higher affinity towards CO2 compared to

rubisco and will not fix O2.

• PEP carboxylase can fix carbon efficiently during hot and dry day

when stomata are partially closed.

• Cause the CO2 concentration in leaf to decrease and O2

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MAIN IDEAS /KEY

FIGURE 16: C4 pathway

1. In mesophyll cells, the enzyme PEP carboxylase add carbon

dioxide to phosphoenolpyruvate (PEP).

✓ Forming 4C product -oxaloacetate (OAA).

2. Oxaloacetate (OAA) reduce to malate.

3. Malate transported into the bundle-sheath cell via plasmodesmata.

4. In bundle-sheath cell, malate release CO2 (decarboxylation)

forming pyruvate

✓ CO2 enters Calvin cycle

✓ Pyruvate transported back to mesophyll cell & phosphorylated to form PEP

✓ Allowing the following cycle to continue

CRASSULACEAN ACID METABOLISM (CAM) MECHANISM

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MAIN IDEAS /KEY

• CAM plants : stomata open during the night and close during the

day.

✓ To minimize photorespiration.

• Closing stomata during the day helps desert plants to conserve

water.

• But, this prevents CO2 from entering the leaves.

FIGURE 18: CAM pathway

1. During the night, stomata are opened.

✓ CO2 enters the leaf tissue

✓ CO2 fixed with PEP catalysed by PEP carboxylase to form

oxaloacetate (OAA)

✓ OAA reduced into malate (NADH oxidized)

2. Reduced malate stored in the vacuole

3. During day, stomata closed, malate passed to the chloroplast

✓ Malate undergo decarboxylation release CO2 and form pyruvate

back (pyruvate converted to starch that helps the closure of stomata)

✓ CO2 then enters Calvin cycle

Differences between C3,