17.3

17.0 Photosynthesis

1) 17.1 Overview of photosynthesis 2) 17.2 Absorption spectrum of

photosynthesis pigments 3) 17.3 Light Dependent Reaction 4) 17.4 Light Independent Reaction/Calvin

Cycle 5) 17.5 Alternative mechanisms of carbon

Fixation : Hatch Slack (C4) and Crassulacean Acid Metabolism (CAM) pathways

Learning outcomes :

17.3 Light Dependent Reaction

Explain the photoactivation of

chlorophyll resulting in the

conversion of light energy into

ATP and reduced NADP+

The process of photosynthesis can be summarized by the equation :

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

carbon water energy glucose oxygen

dioxide

In this equation the six-carbon glucose and oxygen are the products

The energy stored in glucose and other carbohydrates can be used to produce ATP during Cellular Respiration.

OVERVIEW OF PHOTOSYNTHESIS

Learning Outcomes :

17.3 : Explain the photoactivation of chlorophyll resulting in conversion of light energy into ATP and reduced NADP+

STAGES OF PHOTOSYNTHESIS

STAGE 1 :

LIGHT DEPENDENT

REACTIONS

•Energy is captured from sunlight.

•Water is split into hydrogen ions,

electrons, and oxygen (O2).

•The O2 diffuses out of the

chloroplasts (by-product).

Learning Outcomes :Learning Outcomes :



STAGE 2 :

•The thylakoids convert

light energy into the

chemical energy, which

is temporarily stored in

ATP (by

•photophosphorylation

for the Calvin cycle) and

NADPH.

Learning Outcomes :


STAGE 3 :

THE LIGHT

INDEPENDENT

REACTIONS/

CALVIN CYCLE

•The chemical energy

stored in ATP and

NADPH powers the

formation of organic

compounds (sugars),

CO2 are used

Learning Outcomes :


PHOTOSYSTEMS ARE LIGHT COLLECTING UNITS

OF CHLOROPLASTS.

Learning Outcomes :


A photosystem

composed of :

Reaction centre

(protein complex

includes

chlorophyll a &

primary electron

acceptor)

PHOTOSYSTEM

Learning Outcomes :


Surrounded by light-harvesting complexes (particular

protein which bounded with chlorophyll a, chlorophyll b and carotenoid)

PHOTOSYSTEM

Learning Outcomes :


• The chlorophylls and carotenoids are grouped in cluster of a few hundred pigment molecules in the thylakoid membranes

• Each cluster of pigment molecules is referred to as a PHOTOSYSTEM

• There are 2 types of Photosystems known as

PHOTOSYSTEM I (PS I) and PHOTOSYSTEM II (PS II)

PHOTOSYSTEM

Learning Outcomes :


PHOTOSYSTEM

• The chlorophyll a at reaction center of photosystem I is called P700

• Most effectively absorbs light of wavelength 700nm

• The chlorophyll a at reaction center of photosystem II is called P680

• Most effectively absorbs light of wavelength 680nm

• These two photosystems work together to use light energy to generate ATP, NADPH and O2

Learning Outcomes :


THE PHOTOSYSTEM I &

PHOTOSYSTEM II

Learning Outcomes :


PHOTOACTIVATION

A photon/light energy strikes an antenna molecule of photosystem II and photosystem I

Photon/light energy is transferred to the reaction center

Photoactivation occurs to excite the electron to a higher energy level

The excited electron received by the primary electron acceptor

Learning Outcomes :


•Photolysis is a process of splitting water molecules using light

energy with the release of electrons, protons and oxygen.

•The proton (H+) combine with NADP+ to produce NADPH

•Oxygen is given off or used in respiration.

•H2O 2H+ + 1 O2

2

•The important of photolysis is to replace electron in

photosystem II (non-cyclic photophosphorylation)

PHOTOLYSIS OF WATER

Learning Outcomes :


Learning Outcomes :


LIGHT DEPENDENT REACTION

• Light dependent reaction occurs in thylakoid

membrane/grana

• It consists of :

- Non-cyclic photophosphorylation

- Cyclic photophosphorylation

Learning Outcomes : 17.3 : Explain the photoactivation of chlorophyll resulting in conversion of light energy into ATP and reduced NADP+

LIGHT DEPENDENT REACTION

Non-cyclic

photophosphorylation

Cyclic


(Thylakoid membrane/grana)


In non-cyclic photophosphorylation, both

photosystem I and II are used.

Photolysis of water occurs which supply

electron to reaction center

Photon/light energy is absorbed by

photosynthetic pigment (P680)

Photon/light energy is transferred to the

reaction center /chlorophyll a

Photoactivation occur

NON-CYCLIC PHOTOPHOSPHORYLATION


Non cyclic photophosphorylation



•The excited electrons are received by the

primary electron acceptor

•Then are transferred along a series of electron

carrier in Electron Transport Chain (ETC)

•Primary electron acceptor plastoquinone

(Pq)

cytochrome complex plastocyanin (Pc)

•During this process, energy is released

•The energy released is used to synthesize ATP





At the same time, photon/light energy is also absorbed

by chlorophyll within Photosystem I/P700 to excite the

electron in the reaction center.

The electron lost from PS I is replaced by electrons from

PS II

The electron in PS I are released and transferred to

primary electron acceptor

Then transferred to Feredoxin (Fd)

Finally the electrons are received by NADP+

(nicotinamide adenine dinucleotide phosphate)





•At the same time, NADP+ also receives hydrogen

ions from photolysis of water to form NADPH

•This reaction is catalysed by NADP+ reductase

•Positively charged P680 chlorophyll is neutralized

by electron derived from photolysis of water

•The O2 is a by-product and released into

atmosphere throughout stomata




In cyclic

photophosphorylati

on, only involve of

photosystem I

The P700 molecule

on photosystem I

absorbs a photon

and become

energized

CYCLIC PHOTOPHOSPHORYLATION


The electrons are

excited and accepted by

the primary electron

acceptor

Then passes its electron

to the Feredoxin,

plastoquinon

cytochrome, and

complex

plastocyanin



•Then the electrons are

returned back to PS I

reaction center to

neutralize its chlorophyll

•Energy is released during

transferring of electron

•Energy is used to

generate additional ATP



Cyclic Photophosphorylation


CYCLIC vs NON-CYCLIC

Similarities :

•Production of ATP by chemiosmosis

•Both involved in Light Dependent Reaction

Cyclic photophosphorylation Non-cyclic


Cyclic electron flow Non-cyclic electron flow

No NADPH produced NADPH produced

Only ATP produced ATP and NADPH produced

Involve PS I Involve PS I AND PS II

Electron source : PS I reaction center Electron source : Water

Final electron acceptor is PS I Final electron acceptor is NADP+

Photolysis of water does not occur Photolysis occurs


17.3

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