Lect 1 - Light Dependent Reactions _Teacher

Saint Andrew’s Junior College Cellular Physiology and Biochemistry / Photosynthesis – Light Dependent Reactions

2011 H2 Biology 1

Cellular Physiology and Biochemistry

Photosynthesis – Light Dependent Reactions

Essential Reading � Campbell. Biology (7th edition). pp.181-193 � R. Soper. Biological Science 1 (3rd edition). pp.196-207. Objectives: � With reference to the chloroplast structure, explain the light dependent reactions of photosynthesis (no biochemical

details are needed but will include the outline of cyclic and non-cyclic light dependent reactions, and the transfer of energy for the subsequent manufacturing of carbohydrates from carbon dioxide).

REVIEW – STRUCTURE OF CHLOROPLAST

Photosynthesis takes place in the chloroplasts.

Structural features of the chloroplast which adapt it to its in role in photosynthesis.

OVERVIEW OF PHOTOSYNTHESIS

General equation for photosynthesis :

Water + Carbon dioxide → Glucose + Oxygen + Water

12H2O + 6CO2 → C6H12O6 + 6O2 + H2O

Photosynthesis occurs in

(i) green plants

(ii) algae

(iii) photosynthetic bacteria (e.g,cyanobacteria)

Saint Andrew’s Junior College Cellular Physiology and Biochemistry / Photosynthesis – Light Dependent Reactions

2011 H2 Biology 2

Photosynthesis functions to

(a) harvest light energy and transform it into chemical energy available to living organisms in food chains

(b) creates organic molecules used as building blocks for creating more complex molecules.

(c) oxygen, a by-product of photosynthesis is released. Oxygen is required by most other advanced life forms

The reactions of photosynthesis occur in 2 distinct stages.

• Light Dependent Reactions (Light-dependent stage):

• Dark Reactions (Light-independent stage or Calvin Cycle)

Light Reactions (Light-dependent stage)

• Requires light.

• Occurs in the thylakoid membranes of the granum.

• Consist of light harvesting and electron transport.

• Products of light reactions:

� ATP is synthesised from ADP and Pi

� NADPH is formed from the reduction of NADP+

� Oxygen is formed from water.

• The energy for the synthesis of ATP from ADP and Pi comes from light. Hence, the process is called photophosphorylation.

• Roles of the light reactions = To provide ATP and NADPH for the Calvin cycle.

Dark Reactions (Light-independent stage or Calvin Cycle)

• Does not require light, although it can occur in the presence of light.

• Occurs in the stroma.

• Carbon dioxide is reduced to carbohydrate.

• ATP provides the energy to drive the Calvin cycle.

• NADPH provides the “reducing power” required to convert carbon dioxide into carbohydrate.

An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. In the chloroplast, the thylakoid membranes are the sites of the light reactions, whereas the Calvin cycle occurs in the stroma. The light reactions use solar energy to make ATP and NADPH, which function as chemical energy and reducing power, respectively, in the Calvin cycle. The Calvin cycle incorporates CO2 into organic molecules, which are converted to sugar. (Campbell, p.180)

Saint Andrew’s Junior College Cellular Physiology and Biochemistry / Photosynthesis – Light Dependent Reactions

2011 H2 Biology 3

ROLE OF PIGMENTS IN PHOTOSYNTHESIS

• Sunlight is absorbed by chlorophyll molecules. Chlorophyll is a porphyrin in which nitrogen atoms are coordinated to a magnesium ion to form a magnesium porphyrin. (This contrasts with a haem in which nitrogen atoms are coordinated to an iron atom to form an iron porphyrin).

• When a chlorophyll molecule is excited by a quantum of light (a photon), an electron is excited to a higher energy orbital.

• The “excited” chlorophyll can pass on its extra energy to a neighboring chlorophyll molecule while the former returns to the “ground” state (unexcited state).

• Alternatively, the high-energy electron itself may be passed on, with the chlorophyll taking up a low-energy electron from another source.

• The pigment molecules in chloroplast are organized into photosystems.

How a photosystem harvests light. Photosystems are the light-harvesting units of the thylakoid membrane. Each photosystem is a complex of proteins and other kinds of molecules and includes an antenna consisting of a few hundred pigment molecules. When a photon strikes a pigment molecule, the energy is passed from molecule to molecule until it reaches the reaction centre. At the reaction centre, an excited electron from the reaction-centre chlorophyll is captured by a specialized molecule called the primary electron acceptor.

Saint Andrew’s Junior College Cellular Physiology and Biochemistry / Photosynthesis – Light Dependent Reactions

2011 H2 Biology 4

There are 2 types of photosystems – Photosystem I (PS I) and Photosystem II (PS II).

• Each photosystem consists of a light-harvesting system (antenna complex) and a reaction centre.

Light harvesting system (Antenna complex)

• Consists of accessory pigments –chlorophyll a molecules, chlorophyll b, carotene and xanthophylls. These are clustered together in the thylakoid membrane.

• When an accessory pigment molecule absorbs light and is excited, the energy is transferred from one pigment molecule to another, and is finally channeled to special chlorophyll a molecules at the reaction centre.

Reaction centre

• Consist of primary pigments – special chlorophyll a molecules.

• When adequate energy is channeled into the reaction centre from the light-harvesting centre, an electron of chlorophyll a molecule is boosted to a very high energy level.

• The high-energy electron is then passed on to a chain of electron carriers in the thylakoid membrane.

• The chlorophyll in the reaction centre of PS I has an absorption maximum at 700 nm and so is called P700.

• The chlorophyll in the reaction centre of PS II has an absorption maximum at 680 nm and so is called P680.

Saint Andrew’s Junior College Cellular Physiology and Biochemistry / Photosynthesis – Light Dependent Reactions

2011 H2 Biology 5

LIGHT REACTIONS (NON-CYCLIC PHOTOPHOSPHORYLATION)

Step 1: Light harvesting at PS II

• A photon of light strikes a pigment molecule in a light harvesting complex and the energy is channelled to P680 special chlorophyll a molecules in the PS II reaction centre.

• An electron of chlorophyll a molecule is boosted to a very high energy level and captured by PS II’s primary electron acceptor (shown by the letter X).

Step 2: Photolysis of water (Hill reaction)

• The electron displaced from the PS II reaction centre is replaced via the Hill reaction.

2 H2O → 4 e- + 4 H

+ + O2

• A water molecule is split by an enzyme and the electrons are donated to the P680 chlorophyll a molecules in the PS II reaction centre.

• The removal of electrons from water requires light to fall on PS II and leads to the production of H+

ions and oxygen molecule.

• This reaction is called photolysis of water because light energy is required to split water.

Step 3: Electron transport between PS II to PS I

• Photoexcited electrons are passed from the PS II’s primary electron acceptor to PS I via an electron transport chain consisting of a series of electron carriers of progressively lower energy levels.

Step 4: Light harvesting at PS I

• Meanwhile, light is harvested by the accessory pigments in the light harvesting system of PS I and the energy is channelled to P700 chlorophyll a molecules.

• An electron of chlorophyll a molecule is boosted to a very high energy level and captured by PS I’s primary electron acceptor (shown by the letter Y).

Step 5: Electron transport between PS I and NADP+

• Photoexcited electrons are passed from PS I’s primary electron acceptor down a second electron transport chain to NADP

+.

ATP synthase

complex

Saint Andrew’s Junior College Cellular Physiology and Biochemistry / Photosynthesis – Light Dependent Reactions

2011 H2 Biology 6

Step 6: Chemiosmosis: ATP synthesis

• During the operation of the above reactions, a proton gradient is generated, where the H+ ion

concentration is much greater in the thylakoid space than in the stroma.

• This proton gradient is created by:

� Photolysis of water in the thylakoid space generates H+ ions.

� As the electrons are transported along the series of electron carriers of progressively lower energy levels, the energy released from the electron transport is used to pump H

+ from the

stroma, across the thylakoid membrane and into the thylakoid space.

• ATP is synthesized when H+ ions diffuse from the thylakoid space into the stroma of the chloroplast,

through the ATP synthase complex.

• ATP synthase enzyme catalyses the synthesis of ATP from ADP and Pi.

Step 7: NADPH synthesis

• H+ ions from the stroma and the electrons from PS I reduce NADP

+ to NADPH.

• The removal of H+ ions from the stroma when it is taken up by NADP

+ also contributes to the proton

gradient.

NADP+ + 2 H

+ + 2 e

- → NADPH + H

+

NON-CYLIC PHOTOPHOSPHORYLATION

• The entire process above is called non-cyclic photophosphorylation, due to the non-cyclic pathway of electrons from water to PS II to PS I to NADP

+.

• ATP, NADPH and oxygen are generated.

• This is also called the Z scheme.

Saint Andrew’s Junior College Cellular Physiology and Biochemistry / Photosynthesis – Light Dependent Reactions

2011 H2 Biology 7

CYCLIC PHOTOPHOSPHORYLATION

• An alternative electron transport pathway that involves only PS I and a few electron carriers.

• The plant switches to cyclic photophosphorylation when:

� Only ATP is required.

� The NADPH/NADP+ ratio is high and little NADP

+ is available to accept electrons.

PS I is both the donor and acceptor of electrons

• Electrons displaced from PS I are transferred to PS I’s primary electron acceptor and then on to the ETC between PS II and PS I.

• The electrons are then passed to PS I, completing the cycle.

• Energy released from electron transport is used to pump H+ ions from the stroma into the thylakoid

space, creating a proton gradient. The potential energy of the proton gradient is used for the synthesis of ATP.

• No NADPH is formed.

• Since PS II is not involved, no O2 is produced.

What is the function of cyclic electron flow?

• Non-cyclic electron flow produces ATP and NADPH in roughly equal quantities, but the Calvin cycle consumes more ATP than NADPH. Cyclic electron flow makes up the difference.

• The concentration of NADPH in the chloroplast may help regulate which pathway (cyclic versus non-cyclic) electrons take through the light reactions.

• If the chloroplast runs low on ATP for the Calvin cycle, NADPH will begin to accumulate as the Calvin cycle slows down.

• The rise in NADPH may stimulate a temporary shift from non-cyclic to cyclic electron flow until ATP supply catches up with demand.

ATP synthase

complex

Saint Andrew’s Junior College Cellular Physiology and Biochemistry / Photosynthesis – Light Dependent Reactions

2011 H2 Biology 8

What is the difference between non-cyclic and cyclic photophosphorylation?

Non-cyclic photophosphorylation Cyclic photophosphorylation

Occurs when plants require ATP and NADPH. Occurs when plants require only ATP.

Non-cyclic pathway of electrons Cyclic pathway of electrons

Involves PS I and PS II. Involves only PS I.

Water is the first electron donor. PS I is the first electron donor.

NADP+ is the final electron acceptor. PS I is the final electron acceptor.

High H+ ion concentration in thylakoid space is

due to photolysis of water and active transport of H+ ions using energy generated from the ETC.

High H+ ion concentration in thylakoid space is

due only to active transport of H+ ions using

energy generated from the ETC.

Products = ATP, NADPH and oxygen. Product = only ATP.

Comparing non-cyclic and cyclic photophosphorylation. The non-cyclic and cyclic pathways are superimposed for comparision. Enclosed within the grey box is the cyclic pathway.

Saint Andrew’s Junior College Cellular Physiology and Biochemistry / Photosynthesis – Light Dependent Reactions

2011 H2 Biology 9

OVERVIEW OF PHOTOPHOSPHORYLATION