PhotosynthesisLesson 2

Learning Objectives Understand oxidation and reduction. Understand how light energy is

converted to chemical energy. Understand why water is necessary in

the light-dependent stage of photosynthesis.

Success Criteria State where oxidation and reduction

occurs in the light-dependent stage. Explain how ATP is produced. Explain how NADP is produced. Explain why the products of the light-

dependent stage are important.

Starter

Name A, B, C and D

A – Stroma/Ribosome could be hard to tell)

B – Envelope/Inner/Outer Membrane

C – Thylakoid/Lamella

D – Granum

Oxidation & Reduction

Redox Reactions In the light-dependent stage of photosynthesis, the

majority of reactions involve molecules being oxidised and reduced.

Oxidation and reduction can be explained in three ways:

Oxidation and reduction always occur together.

OXIDATION

Gain of OXYGEN

Loss of HYDROGEN

Loss of ELECTRONS

REDUCTION

Loss of OXYGEN

Gain of HYDROGEN

Gain of ELECTRONS

Light Dependent Reactions

The light-dependent stage of photosynthesis – key facts

Takes place in the thylakoids Light energy excites electrons in chlorophyll Some of this energy is used in the photolysis of water

protons, electrons, hydrogen Protons and electrons accepted by NAPH, it becomes....

Reduced or oxidised? Energy from excited electrons generates ATP through

photophosphorylation Photophosphorylation involves transfer of electrons

along an ETC in the thylakoids. The electrons lose energy at each stage.

This energy is used to combine phosphate with ADP.

LDR Outline This stage of photosynthesis occurs on the thylakoid

membranes, unlike the light-independent reaction (which occurs in the stroma).

The reaction involves the capture of light, whose energy is used for three purposes:

Production of ATP in the presence of light is known as photophosphorylation.

There are two types: Cyclic and Non-Cyclic.

To combine ADP and an Pi (inorganic phosphate) to form ATP.

To reduce the coenzyme NADP.

To split water (photolysis)

chlorophyll

ADP Pi

e- e-

e l e

c t

r o

n

e n

e r

g y

l e

v

e l

energy

energy

H2OH+

O2

ATP!

e-

NADPreduced

NADP

by-product

ATP!

Explanation of Animation…

Photolysis of Water Light energy is able to split water molecules. This ‘photolysis’ yields the following products:

The electrons produced are used to reduce a chlorophyll molecule – These will be excited by light.

The oxygen produced is a by-product which is either used in respiration, or leaves the cell.

The proton (hydrogen ion) is very important though…

H2O

H+

O2e-

Continued… First, the protons are used to generate ATP by chemiosmosis. They do this by diffusing across the thylakoid membrane

through membrane proteins associated with the enzyme ATP synthase.

The protons will eventually be accepted by NADP (becoming reduced NADP).

These molecules are used in the Light Independent Reactions.

ATPATPATP

H+

H+

H+

These go off to reduce NADP

Non-cyclicphoto-

phosphorylation

Reducing NADP NADP is also the final acceptor of the pair of electrons

released by chlorophyll.

Gaining electrons and protons causes the NADP to become reduced.

Along with the ATP formed earlier, reduced NADP is vital for the next stage of photosynthesis – the light-independent stage.

e- e-

H+

NADPreduced

NADP

e- e-chlorophyll

Making ATP Light also strikes chlorophyll, boosting a pair of electrons to

a higher energy level. These excited electrons actually leave the chlorophyll

molecule and are taken up by an electron carrier. This is the first of a sequence of redox reactions.

Next, the electrons are passed along a series electron carriers, in a stage known as the electron-transport chain.

oxidised reduced

e- e-

D E C R E A S I N G E N E R G Y L E V E L

Energy is released each time the electrons pass from carrier to

carrier.

Continued… The energy that is released as the electrons pass down the

transfer chain, is used to synthesise ATP.

Again, this is a form of photophosphorylation.

ADP + Pi ATP

Note:Because the electrons eventually get taken up

by NADP, they cannot be reused by the chlorophyll.

Therefore, the ATP produced by their route down the electron transport chain is described as

NON-CYCLIC.

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Location of the reaction

The light-dependent reaction takes place in the thylakoid membranes of the chloroplasts.

Situated in these membranes are light-harvesting systems called photosystems. There are two types, photosystem I (PSI) and photosystem II (PSII). Both have chlorophyll at their centres.

chlorophyll

light

photosystem

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Light-dependent reaction

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Light-dependent reaction: summary

Cyclic photophosphorylation

Non-cyclic photophosphorylation

photolysis

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Products of the light-dependent reaction

Products of the light-dependent reaction that pass into the light-independent reaction:

Products of the light-dependent reaction that leave the plant:

Products of the light-dependent reaction that are re-used in another part of the light-dependent reaction:

reduced NADP

ATP

oxygen

H+ ions

electrons

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Summary Table

Reaction in LDR Cyclic Phosphorylation

Non-cyclic phosphorylation

Photolysis

Location in chloroplast

Thylakoid membrane

Thylakoid membrane

Thylakoid membrane

Photosystem involved

1 1 and 2 no

Products ATP only ATP and reduced NADP

Oxygen, hydrogen ions and electrons

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How much do you know?

Plenary

Answer

Learning Objectives Understand oxidation and reduction. Understand how light energy is

converted to chemical energy. Understand why water is necessary in

the light-dependent stage of photosynthesis.

Success Criteria State where oxidation and reduction

occurs in the light-dependent stage. Explain how ATP is produced. Explain how NADP is produced. Explain why the products of the light-

dependent stage are important.