1 of 36© Boardworks Ltd 2009. Today we are studying this section of the specification:

1 of 36 © Boardworks Ltd 2009

Today we are studying this section of the specification:

Photosynthesis

Two-stage process:

• Light-dependent reaction– Occurs only in the light; involves thylakoids.

• Light-independent reaction (Calvin cycle)– Occurs in dark and light; involves stroma.

Evidence for two stages

• Temperature has a clear effect on the rate of photosynthesis– This suggests that although light levels initially limit

the rate, there is a second stage controlled by temperature-sensitive enzymes.

• A plant given alternating periods of dark and light forms more carbohydrate than one in continuous light– Period of darkness means that substances recycled in

the light independent stage are not limiting for the light dependent stage.


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


REMINDER: NADP and NADPH

Nicotinamide adenine dinucleotide phosphate (NADP) is a coenzyme involved in the photosynthesis reactions.

The compound is a dinucleotide, containing an adenine base and a nicotinamide base. The nucleotides are joined through their phosphate groups. In addition there is an extra phosphate on the ribose of the adenine-containing nucleotide.

NADP can accept electrons to be reduced to reduced NADP, often called NADPH. NADPH can be oxidized back to NADP, releasing electrons.

nicotinamide

adenine

ribose


Light-dependent reaction

Excitation of electronsPhoton of light hits chlorophyll molecule

Energy transferred to the electrons in the molecule

Electrons raised to higher energy levels

If they are raised to a high enough level they will dissociate

Electrons picked up by an electron acceptor

ATP formed as the electron is passed along an electron transport chain

Electron transfer chain

final electron acceptor

ATP

ADP + Pi

electron acceptor

Non-cyclic photophosphorylation

Photosystem II

Photosystem I

electron acceptor

electron acceptor

e-

e-

e-

ATP

ADP + Pi

light light

H2Oe-

2H+

2H

½O2

NADP

e-

2H+

2H

NADP

Reduced NADP

To the light independent

reaction



Cyclic photophosphorylation

Chlorophyll

e-

e-

electron acceptor

light

ATP

ADP + Pi



Photolysis of water

H2O → 2 H+ + 2 e- + ½ O2

Photosystem II

NADP

Reduced NADP


Light-dependent reaction: summary

Cyclic photophosphorylation

Non-cyclic photophosphorylation

photolysis


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

Light-dependent stage• Occurs in the thylakoids• Involves splitting of water by light –

photolysis• ADP is converted to ATP by light –

photophosphorylation– Cyclic photophosphorylation uses only PSI– Non-cyclic photophosphorylation uses both PSI

and PSII• Reduced NADP and ATP needed for light

independent reactions• O2 released as a waste product.


Today we are studying this section of the specification:

Melvin Calvin

• Member of the Radiation Laboratory at Berkeley, University of California

• Studied using radioactively labelled carbon dioxide

• 14CO2 fed to Chlorella algae and its path tracked


Location of the reaction

The light-independent reaction can also be called the “dark reactions”, or the Calvin Cycle.

The light-independent reaction takes place in the stroma of the chloroplasts.

The reaction consists of a cycle of enzyme-controlled processes making complex molecules such as carbohydrates.

stroma

Three phases

1. CarboxylationCarbon dioxide fixation with ribulose bisphosphate (RuBP)

2. ReductionReduction of glycerate 3-phosphate (GP) to glyceraldehyde 3-phosphate (GALP)

3. RegenerationRe-formation of the CO2

acceptor molecules

CO2

RuBP

6C molecule

GP

GP

Phase 1: Carboxylation

Rubisco

RUBISCO

Phase 2: Reduction

GP

GP

GALP

GALP

2ATP 2ADP + 2Pi

2NADPH + H+ 2NADP+

Phase 3: Regeneration

GALP

GALP

RuBP

ATP ADP + Pi


The light-independent reaction


Products of the Calvin cycle

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

Products of the light-independent reaction that are used in other processes:

NADP

ADP

triose phosphate – used to build complex carbohydrates, amino acids and lipids.

inorganic phosphate


Making complex molecules

Pairs of TP molecules combine to form hexose sugars, such as glucose, some of which may isomerize to form fructose.

Triose phosphate is a three-carbon sugar that can be used to make a variety of complex biological molecules.

TP can be converted to glycerol and this may be combined with fatty acids to make lipids.

These monosaccharides can combine to form disaccharides such as sucrose, and polysaccharides such as cellulose and starch.


Summary of photosynthesis

light- dependent reactions

light- independent

reactions

light

carbohydrates, other complex molecules

Synthesis of Organic SubstancesGALP GALP GALP GP

hexose

glycerolfatty acids

lipids aminoacids

GALP

The Maths• 2 molecules of GALP are made each turn• 5 out of 6 molecules of GALP are used to

regenerate RuBP• 2 molecules of GALP are needed to make

a hexose sugar• How many turns are needed to make 1

molecule of hexose sugar?• How many molecules of ATP and reduced

NADP are needed?

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1 of 36© Boardworks Ltd 2009. Today we are studying this section of the specification: