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Photosynthesis

Topic 8
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3
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Biology HL

The regions of organization are called photosystems, which is is a
protein and enzyme complex that carries out the absorption of light
and the transfer of electrons, and include:

➢

➢

●

Chlorophyll a molecules

●

Accessory pigments

●

A protein matrix

The reaction center is the portion of the photosystem that contains:
○

A pair of chlorophyll a molecules

○

A matrix of protein

○

A primary electron acceptor

Plants have 2 types of photosystem
...
3
■

Biology HL

Released energy pumps protons into the thylakoid space against the
concentration gradient, protons diffuse into the stroma through ATP
synthase

○

A photon is absorbed by a pigment in Photosystem I

○

This energy is transferred through several accessory pigments until received
by a chlorophyll a (P700) molecule
■

The pigments get excited during the process

○

Chlorophyll a gets excited and loses electron

○

Chlorophyll a will get the de-energized electron from Photosystem II

○

The electron with the higher energy state is then passed down a second ETC
which involves the enzyme called NADP reductase and the carrier ferredoxin

○

NADP reductase catalyses the transfer of the electron from ferredoxin to the
+

energy carrier NADP

+

■

Two electrons are required to reduce NADP fully to NADPH

■

Occurs to maintain the proton concentration gradient

➢ NADPH, oxygen in gaseous form and ATP are the final products of the
light-dependent reaction
○

Supply chemical energy for the light-independent reaction to occur

➢ ATP production in photosynthesis is similar to ATP production in cell respiration
○

Chemiosmosis allows the phosphorylation of ADP using light energy
■

That is why it is called photophosphorylation

Respiration chemiosmosis

Photosynthesis chemiosmosis

ETC in the membranes of cristae

ETC in the membranes of thylakoids

Energy released when electrons are

Energy released when electrons are

Released energy pumps protons into the

Released energy pumps protons into the

intermembrane space

thylakoid space

Protons diffuse back into the matrix

Protons diffuse back into the stroma

ATP synthase catalyses phosphorylation

ATP synthase catalyses phosphorylation

Protons come from the matrix

Protons come from the stroma

exchanged between carriers

exchanged between carriers

through ATP synthase

through ATP synthase

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Photosynthesis

Topic 8
...
3

Biology HL

➢ For every 12 TP molecules, the cycle produces one 6-carbon sugar and 6 molecules
of 5-carbon RuBP
➢ 18 ATP and 12 NADPH are necessary to produce 6 RuBP and a sugar
Summary of photosynthesis
➢ Light is needed for the light-independent reaction to occur but not directly
+

➢ NADP and ATP move back and forth in the chloroplasts from the thylakoids to the
stroma in their reduced and oxidized forms

Light-dependent reaction

Light-independent reaction

Occurs in thylakoids

Occurs in the stroma

Uses light to form ATP and NADPH

Uses ATP and NADPH to form TP

Photolysis to provide electrons, protons

Returns ADP, Pi and NADP to the

2 ETC, Photosystems I and II

Calvin cycle

+

and oxygen to the atmosphere

light-dependent reaction

Chloroplast structure

Function

Big surface area of thylakoids

Greater absorption of light

Space within thylakoids

Faster creation of concentration gradient

Stroma

Enzymes of Calvin cycle can work here

Double outer membrane

Isolation from the cytoplasm

The Calvin experiment
➢ In the 1940’s the biologist M
...
He called it the Calvin cycle
14

➢ Chlorella (algae) was exposed to radioactive carbon dioxide had the isotope C
➢ Samples the algae were taken out over time and were boiled to stop photosynthesis
➢ The compounds from the samples were separated by chromatography
➢ The radioactive carbon atoms were then identified by autoradiography
➢ At the substrate molecules of the Calvin cycle were found
○

GP, TP, sugar phosphates and RuBP

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