Photosynthesis Light reactions

The Great Oxygenation Event/Great Oxygen Catastrophe
- about 2 billion years ago, aerobic, photosynthetic
cyanobacteria, having evolved the ability to get
energy from light, began dumping the waste product what into the atmosphere

oxygen

70% of oxygen production is in where

oceans

The presence of large amounts of oxygen in the atmosphere
made the what lifestyle much more advantageous

aerobic

Plants colonized the land in the what period (450 Million Years Ago)

Silurian

In the Carboniferous and Permian periods O2 was >?%, vs 21% today

30

Photosynthesis uses light energy to generate high-energy what

electrons

NADPH

The high-energy electrons flow down an electron transport chain
and are used to form what, biosynthetic reducing power

ATP

Photosynthesis’ electron flow is also used to create a proton-motive force that
powers the synthesis of what

Water is what to oxygen, which is released to the environment

oxidized

The reactions powered by sunlight are called the what reactions

light

The ATP and NADPH produced in the light-dependent
reactions provide the energy and the what power to
convert atmospheric CO2 into simple organic compounds

reducing

light

The ATP and NADPH produced in the light-dependent
reactions provide the energy and the reducing power to
convert atmospheric CO2 into simple organic compounds
These are known as the dark reactions because they do
not require what

chloroplast

The what is a double-membrane organelle

The inner membrane surrounds the stroma, which is the site of hexose
synthesis from CO2 and H2O using ATP and NADPH made in the what
reactions

light

thylakoid

In the stroma are membranous sacs called what membranes, where
the light reactions of photosynthesis take place

pigment

Photosynthesis begins with the absorption of light by a
photoreceptor molecule, also called a what

chlorophyll

The primary photoreceptor in the chloroplasts of green plants is
what, a substituted tetrapyrrole bound to a large hydrophobic
20-carbon chain called phytol

Chlorophyll a and b have very slight structural what

differences

antenna

The efficiency of photosynthesis is increased using what molecules,
other pigments that absorb light at wavelengths different from chlorophyll

harvesting

The antenna molecules are arranged in light-what complexes that
surround the reaction center and funnel energy to the reaction center

in the fall, trees does what to chlorophyll production
and breakdown and absorb chlorophyll components for re-use
which allows the presence of other pigments to become apparent

ceases

Light Absorption

When a photon of the appropriate energy is absorbed by a pigment,
an electron in the pigment molecule jumps to a what energy state,
which is generally unstable

higher

resonance

In most cases, the excited electron falls back to its original state,
releasing the energy as a photon of light
The energy in that photon can be absorbed by an electron in another
neighboring pigment molecule, which jumps to a higher energy state.
Transfer of energy by this means is called what energy transfer.

reaction

The energy in that photon can be absorbed by an electron in another
neighboring pigment molecule, which jumps to a higher energy state.
Transfer of energy by this means is called resonance energy transfer.
Eventually the energy is transferred to chlorophyll molecules in the
what center

electron

Another fate for an excited electron is to move to a nearby molecule, a
process called what transfer

Electron transfer results in photoinduced charge separation - the initial
molecule that lost the high-energy electron becomes what charged,
while the molecule that accepted the electron is now what charged

positively, negatively

reaction

Photoinduced charge separation occurs in the what center

chlorophyll

What is part of the special pair in the reaction center that
undergoes charge separation. An electron from chlorophyll can be
transferred to a nearby electron acceptor molecule (the other
member of the pair)

electron

Chlorophyll is part of the special pair in the reaction center that
undergoes charge separation. An electron from chlorophyll can be
transferred to a nearby electron acceptor molecule (the other
member of the pair)
This transferred electron will now flow down an oxidation-reduction
chain, just like in the what transport chain complexes

photosystems

Photosynthesis in green plants consists of two what, arrays of
light-absorbing pigments located in the thylakoid membrane
They are called what I (PS I) and what II (PS II)

generates biosynthetic NADPH which functions as
chemical reducing power

Photosystem I

replenishes the electrons of photosystem I and
generates a proton gradient used to synthesize ATP

Photosystem II

The missing electrons in photosystem ? are replaced by the photolysis
of water

II

PSI is a large what complex
14 proteins in the complex

trimeric

chlorophylls

The PSI reaction center ‘special pair’
is a pair of what

ferredoxin

Photosystem I Uses Light Energy to Generate
Reduced what, a Powerful Reductant

The reaction center in
photosystem what is called P700

I

An electron in P700 is excited to a what energy state by a photon and passes
- to chlorophyll a
- through three Fe-S centers
- to phylloquinone (QK)
- and finally to ferredoxin

higher

The end of the electron transport chain in PSI is ferredoxin,
which is what

reduced

NADPH

Ferredoxin-NADP+ reductase is an enzyme associated with
the thylakoid membrane that transfers electrons from
ferredoxin to form what, biosynthetic reducing power

Ferredoxin-NADP+ reductase contains FAD, which is what to FADH2

reduced

NADP+

Electrons pass from FADH2 to what to generate NADPH

stroma

The formation of NADPH by ferredoxin-NADP+ reductase
occurs in the what

NADP+ and NAD+ differ only in a what group, but this allows the cell
to distinguish between them.

phosphate

What is generally used in biosynthetic reactions

NADPH

What is used in catabolic reactions (breaking down carbs and fats).

NADH

Photosystem what Provides Electrons to Photosystem what

II to I

gradient

Photosystem II Transfers Electrons to
Photosystem I and Generates a Proton what

The reaction center in photosystem what is called P680

II

An electron in P680 is excited to
a higher energy state by a what
and passes down an electron
transfer chain

photon

plastocyanin

An electron in P680 is excited to
a higher energy state by a photon
and passes down an electron
transfer chain
- to pheophytin
- to chlorophyll a
- to plastoquinone
- to the cytochrome b6f complex
- and finally to what

plastoquinone

structurally and functionally similar to ubiquinone in the
mitochondrial ETC. It is an electron carrier that is mobile in the membrane

cytochrome b6f complex = contains a b-type cytochrome
with heme, an iron-sulfur protein, and cytochrome what

f

Cytochrome b6f uses electrons from reduced plastoquinone
to reduce what, a mobile electron carrier which
carries electrons from cytochrome b6f to PS I

plastocyanin

P700+ acquires an electron
from what, a soluble
Cu-containing electron-transfer
protein that received it from
Photosystem II

plastocyanin

regenerated

P700+ acquires an electron
from plastocyanin, a soluble
Cu-containing electron-transfer
protein that received it from
Photosystem II
P700 is now what

plastocyanin

carries electrons from PS II to PS I to replace the electrons
that were used to make NADPH

water

Regeneration of P680 in PSII
the electron removed from
P680 is replaced with an
electron from the oxidation
of what

Which is a strong oxidant that extracts electrons from water

P680

photolysis

P680* is a strong oxidant that extracts electrons from water
This reaction, the what of water, occurs at the water-oxidizing
complex (WOC) of photosystem II

O2

The photolysis of water by PS II is the source of what for all of life

Water Is Split at the Water-Oxidizing Complex
the water-oxidizing complex passes how many electrons, one at a
time, to P680+ (can only accept one electron at a time)

4

Water Is Split at the Water-Oxidizing Complex
the water-oxidizing complex passes four electrons, one at a
time, to P680+ (can only accept one electron at a time)
It gets these electrons by oxidizing ? H2O to O2

2

The Water-Oxidizing Complex Contributes to the Proton Gradient
In the process of oxidizing water to liberate electrons,
four protons are also produced and released into the
thylakoid lumen, further what the proton gradient

increasing

water

Chemiosmotic Mechanism for ATP Synthesis in Chloroplasts
12 protons (2 x 6)
build up in the lumen
per 4 electrons
stripped from what to
make 1 O2

ATP

Chemiosmotic Mechanism for ATP Synthesis in Chloroplasts
chloroplast ATP
synthase uses this
proton gradient to
make what

Mitochondria

The ATP Synthase of Chloroplasts
Closely Resembles That of what

chloroplast

The ATP synthase of the chloroplast is also called the CF1-CF0 complex,
where C stands for what

The chloroplast CF1-CF0 complex is very similar to the mitochondrial F1-F0
complex
– what= transmembrane proton pore that is homologous to mitochondria F0

CF0

The chloroplast CF1-CF0 complex is very similar to the mitochondrial F1-F0
complex
– what = peripheral membrane protein complex similar to mitochondrial F1

CF1

Mechanism of Chloroplast ATP Synthase
Mechanism essentially identical to mitochondrial ATP synthase
– formation of ATP requires a proton-motive force
– ADP and Pi react to form ATP in the enzyme active sites
– rotational catalysis sequentially engages each of the how many
β subunits of the ATP synthase in ATP synthesis, ATP
release, and ADP + Pi binding
– three ATP made per rotation of chloroplast ATP synthase

3

The ATP synthase of the chloroplast is also called the CF1-CF0 complex,
where C stands for chloroplast
Newly synthesized ATP is
released into the stroma, where
it is used in what
synthesis in the dark reactions

carbohydrate

The Approximate Stoichiometry of Photophosphorylation Has Been Established
experimental measurements show eight what drive the
production of one O2, yielding ~3 ATP

photons

cyclic

If the NADPH needs are adequate, what electron flow generates ATP
without forming NADPH

The electrons from photosystem I flow from ferredoxin to cytochrome b6f
with or without making NADP+

without

The protons pumped by
cytochrome b6f are used to
synthesize ATP without
making what

NADPH

photophosphorylation

The electrons from photosystem I flow from ferredoxin to cytochrome b6f without making NADP+
The protons pumped by cytochrome b6f are used to synthesize ATP without making NADPH
This process is called cyclic what

what electron transfer
produces NADPH and ATP

linear

what electron transfer
produces only ATP

cyclic

allows plants to adjust the
ratio of ATP to NADPH to
match its needs

cyclic electron flow