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Photosynthesis
Richard Cogdell
INSTANT
EXPERT
30
Earth’s life support system
You have photosynthesis to thank for every lungful of air you
breathe. In fact, photosynthesis is probably the most important
biochemical process on the planet. Besides pumping oxygen
into the atmosphere, it is the energy source behind all our
food and almost all the heat and power we use. Without it, the
evolution of life on Earth would have followed a very different
path. Yet unpicking the molecular details of photosynthetic
chemistry, and understanding how the process shapes our
environment, remains a key challenge
ii | NewScientist | 2 February 2013
Anatomy of a chloroplast
Photosynthesis:
the basics
CARBOHYDRATE
LIPID
THYLAKOID
Plants and algae use the sun’s
energy to convert water and
carbon dioxide into sugars. This
process – photosynthesis – takes
place inside organelles called
chloroplasts
STROMA
INNER
MEMBRANE
A plant leaf may contain 500,000
chloroplasts per square millimetre
OUTER
MEMBRANE
SUN
Chlorophyll pigments, which give plants
their green colour, absorb a photon from
the sun and pass its energy to a pair of
chlorophylls in the reaction centre
REACTION
CENTRE
The reaction centre then kicks out
an electron – a process called charge
separation
CHARGE
SEPARATION
e–
The electron is used to create
NADPH, a chemical reducing agent,
and ATP, the biological energy
molecule
e–
OXYGEN
EVOLVING
CENTRE
OXYGEN
WATER
NADPH
The reaction centre is reset with an
electron from the oxygen evolving
vcentre which splits water molecules
into electrons, hydrogen ions and
oxygen gas
NADPH and ATP are used in the
Calvin cycle to convert a 3-carbon
sugar (3-phosphoglycerate) into
a 5-carbon sugar (ribulose 1,5
bisphosphate)
Photosynthesis is the process by which plants, algae
and some bacteria convert carbon dioxide and water
into carbohydrates using energy from sunlight. In
most cases, they achieve this by splitting apart the
hydrogen and oxygen in water (H2O), giving off oxygen
(O2) as a by-product. In many ways, photosynthesis is
the reverse of respiration: when we animals respire,
we use O2 to burn up carbohydrates, releasing CO2 and
producing the energy we need to live.
Photosynthesis consists of a complex series of
reactions, but it can be divided into four key stages:
light absorption, charge separation, carbon fixation
and oxygen evolution. First, a photon of sunlight is
absorbed by chlorophyll pigments and passed to a
“reaction centre”, which contains a specially aligned
pair of chlorophyll molecules. Here charge separation
occurs: the chlorophyll pair uses the photon’s energy
to spit out an electron. This triggers the final two
stages. The ejected electron is passed along a chain
of molecules until it is used to convert CO2 into
carbohydrate, a process known as carbon fixation.
Meanwhile the reaction centre is “reset” with a new
electron stripped out of water. This replacement
comes from part of the reaction centre complex
called the oxygen evolving centre, which splits water
molecules into electrons, hydrogen ions and oxygen
gas. The complete process can be summarised in a
simple equation:
ATP
5-CARBON
SUGAR
An enzyme called rubisco adds carbon
from CO2 onto a 5-carbon sugar, making
two molecules of 3-phosphoglycerate
Some of this carbon is returned to the Calvin cycle.
The rest is converted into carbohydrates such as
sucrose and used to build leaves and stems
RUBISCO
CALVIN
CYCLE
CO2
3-CARBON
SUGAR
SUCROSE
Stack ‘em up: thylakoid
“coins” in a chloroplast
(above) are linked by
thin lamellae
H2O + CO2 + light ➝ C(H2O) + O2
All this chemistry, from light absorption to the
synthesis of carbohydrates, occurs in a structure
called a chloroplast.
Chloroplasts have two membranes. The smooth
outer membrane holds the whole structure together.
The inner membrane is folded into a series of stacked
discs called thylakoids that contain the pigments and
Justin Guariglia/Corbis
In the chloroplast, the photosynthetic reactions
that depend on light are physically separated from
those that do not. These are called the “light” and
“dark” reactions, respectively.
All the components of the light reactions are
arranged in or on proteins held in the thylakoid
membrane. Light harvesting antenna, for instance,
are proteins that contain chlorophyll pigments
arranged to absorb light and pass the energy to
nearby reaction centres.
While some bacteria contain one kind of
photosynthetic reaction centre, algae and plants
contain two types – photosystem one and
photosystem two. Charge separation in PS2 pulls
electrons from the oxygen evolving centre and
passes them to PS1. PS1 is activated by a second
photon and the electrons it produces are passed
out of the thylakoid membrane and onto molecules
involved in the dark reactions.
The dark reactions occur in the stroma. Here,
enzymes drive a cyclic reaction that converts CO2
and a sugar containing five carbon atoms into
molecules of 3-phosphoglycerate, a 3-carbon sugar.
A proportion of these sugars are fed back into the
cycle. The rest are used as building blocks to form
carbohydrates such as sucrose, cellulose or starch.
The details of this reaction, known as the Calvin
Cycle, were discovered in 1950 by Melvin Calvin,
James Bassham, and Andrew Benson at the
University of California in Berkeley.
The enzyme responsible for fixing carbon
from CO2 is called rubisco. It is probably the most
abundant protein on the planet. Every atom of
carbon in your body has been captured from the
atmosphere by rubisco yet remarkably it is a
rather inefficient enzyme; it has a low affinity
for CO2, and also reacts with O2 in a process called
photorespiration, with the result that about a
third of the carbon it fixes is released back into
the atmosphere.
Driven by light energy, photosynthetic
chemistry in the thylakoid membrane produces
ATP, a molecular source of energy, along with a
reducing agent called NADPH. These molecules
are then consumed in the dark reactions. NADPH
is formed during the final stage of the electron
transport chain while ATP is created when
energy from photons is used to pump protons
across the thylakoid membrane. This sets up an
electrochemical gradient that pushes the protons
back out, releasing energy and generating ATP.
Peter Fakler/Alamy BIOLOGY PICS/spl
Ashley Cooper/Corbis
The light and
dark reactions
Powerhouses: every year
photosynthesis pumps
300 billion tonnes of
oxygen into the biosphere
protein complexes required to capture
solar energy and release oxygen. The
enzymes and other components involved
in converting CO2 into sugars are located
in the stroma, the fluid-filled space
inside the chloroplast (see diagram, left).
The central role played by chloroplasts
was highlighted 75 years ago, when
Robert Hill, a biochemist at the University
of Cambridge, discovered that these
organelles can generate oxygen when
illuminated in the absence of CO2. This
finding was a key discovery because it
provided one of the first indications that
the ultimate source of electrons is water
and not CO2.
2 February 2013 | NewScientist | iii
Shaping the planet
Present: Fifty shades of green
MATTHEW OLDFIELD/spl
Despite more than 2 billion years of evolution, the
core reactions of photosynthesis have remained
remarkably similar across species. Yet a variety of
subtle physical and biochemical modifications have
also evolved, each optimised to suit conditions in
specific ecological niches.
Plants, for example, have evolved slightly different
forms of photosynthesis. Some 85 per cent of plant
species are known as C3 plants. These use the enzyme
rubisco to fix carbon from CO2 to form 3-carbon sugar
molecules that provide the building blocks for sucrose.
Another group of plants has evolved a way to get
round the inefficiency of rubisco. C4 plants, including
tropical grasses such as sugar cane, supercharge
Not so sluggish: playing
host to chloroplasts gives
Elysia an energy boost
carbon fixation by using an additional enzyme, PEP
carboxylase, to fix CO2 into malic acid, a 4-carbon
molecule. The malic acid is then pumped into
specialised cells where it is broken down to release
CO2. Inside these cells, rubisco is exposed to high
concentrations of CO2 which helps it work more
efficiently. Although this process requires energy,
it allows photosynthesis in C4 plants to be up to
50 per cent more efficient than C3 plants,
giving them a competitive advantage in
hot sunny conditions.
A different adaption is found in
dinoflagellates such as Amphidinium
carterae. These live in the sea at depths
where the only available light is in the
blue-green part of the spectrum. They
have evolved a unique light harvesting
complex that uses pigments called
carotenoids, which absorb blue-green
light. Chlorophyll pigments in plants
absorb weakly at these wavelengths.
A number of creatures including
jellyfish, flatworms, bivalve molluscs
and salamanders also make use of
photosynthesis, thanks to a symbiotic
relationship with photosynthetic algae.
The sea slug Elysia, for example, eats
green algae and keeps their chloroplasts
alive in its body, supplementing its diet
using the carbohydrates they create.
When photosynthetic organisms first began to generate oxygen, it
marked the start of a transformation of our world. Oxygen provided
access to a more efficient source of energy through respiration, a
process that would ultimately allow multicellular animals to evolve.
Now, more than 2 billion years after these changes began, the
world is transforming again. Our emissions are causing damaging
climate change. So how will photosynthetic organisms adapt to a
warmer planet, and what are the implications for our biosphere?
Past: First breath
When the first bacteria began to harness light energy
some 3.4 billion years ago, Earth’s atmosphere
was mainly composed of nitrogen and CO2. These
anaerobic photosynthetic organisms relied on
hydrogen, or organic or sulphur compounds as a source
of electrons. Then, about 2.4 billion years ago, the
“great oxygenation event” kicked off. Photosynthetic
organisms evolved – probably ancestors of presentday cyanobacteria – that were capable of splitting
water to produce oxygen. Now oxygen levels in the
biosphere began to rise (see timeline, below).
The changes that occurred over the next 2 billion
years resulted in the extinction of many anaerobic
organisms – it would have been a dramatic and
cataclysmic transformation. Yet the arrival of oxygen
was not entirely bad news.
Ultraviolet radiation from the sun hit oxygen in the
upper layers of the atmosphere, and the subsequent
reaction created ozone (O3). The layer of it in the
stratosphere filtered out this harmful ultraviolet light,
Since oxygenic photosynthesis evolved some
2.8 billion years ago, the proportion of the atmosphere
that is made up of CO2 has dropped from about 20 per
cent to just 0.04 per cent. The levels started to rise
again at the start of the industrial revolution, and
continue to go up. This is happening because the
capacity of photosynthesis to soak up the huge
volumes of gas we release by burning fossil fuels has
been exceeded. What will this change mean for plants
and for other life forms like us that ultimately depend
on photosynthesis?
Studies show that some trees are already growing
bigger and faster, but perhaps the only certainty is
that the effects on the composition of plant
populations will be highly unpredictable. To find out
more about how elevated CO2 concentrations will
influence crop growth, researchers are simulating
future atmospheric conditions in field experiments
that expose plants to different compositions of air.
These experiments show that C4 and C3 plants
respond differently. C4 plants such as maize (corn)
slightly increase their rate of photosynthesis but there
is little effect on growth, even when CO2 levels reach
0.06 per cent. However at this level of CO2, the rate
of photosynthesis in C3 plants increases by about
40 per cent. This is reflected in the crop, with wheat,
rice and soybean showing increases in yield of up to
14 per cent.
There may also be an impact on water use. Plants
regulate their CO2 uptake using tiny pores in their
leaves called stomata. As CO2 levels rise, stomata will
stay closed for longer. These pores also allow water
vapour to escape, so higher CO2 levels may reduce
plants’ water losses, meaning farmers would not have
to water their crops as often.
These benefits might come at a price. Increasing
rates of photosynthesis mean plant growth may then
be limited by the availability of key nutrients such as
phosphorus and nitrogen. This could prove most serious
for crops such as pulses that have seeds with high
protein content and these may need extra fertiliser.
Farmers can probably mitigate gradual changes
to the atmosphere by altering farming practices –
applying extra fertiliser, say, or changing crop
varieties. What will be harder to deal with are sudden
extremes of weather. Extensive drought in the US
and heavy rain in parts of Europe drastically reduced
grain yields in 2012, and global warming is expected
to bring even wilder fluctuations in weather patterns.
Open and shut case: plants
control the uptake of CO2
via tiny pores in their leaves
iv | NewScientist | 2 February 2013
”The release of oxygen
permitted the evolution
of new life forms that
obtained energy from
respiration”
Relics: stromatolites are
fossilised deposits formed
by ancient cyanobacteria
Nearly all the oxygen on our planet comes from photosynthesis. This gas first began to appear when photosynthetic cyanobacteria evolved.
These bacteria could split water to give them energy, rather than using hydrogen or sulphur compounds
300 mya
40
Atmospheric oxygen levels (%)
right: Frans Lanting/Corbis far right: Mint Images/Rex below: DR JEREMY BURGESS/spl
Future: Plants in a changing world
which damages DNA, helping life spread out of the
deep oceans. The earliest land dwellers were mosses
and liverworts, descended from green algae that
thrived in warm, shallow water. Oxygen also permitted
the evolution of new life forms that obtained their
energy from respiration.
Aerobic respiration is very efficient. The increase
in energy available to support life allowed a great
expansion in the number of species on our planet
and, in particular, the evolution of large multicellular
creatures. By 400 million years ago, oxygen levels had
begun to stabilise at close to current levels, and plants
such as ferns, grasses and cacti had colonised the land.
The release of oxygen by photosynthetic organisms
also altered Earth’s geology. For instance, oxygen in
the oceans triggered the formation of iron oxide,
eventually producing the red bands of iron ore
deposits in sedimentary rocks. Oxygen also generated
thousands of other minerals in the crust, helping to
create the rich variety of materials we exploit today.
30
High oxygen peak during
the Carboniferous
3.4 bya
First
photosynthetic
bacteria evolve
20
Great oxygenation
event begins
2.8 bya
Red and brown
algae evolve
1.9 bya
Oxygen
levels drop
500 mya
First land plants evolve
750 mya
Green algae
evolve
Photosynthetic
cyanobacteria
begin to
release oxygen
10
0
1.2 bya
2.4 bya
4 Billion years ago (bya)
3
2
1
0
2 February 2013 | NewScientist | v
Waste not: sugar cane,
the world’s largest crop,
is turned into biofuel,
with left-over bagasse
burned to make heat
vi | NewScientist | 2 February 2013
The complexity of photosynthesis is a huge challenge for those trying
to unpick its details. And with the twin threats of climate change and
food shortages, scientists are looking to photosynthesis for help. Can
plants and algae be the key to new carbon-neutral fuels, for example?
Might we even be able to supercharge the photosynthetic process
and increase the yields of vital food crops?
New sources of fuel
Global carbon emissions are rising
steadily and if our planet is to avoid
catastrophic warming, we must
work rapidly to replace fossil fuels.
Can photosynthesis help?
Plant power has already been
harnessed for biofuel. US distilleries
produce more than 50 billion litres
of bioethanol annually, mainly from
fermented corn. Most is blended
with conventional petrol and used
to power vehicles.
Yet questions remain over
the sustainability of this biofuel.
The conversion of solar energy
into bioethanol is very inefficient,
meaning huge areas of land
are needed if production is to
be scaled up.
Another way that photosynthesis
can offer us fuels is if we can mimic
the way in which plants and algae
use light to split water, to generate
Derek Lovley (above)
H2 as well as O2 (see “From ocean
hopes to modify a
to atmosphere”, page vii).
bacteria so it produces Scientists already do this in the
hydrocarbon fuel from lab – photovoltaic cells connected
sunlight and CO2
to a pair of platinum electrodes
immersed in water will generate
bubbles of H2 fuel. However, this
technique would be prohibitively expensive on a large scale
because of the high cost of platinum. The challenge is to
mass-produce electrodes at lower cost.
One contender is a system devised by Daniel Nocera and
colleagues at the Massachusetts Institute of Technology.
Their oxygen evolving electrode uses a structure inspired
by the plant’s oxygen evolving centre, but with cobalt
instead of manganese. This splits water to release oxygen,
creating hydrogen ions that combine with electrons at the
other electrode – an alloy of nickel, molybdenum and zinc –
to form hydrogen gas. Electricity is provided by a special
silicon-based solar cell.
Still further off is the “electric leaf”. This is a concept for
a hybrid fuel generation system, using photovoltaic panels
that supply electricity to living cells. These cells will be
engineered to create not H2, but energy-rich hydrocarbons.
A bacteria called Geobacter might provide the basis
for the biological half of this double act. Geobacter isn’t
photosynthetic. Instead, it extracts electrons from minerals
and uses them to power its metabolism. Derek Lovley at
the University of Massachusetts Amherst, near Boston,
has shown that Geobacter can
grow using electrons provided by
a photovoltaic cell and that the
bacteria can extend wire-like
hairs called pili to make electrical
connections.
This raises an intriguing question:
could we modify Geobacter so it
turns electrons into hydrocarbon
fuel? Jay Keasling, of the University
of California, Berkeley, has shown
that the metabolic pathway required
to synthesise hydrocarbons called
terpenes can be engineered into E.
coli. In principle, the same thing
could be done with Geobacter,
creating a hybrid system that
converts sunlight into a petrol
substitute.
From ocean to
atmosphere
”Geobacter bacteria could
provide a hybrid system
that converts sunlight
into a petrol substitute”
Photosynthesis, and therefore all life on
Earth, has been able to proliferate because
of oxygen evolution. Plants, algae and
cyanobacteria release oxygen by splitting
water using a protein structure called the
oxygen evolving centre. At its heart are four
manganese ions, held in specific orientations
by a protein scaffold.
Although scientists can mimic the way the
centre splits water using electricity and a
platinum catalyst, this requires roughly twice
the energy used by photosynthetic organisms.
The oxygen evolving centre reduces its
energy needs by dividing up the chemistry
into a series of small steps. In particular, the
manganese ions have four oxidation states
and give up their electrons one at a time,
steadily increasing their oxidation power
until molecular oxygen is formed.
Yet despite decades of study, scientists
have so far failed to unravel every fine detail
of the way the centre functions. We know the
key features of its structure, thanks to X-ray
measurements. However, these only provide
a static snapshot: how the metal ions and
the protein’s amino acids cooperate as the
reactions proceed is still unknown.
To find out more, researchers are using
techniques such as time-resolved X-ray
crystallography, in which ultrashort X-ray
pulses are beamed through a sample of oxygen
evolving centres. These measurements can
An “electric leaf” constructed from
a genetically engineered bacterium
could convert sunlight and carbon
dioxide into vehicle fuel
LIGHT
PHOTOVOLTAIC
PANELS
CO2
+
–
GEOBACTER
BACTERIA
HYDROCARBON
FUEL
A false colour image
shows how CO2 emission
(red) gives way to CO2
absorption (green)
as photosynthesis
switches on at dawn
Jamison Daniel, ORNL/NCCS
A plant’s efficiency at turning CO2, water and light
into biomass is extremely low – typically around
4 to 5 per cent at best. But where do these limits
come from? Can they be overcome to produce crops
with higher yields?
Plants rely on chlorophyll molecules to collect
light, yet these pigments don’t absorb over the
entire spectrum – light at wavelengths above
750 nanometres is not used. This means plants
waste about half of the energy in the solar
spectrum, so researchers are attempting to
tackle this by combining plant reaction centres
with light harvesting antenna from purple
bacteria which absorb light wavelengths from
800 to 1000 nanometres.
Plants also make far more chlorophyll than
they need. This is a survival mechanism: with
extra chlorophyll in their leaves, little light will
reach competitors growing below. But this also
means that in strong sunshine, plants absorb more
light than they can use. Under these conditions up
to 80 per cent of the light collected is wasted, with
excess energy dissipated as heat. Researchers
hope that reducing a plant’s light harvesting
capacity will increase its photosynthetic efficiency,
so several teams are making mutant green algae
with reduced pigment content.
Another major inefficiency occurs during
carbon fixation, thanks to the enzyme rubisco.
C4 plants, such as sugar cane and sorghum have
partially solved this thanks to a CO2 concentration
mechanism (see “The light and dark reactions”,
page ii) that raises the efficiency of photosynthesis
to about 6 per cent. Cyanobacteria have a similar
strategy: they contain carboxysomes, protein
assemblies containing rubisco where CO2 can be
concentrated while O2 is excluded.
To improve crop yield, researchers are trying to
convert C3 plants into C4 ones, and see if C3 plants
can be modified to make their own carboxysomes.
One team based at the University of Cambridge is
attempting to change the leaf anatomy of a C3 plant
so that it produces carboxysomes in its chloroplasts.
To do this, the plant must not only synthesise all
the components needed but they must also be
delivered and assembled inside the chloroplast.
Even if these efforts succeed, it is hard to envisage
photosynthetic efficiency rising above 10 per cent.
FRONTIERS OF PHOTOSYNTHESIS
Daniel Nocera (below)
is developing a solar cell
(right) to make H2 fuel
bottom: VOLKER STEGER/spl Isaac Hernandez/IsaacHernandez.com top: Dominick Reuter
Jason Larkin/Panos
Improving
crop yields
reveal structural changes on a picosecond
timescale, with a resolution of a few
nanometres. Such experiments could allow
us to follow the detailed changes that occur in
the centre’s structure as a single molecule of
oxygen is released, solving a scientific riddle
and perhaps pointing the way to new kinds of
photovoltaic technology.
2 February 2013 | NewScientist | vii
Richard Cogdell
Richard Cogdell FRS is the Hooker
Professor of Botany and Director of
the Institute of Molecular, Cell and
Systems Biology at the University of
Glasgow, UK. His research focuses on
artificial photosynthesis and the light
reactions of bacterial photosynthesis
Next
INSTANT
EXPERT
Michael O’Shea
The human brain
6 April
The emergence of quantum biology
Despite decades of research,
fundamental questions about
photosynthesis remain unanswered.
One important issue is whether we
can use it to develop artificial systems
capable of turning solar energy into
carbon-neutral fuels (see “New
sources of fuel”, page vi). The world
has plenty of fossil fuels, but our
unrestrained use of them will have
severe consequences for the planet.
Progress here may require a step
change in current approaches.
Strange as it may seem, deciphering
the quantum properties of the
pigments involved may be the key we
will need to master photosynthesis.
Recent measurements have shown
that when the pigments in lightharvesting antenna are excited
by the energy of a photon, their
electrons can jump into a quantum
superposition of excited states. This
“coherent” quantum state can last
hundreds of femtoseconds or so.
Although this effect may seem
subtle, it raises the intriguing
possibility that such quantum states
play a fundamental role in the
early stages of the light reactions.
It may also help to explain why
viii | NewScientist | 2 February 2013
photosynthetic antenna are so
efficient at transferring light energy
to the reaction centres.
Within any light-harvesting
antenna complex, the protein
structure will be changing constantly
because of unavoidable thermal
effects. These fluctuations cause
the precise energies of the
chlorophyll pigments bound to the
protein to change, influencing the
“energy landscape” of the system
and either enhancing energy
transfer processes or making them
less efficient. However a coherent
quantum state could, in principle,
smooth out the effects of these
fluctuations so that energy transfer
always remains highly efficient.
If this hypothesis proves correct,
it raises a key question: can we
learn to harness the power of these
quantum effects and use them to
improve the performance of devices
such as photovoltaic cells? Might
similar quantum states play a key
role elsewhere in photosynthetic
systems or in other places such as
olfactory receptors? These questions
lie at the heart of the emerging field
of quantum biology.
Further READING
Molecular Mechanisms of
Photosynthesis by Robert E.
Blankenship (Blackwell, 2002)
Photosynthesis by P. J. Weaire,
(The Biochemical Society, 1994)
“Crystal structure of oxygen-evolving
photosystem II at a resolution of 1.9 Å”
by Yasufumi Umena et al. Nature,
vol 473, p 55
“Powering the planet: chemical
challenges in solar energy utilization”
by Nathan S. Lewis and Daniel G.
Nocera, PNAS, vol 103, p 15729
“Comparing photosynthetic and
photovoltaic efficiencies and
recognizing the potential for
improvement,” by Robert E. Blankenship
et al. Science, vol 332, p 805
“Lessons from nature about solar
light harvesting” by Gregory D. Scholes
et al. Nature Chemistry, vol 3, p763
“Rubisco: structure, regulatory
interactions and possibilities for a
better enzyme” by Robert J. Spreitzer
and Michael E. Salvucci, Annual Review
of Plant Biology, vol 53, p 449
Cover image
Biology Pics/SPL