Enormous amounts of energy are stored as products of
photosynthesis. Each year at least 1017 kJ of free energy from sunlight is
captured and used for biosynthesis by photosynthetic organisms. This is more
than ten times the fossil-fuel energy used each year by people the world over.
Even fossil fuels (coal, oil, and natural gas) are the products of
photosynthesis that took place millions of years ago. Because of our global
dependence upon solar energy, past and present, for both energy and food,
discovering the mechanism of photosynthesis is a central goal of biochemical
research.
The overall equation of photosynthesis describes an oxidationreduction
reaction in which H20 donates electrons (as hydrogen) for the reduction of COZ
to carbohydrate (CH2O):
CO 2 + H 2O
![]() ------> O 2 +
(CH 2O)
Photosynthesis encompasses two processes: the light reactions,
which occur only when plants are illuminated, and the carbon fixation reactions,
or so-called dark reactions, which occur in both light and darkness (Fig.
18-33). In the light reactions, chlorophyll and other pigments of the
photosynthetic cells absorb light energy and conserve it in chemical form as the
two energy-rich products ATP and NADPH; simultaneously,
O2 is evolved. In the carbon fixation reactions, ATP
and NADPH are used to reduce CO2 to form glucose and
other organic products. The formation of O2, which
occurs only in the light, and the reduction of CO2,
which does not require light, thus are distinct and separate processes. In this
chapter we are concerned only with the light reactions; the reduction of
CO2 is described in Chapter 19.
In photosynthetic eukaryotic cells, both the light and carbon fixation
reactions take place in the chloroplasts (Fig. 18-34). When solar energy is not
available, mitochondria in the plant cell generate ATP by oxidizing
carbohydrates originally produced in chloroplasts in the light.
Chloroplasts may assume many different shapes in different species, and they
usually have a much larger volume than mitochondria.
They are surrounded by a continuous outer membrane, which, like the outer
mitochondrial membrane, is permeable to small molecules and ions. An inner
membrane system encloses the internal compartment, in which there are many
flattened, membrane-surrounded vesicles or sacs, called
thylakoids, which are usually arranged in stacks called
grana. The thylakoid membranes are separate from the inner
chloroplast membrane. Embedded in the thylakoid membranes are the photosynthetic
pigments and all the enzymes required for the primary light reactions. The fluid
in the compartment surrounding the thylakoids, the stroma, contains most of the
enzymes required for the carbon fixation reactions, in which
CO2 is reduced to form triose phosphates and, from
them, glucose.
Our discussion here focuses on the nature of the light-absorbing systems and
their roles in photosynthesis.
Light Produces Electron Flow in Chloroplasts
How do the pigment molecules of the thylakoid membranes transduce absorbed
light energy into chemical energy? The key to answering this question came from
a discovery made in 1937 by Robert Hill, a pioneer in photosynthesis research.
He found that when leaf extracts containing chloroplasts were supplemented with
a nonbiological hydrogen acceptor and then illuminated, evolution of
O2 and simultaneous reduction of the hydrogen acceptor
took place, according to an equation now known as the Hill
reaction:
| 2H2O + 2A
| light >
![]()
| 2AH2 + O2
|
where A is the artiiicial hydrogen acceptor. One of the nonbiological
hydrogen acceptors used by Hill was the dye 2,6-dichlorophenolindophenol, now
called a Hill reagent, which in its oxidized form (A) is blue and in its reduced
form (AH2) is colorless. When the leaf extract
supplemented with the dye was illuminated, the blue dye became colorless and
O2 was evolved. In the dark neither
O2 evolution nor dye reduction took place. This was
the iirst specific clue to how absorbed light energy is converted into chemical
energy: it causes electrons to flow from H2O to an
electron acceptor. Moreover, Hill found that CO2 was
not required for this reaction, nor was it reduced to a stable form under these
conditions. He therefore concluded that O2 evolution
can be dissociated from CO2 reduction. Several years
later it was found that NADP+ is the biological
electron acceptor in chloroplasts, according to the equation
| 2H2O + 2NADP+
| light >
![]()
| 2 NADPH + 2H+ + O2
|
This equation shows an important distinction between
mitochondrial oxidative phosphorylation and the analogous process in
chloroplasts: in chloroplasts electrons flow from H2O
to NADP+, whereas in mitochondrial respiration
electrons flow in the opposite direction, from NADH or NADPH to
O2, with the release of free energy. Because
lightinduced electron flow in chloroplasts is in the reverse or "uphill"
direction, from H2O to
NADP+, it cannot occur without the input of free
energy; this energy comes from light. To understand how this occurs, we must
first consider the effects of light absorption on molecular structure.
Absorption of Light Excites Molecules
Visible light is electromagnetic radiation of wavelengths 400 to 700 nm, a
small part of the electromagnetic spectrum (Fig. 18-35), ranging from violet to
red. The energy of a single photon (a quantum of light) is greater at the violet
end of the spectrum than at the red end. The energy in a "mole" of photons (one
einstein; 6 × 1023 photons) is 170 to 300 kJ, about an
order of magnitude more than the energy required to synthesize a mole of ATP
from ADP and Pi (about 30 kJ under standard conditions). The energy of an
einstein in the infrared or microwave regions of the spectrum is too small to be
useful in the kinds of photochemical events that occur in photosynthesis. UV
light and x rays, on the other hand, have so much energy that they damage
proteins and nucleic acids and induce mutations that are often lethal.
The ability of a molecule to absorb light depends upon the arrangement of
electrons around the atomic nuclei in its structure. When a photon is absorbed,
an electron is lifted to a higher energy level. This happens on an all-or-none
basis; to be absorbed, the photon must contain a quantity of energy (a
quantum) that exactly matches the energy of the electronic transition.
A molecule that has absorbed a photon is in an excited state,
which is generally unstable. The electrons lifted into higher-energy orbitals
usually return rapidly to their normal lower-energy orbitals; the excited
molecule reverts (decays) to the stable ground state, giving up
the absorbed quantum as light or heat or using it to do chemical work. The light
emitted upon decay of excited molecules, called fluorescence,
is always of a longer wavelength (lower energy) than the absorbed light.
Excitation of molecules by light and their fluorescent decay are extremely fast
processes, occurring in about 10-15 and
10-12 s, respectively. The initial events in
photosynthesis are therefore very rapid.
Chlorophylls Absorb Light Energy for Photosynthesis
The most important light-absorbing pigments in the thylakoid membranes are
the chlorophylls, green pigments with polycyclic, planar
structures resembling the protoporphyrin of hemoglobin (see Fig. 718), except
that Mg2+, not Fe2+,
occupies the central position (Fig. 1836). Chlorophyll a, present in the
chloroplasts of all green plant cells, contains four substituted pyrrole rings,
one of which (ring IV) is reduced, and a fifth ring that is not a pyrrole. All
chlorophylls have a long phytol side chain, esterified to a
carboxyl-group substituent in ring IV. The four inward-oriented nitrogen atoms
of chlorophyll a are coordinated with the Mg2+.
The heterocyclic five-ring system that surrounds the
Mg2+ has an extended polyene structure, with
alternating single and double bonds. Such polyenes characteristically show
strong absorption in the visible region of the spectrum (Fig. 18-37); the
chlorophylls have unusually high molar absorption coefficients (see Box 5-1) and
are therefore particularly well-suited for absorbing visible light during
photosynthesis.
Chloroplasts of higher plants always contain two types of
chlorophyll. One is invariably chlorophyll a, and the second in many species is
chlorophyll b, which has an aldehyde group instead of a methyl group attached to
ring II (Fig. 18-36). Although both are green, their absorption spectra are
slightly different (Fig. 18-37), allowing the two pigments to complement each
other's range of light absorption in the visible region. Most higher plants
contain about twice as much chlorophyll a as chlorophyll b. The bacterial
chlorophylls differ only slightly from the plant pigments (Fig. 18-36).
Accessory Pigments Also Absorb Light
In addition to chlorophylls, the thylakoid membranes contain secondary
light-absorbing pigments, together called the accessory
pigments, the carotenoids and phycobilins. Carotenoids
may be yellow, red, or purple. The most important are
β-carotene (Fig. 18-36), a red-orange isoprenoid compound that is the
precursor of vitamin A in animals, and the yellow carotenoid
xanthophyll. The carotenoid pigments absorb light at wavelengths other
than those absorbed by the chlorophylls (Fig. 18-37) and thus are supplementary
light receptors. Phycobilins are linear tetrapyrroles that have
the extended polyene system found in chlorophylls, but not their cyclic
structure or central Mg2+. Examples are phycoerythrin
and phycocyanin (Fig. 18-36).
The relative amounts of the chlorophylls and the accessory pigments are
characteristic for different plant species. It is variation in the proportions
of these pigments that is responsible for the range of colors of photosynthetic
organisms, which vary from the deep bluegreen of spruce needles, to the greener
green of maple leaves, to the red, brown, or even purple color of different
species of multicellular algae and the leaves of some decorative plants.
Experimental determination of the effectiveness of light of different colors
in promoting photosynthesis yields an action spectrum (Fig. 18-38), often useful
in identifying the pigment primarily responsible for a biological effect of
light. By capturing light in a region of the spectrum not used by other plants,
a photosynthetic organism can claim its unique ecological niche. For example,
the phycobilins, present only in red algae and cyanobacteria, absorb in the
region 520 to 630 nm, allowing these organisms to live in niches where light of
lower or higher wavelength has been filtered out by the pigments of other
organisms living in the water above them, or by the water itsel?
The light-absorbing pigments of thylakoid membranes are arranged in
functional sets or arrays called photosystems. In spinach chloroplasts each
photosystem contains about 200 molecules of chlorophylls and about 50 molecules
of carotenoids. The clusters can absorb light over the entire visible spectrum
but especially well between 400 to 500 nm and 600 to 700 nm (Fig. 18-37). All
the pigment molecules in a photosystem can absorb photons, but only a few can
transduce the light energy into chemical energy. A transducing pigment consists
of several chlorophyll molecules combined with a protein complex also containing
tightly bound quinones; this complex is called a photochemical reaction
center. The other pigment molecules in a photosystem are called
light-harvesting or antenna molecules. They
function to absorb light energy and transmit it at a very high rate to the
reaction center where the photochemical reactions occur (Fig. 18-39, p. 578),
which will be described in detail later.
The chlorophyll molecules in thylakoid membranes are bound to integral
membrane proteins (chlorophyll a/b-binding, or CAB, proteins) that orient the
chlorophyll relative to the plane of the membrane and confer light absorption
properties that are subtly different from those of free chlorophyll. When
isolated chlorophyll molecules in vitro are excited by light, the absorbed
energy is quickly released as fluorescence and heat, but when chlorophyll in
intact spinach leaves is excited by visible light (Fig. 18-40 (step l ), p.
579), very little fluorescence is observed. Instead, a direct transfer of energy
from the excited chlorophyll (an antenna chlorophyll) to a neighboring
chlorophyll molecule occurs, exciting the second molecule and allowing the first
to return to its ground state (step 2). This resonance energy transfer is
repeated to a third, fourth, or subsequent neighbor, until the chlorophyll at
the photochemical reaction center becomes excited (step 3). In this special
chlorophyll molecule, an electron is promoted by excitation to a higher-energy
orbital. This electron then passes to a nearby electron acceptor that is part of
the electron transfer chain of the chloroplast, leaving the excited chlorophyll
molecule with an empty orbital (an "electron hole") (step 4 ). The electron
acceptor thus acquires a negative charge. The electron lost by the
reaction-center chlorophyll is replaced by an electron from a neighboring
electron donor molecule (step 5 ), which becomes positively charged. In this
way, excitation by Light causes electric charge separation and initiates an
oxidationreduction chain. Coupled to the light-dependent electron flow along
this chain are processes that generate ATP and NADPH.
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