How can light be used to make food? When a person turns on a lamp, electrical energy becomes
light energy. Like all
other forms of kinetic energy, light can travel, change form, and be
harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy,
which photoautotrophs use to build carbohydrate molecules (Figure 1.1). However, autotrophs only use
a few specific components of sunlight.
Figure 1.1 Photoautotrophs can capture light energy from the sun,
converting
it into the chemical energy
used to build food molecules.
(credit: Gerry Atwell)
What Is Light Energy?
The sun emits an enormous
amount of electromagnetic radiation (solar energy).
Humans can see only a fraction of this energy, which portion
is therefore referred
to as “visible light.” The manner in which solar energy travels is described
as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength, the distance between consecutive points of a wave. A
single wave is measured from two consecutive points, such as from crest to
crest or from trough to trough (Figure 1.2).
Figure
1.2 The wavelength of
a single wave is the distance between two
consecutive points of similar position (two crests or two troughs) along the wave.
Visible light constitutes only one of many types
of electromagnetic radiation emitted from the sun and other stars.
Scientists differentiate the
various types of radiant energy from
the sun within the electromagnetic spectrum. The electromagnetic spectrum is
the range of all possible frequencies of radiation (Figure 1.3). The difference
between wavelengths relates to the amount of energy carried by them.
Figure 1.3 The
sun emits
energy
in
the
form
of
electromagnetic radiation. This radiation exists
at different
wavelengths, each of which has
its own characteristic energy. All electromagnetic radiation, including visible light,
is characterized by its wavelength.
Each type of
electromagnetic radiation travels at a particular wavelength. The longer the
wavelength (or the more stretched out it appears in the diagram), the less energy is carried. Short, tight waves carry
the most energy. This may seem illogical, but think of it
in terms of a piece of moving a heavy rope. It takes little effort by a person to move a rope in long,
wide waves. To make a rope move in
short, tight waves, a person would need to apply significantly more energy.
The electromagnetic
spectrum (Figure 1.4) shows several types of electromagnetic radiation
originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy
waves can penetrate tissues and damage cells and DNA, explaining why both X-rays and UV rays can be
harmful to living organisms.
Absorption of Light
Light energy initiates the process of
photosynthesis when pigments absorb the light. Organic pigments, whether in the human retina or the chloroplast
thylakoid, have a narrow range of energy
levels that they can absorb. Energy
levels lower than those represented by red light are insufficient to raise an orbital electron to a
populatable, excited (quantum) state. Energy
levels higher than those in blue light will physically tear the molecules
apart, called bleaching. So retinal pigments can only “see” (absorb)
700 nm to 400 nm light, which
is therefore called
visible light. For the same reasons, plants
pigment molecules absorb only light in the wavelength range of 700 nm to
400 nm; plant physiologists refer to this range for plants as
photosynthetically active radiation.
The visible
light seen by humans as white light actually exists in a rainbow of colors. Certain
objects, such as a prism or
a drop of water, disperse white light
to reveal the colors to the human eye. The visible light portion of the
electromagnetic spectrum shows the rainbow of colors, with violet and blue having
shorter wavelengths, and therefore higher
energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy (Figure 1.4).
Figure 1.4 The colors of visible light do not carry the same amount of energy. Violet has the
shortest wavelength and therefore carries the most energy, whereas red has the longest wavelength and carries the
least amount of
energy. (credit:
modification of work by NASA)
Understanding Pigments
Different
kinds of pigments exist, and each has evolved to absorb only certain
wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color.
Chlorophylls and carotenoids are the two major classes
of photosynthetic pigments
found in plants and algae; each class has multiple types of pigment
molecules. There are five major chlorophylls: a, b, c and d and a related molecule found in prokaryotes called
bacteriochlorophyll. Chlorophyll a and chlorophyll
b
are found in higher plant
chloroplasts and will be
the focus of the following discussion.
With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids
found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds
(zeaxanthin), or the orange of an orange peel (β-carotene)—are used as
advertisements to attract seed dispersers. In photosynthesis, carotenoids
function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy.
When a leaf is exposed to full sun, the light-dependent reactions are required
to process an enormous amount of energy;
if that energy is not handled
properly, it can do significant
damage. Therefore, many carotenoids reside in the thylakoid membrane, absorb
excess energy, and safely dissipate that energy
as heat.
Each type of pigment can
be identified by the specific pattern of wavelengths it absorbs from visible
light, which is the absorption spectrum.
The graph in Figure 1.5 shows the absorption spectra for chlorophyll a, chlorophyll b, and a
type of carotenoid pigment
called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct
set of peaks and troughs, revealing a highly specific pattern of
absorption. Chlorophyll a absorbs
wavelengths from either end of the visible spectrum (blue and red), but not
green. Because green is reflected or transmitted, chlorophyll appears green.
Carotenoids absorb in the
short-wavelength blue region, and
reflect the longer yellow, red, and orange wavelengths.
Figure
1.5 (a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid
membrane. Chlorophyll
a and
b, which are identical except for the part indicated in the red box, are responsible for the green color of leaves. β-carotene is responsible for the orange color in carrots. Each
pigment
has (d) a unique absorbance spectrum.
Many photosynthetic organisms have a mixture of pigments; using
them, the organism can absorb energy from a wider range of wavelengths. Not
all photosynthetic organisms have
full access to sunlight. Some organisms
grow underwater where light intensity and quality decrease and change with
depth. Other organisms grow in
competition for light. Plants on the rainforest floor must be able to absorb
any bit of light that comes through, because the taller trees absorb most of
the sunlight and scatter the remaining solar radiation (Figure 1.6).
When studying a
photosynthetic organism, scientists
can determine the types of pigments present by generating absorption spectra.
An instrument called a spectrophotometer can differentiate which wavelengths of light a
substance can absorb. Spectrophotometers measure transmitted light and compute
from it the absorption. By extracting pigments
from leaves and placing these
samples into a spectrophotometer,
scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identification of
plant pigments include various types of chromatography that separate the
pigments by their relative affinities to solid and mobile phases.
How Light-Dependent Reactions Work
The overall function of light-dependent
reactions is to convert solar energy
into chemical energy in the form of
NADPH and ATP. This chemical energy
supports the light-independent reactions and fuels the assembly of sugar
molecules. The light-dependent reactions are depicted in Figure 1.7. Protein complexes and pigment
molecules work together to produce NADPH and ATP.
The actual
step that converts
light energy into chemical
energy takes place in a multiprotein complex called a photosystem, two types of which are
found embedded in the thylakoid membrane, photosystem
II (PSII) and photosystem I (PSI)
(Figure 1.8).
The two complexes differ on the
basis of what they oxidize (that is, the source of the low-energy electron supply) and what they reduce
(the place to which they deliver their energized electrons).
Both photosystems have
the same basic structure; a number of antenna
proteins to which the
chlorophyll molecules are bound surround the reaction center where
the photochemistry takes place. Each photosystem is serviced by the light- harvesting complex, which passes
energy from sunlight to the reaction
center; it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules as well as other pigments
like carotenoids. The absorption of a single photon or distinct quantity or “packet” of light by any of the
chlorophylls pushes that molecule into an excited state. In short, the light
energy has now been captured by
biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to
chlorophyll until eventually (after about a millionth of a second), it is
delivered to the reaction center. Up to this point, only energy has been transferred between
molecules, not electrons.
Figure 1.8 In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons from water. The
electrons travel through the chloroplast electron
transport chain to photosystem I (PSI), which
reduces NADP+ to NADPH. The electron transport chain moves protons across the thylakoid
membrane into the lumen.
At the same time, splitting
of water adds protons to the lumen, and reduction of NADPH removes protons from the stroma. The net result
is a low pH in the thylakoid lumen, and
a high pH in the stroma. ATP synthase uses this electrochemical gradient to make ATP.
What is the initial source of
electrons for the chloroplast electron
transport chain?
a. water
b. oxygen
c. carbon dioxide
d. NADPH
The reaction center
contains a pair of chlorophyll a molecules
with a special property. Those two
chlorophylls can undergo oxidation upon excitation; they can actually
give up an electron in a process
called a photoact. It is at this step in the reaction
center, this step in photosynthesis, that light energy is converted into an excited
electron. All of the subsequent steps involve getting
that electron onto the energy
carrier NADPH for delivery to the Calvin cycle where the electron is deposited
onto carbon for long-term storage in the form of a carbohydrate.PSII and PSI
are two major components of the photosynthetic electron transport chain,
which also includes the cytochrome complex. The cytochrome complex,
an enzyme composed of two protein complexes, transfers the electrons from the
carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc), thus
enabling both the transfer of protons across the thylakoid membrane and the
transfer of electrons from PSII to PSI.
The reaction center of
PSII (called P680) delivers its
high-energy electrons, one at the
time, to the primary electron acceptor, and through the electron
transport chain (Pq to cytochrome complex to plastocyanine) to PSI. P680’s missing electron is replaced by
extracting a low-energy electron
from water; thus, water is split and PSII is re-reduced after every photoact.
Splitting one H2O molecule releases two electrons, two
hydrogen atoms, and one atom of oxygen. Splitting two molecules is required to form one molecule of diatomic O2 gas. About 10 percent of the oxygen is used by mitochondria
in the leaf to support oxidative phosphorylation. The remainder escapes to the
atmosphere where it is used by aerobic organisms to support respiration.
As electrons move through
the proteins that reside between PSII and PSI, they lose energy.
That energy is used to move hydrogen
atoms from the stromal side of the membrane to the thylakoid lumen. Those
hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used synthesize ATP in a later
step. Because the electrons have lost energy prior to their arrival at PSI, they
must be re-energized by PSI, hence,
another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center (called P700). P700 is oxidized and sends a
high- energy electron to NADP+ to form NADPH. Thus,
PSII captures the energy to create
proton gradients to make ATP, and PSI captures the energy to reduce NADP+ into
NADPH. The two photosystems work in concert, in part, to guarantee that the
production of NADPH will roughly equal the production of ATP.
Other mechanisms exist to fine tune that ratio to exactly match the chloroplast’s constantly changing
energy needs.
Generating an Energy Carrier: ATP
As in the intermembrane
space of the mitochondria during cellular respiration, the buildup of hydrogen
ions inside the thylakoid lumen creates a concentration gradient. The passive
diffusion of hydrogen ions from high
concentration (in the thylakoid lumen) to low concentration (in the stroma) is
harnessed to create ATP, just as in the electron transport
chain of cellular respiration. The ions build up energy because of diffusion
and because they all have the same electrical charge, repelling each
other.
To release this energy,
hydrogen ions will rush through any opening, similar to water jetting through a
hole in a dam. In the thylakoid, that opening is a passage through a specialized
protein channel called the ATP
synthase. The energy released by the
hydrogen ion stream allows ATP
synthase to attach a third phosphate group to ADP, which forms a molecule of ATP
(Figure 1.8).
The flow of hydrogen ions through ATP
synthase is called chemiosmosis because the ions move from an area of high to
an area of low concentration through
a semi-permeable structure.
Using Light Energy to Make Organic Molecules
After the energy from the sun is converted into chemical energy and temporarily stored
in ATP and NADPH
molecules, the cell has the
fuel needed to build carbohydrate molecules for long-term energy storage. The products of the
light-dependent reactions, ATP and
NADPH, have lifespans in the range of millionths of seconds, whereas the
products of the light- independent reactions (carbohydrates and other forms of reduced
carbon) can survive
for hundreds of millions of years. The carbohydrate molecules made will have
a backbone of carbon atoms. Where does the carbon come from? It comes from
carbon dioxide, the gas that is a waste product
of respiration in microbes, fungi, plants, and animals.
The Calvin
Cycle
In plants, carbon dioxide
(CO2) enters the leaves through stomata, where it
diffuses over short distances
through intercellular spaces until it reaches
the mesophyll cells. Once in the mesophyll
cells, CO2 diffuses into the stroma of the chloroplast—the site of
light-independent reactions of photosynthesis. These reactions actually have
several names associated with them. Another term, the Calvin cycle, is named for the man who discovered it, and because
these reactions function as a cycle. Others call it the Calvin-Benson cycle to include
the name of another scientist
involved in its discovery. The most outdated name is dark
reactions, because light is not directly required (Figure 1.9). However, the term dark reaction can be misleading because it implies
incorrectly that the reaction only occurs at night or is independent of light,
which is why most scientists and
instructors no longer use it.
The light-independent
reactions of the Calvin cycle can be organized
into three basic stages: fixation, reduction, and regeneration.
Stage 1: Fixation
In the stroma, in addition to CO2, two other components are present to initiate the light-independent reactions:
an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and three molecules of ribulose bisphosphate (RuBP), as shown in Figure 1.10. RuBP has five atoms of
carbon, flanked by two phosphates.
Figure 1.10 The Calvin cycle has
three
stages. In stage 1, the enzyme RuBisCO
incorporates carbon dioxide into an organic molecule, 3-PGA. In stage 2, the organic
molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP,
the molecule that starts the cycle, is regenerated so that the cycle can
continue. Only one carbon dioxide molecule is
incorporated at a time, so the cycle must be completed three times to produce a single
three- carbon GA3P molecule, and six times to produce a
six-carbon glucose molecule.
Which of the following statements is true?
a. In photosynthesis, oxygen, carbon dioxide, ATP, and NADPH
are
reactants. GA3P
and
water
are products.
b. In photosynthesis, chlorophyll, water, and
carbon
dioxide
are reactants. GA3P and oxygen
are products.
c. In photosynthesis, water, carbon dioxide, ATP, and NADPH
are
reactants. RuBP
and
oxygen
are products.
d. In photosynthesis, water and carbon dioxide are reactants. GA3P and oxygen
are products.
d. In
photosynthesis, water and carbon dioxide are reactants. GA3P and oxygen
are products.
RuBisCO catalyzes a reaction between CO2 and RuBP. For each CO2 molecule that reacts with one RuBP, two molecules
of another compound (3-PGA) form. PGA has three carbons and one phosphate. Each
turn of the cycle involves only one RuBP and one carbon dioxide and forms two
molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO2 +
15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called carbon
fixation, because CO2 is
“fixed” from an inorganic form into organic molecules.
Stage 2: Reduction
ATP and NADPH are used to convert
the six molecules of 3-PGA into six molecules of a chemical
called glyceraldehyde
3-phosphate (G3P). That is a reduction
reaction because it involves the gain of electrons by 3-PGA. Recall that a reduction
is the gain of an electron by an atom or molecule. Six molecules of both ATP and NADPH are used. For ATP,
energy is released with the loss of
the terminal phosphate atom, converting it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP+. Both of these molecules
return to the nearby light-dependent reactions to be reused
and reenergized.
Stage 3: Regeneration
Interestingly, at this point, only one of the G3P molecules leaves the Calvin
cycle and is sent to the cytoplasm to contribute to the formation of other
compounds needed by the plant. Because the G3P exported from the chloroplast
has three carbon atoms, it takes three “turns”
of the Calvin cycle to fix enough net carbon to export one G3P. But each turn makes two G3Ps,
thus three turns make six G3Ps. One is exported while the remaining five G3P
molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare
for more CO2 to be fixed. Three more molecules of
ATP are used in these regeneration reactions.
The Energy Cycle
Whether the organism is a bacterium, plant, or animal, all living things
access energy by breaking down
carbohydrate molecules. But if plants make carbohydrate molecules, why would
they need to break them down, especially when it has been shown that the gas organisms release as a “waste product” (CO2) acts as a substrate for the formation of
more food in photosynthesis? Remember, living things need energy to perform life functions. In addition, an organism
can either make its
own food or eat another
organism—either way, the food still needs to be broken down. Finally, in the process
of breaking down food, called cellular
respiration, heterotrophs release
needed energy and produce
“waste” in the form of CO2 gas.
In nature, there is no such thing as waste. Every single atom of matter and energy is conserved, recycling
over and over infinitely. Substances change form or move from one
type of molecule to another, but
their constituent atoms never disappear (Figure 1.11).
CO2 is no more a form of waste than oxygen is wasteful to photosynthesis. Both are byproducts of reactions that move
on to other reactions. Photosynthesis absorbs light energy to build carbohydrates in chloroplasts, and aerobic cellular
respiration releases energy by using oxygen
to metabolize carbohydrates in the cytoplasm and mitochondria. Both processes
use electron transport chains to capture the energy necessary to drive other reactions. These two powerhouse
processes, photosynthesis and cellular
respiration, function in biological, cyclical
harmony to allow organisms
to access life-sustaining energy that originates millions of miles
away in a burning star humans call the
sun.
Figure
1.11 Photosynthesis consumes carbon dioxide and produces oxygen.
Aerobic
respiration consumes oxygen and produces carbon
dioxide. These two processes play an important role in the carbon cycle. (credit: modification of work
by Stuart Bassil)
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