Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH Flashcards
Chloroplasts are chemical factories powered by the
sun
Light is a form of energy known as
electromagnetic energy, also called electromagnetic radiation.
Electromagnetic energy travels in rhythmic waves analogous to those created by
dropping a pebble into a pond.
Electromagnetic waves, however, are disturbances of electric and magnetic fields rather than disturbances of a material medium such as .
water
The distance between the crests of electromagnetic waves is called the
wavelength
Wavelengths range from less than a nanometer (for gamma rays) to more than a
kilometer (for radio waves).
This entire range of radiation is known as the
electromagnetic spectrum
The segment most important to life is the narrow band from about 380 nm to 750 nm in wavelength. This radiation is known as
visible light because it can be detected as various colors by the human eye.
The model of light as waves explains many of light’s properties, but in certain respects light behaves as though it consists of discrete particles, called
photons
Photons are not tangible objects, but they act like objects in that each of them has a fixed quantity of
energy
The amount of energy is inversely related to the wavelength of the light: The shorter the wavelength, the greater the energy of each
photon of that light.
Thus, a photon of violet light packs nearly twice as much energy as a
photon of red light
When light meets matter, it may be reflected, transmitted, or
absorbed
Substances that absorb visible light are known as
pigments.
The ability of a pigment to absorb various wavelengths of light can be measured with an instrument called a
spectrophotometer.
A graph plotting a pigment’s light absorption versus wavelength is called an
absorption spectrum
the key light-capturing pigment that participates directly in the light reactions;
chlorophyll a
The accessory pigment ____________; and a separate group of accessory pigments called
chlorophyll b, carotenoids.
The spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis, since they are absorbed, while green is the least effective color. This is confirmed by an _____________________ for photosynthesis (Figure 10.10b), which profiles the relative effectiveness of different wavelengths of radiation in driving the process.
action spectrum
An action spectrum is prepared by illuminating chloroplasts with light of different colors and then plotting wavelength against some measure of
photosynthetic rate, such as CO2 consumption or O2 release.
The action spectrum for photosynthesis was first demonstrated by
Theodor W. Engelmann, a German botanist, in 1883.
hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-green light
carotenoids
a more important function of at least some carotenoids seems to be
photoprotection
These compounds absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen, forming
reactive oxidative molecules that are dangerous to the cell.
Interestingly, carotenoids similar to the photoprotective ones in chloroplasts have a photoprotective role in the
human eye. (Carrots, known for aiding night vision, are rich in carotenoids.)
They are also often advertised in health food products as “phytochemicals” (from the Greek phyton, plant), some of which have
antioxidant properties.
The colors corresponding to the absorbed wavelengths disappear from the spectrum of the transmitted and reflected light, but
energy cannot disappear.
When the electron is in its normal orbital, the pigment molecule is said to be in its
ground state
Absorption of a photon boosts an electron to an orbital of higher energy, and the pigment molecule is then said to be in an
excited state
The only photons absorbed are those whose energy is exactly equal to the energy difference between the
ground state and an excited state, and this energy difference varies from one kind of molecule to another.
Thus, a particular compound absorbs only photons corresponding to
specific wavelengths
Generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground-state orbital in a billionth of a second, releasing their excess energy as
heat.
In isolation, some pigments, including chlorophyll, emit light as well as heat after absorbing
photons
As excited electrons fall back to the ground state, photons are given off, an afterglow called
fluorescence
Chlorophyll molecules excited by the absorption of light energy produce very different results in an intact chloroplast than they do in
isolation
In their native environment of the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into complexes called
photosystems.
is composed of a reaction-center complex surrounded by several light-harvesting complexes
photosystem
is an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor.
reaction-center complex
consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and multiple carotenoids) bound to proteins
light-harvesting complex
Together, these light-harvesting complexes act as an antenna for the
reaction-center complex.
When a pigment molecule absorbs a photon, the energy is transferred from pigment molecule to pigment molecule within a
light-harvesting complex,
is a molecule capable of accepting electrons and becoming reduced.
primary electron acceptor
The solar-powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step of the
light reactions
As soon as the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it; this is a
redox reaction.
The thylakoid membrane is populated by two types of photosystems that cooperate in the light reactions of photosynthesis:
photosystem II (PS II) and photosystem I (PS I).
They were named in order of their discovery, but photosystem II functions first in the
light reaction
Each has a characteristic reaction-center complex—a particular kind of primary electron acceptor next to a special pair of chlorophyll a molecules associated with .
specific proteins
The reaction-center chlorophyll a of photosystem II is known as _________ because this pigment is best at absorbing light having a wavelength of 680 nm (in the red part of the spectrum).
P680
The chlorophyll a at the reaction-center complex of photosystem I is called .
P700 because it most effectively absorbs light of wavelength 700 nm (in the far-red part of the spectrum)
These two pigments, P680 and P700, are nearly identical chlorophyll a molecules. However, their association with different proteins in the thylakoid membrane affects the electron distribution in the two pigments and accounts for the slight differences in their
light-absorbing properties.
Light drives the synthesis of ATP and NADPH by energizing the two types of photosystems embedded in the
thylakoid membranes of chloroplasts.
The key to this energy transformation is a flow of electrons through the photosystems and other molecular components built into the thylakoid membrane. This is called
linear electron flow, And it occurs during the light reactions of photosynthesis
STUDY 10.14
In certain cases, photoexcited electrons can take an alternative path called
cyclic electron flow, which uses photosystem I but not photosystem II
STUDY FIGURE 10.15 AND 10.16
Rather than having both PSII and PSI, several of the currently existing groups of photosynthetic bacteria are known to have a
single photosystem related to either PSII or PSI.
For these species, which include the purple sulfur bacteria (see Figure 10.2e) and the green sulfur bacteria, cyclic electron flow is the one and only means of generating
ATP during the process of photosynthesis.
Evolutionary biologists hypothesize that these bacterial groups are descendants of ancestral bacteria in which photosynthesis first evolved, in a form similar to
cyclic electron flow.
Cyclic electron flow can also occur in photosynthetic species that possess both photosystems; this includes some
prokaryotes, such as the cyanobacteria shown in Figure 10.2d, as well as the eukaryotic photosynthetic species that have been tested thus far.
Although the process is probably in part an “evolutionary leftover,” research suggests it plays at least one beneficial role for these
organisms.
Plants with mutations that render them unable to carry out cyclic electron flow are capable of growing well in low light, but do not grow well where light is
intense.
This is evidence for the idea that cyclic electron flow may be
photoprotective
Chloroplasts and mitochondria generate ATP by the same basic mechanism:
chemiosmosis
An electron transport chain pumps protons (H+) across a membrane as electrons are passed through a series of carriers that are progressively more
electronegative.
Thus, electron transport chains transform redox energy to a proton-motive force, potential energy stored in the form of an H+ gradient across a
membrane.
An ATP synthase complex in the same membrane couples the diffusion of hydrogen ions down their gradient to the phosphorylation of ADP, forming
ATP.
Some of the electron carriers, including the iron-containing proteins called ______________, are very similar in chloroplasts and mitochondria.
cytochromes
there are noteworthy differences between photophosphorylation in chloroplasts and oxidative phosphorylation in mitochondria. Both work by way of chemiosmosis, but in chloroplasts, the high-energy electrons dropped down the transport chain come from water, while in mitochondria,
they are extracted from organic molecules (which are thus oxidized)
Chloroplasts do not need molecules from food to make ATP; their photosystems capture light energy and use it to drive the electrons from water to the top of the
transport chain
mitochondria use chemiosmosis to transfer chemical energy from food molecules to ATP, whereas chloroplasts use it to
transform light energy into chemical energy in ATP.
Electron transport chain proteins in the inner membrane of the mitochondrion pump protons from the mitochondrial matrix out to the intermembrane space, which then serves as a
reservoir of hydrogen ions
Similarly, electron transport chain proteins in the thylakoid membrane of the chloroplast pump protons from the stroma into the thylakoid space (interior of the thylakoid), which functions as the
H+ reservoir.
In the mitochondrion, protons diffuse down their concentration gradient from the intermembrane space through ATP synthase to the matrix, driving
ATP synthesis
In the chloroplast, ATP is synthesized as the hydrogen ions diffuse from the thylakoid space back to the stroma through
ATP synthase complexes (Figure 10.18), whose catalytic knobs are on the stroma side of the membrane.
Thus, ATP forms in the stroma, where it is used to help drive sugar synthesis during the
Calvin cycle.
The currently accepted model for the organization of the light-reaction _____________ within the thylakoid membrane is based on several research studies.
“machinery”
Electron flow pushes electrons from water, where they are at a state of low potential energy, ultimately to NADPH,
where they are stored at a state of high potential energy.
The light-driven electron flow also generates
ATP.
Thus, the equipment of the thylakoid membrane converts light energy to
chemical energy stored in ATP and NADPH.
Oxygen is produced as a
by-product.
