Chapter 7 – Using Light to Make Food
7.1 Autotrophs are the producers of the biosphere
 Plants are autotrophs (“self-feeders” in Greek) and make food to sustain themselves. They are also
considered the producers for the environment because they make the food supply for all other life. All
organisms that produce organic molecules using sunlight are called photoautotrophs.
 Through the process of photosynthesis, plants convert CO2 into H2O and release organic materials for
almost all other organisms. Producers feed the consumers, or heterotrophs. Plants are not the only
autotrophs. Some algae, some prokaryotes, and some protists can also perform photosynthesis.
 Photosynthesis occurs in the chloroplasts, where light energy is used to synthesize organic compounds.
Photosynthetic pigments, enzymes, and other molecules are in the membranes and let the process carry
on efficiently.
7.2 Photosynthesis occurs in chloroplasts in plant cells
 All green parts of the plant can carry out photosynthesis. The most chloroplasts are contained in the
leaves (about half a million per square millimeter). In it is the green pigment, chlorophyll that is designed
to absorb light. Chloroplasts are concentrated in the mesophyll, which is the green tissue in the interior of
the leaf. Carbon dioxide enters and oxygen leaves through tiny pores on the surface of the leaves called
stomata.
 Water is absorbed into the leaves from the roots and travels through veins in the plant tissues. Inside the
chloroplast is a thick fluid called stroma. Suspended in the stroma is a network of membrane bound sacs
called thylakoids. These sacs stack into grana (stacks of thylakoids). Built into the thylakoids is
chlorophyll, and the thylakoids will convert the light energy.
7.3 Scientists traced the process of photosynthesis using isotopes
The basic equation of photosynthesis was worked out in the 1800s and it was assumed that the oxygen
produced was from CO2 gas. In the 1950s, this hypothesis was tested and it was shown that the oxygen
produced actually came from the water that entered the reaction, not the CO2.
7.4 Photosynthesis is a redox process, as is cellular respiration
When carbon dioxide is reduced to sugar, electrons and hydrogen are added to it (reduction). When water
molecules are split apart and yield O2, they lose electrons (oxidation) and hydrogen ions. Redox reactions of
photosynthesis require energy. The light energy captured by chlorophyll molecules in the chloroplasts
provides an energy boost. Photosynthesis coverts light energy to chemical energy and stores it in the
chemical bonds of sugar for later use.
7.5 The two stages of photosynthesis are linked by ATP and NADPH
The equation for photosynthesis is a simple summary of a complex process
6CO2 + 6H2O (light)  C6H12O6 + 6O2 + 6H2O
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The photosynthesis process occurs in two stages with multiple steps each. The light reactions convert
light energy to chemical energy and release O2. The light reactions occur in the thylakoids. Water is split,
providing electrons and giving off oxygen gas. Light energy drives the transfer of electrons to produce
NADPH.
The Calvin Cycle occurs in the stroma and is a series of reactions that assembles sugars using CO2 and the
products of the light reactions. The incorporation of carbon from CO2 is called carbon fixation. After
fixation, enzymes make sugars by reducing carbon compounds.
NADPH from the light reactions provides the electrons for reducing carbon in the Calvin Cycle. ATP from
the light reactions provides the chemical energy to power the Calvin Cycle. Calvin Cycle is called the light
independent reactions because they do not need light directly to occur, though oftentimes, the reactions
do occur during the day.
7.6 Visible radiation absorbed by pigments drives the light reactions
 Sunlight is electromagnetic energy or radiation. This type of energy travels in rhythmic waves. Visible
light is a small portion of the entire electromagnetic spectrum, which ranges from gamma rays to radio
waves. Each wave is measured in a wavelength (from crest to crest on adjacent waves). Shorter
wavelengths have more energy than longer ones.
 Light can also behave as a discrete package of energy called a photon. A photon has a fixed quantity of
energy, and at higher wavelengths, the photon will have greater energy.
 Photosynthetic Pigments: Light absorbing molecules called pigments are found in the thylakoid. They
absorb some wavelengths of light while reflecting others. We cannot see the ones that have been
absorbed, but we perceive the ones that have been reflected. Different pigments absorb different
wavelengths:
Chlorophyll a participates directly in the light reactions. It absorbs red and blue-violet light.
Chlorophyll b broadens the range of light that a plant can use by passing absorbed energy to chlorophyll
a, which puts it to work in the light reactions. It absorbs orange and blue light.
Carotenoids are various shades of yellow and orange. They absorb green-blue and yellow green. They
may broaden the spectrum of light that drives photosynthesis, but also seem to function in photoprotection.
They absorb and dissipate excessive light that could damage the chlorophyll or interact with oxygen.
7.7 Photosystems capture solar energy
 Pigments absorb a photon and jump to an energy level farther from the nucleus. In this state, the
electrons have more potential energy and are considered to be in an “excited” state. This state is unstable
and will release energy causing them to drop back down to their ground state. In the thylakoid,
chlorophyll passes off excited electrons to another molecule before the electrons can drop back to ground
state.
 In the thylakoid, the chlorophyll is organized into photosystems, which have light harvesting complexes
surrounding a reaction center complex. The complexes contain pigments (chlorophylls a and b,
carotenoids) that absorb photons and pass the energy from a primary electron acceptor and down the
electron transport chain.
 Pigments absorb photons and pass the energy until it reaches a reaction center. The reaction-center
complex contains special chlorophyll a molecules and the primary electron acceptor, which can take the
electron and become reduced. The solar-powered transfer of an electron from the reaction center to the
primary electron acceptor is the first step in transforming light energy.
 Two photosystems exist and work together in the light dependent reactions. They are termed
Photosystem I and II for their order of discovery, yet not for the order in which they occur. Each
photosystem has a characteristic reaction center complex. In PSII, it is called P680 because the light
absorbs best at a red wavelength of 680nm. In PSI, it is called P700 because the light absorbs best at a far
red wavelength of 700nm.
7.8 Two Photosystems connected by an electron transport chain generate ATP and NADPH
 Light is transformed into ATP and NADPH. Electrons removed from water pass from Photosystem II to
Photosystem I to NADP+. Between the two systems, the electrons pass down an electron transport chain
to provide energy to make ATP.
 How does this happen?
1. A pigment molecule in a light harvesting complex absorbs a photon of light. Energy passes from one
pigment molecules and finally to the reaction center of Photosystem II where it excites an electron of
chlorophyll P680 to a higher energy state.
2. The electron is captured by the primary electron acceptor
3. Water is split and the electrons are supplied one by one to P680, replacing an electron lost to the
primary electron acceptor. Oxygen combines with an oxygen from the split water to release O2
4. Each electron passes from PSII to PS I via the electron transport chain. The movement of electrons
provides energy for the making of ATP by pumping H+ across the membrane
5. Light excites an electron in the chlorophyll of P700 in the reaction center of PSI, where the primary
electron acceptor captures an electron from PSII to replace the one lost in P700
6. Photoexcited electrons of PS I are passed through the chain to reduce NADH+ to make NADPH
 Products of the photosystems include NADPH, ATP, and O2
7.9 Chemiosmosis powers ATP synthesis in the light reactions
Photoexcited electrons are passed down the electron transport chain connecting the two photosystems.
Hydrogen ions are pumped across the membrane from the stroma into the thylakoid space, making a
concentration gradient across the membrane. The energy of the gradient drives H+ to phosphorylate ADP into
ATP.
7.10 ATP and NADPH power sugar synthesis in the Calvin Cycle
Using carbon from CO2, ATP, and NADPH, the Calvin cycle constructs a three carbon sugar, glyceraldehydes
three phosphate (G3P). A plant uses G3P to make glucose and other organic molecules as needed.
1. The enzyme rubisco attaches CO2 to RuBP (carbon fixation)
2. NADPH reduces the organic acid 3-PGA to G3P using ATP (reduction). To make the G3P, three CO2 are
used
3. For every three CO2, one G3P is made and leaves the cycle. The remaining G3P are rearranged
4. Using ATP to regenerate RuBP from the excess G3P
 The Calvin Cycle consumes 9 ATP and 6 NADPH to run
7.11Other methods of carbon fixation have evolved in hot, dry climates
 The first step of the Calvin Cycle is carbon fixation, with most plants using CO 2 directly from the air and
attaching to RuBP. These plants are called C3 plants because their first product is a 3 Carbon compound
(3-PGA).
 While these plants are widely distributed, they don’t do well in hot, dry climates because they close their
stomata to prevent water loss (which also prevents gas entry or exit). As a result, CO2 levels drop and
photosynthesis slows. Oxygen (O2) also begins to accumulate, making other problems for the plant.
 Photorespiration: As O2 levels build up, rubisco attaches the oxygen to RuBP instead of CO2. This two
carbon product is then broken down in the cell. It consumes O2 and releases CO2, and it uses ATP instead
of making it. It is beneficial though because it will drain away as much as 50% of the carbon fixed by the
Calvin Cycle.
 C4 Plants: In plants of hot, dry climates, carbon fixation has evolved to minimize photorespiration and
maximize the Calvin Cycle. These plants first fix CO2 into a 4-carbon compound. The stomata are still kept
closed, but sugars are still made photosynthetically. The 4-carbon compound moves to the bundle sheath
cells (packed around the veins of the leaf) to release CO2. The CO2 concentration stays high enough for
the Calvin Cycle, but low enough to avoid photorespiration.
 CAM Plants: In many succulent (water storing) plants, another method has evolved. CAM plants
conserve water by only opening their stomata at night. So the CO2 is fixed into a 4-carbon compound that
is stored at night and releases during the day to participate in the Calvin Cycle.
7.12Photosynthesis uses light energy, carbon dioxide and water to make organic molecules
About 50% of the carbohydrates made by the plant are consumed in the mitochondria of plant cells. Plants
use it to feed themselves, but also serve as the ultimate food source for all other organisms.
7.13 Photosynthesis may moderate global climate change
 The greenhouse effect is a natural heating effect that keeps the planet’s surface warm enough to sustain
life. However, human activities do add more gases to the atmosphere, thus increasing this effect. CO2 is a
greenhouse gas, but it is a waste product of cellular respiration and the raw material for photosynthesis.
 Increasing concentrations of greenhouse gases may contribute to global climate change. Photosynthesis
does take some CO2 out of the atmosphere, but alone, it is not enough to offset the imbalance. New
technologies that release less greenhouse gases are being explored.
7.14 Scientific study of Earth’s ozone layer has global significance
The ozone layer shields Earth from UV radiation. CFCs found in aerosols, refrigerators, and Styrofoam have
been linked to destruction of the ozone layer. In 1987, it was agreed that countries would phase out CFCs.
Current CFC use is near 0, but it may take until 2060 for the ozone layer to stabilize.