Ch. 7 Photosynthesis: Using Light to Make Food
Introduction
A. Plants, algae, and certain prokaryotes
1. convert light energy to chemical energy and. store the chemical energy in sugar, made from
a. carbon dioxide and water.
B. Algae farms can be used to produce
1. oils for biodiesel or carbohydrates to generate ethanol.
An Overview of Photosynthesis
7.1 Autotrophs are the producers of the biosphere
1. Autotrophs
a. make their own food through the process of photosynthesis,
b. sustain themselves, and
c. do not usually consume organic molecules derived from other organisms.
2. Photoautotrophs use the energy of light to produce organic molecules.
3. Chemoautotrophs are prokaryotes that use inorganic chemicals as their energy source.
4. Heterotrophs are consumers that feed on
a. plants, animals, or decompose organic material.
5. Photosynthesis in plants
a. takes place in chloroplasts, converts carbon dioxide and water into organic molecules, and
c. releases oxygen.
7.2 Photosynthesis occurs in chloroplasts in plant cells
Leaf Cross-section
Chloroplast from Mesophyll
1. Chloroplasts are the major sites of photosynthesis in green plants.
2. Chlorophyll
1
a. is an important light-absorbing pigment in chloroplasts,
b. is responsible for the green color of plants, and
c. plays a central role in converting solar energy to chemical energy.
3. Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the
interior of the leaf.
4. Stomata are tiny pores in the leaf that allow
a. carbon dioxide to enter and oxygen to exit.
5. Veins in the leaf deliver water absorbed by roots.
6. Chloroplasts consist of an envelope of two membranes, which
a. enclose an inner compartment filled with a thick fluid called stroma and
b. contain a system of interconnected membranous sacs called thylakoids.
7. Thylakoids
a. are often concentrated in stacks called grana and have an internal compartment called the
thylakoid space, which has functions analogous to the intermembrane space of a mitochondrion.
c. Thylakoid membranes also house much of the machinery that converts light energy to chemical
energy.
8. Chlorophyll molecules
a. are built into the thylakoid membrane and
b. capture light energy.
*7.3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopes
1. Scientists have known since the 1800s that plants produce O2. But does this oxygen come from carbon dioxide or water?
a. For many years, it was assumed that oxygen was extracted from CO2 taken into the plant.
b. However, later research using a heavy isotope of oxygen, 18O, confirmed that oxygen produced
by photosynthesis comes from H2O.
7.4 Photosynthesis is a redox process, as is cellular respiration
1. Photosynthesis, like respiration, is a redox (oxidation-reduction) process.
a. CO2 becomes reduced to sugar as electrons along with hydrogen ions from water are added to it.
b. Water molecules are oxidized when they lose electrons along with hydrogen ions.
Reduction-oxidation process
Photosynthesis
Cellular Respiration
2. Cellular respiration uses redox reactions to harvest the chemical energy stored in a
glucose molecule.
a. This is accomplished by oxidizing the sugar and reducing O2 to H2O.
b. The electrons lose potential as they travel down the electron transport chain to O2.
c. In contrast, the food-producing redox reactions of photosynthesis require energy.
3. In photosynthesis,
a. light energy is captured by chlorophyll molecules to boost the energy of electrons,
b. light energy is converted to chemical energy, and chemical energy is stored in the chemical
bonds of sugars.
2
7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH
1. Photosynthesis occurs in two metabolic stages.
a. The light reactions occur in the thylakoid membranes. In these reactions
i. water is split, providing a source of electrons and giving off oxygen as a by-product,
ii. ATP is generated from ADP and a phosphate group, and
iii. light energy is absorbed by the chlorophyll molecules to drive the transfer of electrons and H+
from water to the electron acceptor NADP+, reducing it to NADPH.
iv. NADPH produced by the light reactions provides the electrons for reducing carbon in the
Calvin cycle.
b. The second stage is the Calvin cycle, which occurs in the stroma of the chloroplast.
i. The Calvin cycle is a cyclic series of reactions that assembles sugar molecules using CO2 and
the energy-rich products of the light reactions.
ii. During the Calvin cycle, CO2 is incorporated into organic compounds in a process called
carbon fixation.
iii. After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds.
iv. The Calvin cycle is often called the dark reactions or light-independent reactions, because
none of the steps requires light directly.
The Light Reactions: Converting Solar Energy to Chemical Energy
7.6 Visible radiation absorbed by pigments drives the light reactions
3
1. Sunlight contains energy called electromagnetic energy or electromagnetic radiation.
a. Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths.
b. Electromagnetic energy travels in waves, and the wavelength is the distance
between the crests of two adjacent waves.
2. Light behaves as discrete packets of energy called photons.
a. A photon is a fixed quantity of light energy.
b. The shorter the wavelength, the greater the energy.
3. Pigments
a. absorb light and are built into the thylakoid membrane.
4. Plant pigments
a. absorb some wavelengths of light and reflect or transmit other wavelengths.
5. We see the color of the wavelengths that are transmitted. For example, chlorophyll transmits green
wavelengths.
6. Chloroplasts contain several different pigments, which absorb light of different
wavelengths.
a. Chlorophyll a absorbs blue-violet and red light and reflects green.
b. Chlorophyll b absorbs blue and orange and reflects yellow-green.
c. Carotenoids
i. broaden the spectrum of colors that can drive photosynthesis and
ii. provide photoprotection, absorbing and dissipating excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules.
7.7 Photosystems capture solar energy
1. Pigments in chloroplasts absorb photons (capturing solar power), which
a. increases the potential energy of the pigment’s electrons and sends the electrons into an unstable
state.
c. These unstable electrons
i. drop back down to their “ground state,” and as they do, release their excess energy as heat.
2. Within a thylakoid membrane, chlorophyll and other pigment molecules
a. absorb photons and transfer the energy to other pigment molecules.
3. In the thylakoid membrane, chlorophyll molecules are organized along with other
pigments and proteins into photosystems.
4
4. A photosystem consists of a number of light-harvesting complexes surrounding a
reaction-center complex.
5. A light-harvesting complex contains various pigment molecules bound to proteins.
6. Collectively, the light-harvesting complexes function as a light-gathering antenna.
7. The light energy is passed from molecule to molecule within the photosystem.
a. Finally, it reaches the reaction center where a primary electron acceptor accepts these electrons
and consequently becomes reduced.
b. This solar-powered transfer of an electron from the reaction-center pigment to the primary electron acceptor is the first step in the transformation of light energy to chemical energy in the light
reactions.
8. Two types of photosystems (photosystem I and photosystem II) cooperate in the light reactions.
9. Each type of photosystem has a characteristic reaction center.
a. Photosystem II, which functions first, is called P680 because its pigment absorbs light with a
wavelength of 680 nm.
b. Photosystem I, which functions second, is called P700 because it absorbs light with a wavelength
of 700 nm.
7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH
1. In the light reactions, light energy is transformed into the chemical energy of ATP and NADPH.
2. To accomplish this, electrons are
a. removed from water, passed from photosystem II to photosystem I, and
b. accepted by NADP+, reducing it to NADPH.
3. Between the two photosystems, the electrons
a. move down an electron transport chain and provide energy for the synthesis of ATP.
4. The products of the light reactions are
a. NADPH, ATP, and oxygen.
7.9 Chemiosmosis powers ATP synthesis in the light reactions
1. Interestingly, chemiosmosis is the mechanism that
a. is involved in oxidative phosphorylation in mitochondria and generates ATP in chloroplasts.
2. ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions across a membrane.
3. In photophosphorylation, using the initial energy input from light,
a. the electron transport chain pumps H+ into the thylakoid space, and
5
b. the resulting concentration gradient drives H+ back through ATP synthase, producing ATP.
4. How does photophosphorylation compare with oxidative phosphorylation?
a. Mitochondria use oxidative phosphorylation to transfer chemical energy from food into the
chemical energy of ATP.
b. Chloroplasts use photophosphorylation to transfer light energy into the chemical
energy of ATP.
IV. The Calvin Cycle: Reducing CO2 to Sugar
7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
6
1. The Calvin cycle makes sugar within a chloroplast.
2. To produce sugar, the necessary ingredients are
a. atmospheric CO2 and ATP and NADPH generated by the light reactions.
3. The Calvin cycle uses these three ingredients to produce an energy-rich, three-carbon sugar called
glyceraldehyde-3-phosphate (G3P).
4. A plant cell may then use G3P to make glucose and other organic molecules.
5. The steps of the Calvin cycle include
a. carbon fixation,
b. reduction,
c. release of G3P, and
d. regeneration of the starting molecule ribulose bisphosphate (RuBP).
Photosynthesis Reviewed and Extended
7.12 Review: Photosynthesis uses light energy, carbon dioxide, and water to make
organic molecules
1. Most of the living world depends on the food-making machinery of photosynthesis.
2. The chloroplast
a. integrates the two stages of photosynthesis and makes sugar from CO2.
3. About half of the carbohydrates made by photosynthesis are consumed as fuel for cellular respiration in the mitochondria of plant cells.
4. Sugars also serve as the starting material for making other organic molecules, such as proteins, lipids, and cellulose.
5. Excess food made by plants is stockpiled as starch in roots, tubers, seeds, and fruits.
7
8
Word Parts
auto- = self;
-troph = food
chloro- = green;
-phyll = leaf
electro- = electricity;
magnet- = magnetic
hetero- = other
meso- = middle
photo- = light
Vocabulary
1. autotroph- An organism that makes its own food (often by photosynthesis), thereby sustaining itself
without eating other organisms or their molecules. Plants, algae, and numerous bacteria are autotrophs.
2. wavelength- The distance between crests of adjacent waves, such as those of the electromagnetic
spectrum.
3. stroma- The dense fluid within the chloroplast that surrounds the thylakoid membrane and is involved in the synthesis of organic molecules from carbon dioxide and water. Sugars are made in the
stroma by the enzymes of the Calvin cycle.
4. carbon fixation- The incorporation of carbon from atmospheric CO2 into the carbon in organic
compounds. During photosynthesis in a C3 plant, carbon is fixed into a three-carbon sugar as it enters
the Calvin cycle. In C4and CAM plants, carbon is fixed into a four-carbon sugar.
5. photosynthesis- The process by which plants, autotrophic protists, and some bacteria use light energy to make sugars and other organic food molecules from carbon dioxide and water.
6 stoma- A pore surrounded by guard cells in the epidermis of a leaf. When stomata are open, CO2 enters a leaf, and water and O2 exit. A plant conserves water when its stomata are closed.
7. photophosphorylation- The production of ATP by chemiosmosis during the light reactions of photosynthesis.
8. photoautotroph- An organism that obtains energy from sunlight and carbon from CO2 by photosynthesis.
9. light reactions- The first of two stages in photosynthesis; the steps in which solar energy is absorbed
and converted to chemical energy in the form of ATP and NADPH. The light reactions power the sugar- producing Calvin cycle but produce no sugar themselves.
10. granum- A stack of membrane-bounded thylakoids in a chloroplast. Grana are the sites where light
energy is trapped by chlorophyll and converted to chemical energy during the light reactions of photosynthesis.
11. mesophyll- The green tissue in the interior of a leaf; a leaf's ground tissue system; the main site of
photosynthesis.
12. photorespiration- In a plant cell, a metabolic pathway that consumes oxygen, releases CO2 ,and
decreases photosynthetic output. Photorespiration generally occurs on hot, dry days, when stomata
close, O2 accumulates in the leaf, and rubisco fixes O2 rather than CO2 . Photorespiration produces no
sugar molecules or ATP.
13. electromagnetic spectrum- The entire spectrum of radiation ranging in wavelength from less than
a nanometer to more than a kilometer.
9
14. producer- An organism that makes organic food molecules from CO2, H2O, and other inorganic
raw materials: a plant, alga, or autotrophic prokaryote.
15. chlorophyll- A green pigment located within the chloroplasts of plants, algae, and certain prokaryotes. Chlorophyll a can participate directly in the light reactions, which convert solar energy to chemical energy.
16. thylakoid- A flattened membranous sac inside a chloroplast. Thylakoid membranes contain chlorophyll and the molecular complexes of the light reactions of photosynthesis. A stack of thylakoids is
called a granum.
17. photosystem- A light-capturing unit of a chloroplast's thylakoid membrane, consisting of a reaction-center complex surrounded by numerous light-harvesting complexes.
18. Calvin cycle/reactions- The second of two stages of photosynthesis; a cyclic series of chemical
reactions that occur in the stroma of a chloroplast, using the carbon in CO2and the ATP and NADPH
produced by the light reactions to make the energy-rich sugar molecule G3P.
19. photon- A fixed quantity of light energy. The shorter the wavelength of light, the greater the energy
of a photon.
10