General Biology (BSC 1005)
Broward College
Photosynthesis: Using Light to Make Food
I. Introduction
A. Plants, algae, and certain prokaryotes
1. convert light energy to chemical energy
2. store the chemical energy in sugar, made from
a.
b.
B. Algae farms can be used to produce
1. oils for biodiesel
2. carbohydrates to generate ethanol
II. An Overview of Photosynthesis
A. 7.1 Autotrophs are the producers of the biosphere
1. Autotrophs
a. make their own food through the process of photosynthesis
b. sustain themselves by producing organic food molecules
c. do not
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.
b.
c.
5. Photosynthesis in plants
a. takes place in chloroplasts
b. converts carbon dioxide and water into organic molecules
c.
B. 7.2 Photosynthesis occurs in chloroplasts in plant cells
1. Chloroplasts are the major sites of photosynthesis in green plants
2. Chlorophyll
a. is an important light-absorbing pigment in chloroplasts
b. is responsible for the green color of plants
c. plays a central role in converting
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.
b.
5. Veins in the leaf deliver water absorbed by roots
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General Biology (BSC 1005)
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6. Chloroplasts consist of an envelope of two membranes, which
a. enclose an inner compartment filled with a thick fluid called stroma
b. contain a system of interconnected
7. Thylakoids
a. are often concentrated in stacks called grana
b. 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
b.
C. 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
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,
3. In photosynthesis,
a. light energy is captured by chlorophyll molecules to boost the energy of electrons
b. light energy is
c. chemical energy is stored in the chemical bonds of sugars
D. 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
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
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
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General Biology (BSC 1005)
Broward College
b. The second stage is the Calvin cycle,
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
III. The Light Reactions: Converting Solar Energy to Chemical Energy
A. 7.6 Visible radiation absorbed by pigments drives the light reactions
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
3. Pigments
a. absorb light and
b.
4. Plant pigments
a. absorb some wavelengths of light and
b. 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
B. 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
b. sends the electrons into an unstable state
c. These unstable electrons
i. drop back down to their “ground state,” and as they do,
ii. release their
2. Within a thylakoid membrane, chlorophyll and other pigment molecules
a.
b. transfer the energy to other pigment molecules
3. In the thylakoid membrane, chlorophyll molecules are organized along with other
pigments and proteins into photosystems
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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
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
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
b. Photosystem I, which functions second, is called P700 because it absorbs light with a
wavelength
C. 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
b. passed from photosystem II to photosystem I
c. then accepted by NADP+,
3. Between the two photosystems, the electrons
a. move down an electron transport chain
b. provide energy for the synthesis of ATP
4. The products of the light reactions are
a.
b.
c.
D. 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
b. 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
b. the resulting concentration gradient drives H+ back through ATP synthase,
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IV. The Calvin Cycle: Reducing CO2 to Sugar
A. 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
1. The Calvin cycle makes sugar within a chloroplast
2. To produce sugar, the necessary ingredients are
a.
b.
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.
b. reduction
c. release of G3P
d. regeneration of the starting molecule ribulose bisphosphate (RuBP)
V. Photosynthesis Reviewed and Extended
A. 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
b.
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
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B. 7.13 Photosynthesis may moderate global climate change
1. The greenhouse effect operates on a global scale
a. Solar radiation includes visible light that penetrates the Earth’s atmosphere and warm’s the
planet’s surface
b. Heat radiating from the warmed planet is absorbed by gases in the atmosphere, which then
reflects some of the heat back to Earth
c. Without the warming of the greenhouse effect, the Earth would be much colder and most
life as we know it could not exist
2. The gases in the atmosphere that absorb heat radiation are called greenhouse gases. These
include
a
b.
c.
3. Increasing concentrations of greenhouse gases have been linked to global climate change
(also called global warming), a slow but steady rise in Earth’s surface temperature
4. Since 1850, the atmospheric concentration of CO2 has increased by about 40%, mostly due to
the combustion of fossil fuels including
a.
b.
c.
5. The predicted consequences of continued warming include
a. melting of polar ice
b.
c.
d. droughts
e. increased extinction rates
f. the spread of tropical disease
6. Widespread deforestation has aggravated the global warming problem by reducing an
effective CO2 sink
7. Global warming caused by increasing CO2 levels may be reduced by
a. limiting deforestation,
b. reducing fossil fuel consumption, and
c. growing biofuel crops that
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