AP Biology
R-11
Essential Knowledge: Ch 10 Photosynthesis
10.1 Photosynthesis converts light energy to the chemical energy of food
 Autotrophs (producers) make their own organic molecules (food) from inorganic substances;
heterotrophs (consumers) obtain their organic molecules by eating other organisms
 Plants are autotrophs that make their own organic molecules via photosynthesis
 Photosynthesis converts radiant energy (sunlight) into chemical energy (glucose)
 Photosynthesis occurs in the chloroplasts
 Leaves are the major site of photosynthesis in plants
 Chlorophyll is a green pigment found in the chloroplasts; it is the chlorophyll that actually absorbs
the light energy
 Tiny pores on the underside of leaves, called stomata, are where CO2 enters and O2 exits
 The water needed for photosynthesis is absorbed by the roots of a plant and travels through veins to
reach the leaves
 Like mitochondria, chloroplasts have a double membrane
 The inner membrane encloses a compartment filled with a thick fluid called stroma
 Thylakoids are interconnected membranous sacs suspended in the stroma
 Stacks of thylakoids are called grana (singular is granum); the purpose of the grana is to increase
surface area
 The chlorophyll molecules that capture the light energy are embedded in the thylakoid membranes
 6CO2 + 6H2O + Light Energy → C6H12O6 + 6CO2
 The photosynthesis equation is the reverse of the cellular respiration equation
 In photosynthesis, electrons are added to carbon dioxide to produce sugar
 Hydrogen is moved along with the electrons so hydrogen gets transferred from water to carbon
dioxide to form sugar
 For this to happen, water must be split into hydrogen and oxygen
 O2 escapes through the stomata into the atmosphere
 The two stages of photosynthesis are called the light reactions and the Calvin Cycle
 The light reactions convert solar energy to chemical energy; they use light energy to drive the
synthesis of NADPH and ATP
 NADP+ is an electron carrier similar to the electron carrier NAD+ of cellular respiration
 The light reactions DO NOT produce sugar
 The Calvin Cycle makes sugar from carbon dioxide
 The ATP produced by the light reactions provides the energy needed for the synthesis of sugar by the
Calvin Cycle
 The NADPH produced by the light reactions provides the high energy electrons for the reduction of
carbon dioxide to glucose
10.2 The light reactions convert solar energy to the chemical energy of ATP and NADPH
 Sunlight is a type of energy called radiation or electromagnetic energy
 Electromagnetic energy travels through space as rhythmic waves
 The distance between two adjacent waves is a wavelength
 The entire range of radiation is known as the electromagnetic spectrum
 The part of the spectrum most important to life is visible light because visible light is what drives
photosynthesis
 The colors of visible light are ROY G BIV
 Red has the longest wavelength and least energy; violet has shortest wavelength and highest energy
 The amount of energy in a wavelength is inversely proportional to the wavelength: the shorter the
wavelength, the more energy and the longer the wavelength, the less energy
 The electromagnetic spectrum consists of Gamma rays (shortest wavelength and highest energy), Xrays, UV rays, Visible light, Microwaves, and Radio waves (longest wavelength and least energy)
 Photons are discrete packets of light energy; photons have a fixed quantity of energy
 Light behaves as BOTH particles and waves
 When sunlight shines on a pigmented material, certain wavelengths (colors) of visible light are
absorbed and disappear, the color we see is the color most reflected
 If all the colors of visible light are reflected, we see white; if they are all being absorbed, we see black
 Leaves appear green because violet-blue and red light are being absorbed while green is being
reflected
 Different pigments absorb light of different wavelengths, and chloroplasts contain several kinds of
pigments (chlorophyll a, chlorophyll b, and carotenoids)
 Chlorophyll a absorbs mostly blue-violet and red light and is the pigment that participates directly in
the light reactions; chlorophyll a is the main photosynthetic pigment in plants
 Chlorophyll b absorbs mostly blue and orange light; it does not participate directly in the light
reactions but it picks up energy in those wavelengths and transfers it to chlorophyll a, thus
broadening the range of light conditions under which photosynthesis can occur
 Carotenoids are a family of yellow-orange pigments that absorb mainly blue-green light. Some pass
their energy to chlorophyll a, others protect the plant by absorbing and dissipating excessive light
energy that would otherwise damage chlorophyll
 The pigment molecules (chlorophyll a, chlorophyll b, and carotenoids) are embedded in the thylakoid
membranes and are organized into photosystems
 When a pigment molecule absorbs a photon of light, one of the pigment’s electrons gains energy and
it moves from the ground state to an excited state; the excited state is very unstable
 Because the excited state is so unstable, electrons in isolated pigment molecules usually lose their
energy and immediately fall back to the ground state, releasing heat and/or light
 A photosystem is composed of a reaction center surrounded by a light harvesting complex
 The light harvesting complex is a cluster of different pigment molecules that act as a light gathering
antenna
 When a photon strikes one pigment molecule it is absorbed, and then the energy jumps from one
pigment molecule to the next until it arrives at the reaction center
 Within the reaction center of each photosystem are a pair of special chlorophyll a molecules; they
are special because not only can they accept electrons, they can also transfer them to the primary
electron acceptor
 When the energy from the light harvesting complex arrives at the reaction center, one of the
electrons in the special chlorophyll a molecules moves from the ground state to the excited state;
when it moves to the excited state, it is captured by the primary electron acceptor
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When excited electrons are captured by the primary electron acceptor, the missing electrons are
replaced by the splitting of water (electrons and hydrogen move on to generate ATP and NADPH
while oxygen is released through the stomata)
Photosystem II (water splitting photosystem), contains P680 chlorophyll a molecules which are best
at absorbing light with a wavelength of 680 nm
Photosystem I (NADPH producing photosystem), contains P700 molecules which are best at
absorbing light with a wavelength of 700 nm
Energized electrons move from the primary electron acceptor in the reaction center of PS II , down
an electron transport chain, to PS I
The energy released as electrons move down the ETC is used to pump H+ across the thylakoid
membrane to form a proton gradient
ATP synthase embedded in the thylakoid membrane generates the energy to make ATP by
chemiosmosis
PS II transfers its excited electrons to NADP+, reducing them to form NADPH; the NADPH is used in
the Calvin Cycle
THE PRODUCTS OF THE LIGHT REACTIONS ARE ATP AND NADPH; THE LIGHT REACTIONS DO NOT
PRODUCE SUGAR
10.3 The Calvin Cycle uses ATP and NADPH to convert CO2 to sugar
 The Calvin Cycle occurs in the stroma of the chloroplast
 The Calvin Cycle uses CO2 from the atmosphere, as well as ATP and NADPH from the light reactions
to generate G3P (a three carbon sugar)
 Plant cells use the G3P to make glucose and other organic molecules
 The Calvin Cycle is an anabolic pathway; it uses energy, in the form of ATP, to synthesize more
complex molecules
 Carbon molecules from the atmosphere become part of organic molecules by a process called carbon
fixation
 One spin of the Calvin Cycle fixes one carbon atom; the Calvin cycle must spin three times to make
one molecule of G3P; it takes three CO2 molecules to make one molecule of G3P
 In the first step of the Calvin cycle (carbon fixation), one atom of carbon is attached to a five carbon
sugar named RuBP; the enzyme that catalyzes this reaction is called rubisco
 Rubisco is the most abundant protein in chloroplasts and thought to be the most abundant protein
on Earth (remember enzymes are proteins)
 The six carbon sugar formed when CO2 combines with RuB is unstable, it immediately breaks in half
to form two molecules of 3-PGA
 3-PGA is the first product of the Calvin cycle
 In the second step of the Calvin cycle (reduction), enzymes use ATP and high energy NADPH electrons
to convert 3-PGA into G3P
 In the third step of the Calvin cycle (regeneration of RuBP), five molecules of G3P (3C) are rearranged
to form 3 molecules of RuBP (5C)
 To make one molecule of G3P, the Calvin cycle uses 9 ATP and 6 NADPH
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10.4 Alternative mechanisms of carbon fixation have evolved in hot, arid climates
 Plants that use CO2 from the air to fix carbon via the Calvin Cycle are called C3 plants because the first
organic product formed is 3-PGA
 C3 plants are common and widely distributed, examples of C3 plants are soybeans, oats, wheat, and
rice
 In hot, dry conditions, C3 plants close their stomata to reduce water loss, but this also keeps CO2 from
entering which decreases the rate of photosynthesis
 In most plants, carbon is initially fixed by rubisco catalyzing the reaction to combine RuBP with CO2
 One problem with rubisco is that although it has an affinity for CO2, it also has an affinity for O2
 Low levels of CO2 and high levels of O2 in a leaf with its stomata closed result in photorespiration
 Photorespiration occurs when rubisco adds O2 to the Calvin Cycle instead of CO2; the result is a two
carbon product which leaves the chloroplast, is rearranged, and released as CO2
 Photorespiration uses ATP instead of making it, and when it is occurring, the Calvin Cycle produces
no sugar
 C4 plants have special adaptations that save water without shutting down photosynthesis
 C4 plants produce a four carbon sugar (oxaloacetate) as their first product
 C4 plants include corn, grasses, and sugarcane
 C4 plants have a unique leaf anatomy; they have two types of photosynthetic cells: bundle sheath
cells and mesophyll cells
 CO2 is fixed in the mesophyll cells by an enzyme named PEP carboxylase
 PEP carboxylase has a very high affinity for CO2 and no affinity for O2; PEP carboxylase works much
more efficiently than rubisco when conditions are hot, the stomata are closed, and levels of CO2 drop
while levels of O2 rise
 When PEP carboxylase combines CO2 with the three carbon sugar PEP, a four carbon sugar is formed
 The four carbon sugar is then moved into the bundle sheath cells where the CO2 is released to be
used in the Calvin Cycle
 The mesophyll cells of a C4 plant pump CO2 into the bundle sheath, keeping the CO2 concentration in
the bundle sheath cells high enough for rubisco to bind carbon dioxide rather than oxygen
 C4 photosynthesis minimizes photorespiration and enhances sugar production in hot, dry regions
 Crassulacean Acid Metabolism (CAM) plants are adapted to very dry climates
 Examples of CAM plants include pineapple, many cacti, and succulents such as aloe and jade
 CAM plants open their stomata to admit CO2 at night
 When CO2 enters the leaves of a CAM plant, it is incorporated into four carbon organic acids which
are then stored in the plant’s vacuoles
 During the day, when the stomata are closed, the organic acids are released from the vacuoles
 When the light reactions supply the ATP and NADPH needed to run the Calvin Cycle, the organic acids
release CO2
 C4 plants physically separate carbon fixation in the mesophyll cells from the Calvin Cycle in the
bundle sheath cells; in CAM plants the two steps occur in the same space but are separated by time (
night and day)
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