AP Biology
Ch. 10
Photosynthesis

Photosynthesis
 Transforms solar energy trapped by
chloroplasts into chemical bond energy stored
in sugar and other organic molecules.
 Directly or indirectly supplies energy to most
living organisms
 Uses energy-poor molecules, CO2 and H2O to
make energy-rich molecules such as glucose

Autotrophs (Producers)
 Organisms get organic molecules used for
energy by one of two ways: autotrophic
nutrition or heterotrophic nutrition.
 Autotrophs synthesize organic molecules
from inorganic raw materials.
 Photoautotrophs use light as an energy
source to make food. Ex) plants, algae, and
some prokaryotes
 Chemoautotrophs use the oxidation of
inorganic chemicals, such as sulfur or
ammonia, to make organic molecules. Ex)
some bacteria living on the ocean floor near
volcanic vents.


Purple Sulfur Bacteria
Heterotrophs (Consumers)
 Nutrition is acquired from organic
molecules produced by other organisms
 Examples are animals that eat plants
(herbivores), other animals (carnivores),
or both (omnivores).
 Some heterotrophs decompose and feed
on organic litter (detritivores)
Chloroplasts
 Site of photosynthesis in plants, mainly in the
leaves
 Chlorophyll is the green pigment in
chloroplasts that gives a leaf its color, and that
absorbs light energy used to drive
photosynthesis.
 Chloroplasts are concentrated in cells of
mesophyll, green tissue in the leaf’s interior.
 Carbon dioxide enters and leaves the leaf
through stomata
 Water absorbed by roots is transported to the
leaves through vascular bundles.

Chloroplasts: structure
 Divided into three functional compartments by
a system of membranes:
 1) Intermembrane space- separates the two
layers of the double membrane surrounding
the chloroplast.
 2) Thylakoid space- thylakoids are stacks of
membranes within the chloroplast, contain
chlorophyll, location of light reactions
 3) Stroma- viscous fluid outside the
thylakoids, location of the Calvin cycle
(converts CO2 to sugar)
Tracing atoms through photosynthesis

Overview of Photosynthesis: cooperation of light reactions and Calvin cycle
Pathways of Photosynthesis:
Light reactions
 Convert light energy to chemical bond energy
in ATP and NADPH (NADP+ is reduced).
 Occurs in the thylakoid membranes of
chloroplasts
 Oxygen given off as by-product from the
splitting of water. H+ passed on to the Calvin
cycle, joining CO2 to from sugar.
 ATP is generated by photophosphorylation.
Properties of Light
 Sunlight is electromagnetic energy,
having wavelike and particle-like
properties.
 The visible range of light is the radiation
that drives photosynthesis.
 Blue and red, the two wavelengths most
effectively absorbed by chlorophyll, are
the colors most useful as energy for
photosynthesis.

Photosynthetic Pigments:
Light receptors
 Light may be reflected, transmitted, or
absorbed when is contacts matter.
 Pigments are substances that absorb
different wavelengths of light:
wavelengths absorbed disappear, while
the color seen is reflected.
 For example, leaves look green because
chlorophyll absorbs red and blue light,
but transmits and reflects green light.

Absorption vs. Action spectrum
 Each pigment has a characteristic absorption
spectrum, or pattern of wavelengths that it
absorbs. It is expressed as a graph of
absorption vs. wavelength.
 Absorption spectrums for a pigment are
determined using a spectrophotometer.
 Action spectrums profile the relative
effectiveness of different wavelengths of visible
light for photosynthesis. Graph is wavelength
vs. rate of photosynthesis.

Pigments of Photosynthesis
 Chlorophyll a- participates directly in the
light reactions, accessory pigments can
absorb and transfer energy to chlorophyll a
 Accessory pigments- include chlorophyll b
(yellow-green pigment), and carotenoids
(yellow and orange pigments)
 Accessory pigments expand the range of
wavelengths of light available for
photosynthesis

Photoexcitation of Chlorophyll
 When pigments absorb photons, the
absorbed photons boost one of the
pigment molecule’s electrons in its lowest
energy state (ground state) to an orbital
of higher potential energy (excited state)
 The excited state is unstable, so excited
electrons fall back to the ground state,
releasing heat or by fluorescing.
Photoexcitation of chlorophyll,
cont.
 Pigment molecules in the thylakoid
membrane do not fluoresce because the
primary electron acceptor molecules trap
excited state electrons that have
absorbed photons.
 Isolated chlorophyll fluoresces in the red
part of the spectrum and dissipates heat.

Photosystems
 Chlorophyll in the chloroplast is organized with
proteins into photosystems.
 A photosystem has a light-gathering “antenna
complex” consisting of chlorophyll a,
chlorophyll b, and carotenoid molecules.
 When light energy hits the chloroplast, energy
is transferred to chlorophyll a in the reaction
center.
 The solar-powered transfer of electrons
from chlorophyll a to the primary electron
acceptor is the first step of the light
reactions. It is a redox reaction.

Photosystems I and II
 The thylakoid membrane has two types
of photosystems that cooperate in the
reactions of photosynthesis.
 Photosystem II occurs first, and has
chlorophyll P680 (absorbs at 680 nm
wavelengths of light, red).
 Photosystem I has chlorophyll P700,
which absorbs best at 700 nm (far red
part of the spectrum)
Noncyclic electron flow
 Light drives the synthesis of NADPH and
ATP by energizing the two photosystems
in the thylakoid membranes of
chloroplasts.
 During photosynthesis, there are two
possible routes for electron flow: cyclic
and non-cyclic electron flow (non-cyclic is
most common)
Noncyclic electron flow, cont.
 Photosystem II absorbs light, excites eto a higher level, chlorophyll becomes
oxidized
 Water is split; oxygen is released
 Each photoexcited electron passes from
photosystem II to photosystem I by an
electron transport chain. Electron
carriers are plastoquinone (Pq) and a
cytochrome complex.
 As electrons move down the chain, ATP
is formed by noncyclic
photophosphorylation.
 Primary electron acceptor of
Photosystem I passes photoexcited
electrons to a second electron transport
chain, producing NADPH.
 ATP and NADPH will be used in the
Calvin cycle to make sugar.
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Cyclic electron flow
 Under certain conditions, photoexcited
electrons take an alternate path called cyclic
electron flow.
 Cyclic electron flow uses Photosystem I, but
not Photosystem II.
 ATP is produced by cyclic
photophosphorylation, but no oxygen or
NADPH.
 Noncyclic electron flow produces ATP and
NADPH in equal amounts, but the Calvin cyclic
requires more ATP.
 Cyclic electron flow makes up the difference.

Chemiosmosis: Chloroplasts vs.
Mitochondria (similarities)
 Chloroplasts and mitochondria both
generate ATP by chemiosmosis.
 Both have an electron transport chain
assembled in a membrane
 Both have an ATP synthase complex built
into the same membrane that couples the
diffusion of H+ down their gradient to the
phosphorylation of ADP.
 The ATP synthase complexes of both
organelles are similar.
Chemiosmosis: Chloroplast vs.
Mitochondria (differences)
 Mitochondria transfer chemical energy from food
molecules to ATP, while chloroplasts transform light
energy into chemical energy.
 Spatial organization of chemiosmosis differs in
mitochondria and chloroplasts:
 The inner membrane of the mitochondrion pumps
protons from the mitochondrial matrix out to the
intermembrane space, which is a reservoir of protons
that power ATP synthase.
 The thylakoid membrane of the chloroplast pumps
protons from the stroma into the thylakoid space. ATP
is produced as protons diffuse from the thylakoid
compartment back to the stroma through ATP
synthase. ATP forms in the stroma where it drives
sugar synthesis during the Calvin cycle.

Summary of Light Reactions
 Noncyclic electron flow pushes electrons from
water ( low P.E.) to NADPH (high P.E.)
 Water is split by photosystem II on the side of
the membrane facing the thylakoid space.
 The diffusion of H+ from the thylakoid space to
the stroma (along the H+ gradient) powers ATP
synthase.
 Light-driven reactions across the membrane
store chemical energy in NADPH and ATP,
which shuttle energy to the Calvin cycle.

Melvin Calvin
Calvin cycle
 Carbon enters the Calvin cycle in the form of
CO2 and leaves in the form of sugar.
 It consumes ATP as an energy source, and
NADPH as the reducing agent to add high
energy electrons to form sugar.
 The Calvin cycle produces a 3-C sugar,
glyceraldehyde-3-phosphate (G3P). The cycle
must take place 3 times, fixing three molecules
of CO2, to make one molecule of G3P.
Calvin cycle: Phase I
Carbon fixation
 Each CO2 is incorporated by attaching to
a 5-C sugar, ribulose bisphosphate
(RuBP). Rubisco is the enzyme that
catalyzes this step.
 A 6-C intermediate forms that
immediately splits into two molecules of
3-phosphoglycerate.

Calvin cycle: Phase 2
Reduction
 Endergonic reduction is a 2-step process that
couples ATP hydrolysis with the reduction of 3phosphoglycerate to glyceraldehyde phosphate
(G3P).
 For every three CO2 molecules that enter the
Calvin cycle, six G3P molecules are produced,
only one of which can be counted as net gain.
 The other five G3P molecules are recycled to
regenerate molecules of RuBP.

Calvin cycle: Phase III
Regeneration of CO2 acceptor (RuBP)
 A complex series of reactions rearranges
the carbon skeletons of five G3P
molecules into three RuBP molecules.
 3 ATP molecules are used, RuBP is now
prepared to receive CO2 again, and the
cycle continues.

Energy totals
 To make one G3P molecule, the Calvin cycle
consumes 9 ATPs and 6 molecules of NADPH.
 G3P is the starting material for the synthesis of
glucose and other carbohydrates.
 The Calvin cycle uses 18 ATPs and 12
molecules of NADPH to produce one molecule
of glucose.
 The light reactions and Calvin cycle are both
required to make sugar from CO2.
Carbon fixation: Alternative
methods in hot, dry climates
 In most plants, the fixation of carbon via
rubisco results in a 3-carbon compound.
These are called C3 plants. Examples are
rice, wheat, and soybeans.
 On hot, dry days, these plants close their
stomata to conserve water. CO2 coming into
the plant decreases.
 Rubisco can also bind oxygen, in place of
CO2. O2 enters the Calvin cycle. This is
photorespiration.
Photorespiration
 This process is called photorespiration
because it occurs in light (photo), and uses
oxygen (respiration).
 However, photorespiration does not generate
ATP or make glucose.
 Photorespiration decreases photosynthetic
output by removing organic material from the
Calvin cycle.
 Perhaps photorespiration is an evolutionary
“relic” from a time when there was more CO2 in
the atmosphere than O2, and rubisco did not
have to differentiate between them.
C4 Plants
 C4 plants have adapted to hot, dry climates by
producing a 4-carbon compound as its first
product.
 Sugar cane and corn are examples of C4
plants.
 There two types of photosynthetic cells:
Bundle-sheath cells and mesophyll cells.
 The Calvin cycle is confined to chloroplasts of
the bundle-sheath cells, but CO2 can be
incorporated into organic compounds in the
mesophyll cell to be used later in the Calvin
cycle.
C4 plants, cont.
 CO2 is added to phosphoenolpyruvate (PEP)
by the enzyme PEP carboxylase.
 Compared to rubisco, PEP carboxylase has a
higher affinity for CO2 and can fix it under
conditions (hot and dry) when rubisco cannot.
 The four carbon products are then exported to
the bundle-sheath cells through
plasmodesmata.
 C4 photosynthesis minimizes photorespiration,
and enhances sugar production.

CAM plants
 A second adaptation to arid conditions in
succulent (water-storing) plants is
crassulacean acid metabolism (CAM).
 Cacti and pineapples are examples of CAM
plants.
 The stomata are closed during the day, but
open at night to collect CO2.
 Carbon fixation into organic acids occurs at
night. These molecules are stored in the
mesophyll cell.
 The Calvin cycle operates during the day when
the light reactions can provide ATP and
NADPH to make sugar from the stored CO2.


Review of Photosynthesis
 Light reactions convert light energy to the
chemical energy of ATP and NADPH.
 Pigments and protein molecules that
carry out the light reactions are found in
the thylakoid membranes and include two
photosystems and electron transport
chains.
 The light reactions split water and
release oxygen to the atmosphere.
Review of photosynthesis, cont.
 The Calvin cycle takes place in the
stroma of the chloroplast, and uses ATP
and NADPH to convert CO2 to
carbohydrates (glucose).
 The direct product of the Calvin cycle is
G3P. Enzymes in the chloroplast convert
this molecule into a diverse group of
other organic compounds.
 The Calvin cycle returns ADP and
NADP+ to the light reactions.