CHAPTER 10
PHOTOSYNTHESIS
INTRODUCTION
PHOTOSYNTHESIS- conversion of solar
energy from the sun to chemical
energy stored in sugar and other
organic molecules
- an organism obtains the organic
compounds it needs for energy and
carbon skeletons by one of two ways:
Autotrophic nutrition/ Heterotrophic
nutrition
AUTOTROPHS
Autotrophs can sustain themselves without
eating other organisms or substances that
come from other organisms
- produce organic molecules from CO2 and
other inorganic raw materials that come
from the environment
- they are the main sources of organic
compounds for all other organisms 
called PRODUCERS of the biosphere
- plants are autotrophs 
PHOTOAUTOTROPHS- organisms that
use light as a source of energy to
synthesize organic compounds
- photosynthesis also occurs in algae and
some prokaryotes
HETEROTROPHS
Heterotrophs are unable to make their own
food, so they live on compounds produced
by other organisms
- they are the biosphere’s CONSUMERS
- most common form is when an animal eats
plants or other animals
- some consume the remains of dead
organisms DECOMPOSERS
- almost all heterotrophs are completely
dependent on photoautotrophs for food
and oxygen
CHLOROPLASTS
All green parts of a plant have chloroplasts,
but the main sites of photosynthesis are
leaves
- Color of the leaf is from CHLOROPHYLLgreen pigment found inside the
chloroplast
- light energy absorbed by chlorophyll drives
the synthesis of food molecules in the
chloroplast
SOME RELATED TERMS
- Chloroplasts are found mainly in the cells
of the MESOPHYLL- interior tissue in leaf
- CO2 enters the leaf and oxygen exits by
way of microscopic pores called STOMATA
- 2 membranes enclose the STROMA- the
fluid inside the chloroplast
- inside the inner membrane is another
membrane system called THYLAKOIDS
- in some places the thylakoid sacs are
stacked in columns called GRANA
- chlorophyll is found within the thylakoid
membranes
SUMMARY EQUATION
6 CO2 + 6 H2O + Light energy C6H12O6
+ 6 O2
The overall chemical change is the reverse
of the one that occurs in cellular
respiration
- but plants do not make food by simply
reversing respiration
SPLITTING OF WATER
It was originally thought that the oxygen
given off by plants came from CO2 and not
water
- chloroplasts split water into hydrogen
and oxygen
- original idea challenged by C.B. van Niel in
the 1930s
- studying photosynthesis in bacteria
- 20 years later van Niel’s hypothesis was
confirmed when scientists used an isotope
to trace the fate of oxygen atoms during
photosynthesis
REDOX REACTIONS
In respiration, energy is released from sugar
when electrons are transported by carriers
to oxygen
- electrons lose potential energy as they are
pulled down the chain, and mitochondria
use that energy to make ATP
Photosynthesis REVERSES the flow of
electrons
- water is split and electrons (along with
H+) are transferred from water to CO2,
reducing it to sugar
- electrons increase in potential energy
as they move from water to sugar
OVERVIEW OF LIGHT
REACTIONS AND CALVIN CYCLE
Photosynthesis actually has 2 stages- the
LIGHT REACTIONS and the CALVIN CYCLE
LIGHT REACTIONS
- steps of photosynthesis that convert
solar energy to chemical energy
- light energy that is absorbed by chlorophyll
drives a transfer of electrons and
hydrogen from water to an acceptor called
NADP+, which stores the electrons
- water is split, and O2 is given off
- NADP+ is related to NAD+ (in respiration)
- light reactions use solar power to reduce
NADP+ to NADPH by adding a pair of
electrons and an H+
- the light reactions generate ATP by the
addition of a phosphate to ADP 
PHOSPHORYLATION
- the light reactions produce no sugar
CALVIN CYCLE
- named for Melvin Calvin
- incorporates CO2 from the air into organic
molecules present in the chloroplast >
CARBON FIXATION
- then reduces fixed carbon to carbohydrate
by the addition of electrons
- reducing is provided by NADPH
- Calvin cycle makes sugar, but can only do
so with the NADPH and ATP made in the
light reactions
- sometimes called the dark reactions, or the
light-independent reactions because steps
do NOT require light directly
The light reactions take place in the
THYLAKOIDS
The dark reactions take place in the
STROMA
LIGHT
Light travels in waves of energy  WATER
WAVELENGTH- distance between crests of
electromagnetic waves
- range from very small (gamma rays) to
very large (radio waves)
ELECTROMAGNETIC SPECTRUM- entire
range of radiation
VISIBLE LIGHT- radiation that is most
important to life (380 to 750 nm)
- detected as colors
Sometimes light behaves like it is made up
of particles
- these are called PHOTONS
- act like objects in that each has a fixed
amount of energy
- this energy is inversely related to the
wavelength of the light (shorter
wavelength = greater energy of photon)
- visible light drives photosynthesis
PIGMENTS
PIGMENT- a substance that absorbs
visible light
- different pigments absorb different
wavelengths of light
- wavelengths that are absorbed
disappear
- wavelengths that are transmitted are the
ones we see
- if a pigment absorbs all colors, it appears
black
Wavelength can be measured with a
SPECTROPHOTOMETER
- directs beams of light through a solution of
a pigment and measures the fraction of
the light TRANSMITTED at each
wavelength
ABSORPTION SPECTRUM- a graph
plotting a pigment’s light absorption
http://www.biology.lsu.edu/introbio/tutorial/Spec/spectrophotometer.JPG
- blue and red light work best for
photosynthesis
- green is least effective
ACCESSORY PIGMENTS
Chlorophyll a is not the only important
pigment in chloroplasts
- only chlorophyll a can participate
directly in the light reactions
- other pigments capture light energy and
transfer it to chlorophyll a
- CHLOROPHYLL b- slightly different
structure gives it a different color:
Chlorophyll a = blue-green
Chlorophyll b = yellow-green
Other accessory pigments such as
CAROTENOIDS are various shades of
yellow and orange
- broaden the spectrum of colors used for
photosynthesis
- function in photoprotection- absorb
excess light energy that would damage
chlorophyll
“EXCITING” CHLOROPHYLL
When a pigment molecule absorbs a
photon, one of the molecule’s electrons is
elevated to a higher energy level
- the pigment molecule is then said to be
“excited”
- only photons of specific wavelengths are
absorbed
- the photons are absorbed by clusters of
pigment molecules in the thylakoid
membrane
The electron cannot remain in this excited
state for long
- it is unstable
- usually, when pigments absorb light, their
excited electrons drop to a lower energy
level, releasing energy as heat
* what makes the top of an automobile so
hot on a sunny day
PHOTOSYSTEMS
Inside the thylakoid membrane, chlorophyll
is organized along with proteins and other
organic molecules into PHOTOSYSTEMS
- has a light-gathering complex made up
of chlorophyll a, chlorophyll b, and
carotenoids
- enables the photosystem to harvest more
light over more of the spectrum
When any pigment molecule absorbs a
photon:
- energy is transferred from pigment to
pigment until it reaches a particular
chlorophyll a
- only this chlorophyll a molecule is located
in the REACTION CENTER- where the
first light-driven reaction of
photosynthesis takes place
Along with the chlorophyll a molecule in the
reaction center is the PRIMARY
ELECTRON ACCEPTOR
- In a Redox reaction, chlorophyll a in the
reaction center loses one of its electrons
to the primary electron acceptor
- this happens when light excites the
electron to a higher energy level
- the electron acceptor “catches” the excited
electron before it can return to its ground
state
Each photosystem functions in the
chloroplast as a light-harvesting unit
- the transfer of electrons from chlorophyll to
the primary electron center is the first
step of the light reactions
2 PHOTOSYSTEMS
There are 2 types of photosystems found in
the thylakoid membrane:
PHOTOSYSTEM I & PHOTOSYSTEM II
- each has a unique reaction center
- the reaction center of photosystem I is
called P700 because it most effectively
absorbs light of wavelength 700 nm
- the reaction center of photosystem II is
called P680
- the 2 pigments are identical chlorophyll a
molecules, but they are associated with
different proteins- affects light-absorbing
properties
NEXT: HOW THE 2 PHOTOSYSTEMS
WORK TOGETHER
NONCYCLIC ELECTRON
FLOW
The key to making NADPH and ATP
(needed for dark reactions) is a flow of
electrons through the photosystems and
other molecules in the thylakoid
membrane
There are 2 possible routes for electron flow:
Cyclic and Noncyclic
STEPS OF NONCYCLIC
ELECTRON FLOW
- most common route
1. Photosystem II absorbs light and an
electron moves to a higher energy level,
becoming excited
- this electron is captured by the primary
electron acceptor
- chlorophyll is oxidized; electron “hole” must
be filled
2. An enzyme extracts electrons from water
and supplies them to photosytem II,
replacing the lost electrons
- this reaction splits water into 2 H atoms
and an O atom
- the O immediately combines with another
O to form O2 (released)
3. Each excited electron passes from the
primary electron acceptor of photosystem
II to photosystem I by an electron transport
chain
- very similar to the one in cellular
respiration
4. As electrons “fall” down the chain the
energy that they release is harnessed by
the thylakoid membrane to make ATP
- called photophosphorylation (specifically
noncyclic photophosphorylation)
- this ATP will provide chemical energy to
make sugar in the dark reactions
5. When electrons reach the bottom of the
electron transport chain, they fill a “hole” in
the reaction center of photosystem I
- this hole is created when light energy
drives an electron to the primary acceptor
of photosystem I
6. The primary electron acceptor of
photosystem I gives the electrons to a
second electron transport chain
- this chain transmits electrons to ferredoxin
(Fd), an iron-containing protein
- an enzyme called NADP+ reductase
transfers electrons from Fd to NADP+
- this becomes NADPH, the other
molecule needed in the dark reactions
CYCLIC ELECTRON FLOW
In CYCLIC ELECTRON FLOW,
photosystem I is used, but not
photosystem II
- the electrons cycle back from ferredoxin
(Fd) to the cytochrome complex, and then
to the reaction center of photosystem I
- NADPH is not made, and oxygen is not
released
- ATP is still made!
- this is called CYCLIC
PHOTOPHOSPHORYLATION
Why is cyclic electron flow needed?
- Noncyclic electron flow makes equal
amounts of NADPH and ATP
- the Calvin cycle (dark reactions) use more
ATP than NADPH
- cyclic flow makes up the difference by only
producing ATP
CHEMIOSMOSIS
Chloroplasts and mitochondria both
generate ATP by chemiosmosis
- electron transport chain in a membrane
pumps protons across the membrane as
electrons fall down the chain
- potential energy is stored in the form of
an H+ gradient across a membrane
- an ATP synthase complex is also built into
the membrane
- ATP synthase couples the diffusion of H+
down their gradient to the phosphorylation
of ADP
- some of the electron carriers are very
similar in mitochondria and chloroplasts
(cytochromes)
- ATP synthase complexes are very similar
DIFFERENCES IN
CHEMIOSMOSIS
- In mitochondria, electrons are extracted
from food molecules
- the photosystems of chloroplasts use light
energy to drive electrons to the top of the
chain (do not need food)
- inner membrane of mitochondrion pumps
protons from the mitochondrial matrix out
to the intermembrane space
- it is the thylakoid membrane of the
chloroplast that pumps protons from the
stroma into the thylakoid space
- thylakoid membrane makes ATP as the H+
diffuse from the thylakoid space back to
the stroma
- ATP is made in the stroma
- NADPH is produced on the side of the
membrane facing the stroma
CALVIN CYCLE
The Calvin cycle is similar to the Krebs cycle
in that a starting material is regenerated
after molecules enter and leave the cycle
- Carbon enters the Calvin cycle in the
form of CO2 and leaves in the form of
sugar
- cycle uses ATP and NADPH
- the carbohydrate produced in the Calvin
cycle is not glucose, but a 3-carbon sugar
named GLYCERALDEHYDE-3PHOSPHATE (G3P)
- for one molecule of G3P to be made, the
Calvin cycle must take place 3 times,
fixing 3 molecules of CO2
- the Calvin cycle can be divided into 3
phases:
Phase 1: Carbon fixation
- the Calvin cycle attaches each CO2
molecule to a five-carbon sugar named
RIBULOSE BISPHOSPHATE (RuBP)
- the enzyme that catalyzes this reaction
is RUBISCO
- the product of this reaction immediately
splits to form 2 molecules of 3PHOSPHOGLYCERATE (for each CO2)
Phase 2: Reduction
- each phosphoglycerate receives a
phosphate group from ATP and becomes
1,3-bisphosphoglycerate
- next a pair of electrons from NADPH
reduces the 1,3-bisphosphoglycerate to
G3P (the sugar)
- not all of the G3P made is given off;
some must be recycled to regenerate
RuBP
Phase 3: Regeneration of RuBP
- the carbon skeletons of 5 molecules of
G3P are rearranged into 3 molecules of
RuBP
- 3 ATP are needed for this step
SUMMARY OF CALVIN CYCLE
TOTALS (for making 1 G3P):
- 9 ATP are consumed
- 6 NADPH are consumed
- the light reactions regenerate ATP and
NADPH
- the G3P made is the starting material for
metabolic pathways that make other
organic compounds
- neither the light reactions nor the Calvin
cycle can make sugar alone