Chapter 7 – Photosynthesis: Using light to make food
An overview of photosynthesis
I. Autotrophs are the producers of the biosphere
A. Grouping organisms into nutritional groups…2
questions:
1. Carbon source? – Where do you get your carbon for
growth and development (raw materials to build)?
a) heterotroph – use organic substrates as carbon
source to make other organic bulding blocks (aka:
consumer)
b) autotroph – use inorganic CO2 as carbon source to
make organic building blocks (aka: producer)
2. Energy source? – Where does the energy come from to
produce ATP?
a) phototrophic – uses light as energy source to make
ATP
b) chemotrophic – uses chemical compounds as
energy source to make ATP
3. Classify a bacteria that uses H2S as a source of energy
to make ATP, and uses CO2 as a carbon source to build
organic molecules? …chemoautotrophs
a) certain bacteria and archaea
4. How about humans? chemoheterotrophs
a) animals, fungi, protists, most bacteria and archaea
5. What would be an example of a photoheterotroph?
Purple non-sulfur bacteria
6. How would you classify organisms that perform
photosynthesis and use CO2 as a carbon
source?…Photoautotrophs
a) plants, certain bacteria like cyanobacteria, certain
archaea, and algae.
B. 6 CO2 + 12 H2O + light energy  C6H12O6 + 6 H2O + 6 O2 –
review, reverse of cellular resp.
II. Where does photosynthesis occur? chloroplasts
A. All green parts of plants have chloroplasts –
1. leaves tend to have most - dominant photosynthetic
locations (solar panels of the natural world)
B. The leaf
1. Mesophyll – green tissue in the interior of the leaf
where chloroplasts are concentrated
2. What do plants needs to perform photosynthesis and
how does it enter the leaf?
a) Stomata – tiny pores in the leaf allow for CO2 and
O2 to enter and exit
3. H2O comes in through veins from the roots
C. Chloroplasts
D. Three compartments: intermembrane space, stroma,
and thylakoid compartment
a) Stroma is where CO2 and H2O is built into sugars
b) thylakoid compartments in a granum are
interconnected
c) thylakoid membrane houses much of the
machinery that converts light to chemical energy.
E. Chloroplasts contain chlorophyll
1. Chlorophyll
a) located in the thylakoid membranes of grana
within the chloroplasts
b) absorbs the light energy used in photosynthesis
c) gives plants their green color
III.How can we determine where the Oxygen in O2 is
coming from?
A. Hypothesis – 1700’s – Jan Ingenhousz – plants produce
O2 by extracting is from CO2
B. Experiment – 1950’s – using radioactive 18O to follow
what happens to oxygen
1. Give plant C18O2, did not give off 18O2
2. Give H218O, it gave off 18O2
IV. Photosynthesis is a redox process, as is cellular
respiration
A. What do we want to do in photosynthesis? After cellular
respiration, where did the electrons end up? We want them
back. We want to grab low energy electrons and bring them
to high energy.
1. The electrons are going to be stripped from water
2. Is this easy to do? We need to use light energy.
3. Electrons will be stripped from water along with
hydrogen ions (oxidize) to get O2.
4. These electrons and protons will be added to CO2,
reducing it to glucose. – redox
B. In cell resp we stripped electrons from glucose and let
them fall slowly to O2, putting them in H2O. We will excrete
this H2O and photosynthetic organisms will pick it up, grab
the electrons and put them back into glucose. It’s a circle!!
C. Electrons need to travel uphill in photosynthesis – from
high affinity to low affinity!! This is possible thanks to light
energy captured by chlorophyll.
V. Overview: Photosynthesis occurs in two stages linked
by ATP and NADPH
A. HOW DO PLANTS CAPTURE THE ENERGY OF LIGHT TO
MAKE SUGAR?
B. Light reactions (stage 1)
1. convert light energy to chemical energy
2. produce O2 gas as a waste product
3. occur in thylokoid membranes producing ATP and
NADPH energy shuttles
4. light is necessary, obviously
C. Calvin cycle (stage 2)
1. Assemble glucose from CO2 = carbon fixation incorporation of carbon from CO2 into organic molecules
2. Occurs in the stroma.
3. Requires the energy-containing products (ATP and
NADPH) of the light reactions.
4. NADP+ = NAD+ with a phosphate (P)
a) same function as NAD+
b) 2 electron carrier
5. Don’t strictly need light for calvin, but need ATP and
NADPH, which needs light. So, Calvin usually runs during
daytime as well.
The light reactions: converting solar energy to chemical
energy
VI.
Visible radiation drives the light reactions
A. Light = electromagnetic energy = wave or particle
(photon)
B. Wavelength – the distance between the crests of two
adjacent waves.
C. Humans can only see a very narrow band of
wavelengths – different wavelengths = different colors.
D. Leaves absorb some wavelengths of light (blue-violet
and red-orange) and not others (what we see as green)
using light absorbing pigments in the thylakoid membrane:
1. Chlorophyll A – absorbs blue/violet and red –
participates directly in light reactions
2. Chlorophyll B – absorbs blue and orange – hands
energy off to chlorophyll A to use for light reactions
3. Carotenoids – other pigments found in plants – absorb
mainly blue-green light (appear yellow-orange - carrots)
a) some pass energy to chlorophyll a and other
protect chlorophyll by absorbing damaging light
E. Only chlorophyll A participates directly in the light
reactions. The others broaden the range of energy
absorbed and transfer this to chlorophyll A (mirrors around
a solar panel).
VII. Photosystems capture solar power
A. Photon – a light particle with a fixed quantity energy
(shorter the wavelength = greater the energy)
1. ex) violet light photon has 2X the energy of a red light
photon
B. When a pigment absorbs a photon, the energy of one of
its electrons is raised from a ground state to an excited
state.
1. excited state is very unstable and will fall back to the
ground state almost immediately.
2. as it falls, it releases the energy is absorbed as light or
as heat or both. (FLUORESCENT PAINT)
3. fluorescence – the release of light energy from
electrons falling back to the ground state
C. So should we let it fall? Let’s grab it at the top in its
excited state before it falls back down!!
1. The excited electron is passed to a neighboring
molecule, the primary electron acceptor, which is reduced
as chlorophyll is oxidized
D. Reaction center – A single chlorophyll a molecule plus a
primary electron acceptor
E. However, MANY pigment molecules (200-300 other
chlorophyll a, chlorophyll b, and carotenoids) are grouped
with associated proteins into a type of antenna, which
transmit the absorbed light energy to the reaction center.
F. photosystem
1. reaction center
2. antenna system (light harvesting complex)
3. MANY in a single thylakoid membrane
G. Two photosystems have been identified (named in order
of their discovery):
1. Photosystem I (P700 – named after light best
absorbed, red with wavelength of 700nm)
2. Photosystem II (P680 – 680nm – orange/red)
3. Both have same chlorophyll a in reaction center, but
associated proteins are different.
VIII. In the light reactions, electron transport chains
generate ATP, NADPH, and O2
A. Kinetic energy of light is absorbed
B. Electrons are excited
C. Excited electrons are passed along an electron
transport chain in a series of redox reactions (look familiar)
D. The energy released by these reactions is used to
generate ATP and NADPH
E. The production of NADPH requires 2 electrons.
1. Excite electrons in special pair of ‘chlorophyll a’
molecules in reaction center of photosystem II
2. Give electrons to primary electron carrier
(plastoquinone), which NOW has a higher affinity since
the light “loosened” them from the grips of chlorophyll.
3. Replenish chlorophyll’s electrons with electrons from
water, which becomes O2. The protons from water are
pushed into the thylakoid compartment.
4. Primary electron carrier (plastoquinone) carries
electrons and two protons to cytochrome b6f complex (a
proton pump). The two protons are pushed into the
thylakoid compartment upon hand-off of the electrons.
5. Energy of the MOVING 2 electrons are used to pump 2
protons against their concentration gradient into the
thylakoid compartment. (We have now pumped a total of
6)
6. Electrons are then transferred to the second mobile
carrier (plastocyanine) and brought to photosystem I,
where they are given to the two special chlorophyll a
molecules and re-excited in a similar fashion to
photosystem II.
7. The two electrons are then passed down another,
smaller ETC (no protons pumped here) and NADP+ is the
final electron acceptor as it is converted to NADPH,
carrying the two high energy electrons!
IX. Chemiosmosis powers ATP synthesis in the light
reactions
A. The photosystems and ETC are in the thylakoid
membranes
B. H+ (protons) from water splitting are pumped from the
stroma across the thylakoid membranes by ETC proteins
using the energy released during the electron flow. proton
gradient created (dam analogy again) –discussed above
C. H+ are only allowed to diffuse out through ATP
synthase, which phosphorylates ADP to ATP in the
process!! – this is called PHOTOPHOSPHORYLATION
because it is the energy from light that is used to
phosphorylate ADP.
D. Very similar to ATP generation in mitochondria – here it
is called photophosphorylation.
The Calvin Cycle: Converting CO2 to sugars – IT’S A
SUGAR FACTORY
X. ATP and NADPH power sugar synthesis in the Calvin
cycle
A. Calvin cycle – net result is to make phosphorylated 3Carbon molecules (G3P – glyceraldehyde-3-phosphate)
from CO2 using the energy and electrons provided by the
light reactions (ATP and NADPH).
1. Carbon Fixation
a) 3 CO2 are combined with 3 5-carbon
intermediates (RuBP, ribulose bisphosphate) –
catalyzed by the enzyme RuBP carboxylase
(RuBisCO) – to yield 6 3-phosphoglyceric acid
(3PGA).
2. Energy consumption and redox
a) Consume 6 ATP and 6 NADPH
b) 6 3-PGA are reduced to 6 energy rich
glyceraldehyde-3-phosphate (6 G3P) molecules.
c) Release one molecule of G3P
d) 5 G3P’s remain in cycle
3. Regeneration of RuBP
a) Energy is needed to convert the 5 G3P molecules to
3 RuBP molecules to continue the cycle
4. It takes 2 G3P’s to make a glucose (done in the
cytoplasm) – so it takes 18 ATP and 12 NADPH (24 high
energy electrons).
B. For every 3 CO2 entering the cycle, one phosphorylated
3-carbon molecule is released from the chloroplast –
C. Calvin cycle takes place in stroma
D. Plants using only the calvin cycle to fix carbons are
known as C3 plants.
Photosynthesis reviewed and extended
XI. Review: photosynthesis used light energy to make
food molecules
A. Sugar molecules made by photosynthesis in plants is
the plants own food supply –
1. Sugar goes through cellular respiration in plants just
like in animals!
B. Sugars are used to build other organic compounds
including cellulose, fats, amino acids etc… (Plant of course
will need nitrogen, phosphorus, vitamins (cofactors) from
the soil).
C. Thus, plants and other photosynthesizers are the
ultimate source of food for almost all organisms
XII. C4 and CAM plants have special adaptations that
save water
A. C3 plants – use CO2 directly from air –
1. Problem: On hot, dry days, in order to avoid water loss
and dehydration, these plants close their stomata
a) CO2 can’t easily get in to make sugar
b) O2 builds up from light reactions and can’t escape
c) O2 can fit into active site of RuBisCO. So RuBisCO
can also use O2 if there is a high O2 concentration and
low CO2 concentration.
d) This yields a 2-carbon sugar in the Calvin Cycle
instead of G3P. The 2-carbon is broken down to CO2
and H2O…no sugar is made, no energy stored, no
ATP made via cellular respiration!!
e) BIG problem for agriculture (oats, wheat, rice,
etc…). – plants grow slow when you have little ATP!
2. not a problem in cool, moist condition because
stomata can be left open, CO2 can get in and O2 can get
out. plants need to find a way to keep the CO2 high and
O2 away from RuBisCO!
B. C4 plants – special adaptations to conserve water and
prevent photorespiration
1. These plants are found in tropical locations typically
a) hot and dry weather keeps stomata closed most of
time, conserving water
2. Spacial separation of light reactions and Calvin cycle
a) Light reactions occur in mesophyll cells as usual,
but NOT Calvin Cycle
b) When Stomata close, oxygen made in mesophyll
cells will not affect Rubisco
c) There is a new enzyme present that produces 4carbon compounds from CO2 in these cells – like a
Pre-Calvin cycle.
(1) Enzyme can only use CO2, not O2 – so O2 is not a
problem!
d) 4C compound enters a neighboring bundle sheath
cell where Calvin occurs. Drops off CO2 and goes
back for another (CO2 carrier).
(1) concentration of CO2 stays higher and O2 stays
lower in these cells even when stomata are closed
(2) Drawback – it costs energy to shuttle the CO2
(3) Costs 30 ATP to make a glucose
(4) ADVANTAGE IN CONDITIONS OF
DROUGHT, HIGH TEMP, CO2 LIMITATIONS
(5) sugarcane, maize, sorghum, finger millet,
amaranth, and switchgrass.
(6) 1% of plant species
(7) Concentrated in the tropics
C. CAM (crassulacean acid metabolism) Plants
1. Time separation
2. Stomata ONLY open at night to allow CO2 to enter.
3. The CO2 is fixed into 4-carbon compounds just like C4
plants, banking the CO2 at night, and releasing it to the
Calvin cycle during the day.
4. Everything happens in one cell.
5. Pineapples, many cacti, succulent plants (juicy leaves)
Photosynthesis, Solar Radiation, and Earth’s
atmosphere
XIII. Human activity is causing global warming;
photosynthesis moderates it
A. Greenhouse Effect – CO2 retains heat (IR radiation)
beging radiated from the Earth that would otherwise go out
into space – good thing otherwise we wouldn’t be here
(keeps us warm), but too much of a good thing can be bad
B. Greenhouse gases – gases that reabsorb heat in the
atmosphere (CO2, CFC’s, CH4 and others)
C. Global Warming – steady rise in Earth’s average temp. –
can be from greenhouse effect, solar disturbance, volcanic
action, etc…
D. The burning of fossil fuels and wood releases CO2,
which increases greenhouse effect, which could account
for global warming.
E. Photosynthesis sucks up CO2 – so grow more plants
and stop deforestation!
XIV. Mario Molina talks about Earth’s protective ozone
layer
A. O2 (product of photosynthesis) is broken apart by solar
radiation and combines with free O2 to form O3 (OZONE)
B. Ozone in the upper atmosphere protects the biosphere
from harmful UV radiation
C. Ozone depletion resulted from the widespread use of
CFC’s (Chlorofluorocarbons)