Photosynthesis
Photosynthesis is a producer
• Photosynthesis nourishes most organisms
on earth
• Plants and other autotrophs are producers
• Autotrophs produce their own food
• Heterotrophs feed on organic material
2 Types
1. Anoxygenic: does
NOT produce O2
Bacteria
2. Oxygenic:
produces O2
Cyanobacteria, algae,
and land plants
Both use pigments but differ in function of
pigments.
Chloroplast:
photosynthetic
organelles
3 Processes occur here:
1. Capture light energy
2. Use energy to make ATP and reduce
NADP+ to NADPH
3. Use ATP and NADPH to power the
synthesis of organic molecules from CO2
2 Stages of Photosynthesis
1. Light-dependent reactions: Light energy
stored in ATP and NADPH. Needs light.
2. Light-independent
reactions: cycles
that form organic
molecules using
CO2 in a process
called carbon
fixation. Does NOT
need light.
Equation
6CO2 + 12H2O + light energy  C6H12O2 + 6H2O + 6O2
Carbon Water
Glucose Water Oxygen
Dioxide
Generally, opposite of cell respiration
The Chloroplast
• The chloroplast is the site of
photosynthesis in a cell
• Contains a pigment called chlorophyll
• Chloroplasts are found mainly in the
Mesophyll (leaf tissue)
• CO2 and O2 enter the leaf via the stomata
Structure of Chloroplast
• Like mitochondria, chloroplasts have an
internal and external membrane.
• Thylakoids are structures made of the inner
membranes. A stack of thylakoids is called
granum (grana plural).
• Thylakoid membranes contain pigments like
chlorophyll.
Structure of Chloroplast (cont’d)
• The photosynthetic pigments are clustered
together to form photosystems which
capture energy packets called photons.
• Surrounding the grana is a semiliquid
called stroma which houses the enzymes
that make organic molecules.
Discovery of Photosynthetic Processes
Some bacteria use hydrogen sulfide(H2S)
instead of water for photosynthesis
Thus, the general formula for photosynthesis
is:
CO2 + 2H2X  CH2O + H2O +2X
By using O2 isotopes, C.B. Van Niel proved
that the O2 produced came from the splitting of
water
Tracking Atoms through
Photosynthesis
Pigments
• Molecules that absorb light energy in the
visible range.
Light is a form energy
• Visible light is made of various colors which are
different due to their wavelength.
• Visible light is a small part of the
electromagnetic spectrum.
Photons
• Particle of light
acting as a
bundle of energy.
• Light has a dual
nature as it is
both waves and
energy.
Photoelectric Effect
• When light or photons transfer energy to
electrons, the electrons are removed from
molecules and create an electric current.
• Chloroplasts act as photoelectric devices.
Absorption Spectrum
• The electrons that absorb energy jump to
higher energy levels. The shorter the
wavelength of light the greater the energy
absorbed.
• To boost electrons to discrete energy levels,
specific atoms can only absorb specific
photons of light relative to the atoms energy
levels. The range of photons a molecule can
absorb is called the absorption spectrum.
Action spectrum
• Relative effectiveness of different
wavelengths of light for photosynthesis.
Chlorophyll a and Chlorophyll b
• Chlorophyll a is the main pigment that can
directly convert light energy to chemical
energy.
• Chlorophyll b is a secondary or accessory
pigment that helps to absorb a greater
range of wavelengths of light.
• Both absorb red and blue-violet light.
Thus they reflect green light giving a green
appearance.
Structure of
Chlorophylls
• Chlorophylls are
made of a
porphyrin ring
(rings w/ single and
double bonds) a Mg
atom at the center,
a hydrocarbon
chain, and a side
chain.
Carotenoid
• Pigment that absorbs mostly blue and green
ranges of light capturing energy from
wavelengths of light not absorbed by
chlorophyll.
• Play protective role.
• May contain beta-carotene which is helpful
for vision.
Phycobiloproteins
• Pigment found in cyanobacteria and algae
which absorbs green light.
Photosystem Organization (8.4)
• 1 molecule of O2 for every 2500
molecules of chlorophyll.
• Light absorbed by a cluster of pigment
molecules in a photosystem.
• Each photosystem had 2 parts: antenna
complex and a reaction center.
Antenna Complex “Light-harvesting”
• Made of different pigments like chlorophyll
which absorb photons of light and pass the
energy from one pigment molecule to the
next in the thylakoid membrane.
• Eventually the energy is passed to the
reaction center.
Reaction Center
• When a chlorophyll molecule in the
reaction center absorbs a photon of light,
an electron is excited and moves the an
electron acceptor quinone.
• Quinone then passes the e- to another
acceptor.
• Water donates e- to chlorophyll and is
oxidized to form H+ and O2 a product.
• http://www.sumanasinc.com/webcontent/a
nimations/content/harvestinglight.html
LIGHT-DEPENDENT REACTIONS
4 Parts to Reactions
1. Primary photoevent: photon captured by
pigment and e- excited.
2. Charge separation: energy of e- transferred to
acceptor molecule at reaction center.
3. Electron transport: e- move along carrier
molecules until they reduce NADP+ to NADPH
and H+ moves across membrane to generate a
gradient.
4. Chemiosmosis: H+ diffuses back across
membrane through ATP synthase to generate
ATP.
Bacteria use 1 photosystem
• In sulfur bacteria, e- absorb photons and are
boosted from the reaction center to an eacceptor which happens to be H+ and
together they form an H atom.
• e- are recycled as they are used to make ATP
and return back to chlorophyll molecules. This
is called cyclic photophosphorylation.
Chloroplasts have 2 Photosystems
1. Photosystem I (PS I): has absorption peak of
700 nm so we call it P700. Transfers e- to
NADP+ to make NADPH.
2. Photosystem II (PS II): has an absorption
peak of 680 nm, we call it P680. e- lost in
PS I are replaced by e- from water generated
in PS II.
** Systems are connected by
cytochrome/b6-f complex of e- carriers.
Photosystem
How do they work together?
Noncyclic photophosphorylation
• e- do not return to photosystems, instead they
are used to make NADPH. 2 main products of
PS I and PSII are ATP and NADPH.
NOW DRAW IT
Light-Dependent Reactions
• http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::5
35::535::/sites/dl/free/0072437316/120072
/bio13.swf::Photosynthetic%20Electron%2
0Transport%20and%20ATP%20Synthesis
• Plants will carry out cyclic phosphorylation
when they run low on ATP.
• Cyclic phosphorylation only generates ATP,
(no NADPH or oxygen is made). The e- will
leave PS I and return to the b6-f complex.
Carbon Fixation: Calvin Cycle (8.6)
• Occurs day or night but depends on NADPH
and ATP for energy to make sugar.
• Produces PGAL (phosphoglycerate) a 3C
molecule used to make glucose and other
sugars. Thus it is called C3 photosynthesis.
• 6 turns of cycle produces 2 PGAL or 1 sugar.
• Occurs in the stroma.
NOW DRAW IT
• http://www.sinauer.com/cooper/4e/animati
ons0305.html
Photorespiration and Other
Processes (8.7)
Photorespiration
• Normally rubisco uses CO2 in the Calvin
cycle.
• However during photorespiration, the stomata
are closed increasing O2 levels and
decreasing CO2 levels in the leaf.
• Rubisco will sometimes binds with O2 instead
of CO2.
• ATP is used, CO2 is made and No sugar is
formed.
• This generally occurs when temperatures are
high or it is dray and the stomata are closed
C4 Photosynthesis
• Assists plants that live in hot, dry areas such
as corn, crabgrass, and sugar cane.
• (Phosphoenolpyruvate) PEP carboxylase
joins CO2 and PEP to produce oxaloacetate
(OAA) in the mesophyll cells.
• Now in an organic form, CO2 is transferred to
the bundle sheath cells and there it is used in
the Calvin cycle.
• The advantage is reducing photorespiration.
• What is
similar?
• What is
different?
CAM plants
• CAM = crassulacean acid metabolism
• Adaptation to dry areas.
• At night plants open their stomata to
capture CO2 using PEP carboxylase and
store it in organic compounds.
• During the day, they use light energy and
the CO2 to produce sugar.
• All reactions occur in the mesophyll cells.