Photosynthesis
• Converts solar energy into the potential
chemical energy of a carbohydrate in this
way:
• Solar energy + carbon dioxide + water →
carbohydrate + oxygen
• Photosynthetic organisms include plants,
algae, and certain bacteria.
• These organisms are called producers;
they synthesize organic molecules from
raw materials.
Photosynthetic organisms
•
1)
2)
3)
4)
Nearly all life is dependent on solar energy
because:
Photosynthetic organisms use solar energy to
produce organic nutrients.
Almost all organisms depend either directly
or indirectly on these organic nutrients to
sustain themselves.
Photosynthetic organisms provide food for
other organisms, known as consumers.
The bodies of plants became the coal or other
fossil fuels used today.
Structure and Function of Chloroplasts
• Chloroplasts are the organelles that carry on
photosynthesis.
• Mesophyll cells in the middle of a leaf house
chloroplasts
• Mesophyll cells are protected from drying out
by a waxy cuticle.
• Pores called stomata allow CO2 and O2 to
enter the leaf.
Mesophyll cells of a leaf
The xylem is responsible for
the transport of water and
soluble mineral nutrients from
the roots throughout the plant.
It is also used to replace water
lost during transpiration and
photosynthesis.
The phloem is concerned
mainly with the transport of
soluble organic material made
during photosynthesis.
Structure of Chloroplasts
• Bounded by a double membrane.
• The inner membrane encloses a large central space
called the stroma that houses enzymes used to
reduce CO2 to carbohydrate.
• A membranous system of thylakoids lies within the
stroma; some thylakoids are stacked into grana;
thylakoids contain chlorophyll and other pigments.
• All thylakoids are connected – single inner
compartment within choropasts called the thylakoid
space
• Chlorophyll and other pigments absorb solar energy.
Chloroplast structure
two outer
membranes
Overview of Photosynthesis
inner membrane
system
(thylakoids connected by
channels)
stroma
channel
stacked part of
thylakoid membrane
Stepped Art
Figure 7.3d,e
Page 116
Visible Light
• Radiant energy from the sun (solar energy) can
be described in terms of its wavelength and its
energy content.
• The colors in visible light range from violet
(the shortest wavelength and highest energy) to
blue, green, yellow, orange, and red (the
longest wavelength and lowest energy).
Visible Light Spectrum
• Pigments (chlorophylls and carotenoids) found
within photosynthesizing cells, are capable of
absorbing various portions of visible light.
• Both chlorophyll a and chlorophyll b absorb
violet, blue, and red light best.
• Carotenoids absorb light in the violet-bluegreen range and reflect yellow or orange
• Leaves appear green because green light is
reflected and only minimally absorbed.
In the fall when the pigment chlorophyll breaks down
(chlorophyll needs warm weather to be created; is created
continually during warm months), the remaining pigment
(carotenoids) become unmasked, reflecting the colors like
orange and yellow
http://www.youtube.com/watch?v=AeypaiIoMPI
•Photosynthesis is an oxidation-reduction reaction,
or redox reaction for short.
•Oxidation is the loss of electrons; hydrogen atoms
are removed from glucose.
•Reduction is the gain of electrons; oxygen atoms
gain electrons.
•Remember OIL RIG (oxidation is loss, reduction
is gain)
Overview of Photosynthesis
• A simplified overall equation for
photosynthesis is:
• Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2
• During photosynthesis, water molecules are
oxidized; they lose electrons (e-) along with
hydrogen ions (H+).
• Also, CO2 is reduced and gains electrons given
up by H2O.
• Electrons from H2O are energized by the sun.
Enzyme involved
•NADP+
•Nicotinamide adenine dinucleotide phosphate
•Accepts 2 electrons and H+ to become NADPH
The ATP cycle
Two Sets of Reactions
• Photosynthesis is divided into two sets of
reactions, as implied by the term
“photosynthesis”:
• “Photo” refers to the light-dependent (needs
light) reactions that capture energy from the
sun
• Photosystem II
• Photosystem I
• “Synthesis” refers to the light-independent
(does not need light) reactions that produce
carbohydrate (glucose).
• Calvin Cycle
Photosynthesis – Light-dependent and Light-independent
Reactions
H2O
CO2
Lightindependent
Chloroplast
Light
NADP+
Lightdependent
ADP
+
P
CALVIN
CYCLE
(in stroma)
LIGHT
REACTIONS
(in thylakoids)
ATP
NADPH
O
Sugar
SUNLIGHT
inside a Thylakoid
O
H+
H2O
H+
H+
H+
e–
NADP+ NADPH
H+
Stroma
ATP
ADP + Pi
H+
Light-Independent Reactions
CO2
H2O
P glucose
carbohydrate end product
(e.g., sucrose, starch, cellulose)
Overview of Photosynthesis
Figure 7.3f
Page 117
Animation of light-dependent
reactions
Simplified animated steps of lightdependent reactions
Animation of overall light-dependent and animation of cyclic &
noncyclic flow
Overview of light-independent reactions (Calvin cycle)
Another Calvin cycle animation
Overall process of photosynthesis
A photosynthesis rap!!!!!!!!!
http://www.youtube.com/watch?v=Wi60tQa8jfE
Musical lecture part I
Musical lecture part II
Light-Dependent Reactions
•Light energy is absorbed by chlorophyll.
•Electrons are energized.
•Energized electrons move down an electron
transport system, release energy, and produce ATP.
•NADP+ accepts electrons to produce NADPH.
•Water is split to produce oxygen, hydrogen ions,
and electrons
•Photosystem II occurs first
•Photosystem I occurs second (II and I were named
for the order in which they were discovered, NOT
the order in which they occur).
Light-dependent reactions continued
•Both occur within the thylakoid membranes inside
the chloroplast
•Each photosystem has a pigment system of green
pigments (chlorophylls a and b) and accessory
pigments.
•Closely packed pigments serve as an “antenna” for
gathering solar energy.
•Electrons can follow one of two electron pathways
Electron Pathways
• Electrons can follow a cyclic electron pathway
or a noncyclic electron pathway during the
light-dependent phase of photosynthesis.
• The cyclic electron pathway generates ATP
only.
• Used by some bacteria
• The noncyclic electron pathway generates both
ATP and NADPH.
• Used by some bacteria and plants
Cyclic Electron Pathway
• The cyclic electron pathway begins after the
PS I pigment complex absorbs solar energy.
• High-energy electrons (e-) leave the reactioncenter chlorophyll a molecule and later return
to it.
• But first, the electrons enter an electron
transport system that leads to the production of
ATP.
ATP Production
• Each time water is split, two H+ remain in the
thylakoid space.
• As electrons move down the electron transport
system, they give up energy, which is used to
pump H+ from the stroma into the thylakoid
space.
• Thus, H+ build up in the thylakoid space.
• The flow of H+ through an ATP synthase
complex back into the stroma drives the
chemiosmotic production of ATP.
The light-dependent reactions: the
cyclic electron pathway
Light-dependent Photosystem II and
Photosystem I - Electron transport system
Stroma
Noncyclic Electron Pathway
• In the noncyclic electron pathway, electrons
move from H2O through PS II to PS I and then
on to NADP+.
• The PS II pigment complex first absorbs solar
energy, and high-energy electrons (e-) leave the
reaction-center chlorophyll a.
• PS II takes replacement electrons from water,
which splits, releasing oxygen.
• The oxygen leaves the chloroplast as oxygen
gas (O2), and the hydrogen ions (H+)
temporarily remain in the thylakoid space.
• The high-energy electrons enter an electron
transport system, and energy is used to
produce ATP.
• Low-energy electrons leaving the electron
transport system enter PS I where they replace
electrons that have left the PS I reaction center.
• When the PS I pigment complex absorbs solar
energy, high-energy electrons leaving the
reaction-center chlorophyll a are captured by
an electron acceptor.
• This time, the electron acceptor passes the
electrons on to NADP+, which accepts a
hydrogen ion (H+) and becomes NADPH:
• NADP+ + 2e- + H+ → NADPH
Results of the noncyclic electron pathway:
• Water is split, yielding H+, e-, and O2
• ATP is produced
• NADP+ becomes NADPH
The light-dependent reactions: the
noncyclic electron pathway
Machinery of
Noncyclic Electron Flow (Light
RXN)
Inside THYLAKOID
H2O
photolysis
second electron
transfer chain
e–
e–
first electron
transfer chain
PHOTOSYSTEM II
NADP+
ATP SYNTHASE
NADPH
PHOTOSYSTEM I
STROMA of Chloroplast
ADP
+ Pi
ATP
Figure 7.13a
Page 123
The Thylakoid Membrane
• These four complexes are in the thylakoid
membrane:
• PS II consists of a protein complex and the
light-gathering pigment complex; PS II splits
water and released oxygen
• The cytochrome complex transports electrons
between PS II and PS I, during which pumping
of H+ occurs
• PS I consists of a protein complex and a lightgathering pigment complex; NADP reductase
is an enzyme that reduces NADP+ to NADPH
• ATP synthase complex has a H+ channel and a
protruding ATP synthase; as H+ flows down its
concentration gradient through this channel
from the thylakoid space into the stroma, ATP
is produced.
Light-Dependent Reaction:
Arrangement of Photosystems
water-splitting complex
H2O
thylakoid
compartment
2H + 1/2O2
P680
P700
acceptor
acceptor
PHOTOSYSTEM II
pool of
electron
carriers
stroma
PHOTOSYSTEM I
Figure 7.10
Page 121
Light-dependent Photosystem II and
Photosystem I - Electron transport system
Stroma
LIGHT-DEPENDENT REACTION
Requires Light and
Water
WHAT IT DOES:
SPLITS H2O
Creates ATP and NADPH
For Calvin Cycle
Forms O2 as a byproduct
WHERE IT HAPPENS:
Leaf
Chloroplasts
Inside Thylakoids
Light-Independent Reactions
•
•
•
•
The light-independent reactions do not need light
Take place in the stroma
Reactions make up the Calvin cycle.
Carbon dioxide (CO2) is taken up by the plant
(from the atmosphere)
• ATP and NADPH (made from the light dependant
reactions) add phosphates and electrons to CO2,
and convert it to glyceraldehyde 3- phosphate, or
G3P (PGAL)
• G3P is a type of sugar which a plant can easily
convert to glucose. This glucose can then be used
as a source of stored energy for the plant
The Calvin Cycle
AKA G3P
Details about the Calvin Cycle
• Carbon dioxide is taken up by a 5-carbon
sugar, RuBP.
• The resulting 6-carbon molecule breaks into
two 3-carbon PGA molecules.
• Each 3 carbon molecule PGA is reduced to
G3P/PGAL using NADPH and ATP during a
redox reaction (CO2 is reduced while NADPH
is oxidized).
• Some of the resulting G3P/PGAL is used to
form glucose, the rest is used to re-form RuBP.
The Role of Glucose
• The formation of glucose from G3P/PGAL is
of interest because glucose is used by most
organisms to produce ATP to supply energy
needs.
• Glucose can be stored as starch, or is used to
as cellulose make to strengthen plant cell
walls.
• G3P/PGAL is converted to many organic
molecules besides glucose, including plant
oils.
Stages of the Calvin Cycle
•
1)
2)
3)
4)
The Calvin cycle can be divided into:
Fixation of CO2;
Reduction of CO2; and
Regeneration of RuBP.
Metabolites of the Calvin cycle include:
1)RuBP = ribulose bisphosphate
2)PGA = 3-phosphoglycerate
3)PGAP = 1, 3-bisphosphoglycerate
4)G3P/PGAL = glyceraldehyde-3-phosphate
Fixation of Carbon Dioxide
• The first event of the Calvin cycle is the
fixation of carbon dioxide (CO2).
• This occurs when CO2 is attached to an
organic compound.
• A 5-carbon molecule, RUBP, combines with
CO2 to form a 6-carbon molecule.
• An enzyme, RuBP carboxylase, speeds this
reaction.
Reduction of Carbon Dioxide
• The 6-carbon molecule immediately breaks
down into two 3-carbon PGA molecules.
• Each of the two PGA undergoes reduction to
G3P/PGAL in two steps.
• This is a redox reaction in which CO2 is
reduced to carbohydrate (CH2O) and NADPH
is oxidized to NADP+.
• Energy for this reaction comes from ATP
(generated during light-dependent
photosynthesis).
Regeneration of RuBP
• RuBP must be regenerated for the Calvin cycle
to continue.
• For every three turns of the Calvin cycle, five
molecules of G3P/PGAL are used to re-form
three molecules of RuBP.
• This reaction also utilizes some of the ATP
produced by the light-dependent reactions.
The light-independent reactions of the
Calvin cycle (detailed)
Or G3P
Making 1 molecule of glucose requires:
•6CO2 (from atmosphere)
•18ATP (from light-dependent
reactions)
•12 NADPH (from light-dependent
reactions)
• Photosynthesis, like all energy
transformations, results in a loss of usable
energy.
• Under ideal laboratory conditions, plants
transform 25% of solar energy they absorb into
carbohydrate.
• Under natural conditions, the efficiency of
photosynthesis ranges from less than 1% to a
maximum of 8%.
• Regardless, living things depend on
photosynthesis as their ultimate source of
chemical energy.
Calvin Cycle
• Overall reactants
• Overall products
– Carbon dioxide
– Glucose
– ATP
– ADP
– NADPH
– NADP+
Reaction pathway is cyclic and ribulose bisphosphate
(RuBP) is regenerated
DARK REACTION
Requires ATP, NADPH,
and CO2
WHAT IT DOES:
Produces High
Energy Sugars
Combines CO2, ATP
and NADPH
Forms a 6 carbon sugar
(Glucose)
WHERE IT HAPPENS:
Leaf
Chloroplasts
In the Stroma
(Thick Fluid inside Chloroplast)
Summary of Photosynthesis
• Light-Dependent Reactions
– Subset RXNS:
• Cyclic Electron Flow (1 photosystem)
• Noncyclic Electron Flow (produces O2; 2 photosystems, 2
ETS)
– Location: Thylakoid
– Produces: O2, ATP, NADPH (only ATP for cyclic)
• Light-Independent Reactions
– Aka Calvin Cycle
– Location: Stroma
– Produces: glucose, ADP, NADP+
Overview of Photosynthesis
Photosynthesis Vs. Cellular Respiration
Overall equation for photosynthesis is:
Overall equation for cellular respiration is:
Photosynthesis Vs. Cellular Respiration
Photosynthesis
Occurs in plants,
algae, & some
bacteria
Occurs in
chloroplasts
Glucose is
produced
Cellular
Respiration
Occurs in all
organisms
Occurs in
mitochondria
Glucose is
broken down
Photosynthesis Vs. Cellular Respiration (cont’d)
Photosynthesis
Occurs in plant
cells during day
Cellular
Respiration
Occurs in plant
cells day AND
night
Uses electron
Uses electron
transport system transport system
Uses NADP+ and Uses NAD+ and
NADPH
NADH, FAD and
FADH2
Photosynthesis versus cellular
respiration