Download light reactions of photosynthesis

PHOTOSYNTHESIS
OBTAINING ENERGY
 Organisms can be classified according to how
they get energy
 Organisms that use energy from sunlight are
called autotrophs
 Most autotrophs use the process of
photosynthesis to convert light energy from the
sun into chemical energy in the form of organic
compounds, mostly carbohydrates.
What is photosynthesis?
Photosynthesis involves a series of
chemical reactions (biochemical pathway)
where the product of one reaction is
consumed in the next reaction
Photosynthesis produces chemical energy
in the form of glucose
The ultimate source of energy for all life is
the sun
The next picture…
 Shows how autotrophs use photosynthesis to
produce organic compounds from carbon
dioxide and water
1. O2 & organic compounds produced are used to
create cellular respiration
2. Cellular Respiration, the CO2 & H2O are
produced
3. The products of photosynthesis are reactants in
cellular respiration, & vice versa
Light
Energy
Photosynthesis
by autotrophs
Carbon dioxide
& water
Organic compounds
& oxygen
Cellular Respiration
by autotrophs
& heterotrophs
Photosynthesis can be divided
into 2 stages
1.
2.
Light Dependent Reactions: light energy
(absorbed from the sun) is converted to
chemical energy, which is temporarily stored in
ATP and the energy carrier molecule NADPH
Calvin Cycle or Light Independent Reactions:
organic compounds are formed using CO2 and
the chemical energy stored in ATP and NADPH
Equation for Photosynthesis
6CO2 + 6H20
 Carbon dioxide
water
 C6H1206 + 6O2
light
glucose
oxygen
Light dependent reaction
First stage of photosynthesis
Capturing Light Energy
The 1st stage of photosynthesis includes the
light dependent reaction b/c they require
light to happen
The light reactions begin with the
absorption of light in chloroplasts {found
in cells of plants, bacteria & algae}
Internal Membranes of Chloroplasts
Chloroplast have a double membrane:
inner & outer
Stroma is the solution surrounding the
grana
thylakoid are arranged as flatten sacs
Grana are stacks of thylakoid;
– this is where the dark reaction (light
independent reaction) will occur
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chloroplast
Inner membrane
Outer membrane
Granum
Stroma
Thylakoid
Thylakoid
Stroma
The Stroma
The stroma houses
the enzymes needed
to assemble organic
molecules from
CO2, using energy
from ATP &
NADPH
Granum
Light & Pigments
Where is the energy in light?
Properties of light
 Light from the sun appears white, but is actually
made of a variety of colors
 White light can be separated into its components
by passing it through a prism
 The resulting array of colors, ranging from red to
violet called the visible light spectrum
 ROY G BIV (rainbow) makes up the visible light
spectrum
Light
 When white light strikes an object, its component
colors can be reflected, transmitted or absorbed
by the object.
 Many objects contain pigments, compounds
which absorb light
– Most pigments absorb certain colors more strongly
than others, which subtracts those colors from the
visible spectrum
– Therefore, the light that is reflected or transmitted by
the pigment no longer appears white
White light
White light contains a variety
of colors. Each color has a
different wavelength
measured in nanometers.
Chloroplast Pigments
Located in the membrane of the thylakoids
are several pigments called chlorophylls
There are several different types of
chlorophyll
2 most common are called chlorophyll a &
chlorophyll b
Chlorophyll a & Chlorophyll b
Chlorophyll a absorbs less blue light, but
more red light than chlorophyll b
Neither chlorophyll a nor chlorophyll b
absorbs much green light
– Instead, they allow green light to be reflected
or transmitted
– For this reason, leaves & plants with large
amounts of chlorophyll look green.
Chlorophyll a & Chlorophyll b
Only chlorophyll a is directly involved in
the light reactions of photosynthesis.
Chlorophyll b assist chlorophyll a in
capturing light energy, so chlorophyll b is
called an accessory pigment
Other pigment components
Other compounds found in the thylakoid
membrane include the yellow, orange, and
brown carotenoids, which also function as
accessory pigments
By absorbing colors that chlorophyll a
cannot absorb, the accessory pigments
enable plants to capture more of the energy
in light
The 3 curves show how 3 pigments involved in photosynthesis
differ in the colors of light they absorb. Where a curve has a
peak, much of the light at that wavelength is absorbed. Where
a curve has a trough, much of the light at that wavelength is
reflected or transmitted
In leaves of a plant
 In the leaves of a plant,
the chlorophylls are more
abundant & mask the
color of the other
pigments
 But in the
nonphotosynthetic parts
of a plant (fruits &
flowers) the colors of the
other pigments may be
visible
During the Fall
During the fall many plants lose their
chlorophylls & their leaves take on the rich
hues of the carotenoids.
Why?
–
–
–
–
Shorter day length
Less sunlight
Chlorophyll disintegrates
Other pigments can now be seen
Leaves in the Summer vs. Autumn
Oak leaf in
summer
Oak leaf in
autumn
Converting light energy to
chemical energy
Once the pigments in the chloroplast have
captured light energy, the light energy must
then be converted to chemical energy
The chemical energy is temporarily stored
in ATP & NADPH
O2 is given off
 continue…
Continued…
The chlorophylls and carotenoids are
grouped in clusters of a few hundred
pigment molecules in the thylakoid
Each cluster of pigment & the proteins that
the pigment molecules are embedded in are
referred to as photosystem
2 Types of photosystems
Photosystem I
Photosystem II
They contain similar kinds of pigments,
but have different roles in the light reaction
Both have antenna complexes to harvest
energy
The light dependent reaction
begins…
 Takes place within the thylakoid membranes
within chloroplasts in leaf cells
 accessory pigment molecules in both
photosystems absorb light
 They acquire some energy carried by the light
which is passed quickly to the other pigment
molecules until it reaches a specific pair of
chlorophyll a molecules
 chlorophyll a molecules can absorb light (5 steps)
Light Reaction: step 1
 Light energy forces electrons to enter a higher
energy level in the 2 chlorophyll a molecules of
photosystem II.
 These energized electrons are said to be “excited”
& have enough energy to leave the chlorophyll
molecule
 They lost electrons– oxidation reduction reaction
– So, some substance must accept the e- that the
chlorophyll a molecules have lost
– The acceptor of e- from chlorophyll a is a molecule in
the thylakoid membrane called the Primary Electron
Acceptor
Light Reaction: step 2
Some substance must accept the electrons
lost from chlorophyll a
This acceptor is in the thylakoid called the
primary electron acceptor
Light Reaction: step 3
The primary electron acceptor donates the
electron to the ETC located in the
thylakoid membrane
As they pass from molecule to molecule
in the chain, they lose most of the energy
the acquired when they were excited
The energy they lose is used to move H+
protons into the thylakoid
Light Reaction: step 4
 At the same time light is absorbed by
photosystem II, light is also absorbed by
photosystem I.
 Electrons move from a pair of chlorophyll a
molecules in photosystem I to another primary
electron acceptor.
 The e- lost by chlorophyll a are replaced by the
e- that have passed through the ETC from
photosystem II.
Light Reaction: step 5
The primary e- acceptor of photosystem I
donates e- to a different ETC
This chain brings the e- to the side of the
thylakoid membrane that faces the stroma
There the e- combine with a proton &
NADP+
NADP+ is reduced to NADPH
Calvin cycle animation
http://www.science.smith.edu/departments/Biolog
y/Bio231/ltrxn.html
Light (dependent) Reaction
http://www.youtube.com/watch?v=eY
1ReqiYwYs&feature=related
Restoring Photosystem I
 In step 4, electrons from chlorophyll molecules in
photosystem II replace electrons that leave
chlorophyll molecules in photosystem I
• If this did not happen, both ETC’s would stop &
photosynthesis would not occur!
 The replacement electrons for photo II are
provided by water molecules
 An enzyme inside the thylakoid splits water into
p+, e- & Oxygen
The splitting of water inside the thylakoid releases e- which replace
E- that leave photosystem II when it is illuminated.
Splitting of Water
 2H2O  4H + 4e- + O2
 For every 2 water molecules that are split, 4 ebecome available to replace those lost by
chlorophyll molecules in photosystem II
 The p+ that are produced are left inside the
thylakoid, while oxygen diffuses out of the
chloroplast & can leave the plant
 O2 is not needed for photosynthesis to occur, but
is essential for cellular respiration in most
organisms including plants!
Chemiosmosis
ATP is the main energy currency of cells
An important part of light reactions is the
synthesis of ATP though Chemiosmosis
Chemiosmosis relies on a concentration
gradient of protons across the thylakoid
membrane
How Chemiosmosis is used
 Some p+ are made from the breakdown of water
inside the thylakoid
 Other p+ are pumped from the stroma to the
interior of the thylakoid
 The energy required to pump these p+ is supplied
by the excited e- as they pass along the ETC of
photosystem II
 All of these act to build up a concentration
gradient of protons
 The concentration of protons is higher inside the
thylakoid than in the stroma
ATP synthesis
Located in the thylakoid membrane
Energy driving this reaction is made by the
movement of p+ from inside the thylakoid
to the stroma
Some of the protons in the stroma are used to make NADPH
from NADP+.
Together NADPH & ATP provide energy for the second set
of reactions in photosynthesis
The Calvin Cycle
The Second phase of photosynthesis
~Light Independent Reaction
~or Dark Reaction
Named for
 Melvin Calvin (19111997), American scientists
who received the Nobel
Prize for biochemistry for
his discovery of the
chemical pathways of
photosynthesis
Carbon Fixation
The Calvin Cycle is a series of enzyme-
assisted chemical reactions that make a 3carbon sugar.
A total of 3 CO2 molecules must enter the
Calvin cycle to produce each 3-carbon
sugar that will be used to make the organic
compound.
• The Calvin cycle has 3 major steps, which occur in
the stroma of the chloroplast
Carbon fixation by the Calvin
cycle
In the Calvin cycle, carbon atoms from
CO2 in the atmosphere are bonded or
“fixed” into organic compounds
This incorporation of CO2 into organic
compounds is known as carbon fixation
Calvin cycle: step 1
CO2 diffuses into the stroma from the
surrounding cytosol
An enzyme combines a CO2 molecule with
a 5-carbon carbohydrate called RuBP
The 6-carbon molecule that results are very
unstable & they each immediately split into
2 3-carbon molecules called 3phosphoglycerate (3-PGA)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
THE CALVIN CYCLE
Fig. 10.17a
3 (TEArt)
1
CO
2
P
3
6
3-phosphoglycerate
RuBP
(Starting material)
Ribulose Bishosphate
The Calvin cycle begins when a carbon atom
from a CO2 molecule is added to a five-carbon
molecule (the starting material). The resulting
six-carbon molecule is unstable and immediately
splits into three-carbon molecules.
Calvin cycle: step 2
 Each molecule of 3-PGA is converted into
another 3-carbon molecule, glyceraldehyde 3phosphate (G3P) in a 2 part process:
– 1st each PGA molecule receives a phosphate group
from a molecule of ATP
– The resulting compound then receives a p+ from
NADPH & releases a phosphate group, producing
G3P
– The ADP, NADP+, and PO4 can be used again in light
reaction to make more ATP & NADPH
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
THE CALVIN CYCLE
Fig. 10.17b (TEArt)
2
P
6
3-phosphoglycerate
6 ATP
6 NADPH
P
6
P
1
Glyceraldehyde
3-phosphate
Glyceraldehyde
3-phosphate
Glucose
Then, through a series of reactions, energy from ATP and
hydrogens from NADPH (the products of the light-dependent
reactions) are added to the three-carbon molecules. The nowreduced three-carbon molecules either combine to make
glucose or are used to make other molecules.
Calvin cycle: step 3
One of the G3P molecules leaves the
Calvin cycle & is used to make organic
compounds (carbohydrates)-stored for later
use
Calvin cycle: step 4
The remaining G3P molecules are
converted back into RuBP through the
addition of P from ATP
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 10.17c
(TEArt)
3
3
RuBP
(Starting material)
3
ATP
P
5
Glyceraldehyde
3-phosphate
Most of the reduced three-carbon molecules are
used to regenerate the five-carbon starting material,
thus completing the cycle.
In conclusion
By regenerating the RuBP that was
consumed in step 1, the reactions of step 3
allow the Calvin cycle to continue
operating
Some PGAL molecules are not converted
into RuBP, but leave the cycle & can be
used by the plant cell to make other
organic compounds (cellular respiration)
Calvin cycle animation
http://www.science.smith.edu/departments/Biolog
y/Bio231/calvin.html
Balance sheet for photosynthesis
How much ATP & NADPH required to
make 1 molecule of PGAL from CO2?
Each turn of the Calvin cycle fixes 1-CO2
Since PGAL is a 3-carbon compound-it takes 3
turns of the cycle to produce each molecule of
PGAL
Each turn, 2-ATP & 2-NADPH are used (step 2),
1 for each molecule of PGAL produced & 1
more ATP in step 4.
Total
Three turns of the Calvin cycle use 9
molecules of ATP & 6 molecules of
NADPH
Alternative pathways
Calvin cycle is the most common pathway
for carbon fixation
Plant species that fix carbon thought the
Calvin cycle are known as C3 Plants
because of the 3-carbon compound (PGA)
that is initially formed
Other carbon fixation plants
Other plant species fix carbon through
alternative pathways & then release it to
enter the Calvin cycle
These are generally found in hot, dry
climates.
Under these conditions, plants rapidly lose
water to air
Stomata
 Most of the water loss from
a plant occurs through
small pores called stomata
which are found on the
underside of leaves
Problems in the Stomata
 Are passageways for CO2 enters and O2 exits
plant leaves.
 When stomata’s are partly closed, the level of
CO2 in the plant falls as CO2 is consumed in the
Calvin cycle.
 At the same time, O2 rises b/c the light reactions
split water and generate O2
 Both conditions inhibit carbon fixation by the
Calvin cycle < low CO2 & high O2
C4 pathways
C4 pathways enables plants to fix CO2 into
4-carbons, and plants that use it is known
as C4 plants
C4 plants have their stomata partially
closed the hottest part of the day
They have a certain enzyme that can fix
CO2 into 4-carbon compounds even when
CO2 is low & O2 is high
C4 Plants
 These are transported to other cells where CO2 is
released & enters the Calvin cycle
 Ex: corn, sugar cane, & crabgrass.
 They lose about half as much water as C3 plants
CAM pathways
 CAM pathways open their stomata at night and
close them during the day
 At night- they take in CO2 and fix it into a variety
of organic compounds
 Day-CO2 is released from these compounds &
enters the Calvin cycle
 In low temperatures, they grow fairly slow, but
they lose less water than C3 or C4 plants
CAM plants
Cactuses, pineapples and certain plants that
have a different adaptation to hot, dry
climate