Photosynthesis
Introduction to Biology
Ppt from aurumscience.com
• How does a tree gain mass as it grows?
• Law of Conservation of Mass: Mass cannot be
created or destroyed, it only changes form.
Van Helmont’s Experiment
• Jan Baptista van Helmont, a scientist from
Belgium, conducted an experiment to
determine the source of a tree’s mass.
o He grew a Willow tree in a pot for 5 years and remeasured the mass.
o The Willow tree grew by 74kg, but the mass of the soil
changed very little.
o Van Helmont concluded that the source of the plant’s
mass is water.
Woodward’s Experiment
• John Woodward, a professor at Cambridge
university in the 1600s, decided to test this
conclusion.
o He measured the mass of water he added to the plants.
o He also measured the mass of the plants as they grew.
o After 77 days of plant growth, the plant increased in
mass by 1 gram. Over 76,000 grams of water had been
added.
Priestley’s Experiment
• Joseph Priestley believed that plants changed
the air somehow.
• He placed a small mint plant in a jar with a lit
candle.
o He closed the jar, the candle used up the oxygen, and
the flame extinguished.
o After about a month, he was able to re-light the candle,
proving that the plant had changed the air by
producing oxygen.
Priestley’s Second
Experiment
• In his second experiment, Joseph Priestley
kept a mouse in a closed jar of air until it
collapsed.
• He then repeated the experiment, but
included a large plant in the jar with the
mouse.
o The mouse survived!
The Answer
• What are plants made of?
o Primarily carbohydrates such as cellulose, sucrose,
fructose, etc.
o Carbohydrates are made of carbon, oxygen, and
hydrogen.
• What would be the source of each of these
elements for plants?
o Hydrogen: Water
o Oxygen: Water
o Carbon: ..?
Photosynthesis
• Photo = “light”, Synthesis “to make”
• Photosynthesis is using light energy to make
organic compounds such as sugars.
• Autotrophs are able to produce the molecules
they need for life without eating anything.
o Photoautotrophs use sunlight as their energy source.
o Chemoautotrophs use non-living chemicals (like
Hydrogen sulfide gas) as their energy source
• Almost all plants are photoautotrophs.
o Also includes algae, some protozoa, and some
bacteria.
LE 10-2
Plants
Unicellular protist 10 µm
Purple sulfur
bacteria
Multicellular algae
Cyanobacteria
40 µm
1.5 µm
• Heterotrophs obtain their organic material by
eating other organisms
• Almost all heterotrophs, including humans,
depend on photoautotrophs like plants for
food and oxygen
Energy in Sunlight
• Energy from the sun travels to Earth in the form
of light.
• Sunlight is a mixture of many different types of
energy:
o Ultraviolet: Invisible to us, causes sunburns
o Visible Light: Wavelengths of light we can see,
o Infrared: Energy in the form of heat
Energy
• Our eyes see the different wavelengths of the visible
spectrum as different colors: red, orange, yellow,
green, blue, indigo, and violet.
Pigments
• Plants gather the sun’s energy
with light-absorbing molecules
called pigments.
• The plants’ principal pigment is
chlorophyll.
o Chlorophyll is a green pigment.
o Plants are green because chlorophyll
reflects green light and absorbs every
other wavelength.
Pigments
• There are two types of chlorophyll found in
plants, chlorophyll a and chlorophyll b.
• Chlorophyll absorbs blue-violet and red light
very well, but not green.
o Remember, green light is reflected, and not absorbed.
Measuring Light
Absorption
• A spectrophotometer measures a pigment’s
ability to absorb various wavelengths
• This machine sends light through pigments
and measures the fraction of light transmitted
at each wavelength
LE 10-8a
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
Galvanometer
0
Slit moves to
pass light
of selected
wavelength
Green
light
100
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
LE 10-8b
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
0
Slit moves to
pass light
of selected
wavelength
Blue
light
100
The low transmittance
(high absorption)
reading indicates that
chlorophyll absorbs
most blue light.
• An absorption spectrum is a graph plotting a
pigment’s light absorption versus wavelength
• The absorption spectrum of chlorophyll a
suggests that violet-blue and red light work
best for photosynthesis
LE 10-9a
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
400
500
600
Wavelength of light (nm)
Absorption spectra
700
Pigments
• Plant cells contain other pigments besides
chlorophyll that increase the wavelengths
absorbed.
o These are called carotenoids.
• During the summer, so much chlorophyll is
produced that the green color overwhelms
the other pigments.
• When temperatures drop, the plants stop
producing chlorophyll, and the other
pigments may be seen.
Chloroplasts
• Photosynthesis takes place inside organelles
called chloroplasts.
• Chloroplasts contain stacks called grana.
• The grana contained stacked membranes
called thylakoids, which are interconnected.
Chloroplasts
• Leaves are the major locations of
photosynthesis
• Their green color is from chlorophyll, the green
pigment within chloroplasts
• Light energy absorbed by chlorophyll drives
the reactions needed to produce sugars from
carbon dioxide.
• The plant “breathes” through microscopic
pores called stomata.
o CO2 enters the leaf and O2 exits
Chloroplasts
• Pigments are located in the thylakoid
membranes.
• The fluid portion outside of the thylakoids is
known as the stroma.
Photosynthesis Equation
• Photosynthesis can be summarized in the
following equation:
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
Carbon
dioxide
Water
Sunlight
Glucose
Oxygen
Water
(Less)
LE 10-3
Leaf cross section
Vein
Mesophyll
Stomata
CO2 O2
Mesophyll cell
Chloroplast
5 µm
Outer
membrane
Thylakoid
Thylakoid
Stroma Granum
space
Intermembrane
space
Inner
membrane
1 µm
LE 10-4
Products:
12 H2O
6 CO2
Reactants:
C6H12O6
6 H2O
6 O2
Stages of Photosynthesis
• Photosynthesis consists of the light reactions
(the photo part) and Calvin cycle (the
synthesis part)
• The light reactions occur in the thylakoids of
the chloroplast.
o Splits water, releases O2, produces ATP and NADPH
• The Calvin cycle occurs in the stroma of the
chloroplast.
o Forms sugar from CO2 using ATP and NADPH
LE 10-5_1
H2O
Light
LIGHT
REACTIONS
Chloroplast
LE 10-5_2
H2O
Light
LIGHT
REACTIONS
ATP
NADPH
Chloroplast
O2
LE 10-5_3
H2O
CO2
Light
NADP+
ADP
+ Pi
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
ATP and NADPH
• Chloroplasts are solar-powered chemical
factories
• Their thylakoids transform light energy into the
chemical energy of ATP and NADPH.
o These are small energy-containing molecules that can
be used to make glucose later.
LE 10-7
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Absorption of Sunlight
• When chlorophyll absorbs light, it goes from a
low-energy ground state to an high-energy
excited state, which is unstable.
• When excited electrons fall back to the
ground state, photons are given off causing
fluorescence.
LE 10-11
e–
Excited
state
Heat
Photon
Chlorophyll
molecule
Photon
(fluorescence)
Ground
state
Excitation of isolated chlorophyll molecule
Fluorescence
The Photosystem
• The basic unit of photosynthesis in the
thylakoid is called a photosystem.
• A photosystem contains a reaction center
surrounded by light-harvesting complexes
• The light-harvesting complexes (pigment
molecules) funnel the energy from photons of
sunlight to the reaction center.
• The reaction center contains chlorophyll,
which absorbs the energy from the photon.
• This splits a water molecule into O2 , 2 H+ ions, and 2
electrons.
• These electrons are energized and passed onto another
molecule called the primary electron acceptor.
LE 10-12
Thylakoid
Photosystem
Photon
Thylakoid membrane
Light-harvesting
complexes
Reaction
center
STROMA
Primary electron
acceptor
e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
• There are two types of photosystems in the
thylakoid membrane:
• Photosystem II absorbs wavelengths of sunlight 680nm
long.
• Photosystem I then absorbs wavelengths of sunlight
700nm long.
• The two photosystems work together to use
light energy to generate ATP and NADPH
LE 10-13_1
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
e–
Light
P680
Photosystem II
(PS II)
LE 10-13_2
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Energy of electrons
Primary
acceptor
2
H+
1/ 2
+
O2
Light
H2O
e–
e–
e–
P680
Photosystem II
(PS II)
LE 10-13_3
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Energy of electrons
Pq
2 H+
+
1/ 2 O 2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P680
ATP
Photosystem II
(PS II)
LE 10-13_4
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Energy of electrons
Pq
2
H+
1/ 2
+
O2
Light
H2O
e–
Cytochrome
complex
Pc
e–
e–
P700
P680
Light
ATP
Photosystem II
(PS II)
Photosystem I
(PS I)
LE 10-13_5
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
e–
Pq
Energy of electrons
2
H+
e–
H2O
Cytochrome
complex
+
1/2 O2
Light
Fd
e–
e–
NADP+
reductase
Pc
e–
e–
NADPH
+ H+
P700
P680
Light
ATP
Photosystem II
(PS II)
NADP+
+ 2 H+
Photosystem I
(PS I)
LE 10-14
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Photosystem II
Photosystem I
LE 10-17
H2 O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Light
2
Photosystem I
Light
NADP+
reductase
H+
NADP+ + 2H+
Fd
NADPH + H+
Pq
H2O
THYLAKOID SPACE
(High H+ concentration)
1/2
Pc
O2
+2 H+
2 H+
To
Calvin
cycle
Thylakoid
membrane
STROMA
(Low H+ concentration)
ATP
synthase
ADP
+
Pi
ATP
H+
Building Glucose
• The Calvin cycle builds sugar from smaller
molecules by using ATP and NADPH
• Carbon enters the cycle as CO2 and leaves as
a sugar named glyceraldehyde-3-phospate
(G3P)
o To make one G3P, the cycle must take place three
times, using up three molecules of CO2
• The Calvin cycle has three phases:
o Three atoms of carbon from carbon dioxide
are added to the cycle using an enzyme
called rubisco.
• This creates a 6-carbon molecule
o ATP and NADPH is used to create two
molecules of G3P
Play
• One leaves the cycle, one stays behind
o The original molecules in the cycle are then
regenerated using more ATP
LE 10-18_1
H2 O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
Short-lived
intermediate
P
P
6
3-Phosphoglycerate
3 P
P
Ribulose bisphosphate
(RuBP)
6
6 ADP
CALVIN
CYCLE
ATP
LE 10-18_2
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
CALVIN
CYCLE
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
6 NADP+
6 Pi
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
LE 10-18_3
H2O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
6
P
3-Phosphoglycerate
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
3 ADP
3
CALVIN
CYCLE
6 P
ATP
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+
6 Pi
P
5
G3P
6
P
Glyceraldehyde-3-phosphate
(G3P)
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
Adaptations in Arid
Environments
• Dehydration is a problem for plants, especially
in hot, arid ecosystems.
• On hot, dry days, plants close their stomata,
which conserves water but also limits
photosynthesis.
o Plants are unable to take in CO2 and remove O2.
• These conditions favor a seemingly wasteful
process called photorespiration.
Photorespiration: An
Evolutionary Relic?
• In photorespiration, O2 is added to the Calvin
cycle instead of CO2
• This produces a molecule that must be sent to
the mitochondria before it can be sent back
and the Calvin cycle finished.
o This uses more energy to produce G3P, and is much less
efficient for the plant.
• Photorespiration may be an evolutionary relic
because rubisco first evolved at a time when
the atmosphere had far less O2 and more CO2
• In many plants, photorespiration is a problem
because on a hot, dry day it can drain much
of the plant’s ATP and NADPH.
C4 Plants
• Some plants have an adaptation to manage
life in arid climates. These are called C4
plants.
o Example: Sugar cane, corn
• These plants minimize the cost of
photorespiration by incorporating CO2 into
four-carbon compounds and storing them in
areas of the leaf less exposed to the dry air.
• These four-carbon compounds can be used
to release carbon dioxide when the stomata
are closed, allowing the Calvin cycle to
continue like normal.
CAM Plants
• CAM plants open their stomata at night,
incorporating CO2 into organic acids
• Stomata close during the day, and CO2 is
released from organic acids and used in the
Calvin cycle
LE 10-20
Sugarcane
Pineapple
CAM
C4
CO2
Mesophyll
cell
Organic acid
Bundlesheath
cell
CO2
CO2 incorporated
into four-carbon Organic acid
organic acids
(carbon fixation)
CO2
CALVIN
CYCLE
Sugar
Spatial separation of steps
CO2
Organic acids
release CO2 to
Calvin cycle
Night
Day
CALVIN
CYCLE
Sugar
Temporal separation of steps