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Transcript
Photosynthesis:
Using Light to make Food
OVERALL, PHOTOSYNTHESIS...
SUNLIGHT (light energy)
6 CO2 + 6 H2O  C6H12O6 + 6O2
Capturing Free Energy without Light
Photoautotrophs- organism use sunlight as the source of
energy to drive photosynthesis and produce carbohydrates.
Chemoautotrophs- prokaryotes that use inorganic
substances (hydrogen sulfide, ammonia, nitrite) as a source
of energy.
Example:
CO2 + 4 H2S + O2
CH2O + 4 S + 3 H2O
Important details….
• Light is required.
It is the source of energy.
• The reaction takes place in the chloroplast
of the plant cell.
• The chloroplast is filled with green pigment
called chlorophyll.
PHOTOSYNTHESIS TAKES PLACE IN...
CHLOROPLASTS
Photosynthesis occurs in chloroplasts
in plant cells
 Chloroplasts are the major sites of photosynthesis
in green plants.
 Chlorophyll
– is an important light-absorbing pigment in chloroplasts,
– is responsible for the green color of plants, and
– plays a central role in converting solar energy to
chemical energy.
Chloroplast – Internal Structure
Pigments are located in the membranes of the thylakoids.
Photosynthesis occurs in chloroplasts
in plant cells
 Thylakoids
– Function analogous to the inter-membrane space of
mitochondria (cellular respiration).
– Contain needed enzymes and other proteins.
– Chlorophyll molecules are built into the thylakoid
membrane and capture light energy.
Photosynthesis occurs in chloroplasts
in plant cells
 Stoma (singular), Stomata (plural)
• Guard cells contain chlorophyll.
• Most stomata are on the lower epidermis.
• Lower concentrations of stomata indicate lower rates of
photosynthesis and growth or adaptations for dry weather.
Essential Leaf Parts for Photosynthesis
Leaf Part
Chloroplasts
Veins
Stomata
Structure
Function
Photosynthesis is a redox process,
as is cellular respiration
 Photosynthesis, like respiration, is a redox
(oxidation-reduction) process.
– CO2 becomes reduced to sugar as electrons along with
hydrogen ions from water are added to it.
– Water molecules are oxidized when they lose electrons
along with hydrogen ions.
Photosynthesis is a redox process,
as is cellular respiration
 Cellular respiration uses redox reactions to harvest
the chemical energy stored in a glucose molecule.
– This is accomplished by oxidizing the sugar and
reducing O2 to H2O.
– The electrons lose potential energy as they travel down
the electron transport chain to O2.
– The food-producing redox reactions of photosynthesis
require energy.
Compare Cellular Respiration with Photosynthesis
Becomes oxidized
Becomes reduced
Photosynthesis is a redox process,
as is cellular respiration
 In photosynthesis,
– light energy is captured by chlorophyll molecules to
boost the energy of electrons,
– light energy is converted to chemical energy, and
– chemical energy is stored in the chemical bonds of
sugars.
Go to…….. Photosynthesis Animation
Overview: The two stages of photosynthesis are
linked by ATP and NADPH
 Photosynthesis occurs in two metabolic stages.
1. The light reactions occur in the _________ ________.
– _______ is split, providing a source of electrons and giving
off oxygen as a by-product,
– ______ is generated from ADP and a phosphate group, and
– ________energy is absorbed by the chlorophyll molecules to
drive the transfer of electrons and H+ from water to the
electron acceptor NADP+ reducing it to NADPH.
– NADPH produced by the light reactions provides the
electrons for reducing carbon in the Calvin cycle.
Overview: The two stages of photosynthesis are
linked by ATP and NADPH
2. The second stage is the Calvin cycle, which occurs in
the ______________ of the chloroplast.
– The Calvin cycle is a cyclic series of reactions that
assembles ______________ molecules using CO2 and the
energy-rich products of the light reactions.
– During the Calvin cycle, ____________ is incorporated into
organic compounds in a process called carbon fixation.
– After carbon fixation, enzymes of the cycle make _________
by further reducing the carbon compounds.
– The Calvin cycle is often called the dark reactions or lightindependent reactions, because none of the steps requires
light directly.
H2O
CO2
Light
NADP+
ADP
P
Calvin
Cycle
(in stroma)
Light
Reactions
(in thylakoids)
ATP
NADPH
Chloroplast
O2
Sugar
An overview of the two stages of photosynthesis in a chloroplast
Visible radiation absorbed by pigments drives
the light reactions
 Sunlight contains energy called electromagnetic
energy or electromagnetic radiation.
– Visible light is only a small part of the electromagnetic
spectrum, the full range of electromagnetic
wavelengths.
– Electromagnetic energy travels in waves, and the
wavelength is the distance between the crests of two
adjacent waves.
Light, microwaves, x-rays, and TV
and radio transmissions are all
kinds of electromagnetic waves.
They are all the same kind of wavy
disturbance that repeats itself over
a distance called the wavelength.
One thing that all the forms of
electromagnetic radiation have in
common is that they can travel
through empty space. This is not
true of other kinds of waves; sound
waves, for example, need some
kind of material, like air or water, in
which to move.
Visible Light
is
380-750nm
Increasing energy
105 nm 103 nm
Gamma
rays
X-rays
103 nm
1 nm
UV
106 nm
Infrared
103 m
1m
Microwaves
Radio
waves
Visible light
380 400
500
600
Wavelength (nm)
700
650
nm
750
Relate wavelength of light to color and energy
Visible Light
light travels in waves
shorter
waves
as wavelength increases, energy decreases
more energy
Remember:
•white light separates into color when going thru prism or water droplets
•energy travels in waves
•wavelength refers to distance between crest of two waves
•short wavelengths have more energy, wavelengths below 500nm breaks
chemical bonds in DNA and proteins
•Wavelength
Energy
longer
waves
less energy
Visible radiation absorbed by pigments drives
the light reactions
We see the color of the wavelengths that are transmitted.
For example, chlorophyll transmits green wavelengths.
Visible radiation absorbed by pigments drives
the light reactions
 Chloroplasts contain several different pigments,
which absorb light of different wavelengths.
– _____________absorbs blue-violet and red light and
reflects green.
– _____________absorbs blue and orange and reflects
yellow-green.
– _____________and other pigments
– broaden the spectrum of colors that can drive photosynthesis
– Some provide photoprotection, absorbing and dissipating
excessive light energy that would otherwise damage
chlorophyll
© 2012 Pearson Education, Inc.
Accessory PigmentsAbsorb light that chlorophyll a and b cannot!
Leaves have different pigments.
When (green) chlorophyll begins to breakdown due
to colder temperatures and shorter days,
other pigments become visible.
Reactions of Photosynthesis
2 Reaction Stages (sets)
light dependent reactions
sun energy
chemical energy
light independent reactions
or Calvin Cycle
carbon fixation or the synthesis of glucose
Photosystems capture solar energy
 Pigments in chloroplasts absorb photons (unit of
solar power), which
–
increases the potential
energy of the pigment’s
electrons and
sends the electrons
into an unstable state.
–
These unstable electrons
drop back down to their “ground state,”
and as they do, release their excess
energy as heat.
Cyclic Photophosphorylation
Animation- first part- Cyclic
A light-excited pair of chlorophyll molecules in the reaction center of a photosystem
passing an excited electron to a primary electron acceptor (it becomes reduced)
Photosystem
Reaction-center
complex
Light-harvesting
complexes
Thylakoid membrane
Light
Transfer
of energy
Pair of
chlorophyll a molecules
Primary electron
acceptor
Pigment
molecules
Noncyclic Photophosphorylation
 Two types of photosystems (photosystem I and
photosystem II) cooperate in the light reactions.
 Each type of photosystem has a characteristic
reaction center.
– Photosystem II, which functions first, is called P680
because its pigment absorbs light with a wavelength of
680 nm.
– Photosystem I, which functions second, is called P700
because it absorbs light with a wavelength of 700 nm.
Noncyclic Photophosphorylation-Two photosystems
connected by an electron transport chain generate
ATP and NADPH
 In the light reactions, light energy is transformed
into the chemical energy of ATP and NADPH.
 To accomplish this, electrons are
– removed from water,
– passed from photosystem II to photosystem I, and
– accepted by NADP+, reducing it to NADPH.
 Between the two photosystems, the electrons
– move down an electron transport chain and
– provide energy for the synthesis of ATP.
Noncyclic Photophosphorylation-Two photosystems
connected by an electron transport chain generate
ATP and NADPH
Light
Photosystem II
Stroma
Electron transport chain
Provides energy for
synthesis of ATP
by chemiosmosis
NADP  H
Light
Photosystem I
1
Primary
acceptor
Thylakoid membrane
Primary
acceptor
2
4
P700
P680
Thylakoid
space
3
H2O
1
2
5
O2  2 H
6
NADPH
Chemiosmosis powers ATP synthesis in the
light reactions
 Chemiosmosis is the mechanism that
generates ATP in chloroplasts.
 In photophosphorylation, using the initial energy
input from light,
– the electron transport chain pumps H+ into the
thylakoid space, and
– the resulting concentration gradient drives H+
back through ATP synthase, producing ATP.
3 VERY Important Concepts
• How is oxygen produced in the light
dependent reaction?
• Energy carried by the e- is captured in
_____________.
• Chemiosmosis produces __________
Nicotinamide Adenine Dinucleotide Phosphate
In chloroplasts, NADP is reduced in last step of the
electron chain of the light dependent reactions of
photosynthesis. The NADPH formed is used in the light
independent reactions.
Noncyclic Photophosphorylation
Dependent Reaction Steps
1.
2.
3.
4.
5.
ATP and NADPH power sugar synthesis in the
Calvin cycle (does not need light)
 The Calvin cycle makes sugar within a chloroplast.
 To produce sugar, the necessary ingredients are
1.
2.
3.
 The Calvin cycle produces an energy-rich, threecarbon sugar called glyceraldehyde-3-phosphate
(G3P). A plant cell then uses G3P to make glucose.
Step
1
Carbon fixation
Input:
3
CO2
Rubisco
1
3 P
6
P
RuBP
Step 2 Reduction
P
3-PGA
6
ATP
3 ADP
6 ADP
Step 3 Release of one
molecule of G3P
Calvin
Cycle
4
3 ATP
4
Regeneration of RuBP
6 NADPH
6 NADP
5
P
6
P
G3P
Step
2
G3P
3
Output: 1
P
G3P
Glucose
and other
compounds
P
Calvin-Benson Cycle animation
http://faculty.nl.edu/jste/calvin_cycle.htm
Noncyclic Photophosphorylation
Light inDependent Reaction Steps
1.
2.
3.
4.
5.
Different Carbon Fixing Pathways
• The carbon fixing pathway previously
described is more accurately called the C3
pathway due to the first stable compounds
formed in the Calvin-Benson Cycle being 2, 3carbon molecules (PGA)
• Different environments have adapted a different
pathway.
_____________: high daytime temperatures, high
intensity of sunlight, ex.- corn, sugarcane
_____________: high daytime temperatures, intense
sunlight, low soil moisture, ex.- cacti, pineapples
EVOLUTION CONNECTION: Other methods of
carbon fixation have evolved in hot, dry
climates
 Most plants use CO2 directly from the air, and
carbon fixation occurs when the enzyme rubisco
adds CO2 to RuBP.
 Such plants are called C3 plants because the first
product of carbon fixation is a _______-carbon
compound, 3-PGA.
EVOLUTION CONNECTION:
Other methods of carbon fixation
 In hot and dry weather, C3 plants
– close their stomata to reduce water loss but
– Problem is…._________________________________
– As O2 builds up in a leaf, rubisco adds O2 instead of
CO2 to RuBP, and a two-carbon product of this reaction
is then broken down in the cell.
– This process is called __________________ because it
occurs in the light, consumes O2, and releases CO2.
– But unlike cellular respiration, it uses ATP instead of
producing it.
Photorespiration
• If oxygen levels are high, Calvin’s cycle
slows. Less glucose is produced and CO2
is actually released from the cycle instead
of being fixed by the cycle.
• Why does O2 build up in leaves?
•
•
•
EVOLUTION CONNECTION:
Other methods of carbon fixation
 C4 plants have evolved a means of
– carbon fixation that saves water during photosynthesis
while
– optimizing the Calvin cycle.
 C4 plants are so named because they first fix CO2
into a four-carbon compound.
 When the weather is hot and dry, C4 plants keep
their stomata mostly closed, thus conserving water.
EVOLUTION CONNECTION:
Other methods of carbon fixation
 Another adaptation to hot and dry environments
has evolved in the CAM plants, such as
pineapples and cacti.
 CAM plants conserve water by opening their
stomata and admitting CO2 only at night.
 CO2 is fixed into a four-carbon compound,
– which banks CO2 at night and
– releases it to the Calvin cycle during the day.
Mesophyll
cell
Bundlesheath
cell
CO2
4-C compound
4-C compound
CO2
CO2
Calvin
Cycle
Calvin
Cycle
3-C sugar
C4 plant
Sugarcane
CO2
Night
3-C sugar
Day
CAM plant
Pineapple
Outline of C4 and CAM Pathways
• http://users.rcn.com/jkimball.ma.ultranet/Bi
ologyPages/C/C4plants.html
Review: Photosynthesis uses light energy, carbon
dioxide, and water to make organic molecules
 Most of the living world depends on the foodmaking machinery of photosynthesis.
 The chloroplast
– integrates the two stages of photosynthesis and
– makes sugar from CO2.
Review: Photosynthesis uses light energy, carbon
dioxide, and water to make organic molecules
 About half of the carbohydrates made by
photosynthesis are consumed as fuel for cellular
respiration in the mitochondria of plant cells.
 Sugars also serve as the starting material for making
other organic molecules, such as proteins, lipids, and
cellulose.
 Excess food made by plants is stockpiled as starch
in roots, tubers, seeds, and fruits.
H2O
CO2
Chloroplast
Light
NADP
Light
Reactions
ADP
P
RuBP
Calvin
Cycle 3-PGA
(in stroma)
Photosystem II
Electron
transport chain
Thylakoids
Photosystem I
ATP
NADPH
O2
Stroma
G3P
Sugars
Cellular
respiration
Cellulose
Starch
Other organic
compounds
CONNECTION: Photosynthesis may moderate
global climate change
 The greenhouse effect operates on a global
scale.
– Solar radiation includes visible light that penetrates the
Earth’s atmosphere and warms the planet’s surface.
– Heat radiating from the warmed planet is absorbed by
gases in the atmosphere, which then reflects some of
the heat back to Earth.
– Without the warming of the greenhouse effect, the Earth
would be much colder and most life as we know it could
not exist.
CONNECTION: Photosynthesis may moderate
global climate change
 The gases in the atmosphere that absorb heat
radiation are called greenhouse gases. These
include
– water vapor,
– carbon dioxide
– and methane.
CONNECTION: Photosynthesis may moderate
global climate change
 Increasing concentrations of greenhouse gases
have been linked to global climate change (also
called global warming), a slow but steady rise in
Earth’s surface temperature.
 Since 1850, the atmospheric concentration of CO2
has increased by about 40%, mostly due to the
combustion of fossil fuels including
– coal,
– oil, and
– gasoline.
CONNECTION: Photosynthesis may moderate
global climate change
 Some predicted consequences of continued
warming include
– melting of polar ice,
– rising sea levels,
– extreme weather patterns,
– droughts,
– increased extinction rates, and
– the spread of tropical diseases.
CONNECTION: Photosynthesis may moderate
global climate change
 Widespread deforestation has aggravated the
global warming problem by reducing an effective
CO2 sink.
 Global warming caused by increasing CO2 levels
may be reduced by
– limiting deforestation,
– reducing fossil fuel consumption, and
– growing biofuel crops that remove CO2 from the
atmosphere.
SCIENTIFIC DISCOVERY: Scientific study of
Earth’s ozone layer has global significance
 Solar radiation converts O2 high in the atmosphere
to ozone (O3), which shields organisms from
damaging UV radiation.
 Industrial chemicals called CFCs
(chlorofluorcarbons) have caused dangerous
thinning of the ozone layer, but international
restrictions on CFC use are allowing a slow
recovery.
Ozone hole
Animations for Review (click on the links):
Calvin Cycle animation
Cyclic and Non-Cyclic Photophosphorylation- step by step
C4 and CAM photosynthesis
ALSO read CliffsNotes Chapter 5
You should now be able to
1. Define autotrophs, producers, and
photoautotrophs.
2. Describe the structure of chloroplasts and their
location in a leaf.
3. Explain how plants produce oxygen.
4. Describe the role of redox reactions in
photosynthesis and cellular respiration.
5. Compare the reactants and products of the light
reactions and the Calvin cycle.
You should now be able to
6. Describe the properties and functions of the
different photosynthetic pigments.
7. Explain how photosystems capture solar energy.
8. Explain how the electron transport chain and
chemiosmosis generate ATP, NADPH, and
oxygen in the light reactions.
9. Compare photophosphorylation and oxidative
phosphorylation.
10. Describe the reactants and products of the
Calvin cycle.
You should now be able to
11. Compare the mechanisms that C3, C4, and CAM
plants use to obtain and use carbon dioxide.
12. Review the overall process of the light reactions
and the Calvin cycle, noting the products,
reactants, and locations of every major step.
13. Describe the greenhouse effect.
14. Explain how the ozone layer forms, how human
activities have damaged it, and the
consequences of the destruction of the ozone
layer.