Download RACC BIO Photosynthesis

PHOTOSYNTHESIS
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: The Process That Feeds the Biosphere
• Photosynthesis is the process that converts solar
energy (light energy from the sun) into chemical
energy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Autotrophs – self feeders
• Autotrophs sustain themselves without eating
anything derived from other organisms
• Producers
• Almost all plants are photoautotrophs, using the
energy of sunlight to make organic molecules
from water and carbon dioxide (inorganic)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Producers of the biosphere
• Photosynthesis occurs in plants, algae, certain
other protists, and some prokaryotes
• These organisms feed not only themselves but
also the entire living world
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Examples of Producers
Plants
Unicellular protist 10 µm
Purple sulfur
bacteria
Multicellular algae
Cyanobacteria
40 µm
1.5 µm
Heterotrophs - Consumers
• Heterotrophs obtain their organic material from
other organisms
• Almost all heterotrophs, including humans,
depend on photoautotrophs for food and oxygen
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Basics of Photosynthesis
• Chloroplasts: Sites of Photosynthesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-3
Photosynthesis takes place in the chloroplasts/found in
several organisms
Leaf cross section
Vein
1.Leaves are the
major location for
photosynthesis
Mesophyll
2.Green color is
from chlorophyllthe pigment in
chloroplasts –
absorbs light
energy
Stomata
CO2 O2
3.Through microscopic pores
(stomata) CO2 enters and O2
leaves (stoma means “mouth”)
Mesophyll cell
Chloroplast
4.Chloroplasts are found
mainly in cells of the
mesophyll, the interior
tissue of the leaf.
•6. In the inner
membrane encloses
a second
compartment which
is filled with thick
fluid called stroma.
•Suspended in the
stroma are the
thylakoids, which
contains the
thylakoids space.
•Thylakoids can be
concentrated in
stacks called grana
5 µm
•A typical mesophyll has
30-40 chloroplasts
Outer
membrane
Thylakoid
Thylakoid
Stroma Granum
space
Intermembrane
space
Inner
membrane
1 µm
•5.The chlorophyll is in the
membranes of thylakoids
Equation of Photosynthesis
• Photosynthesis can be summarized as the
following equation:
6 CO2 + 6H2O + Light energy  C6H12O6 + 6 O2
What do you notice about this equation compared to
respiration?
Requires a lot of energy that comes from sunlight
•
Chloroplasts split water into hydrogen and oxygen, incorporating the electrons
of hydrogen into sugar molecules, releasing oxygen.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosynthesis is a Redox Process, like cellular
respiration
• Photosynthesis is a redox process in which water
is oxidized and carbon dioxide is reduced
• The redox process takes the form of hydrogen
transfer from water to CO2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosynthesis Equation
•
Water molecules are split apart, yielding O2, and are oxidized. They
lose electrons along with hydrogen ions.
•
CO2 is reduced to sugar as electrons and hydrogen ions are added to
it.
•
The food-producing redox reactions of photosynthesis involve an
uphill climb
•
The light energy captured by chlorophyll molecules in the chloroplast
provides the boost for the electrons.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Two Stages of Photosynthesis: A Preview
• Photosynthesis consists of the light reactions (the
photo part) and Calvin cycle (the synthesis part)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-5_1
The Light Reactions – converts
solar energy to chemical energy
Take place in the
thylakoids
H2O
Split water
Light
Light energy is absorbed by
chlorophyll and drives the
transfer of electrons and
hydrogen from water to the
electron acceptor….
LIGHT
REACTIONS
Chloroplast
LE 10-5_2
NADPH is an electron
carrier.
H2O
Light
Electron acceptor is NADP+
LIGHT
REACTIONS
ATP is formed
ATP
NADPH
Reduced to
NADPH
Stores energized
electrons
Chloroplast
O2
Oxygen is released
LE 10-5_3
The Calvin Cycle
•The Calvin cycle begins with carbon fixation,
incorporating CO2 into organic molecules
H2O
CO2
Light
NADP+
ADP
+ Pi
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
[CH2O]
(sugar)
•The Calvin cycle (in
the stroma) forms
sugar from CO2, using
ATP and NADPH
Concept 10.2: The light reactions convert solar
energy to the chemical energy of ATP and NADPH
• Chloroplasts are solar-powered chemical factories
• Their thylakoids transform light energy into the
chemical energy of ATP and NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Nature of Sunlight
• Sunlight is a form of electromagnetic energy, also
called radiation
• Like other electromagnetic energy, light travels in
rhythmic waves
• Wavelength = distance between crests of waves
• Light also behaves as though it consists of
discrete particles, called photons
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Visible radiation
• The electromagnetic spectrum is the entire range
of electromagnetic energy, or radiation
• Visible light consists of colors we can
see, including wavelengths that drive
photosynthesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-6
The amount of energy in a photon is inversely related to its wavelength
10–5 nm 10–3 nm
Gamma
rays
1 nm
X-rays
103 nm
UV
Infrared
1m
(109 nm)
106 nm
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
Photosynthetic Pigments: The Light Receptors
• Pigments are substances that absorb visible light
• Different pigments absorb different wavelengths
• Wavelengths that are not absorbed are reflected
or transmitted
• Leaves appear green because chlorophyll reflects
and transmits green light
Animation: Light and Pigments
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-7
Light
Reflected
light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Chloroplast Pigments
• Chlorophyll a is the main photosynthetic pigment
– Participates directly in the light reactions
• Accessory pigments, such as chlorophyll b,
– Absorbs mainly blue and orange light (reflects
yellow-green)
• Broadens the range of light that a plant can use
• Accessory pigments called carotenoids absorb
excessive light that would damage chlorophyll
– Absorb mainly blue-green light (reflect yelloworange)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Excitation of Chlorophyll by Light
• Light behaves as discrete packets of energy called
photons
– A photon is a fixed quantity of light energy
• When a pigment absorbs a photon, the electron
has been raised from a ground state to an excited
state, which is very unstable
• When excited electrons fall back to the ground
state, photons are given off (electrons lose
energy), and emit an afterglow called fluorescence
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A Photosystem: A Reaction Center Associated with
Light-Harvesting Complexes
•
A photosystem consists of a reaction center surrounded by lightharvesting complexes
•
The light-harvesting complexes (pigment molecules bound to proteins)
funnel the energy of photons to the reaction center
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosystem
• A primary electron acceptor in the reaction center accepts
an excited electron from chlorophyll a
• Solar-powered transfer of an electron from a chlorophyll a
molecule to the primary electron acceptor is the first step of
the light reactions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-12
Thylakoid
Photosystem
Photon
Light-harvesting
complexes
Cluster of pigmented
molecules functions as a
light-gathering antenna.
Photons jump from
pigment to pigment and
arrive at the reaction
center
Primary electron
acceptor
The reaction center
chlorophyll a molec
and a primary elect
acceptor molecule
Thylakoid membrane
Pigment molecules
include chlorophyll a
and b.
Reaction
center
STROMA
e–
Acceptor traps li
excited electron
reaction center
chlorophyll.
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
con
The Two Photosystems in the light reactions
• Water Splitting (photosystem II)
– Uses light energy to extract electrons from
water
– Releases O2 as a waste product
• NADPH producing (photosystem I)
– Produces NADPH by transferring lightexcited electrons from chlorophyll to NADP+
• An electron transport chain connecting the two
systems releases energy that the chloroplast
uses to make ATP
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-14
ATP
e–
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
PowerPoint Lectures for
–
Biology, SeventheEdition
Neil Campbell and Jane Reece
Water splitting
system
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
NADPH producing
system
Generating ATP in the Light Reactions
Light reactions take place in the thylakoid
membrane
• The mechanism of ATP production is similar to
ATP production in cellular respiration
• An electron transport chain pumps hydrogen
ions (H+) across the thylakoid membrane
• ATP synthases use the energy stored by the
H+ gradient to make ATP
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Light Reactions
•
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Electrons are stored at a high
state of potential energy in
NADPH. This and ATP will
move on to the Calvin Cycle
Review of light reactions
http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Calvin cycle uses ATP and NADPH to convert
CO2 to sugar
• The Calvin cycle, like the citric acid cycle,
regenerates its starting material after molecules
enter and leave the cycle
• The cycle builds sugar from smaller molecules by
using ATP and the reducing power of electrons
carried by NADPH. It requires more ATP than
NADPH
• Carbon enters the cycle as CO2 and leaves as a
sugar named glyceraldehyde-3-phospate (G3P)
• For net synthesis of one G3P, the cycle must take
place three times, fixing three molecules of CO2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Calvin Cycle
• The Calvin cycle has three phases:
– Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor (RuBP)
• Takes place inPlay
the stroma
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-18_1
Phase 1: Carbon fixation
H2 O
CO2
Input
Light
(Entering one
CO2 at a time)
3
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
CO2 is added to a five-C sugar. The
reaction is catalyzed by the enzyme
Rubisco
ATP
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
Short-lived
intermediate
3 P
P
Ribulose bisphosphate
(RuBP)
splits into 2
molecules of
P
P
6
3-Phosphoglycerate
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
Each molecule of 3… is
then phosphorylated by
ATP to form
2 electrons
from NADPH
1,3-Bisphosphoglycerate
reduce this
6 NADPH
compound to
6 NADP+
create
6 P
6 P
P
i
6
P
Glyceraldehyde-3-phosphate
(G3P)
The cycle must turn 3
times to create a net gain
of one molecule of G3P glucose
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
The rearrangement
of five molecules of
G3P into the 3
molecules of RuBP
requires 3 more
ATP
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)
9 molecules of ATP and 6 of
NADPH are required to
synthesize one G3P
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
Calvin Cycle review
•
http://www.science.smith.edu/departments/Biology/Bio231/calvin.html
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
C4 and CAM plants
• All photosynthetic plants need CO2 to build sugar
– CO2 binds to RuBP (in Calvin Cycle)
– This is called the C3 pathway because it produces 2 –
3carbon molecules that continue in the cycle
• The enzyme that catalyzes this reaction is RUBISCO
– Rubisco is not efficient at grabbing CO2
– If CO2 levels fall too low, rubisco grabs O2 and sugars
are burned up.
– This process is called photorespiration
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photorespiration - problems
• Problem during hot, dry days – or in these
climates, plants keep stomates closed to prevent
water loss
• Different systems evolved to deal with this
problem
– So stomates don’t need to be open too often
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Comparing C4 and CAM plants
C4
•
CAM
Each use a different enzyme to bind CO2
Harvest more CO2 without opening stomates too often
Enzyme is PEP carboxylase instead of Rubisco (4 –C-molecule, hence the name)
Forms OAA (oxaloacetate)
CO2 comes from this to use in the Calvin Cycle
During the day
•
Takes place in mesophyll cells
•
OAA pumped to bundle sheath
(which are other cells)
•
CO2 is then released to Calvin
Cycle
•
•
Happens at night
•
OAA stored in vacuoles within
the cell
•
Stomates can be open in
evening when cooler avoiding
water loss
•
CO2 released to Calvin Cycle
in day and driven by suns
energy
•
More common: ex: cacti and
succulents
Ex. Corn, sugar cane, tropical
grass
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
No structural
separation like C4
Night plants: instead, the
two processes are
separated in time.
Day
CALVIN
CYCLE
Sugar
Temporal separation of steps
The Importance of Photosynthesis: A Review
• The energy entering chloroplasts as sunlight gets
stored as chemical energy in organic compounds
• Sugar made in the chloroplasts supplies chemical
energy and carbon skeletons to synthesize the
organic molecules of cells
• In addition to food production, photosynthesis
produces the oxygen in our atmosphere
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 10-21
Light reactions
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+ Pi
RuBP
Photosystem II
Electron transport
chain
Photosystem I
ATP
NADPH
3-Phosphoglycerate
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
O2
Sucrose (export)