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CHAPTER 10
PHOTOSYNTHESIS
The human brain, so frail, so perishable, so full of inexhaustible
dreams and hungers, burns by the power of the leaf." Loren Eiseley, The Unexpected Universe
Photosynthesis
An anabolic, endergonic, carbon dioxide
(CO2) requiring process that uses light energy
(photons) and water (H2O) to produce organic
macromolecules (glucose).
6CO2 + 6H2O C6H12O6 + 6O2
glucose
SUN
photons
Autotrophs produce their organic
molecules from CO2 and other inorganic
raw materials
Plants and other autotrophs are the
producers of the biosphere
Photoautotroph
Cyanobacteria Protist- Euglena
Chemoautotroph – Purple
Sulphur Bacteria
Photoautotroph
Heterotrophs live on organic compounds
produced by other organisms.
Green organelle = chloroplasts
Half a million chloroplasts mm2 of leaf
Chlorophyll – is the green pigment
inside the chloroplasts.
Chloroplasts are the sites of
photosynthesis in plants
Elodea
Leaf Structure
2 layers of Mesophyll Cells
Stomata
Cuticle
Vein
Air Spaces
Epidermis
6H2O 6CO2 + C6H12O6 + 6O2
Leaf Structure
2 layers of Mesophyll Cells
Contain Chloroplasts
Stomata (CO2 enters, O2 exits)
Cuticle - prevents water loss
Vein (carries water to leaf
in xylem, and glucose away
from leaf in phloem)
Air Spaces
holds gases
Epidermis
(transparent, so light goes
through; makes cuticle)
6H2O 6CO2 + C6H12O6 + 6O2
SPONGY Mesophyll - glucose made here
Palisade Mesophyll - glucose
made here
Ques: Where will the
leaves be thin - top
layer or in the lower
layers of this forest?
Why?
Ques: Where will the
leaves have more
stomata - tropical rain
forest or desert? Why?
Fig. 10.2
Has Chlorophyll in
Membrane to trap light
energy and ETC + ATP
Synthase to make ATP
ATP is used in
the stroma to link
up C, O, and H to
form glucose by
Calvin’s cycle
THYLAKOID- LIGHT DEPENDENT REACTION
STROMA - LIGHT INDEPENDENT REACTION/CALVIN’S CYCLE
Thylakoid
Granum
Thylakoid Membrane has
Chlorophyll + Reaction
Centers that can absorb
light and “energize”
electrons. These electrons
are passed along an ETC to
make ATP
Thylakoid space involved
in creating a Proton
pH Gradient (Chemiosmosis!)
ADP + Pi ATP
H+ H+
H+ H+ H+
Source of Atoms in Photosynthesis
6CO2 + 12H2O + light -> C6H12O6 + 6O2 + 6H2O
Old Hypothesis:
Step 1: CO2 -> C + O2
Step 2: C + H2O -> CH2O
CO2 + H2O + light energy -> CH2O + O2
–CH2O -general formula for a sugar.
Actual:
Step 1: H2O -> 2H+ + 2e- + 1/2O2
Step 2: CO2 + 2H+ + 2e- -> CH2O
Photosynthesis is a redox reaction.
– It reverses the direction of electron flow in respiration.
Water is split and electrons transferred
with H+ (protons) from water to CO2,
reducing it to sugar.
Fig. 10.3
Photosynthesis – photo = light; synthesis
= making of sugar
The light reactions convert
to chemical NRG(ATP)
2 Parts of Photosynthesis are - The
Light reactions and the Calvin cycle
SOLAR ENERGY
CO2 The Calvin cycle incorporates from
the atmosphere
Calvin’s cycle uses energy from the light
reaction to convert the new carbon to
sugar - C6H12O6
Electron Donor
High Energy Electron
Acceptor
Thylakoids (Grana)
Contain Chlorophyll
Stroma
Photophosphorylation
The thylakoid chlorophyll convert light energy
into the chemical energy of ATP and
NADPH.
The light reactions : a closer look
When light meets matter, it may be
reflected, transmitted, or absorbed.
– Different pigments absorb photons of
different wavelengths.
Fig. 10.6
A spectrophotometer measures the
ability of a pigment to absorb various
wavelengths of light.
Absorption spectrum plots a pigment’s
light absorption vs wavelength
Fig. 10.7
– Chlorophyll a, the dominant pigment,
absorbs best in the red and blue
wavelengths, and least in the green.
Fig. 10.8a
Collectively, these photosynthetic pigments determine an
overall action spectrum - plots changes in
photosynthetic rate as light wavelength is changed
Engleman’s Exeriment: You
are the light of my life!
Fig. 10.8c
High O2 High O2
High O2 High O2
Blue and red light = more photosynthesis in algae because? ;
This means more oxygen in that part of the spectum;
This then implies that more bacteria will be supported in
blue/red areas.
When a molecule absorbs a photon, one
of that molecule’s electrons is elevated to
an orbital with more potential energy.
Chlorophyll is in the thylakoid membrane
In chlorophyll a and b, it is an electron from
magnesium in the porphyrin ring that is
excited by light.
Some pigments, including chlorophyll,
release a photon of light, in a process
called fluorescence, as well as heat.
Fig. 10.10
Chlorophyll is organized along with proteins and
smaller organic molecules into photosystems.
A photosystem acts like a light-gathering
“antenna complex” consisting of a few hundred
chlorophyll a, chlorophyll b, and carotenoid.
There are two types of photosystems.
Photosystem I has a reaction center
chlorophyll, the P700 center, that has an
absorption peak at 700nm.
Photosystem II has a reaction center with a
peak at 680nm.
During the light reactions, there are two
possible routes for electron flow: cyclic and
noncyclic.
Noncyclic electron flow, the predominant
route, produces both ATP and NADPH.
CO2
O2
carbohydrate end product (e.g., sucrose, starch, cellulose)
Light-Independent Reactions
glucose P
ADP + Pi ATP
NADPH NADP+
e–
H+
H+
H+ H+
H+
O
H+
H2O
SUNLIGHT
Overview of Photosynthesis
Stroma
PSII PSI ETC
Carbohydrate making phase
(light independent reaction)
ATP and NADPH making phase
(light dependent reaction)
inside a Thylakoid
ATP Synthase
Noncyclic Electron Flow
- Z scheme
P700
Photosystem I P680
Photosystem II
Primary
Electron
Acceptor
Primary
Electron
Acceptor
ETC
Enzyme
Reaction
H2O
1/2O2 + 2H+
ATP
NADPH
Photon
2e-
2e-
2e-
2e-
2e-
SUN
Photon
Noncyclic
photophosphorylation
Photon
Noncyclic Electron Flow
ADP + ATP
NADP+ + H NADPH
Oxygen comes from the splitting of
H2O, not CO2
H2O 1/2 O2 + 2H+
(Reduced)
P (Reduced
)
(Oxidized)
Fig. 10.12
Chemiosmosis
Proton Pump Powers ATP synthesis.
Located in the thylakoid membranes.
Uses Electron Transport Chain and ATP
synthase (enzyme) to make ATP.
Protons are pumped into thylakoid space
from stroma to form a gradient during
Light Reactions. When they flow back
through ATP synthase, ATP is made
Photophosphorylation: addition of
phosphate to ADP to make ATP using the
energy provided by light
Chloroplast
Granum Thylakoid
Stroma
Outer Membrane
Inner Membrane
Chemiosmosis - proton pumping
H+ H+
ATP Synthase
H+ H+ H+ H+
H+ H+ high H+
concentration
H+ ADP + P ATP
PS II PS I E
T C
low H+
concentration
H+
Thylakoid
SUN (Proton Pumping)
Thylakoid
Space
Stroma
Low pH
High pH
Fig. 10.13
The light reactions use the solar power of
photons absorbed by both
photosystem I and
photosystem II to
provide chemical
energy in the form
of ATP and reducing
power in the form
of the electrons
carried by NADPH.
Cyclic Photophosphorylation
Cyclic Electron Flow
P700
Primary
Electron
Acceptor
e-
e-
e-
e-
ATP
produced
by ETC
Photosystem I
Accessory
Pigments
SUN
Photons
Satisfy the higher demand for ATP in Calvin’s cycle
Fig. 10.16
Noncyclic electron flow pushes
electrons from water, where they
are at low potential energy, to
NADPH, where they have high
potential energy.
– This process also produces ATP.
– Oxygen is a byproduct.
Cyclic electron flow converts light
energy to chemical energy in the
form of ATP.
Calvin Cycle
Carbon Fixation (light independent rxn).
C3 plants (80% of plants on earth).
Occurs in the stroma.
Uses ATP and NADPH from light rxn.
Uses CO2. Fixes 1C per turn. In reality
Calvin’s makes a 3C compound (2 of them
join to form the 6C Glucose)
To produce glucose: it takes 6 turns and
uses 18 ATP and 12 NADPH.
Chloroplast
Granum Thylakoid
Stroma
Outer Membrane
Inner Membrane
Calvin Cycle (C3 fixation)
6CO2
6C-C-C-C-C-C
6C-C-C 6C-C-C
6C-C-C-C-C
12PGA
RuBP
12G3P
(unstable)
6NADPH 6NADPH
6ATP 6ATP
6ATP
C-C-C-C-C-C
Glucose
(6C)
(36C)
(36C)
(36C)
(30C)
(30C)
(6C)
6C-C-C 6C-C-C
C3
glucose
RUBISCO
Calvin Cycle
Remember: C3 = Calvin Cycle
Great under normal, cool, moist
conditions
C3
Glucose
Photorespiration
Occurs on hot, dry, bright days.
Stomates close = more O2 and less CO2
Fixation of O2 instead of CO2 because RUBISCO (first enzyme in Calvin’s cycle) acts as an OXYGENASE = acts different when oxygen concentration rises in a leaf cell.
Light reaction and Calvin’s cycle take place, BUT
Produces 2-C molecules (not glucose) instead of 3-C sugar molecules.
Produces no glucose molecules and uses up ATP - what a waste!
Photorespiration
Because of photorespiration: Plants
have special adaptations to limit the effect
of photorespiration.
1. C4 plants
2. CAM plants
C4 Plants
Hot, moist environments - stomata open .
15% of plants (grasses, corn, sugarcane).
Divides photosynthesis spatially to prevent photorespiration.
Light rxn - mesophyll cells (makes ATP and NADPH and oxygen!).
Calvin cycle - bundle sheath cells - cells surrounding xylem and phloem.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
C4 Plants
Mesophyll Cell
CO2
C-C-C
PEP
C-C-C-C
Malate
ATP
Bundle Sheath Cell
C-C-C
Pyruvic Acid
C-C-C-C
CO2
C3
Malate
Transported
glucose
Vascular
Tissue
PEP
Carboxylase RUBISCO
CAM Plants
Crassulacean Acid Metabolism
Hot, dry environments.
5% of plants (cactus and ice plants).
Stomates closed during day.
Stomates open during the night.
Light rxn - occurs during the day.
Calvin Cycle - occurs when CO2 is present.
CAM Plants
Night (Stomates Open
Bring in CO2)
Day (Stomates Closed)
Vacuole
C-C-C-C
Malate
C-C-C-C
Malate Malate
C-C-C-C CO2
CO2
C3
C-C-C Pyruvic acid
ATP C-C-C
PEP glucose
Questions:
The O2 released during photosynthesis comes from (A) CO2 (B) H2O (C) NADPH (D) RuBP (RuDP) (E) C6H12O6
The carbon that makes up organic molecules in plants is derived directly from (A) combustion of fuels (B) carbon fixed in photosynthesis (C) carbon dioxide produced in respiration (D) carbon in the lithosphere (E) coal mines
Carbohydrate-synthesizing reactions of photosynthesis directly require (A) light (B) products of the light reactions (C) darkness (D) O2 and H2O (E) chlorophyll and CO2
If plants are grown for several days in an atmosphere containing 14CO2 in place of 12CO2, one would expect to find (A) very little radioactivity in the growing leaves (B) large amounts of radioactive water released from the stomates (C) a large increase in 14C in the starch stored in the roots (D) a large decrease in the rate of carbon fixation in the guard cells (E) an increase in the activity of RuBP carboxylase (rubisco) in the photosynthetic cells.
Which of the following is an important difference between light-dependent and light-independent reactions of photosynthesis? (A) The light-dependent reactions occur only during the day; the light-independent reactions occur only during the night. (B) The light-dependent reactions occur in the cytoplasm; the light-independent reactions occur in the chloroplasts. (C) The light-dependent reactions utilize CO2 and H2O; the light -independent reactions produce CO2 and H2O. (D) The light-dependent reactions depend on the presents of both photosystems I and II; the light-independent reactons require only photosystem I. (E) The light-dependent reactions produce ATP and NADPH; the light-independent reactions use energy stored in ATP and NADPH.
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