Photosynthesis - Biology Junction

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PHOTOSYNTHESIS
Photosynthesis
An anabolic, endergonic, carbon
dioxide (CO2) requiring process
that uses light energy (photons)
and water (H2O) to produce
organic macromolecules (glucose).
SUN
photons
6CO2 + 6H2O  C6H12O6 + 6O2
glucose
2
Question:
Where does
photosynthesis
take place?
3
Plants
• Autotrophs – produce their own food
(glucose)
• Process called photosynthesis
• Mainly occurs in the leaves:
a. stoma - pores
b.mesophyll cells
Mesophyll
Cell
Chloroplast
Stoma
4
Stomata (stoma)
Pores in a plant’s cuticle through
which water and gases are
exchanged between the plant and
the atmosphere.
Oxygen
(O2)
Carbon Dioxide
(CO2)
Guard Cell
Guard Cell
Found on the underside of leaves
5
Mesophyll Cell of Leaf
Nucleus
Cell Wall
Chloroplast
Central Vacuole
Photosynthesis occurs in these cells!
6
Chloroplast
Organelle where photosynthesis
takes place.
Stroma
Outer Membrane
Inner Membrane
Thylakoid
Granum
Thylakoid stacks are connected together
7
Thylakoid
Thylakoid Membrane
Granum
Thylakoid Space
Grana make up the inner membrane
8
Question:
Why are
plants
green?
9
Chlorophyll Molecules
• Located in the thylakoid membranes
• Chlorophyll have Mg+ in the center
• Chlorophyll pigments harvest energy
(photons) by absorbing certain
wavelengths (blue-420 nm and red660 nm are most important)
• Plants are green because the green
wavelength is reflected, not absorbed.
10
Wavelength of Light (nm)
400
Short wave
(more energy)
500
600
700
Long wave
(less energy)
11
Absorption of Light by
Chlorophyll
Absorption
violet
blue
green
yellow
wavelength
orange
red
12
Question:
During the fall,
what causes the
leaves to change
colors?
13
Fall Colors
• In addition to the chlorophyll
pigments, there are other pigments
present
• During the fall, the green
chlorophyll pigments are greatly
reduced revealing the other pigments
• Carotenoids are pigments that are
either red, orange, or yellow
14
Redox Reaction
The transfer of
electrons from
another
Two types:
1. Oxidation is
2. Reduction is
one or more
one reactant to
the loss of ethe gain of e-
15
Oxidation Reaction
The loss of electrons from a
substance or the gain of
oxygen.
Oxidation
6CO2 + 6H2O 
C6H12O6 + 6O2
glucose
16
Reduction Reaction
The gain of electrons to a
substance or the loss of
oxygen.
Reduction
6CO2 + 6H2O  C6H12O6 + 6O2
glucose
17
Two Parts of Photosynthesis
Two reactions make up
photosynthesis:
1.Light Reaction or Light
Dependent Reaction Produces energy from solar
power (photons) in the form of
ATP and NADPH.
SUN
18
Two Parts of Photosynthesis
2. Calvin Cycle or Light
Independent Reaction
• Also called Carbon Fixation
or C3 Fixation
• Uses energy (ATP and
NADPH) from light reaction
to make sugar (glucose).
ATP
19
Light Reaction (Electron Flow)
• Occurs in the Thylakoid
membranes (inner membrane)
• During the light reaction,
there are two possible routes
for electron flow
A.Cyclic Electron Flow
B. Noncyclic Electron Flow
20
Cyclic Electron Flow
•
•
•
•
Occurs in the thylakoid membrane
Uses Photosystem I only
P700 reaction center- chlorophyll a
Uses Electron Transport Chain
(ETC)
• Generates ATP only
ADP +
P
ATP
21
Cyclic Electron Flow
Primary
Electron
Accepto
r
SUN
e-
e-
ePhotons
P700
ATP
produced
by ETC
e-
Accessory
Pigments
Photosystem I
Pigments absorb photons, excite electrons, which produce
22
ATP
Noncyclic Electron Flow
• Occurs in the thylakoid membrane
• Uses PS II and PS I
• P680 reaction center (PSII) chlorophyll a
• P700 reaction center (PS I) chlorophyll a
• Uses Electron Transport Chain (ETC)
• Generates O2, ATP and NADPH
23
Noncyclic Electron Flow
Primary
Electron
Acceptor
Primary
Electron
Acceptor
SUN
2e-
2e-
Photon
H2O
1/2O2 + 2H+
Enzyme
Reaction
2e-
ETC
2e-
2e-
P700
NADPH
ATP
P680
Photon
Photosystem I
Photosystem II
24
B. Noncyclic Electron Flow

• ADP +
• NADP
P ++ H
ATP

NADPH
(Reduced
)
• Oxygen comes from the splitting
(Reduced)
of H2O, not CO2
H 2O 
(Oxidized)
1/2 O2 + 2H+
25
Chemiosmosis
• Powers ATP synthesis.
• Located in the thylakoid membranes.
• Uses ETC and ATP synthase (enzyme)
to make ATP.
• Photophosphorylation: addition of
phosphate to ADP to make ATP.
26
Chemiosmosis
SUN
H+ H+
Thylakoid
(Proton Pumping)
E
T
PS II
PS I
C
H+
H+ H+
H+ H+
H+
ADP + P
H+
H+
high H+
concentration
ATP Synthase
ATP
Thylakoid
Space
low H+
concentration
27
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.
• To produce glucose: it takes 6
turns and uses 18 ATP and 12
28
Chloroplast
Stroma
Outer Membrane
Inner Membrane
Thylakoid
Granum
29
Calvin Cycle (C3 fixation)
(36C)
6C-C-C-C-C-C
(6C)
6CO2
(unstable)
(30C)
6C-C-C-C-C
RuBP
(30C)
glucose
6C-C-C 12PGA
(36C)
6ATP
6ATP
6NADPH
6NADPH
6C-C-C
6ATP
C3
6C-C-C
(36C)
6C-C-C 12G3P
(6C)
C-C-C-C-C-C
Glucose 30
Calvin Cycle
• Remember: C3 = Calvin Cycle
C3
Glucose
31
Photorespiration
• Occurs on hot, dry, bright days.
• Stomates close.
• Fixation of O2 instead of CO2.
• Produces 2-C molecules instead of 3-C
sugar molecules.
• Produces no sugar molecules or no
ATP.
32
Photorespiration
• Because of photorespiration: Plants
have special adaptations to limit the
effect of photorespiration.
1. C4 plants
2. CAM plants
33
C4 Plants
• Hot, moist environments.
• 15% of plants (grasses, corn,
sugarcane).
• Divides photosynthesis spatially.
• Light rxn - mesophyll cells.
• Calvin cycle - bundle sheath cells.
34
C4 Plants
Malate
C-C-C-C
Malate
C-C-C-C
Transported
CO2
CO2
C3
glucose
C-C-C
PEP
ATP
Mesophyll Cell
Vascular
Tissue
C-C-C
Pyruvic Acid
Bundle Sheath Cell
35
CAM Plants
• 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.
36
CAM Plants
Night (Stomates Open)
Day (Stomates Closed)
Vacuole
CO2
C-C-C-C
Malate
C-C-C-C
Malate
C-C-C-C
Malate
CO2
C3
C-C-C
PEP
ATP
C-C-C
Pyruvic acid
glucose
37
Question:
Why would CAM
plants close
their stomates
during the day?
38
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