Photosynthesis
How Plants Make Food from
Sunlight and Low Energy
Molecules
Photoautotrophs
Carbon and Energy Sources
Photoautotrophs
Carbon source is carbon dioxide
Energy source is sunlight
Heterotrophs
Get carbon and energy by eating autotrophs
or one another
Photoautotrophs
Capture sunlight energy and use it to
carry out photosynthesis
Plants
Some bacteria
Many protistans
T.E. Englemann’s Experiment
Background
Certain bacterial cells will move
toward places where oxygen
concentration is high
Photosynthesis produces oxygen
T.E. Englemann’s Experiment
Hypothesis
Movement of bacteria can be used to
determine optimal light wavelengths for
photosynthesis
T.E. Englemann’s Experiment
Method
Algal strand placed on microscope slide
and illuminated by light of varying
wavelengths
Oxygen-requiring bacteria placed on
same slide
T.E. Englemann’s Experiment
T.E. Englemann’s Experiment
Results
Bacteria congregated where red and
violet wavelengths illuminated alga
Conclusion
Bacteria moved to where algal cells
released more oxygen--areas illuminated
by the most effective light for
photosynthesis
Linked Processes
Photosynthesis Aerobic Respiration
Energy-storing Energy-releasing
pathway pathway
Releases oxygen Requires oxygen
Requires carbon Releases carbon
dioxide dioxide
Focusing in on the location of photosynthesis in a plant
Location and structure of chlorophyll molecules in plants
Photosynthesis Equation
LIGHT ENERGY
12H2O + 6CO2 6O2 + C6H12O6 + 6H2O
water carbon oxygen glucose water
dioxide
Two Stages of
Photosynthesis
sunlight water uptake carbon dioxide uptake
ATP
LIGHT ADP + Pi LIGHT
DEPENDENT- INDEPENDENT-
REACTIONS REACTIONS
NADPH
NADP+
P glucose
oxygen release new water
Sunlight Energy
Continual input of solar energy into
Earth’s atmosphere
Almost 1/3 is reflected back into space
Of the energy that reaches Earth’s
surface, about 1% is intercepted by
photoautotrophs
Electromagnetic Spectrum
Shortest Gamma rays
wavelength X-rays
UV radiation
Visible light
Infrared radiation
Microwaves
Longest Radio waves
wavelength
Visible Light
Wavelengths humans perceive as
different colors
Violet (380 nm) to red (750 nm)
Longer wavelengths, lower energy
Photons
Packets of light energy
Each type of photon has fixed amount
of energy
Photons having most energy travel as
shortest wavelength (blue-green light)
Pigments
Light-absorbing
molecules
Absorb some
wavelengths and
chlorophyll a
transmit others chlorophyll b
Color you see are
the wavelengths Wavelength (nanometers)
NOT absorbed
Pigments in Photosynthesis
Bacteria
Pigments in plasma membranes
Plants
Pigments embedded in thylakoid membrane
system
Pigments and proteins organized into
photosystems
Photosystems located next to electron
transport systems
Pigments in a Photosystem
reaction center
(a specialized chlorophyll a molecule)
Light-Dependent Reactions
Pigments absorb light energy, give up e-
which enter electron transport systems
Water molecules are split, ATP and
NADH are formed, and oxygen is
released
Pigments that gave up electrons get
replacements
Photosystem Function:
Harvester Pigments
Most pigments in photosystem are
harvester pigments
When excited by light energy, these
pigments transfer energy to adjacent
pigment molecules
Each transfer involves energy loss
Photosystem Function:
Reaction Center
Energy is reduced to level that can be
captured by molecule of chlorophyll a
This molecule (P700 or P680) is the
reaction center of a photosystem
Reaction center accepts energy and
donates electron to acceptor molecule
Cyclic Electron Flow
electron acceptor e– electron
transport
system
e–
e–
ATP
e–
Electron Transport System
Adjacent to photosystem
Acceptor molecule donates electrons
from reaction center
As electrons flow through system,
energy they release is used to produce
ATP and, in some cases, NADPH
Cyclic Electron Flow
Electrons
are donated by P700 in photosystem I to
acceptor molecule
flow through electron transport system and
back to P700
Electron flow drives ATP formation
No NADPH is formed
Energy Changes
second
transport
system
Potential to transfer energy (voids)
e– NADPH
first e–
transport
system
e–
e–
(PHOTOSYSTEM I)
(PHOTOSYSTEM II)
H2O 1/2 O2 + 2H+
Noncyclic Electron Flow
Two-step pathway for light absorption
and electron excitation
Uses two photosystems: type I and
type II
Produces ATP and NADPH
Involves photolysis - splitting of water
Figure 10.4 An overview of photosynthesis: cooperation of the light reactions
and the Calvin cycle (Layer 1)
Figure 10.4 An overview of photosynthesis: cooperation of the light reactions
and the Calvin cycle (Layer 2)
Light-Independent Reactions
Synthesis part of
photosynthesis
Can proceed in the dark
Take place in the stroma
Calvin-Benson cycle
Calvin-Benson Cycle
Overall reactants Overall products
Carbon dioxide Glucose
ATP ADP
NADPH NADP+
Reaction pathway is cyclic and RuBP
(ribulose bisphosphate) is regenerated
Melvin Calvin
The Calvin cycle (Layer 1)
The Calvin cycle (Layer 2)
The Calvin cycle (Layer 3)
Using the Products of
Photosynthesis
Phosphorylated glucose is the building
block for:
sucrose
The most easily transported plant carbohydrate
starch
The most common storage form
The C3 Pathway
In Calvin-Benson cycle, the first stable
intermediate is a three-carbon PGA
Because the first intermediate has
three carbons, the pathway is called the
C3 pathway
Photorespiration in C3 Plants
On hot, dry days stomata close
Inside leaf
Oxygen levels rise
Carbon dioxide levels drop
Rubisco attaches RuBP to oxygen
instead of carbon dioxide
Only one PGAL forms instead of two
C4 Plants
Carbon dioxide is fixed twice
In mesophyll cells, carbon dioxide is fixed
to form four-carbon oxaloacetate
Oxaloacetate is transferred to bundle-
sheath cells
Carbon dioxide is released and fixed again
in Calvin-Benson cycle
Figure 10.18 C4 leaf anatomy and the C4 pathway
CAM Plants
Carbon is fixed twice (in same cells)
Night
Carbon dioxide is fixed to form organic
acids
Day
Carbon dioxide is released and fixed in
Calvin-Benson cycle
Figure 10.19 C4 and CAM photosynthesis compared
Figure 10.20 A review of photosynthesis