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PHOTOSYNTHESIS
Chapter 7
A. Light
Visible light makes up only a small
portion of the electromagnetic
spectrum.
Sunlight consists
of:
 4% Ultraviolet
(UV) radiation
 44% Visible light
 52% Infrared
(IR) radiation
Overview of Photosynthesis
Photosynthesis- process by which plants, algae
and some microorganisms harness solar energy
to make biochemicals.
Occur in organelles – chloroplasts
Two stages – light reaction and carbon
reaction
The products of photosynthesis, glucose and
other carbohydrates – photosynthate.
Characteristics of Visible Light:
•
•
•
is a spectrum of colors ranging from
violet to red
consists of packets of energy called
photons
photons travel in waves, having a
measurable wavelength (λ)
λ = distance a photon travels during a
complete vibration [measured in nanometers
(nm)]
A photon’s energy is inversely related to
its wavelength...
...the shorter the λ, the greater the
energy it possesses.
Which of the following photons possess
the greatest amount of energy?
Green photons
Red photons
Blue photons
λ = 530nm
λ = 660nm
λ = 450nm
What happens to light when it strikes an
object?
• reflected
(bounces off)
•
transmitted
•
absorbed
(passes through)
Only absorbed wavelengths of light
function in photosynthesis.
B. Photosynthetic Pigments
Molecules that capture photon energy
by absorbing certain wavelengths of
light.
1. Primary pigments
•
•
Bacteriochlorophyll - green pigment
found in certain bacteria.
Chlorophylls a & b - bluish green
pigments found in plants, green algae
& cyanobacteria.
Chlorophyll a is
the dominant
pigment in plant
cells.
2. Accessory Pigments
•
•
•
•
•
•
Carotenoids - red, orange, yellow pigments
found in plants, algae, bacteria & archaea.
Xanthophylls – red and yellow pigments found
in plants, algae & bacteria.
Fucoxanthin –brown pigment found in brown
algae, diatoms, & dinoflagellates
Phycoerythrin - red pigment found in red algae.
Phycocyanin - blue pigment found in red algae &
cyanobacteria.
Bacteriorhodopsin – purple pigment found in
halophilic archaea
Each pigment absorbs a particular range of
wavelengths.
Light- form of energy- exists as photons.
Photons possess different wavelengths that
represent different energy levels.
Different wavelengths seen as different colors.
Pigment molecules possess different abilities to
absorb wavelengths and appear as different colors.
Chlorophyll is the major pigment molecule and
appears as green.
Plants and other photosynthetic species use
different pigments to absorb different
wavelengths and use light more efficiently.
C. Chloroplasts
Sites of photosynthesis in plants &
algae.
Concentrated in mesophyll cells of
most plants.
Chloroplast structure:
•
Stroma - gelatinous matrix; contains
•
Thylakoid - flattened membranous sac;
ribosomes, DNA & various enzymes.
embedded with photosynthetic pigments.
Chloroplasts – type of plastid- unique
organelles with multiple layers which increase
surface area to improve efficiency.
Chlorophyll is imbedded within the membrane
layers in complexes that maximize the
absorption and transduction of energy.
D. Photosynthesis
6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O
Occurs in two stages:
•
•
Light reactions - harvest photon
energy to synthesize ATP & NADPH.
Carbon reactions (Calvin cycle) - use
energy from light reactions to reduce
CO2 to carbohydrate.
Overview of Photosynthesis
Molecules in thylakoid membrane capture sunlight energy and transfer
energy to molecules of ATP and NADPH. Enzymes of caebon reactions use
this energy to capture CO2 abd build glucose.
1. Light Reactions
•
•
•
require light
occur in thylakoids of chloroplasts
involve photosystems I & II (light
harvesting systems).
Photosystems
contain antenna
complex that
captures photon
energy & passes
it to a reaction
center.
Light Reactions of Photosynthesis
ATP Production by Chemiosmotic
Phosphorylation
•Light reactions of photosynthesis boost
electrons into higher energy levels.
•Transfer them to the carriers NADPH and ATP
for use in the cell.
•Additional energy is used to pump hydrogen ions
into lumen of thylakoids- establishing gradient
called proton motive force.
•Protons escape through a membrane-bound ATP
synthase – uses the energy release to
phosphorylate ATP (chemiosmotic
phosphorylation)
•Electrons – ultimately replaced by converting
water to protons and oxygen.
2. Carbon Reactions (Calvin cycle;
C3 cycle)
• do NOT require light (occur in both
darkness & light as long as ATP & NADPH
are available)
•
•
occur in stroma of chloroplasts
require ATP & NADPH (from light
reactions), and CO2
Calvin Cycle
Plants that use only the Calvin cycle to
fix carbon are called C3 plants.
Ex. cereals, peanuts, tobacco, spinach,
sugar beets, soybeans, most trees &
lawn grasses.
Carbon fixation uses energy from ATP and
NADPH to convert gaseous carbon dioxide to
organic molecules such as glucose.
The enzyme system constantly recycles its
components, forming Calvin cycle.
Key enzyme – rubisco attaches carbon to the
carrier ribulose bisphosphate.
E. Photorespiration
Process that counters photosynthesis.
Occurs when stomata close under hot,
dry conditions:
• O2 levels in plant increase
• CO2 levels in plant decrease
Under these conditions, rubisco fixes O2
(rather than CO2).
Thus, PGAL is NOT produced.
F. C4 and CAM Photosynthesis
Adaptations that allow certain plants
to conserve water and reduce
photorespiration at higher
temperatures.
1. C4 Photosynthesis
C4 plants reduce photorespiration by
physically separating the light
reactions and Calvin cycle.
Leaf anatomy
of a C4 plant
C4 Photosynthesis:
• Light reactions
occur in
chloroplasts of
mesophyll cells.
• Calvin cycle occurs
in chloroplasts of
bundle sheath cells.
2. CAM Photosynthesis
CAM plants reduce photorespiration
by acquiring CO2 at night.
Night:
•
Malic
acid
•
mesophyll cells fix
CO2 as malic acid
malic acid is stored in
vacuoles.
Day:
•
malic acid releases
CO2 which enters
Calvin cycle.
Inefficiency of rubisco causes photorespiration.
To live in hot climates, plants adept at
manipulations that reduce photorespiration.
C4 plants use a intermediate to separate the
light and carbon reactions from each other within
different cell types.
Resulting in higher carbon dioxide
concentration within bundle-sheath cells –
reduce photorespiration.
CAM plants fix carbon at night when
temperatures are lower and water loss is less
of a problem.