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PHOTOSYNTHESIS
Chapter 6
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
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 ()
 = 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 , the greater the
energy it possesses.
Which of the following photons possess
the greatest amount of energy?
Green photons
 = 530nm
Red photons
 = 660nm
Blue photons
 = 450nm
What happens to light when it strikes an
object?
 reflected
(bounces off)
 transmitted
(passes through)
 absorbed
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.
 Phycoerythrin - red pigment found in
red algae.
 Phycocyanin - blue pigment found in
red algae & cyanobacteria.
Each pigment absorbs a particular
range of wavelengths.
C. Chloroplasts
Sites of photosynthesis in plants &
algae.
Concentrated in mesophyll cells of
most plants.
Chloroplast structure:
 Stroma - gelatinous matrix; contains
ribosomes, DNA & various enzymes.
 Thylakoid - flattened membranous sac;
embedded with photosynthetic pigments.
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
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
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.
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:
mesophyll cells fix
CO2 as malic acid
 malic acid is stored in
vacuoles.

Malic
acid
Day:

malic acid releases
CO2 which enters
Calvin cycle.