Chapter 7
Photosynthesis:
Using light energy to produce
organic molecules
I. Study of Photosynthesis
 Before
1600: Ancient Greeks proposed that
plants obtained all nourishment from soil.
 Mid
1600s: Jan Baptista van Helmont
performed experiments to determine where
plants obtain nourishment from.

Planted small willow tree in a pot with soil.

Added water only, for five years.

Plant gained 75 kilograms.

Soil had lost 60 grams.
Helmont’s Experiment
Add water only
for five years
Plant: 75 kg gained
Soil: 0.06 kg lost
I. Study of Photosynthesis
Helmont’s conclusions:
Plants do not obtain all nutrients from soil (correct)
 They obtain most nutrients from water (wrong).

I. Study of Photosynthesis
 1770:
Joseph Priestley consistently observed
that a candle in a closed jar would not burn
out if a mint plant was added to the jar.
Priestley’s conclusion: Plants “restore” the air
depleted by a candle.
 Late 1770s: Jan Ingenhousz showed that
plants require light to “restore” the air in a
closed jar.
 1800s: Photosynthesis reaction elucidated.
 1950s: Discovered that oxygen produced by
plants comes from water.
Photosynthesis Requires Water,
Carbon Dioxide, and Light
II. Net Reaction of Photosynthesis
6CO2 + 6H2O + LIGHT ENERGY ---> C6H12O6 + 6O2
REDOX REACTION
 Carried
out by photosynthetic autotrophs (plants,
algae, and some bacteria). Also called producers.
 Produce 200 billion tons of organic material from CO2
every year.
 Almost all living organisms obtain energy directly or
indirectly from photosynthesis.
 Exceptions: A few bacteria metabolize sulfur or
iron and some animals that eat them, don’t depend
on photosynthesis.
Photosynthesis is a Redox Reaction
III. Chloroplasts are site of photosynthesis in
eucaryotes
 All
green parts of a plant carry out photosynthesis.
 Most
chloroplasts are found in leaves, specifically in
mesophyll, green tissue in interior of leaves.
 Green
color is due to chlorophyll, a light absorbing
pigment.
 In
bacteria, photosynthesis occurs on extensions of the
cell membrane.
 Stomata:
Pores in leaf for exchange of CO2 and O2
Chloroplasts are the Site of Photosynthesis
Areas of Chloroplast Important for Photosynthesis
Thylakoids: Membrane “discs” arranged in
stacks (grana) which contain chlorophyll and
other important molecules.
Site where solar energy is trapped and converted
into chemical energy (light reactions).
Thylakoid Membrane: Site of ATP synthesis.
Stroma: Thick fluid outside thylakoid
membranes, surrounded by interior membrane.
Site of sugar synthesis (dark reactions).
Chloroplasts Have Three Sets of
Membranes
IV. Plants produce oxygen by “splitting” water
 Water is used as a source of H and electrons to
reduce CO2
6CO2 + 6H2O + ENERGY ---> C6H12O6 + 6O2
Where does the free oxygen come from? CO2 or H2O
Label the CO2 or H2O with radioactive O18

CO2 + 2H2O -------> CH2O + H2O + O2

CO2 + 2H2O -------> CH2O + H2O + O2
**** Free oxygen comes from the splitting of water, not
CO2.
Oxygen Generated by Photosynthesis Comes from Water
IV. Light reactions trap energy and electrons
required to make sugar from CO2

Light reactions: Require light. Convert light energy to
chemical energy of ATP and reducing power of NADPH
 Occur in the thylakoid membranes of chloroplast
 Water is split with energy from sun into free O2, H and
electrons.
 Reduce NADP+ to NADPH: High energy electrons and
H obtained from splitting of H2O
 Photophosphorylation: Light energy is used to produce
ATP from ADP + Pi
 ATP synthesis is driven by chemiosmosis
Input: ADP, NADP+, water, and light.
Output: ATP, NADPH, and O2.
Light Dependent Reactions: Light Energy Trapped by
Chlorophyll is Used to Split Water, Make NADPH & ATP
V. Light Independent (Dark) reactions (Calvin
Cycle) make sugar from CO2

Calvin Cycle: Uses ATP and NADPH produced by light
reactions to reduce CO2 to glyceraldehyde-3-phosphate
 Occurs
in the stroma of chloroplast
 Don’t need light directly.
 Carbon fixation: Process of gradually reducing CO2
gathered from atmosphere to organic molecules
 NADPH provides H and electrons to reduce CO2 and
ATP provides energy.
Input: CO2 , ATP, and NADPH.
Output: Sugars, ADP, and NADP+.
Light Independent Reactions: Sugar Synthesis
NOTE:
 Light
reactions: Transform light energy into
usable form of chemical energy (ATP and
NADPH). Water is split to obtain H.
 Light
independent reactions (Calvin cycle):
Use chemical energy (ATP and NADPH) to
drive the endergonic reactions of sugar
synthesis.
Light and Dark Reactions of Photosynthesis
V. Properties of light energy: A portion of the
electromagnetic spectrum

Visible light: A small portion of the
electromagnetic energy spectrum which cells in
our retina can detect ( wavelength: 380 - 750 nm).
 Wavelength:
Distance between two “crests” of
light wave
 Photon: Discrete “particles” of light energy
 Energy: The amount of energy is INVERSELY
proportional to the wavelength of light
Smaller wavelength
Gamma
rays
Longer wavelength
X-rays UV
light
visible infrared microwaves radio
light
light
waves
Visible light spectrum
Wavelength in nanometers:
380
470
520
570
VIOLET
BLUE
GREEN
YELLOW ORANGE
Higher Energy
610
650
RED
Lower Energy
White Light is a Spectrum of Different Lights
Isaac Newton (1642-1727) separated white light by passing it
through a prism.
Chlorophyll Absorbs Portions of Visible Light Spectrum
Green light
is reflected by
chlorophyll
VI. Pigments allow plants to absorb various
wavelengths of light

Pigments: Molecules that absorb light energy
 Black
object: All wavelengths are absorbed
 White object: All wavelengths are reflected
 Green object: All wavelengths BUT green are
absorbed

Absorption spectrum: Shows wavelengths
absorbed by a certain pigment

Plants use different pigments to capture light
energy, each has its own unique absorption
spectrum
 Chlorophyll
a: Primary light absorption
pigment
 Chlorophyll b: Accessory light absorption
pigment
 Carotenoids: Accessory light absorption
pigments
Structure of a Chlorophyll Molecule
IX. How is ATP produced in photosynthesis?

Answer: By chemiosmosis through the
development of a proton gradient in the
chloroplast.

NOTE: The synthesis of ATP in both oxidative
phosphorylation and photosynthesis is linked to the
production of a proton gradient.
 Electron
transport chain pumps H+ to create gradient
 ATP synthase:
Uses energy as H+ flows “downhill” to
drive the synthesis of ATP from ADP and Pi
ATP Production Requires a Proton Gradient
Photosynthesis Helps Counteract the Greenhouse Effect
 The

earth’s atmosphere contains about 0.03% of carbon dioxide.
Carbon dioxide traps solar energy in the atmosphere, making
the earth about 10oC warmer than it would otherwise be.
 Since
the mid 1800s, the atmospheric levels of carbon dioxide
have risen steadily due to the burning of fuels and forests.
 The
“Greenhouse Effect” refers to the global warming that is
caused by increased atmospheric carbon dioxide levels.
 Global
warming may cause polar ice caps to melt, which in turn
could cause massive coastal flooding and other problems.
 Plants
use up about half of carbon dioxide generated by humans
and other organisms.
Greenhouse Effect: Heat is Trapped by
Carbon Dioxide