Photosynthesis
Capturing the Energy In Light
Photosynthesis

The process by which autotrophs
capture energy from sunlight and store
it within organic compounds
(carbohydrates such as glucose and
starch).
sunlight
6CO2 + 6H2O
Carbon Dioxide
Water
C6H12O6 + 6O2
Glucose
Oxygen
What is an autotroph?


Eukaryotic organisms that can produce
their own food using sunlight energy.
Examples: plants, algae, euglena…
Heterotrophs

Heterotrophs have
to consume other
organisms to
meet their energy
needs.

What happens if a
heterotroph
attempts to
absorb sunlight
energy.
Investigating Photosynthesis
•
•
•
Many Scientists Have
Contributed To
Understanding
Photosynthesis
Early Research Focused On
The Overall Process
Later Researchers
Investigated The Detailed
Chemical Pathways
Early Questions on Plants
Several Centuries Ago, The
Question Was:
Does the increase in mass
of a plant come from the
air? The soil? The Water?
Van Helmont’s Experiment 1643



Planted a seed into A
pre-measured amount of
soil and watered for 5
years
Weighed Plant & Soil.
Plant Was 75 kg, Soil The
Same.
Concluded Mass Came
From Water
Priestley’s Experiment 1771


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Burned Candle In Bell
Jar Until It Went Out.
Placed Sprig Of Mint In
Bell Jar For A Few
Days.
Candle Could Be Relit
And Burn.
Concluded Plants
Released Substance (O2)
Necessary For burning.
Ingenhousz’s Experiment
1779
Repeated Priestly experiment with & without sunlight
Results of Ingenhousz’s
Experiment
 Showed
That Priestley’s
Results Only Occurred In
The Presence Of Sunlight.
 Light Was Necessary For
Plants To Produce The
“Burning Gas” or oxygen
Julius Robert Mayer 1845
Proposed That
Plants can
Convert Light
Energy Into
Chemical
Energy
Samuel Ruben & Martin Kamen
1941
Used Isotopes
To Determine
That The
Oxygen
Liberated In
Photosynthesis
Comes From
Water
RUBIN
KAMEN
Melvin Calvin 1948
First
to trace the path
that carbon (CO2) takes in
forming Glucose
Does
NOT require sunlight
Called the Calvin Cycle or
Light Independent Reaction
Also known as the Dark
Reaction
Energy for Life Processes

Photosynthesis is a biochemical pathway



Involves a complex series of chemical reactions in
which the product of one reaction is consumed in
the next reaction.
Autotrophs use the biochemical pathways of
photosynthesis to manufacture organic
compounds from CO2 and water.
In the process molecular oxygen O2 is
released.
Energy for Life Processes


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Energy stored in organic compounds is
released in a process called cellular
respiration.
Both autotrophs and heterotrophs perform
cellular respiration.
In cellular respiration organic compounds are
combined with O2 to produce ATP.
CO2 and water are the waste products of
cellular respiration.
Photosynthetic Reactions
Light Reactions



The initial reactions in photosynthesis are
collectively known as the light dependent
reactions.
Begin with the absorption of light in
chloroplasts.
Chloroplasts are organelles present in plant
cells and algae (some algae may have only one
while a cell in a plant may have as many as 50).
Structure of the Chloroplast
Double membrane organelle
 Outer membrane smooth
 Inside the inner membrane are
another system of membranes
arranged as flattened stacks of
connected sacs called thylakoids
 Thylakoid stack is called the granun
(grana-plural)
 Gel-like material around grana called
stroma

Light and Pigments
Energy From The Sun
Enters Earth’s Biosphere As
Photons
 Photon = Light Energy Unit
 Light Contains A Mixture Of
Wavelengths
 Different Wavelengths Have
Different Colors

Light and Pigments

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Light from the sun appears white
When passed through a prism the white
light’s component colors become visible,
this is the visible spectrum
Component colors may be transmitted,
reflected or absorbed.
Pigments are compounds that absorb
light. Different colored pigments
absorb different wavelengths of light.
Chloroplast Pigments

A variety of pigments are located in the
membrane of thylakoids.

Chlorophyll
The most important are chlorophylls
 Chlorophyll a and chlorophyll b are the most
common.
 Neither abosorb much green light.
 Chlorophyll a is directly involved in the light
reaction.
 Chlorophyll b assists in capturing light energy
and is called an accessory pigment.

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Carotenoids – Accessory pigments found
in the thylakoid membranes

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yellow, brown, and orange in color
Absorb colors that chlorophyll cannot
absorb.
Usually hidden by the overabundance of
chlorophyll.
Apparent in fruits and in fall leaves when
plants lose chlorophyll.
Light Independent (dark) reactions.

Use energy carrying compounds produced
from the light dependent reactions and
CO2 to make carbohydrates (including
glucose).