Download Photosynthesis

Mrs. McCoy
Life in the Sun
• Life on Earth is solar powered!
• Nearly all organisms depend on
photosynthesis
• Photosynthesis is the process in which light
energy is used to make sugar and other food
molecules from carbon dioxide and water.
Photosynthesis
• Photo - means light
• Synthesis - is a chemical reaction that creates
a more complex molecule “puts together”
Plants
• Plants are autotrophs or “self feeders” they make
their own food and thus sustain themselves without
eating other organisms or eating organic molecules
• Plants capture light energy from the sun and convert
it to chemical energy that is stored in glucose and
other organic molecules made from CO2 and H2O.
• They are known as Producers
Autotrophs
• Although they are self-feeders, autotrophs are
not totally self-sufficient. What do they
require from the environment in order to
synthesize sugar?
Photosynthesis Occurs in Chloroplasts
• All green parts of a plant have chloroplast and
can carry out photosynthesis
• Leaves typically have the most chloroplasts
and are the main site for the process
• Green color in plants if from chlorophyll
pigments in chloroplasts
• Chlorophyll absorbs the light energy that the
chloroplasts put to work in making food
molecules
Structure of the Chloroplast
Chloroplasts
• Chloroplast are concentrated in the cells of
the mesophyll
• Mesophyll is the green tissue in the interior of
the leaf
• Each mesophyll cell has many chloroplasts
• Chloroplasts have outer and inner membranes
with intermembrane space between them
• The inner membrane encloses a second
compartment which is filled with stroma
Stroma
• Stroma is a thick fluid, and is where sugars are
made from carbon dioxide and water
• In the stroma is suspended a system of disklike membranous sacs called thylakoids, which
contain the third chloroplast compartment
Thylakoids
• Thylakoids are concentrated in stacks called grana
(singular granum)
• Thylakoid compartments in a granum are
interconnected, forming a continuous space
• Built into the thylakoid membranes are the
chlorophyll molecules that capture light energy
• Thylakoid membranes also house much of the
machinery that convert light energy to chemical
energy
Chlorophyll Molecule
• There are several types
of chlorophyll
• All share the structure
on the right of the
molecule
• This is referred to as a
chlorin magnesium
ligand
Note the chlorin is the large heterocyclic
structure and a ligand is functional group bound
to a central metal (Mg)
Photosynthesis: The Reactants
• Photosynthesis is a chemical reaction that uses
the energy from sunlight to drive the reaction
• A chemical reaction has reactants (the molecules
that break apart and rearrange their atoms) that
form products (the new arrangement of atoms
that form new molecules)
• In photosynthesis 6 CO2 and 12 H2O break apart
and atoms of each of these molecules rearrange
themselves to form C6H12O6 , 6 H2O and 6 O2
Occurs in Two Stages
• The equation for photosynthesis is a simple summary
for a very complex process
• Photosynthesis has two stages each with multiple steps
• The steps of the first stage are known as the
– these are the reactions that convert light
energy to chemical energy and produce Oxygen gas
(O2) “
• The steps of the second stage are know as the
– this is the cyclical series of reactions that
assemble sugar molecules using CO2 and the energy
containing products of the light reaction “
• The light reactions of photosynthesis occur in the thylakoids membranes
of the chloroplast’s grana
• Light absorbed by chlorophyll in the thylakoid membranes furnishes the
energy that eventually powers the food making machinery of
photosynthesis
• Light energy is used to make ATP from ADP and phosphate
• Light energy is also used to drive a transfer of electrons from water to
NADP+ to form NADPH this reaction temporarily stores the energized
electrons
• As the NADPH is formed Water (H2O) is split giving off Oxygen (O2)
• In summary the light reaction of photosynthesis are the steps that:
–
–
–
–
Absorb solar energy
Convert solar energy to chemical energy
Store converted chemical energy in the form of ATP and NADPH
Note – light reactions do not produce sugar
• The Calvin Cycle (named after American Biochemist and Nobel
Laureate Melvin Calvin) occurs in the stroma of the chloroplast
• The incorporation of carbon from CO2 into organic compounds is
called carbon fixation
• Carbon fixation occurs in the Calvin Cycle as carbon dioxide enters
the leaf, and oxygen exits, by way of tiny pores called stomata
(stoma singular) which are openings in the cuticle of he leaf that
lead to the mesophyll; chloroplast are concentrated in the cells of
the mesophyll, and with in the chloroplasts is the thick fluid or
stroma – this is where the Calvin cycle occur and how the CO2 finds
its way to become incorporated in organic compounds, then to
enzymes and finally to sugar
• After Carbon fixation enzymes of the cycle make sugars by further
reducing carbon by adding high-energy electrons to it, along with H+
/
• As the figure suggests, it is NADPH produced by
the light reactions that provides the high-energy
electrons for reduction in the Calvin Cycle
• And the ATP from the light reaction provides
chemical energy that powers several of the steps
of the Calvin cycle.
• The Calvin cycle does not require light directly,
however, the Calvin cycle runs during the
daytime, when the light reactions power the
cycle’s sugar assembly line by supplying it with
NADPH and ATP.