Download Photosynthesis Light Dependent Reactions PPT

Get out your reading that is due today,
Quiz on Thursday
Without using notes, talk to your group about the
answers to these question….
1. Essentially, what is the job of ATP?
2. Why can’t we store ATP for later use?
3. What is the difference between endergonic and exergonic
reactions?
RECAP AND PREQUEL
• All life needs a constant input of energy
– Heterotrophs (Consume)
• get their energy-filled organic molecules from “eating other
organisms”
• make usable cellular energy through cell respiration
– Autotrophs (Producers)
•
•
•
•
produce their own energy-filled organic molecules
Photo-autotrophs convert energy from sunlight
build organic molecules through photosynthesis
Also break down these organic molecules to make energy
through cell respiration.
PHOTOSYNTHESIS
YOU WOULD BE
DEAD WITHOUT IT.
Light
Plants actually take in 12 water molecules, and six water
molecules are created, so to simplify, this is the equation you
usually see only showing the ‘net consumption of water’.
Obtaining the reactants…
Sunlight
Pigments in the
chloroplasts of
(mostly) plant leaves
absorb light energy
called photons.
CO₂
Water
Gas Exchange occurs
at the stomata, tiny
pores on the leaves.
Photosynthesis takes place inside the cell’s
chloroplasts in the leaf of a plant.
PHOTOSYNTHESIS
LIGHT-DEPENDENT PHASE
(Requires Sunlight)
LIGHT-INDEPENDENT PHASE
(Does Not Require Sunlight)
FIRST PHASE
Light Dependent
Reactions:
The first phase involves
reactions that convert:
12 H₂O
and solar light energy
into
6 O₂
which is released as a waste
18 ATP, and 12 NADPH
which are used later (in light independent) to
make carbohydrates….
Light Dependent Reactions
Reactants
Products
8
Photons (light energy)
get absorbed into
pigments, which are
clustered into a protein
complex called a
photosystem.
Which wavelengths of light are not absorbed by each
pigment? Chlorophyll a, Chlorophyll b, Carotenoids?
The feathers of male cardinals
are loaded with carotenoid
pigments. These pigments
absorb some wavelengths of
light and reflect others.
Sunlight minus absorbed
wavelengths or colors
equals the apparent color
of an object.
Why are plants green?
Transmitted light
• PS II chlorophyll molecules absorbs light
– An electron from a chlorophyll molecule, absorbs the
energy and becomes EXCITED, pops out of the molecule
and leaves the photosystem….
Once energized it then goes through an
electron transport chain.
Passing from one protein to the next,
losing a little bit of energy along the way.
(still pretty energized at the end)
The energy that is being released as it
travels through the ETC is being used by
the proteins to actively transport H⁺
(protons) into the inside of the thylakoid.
The electron then enters photosystem I (PSI),
where the electron gets RE-ENERGIZED
again by photon light, enters a second ETC,
where more H⁺ is pumped into the thylakoid…
At the end of the 2nd E.T.C. the 2 electrons
and a H⁺ attach to a molecule called
NADP, which is referred to as the
FINAL ELECTRON ACCEPTOR.
Once the electron and the Hydrogen ion attach, the
molecule is now called NADPH, an electron carrier.
12 of these are created in all for this process…
Let’s go back to PSII for a sec, when our electron first
got energized…
It is important to replace the lost electrons from
the chlorophyll molecules because then the
chlorophyll wouldn’t be able to absorb more
light energy.
• To replace the lost electrons, molecules
of water are split. This reaction is called
photolysis.
Electrons from the
Hydrogens replace
the ones that
popped out of the
chlorophyll
molecules in
Photosystem II.
2
Hydrogen Ions (protons) are
released into the inside of the
thylakoid.
From the ETC and photolysis, you have all
these H⁺ built up causing a concentration
gradient, and they want to GET OVER TO THE
OTHER SIDE OF THE MEMBRANE
(chemiosmosis)….the only channel protein they
can use is an enzyme called ATP Synthase.
ATP SYNTHASE
The Hydrogen ions pass
through ATP synthase to
even out the concentration
of molecules.
As they do this, the
energy they contain is
used to add a Phosphate
to ADP, creating ATP!!!
(PHOSPHORYLATION)
18 ATP are made in all
In Summary:
Light and water are
used to make the
energy charged
molecules
(ATP and NADPH)
that will fuel the second
phase of
photosynthesis.
ATP
And Oxygen is released
as a waste.
18 ATP +
12 NADPH