Download 6-1 Photosynthesis

Photosynthesis
Capturing the Energy in Light
6CO2 + 6H2O -> C6H12O6 + 6O2
I. Energy for Life Processes
A.
B.
C.
Plants are autotrophs (make their own
food)
Photosynthetic organisms: plants,
protists, algae, some bacteria.
Photosynthesis is a complex series of
chemical reactions known as a
biochemical pathway.
II. Light Absorption in Chloroplast
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The first reactions in photosynthesis are
called light reactions
A chloroplast contains thylakoids, grana or
granum (stacks of thylakoids), and stroma
(solution).
A.
Light and Pigments
1.
2.
3.
4.
Light travels through space as waves of energy
White light from the sun is composed of an array of
colors called the visible spectrum (ROY G. BIV)
Different colors have different wavelengths (nm)
Pigments absorb certain colors of light and reflect or
transmit the colors of others
B. Chloroplast Pigments
1. Chlorophylls: the type of pigment present in the
thylakoids (chlorophyll a and chlorophyll b).
2. Only chlorophyll a is directly involved in the light
reactions of photosynthesis\
3. Chlorophyll b is called an accessory pigment (it assist
chlorophyll a in capturing light energy).
4. Carotenoids also function as accessory pigments
5. In leaves (photosynthetic parts)
chlorophylls are more abundant and they
hide the colors of the other pigments
6. Fruits and flowers: colors of other
pigments might show
III. Electron Transport
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Chlorophylls and carotenoids are grouped in
clusters referred to as a photosystem
(photosystem I and photosystem II)
Steps involved in the light reaction (inside
the thylakoid membrane)
1.
2.
3.
Light excites electrons in chlorophyll a molecules
of photosystem II.
These electrons move to the primary electron
acceptor
The electrons are transferred along a series of
molecules called the electron transport chain
(ETC)
4. Light excites electrons in chlorophyll a
(photosystem I) as the electrons move to the
primary electron acceptor, they are replaced by
electrons from photosystem II.
5. The electrons from
photosystem I are
transferred along
a second ETC. At
the end of the chain
they combine with
NADP+ and a proton
(H+) to make NADPH.
A.
Restoring Photosystem II
1.
2.
3.
The replacement electrons (Step 4) are
provided by the splitting of water molecules.
2H2O
4H+ + 4e- + O2
Protons (H+) produced are left inside the
thylakoid, oxygen diffuses out of the
chloroplast and can leave the plant. For this
reason, oxygen is called a byproduct of the
light reaction.
IV. Chemiosmosis
A. This is a process in chloroplasts and
mitochondria where protons move across
a membrane (from high to low
concentrations) which also synthesizes
ATP.
B. There is a build up of protons inside the
thylakoid (from the splitting of water)
C. ADP + P
ATP (by the enzyme ATP
synthase)
D. The energy that drives the above
reaction is from the movement of the
protons inside the thylakoid to the stroma.
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Describe the structure and function of the
thylakoids of a chloroplast.
What role do accessory pigments (hog breath
fresheners) play in photosynthesis.
What happens to the electrons that are lost by
photosystem II ? What happens to the electrons
that are lost by photosystem I.
Name the substances that are produced when
water molecules are broken down during the
light reaction.
How is ATP made in the light reaction?
Explain how the light reactions would be
affected if there were no concentration gradient
of protons across the thylakoid membrane.