Transcript Document

Photosynthesis
Photosynthesis Overview
Energy for all life on Earth ultimately comes
from photosynthesis.
6CO2 + 12H2O
C6H12O6 + 6H2O + 6O2
Oxygenic photosynthesis is carried out by:
cyanobacteria, 7 groups of algae, all land plants
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Photosynthesis Overview
• Photosynthesis takes place in 3 stages:
– Capturing energy from sunlight
– Using the energy to make ATP and reduce
NADP+ to NADPH
• (nicotinamide adenine dinucleotide phosphate)
– Using the ATP and NADPH to synthesize
organic molecules (glucose) from CO2
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Photosynthesis Overview
Photosynthesis is divided into:
light-dependent reactions
-capture energy from sunlight
-make ATP and reduce NADP+ to NADPH
carbon fixation reactions (lightindependent reactions)
-use ATP and NADPH to synthesize
organic molecules from CO2
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Photosynthesis Overview
• Photosynthesis takes place in chloroplasts.
• thylakoid membrane – internal membrane arranged in
flattened sacs
– contain chlorophyll and other pigments
– Organized into photosystems
• Capture light and transfer energy (to pigment molecules)
• grana – stacks of thylakoid membranes
• stroma – semiliquid substance surrounding thylakoid
membranes (houses the enzymes to make organic
molecules)
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Photosynthesis Overview
• Photosynthesis takes place in the green portions of
plants
– Leaf of flowering plant contains mesophyll tissue
– Cells containing chloroplasts
– Specialized to carry on photosynthesis
• CO2 enters leaf through stomata
– Diffuses into chloroplasts in mesophyll cells
– In stroma, CO2 fixed to C6H12O6 (sugar)
– Energy supplied by light
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Discovery of Photosynthesis
The work of many scientists led to the
discovery of how photosynthesis works.
Jan Baptista van Helmont (1580-1644)
Joseph Priestly (1733-1804)
Jan Ingen-Housz (1730-1799)
F. F. Blackman (1866-1947)
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Pigments
photon: a particle of light
-acts as a discrete bundle of energy
-energy content of a photon is inversely
proportional to the wavelength of the light
photoelectric effect: removal of an electron
from a molecule by light
-occurs when photons transfer energy to
electrons
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Electromagnetic Spectrum
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Pigments
Pigments: molecules that absorb visible
light
Each pigment has a characteristic
absorption spectrum, the range and
efficiency of photons it is capable of
absorbing.
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Pigments
chlorophyll a – primary pigment in plants and
cyanobacteria
-absorbs violet-blue and red light
chlorophyll b – secondary pigment absorbing light
wavelengths that chlorophyll a does not absorb
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Pigments
accessory pigments: secondary pigments
absorbing light wavelengths other than
those absorbed by chlorophyll a
-increase the range of light wavelengths that
can be used in photosynthesis
-include: chlorophyll b, carotenoids,
phycobiloproteins
-carotenoids also act as antioxidants
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Photosystem Organization
A photosystem consists of
1. an antenna complex (light harvesting
complex) of hundreds of accessory
pigment molecules that gather
photons and feeds energy to
reaaction center
2. a reaction center of one or more
chlorophyll a molecules pass
electrons out of photosystem
(photochemical reactions)
In summary, energy of electrons is
transferred through the antenna
complex to the reaction center.
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Photosystem Organization
At the reaction center
(transmembrane protein
complex), the energy from the
antenna complex is transferred
to chlorophyll a.
This energy causes an electron
from chlorophyll to become
excited.
The excited electron is transferred
from chlorophyll a to an electron
acceptor.
Water donates an electron to
chlorophyll a to replace the
excited electron.
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Photosynthesis Overview
•Light Reactions
•Two electron pathways operate in the
thylakoid membrane: the noncyclic
pathway and the cyclic pathway.
•Both pathways produce ATP; only the
noncyclic pathway also produces
NADPH.
•ATP production during photosynthesis
is called photophosphorylation;
therefore these pathways are also
known as cyclic and noncyclic
photophosphorylation.
Calvin Cycle Reactions:
Carbon Dioxide Fixation
• CO2 is attached to 5-carbon RuBP
molecule
– Result in a 6-carbon molecule
– This splits into two 3-carbon
molecules (3PG)
– Reaction accelerated by RuBP
Carboxylase (Rubisco)
• CO2 now “fixed” because it is part of a
carbohydrate
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Calvin Cycle Reactions:
Carbon Dioxide Reduction
• 3PG reduced to BPG
• BPG then reduced to G3P
• Utilizes NADPH and some
ATP produced in light
reactions
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Calvin Cycle Reactions:
Regeneration of RuBP
• RuBP used in CO2 fixation must
be replaced
• Every three turns of Calvin Cycle,
– Five G3P (a 3-carbon
molecule) used To remake
three RuBP (a 5-carbon
molecule)
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The Calvin Cycle:
Fixation of CO2
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Importance of Calvin Cycle
• G3P (glyceraldehyde-3-phosphate) can be
converted to many other molecules
• The hydrocarbon skeleton of G3P can
form
– Fatty acids and glycerol to make plant oils
– Glucose phosphate (simple sugar)
– Fructose (which with glucose = sucrose)
– Starch and cellulose
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– Amino acids