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Sylvia S. Mader
Immagini e
concetti
della biologia
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
B1 Photosynthesis
and cellular
respiration
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
ATP and Photosynthesis
Adenosine triphosphate is ancient and universal. All living
organisms produce or use ATP to do work.
Photosynthesizers are autotrophs (protists, cyanobacteria,
plants, algae and mosses) that produce their own food.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Photosynthesis
Photosynthesis converts visible light (the portion of
electromagnetic radiation) from the sun into chemical
energy.
The most used pigments (light-absorbing molecules) in
photosynthesis are:
•Chlorophyll a
•Chlorophyll b
•Carotenoids
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Chloroplasts
Chloroplasts carry out photosynthesis.
CO2 enters the leaf through small pores called stomata.
chloroplast
stoma
CO2
O2
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Chloroplasts
Chlorophyll and other pigments within the thylakoid
membranes absorb solar energy.
Conversion of CO2 to carbohydrates occurs in the stroma.
Chloroplast
outer membrane
inner membrane
granum
stroma
thylakoid membranes
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Chloroplasts
In the thylakoid membranes, pigment complexes absorb
solar energy which excites electrons (e-).
Energized e- pass through an electron transport chain
(ETC) where they release energy for ATP production.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Photosynthesis is a redox
Photosynthesis is a reduction-oxidation reaction (redox)
where CO2 is reduced and H2O is oxidized, resulting in
carbohydrate and oxygen.
During photosynthesis the coenzyme NADP+ is the eacceptor and is reduced to NADPH.
NADP+ + 2e- + H+
NADPH
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
O2 comes from the water
Isotope-based experiments proved that the oxygen
produced during photosynthesis comes from the water
rather that from CO2.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Photosynthesis reactions
•Light reactions: occur in the thylakoid membranes
during the day when light is available.
•Calvin cycle reactions: enzymatic reactions that reduce
CO2 to a carbohydrate in the stroma.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Light reactions
During the light-dependent reactions, e- move from photosystem II
(PS II) down an ETC to the photosystem I (PS I). Here the e- is reenergized and passes to NADP+ that reduces to NADPH.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
The thylakoid membranes
By chemiosmosis NADP reductase passes e- to NADP+
and NDAPH results. H+ flows down gradient through ATP
synthase complex, where ADP binds to phosphate group
and ATP is produced.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Calvin cycle
During the light-independent reactions, the Calvin cycle uses ATP
and NADPH to produce carbohydrates. During this process CO2 is
fixed and then reduced.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Calvin cycle
The Calvin cycle includes three steps.
1.CO2 fixation: the enzyme RuBP carboxylase fixes CO2
to RuBP, producing a C6 molecule that immediately splits
into two C3 molecules (3PG).
2.CO2 reduction: each 3PG is reduced to a
glyceraldehyde 3-phosphate (G3P) molecule.
3.RuBP regeneration: G3P molecules are used to
regenerate RuBP.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Carbohydrates and other molecules
In plants carbohydrates (G3P) are used to synthesize
other important organic molecules as fructose, sucrose
and starch.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Leafs colors in fall
Fall temperatures cause leaves to change the color.
Less sunlight in fall means not as much solar energy to
rebuild chlorophyll, which disintegrates, leaving yellow and
orange pigments visible.
In some plants, as maples, pigments accumulate in acid
vacuoles, leading to brilliant red color.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Deforestation and global warming
Destroying tropical rain forests increases the concentration
of CO2 emissions in the atmosphere. CO2 is a powerful
green house gas.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Cellular respiration
Cellular respiration is a redox reaction that requires O2 and
produces energy.
Glucose is oxidized to CO2, while O2 is reduced to water.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Cellular respiration
Coenzymes NAD+ and FAD remove hydrogen atoms (eand H+) from glucose.
NAD+ + 2e- + H+
NADH
FAD + 2e- + 2H+
FADH2
Slow release of energy at the ETC allows it to be captured
for ATP production.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Cellular respiration
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Glycolysis
Glucose breakdown starts with glycolysis, that occurs in
the cytoplasm and is an anaerobic process.
Pyruvate is the final product of the glycolysis and it can
follow two pathways:
1.Cellular respiration (aerobic pathway)
1.Fermentation (anaerobic pathway)
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Glycolysis
Cellular respiration has three phases:
• Preparatory reaction
• Citric acid cycle
• Electron transport chain (ETC)
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Glycolysis
Glycolysis starts with an energy-investment phase during
which 2 ATP molecules are consumed to produce two C3
molecules (Glyceraldehyde 3-phosphate).
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Glycolysis
The second step is an energy-harvesting phase with the
synthesis of four ATP molecules.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Glycolysis
•
•
•
•
2 ATP molecules are consumed
4 ATP molecules are produced
The net gain is 2 ATP molecules
2 NADH molecules are produced
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
The mitochondrion
Apart from the preparatory reaction, all cellular respiration
reactions (citric acid cycle and ETC) occur in the
mitochondrion.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Cellular respiration
The preparatory reaction occurs before the citric acid
cycle. Here the pyruvate molecule reacts with Coenzyme A
(CoA) and is converted into acetyl CoA (C2 molecule).
Two NADH and two CO2 molecules are also produced.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Cellular respiration
The citric acid cycle (or Krebs cycle) starts when the two
acetyl CoA molecules enter the matrix of the
mitochondrion.
One ATP molecule per cycle (two per glucose) results.
NADH and FADH2 are produced as CO2 is release.d
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Cellular respiration
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Cellular respiration
The Electron transport chain occurs in the mitochondrial
cristae and stores much energy in ATP molecules.
Electrons are carried by the coenzymes NADH and
FADH2.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Cellular respiration
The ETC is composed of
proteins embedded in the
membrane.
Electrons pass through the
proteins and the energy is
captured and used to form
ATP.
O2 is the final acceptor of
electrons and is reduced to
water.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Cellular respiration
Protein complexes form the
ETC. As e- move through the
ETC, H+ are pumped to the
intermembrane space and
create an H+ gradient.
As H+ flow down the gradient,
ATP is synthesized from ADP
+ P through chemiosmosis.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Total ATP production
Complete oxidation of one glucose molecule produces a
total of 36 ATP molecules.
•2 ATP in the cytoplasm (glycolysis)
•2 ATP from the Citric acid cycle
•32-34 ATP from the ETC and chemiosmosis
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Fermentation
Fermentation is an alternative metabolic pathway when
oxygen is in short supply. Depending on the organisms,
after glycolysis two types of fermentation can occur:
•Lactic acid fermentation (the pyruvate is reduced to
lactate)
•Alcoholic fermentation (the pyruvate is reduced to
alcohol and CO2)
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Fermentation
Fermentation results in a net
gain of only 2 ATP molecules,
but provides a quick burst of
energy for short-term
activities.
Yeast fermentation is used to
prepare bread and alcohol;
bacterial fermentation is used
to produce yogurt, sour cream,
cheese, pickles and vinegar.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Metabolic pathways
Metabolism includes
catabolism (break down of
molecules) and anabolism
(synthesis of molecules).
Carbohydrates, fats and
proteins can be all
catabolized to produce ATP.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012