BIOL 1201 Chapter : Biol 1201 Outline10
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Chapter 10
PHOTOSYNTHESIS
Photosynthesis transforms light energy trapped by chloroplasts into chemical bond
energy and stores that energy in sugar and other organic molecules
-Synthesis of energy-rich organic molecules from energy-poor molecules
(CO2 and H2O)
-Uses CO2 as a carbon source and light-energy as the energy source.
-Directly or indirectly supplies energy to most living organisms.
Major Modes of Nutrition Among Organisms
>>>>>Plants and other autotrophs are the producer of the biosphere
Organisms acquire organic molecules used for energy and carbon skeletons (a.k.a.
food) by one of two nutritional modes:
1. Autotrophic nutrition
2. Heterotrophic nutrition
---Autotrophic nutrition – “Autotrophs” – “self-feeders” they sustain themselves
without eating anything derived from other beings. Produce their organic
molecules from CO2 and other inorganic materials obtained from the environment.
They are the ultimate sources of organic compounds for all nonautotrophic
organisms. Producers of the biosphere.
---Photoautotrophic – plants- organisms that use light as a source of energy to
synthesize organic substances.
---Heterotrophic nutrition – unable to make their own food, they live on
compounds produced by other organisms. They are the biosphere’s consumers.
The Leaf: Adaptations for Photosynthesis
>>>>>Chloroplasts are the site of photosynthesis in plants
a typical mesophyll cell has about 30-40 chloroplasts
a chloroplast has an envelope of two membranes surrounding a dense fluid of called the
stroma.
Suspended within the stroma is a third membrane system made up of sacs called
thylakoids which separate the stroma from the thylakoid space inside the sacs.
Leaves are the major organs of photosynthesis (see fig 10.4)
-Chlorophyll is the green pigment that gives a leaf its color chlorophyll
is also responsible for the absorption of the light energy that
drives photosynthesis
-Chloroplasts are primarily in cells of mesophyll (in the leaf interior)
-CO2 and H2O enter the leaf and O2 leacves through pores called stomata
-Water is absorbed by the roots and transported to the leaves through the
vascular bundles (veins).
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Leaves also use veins to export sugar to roots and other nonphotosynthetic parts
of the plant.
Isotopes: Unraveling Photosynthesis
>>>>>Evidence that chloroplasts split water molecules let researchers track atoms
through photosynthesis
overall photosynthetic equation: in the presence of light, the green parts of plants
produce organic compounds and oxygen from carbon dioxide and water.
The summary equation for photosynthesis:
6 CO2 + 12 H2O + light energy => C6H12O6 + 6 O2 + 6 H2O
While glucose is shown, other carbohydrates are the main products of photosynthesis.
The direct product of photosynthesis is actually a three carbon sugar that can be used
to make glucose.
Water is present on both sides of this equation because photosynthesis uses and
produces H2O. 12 molecules are consumed, and 6 molecules are newly formed.
The equation may be written as a net equation:
6 CO2 + 6 H2O + light energy => C6H12O6 + 6 O2
The overall chemical change during photosynthesis is the reverse of the one that occurs
during cellular respiration:
C6H12O6 + 6 O2 => 6 CO2 + 6 H2O + Energy
Plants use both photosynthesis and respiration, however they are NOT the reverse of a
single pathway.
The splitting of water
Initially it was hypothesized that the oxygen generated by photosynthesis came from
carbon dioxide.
CO2 + light energy => C + O2
the freed carbon would then combine with water to from carbohydrates:
C + H2O => CH2O
However C.B van Niel found that some purple-sulfur bacteria use H2S rather than water
for photosynthesis.
These bacteria still produce carbohydrates, but instead of generating oxygen they make
sulfur.
CO2 + 2 H2S + light energy => CH2O + 2 S + H2O
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Using this reaction as a model van Neil proposed a general photosynthesis reaction:
CO2 + 2 H2X + light energy => CH2O + H2O + 2 X
From that model the reactions for photosynthesis in plants can be written
CO2 + 2 H2O* + light energy => CH2O + H2O + O2*
A significant result of the shuffling of atoms during photosynthesis is the
extraction of hydrogen from water and its incorporation into sugar.
The waste product of photosynthesis O2 is released into the atmosphere.
Photosynthesis as a redox process (antithesis of cellular respiration)
Remember in cellular respiration:
-the process is exergonic
-carbon and hydrogen atoms of the sugar are oxidized (lose in their
sharing of electrons)
-oxygen atoms are reduced (gain in their sharing of electrons).
-electrons lose energy as they travel down the electron transport
chain
-energy is used to produce ATP
-water is formed from oxygen
In photosynthesis:
(Reverses the direction of electron flow, water is split, and electrons are
transferred along with hydrogen ions from the water to carbon dioxide reducing it
to sugar)
-the process is endergonic (requires light energy)
-carbon and hydrogen atoms from carbon dioxide and water are reduced
(gain in their sharing of electrons)
-oxygen atoms of water are oxidized (lose in their sharing of electrons).
-electrons gain energy from light energy as they travel up the electron
transport chain
-energy is used to produce sugar and ATP
-oxygen is formed from water
Document Summary
1 (co2 and h2o: autotrophic nutrition, heterotrophic nutrition. Photosynthesis transforms light energy trapped by chloroplasts into chemical bond energy and stores that energy in sugar and other organic molecules. Synthesis of energy-rich organic molecules from energy-poor molecules. Uses co2 as a carbon source and light-energy as the energy source. Directly or indirectly supplies energy to most living organisms. >>>>>plants and other autotrophs are the producer of the biosphere. Organisms acquire organic molecules used for energy and carbon skeletons (a. k. a. food) by one of two nutritional modes: --autotrophic nutrition autotrophs self-feeders they sustain themselves without eating anything derived from other beings. Produce their organic molecules from co2 and other inorganic materials obtained from the environment. They are the ultimate sources of organic compounds for all nonautotrophic organisms. --photoautotrophic plants- organisms that use light as a source of energy to synthesize organic substances. --heterotrophic nutrition unable to make their own food, they live on compounds produced by other organisms.
