Download 5. CHAPTER XI PHOTOSYNTHESIS

CHAPTER XI
PHOTOSYNTHESIS
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
1
The nature of light
• The sun's energy travels through space to
the earth as electromagnetic radiation
waves at the speed of light, about 300,000
Km/s.
• Light is transmitted to the earth in discrete
(separate) bundles of energy called
photons.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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• The wavelengths of this radiation vary from very long radio
waves of more than 1 kilometer to very short cosmic ‫ كونية‬rays
of less than 4-10nm.
• Visible light is that portion of the electromagnetic spectrum
between 400 and 760 nm; however, plants respond to the
somewhat wider spectrum of about 300 to 800 nm.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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• The number of waves passing a given point in space
per second is called its frequency.
• Light traveling in shorter wavelengths at higher
frequencies is more energetic than light at longer
wavelengths and lower frequencies.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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• The rate of photosynthesis can be
determined by measuring the rate at which
carbon dioxide is absorbed by a leaf.
• Rate of total photosynthesis = rate of
net photosynthesis + rate of respiration
in light.
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• Only about 5 % of the radiant energy provided by the
sun is used directly in photosynthesis (Fig. 11-3).
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Carbon atom
• Carbon has six protons, six neutrons, and six electrons
(atomic weight of 12).
• No other element has an ideal combination of sharability
of electrons plus stability of the bonds formed.
1. The two inner electrons (K shell) cannot be shared, but the four
outer electrons (L shell) can be shared. Thus, carbon can form up
to four bonds: As CO2 or Ethane or or……..
1. The strength of the C-C bond is 82.6 kcal/mole (an estimate of
how much energy is needed to break the bond).
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Oxidation/Reduction
• A compound that is reduced is a good electron
acceptor (oxidizing agent) relative to a
compound that is oxidized= electron donor
(reducing agent) .
• Carbon is at the center of this process in living
organisms because it is so versatile ‫ متنوع‬in
Related change sharing electrons.
• One way to look at photosynthesis is to say that
it is the process of converting:
Oxidized carbon (CO2) → Reduced carbon [carbohydrate or C(H2O)].
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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PHOTOSYNTHESIS:
LIGHT ENERGY → REDUCING POWER
• Photosynthesis is the process of conversion
of
Light energy → Chemical energy
• The simplest chemical reaction that can be
written to represent photosynthesis is:
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
9
• Photosynthesis occurs in chloroplasts (Fig. 11-1).
The chloroplast contains a complex array) ‫ (مجموعة‬of
membranes called thylakoids, and many enzymatic
proteins are imbedded in the thylakoid membranes
(Fig. 11-2).
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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• The essential function of photosystem I and
photosystem II in the chloroplast is to utilize light energy
to obtain electrons from water:
• Each 12 molecules of H2O→6O2+24H+ + 24e- → C6H12O6
• Note that the oxygen produced
• in photosynthesis comes from
• water, not carbon dioxide.
• The photosystems use light energy to raise the electrons
to a very high energetic state so that they can be used to
start the cascade)‫ (شالل‬of reduction reactions.
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The protons (H+ generated in the water-splitting
reaction also set up a proton (H+) gradient
across the thylakoid membranes, and this
gradient
is used to drive ATP synthesis. The energized
electrons transferred to Calvin cycle are ultimately
used to reduce NADP+ to NADPH
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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• Multiple photons are
received by multiple
chlorophyll molecules
and shunted to a reaction
center where the
electrons from water are
energized (Fig. 11-4).
The analogy of
chlorophyll as an antenna
molecule is appropriate.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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The light-requiring reactions of
photosynthesis result in
• The conversion of water to O2.
• The generation of NADPH and the energyrich compound adenosine triphosphate
(ATP).
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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PHOTOSYNTHESIS:
CARBON DIOXIDE → CARBOHYDRATE
• The first step in the generation of carbohydrate is the
reaction of CO2 with the compound ribulose 1,5
bisphosphate. (called Carbon Fixation step)
• The reaction is catalyzed by the enzyme ribulose
bisphosphate carboxylase, usually referred to as
RUBISCO.
• This reaction produce two molecules of 3phosphoglycerate.
– Phosphoglycerate + ATP→ 1,3-biphosphoglycerate + ADP
– 1,3-biphosphoglycerate + NADPH → glyceraldehyde 3phosphate + NADP + inorganic phosphate
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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PHOTOSYNTHESIS:
CARBON DIOXIDE → CARBOHYDRATE
• Next is the regeneration of the beginning substrate,
ribulose 1,5-bisphosphate. It is known as the
photosynthetic carbon reduction cycle, or Calvin cycle.
• A totally new molecule of glyceraldehyde 3-phosphate
can be spun off with every three revolutions of the cycle.
• The new glyceraldehyde 3-phosphate molecule is
combined with another 3-carbon phosphorylated sugar
to give fructose-1, 6-bisphosphate
• In general, dead-end metabolites (that do not participate
in many other reactions) are used for long-distance
transport. Example: sucrose
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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FACTORS AFFECTING THE RATE OF
PHOTOSYNTHESIS
1. Light quality (wavelength).
2. Light intensity (the amount of incident light
energy absorbed by the leaf).
3. Carbon dioxide concentration.
4. Heat.
5. Water availability.
6. Plant development and source-sink
relationships.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Light Quality
• Chloroplasts contain pigments
(chlorophyll a, chlorophyll b, and some
carotenoids) that absorb mostly the red
and blue portions of the visible
wavelengths spectrum and reflect the
green portion.
• Blue and red wavelengths are more
photosynthetically active than green.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Light Intensity
The effect varies with different plants:
• Sun-loving plants: Some species require high light
intensities to grow well (corn, potatoes, sugarcane).
• Shade-loving plants: Some plant species that do not
grow well in high light intensities (dense shade of the
forest floor, useful as house ornamentals).
• Intermediate: Other plant species grow well in
moderately intense light.
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Light intensity distinctly affects the size
and shape of leaves:
• The leaves of a given plant species grow
thinner and broader in area at the low light
intensity than leaves grown at the high
light intensity.
• The leaves of plants grown in high light
intensities tend to be darker green than
those grown in low light intensities.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Light compensation point
• Light compensation point
is a light intensity at
which photosynthesis and
respiration rates are
equal and net gas
exchange is zero (CO2 is
still exchanged) (Fig. 116).
• The plant is said to be
light saturated when
further increases in light
intensity increase
photosynthesis little or
not at all.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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• The light intensity at
which saturation occurs
increases as the CO2
concentration
surrounding the plant
rises (Fig. 11-7).
• At very high light
intensities, the rate at
which CO2 is available to
the plant could limit the
photosynthetic rate.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Carbon Dioxide
• The carbon dioxide compensation point is the
carbon dioxide concentration at equilibrium such
that the amount of carbon dioxide evolved in
respiration exactly equals the amount consumed
in photosynthesis.
DMA: Chapter 11 Hartmann's
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• Increasing the CO2 concentration in a closed
system, such as a sealed greenhouse to about
0.10 % approximately doubles the photosynthetic
rate of certain crops
– Applications of organic matter in the form of crop
residues or green manure crops to the soil tends to
increase CO2 levels in the atmosphere above the soil.
– In greenhouses, the use of horizontal airflow fans
(HAFs) increases airflow around plants and exposes
the leaves to a constant supply of CO2. The use of
HAFs may eliminate the need for supplemental CO2 in
most greenhouses.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Heat
If light is not limiting, the rate of photosynthetic
activity approximately doubles for each 10°C
increase in temperature for many plant species
in the temperate climates.
The effect of temperature varies with species:
• Plants adapted to tropical conditions require a
higher temperature for maximum photosynthesis
than those adapted to colder regions.
• Excessively high temperatures reduce the
photosynthetic rate of some plants not
accustomed ‫ معتادة‬to such high temperatures by
causing the stomata of the leaves to close.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Water
• Under conditions of drought (low soil moisture and
hot, drying winds), plants often lose water through
transpiration faster than their roots can absorb it.
This causes:
– The stomata to close and the leaves to wilt temporarily.
– When this occurs, the exchange of CO2 and O2 is
restricted,
– Resulting in a dramatic drop in photosynthesis.
• Excessive soil moisture sometimes creates an
anaerobic condition (lack of oxygen) around the
roots. This causes:
– Reducing root respiration and mineral uptake and
transport of water and minerals to the leaves
– Thus indirectly depressing photosynthesis in the leaves.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
28
Plant development and the Source-Sink
relationship
• When the leaf reaches full expansion, it is often called a
source leaf because the carbohydrate synthesized in
that leaf is in excess of local requirements and is
exported to other parts of the plant that are actively
growing. These sites of active growth and metabolism
are often called sink tissues. (Roots and reproductive
organs)
• In many plant species, the photosynthesis rates in all the
mature leaves begin to decline drastically when the
plants flower and envelop fruits and seeds.
(Proteins in the leaves, and specially photosynthetic
enzyme proteins in the chloroplasts, are degraded to
amino acids as the leaves begin the process called
senescence. The amino acids in the leaves are
transported to the developing seeds).
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Different Photosynthetic Mechanisms
• Plants that evolved in more arid regions
have adaptations that help conserve
water but do not inhibit photosynthesis
during dry periods.
• C4
• CAM: crassulacean acid metabolism
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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The C4 plants:
• The C4 mechanism is found in tropical grasses
such as corn, sorghum. These plants have a
special anatomy that concentrates CO2 in
special areas of the leaf that allow CO2 to be
fixed more efficiently.
• They are called C4 plants because CO2
entering the leaf is temporarily attached to a
three-carbon organic acid making a four-carbon
organic acid.
• The acid is shuttled to the special areas and the
CO2 is released at the areas where it can be
used efficiently.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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The CAM plants
• The CAM plants are water-storing desert
plants such as succulents and cacti.
• As with C4, the CAM process also
attaches CO2 to a three-carbon organic
acid, but this time it happens at night
when the stomata can open without
extreme water loss.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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Phosphoenolpyruvate
carboxylase (PEP Car)
Ribulose-1,5
-bisphosphate
carboxylase
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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RESPIRATION
• The building and maintenance of cells and tissues
require not only carbohydrates, but also proteins,
lipids, nucleic acids, and so forth to accomplish the
thousands of metabolic tasks required for growth
and reproduction.
• Respiration is the conversion of carbohydrate into
energy-rich ATP:
C6H12O6 + 6O2 + 36ADP + 36 PO43-  6CO2 + 6H2O + 36 ATP3-
• So, respiration is the reverse of photosynthesis
with respect to carbon dioxide  carbohydrate.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
35
Steps of respiration:
1. Glycolysis: the conversion of
carbohydrate to a 2 molecules of threecarbon organic acid named pyruvic acid
• C6H12O6 + 2 ADP + 2 PO43- + 2 NAD+
 2 C3H4O3 + 2ATP + 2NADH + H2O
• =2 ATP+ 2NADH x 3ATP= 8ATP
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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2. Tricarboxylic acid (Krebs) cycle:
•
Before entry into the cycle, the pyruvate is first
converted to acetyl coenzyme A, with the loss of
one CO2 and the production of another NADH
(Fig. 11-8).
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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•
The reduced carbon enters
the cycle as an acetyl group
and is degraded to two
more CO2 molecules
coupled to
1. The generation of three more
NADH molecules x 3 ATP.
2. A new type of electron
acceptor—flavin adenine
dinucleotide (FAD)—is also
involved here.
3. Also, one ATP is gained with
each turn of the cycle.
•
Remember: for every
glucose that is processed
through glycolysis, two
pyruvate molecules are
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
formed.
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3. Electron transport system.
•
One way to describe this
process is to imagine that the
electrons in NADH are passed
down a cascade of stairs, with
energy being released at each
step. During this process,
–
–
•
One NADH yields three ATPs while
One FADH2 yields two ATPs,
Note that molecular oxygen is
the final electron acceptor and
that H2O is the final product.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
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• Enzymes for glycolysis are located in the cell cytoplasm.
The remaining two steps are localized in mitochondria.
Thus, pyruvate—the product of glycolysis—must be
transported into the mitochondrion (Fig. 11-10).
Step
NADH
Produ
ced
ATP Synthesis
Direct
From NADH
From FADH2
Total ATP
Productio
n
Glycolysis
2
2
6-2*
-
6
TCA cycle + electron transport
8
2
24
4
30
Grand total
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* Six are produced but two must be subtracted to support the transport of
NADH into the mitochondria
ATP is the energy currency of living organisms.
DMA: Chapter 11 Hartmann's
Plant Science, 4th edition
41