NSS Bio Supplementary notes_Photosynthesis
The Process That Feeds the Biosphere
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Life on Earth is solar powered.
photosynthesis to capture light energy from the sun and convert it to chemical energy stored in
sugars and other organic molecules.
Plants and other autotrophs are the pro____ of the biosphere. Photosynthesis nourishes almost all
the living world directly or indirectly.
All organisms use organic compounds for energy and for carbon skeletons.
Organisms obtain organic compounds by one of two major modes: autotrophic nutrition or
heterotrophic nutrition.
Autotrophs produce their or____ molecules from CO2 and other inorganic raw materials obtained
from the environment. Autotrophs are the ultimate sources of organic compounds for all heterotrophic
organisms. Autotrophs are the producers of the biosphere.
Autotrophs can be separated by the source of energy that drives their metabolism:
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Photoautotrophs use l____ as a source of energy to synthesize organic compounds. Photosynthesis
occurs in plants, algae, some other protists, and some prokaryotes.
Chemoautotrophs harvest energy from oxid_____ inorg____ substances, such as sulfur and
ammonia. Chemoautotrophy is unique to prokaryotes.
Heterotrophs live on organic compounds produced by other organisms. These organisms are the
consumers of the biosphere.
 Some heterotrophs feeds on other organisms. Animals feed this way.
 Other heterotrophs decompose and feed on dead organisms or on organic litter, like feces and
fallen leaves. Most fungi and many prokaryotes get their nourishment this way.
 Almost all heterotrophs are completely dependent on photoautotrophs for food and for oxygen, a
by-product of photosynthesis.
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Photosynthesis converts light energy to the chemical energy of food
All green parts of a plant have chloroplasts. However, the leaves are the major site of photosynthesis
for most plants.
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The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts.
Chlorophyll plays an important role in the absorption of light energy during photosynthesis.
Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf.
O2 exits and CO2 enters the leaf through microscopic pores called stomata in the leaf.
Veins deliver water from the roots and carry off sugar from mesophyll cells to nonphotosynthetic
areas of the plant.
Each chloroplast has two membranes around a central aqueous space, the stroma.
In the stroma is an elaborate system of interconnected membranous sacs, the thylakoids.
Chlorophyll is located in the thylakoids.
Thylakoids may be stacked into columns called grana.
Mesophyll
Chloroplast
5 µm
Outer
membrane
Stroma Granum
Thylakoid Thylakoid
space
Clues to the Mechanism of photosynthesis
Intermembrane
space
Inner
membrane
1 µm
Evidence that chloroplasts split water molecules enabled researchers to track atoms through
photosynthesis.
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Powered by light, the green parts of plants produce organic compounds and O2 from CO2 and
H2O.
The simplified equation of photosynthesis is:
6CO2 + 6H2O + light energy  C6H12O6 + 6O2
In its simplest possible form: CO2 + H2O + light energy  [CH2O] + O2
[CH2O] represents the general formula for a sugar.
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The overall chemical change during photosynthesis is the reverse of cellular respiration.
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One of the first clues to the mechanism of photosynthesis came from the discovery that the O2
given off by plants comes from H2O, not CO2. Before the 1930s, the prevailing hypothesis was
that photosynthesis split carbon dioxide and then added water to the carbon:
Step 1: CO2  C + O2
Step 2: C + H2O  CH2O
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C. B. van Niel challenged this hypothesis. In the bacteria that he was studying, hydrogen sulfide
(H2S), not water, is used in photosynthesis.
These bacteria produce yellow globules of sulfur as a waste, rather than oxygen. Van Niel
proposed this chemical equation for photosynthesis in sulfur bacteria:
He generalized this idea and applied it to plants, proposing this reaction for their photosynthesis:
CO2 + 2H2O  [CH2O] + O2 + H2O
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Thus, van Niel hypothesized that plants split water as a source of electrons from hydrogen
atoms, releasing oxygen as a byproduct.
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Other scientists confirmed van Niel’s hypothesis twenty years later.
They used 18O, a heavy isotope, as a tracer. They could label either C18O2 or H218O.
They found that the 18O label only appeared in the oxygen produced in photosynthesis when water
was the source of the tracer.
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Hydrogen extracted from water is incorporated into sugar, and oxygen is released to the
atmosphere.
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Photosynthesis is a redox reaction.
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Respiration is an exergonic redox process; energy is rel_____ from the oxidation of sugar.
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Electrons associated with sugar's hydrogens lose potential energy as carriers transport them
to ox____, forming water.
Photosynthesis is an endergonic redox process; energy is req____ to reduce carbon dioxide.
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Oxidation is the loss of el_____ or hyd_____ atoms. Oxidation reactions rel_____ energy.
Reduction is gain of electrons or hydrogen atoms and is associated with a g___ of energy.
Oxidation and reduction occur tog_____. When a molecule is oxidized, another must be
reduced.
These coupled reactions are called oxidation-reduction or redox reactions.
Food is highly reduced (has many hydrogens). The chemical pathways of respiration in
oxidize the food (remove hydrogens), producing ATP (usable energy) for the cell.
Light is the energy source that raise pot_____ energy of electrons as they are moved from
water to sugar.
Photosynthesis reverses the direction of electron flow in respiration. Water is split and el______
transferred with H+ from w____ to c_____ di_____, reducing it to s____.
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Because the electrons increase in pot_____ en_____ as they move from water to sugar, the
process requires en____. The energy boost is provided by l____.
Photosynthesis is two processes, each with multiple stages.
The light reactions (photo) convert solar energy to ch_____ energy. The Calvin cycle (synthesis) uses
energy from the light reactions to incorporate CO2 from the atmosphere into s____.
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In the light reactions, light energy absorbed by
chlorophyll in the thy_____ drives the transfer of
electrons and hydrogen from water to NADP+
(nicotinamide adenine dinucleotide phosphate),
forming N____.
NADPH, an electron acceptor, provides reducing
power via energized el____ to the Calvin cycle.
Water is split in the process, and __ is released as a
by-product.
The light reaction also generates A___ (using
chemiosmosis), in a process called
photophosphorylation.
Thus light energy is initially converted to chemical
energy in the form of two compounds: N____ and
A__.
The Calvin cycle begins with the incorporation of CO2
into organic molecules, a process known as car___ fixation.
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The fixed carbon is red____ with electrons provided by NADPH.
A__ from the light reactions also powers parts of the Calvin cycle.
Thus, it is the C____ cycle that makes sugar, but only with the help of ATP and NADPH from the
l____ reactions. The metabolic steps of the Calvin cycle are sometimes referred to as the lightindependent reactions, because none of the steps requires light directly.
Nevertheless, the Calvin cycle in most plants occurs during daylight, because that is when the light
reactions can provide the NADPH and ATP the Calvin cycle requires.
While the light reactions occur at the thy_____, the Calvin cycle occurs in the st____.
The light reactions convert solar energy to the chemical energy of ATP
and NADPH
Light as a source of energy
 The thylakoids convert light energy into the che____ energy of ATP and NADPH.
 Light is a form of electromagnetic radiation. Wavel______ of electromagnetic radiation range
from less than a nanometer (gamma rays) to more than a kilometer (radio waves).
 The entire range of electromagnetic radiation is the electromagnetic spectrum. The most
important segment for life is a narrow band between 380nm (violet) to 750 nm (red), the band of
vi____ light.
 While the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out
most wavelengths, permitting only visible light to pass in significant quantities.
 Visible light is the radiation that drives photosynthesis.
 When light meets matter, it may be refl_____, trans_____, or abs_____.
 Different pigments absorb light of different wavel_____, and the wavelengths that are absorbed
disappear.
 A leaf looks green because chlorophyll, the dominant pig____, absorbs r__ and b___ light, while
transmitting and reflecting g____ light.
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Absorption spectrum
 An absorption spectrum plots a
pigment’s light absorption versus
wavelength. The light reaction can
perform work with those wavelengths
of light that are absorbed.
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There are several pigments in the
thylakoid that differ in their absorption
spectra.
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Chlorophyll a, the dominant pigment,
absorbs best in the red and violet-blue
wavelengths and least in the green. Other pigments have different absorption spectra.
Action spectrum
 Collectively, these photosynthetic pigments determine an overall action spectrum for
photosynthesis.
 An action spectrum measures changes in the r___ of photosynthetic activity (for example, O2
release) as the wave______ is varied.
 The action spectrum of photosynthesis was first demonstrated in 1883 in an elegant experiment
performed by Thomas Engelmann. In this experiment, different segments of a filamentous alga
were exposed to different wavelengths of light.
 Areas receiving wavelengths favorable to photosynthesis produced excess O2.
 Engelmann used the abundance of aerobic bacteria that clustered along the alga at different
segments as a measure of O2 production.
Role of photosynthetic pigments
 The action spectrum of photosynthesis does not match exactly the absorption spectrum of any one
photosynthetic pigment, including chlorophyll a.
 Only chlorophyll a participates directly in the light reaction, but accessory photosynthetic
pigments absorb light and transfer en____ to chlorophyll a.
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Excitation of pigment molecule by light
 When a molecule absorbs light (photon), one of that molecule’s el____ is elevated to an ex____
state.
 Excited electrons are unst___. Generally, they drop to their ground state in a billionth of a second,
releasing heat energy.
 Some pigments, including chlorophyll, can also release a photon of light in a process called
fluorescence. If a solution of chlorophyll isolated from chloroplasts is illuminated, it will fluoresce
and give off heat.
Chlorophyll excited by absorption of light energy produces very different results in an intact
chloroplast than it does in isolation.
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In the thylakoid membrane, chlorophyll is
organized along with proteins and smaller
organic molecules into photosystems. At the
center is a primary electron acceptor, which
accepts an excited electron from the chlorophyll
a molecule.
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The solar-powered transfer of an electron from a
chl_______ a molecule to the primary electron
acceptor is the first step of the light reactions.
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From the primary elecron acceptor, the
photoexcited electrons pass along an el_____
trans____ sy_____ (ETS). As these electrons
“fall” to a l____ energy level along the ETS, their energy is harnessed to produce A__.
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The enzyme NADP+ reductase transfers electrons from ETS to N____. Two electrons are
required for NADP+’s reduction to N_____. NADPH will carry the red____ power of these highenergy electrons to the Calvin cycle.
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The loss of an electron from the chlorophyll molecule leaves it temporarily oxid____ and carries
a pos____ charge.
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An enzyme extracts electrons from w___ and supplies them to the oxidized reaction center.
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This reaction splits water into two hyd_____ ions and an o_____ atom that combines with
another oxygen atom to form O2.
Thus the light reactions use the solar power of photons absorbed to provide chemical energy in the
form of ATP and reducing power in the form of the electrons carried by NADPH.
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Reference: chemiosmosis and ATP production
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Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis.
In both organelles, an electron transport chain /system pumps protons across a membrane as
electrons are passed along a series of increasingly electronegative carriers.
This transforms redox energy to a proton-motive force in the form of an H+ gradient across the
membrane.
ATP synthase molecules harness the proton-motive force to generate ATP as H+ diffuses back across
the membrane.
There are differences between oxidative phosphorylation in mitochondria and photophosphorylation
in chloroplasts.
Mitochondria transfer chemical energy from food molecules to ATP; chloroplasts transform light
energy into the chemical energy of ATP and reducing power in the form of NADPH.
The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
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ATP and NADPH produced by the l____ reactions are used in the Calvin cycle to reduce carbon
dioxide to sugar.
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The Calvin cycle is similar to the Krebs cycle in that the starting material is reg_______ by the end
of the cycle.
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C____ enters the Calvin cycle as CO2 and leaves as sugar.
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The Calvin cycle actually produces a three-carbon sugar glyceraldehyde 3-phosphate
(G3P/PGAL).
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For the Calvin cycle to synthesize one molecule of sugar (G3P), t_____ molecules of CO2
must enter the cycle. The cycle may be divided into three phases:
Phase 1: Carbon Fixation.
This reaction is catalyzed by the enzyme RuBP carboxylase (rubisco) – one of the most abundant
proteins on Earth.
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The Calvin cycle begins when each molecule of CO2 is attached to a f___-carbon sugar,
ribulose biphosphate (RuBP).
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The product of this reaction is an unstable s__-carbon intermediate that immediately
spl__ into two molecules of 3-phosphoglycerate (PGA).
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For every three CO2 molecules that enter the Calvin cycle, t____ RuBP molecules are
carboxylated forming s__ molecules of 3-phosphoglycerate.
Phase 2: Reduction.
This endergonic reduction phase is a two-step process that couples ATP hydrolysis with the reduction
of 3-phosphoglycerate (PGA) to glyceraldehyde phosphate (PGAL).
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An enzyme phosphorylates PGA by transferring a phosphate group from ATP. This reaction:
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produces 1, 3-bisphosphoglycerate (DPGA).
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uses six A__ molecules to produce six molecules of DPGA.
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prepare DPGA for the addition of high-energy el_____ from NADPH.
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Electrons from NADPH red___ the DPGA to glyceraldehyde 3-phosphate (G3P /PGAL). G3P
is the same three-carbon sugar produced when glycolysis splits glucose.
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For every three CO2 molecules that enter the Calvin cycle, six G3P molecules are produced,
only one of which can be counted as net gain.
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The cycle begins with three five-carbon RuBP molecules – a total of 15 carbons. The six G3P
molecules produced contain 18 carbons, a net gain of three carbons from CO2.
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One G3P molecule exits the cycle; the other 5 are recycled to regenerate 3 molecules of RuBP.
G3P produced by the Calvin cycle is the r__ material used to synthesize glucose and other
carbo_________.
Phase 3: Regeneration of Starting Material (RuBP).
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A complex series of reactions rearranges the carbon skeletons of five G3P molecules into three
R___ molecules.
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These reactions require three A__ molecules.
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RuBP is thus reg_______ to begin the cycle again.
A review of the importance of photosynthesis as producer of organic food
and metabolic energy
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On a global scale, photosynthesis makes about 160 billion metric tons of carbo_______ per
year. No other chemical process on Earth is more productive or is as important to life.
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Light reactions capt___ solar energy and use it to:
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produce ATP
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transfer electrons from water to NADP+ to form NADPH
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The Calvin cycle uses ATP and NADPH to fix CO2 and produce sugar.
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Photosynthesis transforms light energy to che____ bond energy in sugar molecules.
10-11
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Sugars made in chloroplasts supply the entire plant with chemical energy and carbon skeletons
to synthesize org____ molecules.
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Nonphoto________ parts of a plant depend on organic molecules exported from leaves in veins.
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The disaccharide sucr___ is the transport form of carbohydrate in most plants.
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Sucrose is the raw material for cellular resp______ and many anabolic pathways in
nonphotosynthetic cells.
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Much of the sugar is glucose – the monomer linked to form cellulose, the main constituent of
plant cell w__.
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Most plants make more organic material than needed. Plants consume about 50% of the
photosynthate as fuel for cellular respiration.
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Extra sugars are synthesized into st____ and stored in storage cells of roots, tubers, seeds, and
fruits.
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Hetero____ depends directly or indirectly on plants as food for sources of org____ molecules
and metabolic en____.
FACTORS WHICH AFFECT THE RATE OF PHOTOSYNTHESIS
A) General Factors
a) Light Intensity
In general, increased light intensity increases the rate of photosynthesis until another factor, usually
carbon dioxide, becomes lim_____.
At the usual carbon dioxide levels in the atmosphere (~0.03%) the maximum photosynthetic rate is
reached in many plants, at only quarter to half full sunlight.
If we starts from darkness, as the light intensity increases, a point is reached when the intake of carbon
dioxide for photosynthesis exactly b______ its output in respiration. This is called the c__________
point. Further increase in light intensity will result in carbon dioxide being ab____ and oxygen
ev____.
10-12
Q. Trees in woodland communities are
usually s__ plants, adapted to a
high light intensity. However, their
seedlings are often capable of
tolerating shade conditions.
The shade plants usually reach
their compensation point at a
l_____ light intensity than the sun
plants. (The compensation point for
the former may be as low as 0.3 to
1.0% of maximum daylight, as
compared with 1.3 to 7.5% for sun
plants. This is shown diagrammatically)
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Comment on the ecological significance of the different compensation points for sun and shade plants.
2
Draw a table to show the morphological adaptations in sun and shade plants?
3
The utilisation of carbon dioxide in photosynthesis is much less temperature sensitive than the production of carbon
dioxide in respiration. This fact has important bearings on glasshouse management. In seasons of low light intensity
it is essential to keep temperatures at the safe minimum for the crop concerned. Explain why it is essential to do this.
4
How could the safe minimum temperature be defined?
b) Quality of light
The action spectrum for photosynthesis has peaks in the red and blue regions, yet the wavelengths
available to a plant may vary in different habitats.
Q. What would you expect to changes in the quality of light available under a woodland? What physiological adaptations are
expected in the plants striving at the lower strata inside a woodland?
c) Carbon dioxide Concentration
Q.1
The normal CO2 concentration in air is ~0.03%. What does the
figure indicates about the role of CO2 concentration in the process of
photosynthesis ?
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Under bright sunlight and a temperature of 20--25°C, with a plentiful supply of water, the amount
of carbon dioxide in the atmosphere (0.03%) seems likely to be the l_________ factor for most
plants. Increasing the level of the gas up to 1%* shows an increase in the photosynthetic rate of most
species.
d) Temperature
Q. Comment on the relationship between the rate of photosynthesis and
temperature.
Since the ‘dark’ reaction of photosynthesis is a series of en_______-controlled reactions, it is not
surprising that increasing the temperature speeds up the rate of the reaction. Photosynthesis usually
doubles in rate for every 10°C rise in temperature up to about 30°C, which is probably the opt_____
temperature for most plants for sh___ periods (about 30 minutes). Rates at higher temperatures
for longer periods soon begin to drop, probably due to e______ destruction and lim______
availability of carbon dioxide.
About 40° C the rate falls off rapidly as the enzymes are de_______. In temperate regions the range
of temperature for photosynthesis for the majority of the plants is probably around 10- 35° C. Tropical
species do not usually photosynthesise below about 5° C, whereas some conifers may continue to do so
around freezing point.
e) Water
Less than 1% of the water absorbed by most plants is needed for photosynthesis, so it is n__ likely that
its deficiency would affect the process directly. The indirect effects of water shortage however, are
likely to retard photosynthesis, causing dehy_______ which will affect the structure of protoplasm
and hence its metabolism. The structural arrangement of the chloroplast is likely to be affected too.
It was originally thought that water deficit closing the stomata would reduce carbon dioxide absorption,
but there is much experimental evidence to suggest that even stomata appearing closed when viewed
microscopically are still sufficiently open to admit the gas.
f) Mineral salts
Mineral salts are not directly involved as raw materials for photosynthesis, but shortage of either ir___
or m_________ affects the rate of chlorophyll production and hence, indirectly, the photosynthetic
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rate. Some ions, such as potassium and some trace elements, such as molybdenum, are also required,
though the exact function of some of them is obscure.
Macronutrients are required in large amounts
Carbon
C
Component of all organic compounds
Oxygen
O
supplied by air & water
Hydrogen
H
Component of all organic compounds
Nitrogen*
N
Part of chlorophyll, amino acids, proteins, nucleic acids
Phosphorus*
P
Used in photosynthesis and almost all aspects of growth, ATP, NADP+, nucleic acids
Potassium*
K
Activates enzymes, used in formation of sugar and starch
Calcium
Ca
Used in cell growth and division, part of cell wall
Magnesium#
Mg
Part of chlorophyll, activates enzymes
Sulfur
S
Part of amino acids and proteins
Micronutrients are required in trace amounts
Boron
B
Affects reproduction
Chlorine
Cl
Aids in root growth
Copper
Cu
Used in clorophyll, activates enzymes
Iron
Fe
Used in Photosynthesis, cytochromes
Manganese
Mn
activates enzymes
Sodium
Na
Used for water movement
Zinc
Zn
Part of enzymes, used in auxins
Molybdenum
Mo
Used in nitrogen fixation
Nickel
Ni
Liberates Nitrogen
Cobalt
Co
Fixates Nitrogen
Silicon
Si
Makes tougher cell walls: enhances heat and drought tolerance
B) Limiting Factors
Concept: the functioning of an organism is controlled or limited by that essential environmental
factor or combination of factors present in the least favorable amount.
Photosynthesis is a complex process, involving many fa___ and raw materials_____. If any one of
these factors is NOT at the opt______ level or is in short sup___ then it may hold up the rest of the
process and it is called a limiting factor.
Review questions:
1. Explain the concept of limiting factors with reference to light intensity, temperature, and
concentration of CO2.
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light:
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rate of photosynthesis increases as light intensity increases
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photosynthetic rate reaches plateau at high light levels
o
because photosystems are absorbing light at a maximum rate
CO2:
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photosynthetic rate reaches plateau at high light levels
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up to a maximum when rate levels off
o
because ribulose bisphosphate carboxylase is fixing carbon at a maximum rate
temperature:
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rate of photosynthesis increases with increase in temperature
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up to optimal level / maximum
o
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because greater kinetic energy creates more collisions between enzymes and substrates
high temperatures reduce the rate of photosynthesis
o
because high temperature denatures enzymes
o
by breaking intermolecular forces
o
altering the active sites
o
reducing the fit between enzymes and substrates
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Q. The effect of different light intensity on the photosynthetic rate of an intact leaf of a flowering plant measured at two
different concentrations of carbon dioxide (300 and 600 parts per million (ppm) was studied and the results summarized in
the following table:
Photosynthetic rate (Net C02 fixed in mg per 100 cm2 leaf area per hour)
Light intensity (kilolux)
300 ppm C02
0
-2.0
4
1.5
8
5.0
14
10.4
18
14.0
22
15.0
28
15.0
32
15.0
50
15.0
80
15.0
a) Present the results in graphical form.
600 ppm C02
-2.0
2.4
6.8
13.4
17.8
22.3
29.0
30.0
30.0
30.0
5m
b) Describe how the light intensity affects the rate of photosynthesis of the leaf.
3m
c) How does the performance of the leaf differ when put into the two different carbon dioxide concentrations?
4m
d) Account for the photosynthetic rate at zero light intensity. Identify and explain the points at which there is no net
of carbon dioxide by the leaf.
fixation
4m
e) With light intensity at 10 kilolux and CO2 concentration at 300 ppm, explain how you would expect the photosynthetic
rate of the leaf to change under
i) light wind, and
ii) moderate water deficiency conditions.
4m
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