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Chapter 11 – Photosynthesis and plant nutrition
CHAPTER 11 - PHOTOSYNTHESIS AND PLANT
NUTRITION
All living organisms need energy to sustain life. This energy is obtained from organic
molecules such as carbohydrates, fats and proteins. These molecules contain chemical
energy. The initial formation of such molecules takes place in green plants which are
able to transfer energy from sunlight into energy in carbohydrates. This process is
called photosynthesis.
11.1 Leaf structure
The leaf is the main photosynthetic structure of a plant, although stem, sepals and other
parts may also photosynthesize. The leaf is adapted to bring together the three raw
materials, water, carbon dioxide and light, and to remove the products oxygen and
glucose. The structure of a leaf is shown in Fig 11.2.
The equation for photosynthesis may be summarized as
6CO2 + 6H2O+ SUNLIGHT 
C6H12O6 + 6O2
Carbon dioxide + water + sunlight  glucose + oxygen
Gas + liquid + energy -- liquid + gas (solution in water)
The adaptations of the leaf to photosynthesis are therefore:
1. To obtain energy (sunlight).
2. to obtain and remove gases (carbon dioxide and oxygen)
3. to obtain and remove liquids (water and sugar solution)
11.2 Energy and living organisms
Many processes which take place in living organisms require a supply of energy. These
include:
 movement, for example locomotion of the whole organism using muscles or cilia,
circulation of fluids within animal bodies or movement of organelles within cells,
 the synthesis of large molecules such as proteins from smaller molecules such as
amino acids,
 active transport of substances across cell surface membranes
 in mammals and birds, the production of heat for the maintenance of body
temperature above that of the environment.
For all of these processes except heat production, the immediate source of energy is
almost always the chemical substance adenosine triphosphate, or ATP. ATP molecules
contain a relatively large amount of chemical energy in their molecular structure. This
property is shared with many other molecules, but what makes ATP special is the ease
with which living organisms can make use of some of this energy.
When one of the phosphate groups is removed from an ATP molecule, some energy is
released.
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The reaction is a hydrolysis reaction, and if it took place in a test tube the released
energy would be dispersed, mostly as heat, to the environment. Living organisms,
however, can use the energy that ATP releases in many different ways.
You can think of ATP as the 'energy currency' of living organisms. Like money, it is
constantly 'recycled'. ATP is the molecule from which energy can be quickly and easily
used by a living cell. Other molecules, such as carbohydrates and fats, also contain a lot
of energy, but the energy from these molecules cannot be accessed anywhere near as
readily or directly as can the energy in ATP. Carbohydrates and fats act as energy
stores, while ATP is energy currency.
Another advantage of ATP is that it provides energy in small 'packets'. The energy
released from a single ATP molecule is often enough to supply the energy needs of a
single chemical reaction.
As every cell needs a constant supply of energy, it must have a constant supply of A
TP. Each cell has to make its own ATP. All living cells make ATP using energy from other
molecules, especially glucose, in the process of respiration. In respiration, some of the
chemical energy in the molecular structure of glucose is transferred to ATP. When ATP is
used, this transferred energy is lost and so A TP has to be regenerated. The ATP is
recycled.
PHOTOSYNTHESIS
11.3 Energy transfers in photosynthesis
In order to make large amounts of ATP, living cells need a supply of glucose or
other organic molecules. Where do these glucose molecules come from? Where did the
energy in the glucose molecules, which is transferred into the ATP molecules, originate?
Almost all the energy transferred to all the ATP molecules in living organisms
originally came from energy in sunlight. Green plants and some kinds of bacteria are
able to transfer sunlight energy into energy trapped in the molecular structure of
carbohydrates. This process is called photosynthesis.
In photosynthesis, green plants use the energy in sunlight to combine water and
carbon dioxide to produce carbohydrates. The overall equation for this process is:
6C02 + 6H20 - C6H1206 + 602
Or
nC02 + nH20 - (CH20) n + n02
Once carbohydrates such as glucose have been made, the plant can then
convert some of them to other organic substances, such as oils, nucleic acids
and proteins. Animals cannot make organic substances from inorganic ones,
and rely entirely on plants for their supply of organic molecules. Without
plants, animals would have no energy supply, nor a supply of carbon from
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which to build their bodies.
Photosynthesis happens inside chloroplasts, in the leaves of green plants.
11.4 A summary of photosynthesis
Photosynthesis uses two raw materials (water and carbon dioxide) and
an energy source (sunlight). Some of the energy from sunlight is used to begin
a chain of events in which hydrogen atoms from the water molecules are
combined with carbon dioxide molecules to produce sugars. These sugars
contain energy which was originally energy in the sunlight. The oxygen from
the water molecules is given off as a waste product.
The conversion of water and carbon dioxide to sugars and oxygen takes place in
several steps. These steps can be divided into two stages (Fig. 11.4).
Fig 11.4 input & outputs in the stages of photosynthesis
• The light reaction or the light-dependent stage
This is the first stage of photosynthesis. Its function is to make reduced NADP (NADPH)
and to transfer light, energy to ATP.
Energy from sunlight causes electrons to be emitted from chlorophyll
molecules. Some of the energy from these electrons is used to make ATP
molecules. Some is used to make reduced NADP. Water molecules are split to
produce electrons, which replace the electrons that were ejected by light from the
chlorophyll molecules, and hydrogen ions and electrons to help with the production
of ATP. Oxygen is also produced and given off as a by-product.
• The light-independent stage, or the Calvin cycle
This stage incorporates the inorganic molecule carbon dioxide into a carbohydrate.
The reduced NADP and ATP which have been made in the light-dependent
stage are used to convert carbon dioxide into sugars. No further input of light is
needed for this stage, although, of course, it cannot take place unless the lightdependent stage is producing a good supply of reduced NADP and ATP.
In green plants these processes all occur inside chloroplasts, mostly in the leaves.
The light-dependent stage takes place in the membranes inside the chloroplast, which
form thylakoids that are stacked in layers called grana. The Calvin cycle takes place in
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the stroma of the chloroplast.
8.4 A more detailed look at the light-dependent reaction
White light, such as which reaches the Earth from the Sun, is made up of light of many
different wavelengths. We see different wavelengths of light as different colours. Light
with short wavelengths, of around 450 to 500 nm, is seen as blue light, while longer
wavelengths, of around 650 to 700 nm, appear as red light.
Green plants contain coloured substances, or pigments, which can absorb some
wavelengths of light. The main pigment is chlorophyll, which looks green because it
reflects green light. It can absorb most of the other wavelengths of light, especially red
and blue light.
Chlorophyll molecules are found on the membranes inside chloroplasts. When light hits
a chlorophyll molecule, some of the energy in the light is absorbed by the chlorophyll.
This absorbed energy causes an electron to be emitted from the chlorophyll. The
electron possesses some of the energy from the light. It is a high-energy electron (Fig.
11.5).
The electron is passed along a chain of electron carriers.
These are molecules which can pick up the electron and then pass it on to another
molecule. When they pick up the electron they are reduced, and when they lose it again
they are oxidised. In the chloroplast the electron carriers lie in the membrane of the
thylakoid, so that the electron can easily be passed from one to the next.
As the electron is passed along, some of its energy is used to synthesise ATP. The
making of ATP from ADP and inorganic phosphate (P;) is called phosphorylation. As this
is being done using energy from light, it is given the name photophosphorylation.
The electron is then passed on to a second, different chlorophyll molecule which has
also been excited by light to emit an electron. Because the electron does not go back to
where it came from, this process is called non-cyclic photophosphorylation.
The electron from the second chlorophyll molecule is not used to make ATP. Instead,
two of these electrons (e-) are taken up by a molecule of oxidised NADP (NADP+),
together with a hydrogen ion (H+), to form reduced NADP (NADPH).
The first chlorophyll molecule is still short of a electron. It gets one from a water
molecule which is to produce hydrogen ions, electrons and oxygen.
This water-splitting process is catalysed by an enzyme in the chloroplast. It only
happens when light falls onto the chloroplast, and is called photolysis of water.
Overall, the process can be summarised by the equation:
2NADP++ 2H2O- 2NADPH +O2+2H+
Some of the oxygen is used by the plant in respiration, but much of it is released to
the environment as a gas.
So, at the end of the light-dependent reaction, two new substances have been made.
Both of them - ATP and reduced NADP - contain energy which was derived from the
sunlight which fell onto the chlorophyll molecules. These two substances will be used in
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the next stage - the light-independent reactions.
Fig 11.5 Non- cyclic photophosphorylation
8.5 A more detailed look at the light-independent reaction, or Calvin cycle
In the Calvin cycle, hydrogen is added to carbon dioxide to make carbohydrates. The
hydrogen comes from reduced NADP, and the energy needed to drive the reactions
comes from ATP and reduced NADP. The reduced NADP and A TP were made in the lightdependent reactions.
Fig. 11.13 shows the main stages in the Calvin cycle.
Carbon dioxide from the air diffuses through the stomata into the leaf, into the air
spaces in the mesophyll, into a palisade cell and into a chloroplast. Here, in the stroma,
it meets an enzyme called ribulose bisphosphate carboxylase. This enzyme, otherwise
known as RuBP carboxylase, or Rubisco, is thought to be the most abundant enzyme in
the world. The stroma of every chloroplast in every green plant is full of this enzyme.
Rubisco catalyses the combination of carbon dioxide with a five-carbon sugar called
ribulose bisphosphate, or RuBP.
At this point the carbon becomes part of a molecule made by the plant, and is said to
be fixed. The addition of a carbon dioxide molecule to the ribulose bisphosphate briefly
forms a six-carbon molecule, but this immediately splits to form two molecules which
each contain three carbons. They are called glycerate 3-phosphate, or GP for short.
Rubisco
RuBP + carbon dioxide --------- 2 glycerate 3-phosphates
5 carbons
1 carbon
2 X 3 carbons
At this point, the products of the light –dependent reaction, reduced NADP and ATP, are
needed. The reduced NADP and some of the ATP used supply energy and hydrogen
atoms needed to reduce the GP to produce triose phosphate. Triose phosphate is a
three-carbon phosphorylated sugar.
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About one-sixth of the total amount of triose phosphate made is then used to make
other carbohydrates (such as glucose, cellulose or starch), amino acids, fatty acids or
glycerol. The remaining five-sixths are converted back to RuBP again. If this was not
done, the plant would quickly run out of RuBP. The regeneration of RuBP uses up more
of the ATP from the light reaction. It is this regeneration which makes this process a
cycle.
Fig 11.6 overview of the pathways of photosynthesis in a chloroplast
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Fig 11.13 the calvin cycle
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The importance of photosynthesis
Photosynthesis makes life possible for most organisms:
1. During photosynthesis the light energy of the sun is transformed into chemical potential
energy which is stored in glucose and other carbohydrates.
2. Fats and proteins are synthesised from these carbohydrates. Animals, fungi, bacteria (all
the heterotrophic organisms) are therefore dependent on photosynthesis for their
carbohydrates, fats and proteins.
3. During photosynthesis, carbon dioxide is absorbed. Too much carbon dioxide in the water
and in the air is poisonous to animals and this is how carbon dioxide is reduced in nature.
4. Photosynthesis also released large amounts of oxygen, which most plants and animals
require for cellular respiration.
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5. Fuels such as oil, coal and petroleum are required by machines and vehicles. These fuels
are derived from chlorophyll-containing organisms which lived on earth centuries ago
and obtained their energy for photosynthesis from the sun.
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