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Page 1 of 15 JHM March 2016 Unit 3: Sustainability and Interdependence Sub-topic 3.1 (b) Plant growth and Productivity
Page 1 of 15 JHM March 2016
Unit 3: Sustainability and Interdependence
Sub-topic 3.1 (b) Plant growth and Productivity
Page 2 of 15 JHM March 2016
On completion of this sub-topic I will be able to:
• explain that leaves trap light energy;
• explain that this light energy is used in the process of photosynthesis;
• outline how carbohydrates are produced as a result of the process of photosynthesis;
• state that visible light is made up of a range of wavelengths;
• explain that different wavelengths have different colours - red, orange, yellow, green,
blue indigo and violet;
• state that these colours are known as the visible spectrum of light;
• describe that light that strikes the surface of a leaf can be absorbed, reflected or
transmitted;
• describe how synthetic pigments in the leaves absorb light energy and change it into
chemical energy;
• state that the main photosynthetic pigment is chlorophyll;
• describe how the wavelengths of light that are absorbed by a pigment are called its
absorption spectrum;
• explain that the absorption of light by chlorophyll occurs mainly in the blue and red
regions of the spectrum;
• state that the accessory pigments are known as the carotenoids;
• describe how the accessory pigments (carotenoids) absorb light from other regions of
the spectrum and pass the energy onto chlorophyll;
• explain that photosynthesis takes place in the chloroplasts of plant cells;
• explain how the wavelengths of light actually used by a pigment in photosynthesis are
called its action spectrum;
• outline that photosynthesis occurs in two stages : the light dependent stage and the
carbon fixation stage;
• explain that absorbed energy excites electrons in the pigment molecule;
• state that there is transfer of these high-energy electrons through an electron transport
chain (ETC);
• outline how the ETC releases energy to generate ATP by ATP synthase;
• state that some light energy is used in the photolysis of water;
• describe how this is the splitting of water releasing oxygen as a by-product and
hydrogen;
• explain that hydrogen is transferred to the coenzyme NADP;
• describe how ATP and NADPH from the light dependent stage are transferred to the
carbon fixation stage;
• explain that the enzyme RuBisCO fixes carbon dioxide by attaching it to ribulose
bisphosphate (RuBP);
• state that this occurs in the Calvin Cycle;
• explain that the 3-phosphoglycerate produced is phosphorylated by ATP and combined
with hydrogen from NADPH to form glyceraldehyde-3-phosphate (G3P);
• outline how G3P is used to regenerate RuBP and for the synthesis of sugars;
Page 3 of 15 JHM March 2016
• describe how these sugars may be synthesised into starch or cellulose or pass to
other biosynthetic pathways to form a variety of metabolites;
• state that net assimilation is the increase in mass due to photosynthesis minus the
loss due to respiration;
• state that this can be measured by the increase in dry mass per unit leaf area;
• explain that productivity is the rate of generation of new biomass per unit area per
• unit of time;
• outline how biological yield of a crop is the total plant biomass;
• state that economic yield is the mass of desired product;
• show that harvest index is calculated by dividing the dry mass of economic yield
by the dry mass of biological yield.
Prior Learning
Unit (1.7) Photosynthesis
State that photosynthesis is a series of enzyme-controlled reactions which allow green plants to
make their own food. Write the summary equation for photosynthesis. Light energy
CO2 + H2O → Sugar + Oxygen
Chlorophyll State that photosynthesis takes place in the chloroplast.
State that photosynthesis occurs in two stages:
1. Light reactions (Photolysis).
2. Carbon fixation.
State that in stage one, light energy from the sun is trapped by chlorophyll in the chloroplasts and is
converted into chemical energy in the form of ATP.
State that in stage one, water is split to produce hydrogen and oxygen.
State that oxygen is released and diffuses out of the cell.
State that in stage one, hydrogen is picked up by a hydrogen acceptor molecule and passed to the
second stage. In summary photolysis is the process by which energy from sunlight is used to split water molecules
into hydrogen and oxygen and provide ATP.
State that in stage two, the hydrogen from stage one is combined with carbon dioxide from the air
to form sugar. State that ATP (from stage one) provides the energy needed to carry out a series of enzyme
controlled reactions to produce this sugar.
The chemical energy in sugar is available for respiration.
Sugar can be converted into other plant products e.g. starch as a storage carbohydrate and cellulose
as a structural component of the cell wall. State that a limiting factor in photosynthesis is anything which, when in short supply, reduces or
limits the rate of photosynthesis taking place, e.g. carbon dioxide concentration, light intensity and
temperature affect photosynthesis and therefore cell growth.
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Before you can understand in more detail how photosynthesis occurs in the plant cell, you
must first understand a bit about light.
Light is a form of electromagnetic radiation which travels in waves.
The distance between two crests on a wave is called the wavelength and is measured in
nanometres (nm). (1 nm = 10-9 m)
Visible light is made up of a spectrum of colours. Each colour of light has a different
wavelength.
400 nm 500 nm 600 nm 700 nm
When white light hits a leaf, it has three possible fates:
Violet indigo blue green Yellow orange red
Remember ART • Absorbed: Light is taken into the leaf
Reflected: Light is bounced back from the leaf surface
• Transmitted: Light passes right through
the leaf
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The light energy that is absorbed by the leaves is captured by several different pigments
that are present.
There are four pigments in the leaf:
1. Chlorophyll a
2. Chlorophyll b
3. Carotenoids
The different pigments absorb different wavelengths of light.
When viewing the spectrum through chlorophyll, it looks like this:
The blue and violet are no longer visible and only some of the red is still seen.
These have been absorbed by the leaf pigments. – chlorophyll absorbs in the blue and
red regions of the visible spectrum.
These are most important wavelengths for a plant in photosynthesis.
Each pigment absorbs a different wavelength of light – this is shown by an absorption
spectrum.
Comparison of absorption and action
spectra reveals a close match – this
is good evidence for the importance
of leaf pigments in photosynthesis.
Pigments 2 and 3 are accessory pigments
The carotenoids (carotene and xanthophyll) extend the range
of wavelengths absorbed for photosynthesis and pass the
energy to chlorophyll
An absorption spectrum is a graph showing the wavelengths of light absorbed by
different photosynthetic pigments
An action spectrum is a graph showing the rate of photosynthesis at different
wavelengths of light.
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The main pigments are Chlorophyll a and b. However, the carotenoids allow the plant to
carry out photosynthesis in a wider range of light wavelengths.
They are known as accessory pigments as they pass the energy they capture onto
chlorophyll a.
Why does chlorophyll appear green in colour?
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Which wavelengths of light are absorbed by chlorophyll a and b?
Which pigments absorb light in other regions?
What do carotenoids do with the light energy they absorb?
Explain the difference between the absorption spectrum and the action spectrum.
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Why is it an advantage to the plant to have more than one photosynthetic pigment?
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Page 7 of 15 JHM March 2016
Photosynthesis: Stage 1 - Capturing Light Energy
Photosynthesis is a series of enzyme-controlled reactions which occur in the chloroplasts of plants. In photosynthesis, organic molecules such as carbohydrates are synthesised. The energy to drive the reactions comes from light. Light energy is absorbed into the pigments contained in plant cell chloroplasts. The first stage of photosynthesis is dependent on light and is often referred to as the light reactions. Light is absorbed into organelles called chloroplasts that are found mainly in the palisade layer of the leaf.
Chlorophyll is contained in the grana of the chloroplast (singular – granum).
The chloroplast
The photosynthetic pigments are contained within the grana. Therefore this is where
absorption of light energy and photosynthesis take place.
Page 8 of 15 JHM March 2016
The capture of energy and photolysis
In the chloroplast, when light energy is absorbed by the leaf pigments, its electrons become
excited and are raised to a higher energy state.
The electrons are then transferred along an electron transport chain releasing
energy.
This energy is used by the enzyme ATP synthase to generate ATP. This is called
photophosphorylation.
Some of the energy is also used to split water, (photolysis of water), into hydrogen –
this is picked up by the hydrogen acceptor NADP to make NADPH – and oxygen.
The ATP and NADPH produced in the light reaction of photosynthesis are used in the next
stage of photosynthesis referred to as the Calvin Cycle.
Page 9 of 15 JHM March 2016
Photosynthesis: Stage 2 - The Calvin Cycle (Carbon fixation)
At the end of the first stage of photosynthesis (the light dependent stage), the hydrogen (in
the form of NADPH) and the ATP are essential for the second stage – known as the Calvin
cycle.
The Calvin cycle (the light-independent part of photosynthesis) occurs in the stroma of the
chloroplast. It consists of a series of enzyme controlled reactions which bring about:
• carbon fixation;
• regeneration of ribulose bisphosphate (RuBP).
1. Carbon dioxide is obtained by the diffusion of atmospheric carbon dioxide through the
leaf to the chloroplast. The carbon dioxide enters the cycle and becomes attached to
RuBP (ribulose bisphosphate). This reaction is controlled by the enzyme RuBisCO
(ribulose bisphosphate carboxylase/oxygenase). The enzyme RuBisCO fixes CO2 by
attaching it to the RuBP.
2. The Carbon dioxide and RuBP combine to make 3-phosphoglycerate.
3. The 3-phosphoglycerate then joins with the hydrogen from NADPH and is
phosphorylated by the addition of inorganic phosphate (Pi) from ATP which supplies the
energy.
4. This process produces glyceraldehyde-3-phosphate (G3P).
5. Some G3P is then used to regenerate RuBP (to continue the process). The remainder is
used to synthesis glucose.
The fate of glucose after photosynthesis:
The sugar formed during photosynthesis goes onto the following uses:
Amino acids
Page 10 of 15 JHM March 2016
Complete the diagram using the terms listed below;
G3P
RuBp
Intermediate compound
(3 phosphoglycerate)
Page 11 of 15 JHM March 2016
Describe what happens to the light energy that is trapped by chlorophyll.
How does the enzyme RuBisCO fix carbon dioxide?
How is G3P formed?
What is G3P used for?
What is the fate of the glucose produced during photosynthesis?
Page 12 of 15 JHM March 2016
Plant productivity
The biomass of a population of plants is its total mass. This is normally measured as dry
mass – as the water content of living organisms varies greatly throughout the year.
Productivity is the rate at which plants in an ecosystem generate new biomass,
per unit area per unit of time e.g. grams per square metre per year ( gm-2y-1 ).
The conversion of the glucose produced by photosynthesis into complex components of the
plant cell is called assimilation. This process causes an increase in the plants biomass.
However, some of the sugar produced in photosynthesis is used up during respiration,
therefore:
Net assimilation is the increase in mass due to photosynthesis minus the loss due to
respiration.
This can be measured by the increase in dry mass per unit leaf area.
Factors affecting plant productivity:
The rate of photosynthesis (and productivity) is affected by several environmental factors
known as limiting factors.
These include: temperature, light intensity and carbon dioxide concentration.
The rate at which photosynthesis proceeds is limited by whichever one of these factors is in
short supply.
e.g. Light intensity would be the limiting factor on a dull summer day.
Assessing how productive an area of land could be:
In order to grow crops efficiently to ensure the maximum yield, farmers/scientists/policy
makers need to be able to assess the growing potential of the area.
The biological yield of an area is the total biomass of plant produced this is useful
because tells you the total producing power of the land.
The economic yield of an area is the mass of the desired product (e.g. mass of just the
barley grains from a barley field)
Increase in
dry
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The harvest index is the proportion of economic yield to the total biological yield (total biomass). It is calculated using this formula:
This gives a useful estimate of what is wasted during the growth of a crop.
Q1: Link the correct term to each definition.
Terms: • harvest index
• net assimilation
• biological yield
• productivity
• economic yield
Definitions 1. Increase in mass due to photosynthesis minus the loss due to respiration 2. Rate of generation of new biomass 3. Total plant biomass 4. Mass of desired product 5. Calculated by dividing the dry mass of economic yield by the dry mass of biological yield
Q2: What is the measurement for net assimilation? Q3: What is the measurement for productivity? ____________________________________________________________________________
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Q4: A field of grain was harvested and the following results obtained from the results of separating the dry mass of economic yield from the dry mass of biological yield.
Calculate the harvest index for the grain.
Page 14 of 15 JHM March 2016
3.2 Plant growth and productivity
I can state that Traffic light
Photosynthesis traps light energy to produce carbohydrates.
Light striking a leaf is transmitted, reflected or absorbed.
• Of the absorbed light, a small percentage is used to drive the reactions of photosynthesis.
• The wavelengths of light that are absorbed by a photosynthetic pigment are called its absorption spectrum.
• Absorption of light by chlorophyll occurs mainly in the blue and red regions of the spectrum
The green region of the spectrum has the least absorption.
Accessory pigments absorb some light from other regions of the spectrum and pass the energy onto chlorophyll.
• The accessory pigments are the carotenoids. They absorb light from different parts of the spectrum and pass the energy on to chlorophyll.
• The wavelengths of light actually used by a pigment in photosynthesis are called, its action spectrum.
• Photosynthesis is a series of enzyme-controlled reactions that synthesise carbohydrate from carbon dioxide and water.
The process of photosynthesis uses light energy to drive the reactions.
The light is absorbed by the pigment chlorophyll, found in chloroplasts.
• In photosynthesis, light energy is used to regenerate ATP and to split a water molecule in a process called the photolysis of water.
• Chlorophylls a is the major pigment involved in absorption for photosynthesis.
• The absorption spectrum of chlorophyll is closely related to the rate of photosynthesis.
• An action spectrum can be constructed by using the number of oxygen bubbles produced by a photosynthesising plant
• The action spectrum is closely related to the absorption spectrum of all photosynthetic pigments.
• Chromatography is a practical technique, using absorbent material and a solvent, to separate the pigments that are contained in the chloroplasts.
• Photosynthesis occurs in two stages: the light dependent stage and the Calvin Cycle.
• Light energy absorbed by the pigments is used to produce ATP for use in the Calvin Cycle.
Page 15 of 15 JHM March 2016
I can state that Traffic light
Some light energy is used in photolysis. This is the splitting of water releasing oxygen as a by-product and hydrogen.
The hydrogen is transferred to the Calvin Cycle by the hydrogen acceptor NADP that becomes NADPH.
The ATP and NADPH from the light dependent stage are transferred to the Calvin Cycle.
The Calvin Cycle takes place in the stroma of the chloroplast.
The Calvin Cycle requires energy provided by ATP from the light dependent stage.
The enzyme RuBisCO fixes carbon dioxide from the atmosphere by attaching it to RuBP.
The 3-phosphoglycerate produced is phosphorylated by ATP and combines with the hydrogen from NADPH to form glyceraldehyde 3 phosphate (G3P)
G3P sugar may be synthesised into starch, cellulose or other metabolites
• Some G3P is used to regenerate RuBP to continue the cycle
• Major biological molecules in plants such as proteins, fats, carbohydrates and nucleic acids are derived from the photosynthetic process.
• The rate of photosynthesis is affected by environmental conditions, such as carbon dioxide availability, light intensity and temperature.
• The rate of photosynthesis can be measured by: – oxygen evolution; – production of carbohydrate; – carbon dioxide uptake.
• Net assimilation is the increase in mass due to photosynthesis minus the loss due to respiration.
• Net assimilation can be measured by increase in dry mass per unit leaf area.
• Productivity is the rate of generation of new biomass per unit area per unit of time.
• Biological yield of crop is the total plant biomass.
• Economic yield is the mass of desired product.
• The harvest index is calculated by dividing the dry mass of economic yield by the dry mass of biological yield.
