Download 4. Photosynthesis Booklet [A2]

SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Photosynthesis
Learning objectives:
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Define the terms autotroph and heterotroph;
State that light energy is used during photosynthesis to produce complex organic molecules;
Explain how respiration in plants and animals depends upon the products of photosynthesis;
State that in plants photosynthesis is a two-stage process taking place in chloroplasts;
Explain, with the aid of diagrams and electron micrographs, how the structure of
chloroplasts enables them to carry out their functions;
Key definitions:
Compile a glossary by writing your own definitions for the following key terms related to the
learning objectives above.
Key term
autotroph
heterotroph
chloroplast
envelope
lamellae
granum
stroma
thylakoid
light-dependent stage
photosynthetic pigment
Definition
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Key term
A2 BIOLOGY
Definition
photosystems
light-independent stage
chlorophyll
Photosynthesis is of fundamental importance to living things because it transforms sunlight energy
into chemical energy stored in molecules. This becomes part of the energy available in food chains.
The molecules that trap the energy in their chemical bonds are also used as building blocks to
create other molecules. Finally, photosynthesis releases free oxygen gas, essential for the survival of
advanced life forms.
Triose phosphate (a 3-carbon sugar) is converted by a number of steps to:
Glucose – used as the fuel for cellular respiration; supplies energy for metabolism.
Cellulose – glucose is used as a building block for creating cellulose, a component of plant
cell walls.
Starch – stored as a reserve supply of energy in starch granules, to be converted back into
glucose when required.
Disaccharides – glucose is converted to other sugars such as fructose, found in ripe fruit, and
sucrose, found in sugar cane.
Lipids
Amino acids
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Structure and role of chloroplasts
Photosynthesis takes place within organelles called chloroplasts. They vary in shape and size but
most are disc-shaped and between 2-10µm long. Each chloroplast is surrounded by a double
membrane, an envelope. There is an intermembrane space, about 10-20nm in width, between the
inner and outer membrane. The outer membrane is permeable to many small ions. The inner
membrane is less permeable and has transport proteins embedded in it. It is folded into lamellae
(thin plates) which are stacked up like piles of pennies. Each stack of lamellae is called a granum
(plural, grana).
There are two distinct regions inside each chloroplast – the stroma and the grana. Both of these
regions can be seen when chloroplasts are viewed under a light microscope.
The grana are stacks of thylakoid membranes which contain chlorophyll and are the site of
the light dependent phase. The light dependent phase is the process of energy capture via
photosystems I and II.
The stroma is the fluid-filled matrix of the chloroplast in which the reactions of the light
independent phase of photosynthesis takes place. The light independent phase is the
process of carbon fixation via the Calvin cycle.
The thylakoid membranes provide a large surface area for the photosynthetic pigments, electron
carriers and ATP synthase enzymes, all of which are involved in the light-dependent reaction. The
photosynthetic pigments are arranged into special structures called photosystems, which allow
maximum absorption of light energy. Proteins embedded in the grana hold the photosystems in
place.
The fluid-filled stroma contains the enzymes needed to catalyse the reactions of the lightindependent stage. The grana are surrounded by the stroma so the products of the light-dependent
reaction, which are needed in for the light-independent reaction can readily pass into the stroma.
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A2 BIOLOGY
1. Fig. 3.1 is an electron micrograph of a chloroplast from a tobacco leaf.
(a) Identify the structures labelled W to Z.
W ___________________________________________________________
X ___________________________________________________________
Y ___________________________________________________________
Z ________________________________________________________ [4]
(b) In addition to the structures seen in Fig. 3.1, a chloroplast also contains DNA
and ribosomes.
Suggest the role of DNA and ribosomes in this organelle.
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2. Explain what is meant by the terms autotroph and heterotroph.
autotroph _____________________________________________________
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
heterotroph ___________________________________________________
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[2]
3. Fig. 3.1 is a transmission electron micrograph showing part of a chloroplast
including some of the internal membranes.
(a) Identify E and F in Fig. 3.1.
E ___________________________________________________________
F ___________________________________________________________
[2]
(b) The chloroplast contains fat droplets, as shown in Fig. 3.1. These act as a
reserve of raw material for the chloroplast.
Suggest what this raw material might be used for in the chloroplast.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Pigments and light absorption
Learning objectives:
 Define the term photosynthetic pigment;
 Explain the importance of photosynthetic pigments in photosynthesis;
 State that the light-dependent stage takes place in thylakoid membranes and that the lightindependent stage takes place in the stroma;
As light meets matter, it may be reflected, transmitted or absorbed. Substances that absorb visible
light are called pigments, and different pigments absorb light of different wavelengths. The ability
of a pigment to absorb particular wavelengths of light can be measured with a spectrophotometer.
The light absorption vs wavelength is called the absorption spectrum of that pigment. The
absorption spectrum of different photosynthetic pigments provides clues to their role in
photosynthesis, since light can only perform work if it is absorbed. An action spectrum profiles the
effectiveness of different wavelengths of light in fuelling photosynthesis. It is obtained by plotting
wavelength against some measure of photosynthetic rate (e.g. CO2 production).
The electromagnetic spectrum
Light is a form of energy known as electromagnetic radiation. The segment of the electromagnetic
spectrum most important to life is the narrow band between about 380 and 750 nanometres (nm).
This radiation is known as visible light because it is detected as colours by the human eye (although
some other animals such as insects, can see in the ultraviolet range). It is the visible light that drives
photosynthesis.
Electromagnetic radiation (EMR) travels in waves, where wavelength provides a guide to the energy
of the photons; the greater the wavelength of EMR, the lower the energy of the photons in that
radiation.
SACKVILLE SCIENCE DEPARTMENT
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The photosynthetic pigments of plants
The photosynthetic pigments of plants fall into two categories: chlorophylls (which absorb red and
blue-violet light) and carotenoids (which absorb strongly in the blue-violet and appear orange,
yellow or red). The pigments are located on the chloroplast membranes (the thylakoids) and are
associated with membrane transport systems.
The pigments of chloroplasts in higher plants (chlorophylls and carotenoids) absorb blue and red
light (which is reflected). Each photosynthetic pigment has its own characteristic absorption
spectrum (see graph (a)).
The absorption spectra of photosynthetic pigments shows the relative amounts of light absorbed at
different wavelengths.
(a)
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Although only chlorophyll a can participate directly in the light reactions of photosynthesis, the
accessory pigments (chlorophyll b and carotenoids) can absorb wavelengths of light that chlorophyll
a cannot. The accessory pigments pass the energy (photons) to chlorophyll a, thus broadening the
spectrum that can effectively drive photosynthesis.
(b)
The action spectrum for photosynthesis (see graph (b)) shows the effectiveness of different
wavelengths in fuelling photosynthesis.
The action spectrum and the absorption spectrum for the photosynthetic pigments (combined)
match closely.
1. Explain what is meant by the absorption spectrum of a pigment.
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2. Explain why the action spectrum for photosynthesis does not exactly match the absorption
spectrum of chlorophyll a.
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SACKVILLE SCIENCE DEPARTMENT
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1. Some aquatic photosynthetic organisms, for example seaweeds, contain
pigments such as fucoxanthin and phycoerythrin, in addition to chlorophyll.
These pigments give seaweeds a brown or red colour and are produced in
larger quantities in those seaweeds that live in deeper water.
Suggest why the presence of these pigments is an advantage to seaweeds that
live in deeper water.
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2. An experiment was carried out into the effect of different wavelengths of light
on the rate of photosynthesis.
Four sealed test tubes were set up, each containing three leaf discs from the
same plant suspended above hydrogencarbonate indicator solution. This
solution changes colour at different pH values, as show below.
At the start of the experiment, the contents of all four tubes were orange-red.
Each tube was illuminated by a lamp with a coloured filter in front of it. The
tubes were illuminated for the same length of time. The colour changes were
noted and the results are shown in the table below:
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A fifth tube was set up in the same way as the other tubes. This tube was then
covered in black paper before being illuminated for the same length of time.
The final colour of the hydrogencarbonate indicator in this tube was yellow.
(a) State the purpose of the tube covered with black paper.
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(b) State two precautions that need to be taken when designing and carrying
out this experiment in order to obtain results from which valid conclusions
can be drawn. Explain the need for each precaution.
precaution 1 ___________________________________________________
explanation ____________________________________________________
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precaution 2 ___________________________________________________
explanation ____________________________________________________
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(c) Name the pigment at the reaction centre of photosystems I and II.
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(d) Explain the change observed in the tube exposed to green light.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
The biochemistry of photosynthesis
Like cellular respiration, photosynthesis is a redox process, but the electron flow evident in
respiration is reversed. In photosynthesis, water is split and electrons are transferred together with
hydrogen ions from water to CO2, reducing it to sugar. The electrons increase in potential energy as
they move from water to sugar. The energy to do this is provided by light. Photosynthesis comprises
two phases. In the light dependent phase, light energy is converted to chemical energy (ATP and
reducing power). In the light independent phase (or Calvin cycle), the chemical energy is used for
the synthesis of carbohydrate. The light dependent phase shows non-cyclic phosphorylation. In
cyclic phosphorylation, the electrons lost from photosystem II are replaced by those from
photosystem I. ATP is generated, but not NADPH.
Learning objectives:
 Outline how light energy is converted to chemical energy (ATP and reduced NADP) in the
light-dependent stage (reference should be made to cyclic and non-cyclic
photophosphorylation, but no biochemical detail is required);
 Explain the role of water in the light-dependent stage;
 Outline how the products of the light-dependent stage are used in the light-independent
stage (Calvin cycle) to produce triose phosphate (TP) [reference should be made to ribulose
bisphosphate (RuBP), ribulose bisphosphate carboxylase (rubisco) and glycerate 3phosphate (GP), but no other biochemical detail is required;
 Explain the role of carbon dioxide in the light-dependent stage (Calvin cycle);
 State that TP can be used to make carbohydrates, lipids and amino acids;
 State that most TP is recycled to RuBP;
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Key definitions:
Compile a glossary by writing your own definitions for the following key terms related to the
learning objectives above.
Key term
NADP
reduced NADP
electron acceptors
electron carriers
photophosphorylation
ribulose bisphosphate
(RuBP)
glycerate 3-phosphate
(GP)
triose phosphate (TP)
photolysis
Calvin cycle
ATP synthetase
NADP+ reductase
chemiosmosis
Definition
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A2 BIOLOGY
Light dependent phase (energy capture)
The light dependent phase of photosynthesis takes place on the thylakoid membranes of the
chloroplasts. The photosystems, with the photosynthetic pigments, are embedded in these
membranes. Photosystem I (PSI) occurs mainly on the intergranal lamellae and PSII occurs almost
exclusively on the granal lamellae. These pigments trap light energy (usually from sunlight) so that it
can be converted to chemical energy in the form of ATP.
Photophosphorylation is the making of ATP from ADP and Pi in the presence of light. Light has a
dual nature: it can be thought of as travelling in waves, and as travelling in particles called photons.
When a photon hits a chlorophyll molecule the energy of the photon is transferred to two electrons
and they become excited.
Photosystem complexes comprise hundreds of pigment molecules, including chlorophyll a and b.
Photosystem II absorbs light energy to elevate electrons to a moderate energy level.
Photosystem I absorbs light energy to elevate electrons to an even higher level. Its electrons are
replaced by electrons from photosystem II.
These electrons are captured by electron acceptors and passed along a series of electron carriers
embedded in the thylakoid membranes. The electron carriers are proteins that contain iron atoms.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
When chlorophyll molecules absorb light, an electron is excited to a higher level. This electron ‘hole’
must be filled. Each electron is passed from one electron carrier to another, losing energy as it goes.
This energy is used to pump hydrogen ions across the thylakoid membrane. In non-cyclic
phosphorylation, the electrons lost to the electron transport chain are replaced by splitting a water
molecule (photolysis), releasing oxygen gas and hydrogen ions.
The thylakoid space is a hydrogen ion reservoir (low pH); flow of H+ back across the membrane is
coupled to ATP synthesis (by chemiosmosis). ATP synthetase converts ADP and inorganic phosphate
(Pi) into ATP. NADP is a hydrogen carrier picking up H+ from the thylakoid and transporting them to
the Calvin cycle.
Non-cyclic photophosphorylation
Light strikes photosystem II, exciting a pair of electrons that leave the chlorophyll molecule
from the primary pigment reaction centre.
The electrons pass along a chain of electron carriers and the energy released is used to
synthesise ATP.
Light has also struck photosystem I and a pair of electrons has been lost.
These electrons, along with protons (produced at photosystem II by the photolysis of water),
join NADP, which becomes reduced NADP (NADPH).
The electrons from the oxidised photosystem II replace the electrons from PSI.
Electrons from photolysed water replace those lost by the oxidised chlorophyll in PSII.
Protons from photolysed water take part in chemiosmosis to make ATP and are then
captured by NADP, in the stroma. They will be used in the light independent phase.
The role of water
Photosystem II also contains an enzyme that, in the presence of light, can split water into H + ions
(protons), electrons and oxygen. This splitting of water is called photolysis.
2H2O  4H+ + 4e- + O2
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Some of the oxygen produced in this way is used by the plant for its aerobic respiration but much of
it diffuses out of the leaves, through stomata, into the air.
Water is a source of :
hydrogen ions, which are used in chemiosmosis to produce ATP. These protons are then
accepted by a coenzyme NADP (nicotinamide adenine dinucleotide phosphate), which
becomes reduced NADP, to be used during the light dependent stage to reduce carbon
dioxide and produce organic molecules.
electrons to replace those lost by the oxidised chlorophyll;
Light independent phase (carbon fixation)
The light independent reaction, called the Calvin cycle, has also been labelled the ‘dark phase’ of
photosynthesis. This is not a good label as it is not necessary that the phase occur in darkness; it
simply does not need light to proceed. In the Calvin cycle, hydrogen (H+) is added to CO2 and a 5C
intermediate to make carbohydrate. The H+ and ATP are supplied by the light dependent phase.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
1. The table below contains statements that refer to the light-dependent stage of
photosynthesis.
Complete the table, indicating with the letters C, N or B, whether each
statement applies to:
 cyclic photophosphorylation only (C) or
 non-cyclic photophosphorylation only (N) or
 both cyclic and non-cyclic photophosphorylation (B)
The first one has been completed for you.
[5]
2. Fig. 3.2 represents the light harvesting system found on the surface of the
internal membranes of the chloroplast.
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A2 BIOLOGY
Use the information in Fig. 3.2 to describe how light is harvested in the
chloroplast membranes.
In your answer, you should use appropriate technical terms, spelled correctly
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
3. The molecules listed below are all associated with photosynthesis.
amino acid
ATP
carbon dioxide
glycerate-3-phosphate (GP)
oxygen
reduced NADP
ribulose bisphosphate (RuBP)
rubisco
triose phosphate (TP)
water
From these molecules, identify:
(a) the enzyme
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(b) a product of the light-dependent reaction that is used in the lightindependent reaction
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(c) a 3-carbon compound
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(d) a compound that can be made from TP but is not part of the Calvin cycle
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(e) a 5-carbon compound
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(f) a product of the light-dependent reaction that is not used in the lightindependent reaction
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
4. The Calvin cycle is the stage of photosynthesis during which carbon dioxide is
fixed. The Calvin cycle uses the products of the light dependent stage.
(a) Name the products of the light dependent stage that are used in the Calvin
cycle.
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(b) Discuss the fate of triose phosphate (TP) in the Calvin cycle.
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(c) A process known as photorespiration also takes place in photosynthetic
cells. In this process, oxygen competes with carbon dioxide for the active
site of the enzyme RuBP carboxylase (Rubisco).
Fig. 3.1 (a) and Fig. 3.1 (b) outline the processes of photosynthesis and
photorespiration.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Suggest why the process outlined in Fig. 3.1 (b) is known as
photorespiration.
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(d) Using Fig. 3.1 (a) and Fig. 3.1 (b), describe and explain the likely effect on
photosynthesis of an increase in the oxygen concentration.
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(e) Some plants, known as C4 plants, use an enzyme called PEP carboxylase,
instead of Rubisco, to fix carbon dioxide.
Suggest why these plants do not show photorespiration.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Photosynthetic rate
Learning objectives:
 Describe the effect on the rate of photosynthesis, and on levels of GP, RuBP and TP, of
changing carbon dioxide concentration, light intensity and temperature;
 Discuss limiting factors in photosynthesis with reference to carbon dioxide concentration,
light intensity and temperature;
 Describe how to investigate experimentally the factors that affect the rate of
photosynthesis;
The rate at which plants can make food (the photosynthetic rate) is dependent on environmental
factors, particularly the amount of light available, the level of carbon dioxide (CO2) and the
temperature. The effect of these factors can be tested experimentally by altering one of the factors
while holding others constant (a controlled experiment). In reality, a plant is subjected to variations
in all three factors at the same time. The interaction of the different factors can also be examined in
the same way, as long as only one factor at a time is altered. The results can be expressed
graphically.
1. In order to maximise production, market gardeners often grow plants in
glasshouses.
Light conditions can be controlled along with a number of other factors.
How can factors other than light conditions be controlled to increase the rate
of photosynthesis and maximise production?
In your answer you should explain why the rate of photosynthesis is affected
by the controlled factors you have discussed.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
2. A student carried out an experiment to investigate the effect of light intensity
on the rate of photosynthesis in an aquatic plant, using the apparatus shown in
Fig. 2.1.
The student decided to measure the rate of photosynthesis by measuring the
gas produced over a five minute period. The gas collected in the capillary tube.
After five minutes, the length of the bubble was measured along the scale.
The light intensity was varied by altering the distance (d) between the lamp
and the photosynthesising plant.
The student prepared Table 2.1 to calculate the light intensity.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(a)(i) Calculate the light intensity when the lamp was 24cm from the plant.
Show your working.
Answer = ____________________ [2]
(ii) The length of the gas bubble was measured (in mm).
State what additional information would be required to calculate the
volume of gas produced.
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(iii) Suggest how the student supplied the aquatic plant with a source of
carbon dioxide.
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(b) Certain assumptions are made when using the apparatus shown in Fig. 2.1
to measure the rate of photosynthesis.
(i) One of these assumptions is that all of the oxygen produced by the plant
during photosynthesis is collected.
Suggest why not all of the oxygen produced by the plant is collected.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(ii) Another assumption is that all of the gas collected is oxygen.
Analysis of the gas collected reveals that it has the following
composition:
 oxygen 50%
 nitrogen 44%
 carbon dioxide 6%
Suggest a reason for the presence of nitrogen in the gas collected.
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(iii) Comment on the percentage of carbon dioxide present in the gas
collected and give reasons for this figure.
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3. One way to determine the rate of photosynthesis is to measure the uptake of
carbon dioxide.
(a) Discuss why measuring carbon dioxide uptake may or may not give a better
indication of photosynthetic activity than measuring oxygen production.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(b) Fig. 4.1 shows the relationship between light intensity and the relative
carbon dioxide uptake and production in a plant.
(i) State the factor that is limiting the rate of photosynthesis at A on the
graph.
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(ii) Suggest one factor that may limit the rate of photosynthesis at B.
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(iii) Carbon dioxide is given off by the plant when the light intensity is lower
than X.
Name the process that produces carbon dioxide in the plant.
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(iv) With reference to Fig. 4.1, explain the biochemical processes that are
occurring in the plant:
 as light intensity increases from 0 (zero) to X;
 at light intensity, X;
 at light intensities greater than X;
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
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(c) (i) Name the products of the light-independent stage of photosynthesis.
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(ii) Paraquat is a weedkiller. It binds with electrons in photosystem I.
Suggest how paraquat results in the death of a plant.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
4. Many herbicides act by inhibiting photosynthesis in weeds. A series of research
studies were carried out to evaluate the effectiveness of a triazine herbicide on
the yield of a crop of corn, Zea mays. Some of the data obtained is shown in
Table 3.1.
(i) Calculate the yield difference caused by the application of herbicide in
study G.
Show your working.
Answer = ____________________ kgha-1
____________________ % [2]
(ii) Suggest why the researchers concluded that the data obtained from Study
E was not useful in evaluating the effectiveness of the herbicide.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(iii) Triazine herbicide acts on weeds by binding to a specific protein associated
with photosystem II, blocking the movement of electrons between electron
carriers.
Explain the effect that the herbicide binding to this protein will have on
photosynthesis.
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(iv) Plants treated with triazine herbicide can, when illuminated under
experimental conditions, be seen to fluoresce (emit light) and give off small
quantities of heat.
Suggest how this experimental finding could be explained.
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