1
1 PHOTOSYNTHESIS PRIOR KNOWLEDGE You will probably remember the equation for photosynthesis, in which plants use light energy, absorbed by chlorophyll, to react water and carbon dioxide together to produce carbohydrates and oxygen. You may also remember that ATP is the molecule used as the energy currency in cells, and that it is made by combining ADP and inorganic phosphate.
LEARNING OBJECTIVES
enable us to transfer energy from light to produce fuels such as methanol, using processes similar to photosynthesis, but without involving plants. For example, in 2014, researchers at Monash University, in Melbourne, Australia, described the significant progress that they have made in developing a kind of artificial photosynthesis. The researchers are working on developing two new catalysts – one that can split water, and one that can absorb carbon dioxide and react it with the hydrogen from the water to form methanol. When supplied with carbon dioxide dissolved in water, and irradiated with sunlight, these new catalysts can produce methanol.
In this chapter, we look at the process of photosynthesis, in which plants harness light energy to split water and combine hydrogen atoms from it with carbon dioxide, forming carbohydrates that contain some of the energy from the light. (Specification 3.5.1) Humans have an insatiable demand for energy. Today, much of our energy supply still comes from fossil fuels, despite the universal acknowledgement that these will eventually run out, and that burning fossil fuels is causing an increase in the carbon dioxide concentration in the atmosphere, which is leading to increased global temperatures. Artificial photosynthesis represents one of several potential options for obtaining more of our energy supplies from renewable sources. Photosynthesis is a very efficient energy-transfer process, in which energy from light is transferred to energy in carbohydrates. We can already grow plants and algae (Figure 1) to produce biofuels that can be burnt. Now, researchers are looking into various approaches that would
Figure 1 These tanks contain the green unicellular alga, Chlorella, which is being grown to produce biofuels.
As yet, this process needs much more development before it can be scaled up and used commercially. But the promise is there, and other research groups working on different systems are also reporting success. It is a tall order, however, to try to produce a system for transferring light energy to fuels that betters that of plants. After all, they have millions of years of evolution behind them, which have helped to make photosynthesis work with such efficiency.
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1 Photosynthesis
1.1 ENERGY FOR LIVING ORGANISMS All living cells need a constant supply of energy in order to remain alive. Cells use energy for a whole range of processes – for example, active transport to move substances into and out of the cell, moving substances around inside the cell, building protein molecules, replicating DNA – the list is almost endless. When we look at a whole organism (for example, yourself), we can identify many other uses of energy. At this moment, you are using energy to contract the muscles between your ribs, to draw air into your lungs. Your brain cells are using energy to transfer electrical signals, helping you to make sense of what you are reading. The immediate source of almost all of this energy is ATP. You learned a little about ATP in your AS level course (see Chapter 4 of Year 1 Student Book). ATP is a phosphorylated nucleotide. ATP can be hydrolysed by the enzyme ATP hydrolase, separating one of the phosphate groups from the ATP molecule to produce ADP and inorganic phosphate (Figure 2).
P
P
P
P
+
H 2O
P
Pi
ADP Figure 2 The hydrolysis of ATP
This reaction releases energy that the cell can then use for any process that requires energy. The energy is in a small enough ‘packet’ to make sure that not too much is wasted – exactly the right amount of ATP can be hydrolysed to provide just the required amount of energy. Moreover, it can be released from ATP exactly when and where it is required.
P ADP
ATP synthesis Because energy cannot be created or destroyed, the production of an energy-containing substance such as ATP requires an input of energy. The reaction is catalysed by an enzyme called ATP synthase (often just called ATPase) (Figure 3). There are two main ways in which cells produce ATP. In green plants, and also in many types of micro-organisms, ATP is made during photosynthesis. In this case, the energy used to synthesise the ATP comes from light. In this chapter, you will learn where and how this happens. The ATP made in photosynthesis, however, is very short-lived. It is broken down almost straight away to release energy that is then used to produce other energy-containing compounds, in particular carbohydrates. These carbohydrates can be stored, or transported to other parts of the plant, where they can later be broken down to release their energy again. This energy is then used to make more ATP, in every cell of the plant. The reactions involved are called respiration. The reactions of respiration are described in Chapter 2.
ATP ATP hydrolase
In this chapter and the next, we will look, in some detail, at how the cell produces ATP. Every cell makes its own ATP, synthesising it by recombining inorganic phosphate groups with ADP molecules. The quantity of ATP used each day is quite staggering. Your body contains approximately 250 g of ATP, but over the course of one day you probably turn over your own body weight of ATP. This means that the ATP molecules making up that 250 g are constantly being hydrolysed and reformed, over and over again.
P
+
Pi
Animals, such as ourselves, obtain energy-containing compounds by eating them. The ultimate source of the energy in our bodies is the carbohydrates and other organic compounds made by plants. Just as in the plant cells, these are broken down by respiration, inside our body cells, releasing energy to produce ATP (Figure 4).
ATP synthase water released
P
P
P
ATP
Figure 3 The formation of ATP
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1.2
An overview of photosynthesis
c. a reaction that is involved in synthesising large molecules, and which uses energy. 2. Cells use ATP rather than glucose as their direct energy supply. Explain the advantage of this.
Oxygen Energy from sunlight
Photosynthesis
Carbon dioxide Water
Carbohydrates Respiration
Carbon dioxide
Water
Oxygen
Energy from carbohydrates (ATP)
Figure 4 Energy transfers in photosynthesis and respiration
Oxidation and reduction reactions Photosynthesis and respiration involve two types of chemical reaction: 1. Reduction reactions involve the addition of electrons or hydrogen atoms to a molecule, or removal of oxygen. Molecules that supply these electrons or hydrogen atoms are called reducing agents. 2. Oxidation reactions involve the removal of electrons or hydrogen atoms, or the addition of oxygen. Molecules that accept these electrons or hydrogen atoms are called oxidising agents. You can probably see that reduction and oxidation reactions have to occur together. If one substance is giving away electrons to another, then the giver is being oxidised, and the acceptor is being reduced. In this chapter and the next, you will meet many reactions in which electrons or hydrogen atoms are passed from one molecule to another. A very important oxidising agent in photosynthesis is a phosphorylated nucleotide called NADP, which has the essential role of accepting protons (hydrogen ions) and electrons from one substance, and then passing them on to others.
QUESTIONs 1. Give an example of: a. a part of the body that generates a lot of heat b. a part of the body where active transport is used to pump substances into or out of cells
KEY IDEAS
›› Cells require energy for many life-maintaining
processes. The immediate source of this energy is ATP.
›› Every cell makes its own ATP. ›› ATP is synthesised during photosynthesis
and respiration, in a reaction catalysed by ATP synthase.
›› NADP is an oxidising agent that is involved in
photosynthesis, where it accepts and passes on protons and electrons.
1.2 AN OVERVIEW OF PHOTOSYNTHESIS In the rest of this chapter we will look in detail at exactly what happens inside a chloroplast during photosynthesis. The fundamental event is the transfer of energy from light into chemical energy in glucose. This involves two main stages – the light-dependent reaction and the light-independent reaction – each of which is itself made up of many smaller steps. As you will see, ATP is involved in this process. ATP is made in the light-dependent reaction using energy from sunlight. It is then used in the light-independent reaction, releasing energy to be stored in a glucose molecule. Figure 5 summarises what happens during these two stages. In the light-dependent reaction, energy from sunlight is used to split water into hydrogen and oxygen, and to cause electrons to be ejected from chlorophyll molecules. The oxygen is given off as a waste product. The hydrogen and electrons are used to make ATP and reduced NADP. The reduced NADP and the ATP are then used in the light-independent reaction. In this stage, carbon dioxide is converted to carbohydrates (sugars), using the reducing power and energy of the reduced NADP and ATP.
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1 Photosynthesis
energy from sunlight water
light-dependent reaction
oxygen
reduced NADP + ATP carbon dioxide
KEY IDEAS
›› The light-dependent reactions of
light-independent reaction
carbohydrates
Figure 5 The two stages of photosynthesis
Both stages of photosynthesis take place inside chloroplasts. The light-dependent stage happens on the thylakoids – membranes in which chlorophyll and other pigments are embedded. The light-independent stage takes place in the stroma. Figure 6 shows the structure of a chloroplast. Both sets of reactions are kept separate from other reactions in the cell by the outer chloroplast membranes. There are two of these, forming an envelope. Carbon dioxide readily diffuses through the outer membranes of the chloroplasts into the stroma. Sugars that are made in photosynthesis can be transported out though these membranes. However, not all the sugar is exported from the chloroplast; some is converted into starch and stored in starch grains inside the stroma. When needed, carbohydrate may be released from the starch grains and translocated to other parts of the plant, in the form of sucrose.
Lipid droplet Stroma: the site of the light-independent reaction. The stroma is a fluid containing enzymes that use ATP generated in the light-dependent reaction of photosynthesis to convert carbon dioxide into sugar.
photosynthesis take place on the membranes of the thylakoids, inside a chloroplast.
›› During the light-dependent reactions, energy from light is used to split water molecules; the oxygen from the water is given off as a waste product.
›› Also during the light-dependent reactions,
energy from light causes electrons to be ejected from chlorophyll molecules.
›› The hydrogen from the water and the
high-energy electrons from the chlorophyll are used to make ATP and reduced NADP.
›› The light-independent reactions of
photosynthesis take place in the stroma of a chloroplast.
›› During the light-independent reactions, energy from ATP and reduced NADP are used to convert carbon dioxide to carbohydrates.
Outer and inner membranes Inner and outer membranes have phospholipid bilayers. They control movement of molecules into and out of the chloroplast.
Starch grain Excess carbohydrate made during photosynthesis is temporarily stored as starch grains. Membrane-bound ribosomes Free ribosomes
Thylakoid: the light-dependent reaction happens here. Each thylakoid membrane is a phospholipid bilayer that has many chlorophyll molecules embedded in it. These absorb light energy and then transfer it to other molecules.
Granum The thylakoids are arranged in stacks, each called a granum. This greatly increases the efficiency of the light-dependent reaction by capturing most of the light energy that enters the chloroplast.
Figure 6 Inside a chloroplast
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1.3
The light-dependent reactions
1.3 THE LIGHT-DEPENDENT REACTIONS In this stage of photosynthesis, energy captured from sunlight by pigment molecules is used to split water, to synthesise ATP, and to produce reduced NADP (Figure 7).
Photoionisation The thylakoid membranes have a similar basic structure to other cell membranes. They are phospholipid bilayers with various proteins within them. However, thylakoid membranes also contain many pigment molecules. These are arranged in little groups called photosystems. Each photosystem contains numerous chlorophyll molecules, of two types – chlorophyll a and chlorophyll b – and other pigments known as carotenoids.
When light falls on any of these pigments, the energy is passed through the photosystem and transferred to a chlorophyll a molecule. Chlorophyll a is one of a special group of compounds that can absorb light energy and use it to boost the energy level of electrons. Electrons can be thought of as orbiting around the nucleus of an atom. If the right amount of energy is absorbed, this can boost an electron into a higher orbit, or even remove it from the atom completely. The atom that has lost the electron has become a positively charged ion. When the energy that makes this happen comes from light, the process is known as photoionisation. When a chlorophyll a molecule absorbs energy from sunlight, photoionisation takes place and an electron is ejected from the molecule. As we will see, these high-energy electrons have a key role to play.
Light absorption Light energy (photons) is transferred to chlorophyll a molecules.
O2
Photolysis Energy is used to break water, releasing H+, O2 and electrons.
Synthesis of reduced NADP H+ and electrons are used to make reduced NADP.
Photoionisation Energy is transferred to electrons, which are released from chlorophyll molecules.
Photophosphorylation Energetic electrons provide energy for the synthesis of ATP.
Figure 7 Overview of the light-dependent reactions
ASSIGNMENT 1: PIGMENTS (PS 3.1) White light is a mixture of light of different colours; these are easily seen in a rainbow, or when light is passed through a prism. Each colour of light has a different wavelength. Blue light has a wavelength of approximately 450 nm, red light has a wavelength of approximately 650 nm, and the other visible colours have wavelengths in between.
Objects appear to be different colours because of the wavelengths of light that they absorb and reflect. A white object reflects all wavelengths of light, while a black object absorbs all and reflects none. A yellow object reflects yellow light and absorbs other colours. An absorption spectrum shows how much light specific objects or molecules absorb at each
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1 Photosynthesis
wavelength. Figure A1a shows the absorption spectra of the different pigments in a chloroplast. Figure A1b shows a complete absorption spectrum for a leaf, involving all its different pigments.
This graph also shows an action spectrum for photosynthesis – that is, the rate of photosynthesis at different wavelengths of light.
b Absorption spectrum for a whole leaf and the action spectrum for photosynthesis
a Absorption spectra for different pigments Chlorophyll a Chlorophyll b Carotenoids
Absorbance (absorption spectrum)
Absorbance
400 UV violet
Absorption spectrum
500 600 Wavelength / nm green
700 red IR
Rate of photosynthesis (action spectrum)
Action spectrum
400 UV violet
500 600 Wavelength / nm green
700 red IR
Figure A1 Absorption spectra for some pigments in a leaf (a) and absorption and action spectra for a leaf (b)
As can be seen in the absorption spectrum, chlorophyll is not one single compound but a mixture of several different pigments.
Questions A1. What are the main colours of light absorbed by chlorophyll b?
water when the tide is in, seaweeds tend to be red or orange. Higher up the shore, where they are covered only by shallow water for short periods of the day, they are usually green (Figure A2). Suggest how these different colours may be useful adaptations to their environments.
A2. Explain why chlorophyll appears green. A3. Copy the graph axes in Figure A1. On your copy, draw the action spectrum for a leaf if it contained only chlorophyll a and no other pigments. A4. Suggest why it is an advantage to a plant to have several different pigments in its chloroplasts. A5. Light does not penetrate very far into water. Short wavelengths penetrate to greater depths than long wavelengths. Different types of seaweeds contain different photosynthetic pigments. Low down on the shore, where they are covered by deep
Figure A2 Seaweeds come in many colours. Here you can see green, brownish-orange and dark red species.
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The light-dependent reactions
REQUIRED PRACTICAL ACTIVITY 7: APPARATUS AND TECHNIQUES (MS 4.1, PS 2.3, PS 2.4, AT a, AT b, AT g) Use of chromatography to investigate the pigments isolated from leaves of different plants, for example, leaves from shade-tolerant and shade-intolerant plants, or leaves of different colours. This practical activity gives you the opportunity to show that you can:
›
use appropriate instrumentation to record quantitative measurements
› ›
use laboratory glassware apparatus separate biological compounds using thin layer/ paper chromatography.
We have seen that chloroplasts contain several different pigments, including chlorophyll a, chlorophyll b and carotenoids. We can separate these pigments using chromatography. To do this, you first have to extract the pigments from the leaves. The pigments are not soluble in water, so instead you use an organic solvent, such as propanone. Care is needed, because propanone is very volatile (it forms a vapour), highly flammable and is an irritant. You therefore must not have any flames in the vicinity of your experiment and must always wear eye protection. A good way of extracting the pigments is to grind some leaves in a pestle and mortar, adding a little sand to help to crush them and break open the cells, and some propanone to dissolve the pigments. Use only a small amount of propanone, and crush the leaves very thoroughly, to try to get a really concentrated solution of pigments.
1.3
Handle the chromatography paper with forceps rather than with your fingers, as lipids and other substances on your skin may interfere with the movement of the pigment up the paper, or show up as spots on the chromatogram. Use a pencil and ruler to draw a straight line a couple of centimetres up from the base of the strip of chromatography paper. Pour some propanone into the base of a beaker, just deep enough so that you will be able to place the paper into it without the propanone reaching the pencil line. Now you are ready to load the pigment extract onto the paper. Use a narrow-ended glass pipette, or the head of a large pin, for this. Dip it into the pigment extract, and very carefully make a small spot of the pigment exactly on the pencil line on the paper. Repeat many times, allowing each drop to dry before adding the next one. Try to make each spot as small as possible, containing as much pigment as possible. Support the paper in the beaker as shown in Figure P1, and cover it. As the propanone soaked into the chromatography paper is drawn upwards, it passes through the pigment spots that you made, and the pigments dissolve in the propanone. The pigments are carried upwards as the solvent moves upwards. Due to a combination of, for example, different particle size, solubility in propanone and adhesion/attraction to paper, the different pigments move different distances (Figure P2). When the solvent has reached a level close to the top of the paper, carefully remove the paper, and draw a pencil line at the highest level reached by the solvent. This is known as the solvent front. Solvent front Carotenoids
Next, you need to prepare the chromatography paper and apparatus. This is shown in Figure P1. Tape
Tape
Chromatography solvent (propanone)
Cover
Chlorophyll a
Beaker
Chlorophyll b
Chromatography paper formed into a cylinder Pencil marks for the origin Spot of pigment at the origin
Origin Figure P2 A chromatogram of chloroplast pigments
Figure P1 Apparatus for chloroplast pigment chromatography
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1 PHOTOSYNTHESIS Rf values for each spot on the chromatogram are calculated by the equation: Rf =
distance travelled by pigment spot dista ance travelled by solvent
Each type of pigment has its own characteristic Rf value, so even if you cannot see the colours clearly, you can still identify the pigment by calculating its Rf. Although Rf values vary for different solvents, in general carotenoids tend to have values very close to 1 (that is, they are so mobile that they are carried all the way up the paper with the solvent), chlorophyll b has a fairly small Rf value, and chlorophyll a is somewhere in between. With some kinds of leaves and some solvents, you may be able to see even more pigment spots. For example, you may see yellow spots as well as the chlorophyll ones, which are formed by pigments called xanthophylls.
P3. Figure P3 shows chromatograms obtained from seaweed and from spinach leaves. The same solvent was used in both cases.
E A Solvent front
B F C
G
D H
Origin
Spinach
Seaweed
Figure P3 Chromatograms for seaweed and spinach leaves
QUESTIONS P1. Suggest why: a. the pigment spot is loaded onto a pencil line on the chromatography paper b. the pigment spot is made as small and as concentrated as possible c. the pencil line must not be below the surface of the solvent d. a pencil line is drawn to show the solvent front immediately the paper is removed from the solvent. P2. Outline how you could use chromatography to compare the pigments found in the leaves of two different plants.
a. Calculate the Rf value for each pigment spot. Measure to the middle of the spot, as indicated by the pencil dot. b. The table shows the Rf values for each pigment, using the solvent that produced the chromatograms in the diagrams. Identify each pigment.
Pigment
Rf value
beta carotene
almost 1
chlorophyll a
0.45
chlorophyll b
0.38
chlorophyll c
0.01
fucoxanthin
0.32
xanthophyll
0.74
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The light-dependent reactions
1.3
H+
ATP
Detail of part of a thylakoid lamella Light absorption
Stroma
ATP synthase
During electron transfer, H+ ions (protons) are moved into the thylakoid H+ e– spaces.
ADP + Pi
Photosystem Electron transfer chain
Thylakoid space
e–
H+ H+ The passage of the protons through the ATP synthase molecule causes a conformational (shape) change that leads to the synthesis of ATP from ADP and Pi.
Figure 8 Photophosphorylation
Photophosphorylation The high-energy electrons that are ejected from chlorophyll molecules are used to make ATP. As well as the photosystems described earlier, the thylakoid membranes inside the chloroplast contain a number of different electron carriers. These are arranged in a particular sequence, so that the electron is passed from one to the next (Figure 8). Each time the electron is passed on, it loses a little energy. The energy that is released is used to pump protons, H+, across the membranes and into the spaces between them. The result of this is a higher concentration of protons inside the spaces between the membranes than outside them, producing a diffusion gradient for protons. There is also an electrical gradient, because protons carry a positive charge. Thinking about the two gradients together, we say that there is an electrochemical gradient. Protons can diffuse down this gradient, from their high concentration to their lower concentration, and from a place with a high positive charge towards a less positively charged area, through the thylakoid membranes. They can only do this through particular channels, which are formed by ATP synthase molecules. As
the hydrogen ions diffuse through, losing the energy that was given to them as they were pumped through the membrane, the ATP synthases use this energy to catalyse the synthesis of ATP from ADP and Pi. The movement of the protons through the membrane, down their electrochemical gradient, is called chemiosmosis. This is rather a confusing name, because of course it has nothing at all to do with ‘normal’ osmosis, which involves the movement of water molecules. The whole process, in which the high-energy electrons are used to make ATP, is called photophosphorylation.
Photolysis As well as containing light-absorbing pigments, one of the types of photosystem found in chloroplasts contains an enzyme that can split water molecules in the presence of light. This produces protons (hydrogen ions, H+), electrons (e–) and oxygen. water
→ protons
+
electrons +
oxygen
2H2O
→ 4H+
+
4e–
O2
+
This reaction is known as photolysis (photo=light, lysis=splitting). The oxygen is released into the
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1 Photosynthesis atmosphere. This reaction is the source of almost all the oxygen that makes up 20 per cent of the air around us. Millions of tonnes of oxygen are present in the atmosphere, almost all produced by photolysis. The electrons from the water are used to replace the electrons ejected from the chlorophyll molecules during photoionisation. The protons are picked up by NADP, as described below.
Production of reduced NADP At the end of the electron transport chain, the electrons are picked up by NADP, along with the protons from the split water molecules. In the process, the NADP is reduced, becoming reduced NADP. NADP+protons+electrons → reduced NADP Remember that a hydrogen atom is made up of a proton and an electron, so effectively the NADP is combining with hydrogen, which originally came from water.
Products of the light-dependent reaction So, at the end of the light-dependent reaction we have three products:
KEY IDEAS
›› An enzyme present in the thylakoid membranes splits water molecules into protons, electrons and oxygen in the presence of light. This is called photolysis.
›› Energy from light also causes electrons to
be ejected from chlorophyll a, in a process called photoionisation.
›› These high-energy electrons are used to make
ATP, in a process called photophosphorylation.
›› ATP is made when electrons are passed along
the electron transfer chain, on the thylakoid membranes. The energy from these electrons is used to pump protons (hydrogen ions) across the membrane. As they diffuse back, the protons pass through an ATP synthase enzyme, which uses the energy of the gradient to combine ADP with inorganic phosphate.
›› The products of the light-dependent reaction are oxygen, reduced NADP and ATP.
›› oxygen, which is a waste product from the
photolysis of water, and which diffuses out of the chloroplast and eventually into the atmosphere
›› ATP, which was made by photophosphorylation, and contains energy that originated in the light energy absorbed by chlorophyll
›› reduced NADP, which contains protons and
electrons (hydrogen) derived from water molecules that were split by photolysis.
The ATP and reduced NADP are now used in the light-independent reaction.
QUESTIONs 3. Which of these terms matches each description below? photoionisation
photolysis
photophosphorylation
a. The formation of ATP using energy that originally came from light. b. The emission of an electron from a molecule, as a result of the absorption of light energy. c. The splitting of water in the presence of light.
1.4 THE LIGHT-INDEPENDENT REACTION As we have seen, the light-dependent reactions of photosynthesis require energy from sunlight. They can therefore occur during the day, but not at night. However, the next stage of photosynthesis does not need an input of light energy. It can therefore take place even in the dark. In practice, however, it cannot go on all night, because it needs the ATP and reduced NADP that have been made in the light-dependent reactions. Once these have been used up, the light-independent reaction grinds to a halt. In the light-independent stage of photosynthesis, the energy from ATP and the electrons from reduced NADP are used to reduce carbon dioxide from the atmosphere and build it into carbohydrate. This carbohydrate can provide a long-term store of energy. It can also be used as building blocks to produce all the other organic molecules – such as lipids, proteins and nucleic acids – that are needed for growth. Figure 9 summarises the events that take place during the light-independent reaction. It is often known as the Calvin cycle, after the person who first worked out the series of reactions involved in the cycle.
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1.4
The light-independent reaction
Carbon dioxide
phosphate (TP). This is a carbohydrate – it is a 3-carbon sugar.
Rubisco
All of these reactions take place in the stroma of the chloroplast. Ribulose bisphosphate (RuBP)
ADP + Pi ATP
Calvin cycle
Glycerate 3-phosphate (GP) Reduced ATP NADP
5 6
1 6 Glucose
Triose phosphate (TP)
ADP + Pi Oxidised NADP
Other carbohydrates and other organic compounds
Various things can now happen to the TP. Most of it – about five-sixths – is used to regenerate RuBP. If this did not happen, the chloroplast would quickly run out of RuBP and the reaction would stop. The rest is used to make other kinds of useful organic molecules. TP is a 3-carbon compound, so two molecules of it can be combined to form the 6-carbon hexose sugar, glucose. Other carbohydrates, such as sucrose, starch or cellulose, can be synthesised, dependent upon the cell’s needs. TP can also be converted to fats (lipids) and – with the addition of nitrate or ammonium ions – to amino acids and proteins.
Figure 9 The light-independent reaction
As this is a cycle, we could start anywhere, but let’s begin at the top left, where carbon dioxide first comes into the picture. The carbon dioxide combines with a substance called ribulose bisphosphate, generally known as RuBP for short. The reaction is catalysed by an enzyme called rubisco. Rubisco is the most abundant enzyme in the world. An RuBP molecule contains five carbon atoms, and a carbon dioxide molecule contains one, so this reaction produces a 6-carbon molecule. This instantly splits into two, forming two 3-carbon molecules, called glycerate 3-phosphate (GP). The carbon dioxide is now said to be fixed, meaning that it has become part of a compound within the plant. GP is not actually a carbohydrate. It is converted to carbohydrate using energy from ATP and hydrogen from reduced NADP, both of which are supplied from the light-dependent stage of photosynthesis. This is a reduction reaction. The GP is reduced to form triose
QUESTIONs 4. Students sometimes write: In photosynthesis, carbon dioxide is converted to oxygen. Explain why this is not correct. 5. Look at the diagram of the Calvin cycle in Figure 9. Give an example of a redox reaction. State which substance is being oxidised, and which is being reduced. 6. Figure 9 shows that ADP, Pi and oxidised NADP are formed. Suggest what happens to these substances. 7. In your own words, summarise how energy from light is transferred to energy in glucose, during photosynthesis. 8. Using Figure 9 and what you now know about the light-dependent and light-independent reactions of photosynthesis, explain how the structure of a chloroplast is adapted to its functions.
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1 Photosynthesis
ASSIGNMENT 2: WORKING OUT THE CALVIN CYCLE (PS 1.2, 2.1, 3.1, 4.1) In the 1950s, researchers led by Melvin Calvin were the first to find out what happened during the light-independent stage of photosynthesis. The researchers used apparatus similar to that shown in Figure A1.
on the paper because they contained radioactive carbon. Figure A2 shows two of the chromatograms he obtained. The dark spots contain radioactive carbon compounds.
Y
X
Radioactive carbon dioxide added as H14CO3– solution Chlorella
After 5 seconds Light
Glass vessel in the shape of a lolipop
Y
X
After 30 seconds Figure A2 Chromatograms obtained from the Chlorella experiments Hot ethanol
The glass-sided disc contained cells of a tiny one-celled photosynthetic organism called Chlorella. The apparatus was placed in a dark room and the Chlorella cells were supplied with carbon dioxide that contained radioactive carbon. The contents of the apparatus were mixed thoroughly and then a light was switched on. At five-second intervals a member of Calvin’s team extracted a few of the Chlorella cells from the apparatus and placed them into hot ethanol to kill them. The cells were then homogenised (mashed up to form a liquid). The liquid was analysed to find out what it contained. This was done using two-way paper chromatography. This technique involves running the chromatogram with one solvent, then turning the paper through 90° and running it again with a different solvent. Although the substances being separated were not coloured, Calvin could find where they were
The researchers were able to collect samples of these compounds and find out exactly what they were. By using this technique, Calvin’s team was able to investigate the sequence in which different organic compounds were produced. In another series of experiments, Calvin’s team used similar techniques to investigate the effect of light and dark periods on the compounds formed in the light-independent reaction. Figure A3 shows the results from one of these investigations. Light
Amount of substance
Figure A1 The apparatus used by Melvin Calvin’s team
Dark
Glycerate 3-phosphate
Ribulose bisphosphate Time after application of radioactive carbon dioxide
Figure A3 Changes in concentrations of GP and RuBP during light and darkness
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1.4
The light-independent reaction
TASKS
A3. Using the graphs in Figure A3, explain why:
A1. Suggest the advantages of using two-way chromatography, rather than just using one solvent that carries the compounds in one direction.
a. When the light is switched off, the curve for ribulose bisphosphate falls.
Stretch and challenge
c. Eventually, the curve for glycerate 3-phosphate levels off.
A2. Using your knowledge of the lightindependent stage of photosynthesis, suggest the identities of compound X and compound Y in Figure A2.
KEY IDEAS
›› During the light-independent reaction, energy from ATP and electrons from reduced NADP are used to reduce carbon dioxide and produce carbohydrates.
›› The enzyme rubisco, which is present
in the stroma of chloroplasts, catalyses the combination of carbon dioxide with ribulose bisphosphate (RuBP), to produce a 6-carbon compound.
b. When the light is switched off, the curve for glycerate 3-phosphate rises.
a. Express the approximate speed of light in kilometres per hour, using standard form. b. At this speed, how far can light travel in four seconds? Express your answer in standard form. c. Express the speed of sound as a fraction and a decimal of the speed of light, using standard form. Typically, most of the air around us is made up of 1 oxygen and nitrogen. Oxygen is approximately 5 of the air we breathe, and nitrogen makes up about 39 . 50
›› The 6-carbon compound immediately splits into
d. Express the relative quantities of oxygen and nitrogen in the air, as percentages.
›› GP is reduced by reduced NADP, using energy
e. Air also contains small quantities of other gases, such as carbon dioxide and argon. According to your answer for d, what fraction of the air must these gases occupy?
two molecules of glycerate 3-phosphate (GP). from ATP. This produces the carbohydrate triose phosphate (TP).
›› Five-sixths of the TP is used to regenerate
RuBP. The remainder is used to synthesise other carbohydrates, lipids and amino acids.
Part a First, it is important to understand how a kilometre relates to a mile. 1 mile equates roughly to 1.61 kilometres:
Light and air (MS 0.1, 0.2, 0.3) Light is very important for humans and for many other living things. Plants use light to photosynthesise, and the resulting energy accumulates in the food chain (see Chapter 3). Light has inspired people, including artists and scientists, throughout history. Light travels at approximately 186 000 miles per second. The speed of sound is approximately 340 metres per second.
1÷1.61=0.62 So, the value in miles÷0.62 =the equivalent in kilometres Similarly, the value in kilometres×0.62 =the value in miles 186 000 miles per second÷0.62 =300 000 kilometres per second 186 000=186×1000=1.86×100 000 This can be expressed as 1.86×105 in standard form. 300 000=3.0×105 kilometres per second, in standard form.
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1 Photosynthesis Part b Speed is a measure of the distance travelled over the time it has taken, and is generally shown algebraically as: S (m s–1) =
D (m ) T (s )
The equation needs to be re-arranged, taking care to use the correct units, to calculate the distance: Distance=3.0×105 km s−1×4 s =1 200 000=1.2×106 kilometres
reacting molecules in the Calvin cycle. On the other hand, very high temperatures denature proteins in living organisms and therefore stop enzyme-catalysed reactions from taking place at all. The rate of photosynthesis is important in food production because it determines the crop yield. All the factors mentioned above can affect the rate of photosynthesis of a crop. Good crop management involves the manipulation of these factors so as to maximise photosynthesis and achieve good yields. A factor that is in short supply will limit the rate of photosynthesis, and is called a limiting factor.
Part c First, it is important for the values to have the same units. This can be done by converting the speed of sound to miles per second: 340 meters per second÷1000 =0.34 kilometres per second
Light The effect of light on photosynthesis depends on:
›› light quality – that is, the wavelengths that it
contains; plants can use only certain wavelengths for photosynthesis
0.34×0.62=0.21 miles per second =0.21÷186 000=0.0000011=1.1×10−6 So, sound travels at approximately 1.1×10−6 of the speed of light. Part d Oxygen: 1÷5=0.2 0.2×100 (to convert to a percentage)=20% Nitrogen: 39÷50=0.78 0.78×100 (to convert to a percentage)=78% Part e The total percentage of air occupied so far by oxygen and nitrogen is 98% (20%+78%).
›› light duration – that is, the day length ›› light intensity – that is, how strong the light is. At low light intensities, an increase in the rate of photosynthesis is directly proportional to increasing light intensity (Figure 10). But as light intensity increases, photosynthesis reaches a maximum rate and fails to increase further. This could be because:
›› The photosynthetic reactions are proceeding as fast as is possible for the photosynthetic pathways in that particular plant.
›› Some other environmental factor is now limiting the rate, such as carbon dioxide concentration or temperature.
100% (total air) – 98% =2% remaining for the other gases 1 50
1.5 FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS What determines how fast a plant can photosynthesise? Obviously, light intensity will have an effect, as this is the energy source that drives the whole process. Carbon dioxide and water are the two raw materials, so supplies of these will be critical. Temperature is also likely to have an effect, because high temperatures speed up particle movement and therefore increase the frequency of collisions between
Rate of photosynthesis
2% as a fraction=2÷100=1÷50=
Light intensity Figure 10 Effect of light intensity on the rate of photosynthesis
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Factors affecting the rate of photosynthesis
The maximum light that can be used in photosynthetic reactions is estimated to be about 10 000 lux (the SI unit of illuminance, equal to one lumen per square metre). On a clear summer day in Britain, solar illumination may reach 100 000 lux. At this time of year in a sunny field, therefore, light intensity is not the limiting factor. The plants have as much light as they can possibly use. At very high light intensities there may be damage to the chlorophyll molecules, resulting in a drop in the rate of photosynthesis.
It is interesting to note that temperature does not have a direct effect on the light-dependent reactions of photosynthesis. This is because, unusually for a metabolic reaction, the energy that drives the reaction is light energy. The kinetic energy of the particles involved is almost irrelevant. However, temperature does affect the rate of the light-independent stage, just as it does any other metabolic reaction.
Carbon dioxide In tropical areas, temperature and light intensity are not usually the limiting factors for photosynthesis, but carbon dioxide may be limiting. Carbon dioxide is the source of carbon atoms used to make all the organic products of photosynthesis. Carbon dioxide is needed in the light-independent reactions of photosynthesis, where it is reduced to carbohydrate and other organic compounds. Atmospheric carbon dioxide usually makes up only about 0.04% of the volume of the air. This is a tiny proportion. For most plants, this is lower than the optimum value for photosynthesis (Figure 12).
Temperature When light intensity and carbon dioxide are not limiting factors, increasing temperature can increase the rate of photosynthesis. Between the range 10°C to about 35°C, a 10°C rise in the temperature will double the rate of photosynthesis (Figure 11).
High light intensity Rate of photosynthesis
1.5
Crop management in glasshouses Imagine that a grower in the UK produces tomatoes to sell in bulk to supermarkets (Figure 13). In the UK climate, tomatoes can only be grown outdoors for a few months of the year, as they are killed by frost and the fruits grow poorly in low temperatures. So the grower produces the crop in glasshouses.
Low light intensity
Temperature Figure 11 Effect of temperature on the rate of photosynthesis
Rate of photosynthesis
Rate of photosynthesis
CO2 concentration 700 arbitrary units
Concentration of carbon dioxide
CO2 concentration 500 arbitrary units
CO2 concentration 350 arbitrary units
Light intensity
Figure 12 Effect of carbon dioxide concentration on the rate of photosynthesis
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1 Photosynthesis The faster the plants can photosynthesise, the more carbohydrates they can make. This provides them with both the materials and the energy required to make new cells for growth, and for fruit formation. Rapid photosynthesis means rapid production of a large crop of tomatoes. Over short periods, 0.5% carbon dioxide has been found to be the optimum concentration for photosynthesis. Over longer periods, however, this concentration may cause the stomata to close, resulting in a drop in photosynthesis. For glasshouse crops such as tomatoes, 0.1% carbon dioxide is the optimum over long periods.
Glasshouse cultivation allows:
›› better yields to be achieved ›› some crops to be grown out of season and so provide a better economic return
›› some plants to be grown in regions where they would not normally grow.
In order to achieve maximum yields, possible limiting factors inside the glasshouse need to be controlled.
Sunlight heats the inside of the glasshouse. The glass prevents a lot of this heat from escaping.
It is just as important to stop temperatures from rising too high in the summer as it is to keep them from dropping too low in the cooler spring and autumn months. At the ideal temperatures for tomato growth, water loss by transpiration is likely to be high, so the plants will need to be kept well watered. Excessive water loss can lead to closure of the stomata,
Average daily temperature / °C
Figure 13 Growing crops in glasshouses can produce very high rates of photosynthesis, and therefore high yields.
Tomato plants die when the temperature falls below about 2°C. At the other extreme, temperatures above 30°C can damage tomato plants. The grower aims to keep the temperature somewhere between 15°C and 25°C. This allows the plants to photosynthesise rapidly, without there being any danger of cells being damaged.
20 Inside 10 Outside 0
J F M A M J J A S O N D Month Ventilators allow fresh air in to replenish carbon dioxide levels.
Optimum temperatures can be obtained with additional heating in winter and shading in summer. Electric and paraffin heaters can be used in cold weather.
Large commercial glasshouses control temperatures in summer by automatically opening and closing ventilator flaps. Carbon dioxide concentrations can be increased by pumping carbon dioxide into the glasshouse. Paraffin heaters can increase both the temperature and carbon dioxide.
Figure 14 Glasshouse cultivation
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Factors affecting the rate of photosynthesis
meaning that carbon dioxide will not be able to enter the leaves and photosynthesis will come to a halt. Many glasshouses have automatic watering systems with sprinklers and humidifiers. However, it is also important to regulate humidity in order to limit fungal diseases, which can increase when the humidity is too high. Artificial lighting can be used in glasshouses when the natural light intensity falls too low. The light sources that are used provide a mix of the wavelengths that can be absorbed by the plant pigments – a spectrum similar to the absorption spectrum shown in Figure A1 in Assignment 1. This way, the grower is not spending money on electricity to produce light of wavelengths that will be wasted. All of these factors can be controlled by computers. Sensors are used to monitor the level of each factor and the feedback is processed by the computer. Keeping carbon dioxide concentration, temperature and light intensity at their optima is expensive. Carbon dioxide can be added from generators or from cylinders, which have to be bought in. Sometimes, it is generated by burning fuels such as paraffin, which has the added bonus that this also increases temperature. Lights run from electricity. The grower has to calculate the expense of controlling all of these factors, and balance it against the price that he or she expects to get from the crop. The increase in yield has to be worth more than the costs of environmental control, if it is to be worth the investment.
Crop management in open fields It is much less easy to control potential limiting factors when crops are growing in fields. However, there are still measures that can be taken to keep rates of
1.5
photosynthesis, and therefore growth rates, as high as possible. Weeds are plants that grow among the crop, but are not wanted. They compete with the crop plants for light, water and carbon dioxide, as well as for mineral ions from the soil, some of which will be needed to produce the enzymes, chlorophyll and cell structures that are needed for photosynthesis. Killing weeds and applying fertilisers therefore helps to increase photosynthesis in the crop plants. In dry conditions, the stomata on the leaves of the crop plants may close, so that the plant can conserve water. This decreases rates of photosynthesis, because carbon dioxide cannot diffuse into the leaves. Irrigation to keep an adequate water supply can prevent this from happening. Crops that are sensitive to temperature, such as strawberries, can be covered with fleece or polythene to help to increase the temperature around them and allow them to grow earlier in the year than might otherwise be possible.
Figure 15 Spraying herbicides to kill weeds in a field of young wheat plants
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1 PHOTOSYNTHESIS REQUIRED PRACTICAL ACTIVITY 8: APPARATUS AND TECHNIQUES Investigation into the effect of a named factor on the rate of dehydrogenase activity in extracts of chloroplasts (PS 2.4, PS 3.1, PS 4.1, MS 3.2, AT a, AT b, AT c) This practical activity gives you the opportunity to show that you can:
›
use appropriate apparatus to record a range of quantitative measurements
›
use laboratory glassware apparatus for a variety of experimental techniques.
During the light-dependent reaction, NADP acts as an electron acceptor, becoming reduced in the process. The reaction is catalysed by a dehydrogenase enzyme. We can investigate this reaction using a different electron acceptor – one that changes colour when it accepts electrons.
1
2
A good substance to use is a blue dye that you may have used before to test for the presence of vitamin C. This dye is called DCPIP. It is blue when oxidised, but becomes colourless when it accepts electrons and is reduced. The first step is to extract some chloroplasts (Figure P1). Chop up some leaves, add them to a small amount of an ice-cold solution containing sucrose, potassium chloride and a buffer (called an isolation medium), and grind them vigorously in an ice-cold pestle and mortar. The sucrose and potassium chloride ensure that the solution has a similar water potential to the cells in the leaves. The buffer keeps the pH at 7. The resulting mixture is then filtered through muslin to get rid of any big bits of leaves. The green filtrate contains the chloroplasts.
3
4
5 DCPIP
Figure P1 Procedure for investigating dehydrogenase activity in chloroplasts
The filtrate can now be centrifuged. This produces a little green pellet at the base of each centrifuge tube. The pellet contains most of the chloroplasts. Pour off the liquid (called the supernatant) and replace with a little of the ice-cold isolation medium. Mix the chloroplasts into it, using a glass rod or a pipette. (You might like to look at this mixture under a microscope, where you should be able to see the chloroplasts.) Stand each tube containing these resuspended chloroplasts in an ice-cold water bath. You now have a suspension of chloroplasts in each tube. If you place some of this suspension into a clean test tube, add a measured quantity of DCPIP, and
shine light onto it, you will see that the blue DCPIP quickly loses its colour. This happens because the chloroplasts are absorbing light and emitting electrons, which are being picked up by the DCPIP and reducing it. If you do the same in a tube wrapped in black paper, the DCPIP will remain blue. You can now investigate how different factors affect the rate at which this happens. For example, you could shine light of different colours (wavelengths) onto the tubes, and time how long it takes for the DCPIP to decolourise. Keep all other factors constant. You could also investigate the effect of light intensity, or temperature.
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1.5
Factors affecting the rate of photosynthesis
QUESTIONS P1. Suggest why the isolation medium: a. is kept ice cold b. contains a buffer to keep the pH at 7 c. has a water potential similar to that of the leaf cells. P2. If you have access to a colorimeter, you can measure the depth of colour every minute, rather than just timing how long it takes for the DCPIP to change from blue to colourless. The reading on the colorimeter is called the absorbance. The greater the absorbance, the darker the blue colour of the DCPIP. Table P1 shows the results obtained when DCPIP was added to a tube containing resuspended chloroplasts, and another containing the supernatant. Both tubes were kept in light of the same intensity.
Time / minutes
Absorbance / arbitrary units Resuspended chloroplasts
Supernatant
0
10.0
10.0
2
8.4
9.3
4
6.7
8.4
6
5.2
7.1
8
4.9
6.3
10
2.4
4.5
12
1.0
3.2
14
0.3
2.6
Table P1 a. Plot line graphs for these results, drawing lines of best fit. Draw both lines on the same pair of axes. b. Explain the results for the resuspended chloroplasts. c. Suggest reasons for the differences between the two sets of data.
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1 Photosynthesis
ASSIGNMENT 3: MAXIMISING THE YIELD (PS 1.2, 2.1, 2.4, 3.1) Tomatoes are an important crop in many parts of the world. They may be used locally for food, but more often they are an important export crop, bringing in significant income for farmers. Growers have to decide whether it is best to use a low-tech approach and simply growing the tomatoes in open fields, or whether it may be worth making a substantial investment to build glasshouses where they can control the conditions in which the plants are growing. An investigation was carried out into the effect of temperature on the growth of tomatoes. Tomato plants of the same variety – one that grows well at higher temperatures – were grown in an open field and also in three different glasshouses. The experiment was carried out in a subtropical country, where the outside temperature never dropped below 29.5°C. Even when not heated, the temperature inside a glasshouse is generally higher than outside. This is because of the greenhouse effect – short wavelength rays from the Sun pass through the glass into the glasshouse, and are reflected from the surfaces inside the glasshouse as longer wavelengths, which cannot escape. These longer wavelengths warm the air inside the glasshouse. The glasshouses in the experiment had different coverings. Figure A1 shows the variations in temperature in the four growing environments throughout the 10 months of the investigation. Air temperature / °C
43 41
Growing environment
Mean plant height / cm
Mean fruit yield per plant / g
T1
97.50
1348.5
T2
105.83
2145.2
T3
104.83
2055.0
T4
75.40
981.0
Table A1
Questions A1. a. C ompare the temperatures in growing environments T1 and T2. b. Explain the reasons for the differences that you have described. A2. What appears to be the temperature range at which this variety of tomato plants produces the highest yield? Use the information in the graphs and in the table to support your answer. A3. Suggest one factor, other than temperature, which may have been partly responsible for the differences in yield between T1 and T4. How significant might this factor have been? Do you think that it brings the reliability of the results into question? A4. Discuss the extent to which the results of this investigation could be used by growers in a temperate country such as the UK to help them to determine the best environment in which to grow tomatoes.
39 37 35 33 31 29
Equal numbers of one-month-old plants of the same tomato variety were planted into each of the growing environments in February. Table A1 shows the growth yields of fruit from the plants in each of the four growing environments.
Jan Feb Mar Apr May June July Aug Sept Oct Month
T1: roof and walls completely covered with transparent glass T2: roof covered with transparent glass; all four walls covered with shading allowing in only 75% of the light T3: roof and two walls covered with transparent glass; two walls covered with shading allowing in only 75% of the light T4: open field Figure A1 Temperature variation in four growing environments
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Practice questions
KEY IDEAS
›› Low light intensity, low concentrations of carbon
temperature, lighting and carbon dioxide concentration can be controlled. In open fields, the rates of photosynthesis, and therefore yields of crop plants, can be increased by removing weeds that compete for light, carbon dioxide and minerals, and by providing irrigation and adding fertilisers.
dioxide and temperatures that are above or below the optimum can limit the rate of photosynthesis.
›› Agricultural practices can help to reduce the
effects of these limiting factors. In glasshouses,
PRACTICE QUESTIONS 1. An experiment was carried out to investigate the effect of two different carbon dioxide concentrations, and three different day and night temperatures, on the rate of growth of cereal plants. The results are shown in Table Q1.
CO2 concentration / mmol mol-1
Day and night Mean grain temperatures / ºC yield per plant / g
330
660
Mean above-ground Harvest index biomass per plant / g
26/19
9.0
17.1
31/24
10.1
19.8
36/29
10.1
22.2
26/19
13.1
26.6
31/24
12.5
27.6
36/29
11.6
70.1
0.53
Table Q1
a. The harvest index is calculated by dividing the mean grain yield per plant by the mean above-ground biomass per plant. One value for harvest index is included in the table. Calculate the other five values for the harvest index. b. With reference to the data in the table, describe the effect of carbon dioxide concentration on: i. the mean grain yield per plant ii. the harvest index. c. With reference to the data in the table, describe the effect of temperature on: i. the mean grain yield per plant ii. the harvest index.
d. Suggest reasons for the patterns that you have described in your answers to b and c. e. Suggest why farmers may be more interested in achieving a high harvest index rather than a high mean grain yield per plant. 2. Scientists investigated the effect of iron deficiency on the production of triose phosphate in sugar beet plants. They grew plants under the same conditions with their roots in a liquid growth medium that contained all the necessary nutrients. Ten days before the experiments, they transferred half the plants to a liquid growth medium that contained no iron. The scientists measured the concentration of triose phosphate (continued)
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1 Photosynthesis
produced in these plants and in the control plants:
› at the end of six hours in the dark › then for 16 hours in the light.
ii. Explain why it was important to grow the plants under the same conditions up to ten days before the experiment.
Their results are shown in Figure Q1. Mean concentration of triose phosphate / arbitrary units
a. i. The experiments were carried out at a high carbon dioxide concentration. Explain why.
iii. The plants were left in the dark for six hours before the experiment. Explain why.
40
b. Iron deficiency reduces electron transport. Use this information and your knowledge of photosynthesis to explain the decrease in production of triose phosphate in the iron-deficient plants.
30 Control plants 20
c. Iron deficiency results in a decrease in the uptake of carbon dioxide. Explain why. AQA June 2013 Paper 4 Question 5
10
0
Iron-deficient plants
0
4
8
12
Time after light switched on / hours Figure Q1
16
3. During photosynthesis, carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reaction is catalysed by the enzyme rubisco. Rubisco can also catalyse a reaction between RuBP and oxygen to form one molecule of GP and one molecule of phosphoglycolate. Both the reactions catalysed by rubisco are shown in Figure Q2.
+ CO2 RuBP (5-carbon compound)
2 × glycerate 3-phosphate
Rubisco + O2
glycerate 3-phosphate and phosphoglycolate
Figure Q2
a. i. W here exactly in a cell is the enzyme rubisco found? ii. Use the information provided to give the number of carbon atoms in one molecule of phosphoglycolate.
b. Scientists investigated the effect of different concentrations of oxygen on the rate of absorption of carbon dioxide by leaves of soya bean plants. Their results are shown in Figure Q3.
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Practice questions
Rate of absorption of CO2 / μg min–1
60
50
40
Tube
Contents of tube
Uptake of radioactively labelled CO2 / counts per minute
A
Stroma and grana
96 000
B
Stroma, ATP and reduced NADP
97 000
C
Stroma
4000
30
20
10
0
0
20
40
60
80
a. Name the substance that combines with carbon dioxide in a chloroplast.
b. Explain why the results in tube B are similar to those in tube A.
c. Use the information in the table to predict the uptake of radioactively labelled carbon dioxide if tube A was placed in the dark. Explain your answer.
d. Use your knowledge of the light-independent reaction to explain why the uptake of carbon dioxide in tube C was less than the uptake in tube B.
e. DCMU is used as a weed killer. It inhibits electron transfer during photosynthesis. The addition of DCMU to tube A decreased the uptake of carbon dioxide. Explain why.
100
Concentration of oxygen / % Figure Q3
Use Figure Q2 to explain the results obtained in Figure Q3.
c. Use the information provided and your knowledge of the light-independent reaction to explain why the yield from soya bean plants is decreased at higher concentrations of oxygen. Phosphoglycolate is not used in the light-independent reaction. AQA January 2013 Unit 4 Question 5 4. A scientist investigated the uptake of radioactively labelled carbon dioxide in chloroplasts. She used three tubes, each containing different components of chloroplasts. She measured the uptake of carbon dioxide in each of these tubes. Her results are shown in the table.
AQA June 2012 Unit 4 Question 4 5. Researchers have investigated the effect of introducing a form of rubisco from cyanobacteria into tobacco leaves, using gene technology. They measured the amount of CO2 fixed in leaves of normal tobacco plants, and in leaves of tobacco plants containing the cyanobacterial rubisco, exposed to different concentrations of CO2. The rate was measured as the amount of CO2 fixed per mole of rubisco per second. (continued)
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1 Photosynthesis
7
a. Describe the role of rubisco in the light-independent reaction of photosynthesis.
6
b. In which part of the chloroplast does the reaction catalysed by rubisco take place?
The results are shown in Figure Q4.
Rubisco activity / mol CO2 fixed per mol of rubisco per second
5
Tobacco leaves containing rubisco from a cyanobacterium
4 3
Tobacco leaves containing normal rubisco
2
c. Compare the effects of increasing carbon dioxide concentration on tobacco leaves containing normal and those containing cyanobacterial rubisco. d. Suggest how growing genetically engineered plants containing cyanobacterial rubisco might increase yields.
1 0
0
200 400 600 800 Concentration of carbon dioxide / Οmol dm–3
Figure Q4
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