OCR F214 UNIT 1 MODULE 3



OCR F214 MODULE 3: PHOTOSYNTHESIS - Specification (a)-(q)

Types of nutrition

Autotrophic Nutrition

Autotrophs are organisms that can synthesise their own complex organic molecules from simple inorganic molecules and ions

The complex organic molecules include

• amino acids and proteins

• monosaccharides, disaccharides and polysaccharides

• fatty acids and glycerol

• nucleic acids and photosynthetic pigments

The simple inorganic molecules and ions include

• carbon dioxide

• water

• mineral ions

There are two groups of autotrophs – photoautotrophs and chemoautotrophs

Examples of Autotrophs and their Energy Sources

| |PHOTOAUTOTROPHS |CHEMOAUTOTROPHS |

|Energy source |Light energy |Chemical energy |

|Examples of organisms |Plants, algae, cyanobacteria, some |Some bacteria including bacteria in the |

| |photosynthetic bacteria |nitrogen cycle (Nitrosomonas and |

| | |Nitrobacter) |

Autotrophs are the producers in food chains. The majority of food chains on Earth have photoautotrophs as producers

Heterotrophic Nutrition

Heterotrophs are organisms that cannot synthesise their own complex organic molecules from simple inorganic molecules and ions

Heterotrophs have to obtain their complex organic molecules and energy by feeding on other organisms (either autotrophs or other heterotrophs)

• Animals, fungi, some bacteria and some protozoa are heterotrophs

• Heterotrophs digest the complex organic molecules from other organisms into simple organic molecules that they absorb and use to make their own complex organic molecules

Respiration and Photosynthesis in Photoautotrophs and Heterotrophs

• The cells of both autotrophs and heterotrophs respire at all times using oxygen from photosynthesis

• Only photoautotrophs have cells that photosynthesise, and this only occurs during daylight

Comparative Rates of Respiration and Photosynthesis in Photoautotrophs

Equation for Photosynthesis

6CO2 + 6H2O ( C6H12O6 + 6O2

Equation for Respiration

C6H12O6 + 6O2 ( 6CO2 + 6H2O + Energy

During the Day (Spring and Summer in temperate climates)

Rate of Photosynthesis > Rate of Respiration

Net Gas Exchange between the Leaves and the Atmosphere:

CO2 diffuses into the leaves

O2 diffuses out of the leaves

At Night

Only Respiration occurs

Net Gas Exchange between the Leaves and the Atmosphere:

O2 diffuses into the leaves

CO2 diffuses out of the leaves

At Dawn and Dusk (Spring and Summer in temperate climates)

Rate of Photosynthesis = Rate of Respiration

Net Gas Exchange between the Leaves and the Atmosphere:

No net diffusion of O2 and CO2 between the leaves and the atmosphere

The O2 produced by photosynthesis in the chloroplasts:

• diffuses out of the chloroplasts and into the mitochondria within the same cell, to be used for respiration

• or diffuses into the leaf air spaces where it is stored for diffusion into other cells

The CO2 produced by respiration in the mitochondria:

• diffuses out of the mitochondria and into the chloroplasts of the same cell, to be used in photosynthesis

• or, diffuses into the leaf air spaces where it is stored for diffusion into other cells

Experiment to Investigate the Release or Uptake of CO2 by Elodea (pond weed) in Light and Dark Conditions

• Sodium hydrogen carbonate indicator solution is used in this experiment

• This indicator solution changes colour according to its pH:

Red at pH 7 (neutral)

Purple/dark red when the pH is slightly above 7

Yellow at pH 6

• Sodium hydrogen carbonate solution releases CO2 for photosynthesis, this is an acidic gas in solution

The following tubes are set up and left for a few hours

Observations/Explanations

1. The Elodea tube left in light conditions shows a purple colour since CO2 is being taken up from the solution by the Elodea and used in photosynthesis, increasing the solution pH.

2. In the light, the rate of photosynthesis > rate of respiration

3. The Elodea tube left in dark conditions shows a yellow colour since CO2 is evolved from respiration by the Elodea, decreasing the solution pH.

4. In the dark, only respiration occurs in the pond weed

5. Tubes 2 and 4 are controls without Elodea. They show that there is no colour change in the indicator without Elodea

Quantitative Measurement of the Rate of CO2 Uptake or Release using Elodea in Sodium Hydrogen Carbonate Solution

1. A sample of the indicator can be placed in a colorimeter at known timed intervals, using a blue or green filter and the absorbance of light measured

2. The change in absorbance divided by time taken is a measure of CO2 uptake (the tube in the light) or CO2 release (the tube in the dark).

Photosynthesis - background

• CO2 is fixed in photosynthesis (fixation is the synthesis of complex molecules from simpler molecules

• CO2 is reduced in photosynthesis by hydrogen atoms from water

• The reactions involve the conversion of light energy to chemical energy. Light energy is transduced in photosynthesis (transduction is the conversion of one form of energy to another)

• Simple inorganic molecules and ions are converted into complex organic molecules

Location of Photosynthesis in Plants

The Organ: the leaf

The Tissues: palisade mesophyll, spongy mesophyll (note: don’t use the word ‘cell’ when describing a tissue)

The Cells: palisade mesophyll cells, spongy mesophyll cells, guard cells (of epidermal tissue)

The Organelles: chloroplasts

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CHLOROPLAST STRUCTURE

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Details of Chloroplast Structure

• Biconvex disc shaped organelles, 2-10 µm long

• Surrounded by a double membrane, the chloroplast envelope with a fluid-filled intermembrane space between the two membranes

• Outer membrane is more permeable than the inner membrane, since the outer membrane has more transport proteins embedded within it

• The inner membrane is also continuous with membrane bound compartments called lamellae or thylakoids. A stack of flattened thylakoid membranes is called a granum

• Intergranal lamellae link grana together

• The stroma is a fluid filled matrix in the chloroplast

• The grana and stroma can be seen under a higher power light microscope. Thylakoids are visible using an electron microscope

• The chloroplast also contains a loop of DNA and 70S ribosomes

• Starch granules (sometimes very large) and lipid droplets (products of photosynthesis) are stored in the chloroplast

Note: adaptations of chloroplast features to their function is covered on pages 42-43

PHOTOSYNTHETIC PIGMENTS

Definition:

• Photosynthetic pigments are coloured compounds, located in the thylakoid membranes of a chloroplast

• Photosynthetic pigments absorb light energy that is used in photosynthesis.

• There are a number of different pigments in chloroplasts and each pigment absorbs a distinct range of wavelengths of light, with absorption peaks at certain wavelengths

• Wavelengths of light not absorbed are reflected or transmitted (these are the colours that we see)

The Range of Wavelengths that make up Visible Light

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Table Comparing Photosynthetic Pigments

|PIGMENT |FEATURES |

|Chlorophyll a |Blue-green pigment. |

| |Primary pigment at the reaction centre of the photosystems. |

| |Peaks of absorption 420-430nm (blue-violet) and 670nm (red) |

| |Consist of a long hydrocarbon chain (hydrophobic) and a porphyrin|

| |ring (hydrophilic) containing magnesium |

|Chlorophyll b |Yellow-green pigment. |

| |Accessory pigment. |

| |Peaks of absorption 460nm (blue) and 650nm (red-orange) |

| |Very similar structure to chlorophyll a |

|Carotenoids including the carotenes and the xanthophylls |Red ,orange, yellow or brown pigments |

|Brown seaweeds contain fucoxanthin, a brown pigment, as well as |Accessory pigments. |

|chlorophyll |Peaks of absorption 450-510nm. |

|Red seaweeds contain phycoerythrin, a red pigment, as well as |Also absorb UV radiation – protect plant cells against UV |

|chlorophyll |mutagenic effects |

| |Consist of two small hydrocarbon rings linked by a long |

| |hydrocarbon chain (no magnesium containing porphyrin rings) |

Why Seaweeds have more Red or Brown pigments than Terrestrial Plants

• Seaweeds are algae in the Protoctista kingdom. They are not classified as plants

• Seaweeds are autotrophic and important producers in marine ecosystems

• Seaweeds with brown and red pigments can live in deeper water where light intensity is lower

• Fucoxanthin and phycoerythrin can absorb the little light that does penetrate into deeper water

• In addition, not all wavelengths of light can penetrate deeper water. The shorter wavelengths of light, that is blue light, penetrates more

• Fucoxanthin and phycoerythrin absorb the blue light that does penetrate deeper water

PHOTOSYSTEMS

• A photosystem is a cluster of photosynthetic pigments arranged in a funnel shaped antenna complex, embedded in the thylakoid membrane

• Each photosystem has a primary pigment. This is one molecule of chlorophyll a at the reaction centre of the photosystem

• Each photosystem also has 200 or more molecules of accessory pigments. These include chlorophyll b, carotenoids and xanthophylls. Accessory pigments are located in the funnel arrangement of the photosystem, to absorb light energy

• All the pigment molecules absorb photons of light energy. The range of accessory pigments enables a range of wavelengths to be absorbed

• The photons of energy absorbed by accessory pigments are transferred to chlorophyll ‘a’ at the reaction centre in the form of resonance energy (chlorophyll a also absorbs its own specific wavelengths of light)

• The pigments are held in position in the thylakoid membranes by a framework of protein molecules that support the pigments in the best possible positions for energy transfer

• When chlorophyll a absorbs light and resonance energy, its electrons become excited. Each chlorophyll a molecule emits two electrons (previously associated with magnesium in the chlorophyll molecule). These electrons are accepted by an electron acceptor molecule and the light dependent stage is initiated

• There are two types of photosystem, PSI and PSII

• These photosystems differ only in their reaction centres, that contain slightly different forms of chlorophyll ‘a’ with slightly different absorption peaks at the red end of the spectrum

• PSI contains chlorophyll a with an absorption peak of 700nm (at the red end). PSII contains chlorophyll a with an absorption peak of 680nm (at the red end). PSI is sometimes written as P700 and PSII as P680

• PSI is mainly located on the intergranal lamellae. PSII is almost exclusively located on the granal lamellae

The diagram below represents a birds eye view of a photosystem for harvesting light

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ABSORPTION SPECTRA FOR PHOTOSYNTHETIC PIGMENTS

Definition

An absorption spectrum is a graph showing the absorption of light by photosynthetic pigments, either separately or as a mixture, at different wavelengths of visible light

Absorption Spectrum of Individual Photosynthetic Pigments

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Indicate the absorption peaks of:

Chlorophyll a………………………………………………………………………..

Chlorophyll b……………………………………………………………………….

Carotenoids…………………………………………………………………………

Absorption Spectrum of a Mixture of Photosynthetic Pigments

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Describing Absorption Spectra

You may be asked to describe an absorption spectrum for a particular pigment.

Approach this type of question as follows:

• Quote the wavelengths of the spectrum where the pigment has maximum absorption (ie the peaks of absorption). This may require a range of wavelengths to be quoted

• Indicate which wavelengths give the higher absorption peak if there is more than one peak of absorption

• Quote the range of wavelengths where the absorption is lowest (the trough of absorption). Perhaps there is no absorption at particular wavelengths, quote these also

ACTION SPECTRUM FOR PHOTOSYNTHESIS

An action spectrum is a graph showing the rate of photosynthesis at different wavelengths of visible light

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Comparison of Action and Absorption Spectra

The shapes of the spectra are very similar indicating that the photosynthetic pigments absorb the wavelengths of light that are used in photosynthesis

Summary of the Importance of Photosynthetic Pigments in Photosynthesis

• Chlorophyll ‘a’ is the primary pigment in photosystems. It absorbs light energy and resonance energy from accessory pigments and releases electrons. The passage of these electrons results in ATP synthesis

• Chlorophyll ‘b’, carotenoids and xanthophyll pigments are accessory pigments. They each absorb a specific range of wavelengths of light and transfer energy to chlorophyll ‘a’ molecules

• Carotenoid pigments also absorb UV radiation which protects leaf cell DNA from mutations

STAGES OF PHOTOSYNTHESIS

|FEATURES |LIGHT DEPENDENT STAGE |LIGHT INDEPENDENT STAGE/THE CALVIN CYCLE |

|Location |Thylakoid membranes/grana |Stroma |

|Energy source |Photons of light energy |Chemical (ATP) |

|Reactants |Water and NADP |Carbon dioxide, ribulose bisphosphate and |

| | |reduced NADP |

|Products |Oxygen , ATP and reduced NADP |Complex organic molecules such as |

| | |monosaccharides, amino acids, fatty acids, |

| | |glycerol |

|Other requirements |Thylakoid membrane bound PSI and PSII, |A sequence of enzymes to catalyse the |

| |electron transport carriers and ATP |metabolic pathway including ribulose |

| |synthetase associated with protein channels|bisphosphate carboxylase (Rubisco) |

| | | |

|Time that stage occurs |Only during daylight hours |Occurs at any time of the day or night |

THE LIGHT DEPENDENT STAGE OF PHOTOSYNTHESIS

This stage includes:

1. Non-cyclic photophosphorylation

2. Cyclic photophosphorylation

NON-CYCLIC PHOTOPHOSPHORYLATION

This type of photophosphorylation is illustrated below in the Z diagram

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In non-cyclic photophosphorylation, light energy is used for two processes:

• Phosphorylation of ADP to produce ATP (ADP + Pi -> ATP)

• Photolysis of water (H2O -> ½O2 + 2e- + 2H+)

Sequence of Events

• Light energy is absorbed by the primary and accessory pigments in PSI and PSII

• Chlorophyll a molecules at the reaction centres become excited and emit two electrons

• The electrons are raised to a higher energy level and are transferred to electron acceptor A (from PSII) and electron acceptor B (from PSI)

• Electron acceptors A and B are now reduced and the original chlorophyll a molecules at the reaction centres of PSI and PSII are unstable and oxidised

• The electrons from electron acceptor A are transferred along a chain of electron acceptor molecules, each at a lower energy level than the previous electron acceptor. This chain of molecules are also embedded in the thylakoid membrane and are called an electron transport chain

• During this electron transfer, sufficient energy is released to synthesise ATP from ADP and Pi

• The final electron acceptor of the electrons from PSII is the chlorophyll a molecule at the reaction centre of PSI. This chlorophyll a molecule is now reduced and stable

• The electrons originally emitted from PSI are passed to an even higher level that those from PSII, when accepted by electron acceptor B. These electrons are then transferred to NADP

• The enzymes associated with the photolysis of water are within the PSII complex. When water is split by this reaction, using light energy:

H2O -> ½O2 + 2e- + 2H+

-the protons are used to reduce NADP (along with the electrons from PSI)

-the electrons are accepted by the unstable chlorophyll a molecule at PSII

-oxygen is released as a waste product

• The products of non-cyclic photophosphorylation, ATP and reduced NADP are used in the light independent reactions of photosynthesis

OCR Representations of Non-Cyclic Photophosphorylation

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Chemiosmosis and the Synthesis of ATP

• Protons (H+) flow from the thylakoid spaces/lumens through the protein channels of the stalked particles into the stroma

• This proton diffusion, down an electrochemical gradient provides the energy for the synthesis of ATP from ADP + Pi. ATP synthetase catalyses ATP synthesis. ATP synthetase is part of the stalked particle

CYCLIC PHOTOPHOSPHORYLATION

• Only PSI is needed for this process

• When light is absorbed by the pigments in PSI, electrons in chlorophyll a at the reaction centre are excited and emitted to a higher energy level. These electrons are accepted by an electron acceptor molecule

• The electrons are then transferred down an electron transport chain, but the final electron acceptor is the chlorophyll a molecule in PSI, that released the electrons originally. The electrons are re-cycled

• Sufficient energy is released as the electrons are transferred down the electron transport chain to synthesise ATP from ADP and Pi.

• This ATP can be transferred to the light independent stage but since no reduced NADP is synthesised in this process, cyclic photophosphorylation does not contribute very much to the synthesis of organic molecules

• Only PSI photosystems are present in guard cell chloroplasts and it is suggested that the ATP released is used in the active processes required for stomatal opening. It appears that no organic molecules are synthesised in the guard cell chloroplasts

Diagram that Shows both the Cyclic and Non-cyclic Photophosphorylation

Comparing Cyclic and Non-Cyclic Photophosphorylation

| |Cyclic Photophosphorylation |Non-Cyclic Photophosphoylation |

|Photosystem(s) |Only PSI |PSI and PSII |

|Inputs |Light energy, ADP Pi |Light energy, ADP, Pi, water, NADP |

|Products |ATP |ATP, reduced NADP, oxygen |

|Photolysis of water |Does not occur |Does occur |

LIGHT INDEPENDENT STAGE OF PHOTOSYNTHESIS

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• The reactions in this stage are cyclic – the Calvin Cycle

• The stroma is the site of the Calvin cycle

• The light independent reaction can occur at any time of the day or night, provided the inputs (ATP and reduced NADP from the light dependent reactions) are available

• CO2 from the atmosphere diffuses through stomata, leaf air spaces and into palisade mesophyll cells through the thin cell walls, the cell surface membrane, the chloroplast envelope into the stroma

• Carbon dioxide is fixed by a 5 carbon sugar intermediate, ribulose bisphosphate (RuBP). RuBP is a CO2 acceptor

• Ribulose bisphosphate carboxylase oxygenase (rubisco) is the enzyme that catalyses this fixation of carbon dioxide. RuBP is carboxylated (it combines with CO2 in this reaction)

• The product of CO2 fixation is an unstable 6C intermediate compound that breaks down immediately into two molecules of glycerate 3 phosphate (GP) – a 3 carbon compound

• Plants in which GP is the first stable product of CO2 fixation are called C3 plants

• Using ATP as an energy source and reduced NADP as a hydrogen source (both products of the light dependent stage), glycerate 3 phosphate is reduced to triose phosphate (3 carbon sugar) abbreviated to TP

• Some GP is converted to amino acids and fatty acids. The synthesis of amino acids requires a nitrogen source such as nitrates, absorbed by plant roots

• Triose phosphate does not accumulate. It is immediately converted to other products

• About one-sixth of the triose phosphate is converted to hexose (six carbon sugar) or to glycerol for use in lipid synthesis. Each hexose molecule is synthesised from two TP molecules

• Most of the TP is used to regenerate RuBP, completing the cycle. This regeneration requires ATP as an energy and a phosphate source

• Five out of every six molecules of TP are recycled to produce three molecules of RuBP

• The hexose sugars can be polymerised into cellulose for cell wall synthesis, or starch for energy storage

• Glucose can be isomerised to fructose and these two hexose sugars can be combined to form sucrose, the transport sugar in phloem sieve tubes

• The glycerol produced from TP combines with the fatty acids produced from GP to form triglycerides

• Glucose may be used in glycolysis, in the mitochondria as a respiratory substrate

• Hexose sugars are also used to produce pentose sugars for nucleic acid synthesis

Further Information on rubisco

• Rubisco has been described as the most important enzyme on Earth. This is because plants need the enzyme to survive and plants are at trophic level 1of many food chains

• Rubisco is also the most abundant enzyme on Earth since it makes up 50% of leaf protein

• Rubisco is an abbreviation of ribulose bisphosphate carboxylase-oxygenase

• Oxygen can also attach to the active site of rubisco and then the enzyme catalyses a process called photorespiration, a process involving oxygen as a substrate and carbon dioxide as a product

• When photorespiration is occurring, CO2 is not being fixed. Photorespiration is a waste of ATP and does results in the synthesis of less GP and TP. Photorespiration also leads to the formation of hydrogen peroxide, a toxic oxidising agent

• As temperature increases, the oxygenase activity of rubisco increases more than its carboxylase activity

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Question 1

Herbicides interfere with electron transport by accepting electrons

Suggest how this causes plants to die

Question 2

Atrazine is a widely used herbicide. It binds to a chloroplast protein involved in electron transfer between photosystems.

a) Explain how atrazine works as an effective herbicide

b) Maize plants are insensitive to atrazine

The cytoplasm of maize plant cells contains a tripeptide which binds to atrazine. The product is then transported to the vacuole

c) Explain why maize plants are insensitive to atrazine

d) How is this insensitivity to atrazine of benefit to maize farmers?

LIMITING FACTORS

Definition of a limiting factor:

1. A factor that controls/limits the rate of a reaction

2. A factor that is nearest its minimum value or nearest to its least favourable level

3. A factor that if increased, would speed up the reaction

Environmental Factors that may limit the rate of photosynthesis

1. light intensity

2. wavelength of light

3. carbon dioxide concentration

4. temperature

5. water availability

6. mineral ion availability

The Effect of Light Intensity on the Rate of Photosynthesis

Light is involved in the following processes:

• it causes stomata to open so that CO2 can diffuse into the leaves

• it is absorbed by photosynthetic pigments and used to excite electrons in chlorophyll a molecules. Light energy is transduced to chemical energy (ATP) in chloroplasts

• it is needed for the photolysis of water to release protons

Description and Explanations for Graph 1

For graph 1 (left hand graph) the temperature and carbon dioxide concentrations are kept constant

• at zero light intensity, there is no photosynthesis

• at lower light intensities, as light intensity increases, the rate of photosynthesis increases, in a directly proportional way. At these lower light intensities, light intensity is limiting the rate

• at higher light intensities, the rate plateaus. Light intensity no longer limits the rate, since increasing its intensity does not increase the rate. Some other factor is limiting the rate, it may be CO2 concentration

• if the CO2 concentration is increased, the rate will increase, as light intensity increases, but the rate will plateau again, this time at a higher light intensity, because again, another factor (perhaps temperature) is limiting the rate

• assuming that the existing curve in graph 2 (right hand graph) is at a lower carbon dioxide concentration, draw another curve for rate at a higher CO2 concentration

Graph 1 Graph 2

The effect of CO2 concentration on the rate of photosynthesis

• CO2 concentration in the atmosphere is approx, 0.039% by volume

• In greenhouses, the CO2 levels can fall, even with ventilation. Plant growers can increase the CO2 concentrations by burning methane or using oil-fired heaters

Description and Explanation for Graph 3 – the effect of CO2 concentration on the rate of photosynthesis

• At lower CO2 concentrations, increasing the CO2 concentration increases the rate of photosynthesis. Over this lower range of CO2 concentrations, CO2 concentration is rate limiting

• At higher CO2 concentrations, the rate plateaus because some other factor is limiting the rate

• Graph 3 below, shows the effect of increasing the light intensity on the rate. At higher light intensity, CO2 concentration is rate limiting at higher CO2 concentrations, but eventually, the rate plateaus again, at higher CO2 concentrations when another factor limits the rate

Graph 3

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Effect of temperature on the rate of photosynthesis

• The photochemical reactions of the light dependent reactions of photosynthesis are not greatly affected by temperature changes

• The enzyme catalysed reactions of the Calvin cycle are affected by temperature changes

• Between the temperatures 0 to 25oC, the rate of photosynthesis doubles for each 10oC rise in temperature (Q10 = 2)

• Above 25oC, the rate levels off and then falls as more enzymes are denatured

• At higher temperatures, O2 competes with CO2 more successfully for the active sites of rubisco, reducing CO2 fixation

• Since higher temperatures increase transpiration rate, the plant has a stress response leading to closure of more stomata, limiting uptake of CO2

Description and explanation for graph 4

• At low light intensities, increasing temperature has little effect on the rate of photosynthesis, because light intensity is the limiting factor

• At higher light intensity, increasing temperature up to 20oC, increases the rate, because temperature is the limiting factor

Graph 4

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LIMITING FACTORS AND THE CALVIN CYCLE

Effect of Carbon dioxide concentration on the concentrations of Calvin Cycle Molecules

• Increased CO2 concentrations will lead to an increase in CO2 fixation in the Calvin cycle, provided light intensity is not limiting the rate

• Increased CO2 concentration results in more GP synthesis and more TP synthesis

• If CO2 concentrations are reduced below 0.01%, RuBP will accumulate as there is less CO2 to fix. Levels of GP and TP will decrease and fewer GP and TP molecules are synthesised

• Levels of CO2 in leaf air spaces could decrease to these low levels if the plant is suffering from water stress. The plants response would be to close stomata, reducing CO2 uptake

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Effect of Light Intensity on the Concentrations of Calvin Cycle Molecules

• Increasing light intensity increases the rate of the light dependent reactions

• More light energy is available to excite more electrons resulting in more photophosphorylation, producing more ATP and more reduced NADP

• There is now more ATP and more reduced NADP to reduce GP to TP and to phosphorylate TP to RuBP

• At any point in time in bright light, you might expect lower levels of GP and higher levels of TP and RuBP

• At very low light intensities, the light dependent reactions will stop

• No ATP and reduced NADP will be produced at low light intensity. Therefore, the light independent reactions will also stop.

• At any point in time at low light intensity, you might expect high levels of GP and lower levels of TP and RuBP

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Application of Limiting Factors

• In our temperate Summer climate, CO2 concentration in the atmosphere is usually the limiting factor in photosynthesis

• In our Winters, light intensity and temperature are more likely to be the limiting factors for photosynthesis

• The abiotic factors in a greenhouse can be manipulated to increase photosynthesis rates of the crop plants and therefore increase crop yield

• CO2 concentrations in the greenhouse atmosphere can be very low. By burning methane or another fuel in greenhouse burners, the CO2 concentration can be increased

• Light intensity and temperature can also be modified. Sometimes, artificial lights are left on during the night as well as the day

Measuring the Rate of Photosynthesis using a Photosynthometer

• The rate of photosynthesis is usually measured using a photosynthometer as illustrated on page 35

.

• This apparatus can be used to calculate the volume of oxygen released from an aquatic plant per minute

• This method is useful for aquatic plants since the oxygen gas they release as a waste product of photosynthesis is not very soluble in water. The oxygen gas can be seen as bubbles and can be collected

• This apparatus is not suitable for terrestrial plants. To measure their rate of photosynthesis would require measuring rate of uptake of CO2 (perhaps using a radioactive tracer) or the increase in mass of the plant over time

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Procedure to investigate the effect of light intensity on photosynthesis rate

Fill the apparatus with tap water by removing the plunger from the syringe and running a gentle stream of water into the barrel of the syringe, until the barrel and tubing is full of water

1. Replace the syringe plunger and gently push water out of the flared end of the tubing to remove any air bubbles in the water. The plunger should be almost at the end of the syringe barrel

2. Cut a piece of pond weed about 7cm long

3. Place the pond weed into a test tube of water, with the cut end upwards. . Make sure that gas bubbles are being released from the cut end

4. Add several drops of sodium hydrogen carbonate solution to the water. NaHCO3 releases CO2 into the water for the plant’s photosynthesis

5. Stand the test tube in a beaker of water maintained at 20oC. This beaker of water acts as a water bath. The thermometer in the water bath should be checked regularly to monitor the temperature. Cold water should be added if the temperature rises

6. The room needs to be darkened so that the only light source is the lamp positioned close to the apparatus. This controls light intensity

7. Position a lamp close to the pondweed. Measure the distance of the lamp to the pondweed. Calculate light intensity as 1/d2

8. Leave the apparatus with the capillary tube positioned so that it is not collecting gas for 10 mins to allow the pondweed to acclimatise to the conditions

9. Position the capillary tube over the cut end of the pondweed. Start the stopclock and leave the apparatus for 10 mins

10. A bubble of gas will be collected in the bend of the capillary tube. Gently pull back the syringe plunger to move the gas bubble along the scale. Note the length of the bubble

11. Gently push in the plunger to expel the bubble from the capillary tube into the water

12. Reposition the flared end of the capillary tube over the pondweed and repeat the experiment

13. Continue the experiment with several light intensities by moving the lamp further away each time and allowing time for acclimatisation at each light intensity

Calculating the Rate of Photosynthesis

Rate of photosynthesis = volume of gas collected per minute

Volume of gas collected = length of bubble x πr2

(where r is the internal radius of the capillary bore and πr2 is its cross sectional area)

In theory, the rate can be calculated as:

length of bubble x πr2 /Time taken to collect bubble of gas (min)

Units are mm3min-1

Limitations of this Experiment

1. Not all the oxygen produced in photosynthesis during the time period studies is collected

Reasons

• Some of the oxygen produced is used in the plant’s respiration

• Some dissolves in the water in the test tube

• Some remains in the leaf air spaces

• Some escapes from being collected in the flared part of the capillary tube

2. Some of the gas collected is nitrogen

Reasons

• Nitrogen was present in the leaf air spaces and diffuses out of these spaces into the water, along with oxygen

• The water also contains air with dissolved nitrogen gas that comes out of solution. Nitrogen gas is less soluble in warmer water

3. Some of the gas collected is carbon dioxide

Reasons

• Carbon dioxide is a product of the plant’s respiration

• NaHCO3 solution has been added to the water to release CO2. Not all of the CO2 released has been used in photosynthesis

• Some of the dissolved CO2 in the water has come out of solution since it is less soluble in warmer water

Interactions between Respiration and Photosynthesis in Plants

Metabolic pathways interact in organisms since substrates and products are often common between them. The diagram below shows part of these interactions

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On the diagram, name pathways W, X and Y and indicate if they are part of respiration or photosynthesis

Explain how these three pathways are able to work independently of each other in the same leaf cell

Compensation Point and Graphs Linking Photosynthesis and Respiration

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In this experiment, CO2 uptake/production is measured when experiments are carried out at different light intensities

Which biochemical process(es) are occurring at 0 light intensity?

……………………………………………………………………………………………..

Why does the concentration of carbon dioxide produced decrease as the light intensity increases from 0 to X?

………………………………………………………………………………………………

………………………………………………………………………………………………

The light intensity at X is called the compensation point. Explain why there is no net production or uptake of CO2 at X

………………………………………………………………………………………………

Explain the increasing uptake of CO2 at light intensities higher than X

………………………………………………………………………………………………

State the limiting factors at A and B on the graph

………………………………………………………………………………………………

Shade and Sun Leaves

• Some plants have both sun and shade leaves

• The beech tree (Fagus sylvatica) is an example

• Sun leaves are found on the most exposed parts of the tree and shade leaves are found on more sheltered parts

• The diagrams below show transverse sections through the lamina (leaf blade) of a sun and shade leaf

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1) Suggest two ways in which the appearance of the sun leaves would differ from that of the shade leaves. Give a reason for each of your answers

2) Explain why the net rate of photosynthesis of the shade leaves might differ from that of the sun leaves at high light intensities, when light is not a limiting factor

3) State one function of A

4) State one function of B

Adaptations of the Organ, Tissues, Cells and Chloroplasts to Photosynthesis

These adaptations need to be known in detail, since past OCR questions on photosynthesis have required prose-type questions on these aspects

Adaptations of the Chloroplast for Photosynthesis

1. Biconvex shape that increases the surface area for light absorption and gas diffusion

2. Many thylakoid membranes also increase the surface area, containing more photosynthetic pigments

3. Chloroplasts can transduce energy (convert light to chemical energy)

4. A range of photosynthetic pigments absorb light energy. Chlorophyll a loses electrons when light energy is absorbed

5. Chlorophyll a and b absorb red and blue/violet wavelengths of light

6. Carotenoid and xanthophyll pigments absorb other wavelengths of light (blue and green). The range of pigments in chloroplasts ensures that a range of wavelengths of light are absorbed for photosynthesis

7. Carotenoid pigments also absorb UV radiation, protecting the cell DNA from its mutagenic effects

8. The photosynthetic pigments are embedded in the thylakoid membranes, arranged in light-harvesting clusters called the photosystems

9. Chlorophyll a is the primary pigment at the reaction centre of each photosystem

10. Accessory pigments funnel resonance energy to the chlorophyll a molecule at the reaction centre, causing it to lose two electrons

11. Electron carriers (in the electron transport chain) are also embedded in the thylakoid membranes. As the electrons from chlorophyll a are passed along these carriers, ATP is synthesised. For ATP synthesis, ATP synthetase enzymes are associated with proton channels within the thylakoid membranes

12. Proton pumps are also located in the thylakoid membranes for pumping of protons into the thylakoid space; this leads to ATP synthesis

13. NADP is present in chloroplasts

14. The enzymes for the Calvin cycle are present in the chloroplast stroma, including ribulose bisphosphate carboxylase (Rubisco)

15. Starch granules and lipid droplets are stored in chloroplasts

16. DNA and 70S ribosomes in chloroplasts are used in the synthesis of proteins required for photosynthesis such as rubisco and other enzymes

17. The double membranes are partially permeable and compartmentalise the reactions of photosynthesis

Adaptations of Palisade Mesophyll Cells for Photosynthesis

Palisade mesophyll cells are the main sites of photosynthesis in leaves.

1. These cells are closely packed to absorb more incident light

2. Column shaped cells, orientated at right angles to the leaf surface, to reduce the number of light absorbing cross walls

3. Large central vacuoles push the chloroplasts to the edges of the cells. At the cell periphery, there is a shorter distance for gas diffusion (CO2 and O2) and a shorter pathway for light absorption, into or out of the chloroplasts

4. Many chloroplasts with many different photosynthetic pigments in increase light absorption

5. Chloroplasts are able to move within the cytoplasm to absorb as much light as possible and to avoid damage in high light intensities

6. These cells are cylindrical, creating air spaces when packed together

7. Thin cell walls provide a short diffusion distance for gases and a short absorption distance for light

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