Topic 5A - Photosynthesis And Respiration

Question 1

Which biological processes need energy?

Answer 1

Plants need energy for photosynthesis, active transport, DNA replication, cell division and protein synthesis. Animals need energy for things like muscle contraction, maintenance of body temperature, active transport, DNA replication, cell division and protein synthesis.

Question 2

What happens in photosynthesis?

Answer 2

Energy from light is used to make glucose from H2O and CO2 (the light energy is converted to chemical energy in the form glucose).

6CO2 + 6H2O + energy -> C6H12O6 + 6O2

Energy is stored in the glucose until the plants release it by respiration. Animals obtain glucose by eating.

Question 3

What happens in respiration?

Answer 3

Plant and animal cells release energy from glucose. This energy is used to power all the biological processes in a cell. Aerobic respiration:
C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy.
Anaerobic respiration in plants and yeast produces ethanol and carbon dioxide and releases energy. In humans anaerobic respiration produces lactate and releases energy.

Question 4

Describe how ATP produces energy?

Answer 4

ATP carries energy around the cell to where it’s needed. ATP is synthesised via a condensation reaction between ADP and inorganic phosphate using energy from an energy-releasing reaction, e.g breakdown of glucose in respiration. The energy is stored as chemical energy in the phosphate. The enzyme ATP synthase catalyses this reaction. ATP diffuses to the part of the cell that needs energy. Here’s its hydrolysed back into ADP and inorganic phosphate. Chemical energy is released from the phosphate bond and used by cell. ATP hydrolyse catalyses this reaction. The ADP and inorganic phosphate are recycled and the process starts again.

Question 5

Why is ATP a good energy source?

Answer 5

ATP stores or releases only a small, manageable amount of energy at a time, so energy is wasted as heat. It’s a small, soluble molecule so can be easily transported around the cell. It’s easily broken down, so energy can be easily released instantaneously. It can be quickly re-made. It can make other molecules more reactive by transferring one of its phosphate groups to them (phosphorylation).
ATP can’t pass out of the cell, so the cell always has an immediate supply of energy.

Question 6

What are the definitions of metabolic pathway, photophosphorylation, photolysis, and photoionisation?

Answer 6

Metabolic pathway - a series of small reactions controlled by enzymes e.g respiration and photosynthesis.
Photophosphorylation - adding phosphate to a molecule using light.
Photolysis - the lysis of a molecule using light energy.
Photoionisation - light energy excites electrons in an atom or molecule, giving them more energy and causing them to be released. The release of electrons causes the atom or molecule to become a positively-charged ion.

Question 7

What are coenzymes?

Answer 7

A molecule that aids the function of an enzyme. They work by transferring a chemical group from one molecule to another. NADP is used in photosynthesis. NADP transfers hydrogen from one molecule to another - this means it can reduce (give hydrogen to) or oxidise (take hydrogen from) a molecule. Examples of coenzymes in respiration are NAD, coenzyme A, and FAD.

NAD and FAD transfer hydrogen from one molecule to another. Co enzyme A transfers acetate between molecules.

Question 8

Where does photosynthesis occur?

Answer 8

In chloroplasts, which are flattened organelles surrounded by a double membrane. They are found in plant cells. Thylakoids are stacked up in the chloroplast into structures called grana. The grana are linked by lamellae. Chloroplasts contain photosynthetic pigments e.g chlorophyll a, chlorophyll b and carotene which are coloured pigments that absorb the light energy needed for photosynthesis. The pigments are found in the thylakoid membranes - they’re attached to proteins. The protein and pigment is called a photosystem. PS1 absorbs light best at 700nm and PS2 absorbs light best at 680nm. Stroke contains enzymes, sugars and organic acids. Carbohydrates produced by photosynthesis and not used straight away are stored as starch grains in the stroma.

Question 9

What briefly happens in the light dependent reaction?

Answer 9

Needs light energy. Takes place in the thylakoid membranes. Light energy is absorbed by chlorophyll in photosystems. The light energy excites the electrons in the chlorophyll, leading to their eventual release from the molecule. The chlorophyll has been photoionised. Some of the energy from the released electrons is used to add a phosphate group to ADP to form ATP, and some is used to reduce NADP to form reduced NADP. ATP transfers energy and reduced NADP. ATP transfers energy and reduced NADP transfers hydrogen to the light-independent reaction. H2O is oxidised to O2.

Question 10

What briefly happens in the light-independent reaction?

Answer 10

Calvin cycle doesn’t use light energy but it relies on the products of light-dependent reaction. It takes place in the stroma. ATP and reduced NADP from the light-dependent reaction supply the energy and hydrogen to make simple sugars from CO2.

Question 11

How is the energy resulting from the photoionisation of chlorophyll used?

Answer 11

Making ATP from ADP and inorganic phosphate - photophosphorylation.
Making reduced NADP from NADP.
Splitting water into protons, electrons and oxygen - photolysis.

Question 12

What happens in non-cyclic photophosphorylation?

Answer 12

Photosystems in the thylakoid membrane are linked by electron carriers. Electron carriers are proteins that transfer electrons. The photosystems and electron carriers form an electron transport chain - a chain of proteins through which excited electrons flow.

1) light energy is absorbed by psII. The light energy excites electrons in chlorophyll. The electrons move to a higher energy level. These high-energy electrons are released from the chlorophyll and move down the electron transport chain to PSI.

2) As the excited electrons from chlorophyll leave PSII to move down the electron transport, they must be replaced. Light energy splits water into protons, electrons, and oxygen - photolysis. H2O -> 2H^+ + 1/2 O2

3) The excited electrons lose energy as they move down the electron transport chain. This energy is used to transport protons into the thylakoid, so that the thylakoid has a higher concentration of protons than the stroma. This forms a proton gradient across the thylakoid membrane. Protons move down their concentration gradient into the stroma. This forms a proton gradient across the thylakoid membrane. Protons move down their concentration gradient, into the stroma via ATP synthase which is embedded in the thylakoid membrane. The energy from this movement combines ADP and pi to form ATP.
4) light energy is absorbed by PS1, which excites the electrons again to an even higher energy level. Finally the electrons are transferred to NADP, along with a proton from the stroma, to form reduced NADP.

Question 13

What happens in cyclic photophosphorylation?

Answer 13

Only uses PSI. The electrons from the chlorophyll molecule aren’t passed onto NADP, but are passed back to PSI via electron carriers. This means the electrons are recycled and can repeatedly flow through PSI. This process doesn’t produce any reduced NADP or O2 - it only produces small amounts of ATP.

Question 14

Describe the light-independent reaction.

Answer 14

1) CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast. It combines with ribulose bisphosphate (RuBP), a 5-carbon compound, and this reaction is catalysed by rubisco. This gives an unstable 6-carbon compound, which quickly breaks down into two molecules of a 3-carbon compound called glycerate-3-phosphate (GP).
2) the hydrolysis of ATP (from light dependent) provides energy to turn GP into triose phosphate (TP). This reaction also requires H^+ ions, which come from reduced NADP (also from light dependent). Reduced NADP is recycled to NADP. Some triose phosphate is then converted into useful organic compounds (e.g glucose) and some continues in the Calvin cycle to regenerate RuBP. Five out of every 6 molecules of TP produced in the cycle aren’t used to make hexose sugars, but to regenerate RuBP. Regenerating RuBP uses the rest of the ATP produced by the light-dependent reaction.

Question 15

What happens to TP and GP?

Answer 15

Hexose sugars (e.g glucose) are made by joining two triose phosphate molecules together and larger carbohydrates are made by joining hexose sugars together in different ways. Lipids are made using glycerol, which is synthesised from TP and fatty acids, which are synthesised from glycerate-3-phosphate. Some amino acids are made from GP.

Question 16

How many times does the Calvin cycle need to turn?

Answer 16

Three turns of the cycle produces 6 TP. Five out of six of these TP molecules are used to regenerate RuBP. A hexose sugar has six carbons though, so two TP molecules are needed to form one hexose sugar. 6 turns of the cycle need 18 ATP and 12 reduced NADP from the light-dependent reaction. 6 turns of the cycle need 18 ATP and 12 reduced NADP from the light-dependent reaction.

Question 17

What are the optimum conditions for photosynthesis?

Answer 17

High light intensity of a certain wavelength. Light is needed to provide the energy for the light-dependent reaction - the higher the intensity of the light, the more energy it provides. Only certain wavelengths of light are used for photosynthesis. The photosynthetic pigments chlorophyll a, chlorophyll b, and carotene only absorb the red and blue light in sunlight while green light is reflected.
Temperature around 25°C - photosynthesis involves enzymes, so if the temperature falls below 10°C the enzymes become inactive, but if the temperature is more than 45°C they may start to denature. Also at high temperatures stomata close to avoid losing too much water. So less CO2 enters.
Carbon dioxide at 0.4% - makes up 0.04% of the gases in the atmosphere. Increasing this to 0.4% gives a higher rate of photosynthesis, but any higher and the stomata start to close.
Plants also need a constant supply of water - too little and photosynthesis has to stop but too much and the soil becomes waterlogged (reducing the uptake of minerals such as magnesium, which is needed to make chlorophyll a).

Question 18

What can limit photosynthesis?

Answer 18

On a warm sunny windless day it’s usually CO2 that’s the limiting factor, at night it’s the light intensity.
Either light intensity, temperature, CO2 concentration.

Question 19

How are glasshouses adapted for plant growth?

Answer 19

Burning a small amount of propane in a CO2 generator.
Lamps provide light at night.
Glasshouses trap heat from sunlight, and the circulation systems make sure the temperature is even throughout the glasshouse.

Question 20

How can you investigate the pigments in leaves?

Answer 20

All plants contain several different photosynthetic pigments, and each pigment absorbs a different wavelength, so having more than one type of pigment increases the range of wavelengths of light that a plant can absorb. Some plants also have other pigments in their leaves, which play other essential roles e.g. protecting the leaves from excessive UV radiation. Different species of plants contain different proportions and mixtures of pigments. You can use thin layer chromatography to determine what pigments are present in the leaves of a plant. It involves a mobile phase where molecules can move. In TLD, this is liquid solvent. There’s also a stationary phase, where molecules can’t move. In TLC this consists of a solid (e.g glass) plate with a thin layer of gel on top. A sample of pigments can be extracted from the plant and put on the TLC plate. When the plate is placed vertically in the solvent, the solvent moves upwards through the gel, carrying the dissolved pigments with it. Some pigments will travel faster or further through the gel than others, which separates them out. It’s possible to identify a certain pigment by calculating its Rf value, and looking it up in a database. Rf is distance a substance has moved through the gel in relation to the solvent.

Question 21

How can TLC compare the pigments in different plants?

Answer 21

Grind up several leaves from the shade-tolerant plant you’re investigating with some anhydrous sodium sulfate, then add a few drops of propanone. Transfer the liquid to a test tube, add some of petroleum ether (in fume cupboard to protect volatile solvents) and gently shake the tube. Two distinct layers will form in the liquid - the top layer is the pigments mixed in with the petroleum ether.
Transfer some of the liquid from the top layer into a second test tube with some anhydrous sodium sulfate. Draw a horizontal pencil line near the bottom of a TLC plate. Build up a single concentrated spot of the liquid from step 3 on the line by applying several drops and ensuring each one is dry before the next is added. This is the point of origin. Once the point of origin is completely dry, put the plate into a small glass container with some prepared solvent (e.g a mixture of propanone, cyclohexane and petroleum ether) - just enough so that the point of origin is a little bit above the solvent. Put a lid on the container and leave the plate to develop. As the solvent spreads up the plate, the different pigments move with it, but at different rates - so they separate. When the solvent has nearly reached the top, take the plate out and mark the solvent front with a pencil and leave the plate to dry in a well-ventilated place. There should be several new coloured spots on the chromatography plate between the point of origin and the solvent front. These are the separated pigments. You can calculate their Rf values and look them in a database.

Distance travelled by spot/ distance travelled by solvent.

Repeat the process for three shade-intolerant plant and compare the pigments. Shade-tolerant plants sometimes produce dark red, and purple pigments called anthocyanins, which are thought to protect their chloroplasts from brief exposure to higher light levels.

Question 22

How can you investigate the activity of dehydrogenase in chloroplasts?

Answer 22

In PSI, during the light-dependent stage, NADP acts as an electron acceptor and is reduced. The reaction is catalysed by a dehydrogenase enzyme. The activity of this enzyme can be investigated by adding a redox indicator dye to extracts of chloroplasts. Like NADP, the dye acts as an electron acceptor and gets reduced by the dehydrogenase in the chloroplasts. As the dye gets reduced, you’ll see a colour change. For example, the dye DCPIP changes from blue to colourless when it gets reduced. You can measure the rate of the hydrogenate activity by measuring the rate at which DCPIP loses its blue colour. To do this, you need a colorimeter. Cut a few spinach leaves into pieces, remove any tough stalks. Using a pestle and mortar, grind up the leaf with some chilled isolation solution (a solution of sucrose, potassium chloride and phosphate buffer at pH7). Filter the liquid you make into a beaker through a funnel lined with muslin cloth. Transfer the liquid to centrifuge tubes and centrifuge them at high speed for 10 minutes. This will make the chloroplasts gather at the bottom in a pellet. Get rid of the liquid from the top of the tubes, leaving the pellets in the bottom. Re-suspend the pellets in fresh, chilled isolation solution. This is your chloroplast extract. Store it on ice for the rest of the experiment.
Set up a colorimeter with a red filter and zero it using a cuvette containing the chloroplast extract and distilled water. Set up a test tube rack at a set distance from a bench lamp. Switch the lamp on. Put a test tube in the rack, add a set volume of chloroplast extract to the tube and a set volume of dcpip. Mix the contents of the tube together. Immediately take a sample of the mixture from the tube and add it to a clean cuvette. Then place the cuvette in your colorimeter and record the absorbance. Do this every 2 mins for the next 10 mins. Repeat steps 7 to 9 for each distance. Use two negative controls. First should only contain Dcpip and chilled isolation solution and the second should contain both dcpip and chloroplast extract, but it should be wrapped in tin foil.

Question 23

What are the two types of respiration, and describe glycolysis.

Answer 23

Both produce ATP but anaerobic produces less. Both start with glycolysis. The stages after differ.
The process happens in the cytoplasm.
1) phosphorylation- glucose is phosphorylated using a phosphate from a molecule of ATP. This creates 1 molecule of glucose phosphate and 1 ADP. ATP is then again used to add another phosphate, forming hexose bus phosphate. Hexose bisphosphate is then split into 2 molecules of triose phosphate.
2) oxidation - triose phosphate is oxidised (loses hydrogen) forming two molecules of pyruvate. NAD collects the hydrogen ions, forming 2 reduced NAD. 4 ATP are produced, but 2 were used in stage one, so there’s a net gain of 2 ATP. In aerobic respiration, the two molecules of reduced NAD go to oxidative phosphorylation. The two pyruvate molecules are actively transported into the matrix of the mitochondria for the link reaction.

Question 24

What happens in anaerobic respiration?

Answer 24

Pyruvate is converted into ethanol (in plants and yeast) or lactate (in animal cells and some bacteria) using reduced NAD. In alcoholic fermentation, pyruvate is converted into ethanal using CO2, and then into ethanol using reduced NAD.

Question 25

What happens in the link reaction?

Answer 25

Pyruvate is decarboxylated and is then oxidised to form acetate and NAD is reduced to form reduced NAD. Acetate is combined with coenzyme A to form acetyl coenzyme A. No ATP is produced in this reaction. Two pyruvate molecules are made for every glucose molecule. This means the link reaction and the Krebs cycle happen twice for every glucose molecule. So for each glucose molecule:
Two molecules of acetyl coenzyme A go into the Krebs cycle. Two CO2 molecules are released as a waste product of respiration. Two molecules of reduced NAD are formed and go to oxidative phosphorylation.

Question 26

What happens in the Krebs cycle?

Answer 26

A series of oxidation-reduction reactions, which take place in the matrix of the mitochondria. This happens once for each pyruvate molecule.
1) Acetyl CoA from the link reaction combines with a four-carbon molecule (oxaloacetate) to form a six-carbon molecule (citrate). Coenzyme A goes back to the link reaction to be used again.
2) The 6C citrate is converted into a 5C molecule. Decarboxylation occurs. Dehydrogenation also occurs where hydrogen is removed. The hydrogen is used to produce reduced NAD from NAD.
3) The 5C molecule is then converted to a 4C molecule. Decarboxylation and dehydrogenation occur, producing one molecule of reduced FAD and two of reduced NAD. ATP is produced by the direct transfer of a phosphate group from an intermediate compound to ADP. When a phosphate group is directly transferred from one molecule to another it’s called substrate-level phosphorylation. Citrate has no been converted into oxaloacetate.

Question 27

What happens to products of the Krebs cycle?

Answer 27

Coenzyme A is reused in the next link reaction
Oxaloacetate is regenerated for use in the next Krebs cycle.
2CO2 is released as a waste product
1 ATP is used for energy
3 reduced NAD and 1 reduced FAD to oxidative phosphorylation.

Question 28

What happens in oxidative phosphorylation?

Answer 28

Energy is carried by electrons, from reduced coenzymes (reduced NAD and reduced FAD) is used to make ATP. Oxidative phosphorylation involves the electron transport chain and chemiosmosis.

Hydrogen atoms are released from reduced NAD and reduced FAD as they’re oxidised to NAD and FAD. The H atoms split into protons and electrons. The electrons move down the electron transport chain (made up of electron carriers) losing energy at each carrier. This energy is used by electron carriers to pump protons from the mitochondrial matrix into the intermembrane space into the intermembrane space (between the inner and outer mitochondrial membranes). The concentration of protons is now higher in the intermembrane space than in the mitochondrial matrix - this forms an electrochemical gradient (a concentration gradient of ions). Protons then move down the electrochemical gradient, back across the inner mitochondrial membrane and into the mitochondrial matrix, via ATP synthase (which is embedded in the inner mitochondrial membrane). This movement drives the synthesis of ATP from ADP and Pi. This process of ATP production driven by the movement of H^+ ions across a membrane (due to electrons moving down an electron transport chain) is called chemiosmosis. In the mitochondrial matrix, at the end of the transport chain, the protons, electrons and O2 (from the blood) combine to form water. Oxygen is said to be the final electron acceptor.

Question 29

How many ATP can be made from one glucose molecule?

Answer 29

2.5 ATP are made from each reduced NAD and 1.5 ATP are made from each reduced FAD. So 32 ATP.

Question 30

How can ATP production be affected? What

Answer 30

Mitochondrial diseases affect the functioning of mitochondria. They can affect how proteins involved in oxidative phosphorylation or the Krebs cycle function, reducing ATP production. This may cause anaerobic respiration to increase to try and make up some of the ATP shortage. This results in lots of lactate being produced, which can cause muscle fatigue and weakness. Some lactate will also diffuse into the bloodstream, leading to high lactate concentrations in the blood. Some products resulting form the breakdown of other molecules, such as fatty acids from lipids and amino acids from proteins, can be converted into molecules that are able to enter the Krebs cycle (usually acetyl CoA).

Question 31

How can you investigate factors affecting respiration in single-celled organisms?

Answer 31

Yeast are single-celled organisms that can be grown in culture. They can respire aerobically when plenty of oxygen is available and anaerobically when oxygen isn’t available.
Methylene blue is a redox indicator dye that can take the place of electron acceptors in oxidative phosphorylation, causing its colour to change from blue to colourless. The rate at which this colour change happens can give an indication of the rate of respiration of the yeast.
Put a known volume and concentration of substrate solution (e.g glucose) in a test tube. Add a known volume of buffer solution to keep the pH constant. Place the test tube in a water bath set to one of the temperatures being investigated. Leave it there for 10 minutes to allow the temperature of the substrate to stabilise. Add a known volume of yeast suspension to the test tube and stir for two minutes. Add a known volume of yeast suspension to the test tube and stir for two minutes. Add a known volume of methylene blue and seal the tube with a bung. Shake the test tube for a set number of seconds (e.g 10 seconds) and place it back in the water bath. Start a stopwatch immediately afterwards.

Question 32

How can you investigate factors affecting respiration in single-celled organisms?

Answer 32

Yeast are single-celled organisms that can be grown in culture. They can respire aerobically when plenty of oxygen is available and anaerobically when oxygen isn’t available.
Methylene blue is a redox indicator dye that can take the place of electron acceptors in oxidative phosphorylation, causing its colour to change from blue to colourless. The rate at which this colour change happens can give an indication of the rate of respiration of the yeast.
Put a known volume and concentration of substrate solution (e.g glucose) in a test tube. Add a known volume of buffer solution to keep the pH constant. Place the test tube in a water bath set to one of the temperatures being investigated. Leave it there for 10 minutes to allow the temperature of the substrate to stabilise. Add a known volume of yeast suspension to the test tube and stir for two minutes. Add a known volume of yeast suspension to the test tube and stir for two minutes. Add a known volume of methylene blue and seal the tube with a bung. Shake the test tube for a set number of seconds (e.g 10 seconds) and place it back in the water bath. Start a stopwatch immediately afterwards. Record how long it takes for the solution in the test tube to change from blue to colourless. You can use a control to compare colours. Repeat steps 1-5 three times for each temperature being investigated, and calculate a mean time for the colour change to occur at each temperature. Calculate the mean rate of respiration of the yeast at each temperature using:

Mean rate of respiration = 1/ mean time for colour change to occur.

Add a separate test tube containing water to act as a control. After the yeast and methylene blue is added, it shouldn’t decolourise.

Yeast produces CO2 when it respires anaerobically, so the rate at which CO2 is produced gives an indication of the yeast’s respiration rate. You can measure CO2 production, and therefore respiration rate, using a gas syringe:

Set up the apparatus using steps 1-3 of the experiment above. Trickle some liquid paraffin down the inside of the test tube so that it settles on and completely covers the surface of the solution. This will stop oxygen getting in, which will force the yeast to respire anaerobically. Put a bung, with a tube attached to a gas syringe, in the top of the test tube and start a stopwatch. The gas syringe should be set to zero. As the yeast respire, the CO2 formed will push gas into the syringe, which is used to measure the volume of CO2 released. Record the volume of gas in the gas syringe at regular time intervals (e.g every minute). Do this for a set amount of time. Repeat the experiment three times at each temperature you’re investigating. Calculate the mean rate of CO2 production at each temperature. You can also easily adapt these methods to investigate the effects of other variables.

Question 33

How can the rate of oxygen consumption be measured?

Answer 33

Respirometers can be used to indicate the rate of aerobic respiration by measuring the amount of oxygen consumed by an organism over a period of time.

The apparatus is set up partially submerged in a water bath at 15°C to provide the optimum temperature for the woodlice and therefore, the optimum temperature for the enzymes involved in their respiration. Control tube has glass beads of the same mass. The tap is left open and the syringe is removed to allow the apparatus to equilibrate (accounting for any expansion that might cause the pressure to change inside) and the respiration rate of the woodlice to stabilise in their new environment. When the ten minutes is up, the tap is closed and the syringe is attached. The syringe is used to reset the manometer, so that the ends of the fluid are at the same level on either side of the U and the reading from the volume scale on the syringe is recorded. As respiration occurs, the volume of the air in the test tube containing woodlice will decrease.