5.2.1 Photosynthesis

Question 1

What is the definition of photosynthesis?

Answer 1

• Photosynthesis is the process where energy from light is used to make glucose from H2O and CO2

Question 2

Write the equation for photosynthesis

Answer 2

6CO2 + 6H2O + energy from photons = (chlorophyll) C6H12O6 + 6O2

Question 3

What is NADP?

Answer 3

• a coenzyme used in photosynthesis
• transfers hydrogen from one molecule to another
• can reduce or oxidise a molecule

Question 4

Describe the structure of a chloroplast

Answer 4

• they are small, flattened organelles found in plant cells
• double membrane called chloroplast envelope
• thylakoids stack up in the chloroplast into structures called grana
• grana are linked together by lamellae
• the stroma contains enzymes, sugars and organic acids
• DNA is found in the stroma, often circular
• starch grains are stored in the stroma

Question 5

Describe the photosynthetic pigments and photosystems in chloroplasts

Answer 5

• photosynthetic pigments are coloured substances that absorb light energy for photosynthesis
• these pigments are found in the thylakoid membranes, attached to proteins
• the protein and pigment is called a photosystem

Question 6

What is the difference between a primary and accessory pigment?

Answer 6

• primary pigments are reaction centres, where electrons are excited during the LDR
• accessory pigments make up light-harvesting systems
• they surround reaction centres and transfer light energy to them to boost the energy available for electron excitement to take place

Question 7

What wavelength does photosystem I absorb?

Answer 7

• 700nm

Question 8

What wavelength does photosystem II absorb?

Answer 8

• 680nm

Question 9

Describe the basis of light-dependent reaction

Answer 9

• requires light energy
• takes place in the thylakoid membranes of the chloroplasts
• light energy is absorbed by photosynthetic pigments in the photosystems and converted to chemical energy
• light energy is used to add a phosphate group to ADP to form ATP, and to reduce NADP to form reduced NADP
• ATP transfers energy and reduced NADP transfers hydrogen to the light-independent reaction
• during the process H2O is oxidised to O2

Question 10

Describe the basis of light-independent reaction

Answer 10

• doesn’t involve light energy directly
• takes place in the stroma of the chloroplasts
• the ATP and reduced NADP from light-independent reaction supply energy and hydrogen to make glucose and O2

Question 11

Describe thin layer chromatography

Answer 11

• Cut leaves into small pieces and place them in a mortar
• Add propanone and some sand, then grind with a pestle
• pour a few cm^3 of propanone-hexane solvent intp boiling tube. seal and leave for 15 mins to saturate inside with solvent vapour
• cut a piece of chromatography paper
• use a fine glass pipette to place a small drop of pigment mixture on the centre of the line
• allow spot to dry and repeat this several times, allowing successive drops to dry
• builds up a small and concentrated spot of pigments

Question 12

How do you calculate the Rf value?

Answer 12

• distance travelled by spot / distance travelled by solvent

Question 13

What three things is light energy absorbed by photosystems used for in the LDR?

Answer 13

• making ATP from ADP and inorganic phosphate (photophosphorylation)
• making reduced NADP from NADP
• splitting water into protons, electrons and oxygen (photolysis)

Question 14

What are electron carriers?

Answer 14

• proteins that transfer electrons

Question 15

What is an electron transport chain?

Answer 15

• a chain of proteins through which excited electrons flow

Question 16

Describe non-cyclic photophosphorylation

Answer 16

1. Light energy excites electrons in chlorophyll
   - light energy is absorbed by PSII
   - the light energy excites electrons in chlorophyll
   - electrons move to a higher energy level
   - these high-energy electrons move along the electron transport chain to PSI
2. Photolysis of water produces protons (H+ ions), electrons and O2
   - as the excited electrons from chlorophyll leave PSII to move along the electron transport chain, they must be replaced
   - light energy splits water into protons, electrons and oxygen
   - H2O = 2H+ + 0.5O2
3. Energy from excited electrons make ATP
   = the excited electrons lose energy as they move along the electron transport chain
   - this energy is used to transport protons into the thylakoid, bia membrane proteins called proton pumps, so that the thylakoid has a higher concentration of protons than the stroma
   - this form a proton gradient across the membrane
   - protons move down their concentration gradient into the stroma, via the enzyme ATP synthase
   - the energy from this movement combines ADP and inorganic phosphate (Pi) to make ATP
4. and generates reduced NADP
   - light energy is absorbed by PSI which excited the electrons again to an even higher energy level
   - electrons are transferred to NADP, along with a proton from the stroma to form reduced NADP

Question 17

Describe cyclic phosphorylation

Answer 17

• only uses PSI
• electrons from the chlorophyll aren’t passed onto NADP but are passed back to PSI via electron carriers
• electrons are recycled and can repeatedly flow through PSI
• this process doesn’t produce NADP or O2, only small amounts of ATP

Question 18

Describe the basis of the Calvin cycle

Answer 18

• takes place in the stroma of the chloroplasts
• makes a molecule called triose phosphate from CO2 and ribulose bisphosphate
• TP can be used to make glucose and other useful organic substances
• needs ATP and H+ ions

Question 19

Describe the stages of the Calvin cycle

Answer 19

1. CO2 is combined with ribulose bisphosphate to form two molecules of glycerate 3-phosphate
   - CO2 enters the leaf through the stomata and diffuses into the stroma
   - it combines with RuBP, a 5-carbon compound
   - this gives an unstable 6-carbon compound, which quickly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP)
   - RuBisCO catalyses the reaction between CO2 and RuBP
2. ATP and reduced NADP are required for the reduction of GP to triose phosphate
   - ATP from the light-dependent reaction, provides energy to turn the 3-carbon compound GP, into a different 3-carbon compound called triose phosphate, TP
   - also requires H+ ions, which come from reduced NADP
   - the reduced NADP is recycled into NADP
   - triose phosphate is converted into useful organic compounds e.g. glucose
3. Ribulose bisphosphate is regenerated
   - Five out of every six molecules of TP produced in the cycle aren’t used to make hexose sugars, but to regenerate RuBP
   - regenerating RuBP uses the rest the ATP produced by the LDR

Question 20

What are GP and TP converted to?

Answer 20

• Carbohydrates: hexose sugars (e.g. glucose) are made by joining two triose phosphate molecules together and larger carbohydrates (e.g. starch) are made by joinging hexose sugars in different ways
• Lipids: made using glycerol, which is synthesised from triose phosphate and fatty acids, synthesised from glycerate 3-phosphate
• Amino acids: some amino acids are made from glycerate 3-phosphate

Question 21

How many turns of the Calvin cycle are need to make one hexose sugar and why?

Answer 21

• three turns of the cycle produces 6 molecules of TP, because 2 molecules of TP are made for every one CO2
• five out of six of these TP molecules are used to regenerate RuBP
• for three turns of the cycle, only one TP is produced that’s produced to make a hexose sugar
• two TPs are needed to form one hexose sugar because hexose sugars need six carbons
• therefore cycle needs to turn six times to produce two molecules of TP that can be used to make one hexose sugar
• six turns of the cycle need 18ATP and 12 reduced NADP from the LDR

Question 22

What are the optimum conditions for photosynthesis

Answer 22

High light intensity of certain wavelength

• Light is needed to provide energy for LDR
• the higher the intensity of light, the more energy is provided
• only certain wavelengths of light are used for photosynthesis
• chlorophyll a and chlorophyll b and carotene only absorb red and blue in sunlight

Temperature around 25 degrees

• photosynthesis involves enzymes
• if it falls below 10, enzymes become inactive

at high temps, they start to denature:

• stomata close to avoid losing too much water
• photosynthesis slows down because less CO2 enters leaf when stomata are closed
• thylakoid membranes may be damaged, reducing the rate of LDR by reducing the number of sites for electron transfer
• membranes around chloroplasts could be damaged, causing enzymes for the Calvin cycle to be released into the cell, reducing the rate of LIR
• chlorophyll may be damaged, reducing amount of pigment absorbed, reducing the rate of LDR

CO2 at 0.4%:

• CO2 makes up 0.04% of the gases in atmosphere
• increasing 0.4% gives a higher rate of photosynthesis but any higher and stomata will start to close

Question 23

Interpret a light intensity and photosynthesis rate graph

Answer 23

• between points A and B, the rate of photosynthesis is limited by the light intensity
• as light intensity increases, so does the rate of photosynthesis
• B is the saturation point, because it is no longer the limiting factor
• graph levels off

Question 24

Interpret different temperature and rate of photosynthesis graph

Answer 24

• both graphs level off when light intensity is no longer the limiting factor
• graph at 25 levels off at a higher point than at 15 showing that temperature must have been a limiting factor at 15 degrees

Question 25

Interpret CO2 conc and rate of photosynthesis graph

Answer 25

• both graphs levels off when light intensity is no longer the limiting factor
• graph at 0.4% CO2 levels off at a higher point than one at 0.04%
• we know limiting factor isn’t temp because it is same for both graphs

Question 26

How does light intensity affect levels of GP, RuBP and TP?

Answer 26

• in low light intensities, the products of the LDR, reduced NADP and ATP, will be in short supply
• this means that the conversion of GP to TP and RuBP is slow
• so levels of GP will rise and levels of TP and RuBP will fall as they’re used to make GP

Question 27

How does temperature affect levels of GP, RuBP and TP?

Answer 27

• all the reactions in the Calvin cycle are catalysed by enzymes
• at low temps, all the reactions will be slower as enzymes have less kinetic energy. therefore fewer enzyme substrate complexes form
• this means levels of RuBP, GP and TP will fall
• At high temps, levels also fall as enzymes denature

Question 28

How does CO2 concentration affect levels of GP, RuBP and TP?

Answer 28

• at low CO2 concentrations, conversion of RuBP to GP is also slow, as there is less CO2 to combine with RuBP to make GP
• so the level of RuBP will rise and levels of GP and TP will fall

Question 29

Describe an experiment to measure the effect of light intensity on photosynthesis

Answer 29

• Elodea’s oxygen produced can be measured
• a test tube containing pondweed and water is connected to a capillary tube full of water
• tube of water is connected to a syringe
• source of white light is placed at a specific distance from the pondweed
• pondweed is left to photosynthesise for a set amount of time
• oxygen released will collect in capillary tube
• experiment is repeated to calculate average length of gas bubble
• repeat with difference distances of lamp source

Question 30

How can you measure the effect of temperature on photosynthesis in an experiment?

Answer 30

• do the same as light intensity pondweed experiment but place in a beaker of water at a set temp

Question 31

What is the definition of Rf value?

Answer 31

• the ratio of the distance travelled by the pigment to that of the solvent front for a specific solute in a specific solvent