5.2.1 Photosynthesis Flashcards
1
Q
What is the definition of photosynthesis?
A
- Photosynthesis is the process where energy from light is used to make glucose from H2O and CO2
2
Q
Write the equation for photosynthesis
A
6CO2 + 6H2O + energy from photons = (chlorophyll) C6H12O6 + 6O2
3
Q
What is NADP?
A
- a coenzyme used in photosynthesis
- transfers hydrogen from one molecule to another
- can reduce or oxidise a molecule
4
Q
Describe the structure of a chloroplast
A
- 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
5
Q
Describe the photosynthetic pigments and photosystems in chloroplasts
A
- 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
6
Q
What is the difference between a primary and accessory pigment?
A
- 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
7
Q
What wavelength does photosystem I absorb?
A
- 700nm
8
Q
What wavelength does photosystem II absorb?
A
- 680nm
9
Q
Describe the basis of light-dependent reaction
A
- 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
10
Q
Describe the basis of light-independent reaction
A
- 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
11
Q
Describe thin layer chromatography
A
- 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
12
Q
How do you calculate the Rf value?
A
- distance travelled by spot / distance travelled by solvent
13
Q
What three things is light energy absorbed by photosystems used for in the LDR?
A
- making ATP from ADP and inorganic phosphate (photophosphorylation)
- making reduced NADP from NADP
- splitting water into protons, electrons and oxygen (photolysis)
14
Q
What are electron carriers?
A
- proteins that transfer electrons
15
Q
What is an electron transport chain?
A
- a chain of proteins through which excited electrons flow
16
Q
Describe non-cyclic photophosphorylation
A
- 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 - 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 - 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 - 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
17
Q
Describe cyclic phosphorylation
A
- 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
18
Q
Describe the basis of the Calvin cycle
A
- 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
19
Q
Describe the stages of the Calvin cycle
A
- 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 - 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 - 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
20
Q
What are GP and TP converted to?
A
- 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
21
Q
How many turns of the Calvin cycle are need to make one hexose sugar and why?
A
- 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
22
Q
What are the optimum conditions for photosynthesis
A
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
23
Q
Interpret a light intensity and photosynthesis rate graph
A
- 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
24
Q
Interpret different temperature and rate of photosynthesis graph
A
- 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
25
Q
Interpret CO2 conc and rate of photosynthesis graph
A
- 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
26
Q
How does light intensity affect levels of GP, RuBP and TP?
A
- 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
27
Q
How does temperature affect levels of GP, RuBP and TP?
A
- 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
28
Q
How does CO2 concentration affect levels of GP, RuBP and TP?
A
- 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
29
Q
Describe an experiment to measure the effect of light intensity on photosynthesis
A
- 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
30
Q
How can you measure the effect of temperature on photosynthesis in an experiment?
A
- do the same as light intensity pondweed experiment but place in a beaker of water at a set temp
31
Q
What is the definition of Rf value?
A
- the ratio of the distance travelled by the pigment to that of the solvent front for a specific solute in a specific solvent
