3.2 photosynthesis uses light energy to synthesise organic moleucles Flashcards
what is the photosynthesis equation?
6CO2 + 6H2O —> C6H12O6 + 6O2
carbon dioxide + water —> glucose + oxygen
during the day (respiration/photosynthesis) takes place?
both - respiration releases carbon dioxide which, in the leaves, is used in photosynthesis
during the night (respiration/photosynthesis) takes place?
only respiration takes place
as light intensity increases, the rate of photosynthesis (increases/decreases)?
also increases
what is the compensation point?
- up to a certain point, respiration can provide the CO2 needed by photosynthesis until the rates of these reactions are the same
- this is called the compensation point
(the light compensation point is the light intensity at which a plant has no net gas exchange as the volume of gases used and produced in respiration and photosynthesis are equal)
plants that are adapted to live in the shade have a higher rate of photosynthesis at lower light intensities.
do they have a higher or lower compensation point than plants adapted to living in full sunlight?
have a lower compensation point than plants adapted to living in full sunlight
what is a limiting factor?
a variable that limits the rate of a particular reaction
what are some factors required for photosynthesis?
- energy in the form of light (usually from the sun)
- chlorophyll and other pigments to absorb light energy
- water to combine with carbon dioxide to form organic compounds
- a suitable temperature to provide optimum conditions to one for the enzymes that control photosynthetic reactions
•enzymes e.g Rubisco to catalyse reactions
a shortage of any of these factors will limit the maximum rate at which photosynthesis can take place. they are therefore referred to as limiting factors
describe and explain a graph showing the change in rate of photosynthesis against an increasing light intensity
- A = light intensity is low and prevents photosynthesis reaching a maximum rate - as light intensity increases so does the rate. this shows that light intensity is the limiting factor
- B = the amount of photosynthetic pigment or another factor such as co2 concentration or rubisco concentration, becomes the limiting factor
- C = has reached max rate of photosynthesis
(saturation point = an increase in light intensity causes no further increase in the rate of photosynthesis)
what is the saturation point on a graph?
- an increase in e.g light intensity causes no further increase in the rate of photosynthesise
what is the main site of photosynthesis?
- palisade tissue in the leaves
- (chloroplasts)
what are some adaptations of angiosperm leaves to increase the efficiency of photosynthesis?
- leaves can change their position during the day so that the upper surface is always at right angles to the direction of light - maximises the amount of light that they can absorb for photosynthesis
- within the palisade mesophyll cells chloroplasts can move intracellularly in response to light intensity - they can orientate themselves so that they absorb more light and can also move around if more light is passing through one part of a cell to another
- transparent cuticle
- large surface area to maximise light absorption
- thin - to reduce diffusion distance for CO2
- densely packed chloroplasts in the palisade cells
- upper epidermis transparent allowing light to strike mesophyll layers
- air spaces allow for co2 to diffuse to the photosynthesising cells
- vein network transports water and minerals to the leaf and takes sugars away
- stomata allow CO2 to diffuse into the leaf
learn structure of leaf from AS
learn structure of chloroplasts from AS
chloroplasts contain a number of different photosynthetic pigments including:
- chlorophyll a
- chlorophyll b
- carotene
- xanthophyll
what is the function of the photosynthetic pigments?
to absorb light energy
how can the photosynthetic pigments be separated?
- using chromatography
- the different pigments dissolve in the solvent and are absorbed into and adsorbed onto chromatography paper
- the pigments are carries with the solvent
(chromatography is used to separate different products from a mixture)
in chromatography:
- smaller molecules tend to travel further
- larger molecules don’t travel as far
what is the Rf value?
- in a particular solvent, the movement of the pigment (solute) relative to the solvent front (how far the solvent travels)
what is the Rf value equation?
Rf = distance moved by solute / distance moved by solvent front
what are some of the Rf values for the photosynthetic pigments:
- carotene
- phaeophytin
- xanthophyll
- chlorophyll a
- chlorophyll b
dont need to learn
- carotene = 0.95 = yellow spot
- phaeophytin = 0.83 = yellow-grey spot
- xanthophyll = 0.71 = yellow-brown
- chlorophyll a = 0.65 = blue-green
- chlorophyll b = 0.45 = green
do the different photosynthetic pigments absorb light of different wavelength?
yes
if you shine light of different wavelengths through a solution of chlorophyll molecules, short/blue wavelengths and long/red wavelengths are absorbed but green light is reflected/transmitted
how is having multiple photosynthetic pigments an advantage to the plant?
- each pigment absorbs light of slightly different wavelengths
- overall the plant can carry out photosynthesis at a wider range of wavelengths and is more effective
the rate of photosynthesis at different wavelengths of light corresponds closely to the wavelengths absorbed - this shows that it is the wavelengths of light that are absorbed that are used in photosynthesis
what is the rate of photosynthesis usually measured as?
the volume of O2 produced per minute
- an absorption spectrum shows the wavelengths of light absorbed by a pigment at different wavelengths
- an action spectrum shows the rate of photosynthesis at different wavelengths (by showing the wavelength of light absorbed by a pigment (actually used in photosynthesis))
in what wavelength/colour is most light absorbed? least light?
in what wavelength/colour is the highest rate of photosynthesis? lowest rate?
- most light is absorbed in the blue (short) and red (long) wavelengths
- little light is absorbed in green wavelengths (green light is mostly reflected)
- the highest rates of photosynthesis are in blue and red wavelengths
- the lowest rate of photosynthesis is in green wavelengths
how have plants adapted in terms of photosynthesis pigments?
- plants which are adapted to live in habitats with different light availabilities may produce different proportions of photosynthetic pigments and even contain different pigments
- this way, plants can maximise the amount of light energy they can absorb and ensure that photosynthesis can continue
what are some differences in adaptations of sun vs shade plants?
SUN:
- large palisade cells with small chloroplasts
- few grana
- low chlorophyll content
- high compensation point
- more chlorophyll a than b
- rate of photosynthesis higher in shorter wavelengths of light
- small, thick leaves held vertically
SHADE:
- shorter palisade cells with large chloroplasts
- many grana
- high chlorophyll content
- low compensation point
- less chlorophyll a than b
- rate of photosynthesis higher in longer wavelengths of light
- large, thin leaves held horizontally
do the light dependent reactions depend on the photosynthetic pigments absorbing light energy?
yes
how are the photosynthetic pigments arranged in the light dependent reactions?
in collections of molecules called photosystems
(there are 2 different photosystems found embedded in the thylakoid membrane)
what are the two main parts of photosystems?
- an antenna/light-harvesting complex
- reaction center
what does the antenna/light-harvesting complex of the photosystems contain?
- contains most of the pigment molecules that absorb light energy
what does the reaction center of the photosystems contain?
- contains two special chlorophyll a molecules that are sensitive to certain wavelengths of light
- these release high energy electrons to provide energy for subsequent reactions
photosystems II info:
- was the second photosystem to be discovered but is the first stage of the light dependent reactions in photosynthesis
- has an absorption maximum at 680-690nm so it is called P680 or P690
photosystem I info:
- was the first to be discovered
- has an absorption maximum at 700nm so it is called P700
in both photosystems, what is used to excite electrons which are transferred from chlorophyll a in the reaction centre to electron carriers?
light energy, from photons of light at different wavelengths
what happens to the photosystems when the electrons become excited?
- they become oxidised
- because they lose electrons
- but are then reduced by gaining electrons:
• photosystem II from photolysis of water
• photosystem I from photosystem II (via electron carriers)
how is photosystem II reduced again after losing some electrons?
by the photolysis of water
- as the electrons pass from one electron carrier to another, they lose energy
- as electrons pass from PSII to PSI, energy from the electrons is used to power proton pumps
in chloroplasts, an electron transport system is used to produce ATP
what are the 2 paths that the electrons can follow?
- non-cyclic photophosphorylation
- cyclic photophosphorylation
what happens in non-cyclic photophosphorylation?
- excited electrons enter the electron transport chain to produce ATP
- NADP acts as a final electron acceptor and is reduced
- water is photolysed to compensate for electrons lost from photosystem II
- forming both ATP and reduced NADP and involving both photosystem I and II
what happens in cyclic photophosphorylation?
- excited electrons from PSI pass from the electron acceptor, through electron carriers and then back to PSI
- this produces ATP
- no reduction of NADP and no water required to replace lost electrons
what are the useful products of the light dependent stage of photosynthesis?
ATP
reduced NADP
what are the waste products of the light dependent stage of photosynthesis?
oxygen
what is produced from the light dependent stage that is used in the next stage of photosynthesis - the light independent stage?
ATP
reduced NADP
(some oxygen is used in respiration but during the day the rest diffuses out of the leaf through the stomata)
ATP is produced through what type of photophosphorylation in the light dependent stage?
both types
(cyclic and non-cyclic)
reduced NADP is produced through what type of photophosphorylation in the light dependent stage?
only produced via non-cyclic photophosphorylation
differences in cyclic and non-cyclic photophosphorylation:
CYCLIC:
- only PS I is involved
- no photolysis of water takes place
- oxygen not evolved
- reduced NADP is not synthesised
- electron is cycled back to reaction centre
NON-CYCLIC:
- PS I and PS II are involved
- photolysis of water takes place
- oxygen evolved
- reduced NADP is synthesised
- electron doesn’t return to reaction centre
what were the experiments Calvin and his colleagues conducted to determine how carbon dioxide was converted into glucose in photosynthesis?
DONT LEARN
- they used suspensions of the unicellular eukaryotic green algal Chlorella to trace the path of carbon
- he exposed the algal cells to constant conditions of light and CO2 to establish steady-rate photosynthesis
- the lollipop vessel used was thin to allow light to penetrate the algae
- they then added radioactive 14-CO2 for a brief period to label the intermediates of the cycle
- at short time intervals they took samples of the cells and killed them by plunging the suspension into boiling alcohol. This also inactivated their enzymes
- they separated the 14-C-labelled compounds from one another and identified them by the positions on two-dimensional paper chromatograms
- this involved extracting chemicals from the algae and then separating them using chromatography in one solvent
- some substances have the same Rf value using one solvent but a different Rf value using a different solvent
- so they then turned the chromatography paper through 90° and used a different solvent to separate substances which had travelled the same distances using the first solvent
- X-ray film was exposed to the radiation from the 14-C in the chemicals on the chromatogram. dark spots appeared whenever a chemical containing 14-C was present. the darker the spot the higher the concentration of the chemical
-Calvin and his colleagues exposed the algae to radioactive CO2 for different periods of time, extracted radioactive substances from the algae and separated them using 2-D chromatography
- an image shows drawings of the autoradiograms prepared from the chromatograms obtained after five seconds and after 30 seconds
- after five seconds they discovered a mixture of compounds containing 14 C. the darkest spot was identified as glycerate-3-phosphate, a 3 carbon compound followed by sugar phosphates and diphosphates. some triose phosphate was also found
- by repeating the experiment at shorter and shorter time intervals they showed that glycerol-3-phosphate was the first stable compound to be made and therefore the other labelled sugar phosphates must’ve been made from the glycerate-3-phosphate
- after 30 seconds, as long as there was a supply of nitrates, sulphates and phosphates in the culture medium, amino acids, nucleic acids and phospholipids were synthesised together with sugars such as glucose and sucrose
what did Calvin’s findings conclude that the main purposes of the light-independent stage are?
- the uptake of carbon dioxide
- the production of triose phosphate
- to be the starting point for the synthesis of all organic compounds needed by a plant
the steps on the light independent stage form a cycle - a series of reactions in which most of the substances are regenerated
the series of reactions in the light independent stage could only proceed if…
ATP and reduced NADP from the light dependent stage of photosynthesis were present
what are the main steps of the calvin cycle?
- carbon fixation - uptake CO2 by 5C ribulose bisphosphate; this is catalysed by the enzyme Rubisco
- this forms an unstable 6C intermediate which rapidly breaks down to form 2x 3C glycerate-3-phosphate (GP)
- ATP and reduced NADP from the light dependence stage are used to reduce glycerate-3-phosphate to the 3C carbohydrate, triose phosphate (TP)
- through a complex series of reactions, most of the triose phosphate is converted into ribulose-5-phosphate
- ribulose-bisphosphate is regenerated from ribulose-5-phosphate which requires ATP as a source of phosphate
- from triose phosphate glucose, lipids, nucleic acids and amino acids may be manufactured
amino acids need a source of …
nitrogen and sulphur
nucleic acids need a source of …
nitrogen and phosphorus
phospholipids need a source of …
phosphorus
what is carbon fixation catalysed by?
the enzyme Rubisco
(ribulose bisphosphate carboxylase oxidase)
Rubisco enzyme info:
dont need to learn
- most abundant enzyme on the planet
- made of 8 large polypeptides and 8 small polypeptides
- the genes for the large polypeptides are contained in the chloroplast DNA while thr genes for the small polypeptides are found in the nuclear DNA
- in some photosynthetic bacteria, the equivalent of Rubisco contains only large polypeptide chains
- this provides additional evidence for the endosymbiotic theory
various inorganic ions are needed by plants and may be limiting factors to metabolism if in short supply
what is the function of nitrogen in plants?
- synthesis of amino acids, nucleotides and chlorophyll
what are the nitrogen deficiency symptoms in plants?
- stunted growth (as plants cant synthesise proteins)
- yellow leaves (chlorosis)
what is the function of magnesium in plants?
- component of/synthesis of chlorophyll
- enzyme cofactor
what are the magnesium deficiency symptoms in plants?
chlorosis (as chlorophyll cannot be synthesised)
what is the function of potassium in plants?
what are the potassium deficiency symptoms in plants?
- opening and closing of stomata
- enzyme cofactor
- stunted growth
- yellow leaves (chlorosis)
- reduced flowering and fruit formation
what is the function of phosphorus in plants?
what are the phosphorus deficiency symptoms in plants?
- nucleic acid and phospholipid synthesis
- small dark-green leaves often with a red-purple edge
- reduced root growth
what is the function of sulphur in plants?
what are the sulphur deficiency symptoms in plants?
- some amino acids
- chlorosis in new leaves
- reduced root growth
what is the function of calcium in plants?
what are the calcium deficiency symptoms in plants?
- formation of middle lamella between plant cells
- enzyme cofactor
- small leaves
- death of buds at ends of stems
how can you investigate mineral deficiencies in plants?
- a seeding can be grown in solutions containing all the mineral ions required for healthy growth except for the one being investigated
- however it is often difficult to assess the effects of mineral deficiencies unless you can compare the appearance of leaves/stems/roots to those of a healthy plant
- so when investigating mineral deficiencies is useful to include both a positive control and - negative control:
• a positive control includes all the minerals in the correct concentrations for healthy normal growth of the plant
• a negative control would have no minerals present - i.e distilled water - so that the effect of no minerals can be seen
what is hydroponics?
the growing of plants in water containing the correct concentrations of mineral ions
each species will show some variation in the way that mineral deficiencies affect them
what is a chloroplast?
- an organelle found in plants and algae that is the site of photosynthesis
describe the structure of a chloroplast:
- double membrane
- grana - stacks of flattened disks (thylakoids) that contain photosystems, electron transport chain, ATP synthase
- grana connected by intergranal lamellae
- stroma - fluid filled matrix containing enzymes
where are chloroplasts located in an angiosperm leaf?
- mainly found in the (palisade) mesophyll layers
how are chloroplasts adapted for photosynthesis?
- thylakoids give a large surface area for light independent reactions
- photosynthetic pigments arranged into photosystems to maximise light absorption
- stroma directly surrounds grana - products of photosynthesis diffuse directly into the stroma
- contain their own DNA (cpDNA) and ribosomes
- inner chloroplast membrane less permeable than outer enabling control over the movement of substances
what is a transducer? why are chloroplasts described as transducers?
- something that converts one type of energy into another
- chloroplasts transduce light energy into the chemical energy of ATP
what is a photosynthetic pigment?
- a molecule present in chloroplasts that absorb certain wavelengths of light
what is photosynthesis?
- a complex metabolic pathway that synthesises organic molecules in the presence of light
- 6CO2 + 6H2O —> C6H12O6 + 6O2
name the 2 stages of photosynthesis:
- light-dependent stage
- light-independent stage
what is a photosystem?
- protein complex consisting of an antenna complex and reaction centre
- involved in the absorption of light and transfer of electrons in photosynthesis
- two types of: photosystem I and II
how do photosystems I and II differ?
- they absorb different wavelengths of light
describe the process of light harvesting?
- antenna complex absorbs light energy of varying wavelengths and transfers it to the reaction centre
- energy absorbed by two chlorophyll a molecules which emit ‘excited’ electrons
where does the light-dependent stage of photosynthesis take place?
- on the thylakoid membranes of the chloroplast
light-dependent stage very brief summary:
- used light energy to produce ATP, reduced NADP and oxygen
what is the purpose of cyclic photophosphorylation?
- produces additional ATP to meet surplus energy demands of the cell
what is the purpose of non-cyclic photophosphorylation?
- produced ATP and reduced NADP for the calvin cycle
how does chemiosmosis produce ATP in the light-dependent stage?
- protons flow down their concentration gradient from the thylakoid space into the stroma via ATP synthase
- ATP synthase phosphates ADP to form ATP as protons flow through it
describe photolysis:
- the splitting of a molecule of water in the presence of light that occurs during the light-dependent stage of photosynthesis
- this produces protons, electrons and oxygen:
H2O —> 2H+ + 2e- + 1/2 O2
what happens to the products of photolysis?
- H+ - used in proton pumping and to reduce NADP
- e- - replaces electrons lost from chlorophyll a in PS II
- O2 - byproduct, used for respiration or diffuses out of the leaf as waste gas
explain how the electron transfer chain results in the production of reduced NADP?
- NADP acts as a final electron acceptor and is subsequently reduced
where does the light-independent stage of photosynthesis take place?
in the stroma
describe the light-independent stage of photosynthesis:
- uses co2 and the products of the light-dependent stage to build organic molecues
what are the three main stages of the calvin cycle?
- carbon (dioxide) fixation
- regeneration (/synthesis of RuBP)
- reduction
what happens during the carbon fixation of the calvin cycle?
- reaction between co2 and ribulose bisphosphate (RuBP) catalysed by enzyme Rubisco
- forms unstable 6C intermediate that breaks down into two molecules of glycerate 3-phosphate (GP)
what happens during reduction of the calvin cycle?
- 2x GP are reduced to 2x triose phosphate (TP)
- requires 2x reduced NADP and 2x ATP formed during the light-dependent reaction
- forms 2x NADP and 2x ADP that enter the light-dependent reactions
what happens during regeneration of the calvin cycle?
- after 1C leaves the cycle, the 5C compound RuP forms
- RuBP is regenerated from RuP using 1x ATP
- forms 1x ADP
how are nutrients produced as a result of photosynthesis?
- formation of amino acids from GP (requires nitrates and sulfates)
- TP molecules used to produce sugars e.g glucose, fructose, sucrose
name the factors that limit the rate of photosynthesis. what stage do they limit?
- light intensity - light-dependent stage
- light wavelength - absorption by chlorophyll
- co2 levels - light independent stage
- temperature - enzyme controlled reactions
- pH - enzyme controlled reactions
don’t just say ‘pigments absorb light’ SAY ‘pigments absorb light energy’
the chlorophylls and accessory pigments are found lying in the thylakoid membranes, grouped into structures called antenna complexes
how can photosynthetic pigments be extracted from chloroplasts?
- by grinding plant material in a suitable solvent
(then separated by paper chromatography)
non-cyclic photophosphorylation steps (detailed) :
- light energy (photons) strikes chlorophyll (PSII) exciting its electrons, boosting them to a higher energy level
- electrons are accepted by an electron carrier in the thylakoid membrane
- the oxidised chlorophyll removes electrons from water, producing protons and oxygen (photolysis). this occurs in the thylakoid space
- as electrons pass from carrier to carrier, electron energy is lost, which pumps protons from the stroma into the thylakoid space. as protons flow back through the stalked particle, ADP is phosophorylated; 2 ATP are made in total
- electrons enter photosystem 1 where light excites them, boosting them to an even higher energy level
- electrons enter a final electron carrier
- electrons and protons reduce NADP to reduced NADP which pass to the Calvin cycle with the two ATP made
why isnt oxygen released in cyclic photophosphorylation?
as photolysis doesnt occur
- if there is no NADP available for non-cyclic photophosphorylation, then the electrons fall back into the electron transport chain (at an intermediate energy level) and generate 1 ATP
- this cycle continues until NADP is available
how many carbon molecules is made per turn of the calvin cycle?
1
(so x6 to make a glucose)
calvin cycle steps (detailed):
- CO2 diffuses into leaf via stomata, dissolving in the water surrounding palisade mesophyll cells before diffusing into the cells
- CO2 combines with the 5 carbon compound ribulose bisphosphate (RuBP) using the enzyme Rubisco to form an unstable 6C compound
- unstable 6C compound immediately breaks down into 2 molecules of glycerate-3-phosphate (GP)
- using one ATP molecule from the light reaction, GP is reduced to triose phosphate (TP) using hydrogen atoms from reduced NADP
- triose phosphate molecules combine in pairs to form hexose sugars
- five out of every six triose phosphate molecules produced are used to regenerate RuBP (via the intermediate ribulose phosphate) using ATP from the light-dependent reaction to supply energy and phosphate. this allows the cycle to continue
plants must produce all the carbohydrates, fats and proteins they need from the products of the Calvin cycle:
- fructose phosphate formed from the two molecules of triose phosphate can be converted to glucose, or combined with glucose to produce sucrose
- sucrose is then translocated in the phloem to the growing regions of the plant
- some α glucose is stored as starch, β glucose forms cellulose in cell walls
- fatty acids can be formed from glycerate-3-phosphate, and glycerol from triose phosphate, the building blocks of triglycerides
- proteins can be formed from glycerate-3-phosphate, but the amino group requires nitrogen from nitrate ions
what is the rate-limiting step?
- the slowest reaction in a sequence, and determines the overall rate of the reaction
although water is a reactant, long before it becomes a limiting factor, the guard cells lose turgidity and the stomata close, reducing the supply of co2
can several factors be limiting at the same time?
yes - but the one in shortest supply controls the rate-limiting step
explanation for a graph when co2 is a limiting factor:
- at low concentrations, carbon dioxide concentration is limiting
- but as it increases(above 0.5%), the rate plateaus, showing that something else must be limiting
- above 1% the stomata close, preventing uptake of co2
explanation for a graph when light intensity is a limiting factor:
- as light intensity increases the rate of photosynthesis increases up to about 10,000 lux (SI unit of illuminance) when some other factor becomes limiting
- at very high light intensity, the rate decreases as chloroplast pigments become bleached
- different plants have evolved to be most efficient at light intensity found in the environment e.g sun and shade plants
explanation for a graph when temperature is a limiting factor:
- temperature increases the kinetic energy of the reactants and enzymes involved in photosynthesis
- unlike other factors, a plateau is not reached as enzymes e.g Rubisco, begin to denature so the rate of photosynthesis decreases above the optimum temperature
- this will be higher in species adapted to hot, dry environments
why are aquatic plants a good subject to use when investigating how different factors affect photosynthesis?
- temperature and co2 concentrations are more easily controlled than with terrestrial plants, by using a water bath and controlling hydrogen carbonate concentration
- it is also easy to collect and accurately measure the oxygen produced in a capillary tube
(aquatic plants can photosynthesise in water)
the volume of a bubble produced in an experiment investigating how different factors affect photosynthesis is calculated by what formula?
volume = 𝛑r^2 x length of bubble
plants take up minerals from the soil. these are combined with the products of the light independent stage to produce other molecules needed by the plant
give three examples of minerals taken up from the soil and state what molecules are produced when each example combines with the products of the light independent stage [3]
- synthesis of amino acid/proteins using a nitrogen source
- synthesis of phospholipids with phosphate
- synthesis of chlorophyll with magnesium
- synthesis of nucleotides with a nitrogen source and phosphate source
explain how co2 is used in the production of glycerate-3-phosphate [2]
- {RuBP/5C compound} and carbon dioxide linked together
- rubsico is the enzyme that catalyses the reaction
- unstable 6C compound initially formed - then splits into two 3C
describe how glycerate-3-phosphate is converted to triose phosphate [3]
- glycerate-3-phosphate reduced
- using reduced NADP
- ATP also required (to supply energy)/glycerate-3-phosphate is phosphorylated
- {reduced NADP/ATP} from the light dependent reactions
state why only some of the triose phosphate produced by the calvin cycle can be used to produce hexose phosphate [1]
some (triose phosphate) needed to (regenerate/make more) RuBP
- Diuron is a weed killer which is a very specific and sensitive inhibitor of photosynthesis. it blocks the electron carrier binding site on photosystem II. this stop the electron flow from where it is generated in photosystem II, to the electron carrier. this reduces the ability of the plant to convert light energy into chemical energy.
- Diuron only blocks electron flow from photosystem II. It has no effect on photosystem I or other reactions in photosynthesis, such as light absorption or carbon fixation in the calvin cycle
explain the effects of Diuron on non-cyclic photophosphorylation and why cyclic photophosphorylation is not affected [4]
- it stops electrons from PSII being moved to PSI
- so blocking the reduction of NADP to reduced NADP
- cyclic photophosphorylation only involves PSI
- is not stopped as the electrons pass from PSI and return to PSI
- and the carrier involved in this is not affected
- Diuron is a weed killer which is a very specific and sensitive inhibitor of photosynthesis. it blocks the electron carrier binding site on photosystem II. this stop the electron flow from where it is generated in photosystem II, to the electron carrier. this reduces the ability of the plant to convert light energy into chemical energy.
- Diuron only blocks electron flow from photosystem II. It has no effect on photosystem I or other reactions in photosynthesis, such as light absorption or carbon fixation in the calvin cycle
suggest why a plant would die when the weedkiller Diuron is sprayed onto it [3]
- plant cannot generate reduced NADP (so calvin cycle cannot work)
- (no glucose/hexose sugar) will be formed
- for respiration
in the calvin cycle, out of ATP and reduced NADP, which occurs (is used) first?
ATP
??
state precisely where chlorophyll a is found in a photosynthetic cell [2]
- reaction centre/antenna complex
- in a photosystem
- in the thylakoid (membrane)
on an absorption spectra, the % absorbance in the 500-600nm wavelength range is very low. explain these results [1]
(green) light is reflected (by the pigments)
explain the advantage to plants having more than one pigment in their leaves [2]
- {light/photons} can be absorbed over {a greater range or/more} wavelengths
- more {light/photons} absorbed means more products from the light dependent stage
- {greater rate of/faster} photosynthesis / photosynthesis is more efficient
use your knowledge of photosynthesis to explain the role of pigments in the production of ATP [3]
any 3 from:
- light absorbed by (pigment in) {photosystems/PS1/PS2 }
- excites electrons to high energy level
- electrons emitted from reaction centre/chlorophyll a
- used to power proton pumps/generate an EC gradient
- movement of protons through a stalked particle / ATP synthase allows ATP generation
a graph shows the action spectrum and absorption spectrum for photosynthesis.
describe the relationship between the absorption spectrum and the action spectrum and explain what this relationship tells us about light absorption and photosynthesis [2]
- since they follow a similar trend/pattern/shape; it suggests that the pigments/wavelengths responsible or used in light absorption are used in photosynthesis
state precisely how the process of photosynthesis reduces carbon dioxide levels [2]
- light independent
- calvin cycle
- RuBP
- rubisco
- (unstable 6C substance) 2 mols of GP
state why the following processes are required for cellular respiration to continue:
- light independent stage of photosynthesis
- light dependent stage of photosynthesis
- produces glucose
- produces oxygen
glycine is an amino acid. which chemical element would have been needed in the mineral solution in order for the algae to have made this compound? [1]
nitrogen
identify the pigment found in the reaction centre of the photosystems [1]
chlorophyll a
- herbicides inhibit photosynthesis in many ways
- one group of herbicides block electron transport, so chlorophyll continues to absorb light energy but cannot pass this energy on
- light energy not used in electron emission damages chlorophyll leading to chlorosis
- desiccation occurs because of the formation of oxygen free-radicals, which are highly destructive to cell membranes
- use the information given and your own knowledge to explain how blocking electron transport from photosystems with this herbicide could lead to the death of a plant [4]
- destruction of chlorophyll - no PS/unable to absorb light (energy)
- {no/less} (reduced NADP/ATP) for (Calvin cycle/light independent reactions)
- {no/less} (carbohydrate) synthesised for {respiration}
- {no/less} respiration, therefore {no/less} ATP for {cell division/protein synthesis/ active transport}
- destruction of cell membranes - kill/dries out cells
- an experiment was carried out to investigate the effect of leaf colour on the rate of photosynthesis
- leaves placed in water naturally float, but if small discs of leaves are punched out of the leaves and the air in the discs is replaced by hydrogen carbonate solution (a CO2 source), they will sink
- the time taken for 15 light green leaf discs to float to the top of the solution when illuminated from below can be determined and gives an estimate of the rate of photosynthesis
- this was repeated for dark green leaf discs
explain what caused the discs to rise in the solution [2]
- oxygen produced from (photosynthesis/photolysis of water)
- (O2 fills the airspaces in the leaf so) the leaf is {less dense/lighter/more buoyant} (and so rises)
- an experiment was carried out to investigate the effect of leaf colour on the rate of photosynthesis
- leaves placed in water naturally float, but if small discs of leaves are punched out of the leaves and the air in the discs is replaced by hydrogen carbonate solution (a CO2 source), they will sink
- the time taken for 15 light green leaf discs to float to the top of the solution when illuminated from below can be determined and gives an estimate of the rate of photosynthesis
- this was repeated for dark green leaf discs
- explain why the darker green leaf discs rose faster than the lighter green discs [4]
any 4 from:
- (the darker leaves rise more quickly because they have) more (chloroplasts/chlorophyll/pigment) (in the palisade mesophyll)
- so more {photons will be trapped/light (energy) absorbed}/more energy transferred to high energy electrons
- more light dependence stage
- more photolysis of water
- more o2 production
in the absence of light, the concentration of glycerate-3-phosphate (GP) in the chloroplast stroma increases. this is shown on a graph
- explain the shape of the graph for both GP and TP (decreases in dark) when the plant is in the dark [5]
- (intitially), co2 + RuBP reaction continues / GP continues to be produced
- GP cannot be (converted/reduced) to TP
- because ATP AND reduced NADP are needed
- ATP and reduced NADP (only produced in light/not produced in dark)
- therefore less TP available (to regenerate RuBP)
- therefore rate of reaction of co2 and RuBP decreases
describe the process by which algae produces oxygen [5]
- light dependent stage
- reference to PS II
- photolysis
- use of photolysis equation/description of
- replace electrons lost (from PS II)
- oxygen released
give an example of a biological molecule that contains sulfur [1]
protein / any named protein / enzyme
describe the light dependent reactions of photosynthesis [10]
- absorption of light {in photosystems/by pigments}
- energy transferred to reaction centre of photosystem/antenna complex
- a molecule of chlorophyll a is the reaction centre
- electrons excited/electrons {raised to higher energy level/emitted}/high energy electrons produced
- {high energy/excited} electrons passed to electron acceptor / first carrier in chain
- electrons (from photosystem II) pass along {a chain of electron carriers/electron transport chain}
- energy from electrons used to pump protons
- higher concentration of protons INSIDE THYLAKOID (than in the stoma)/ concentration gradient of protons from thylakoid to stoma
- used to produce ATP
- photosystem I receives electrons from {the chain of carriers/from photosystem II}
- electrons {used to reduce NADP/ to produce reduced NADP}
- photolysis of water provides electrons to replace those lost by photosystem II
- oxygen produced {by photolysis/by spitting of water}
- cyclic photophosphorylation only involves photosystem I/ non-cyclic involves both photosystems
- light dependent reactions take place {in thylakoid (membranes)/ in (membranes of) grana}
explain why the student repeated the experiment three times? [1]
- increase reliability / minimise error due to chance / identify anomalous results / check repeatability / check consistency
- REJECT ACCURACY
a group of students extracted chloroplast from spinach leaves. to do this they ground the spinach leaves in a mortar containing an ice cold, isotonic solution.
explain why ice cold, isotonic solution was used when extracting the chloroplasts [2]
- ice cold: (to prevent/slow down) any ENZYME reactions (from the broken cell)
- isotonic : prevent lysis / prevent osmotic activity
