photosynthesis Flashcards
what organisms are capable of photosynthesising
plants, algae, cyanobacteria
- these organisms are autotrophic
give the equation for photosynthesis
6CO2 + 6H2O >light> C6H12O6 + 6O2
define autotroph + give 1 example
organisms that produce food (complex organic compounds) from inorganic molecules using light energy through photosynthesis
e.g. plants
what type of reaction is photosynthesis
anabolic (building things up)
what is the relationship between photosynthesis and respiration
- they are reverse processes
- all organisms respire but not all photosynthesise
compensation point definition
when rates of photosynthesis and respiration are balanced
compensation period definition
time it takes to reach compensation point
when does photosynthesis take place
during the day - the intensity of light has to be sufficient to allow photosynthesis to replenish carbs used in respiration
when does respiration take place
day and night
how many stages are there in photosynthesis + what are they called
2 stages
- light dependent stage
- light independent stage aka calvin cycle
where does photosynthesis take place
in the chloroplasts
what 3 cell types in the leaf contain chloroplasts
- palisade mesophyll
- spongey mesophyll
- stomatal guard cells
outline the general structure of a chloroplast
- double membrane
- thick fluid called stroma
- contains an internal network of thylakoid membranes
- these flatten into sacs called thylakoids
- many thylakoids stacked together = granum
- contained within the grana is the chlorophyll pigments
- grana are linked together by thin pieces of thylakoid membrane called lamellae
what is the purpose of lamellae
ensures grana are connected but distanced
1 adaptation of grana
- high SA:V to maximise photosynthesis
2 adaptations of the stroma
- contains photosynthetic enzymes
- also contains DNA and ribosomes
relative size of chloroplasts compared to mitochondria
chloroplasts are bigger
give the 2 main photosynthetic pigments + the colours they appear
- chlorophyll a (p680)
- chlorophyll a (p700)
these work best at these wavelengths of light
they appear green (meaning green light is reflected, not absorbed)
chlorophyll a - what light is absorbed + reflected, where is it found
- reflects blue green light
- found at reaction centres of both photosystems
- 2 forms which absorb light at wavelengths 680nm in PS2 and 700nm in PS1
- absorbs mostly red light, only some blue (400nm)
give the 3 types of accessory pigments + what colours they appear
- xanthopyll
- carotenoids e.g. beta carotene
appear yellow (meaning yellow light is reflected, not absorbed) - chlorophyll b
appears yellow/green (meaning yellow/green light is reflected, not absorbed)
what is the function of accessory pigments
these pass emitted electrons to the primary pigments in photosystems
chlorophyll b - what light is absorbed + reflected
- reflects yellow + green light
- absorbs wavelengths 400-500nm and 640nm which is blue and red respectively
carotenoids - what light is absorbed + reflected
- reflects yellow light
- absorbs wavelengths 400-500nm which is blue light
xanthophylls - what light is absorbed + reflected
- reflects yellow light
- absorbs blue and green light between wavelengths 375-550nm
why do plants contain a mixture of different pigments
light is made up of many different wavelengths, so it allows plants to maximise light absorption for photosynthesis
outline the structure of a chlorophyll molecule
- hydrophilic porphyrin head which lies parallel to thylakoid membrane for maximum light absorption
- hydrophobic lipid soluble tail which lies within thylakoid membrane
- side chains which determine which wavelengths are absorbed
outline a method to separate pigments
paper chromatography
- dissolve pigments in solvent (e.g. propanol)
- allow solvent to move up chromatography paper
- pigments will separate due to different relative adsorption
- calculate Rf values and compare to database
where does light dependent stage take place
thylakoid membranes
photosystems definition
contains photosynthetic pigment molecules arranged in light harvesting clusters, which are funnel shaped
- this allows each pigment molecule to pass energy down to the next within the cluster until it reaches the primary pigment reaction centre
which type of chlorophyll is used in PS1
chlorophyll a p700
which type of chlorophyll is used in PS2
chlorophyll a p680
NADP definition
a co enzyme which can be reduced to NADPH by the addition of H+
name 2 reaction pathways that can occur during light dependent stage
cyclic photophosphorylation and non-cyclic photophosphorylation
what photosystems are involved in non-cyclic photophosphorylation
PS1 and PS2
what photosystems are involved in cyclic photophosphorylation
PS1 only
outline the process of non-cyclic photophosphorylation
- PS2 absorbs light which excites electrons in chlorophyll to a higher energy level
- high energy electrons are transferred to electron carrier proteins in thylakoid membranes
- these electrons are transferred along the electron transport chain to PS1
- as electrons move down ETC they lose energy
- this energy is used to pump protons from the stroma into the thylakoids against their conc gradients
- this forms a proton gradient across thylakoid membrane (higher [H+] inside than outside in stroma)
- protons diffuse down conc gradient through ATP synthase
- this releases energy which is used to convert ADP into ATP
- when light energy is absorbed by PS1, high energy electrons are transferred directly to NADP (not along ETC)
- these electrons combine with a proton in the stroma to produce NADPH
- electrons are replaced in PS2 by the photolysis of water
outline the process of cyclic photophosphorylation
- light energy is absorbed by PS1 which excites electrons in chlorophyll to a higher energy level
- high energy electrons are transferred to electron carrier proteins in thylakoid membranes
- these electrons cycle through the electron transport chain continuously to PS1
- as electrons move down ETC they lose energy
- this energy is used to pump protons from the stroma into the thylakoids against their conc gradients
- this forms a proton gradient across thylakoid membrane (higher [H+] inside than outside in stroma)
- protons diffuse down conc gradient through ATP synthase
- this releases energy which is used to convert ADP into ATP
1 similarity and 3 differences between cyclic (C) and non-cyclic (NC) photophosphorylation
- ATP is produced in both C and NC
- NADPH is produced in NC but not in C
- in C electrons are continuously recycled but in NC they need to be replaced
- NC involves the photolysis of water, whereas C doesn’t
chemiosmosis definition
the process by which the movement of protons/H+ down their concentration gradient releases energy which is then used for ATP synthesis
outline the role of chlorophyll in photolysis of water
lost electrons from photolysis that go to the chlorophyll after absorbing light
this causes more water to dissociate
give the equation for the photolysis of water
H2O»_space; 1/2O2 + 2H+ + e-
where does oxygen from the photolysis of water go
- released through stomata
- used in respiration
what are the products of light dependent stage
- NADPH
- ATP
where does the calvin cycle take place
stroma
what are the 3 stages of the calvin cycle
1 carbon fixation
2 reduction
3 regeneration
fixation definition
the process by which a small molecule is incorporated into an organic molecule
outline the process of the calvin cycle
1 CARBON FIXATION
- 5C molecule RuBP is combined with CO2 in a reaction catalysed by enzyme RuBisCO
- this forms an unstable 6C intermediate which immediately breaks down into 2 3C GP molecules
2 REDUCTION
- each GP molecule reduced once using NADPH»NADP+ using energy from ATP»ADP
- this forms 2 3C TP molecules
3 REGENERATION
- 1/6th of the 2 TP molecules formed is taken out of the cycle and further metabolised
- the other 5/6ths are phosphorylated again using ATP»ADP to reform RuBP
what are 2 limitations of RuBisCO
- not very efficient so plants need to have a lot of it (makes up around 50% of protein in chloroplasts)
- not very specific meaning it can combine with O2 instead of CO2, creating a useless intermediate - this means the calvin cycle cannot happen in high [O2]
how many turns of the calvin cycle are needed to produce 1 glucose molecule
6 turns
of the 12 TP molecules produced in 6 turns of the calvin cycle, how many are removed to make glucose + what happens to the rest
1/6th = 2 TP molecules are removed
5/6ths = 10 TP reform RuBP
what can TP be metabolised into
- glucose + RuBP mainly
- from here it can be synthesised into sucrose, starch, cellulose
- it can also be a starting material for fatty acids, glycerol, amino acids
when does the calvin cycle occur
it only happens during the day, as it still relies on a continuous supply of NADPH and ATP from light dependent stage
where does the CO2 needed for calvin cycle come from
- respiration
- other organisms + the air, entes plant through the stomata
give 4 factors affecting photosynthesis
- light intensity
- CO2 concentration
- temperature
- water stress (not a main one)
how does light intensity affect photosynthesis
greater light intensity = greater rate of photosynthesis
- light provides energy to produce ATP and NADPH in light dependent stage
- allows stomata to open, enabling gas exchange
- transpiration occurs, allowing water from roots to travel + be evaporated
how does light intensity affect concentrations of RuBP TP and GP
decreased light intensity causes TP + RuBP conc to decrease, and GP to increase slightly
less light = decreased rate of light dependent stage = less ATP and NADPH made meaning GP builds up as it cannot be converted into TP, and so there is less TP and less RuBP formation
how does CO2 concentration affect the rate of photosynthesis
greater CO2 concentration = greater rate of photosynthesis
- CO2 levels in the atmosphere and aquatic habitats are usually high enough to not become a limiting factor
how does CO2 concentration affect concentrations of RuBP TP and GP
decreased CO2 concentration causes GP + TP conc to decrease and RuBP conc to increase
less CO2 = less carbon fixation causing RuBP to build up as it cannot be converted into GP, so less GP so less TP
how does temperature affect photosynthesis
generally as temperature increases, rate of photosynthesis increases, however if it continues to increase past optimum temps rate of photosynthesis decreases
25-30C - rate increases as enzymes have more energy so move faster
30-45C - O2 more successfully competes with CO2 for active site of RuBisCO causing rate to decrease
45C+ - enzymes denature causing rate to decrease
how does temperature affect concentrations of RuBP TP and GP
as temp increases, rate of calvin cycle increases as it is an enzyme controlled reaction
too high temps = O2 filling binding sites or enzymes becoming denatured = no carbon fixation so GP cannot be formed, so less GP so less TP, RuBP initially builds up slightly but it cannot be regenerated
outline an experiment to measure the impact of an abiotic factor on the rate of photosynthesis
1 - make leaf extract
- grind up leaves with a pestle and mortar in isolation medium
- filter then cool and centrifuge filtrate to get a pellet of chloroplasts
- transfer pellets to boiling tubes with isolation medium and use a pipette to squirt the mixture in and out several times to get a uniform suspension
2 - DCPIP
- set up several tubes with the same volume of leaf extract and different environments - e.g. water baths, around a lamp
- add a few drops of DCPIP to each test tube and set up in different environments - e.g. differing distances from the lamp + in the dark, in different temps in water/ice baths
- record how long it takes for DCPIP in each tube to decolourise
- use this to measure rate of decolourisation = rate of photosynthesis
- set up some controls to increase validity
how does the DCPIP experiment work
DCPIP is an electron acceptor, so can accept electrons released from photolysis of water during LDS causing it to be reduced and turn colourless
faster rate of LDS = faster rate of photosynthesis = faster decolourisation of DCPIP
what will occur when DCPIP is reduced
a colour change from dark blue to colourless
what is the purpose of a control
to validate results by proving that the change being observed is not being caused by other factors
describe 3 control experiments that could be used when investigating the rate of photosynthesis with DCPIP
1 - wrapping a test tube in aluminium foil
this shows that the colour change only occurs when there is light present
2 - replacing chloroplast suspension with buffer solution
this shows that chloroplasts are required for the colour change
3 - leaving DCPIP out of a test tube
this shows that the DCPIP is causing the colour change
which experimental technique could be used to separate various photosynthetic pigments
chromatography
