topic 2.9/8.3- photosynthesis Flashcards
define photosynthesis
the production of carbon compounds in cells using light energy (light->chemical)
how can photosynthetic pigments be separated?
by chromatography
- pigments absorb different wavelengths of light and so look a different colour to us
- plastic strip that has been coated with a thin layer of porous material
- spot containing pigments placed near one end
- solvent is allowed to run up the strip to separate the different pigments
Rf =
distance run by the pigment/distance run by solvent
shortest wavelength of visible light
violet
longest wavelength of visible light
red
middle wavelength of visible light
green (525-575nm)
range of wavelengths of visible light
400-700nm
chlorophyll absorbs —- and —- light most effectively and reflects —— light more than other colours
red; blue
green
draw an absorption spectrum for chlorophyll and an action spectrum for photosynthesis
define an absorption spectrum
a graph showing the percentage of light absorbed at each wavelength by a pigment or group of pigments
define an action spectrum
a graph showing the rate of photosynthesis at each wavelength of light
why are action and absorption spectra so similar?
photosynthesis can only occur in wavelengths of light that chlorophyll or other photosynthetic pigments can absorb
effect of the start of photosynthesis on the earth’s atmosphere
For the first 2 billion years after the Earth was formed, its atmosphere was anoxic (oxygen-free)
The current concentration of oxygen gas within the atmosphere is approximately 20%
effect of the start of photosynthesis on the earth’s oceans
- Earth’s oceans initially had high levels of dissolved iron (released from the crust by underwater volcanic vents)
- When iron reacts with oxygen gas it undergoes a chemical reaction to form an insoluble precipitate (iron oxide)
- When the iron in the ocean was completely consumed, oxygen gas started accumulating in the atmosphere
effect of the start of photosynthesis on the earth’s rock deposition
- The reaction between dissolved iron and oxygen gas created oceanic deposits called banded iron formations (BIFs)
- when BIF deposition slowed in oceans, iron rich layers started to form on land due to the rise in atmospheric O2 levels
what type of reaction is photosynthesis and what does this mean?
endothermic reaction; this means it requires energy
state the 3 possible limiting factors on the rate of photosynthesis
- temperature
- light intensity
- carbon dioxide concentration
give 3 ways in which experiments on the limiting factors of photosynthesis can be carried out
Measuring CO2 Uptake (adding sodium hydrogen carbonate raises CO2 conc and you can cause a change in pH)
Measuring O2 Production (gas syringe)
Measuring Biomass (Indirect) as glucose production takes place
where do light-dependent reactions take place?
in the thylakoid space and across the thylakoid membranes
where do light independent reactions take place?
in the stroma - a thick, protein-rich medium enclosed by the inner membrane of the chloroplast
what are the steps and final products of light-dependent reactions ?
- photoactivation
- photolysis
- electron transport
- proton gradient
- chemiosmosis
- ATP synthesis
- reduction of NADP
so the final products are reduced NADP and ATP
describe photosystems
- located in the thylakoid membranes
- consist of chlorophyll and accessory pigments being grouped together in large light-harvesting arrays that have reaction centres
- two types of arrays: Photosystems I and II
photoactivation
- chlorophyll molecules absorb light energy and pass it to two special chlorophyll molecules in the reaction centre of the photosystem
- these absorb the energy from a photon of light, causing an electron within the molecule to become excited- the chlorophyll is then photoactivated
what special property do the chlorophylls at the reaction centre have?
they are able to donate excited electrons to an electron acceptor
which photosystem do light-dependent reactions begin in?
photosystem II
describe what happens after photo activation in photosystem II
- plastoquinone (electron acceptor) collects two excited electrons from photosystem II and then moves to another position in the membrane
- this process can be repeated to produce a second reduced plastoquinone
although it is not in a fixed position, plastoquinone remains in the membrane; why?
as it is hydrophobic
photolysis
- once the plastoquinone becomes reduced, the chlorophyll in the reaction centre is then a powerful oxidising agent and causes the water molecules nearest to split and give up electrons to replace those it has lost:
2H2O –> O2 + 4H+ + 4e-
why is the reduced plastoquinone so important
it not only carries a pair of electrons, but also much of the energy absorbed from light - this drives all the subsequent photosynthetic reactions
list 4 structures contained in the thylakoid membrane
- photosystem II
- ATP synthase
- chain of electron carriers
- photosystem I
electron transport chain and proton gradient
- reduced plastoquinone carries the electrons to the start of the chain of electron carriers
- transfers its electrons
- these are passed from carrier to carrier, releasing energy which is used to pump protons across the thylakoid membrane into the space inside the thylakoids
- a concentration gradient of protons develops across the membrane (storing potential energy)
chemiosmosis
- the protons travel back across the membrane, down the conc gradient through the enzyme ATP synthase
- the energy released by this passage is used to make ATP from ADP + phosphate
- when the electrons reach the end of the chain of carriers they are passed to plastocyanin (an electron acceptor in the fluid of the thylakoid) which is reduced
reduction of NAD
- chlorophyll molecules within photosystem I absorb light energy and pass it to the special two chlorophyll molecules in the reaction centre
- this excites an electron in one of the chlorophylls (photoactivtion)
- the electron passes along a chain of carriers in PI, at the end of which it is passed to ferredoxin, a protein in the fluid outside the thylakoid
- two molecules of reduced ferredoxin are then used to reduce NADP
how are photosystem I and II linked?
electrons excited in photosystem II are passed along the chain of carriers to plastocyanin, which transfers them to photosystem I to replace the electron donated to the chain of electron carriers by photosystem I
difference between cyclic and non-cyclic photophosphorylation
- NADP sometime runs out
- PSI absorbs light energy
- Electrons are excited and released from PSI
- Energy from electrons is used to create an H+ gradient
- H+ diffuses out of the stroma through ATP synthase, making ATP
- Electrons are taken back up by PSI (recycled).
steps of light independent reactions
- carbon fixation
- carboxylation of RuBP
- production of triose phosphate
- ATP and NADPH as energy sources
- ATP used to regenerate RuBP
- ATP used to produce carbohydrates
where does carbon fixation occur?
in the stroma (the fluid that surrounds the thylakoids in the chloroplasts)
carbon fixation
carbon dioxide reacts with ribulose bisphosphate (RuBP), a 5 carbon compound to produce two molecules of glycerate 3-phosphate
ribulose bisphosphate carboxylase (usually abbreviated to rubisco) catalyses this reaction
production of triose phosphate
hydrogen is added to glycerate 3-phosphate to produce triose phosphate, a carbohydrate.
- ATP produced by the light-dependent reactions provides energy needed to perform the reduction
- reduced NADP provides the hydrogen atoms
the use of triose phosphate
- two triose phosphate molecules can be combined to form hexose phosphate, which can be combined by condensation reactions to form starch
- some triose phosphate molecules are converted into RuBP using enzymes and ATP
draw the Calvin cycle
draw a diagram of a chloroplast and label it, describing the structure-function relationships
Thylakoids- flattened discs have a small internal volume to maximise hydrogen gradient upon proton accumulation
Grana – thylakoids are arranged into stacks to increase SA:Vol ratio of the thylakoid membrane
Photosystems – pigments organised into photosystems in thylakoid membrane to maximise light absorption
Stroma – central cavity that contains appropriate enzymes and a suitable pH for the Calvin cycle to occur
describe Calvin’s experiments and how they elucidated the carboxylation of RuBP
Radioactive carbon-14 is added to a ‘lollipop’ apparatus containing green algae (Chlorella)
Light is shone on the apparatus to induce photosynthesis (which will incorporate the carbon-14 into organic compounds)
After different periods of time, the algae is killed by running it into a solution of heated alcohol (stops cell metabolism)
Dead algal samples are analysed using 2D chromatography, which separates out the different carbon compounds
Any radioactive carbon compounds on the chromatogram were then identified using autoradiography (X-ray film exposure)
By comparing different periods of light exposure, the order by which carbon compounds are generated was determined = CC
