5.6 photosynthesis Flashcards
define autotrophic nutrition
converting light energy into chemical energy to synthesis large organic molecules
What is the general equation for photosynthesis
6CO2 + 6H2O —> C6H12O6 + 6O2
How do photosynthesis and respiration interrelate
the products of one process are the raw materials for the other
What is the compensation point
when photosynthesis and respiration occur at the same rate
Define the compensation period
time taken for a plant to reach its compensation point
Describe the structure of a chloroplast
2-10micrometres long surrounded by a double membrane
intermembrane space
fluid filled stroma
thylakoids = granum is thylakoid stacks
intergranal lamellae
How are chloroplasts adapted
stroma contains enzymes needed to catalyse reactions, starch, oils and ribosomes and DNA
outer membrane is permeable to ions
inner membrane less permeable to ions but has transport proteins embedded
granum provide a large surface area for electron carriers and ATP synthase and photosystems
How are photosystems held in place
proteins embedded in thylakoid membranes hold photosystems in pace
Describe photosystems
within thylakoid membranes are funnel shaped structures called photosystems which contain photosynthetic pigment which absorbs light at a particular wavelength
Describe how light travels in a photosystem
The energy of the wavelengths of light captures is funnelled down to the primary pigment centre consisting of a type of chlorophyll at the base
What is the difference between chlorophyll a and b
A has absorption peaks of P700 and P680 and appears blue-green
B absorbs 400-500nm and around 640nm and appears yellow-green
Describe accessory pigments
carotenoids absorb blue light wavelengths s400-500nm and reflect orange yellow light
xanthophyll’s absorb blue and green light 375-550nm and reflect yellow light
where does the light-dependant stage of photosynthesis occur
the granum / thylakoid membranes
what stages does the light dependant stage of photosynthesis consist of
light harvesting at the photosystems
photolysis of water
photophosphorylation
formation of reduced NADP
Describe photolysis
there is an enzyme in PS2 that in the presence of light splits water molecules into protons, electrons and oxygen
Give the equation for photolysis
2H20 –> 4H+ + 4e- + O2
What is the role of water in photosynthesis
source of protons used in photophosphorylation
donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
source of by-product oxygen
keeps plant cells turgid allowing them to function
Define photophosphorylation
the generation of ATP from ADP and Pi in the presence of light
What are the two types of photophosphorylation
cyclic and non-cyclic
What is the difference between cyclic and non-cyclic photophosphorylation
non-cyclic involves PS1 and PS2 it produces ATP NADPH and oxygen
whereas cyclic involves only PS1 and produces only ATP but in smaller quantities
What are the similarities between cyclic and non-cyclic photophosphorylation
Both produce ATP and both involve iron-containing proteins embedded in the thylakoid membranes
Describe non-cyclic photophosphorylation
When a photon of light strikes PS2 its energy is channelled to the primary pigment which excites a pair of electrons in the chlorophyll A and escape from the molecule which are then captured by an electron carrier - these electrons are replaced by those derived from photolysis
Electrons are passed along the electron transport chain and this generates some energy which is used to pump the protons into the thylakoid membrane space across the thylakoid membrane
Eventually electrons are captured by another molecule in PS1 which replace the electrons lost due to excitation by light
A protein-iron-sulphur complex called FERREDOXIN accepts the electrons from PS1 and passes it to NADP in the stroma
As protons accumulate in the thylakoid membrane space a proton gradient forms and protons diffuse down their concentration gradient through special channels in the membrane associated with ATP synthase and the flow of protons causes ADP+Pi –>ATP
Protons are accepted by NADP and this reduces the NADP (catalysed by NADP reductase)
Describe cyclic photophosphorylation
This uses only PS1
As light strikes it it excites a pair of electrons in the primary pigment Chlorophyll A p700
The electrons escape and pass to an electron carrier and then pass back to PS1 and during the passage of electrons along the electron carriers some ATP is generated
Where does only cyclic photophosphorylation occur
In the stomata because it only contains PS1
Describe the Calvin cycle
6CO2 combines with a CO2 acceptor called RuBP which is catalysed by the enzyme RuBisCO and by accepting the carboxyl group the ribulose biphosphate becomes carbonylated forming an unstable intermediate 6 carbon compound that immediately breaks down into a 3 carbon molecule GP and the CO2 has now been fixed
12GP is then reduced using hydrogens from the NADPH into 12TP and energy is used at a rate of 2 molecules of ATP/CO2
In 10 of every 12 TP is rearranged to regenerate 6 of the 5carbon RuBP
The remaining 2 TP are the product
What is GP
Glycerate-3-phosphate
What is TP
Triose phosphate
What is used to reduce the 12 GP to 12 TP
12 ATP to 12 ADP + 12Pi
12 NAPH to 12 NADP
What is needed to turn 10 TP to 6 RuBP
6 ATP to 6 ADP and 6Pi
Describe why there is a low concentration of RuBP
Because it is continuously being recycled
Why does the calvin cycle only run in the daylight
1 - Products from light-dependant stage is required for the Calvin cycle to run
2-Hydrogen ions are pumped into the thylakoid space from the stroma decreasing its pH to 8 which is optimum for RuBisCO
3- In daylight Magnesium ion concentration increases which acts as a cofactor for RuBisCO
4- Ferredoxin that is reduced by electrons from PS1 activates enzymes involved in reactions for the Calvin cycle
What are the uses of triose phosphate
Used to synthesis organic molecules e.g.
glucose converted to sucrose, starch or cellulose
used to synthesise amino acids, fatty acids and glycerol
What are the products of the light - independent stage
2TP -> 6C (glucose )
Name the limiting factors
CO2 concentration
light intensity
temperature
What are the effects of light intensity on the Calvin cycle
GP cannot be reduced to TP due to no ATP or NADPH produced from the light-dependant stage so TP levels fall and GP accumulated and RuBP cannot be regenerated
What are the effects of CO2 concentration on the Calvin cycle
RuBP has nothing to accept so GP and TP cant be made and RuBP accumulates
What is photorespiration
When RuBisCO accepts O2 instead of CO2 so less GP produced and although initially creating an accumulation of RuBP it can no longer be regenerated due to lack of TP
efine water stress
When not enough water is available to the plant
How do plants respond to water stress and how does this affect their ability to photosynthesise
Cells lose water and become plasmolysed, plant roots produce abscisic acid that when translocated to leaves causes stomata to close reducing gaseous exchange so tissues become flaccid and leaves wilt so photosynthesis greatly reduces
