Photosynthesis Flashcards
Autotroph
Photoautotroph
Chemoautotroph
-Organism that synthesises organic molecules from small inorganic ones eg.CO2
-Autotroph that synthesises organic molecules using light energy eg. plants doing photosynthesis
-Autotroph that synthesises organic molecules using chemical energy
Heterotroph
Organism deriving its nutritional requirements from complex organic substances eg. animals eating
Compensation point-
Compensation period-
-Where photosynthesis and respiration occur at same rates
-Time a plant takes to reach compensation point
Structure of chloroplast
-outer and inner membrane making up chloroplast envelope
-Fluid filled matrix is the stroma (light-independent stage takes place)
-stroma contains enzymes for light independent stage as well as starch grains, oil, ribosomes, DNA
-Grana is a stack of thylakoids
-grana connected by intergranal lamallae
Photosystems
-found in thylakoid membrane
-funnel shape with many accessory pigments and one primary pigment/ reaction centre
-primary pigment always chlorophyll a
-PS1 peak absorption= 700nm
-PS2 peak absorption= 680nm
-having different photosynthetic pigments allows maximum absorption of light as each have a different peak absorption
Light dependent stage
Non-cyclic photophosphorylation(Z scheme)
-light energy excites electrons in PS2
-electrons are released and pass through electron carriers containing Fe3+ which is reduced and reoxidised producing energy
-electrons then pass to PS1
-then they pass through another electron carrier of ferredoxin
-electrons end up at NADP reductase to reduce NADP
-electrons lost from PS2 are replaced by photolysis (splitting of water) 2H2O -> 4E- + O2 + 4H+
-hydrogen ions from photolysis end up at NADP reductase
Cyclic photophosphorylation
-electrons excited by light at PS1
-electrons are released and pass through electron carriers producing energy
-electrons return to PS1
-energy produced is used to pump H+ into the thylakoid space, creating a proton gradient
-H+ then diffuses out through ATP synthase which combines ADP and phosphate to produce ATP
-as H+ pass thrpugh the channel they are accepted along with electrons by NADP which is reduced
Grana
-light dependent stage takes place in the grana
-thylakoid membrane is less permeable and folded into flattened disc like sacs
-large SA for distribution of photosystems, electron carriers and ATP synthase
-proteins embedded in thylakoid membrane hold photosystems in place
Light independent stage
Calvin cycle
-CO2 combines with 5 carbon compound ribulose biphosphate RuBp catalysed by RuBisCO
-RuBp by acception carboxyl group forms unstable intermediate 6 carbon compound that immediately breaks down
-produces 2 molecules of 3 carbon compound glycerase-3-phosphate (GP). CO2 is now fixed
-GP is reduced using H+ from reduced NADP to triose phosphate (TP), ATP is also used at the rate of 2 ATP for every CO2 fixed
-10 of every 12 TP regenerate RuBp
-2 TP remaining are the product
2 uses of TP
-Glucose which is converted into sucrose, starch or cellulose
-Synthesise amino acids, fatty acids, glycerol
Calvin cycle only runs during daylight because..
-products from light dependent (ATP + NADP) are needed for calvin cycle
-During light dependent stage H+ are pumped into thylakoid space from the stroma, increasing pH of stroma to 8. This is optimum pH for RuBisCO which is also activated by ATP in the stroma
-in daylight the conc. of Mg2+ in stroma increases, which acts as a cofactor for RuBisCO
-Ferredoxin that is reduced by electrons from PS1 activate enzymes involved in reactions of calvin cycle
Factors affecting photosynthesis rates
-light intensity
-carbon dioxide concentration
- temperature
-water stress
Effect of changing light intensity on calvin cycle
-GP can’t be reduced to TP
- TP levels fall and GP accumulates
-If TP levels fall, RuBp can’t be regenerated
Effect of changing CO2conc on calvin cycle
-RuBp can’t accept carbon dioxide so accumulates
-GP can’t be made
-So TP can’t be made
Effect of changing temperature on calvin cycle
-At low temps 25-30, the rate increases as temperature increases
-at temps above 30, growth rates decrease due to photorespiration (oxygen competes for RuBisCO active site), therefore reducing amount of CO2 accepted by RuBP, reducing amount of GP and TP made
-at temps above 45, enzymes involved in photosynthesis may denature, therefore reducing GP and TP which in turn reduces RuBP
Water stress
-if not enough water available, roots can’t uptake enough water to replace water lost in transpiration
- cells lose water and plasmolyse
-roots produce abscisic, which when translocated to leaves cause stomata to close reducing gaseous exchange
-tissues become flaccid, leaves wilt
-rate of photosynthesis decreases
