5.6 Photosynthesis Flashcards
What is photosynthesis?
-converts light energy to chemical energy, synthesise large organic molecules
- used by photoautotrophs/producers
What is the equation?
6CO2 + 6H2O -> C6H12O6 + 6O2
What is the Calvin cycle an example of?
carbon fixation
Endothermic or exothermic?
- endothermic
Importance of photosynthesis?
-regulates food chains
- regulates CO2 in atmosphere and oceans
What is the relationship between photosynthesis and respiration?
products of p = reactants of r and vise versa
What are compensation points?
When photosynthesis and respiration occur at the same rate
Time taken for this to happen = compensation period
Which type of plant would reach its compensation point sooner?
Shade plants since they utilise light of lower intensity and so reach it quickly when exposed to light
What is the structure of chloroplasts?
outer membrane ( highly permeable)
inner membrane (less permeable)
intermembrane space
intergranal lamellae
thylakoids = highly folded thylakoid membranes
granum = stack of thylakoids
stroma = fluid- filled matrix
What are some features of the grana?
- thylakoid membrane is less permeable
- LARGE SA due to many grana in each chloroplast and many chroroplasts in each cell -> distribution of photosystems containing photosynthetic pigments AND electron carriers and ATP synthase enzymes are found
Products can —— pass from the grana into the stroma
EASILY
What does the stroma contain?
enzymes needed to catalyse reactions of LI stage
starch grains, small ribosomes, DNA
What is the role of the photosynthetic pigments?
within photosystems in the thylakoid membranes
There is energy associated with wavelengths of light absorbed by these pigments which is then funnelled to primary pigment reaction centre.
What is the primary pigment? What is found in PS I and PS II?
- Chlorophyll a
- PS2 has p680
- PS1 has p700
What do chlorophylls contain?
Porphyrin group (Mg and hydrocarbon chain)
What colours do chlorophyll a and b reflect?
two forms of chlorophyll a appear blue-green
chlorophyll b: Appears yellow-green.
What are some examples of accessory pigments? What colour do they appear?
Carotenoids: absorb blue light and reflect yellow/orange
Xanthophylls: absorb blue + green , reflect yellow
What is photophosphorylation? What do both non-cyclic and cyclic photophosphorylation both involve in thylakoid membranes?
ADP + Phosphate -> ATP with light energy
iron-containing proteins embedded that accept and donate electrons
Describe non cyclic photophosphorylation
- Photon strikes PS2 (P680) and its energy is funnelled to the primary reaction centre via accessory pigments
- Light energy excites a pair of electrons in chlorophyll a
- E- are emitted and accepted by an electron acceptor and pass through the ETC in a series of redox reactions
- These are replaced by electrons from the photolysis (splitting with light) of water.
- Energy associated with the electrons is released and used to pump H+ into the thylakoid space
- Electrons are captured by another chlorophyll a in PS1 to replace the 2 lost due to excitation
- Ferredoxin accepts these and passes them to NADP
- Protons diffuse through channels with ATP synthase down their electrochemical gradient -> forms ATP
- NADP accepts H+ and 2e- to become reduced NADP, catalysed by NADP reductase. (FINAL E- ACCEPTOR)
What are the products of non-cyclic photophosphorylation?
ATP, oxygen and reduced NADP
Describe cyclic photophosphorylation - what are the products?
chloroplasts in guard cells contain only —-
electrons pass through a carrier system BACK to PS1 after becoming excited.
no reduced NADP or oxygen
PS1
What are the products of cyclic photophosphorylation?
less ATP
Where does the Calvin cycle occur?
stroma
Describe the Calvin cycle
- RuBisCO catalyses CARBON FIXATION of carbon dioxide with RuBP
- The resulting sugar is unstable and breaks down into 3C molecules.
- Phosphorylation occurs where ATP is broken down to ADP and inorganic phosphates
- Reduction of the GP molecules also occurs ( 12 reduced NADP to 12 NADP)
- some TP leaves the cycle ( if 6 spins, then two molecules) which can be used to synthesise glucose or other organic molecules
- Using energy from ATP (5 molecules to be specific) and TP phosphorylation, RuBP is regenerated.
How many spins are needed for 2 TPs ( so one glucose)?
6
Why does it only run in the daylight?
1) H+ ions would be pumped from stroma = pH raises to around 8 -> optimum for RuBisCO. Also activated by extra ATP in stroma.
2) Magnesium ion conc increases in daylight which act as cofactors
What is a limiting factor?
A factor that limits the rate of a reaction when its at its least favourable,sub-optimal ( usually lowest) supply/ level.
How does light intensity affect the rate of photosynthesis generally?
- inc = inc in rate
- inc in light energy to excite more electrons = inc rate of phosphorylation and more ATP and NADPH produced.
- also causes stomata to open
- plateaus since another factor becomes limiting
How does light intensity affect conc of RuBP, TP and GP?
LIGHT: more ATP and NADPH increases GP reduction into TP and RuBP regeneration
DARK: GP cannot be reduced to TP. TP levels fall = RuBP cannot regenerate.
How does carbon dioxide affect rate of photosynthesis generally?
- increases, since more available for Calvin cycle
- plateaus since another factor becomes limiting
How does carbon dioxide affect conc of RuBP, TP and GP?
- HIGH: more fixation to GP, more TP. RuBP reacting too quickly and levels fall.
- LOW: RuBP cannot accept it and accumulates. Less GP = less TP
How does temperature affect the rate of photosynthesis?
- if other factors in excess, then rate increases due to more KE and more frequent collisions
- ABOVE 30: photorespiration where O2 competes with CO2 for RuBisCOs active site. RuBP eventually won’t regenerate and rate decreases
- ABOVE 45: denaturing, processes cannot go ahead
How does water stress affect photosynthesis rate?
- roots unable to take up enough water to replace water lost
- cells become plasmolysed and tissues become flaccid
- plant roots produce abscisic acid that causes stomata in leaves to close
rate decreases greatly
