Photosynthesis Flashcards
what’s light energy converted to and what is it used for?
chemical energy which is used to synthesise large organic molecules from inorganic ones like H2O, CO2 (autotrophic nutrition)
what are organisms that photosynthesise called?
photoautotrophs
equation for photosynthesis
6 CO2 + 6 H2O + energy from photons —-> C6H12O6 + 6 O2
what’s a photon?
a particle of light containing energy
What’s the main product of photosynthesis?
- monosaccharide sugar which is converted to disaccharides for transport then starch for storage
photosynthesis is an example of…
carbon fixation
do plants respire?
YES
what are non-photosynthetic organisms called?
- heterotrophs
- they obtain energy from digesting organic molecules of food into smaller ones that can be used as respiratory substrates
what type of reaction is respiration ( endothermic/exothermic)?
exothermic
respiration equation
C6H12O6 + 6 O2 —> 6H2O + 6CO2 + energy
how do photosynthesis and respiration interrelate?
- important in recycling CO2 + O2 in atmosphere
- products of photosynthesis are the raw materials for aerobic respiration
- aerobic respiration removes O2 from the atmosphere + adds CO2. Photosynthesis does the opposite
define compensation point
when photosynthesis and respiration proceed at the same rate so there’s no net gain or loss of carbohydrate
do plants respire all the time?
YES
do plants photosynthesise all the time?
NO
- only during daylight
- light intensity must be sufficient to allow photosynthesis at a rate that replenishes carbohydrate stores used up by respiration
define compensation period?
time a plant takes to reach its compensation point
how will the compensation point of shade plants differ from that of sun plants?
shorter compensation period than sun plants which need higher light intensity to achieve their optimum rate of photosynthesis
what’s the granum (pl. grana)?
inner part of chloroplasts made of stacks of thylakoid membranes where light- dependent stage happens
define photosynthetic pigment?
pigment that absorbs specific wavelengths of light and traps energy associated with the light whilst reflecting other wavelengths of light (what we see)
define photosystem?
funnel shaped system of photosynthetic pigments in thylakoids
define stroma
fluid filled matrix
define thylakoid
flattened membrane bound sac, contains photosynthetic pigments + photosystems
feature of outer membrane
highly permeable
what are the structures found between grana?
intergranal lamellae
what are the 3 different internal compartments and the membranes that create them?
- outer, inner, thylakoid membranes
- intermembrane space, stroma, thylakoid space
feature of thylakoid membrane
less permeable than outer membrane
why are there many and chloroplasts?
to provide large SA for:
- photosystems containing photosynthetic pigments that trap sunlight energy
- electron carriers and ATP synthase enzymes needed to convert light to ATP
features of the stroma
- enzymes that catalyse reactions in light dependent stage
- starch grains, oil droplets, small ribosomes and DNA
- loop of DNA has genes that code for some proteins needed for photosynthesis to occur which are assembled at chloroplast ribosomes
what are the 2 types of chlorophyll a?
- P680 in photosystem II, peak of absorption is light of wavelength 680nm
- P700 in photosystem I, peat 700nm
- both appear blue-green and absorb red light
what colour does chlorophyll b appear?
yellow green
name some accessory pigments
- carotenoids
- xanthophyll
define hydrolysis
the splitting of a molecule of a molecule using water
define dehydration
the removal of H from a molecule
define metabolic pathway
a series of small reactions controlled by enzymes (respiration + photosynthesis)
define phosphorylation
adding phosphate to a molecule (ADP is phosphorylated to ATP)
define photolysis
the splitting of a molecule using light energyd
define decarboxylation
removal of CO2 from a molecule
define phosphorylation
adding phosphate to a molecule using light
define NADP
a coenzyme + electron + H carrier
define electron carriers
molecules that accept e- then donate those e- to another carrier. Form an electron transport system. e.g Ferredoxin, NAD (respiration), NADP(photosynthesis)
what are the main stages in the light dependent stage?
- light harvesting at the photosystems
- photolysis of water
- photophosphorylation
- the formation of NADPH
O2 produced
what’s the role of water in the light dependent stage?
- source of H+ used in aerobic respiration
- donates e- to chlorophyll to replace those lost when light strikes
- source of O2 (by-product)
- keeps cells turgid
photolysis equation
2 H2O —> 4H+ + 4e- + O2
photosystem(s) involved in non cyclic photophosphorylation
PS II AND PS I
photosystem(s) involved in cyclic photophosphorylation
PS I only
products of non cyclic photophosphorylation
- ATP
- O2
- NADPH
products of cyclic photophosphorylation
- ATP in smaller quantities
cyclic and non cyclic photophosphorylation involve….
electron carriers
stages in non- cyclic photophosphorylation
- photon strikes PS II (P680), its energy is channelled to the primary pigment reaction centre
- light energy excites a pair of e- inside chlorophyll molecule
- energised pair e- escape from chlorophyll and are captured by e- carriers ( proteins containing Fe cores)
- these e- replaced by e- from photolysis
- Fe3+ ion reduced when e- accepted to Fe2+. It then donates the e- becoming reoxidised, to the next carrier in chain
- during redox reactions, some energy associated with the e- is released and use to pump H+ into thylakoid space from the stroma
- eventually, e- captured by another chlorophyll in PS I. These e- replace those lost from PS I due to excitation by light energy
- Ferredoxin (protein-Fe-S complex) accepts the e- from PS I and passes them to NADP in the stroma
- as H+ accumulate in thylakoid space, electrochemical gradient forms
- H+ diffuses down conc gradient through channels in membrane associated with ATP synthase enzymes and, as they do so, flow of protons causes ADP and Pi to re-join ( phosphorylation
- as H+ pass through channel, they’re accepted along with e- by NADP which becomes reduced, catalysed by NADP reductase
- light energy converted to ATP (chemical energy)
what products end up in the stroma after photophosphorylation for the lis?
- ATP
- NADPH
Define rate
quantity taken up or produced per unit time
How can rate of photosynthesis be measured and limitations?
- by measuring the volume of O2 produced per minute by aquatic plant
- limitations:
some O2 used in respiration
dissolved N2 gas collected
what’s cyclic and non-cyclic photophosphorylation?
The Z scheme
stages in cyclic phosphorylation
- light strikes PS I, pair of e- in chlorophyll at the reaction centre gain energy + become excited
- e- escape and passed on to an e- carrier system and then back to PS I
- During passage of e- along system, small amount of ATP is generated
what are some differences between cyclic and non-cyclic photophosphorylation?
- no photolysis
- no H+ or O2 produced
- no NADPH generated
- less ATP made
why do guard cells only have PS I?
- ATP for active transport of K+ into cells, lowering water potential so that water moves in by osmosis.
- guard cells swell and stomata open
what’s the Calvin cycle?
metabolic pathway of the light-independent stage of photosynthesis, occurring (in eukaryotic cells) in the stroma of chloroplasts where CO2 is fixed, with the products of lds to make organic compounds
What occurs when there’s no light?
- lis ceases as there’s no ATP and H to reduce carbon and synthesise large complex organic molecules
what’s the role of CO2?
- source of C for all organic molecules found in all carbon based - life forms on Earth
- organic molecules used as structures (cellulose cell wall, enzymes, antigens) or as energy stores (starch + glycogen)
why is carbon fixation on stroma important for diffusion?
- maintains a conc gradient
- (air) stomata –> spongy mesophyll –> palisade layer
stages of Calvin cycle
- CO2 combines with ribulose bisphosphate RuBP ( 5 C, CO2 acceptor), catalysed by RuBisCO
- RuBP accepts the carboxyl (COO-) group, becomes carboxylated forming an unstable 6C intermediate compound that immediately breaks down
- product is 2 GP (glycerate-3-phosphate) (3C), CO2 has been fixed
- GP is reduced, using H+ from NADPH to triose phosphate (TP). Energy from ATP is also used at the rate of 2 ATP for every CO2 molecule fixed during stage 3
- in 10 of every 12 TP molecules (3C), the atoms are rearranged to 6 RuBP (5C) which requires phosphate groups.
- the remaining 2 of the 12 TP are the product
why do plants contain little RuBP?
its converted to GP which is continually being regenerated
how many turns of the Calvin cycle are needed to make 2TP (to make one glucose molecule)
2
Outline the main processes in the Calvin cycle
- Carbon fixation (RuBP combining with CO2 to form GP)
- Reduction (of GP to TP)
- Regeneration ( TP to RuBP)
why is RuBisCo only activated during daylight?
- H+ pumped from stroma into thylakoid space in lds raises pH to 8 which is optimum for RuBisCO
- extra ATP in stroma also activate it
- in daylight Mg2+ ions in stroma increase which are cofactors
- Ferredoxin that’s reduced by e- from PS I activates enzymes used in Calvin cycle
what are the uses of TP
- synthesis of aa, fatty acids, glycerol
- some glucose converted to sucrose, starch, cellulose
- the rest us recycled to regenerate RuBP. 5TP (3C) form 3 RuBP (5C)
What’s water stress?
condition a plant faces when water supply becomes limited
What’s a limiting factor?
the factor that will limit the rate when its at lower levels
examples of limiting factors
- CO2 conc
- H20
- light intensity
- availability of chlorophyll, e- carries, relevant enzymes
- Temperature
- turgidity of cells
How does light intensity affect rate of ps?
- light provides energy for lds to produce ATP + NADPH
- causes stomata to open for gaseous exchange which causes transpiration (increase H2O uptake)
- at constant favourable temp + CO2 conc, light intensity is the limiting factor but as it increases rate of ps increases until another factor becomes the limiting factor
How does temperature intensity affect rate of ps?
- enzyme catalysed reactions sensitive to temp
- 25-30C, rate of ps increases as temp increases ( if other factors are at sufficient levels)
- above 30C, growth rates reduce due to photorespiration: O2 competes with CO2 for RuBisCO’s active site
- above 45C, enzymes denature so conc of GP + TP reduce and RuBP not regenerated
What happens to levels of RuBP, TP, GP if there’s little or no light?
- GP can’t be reduced to TP (GP increases)
- TP levels fall + GP accumulates
- if TP levels fall, RuBP can’t be regenerated
What happens to levels of RuBP, TP, GP if there’s little or no CO2?
- RuBP can’t accept CO2 (to form GP) and accumulates
- GP can’t be made
- so TP can’t be made
what happens when a plant is under stress?
- water lost via transpiration not replaced
- cells become plasmolysed
- roots produce abscisic acid which is translocated to leaves + closes stomata
- tissue becomes flaccid + leaves wilt
-rate of ps decreases
How to set up and use a photosynthometer ( Audus microburette)
- set up so its airtight + no bubbles in capillary tubing
- gas collected st flared end of capillary tube over a known time period
- gas bubble can be moved into the part of the tube against the scale and its length is measured
- if radius of tube bore is known :
volume of gas collected = length of bubble x pir2
