C1.3 Photosynthesis Flashcards
What is photosynthesis?
- the conversion of light energy to chemical energy -> pigments (coloured substances e.g chlorophyll)
- Carbon compounds produced contain chemical energy previously light. (glucose)
Equation for photosynthesis?
Carbon dioxide + water (+light) -> glucose + oxygen
6CO2 + 6H2O -> C6H12O6 + 6O2
What are light dependent reactions?
reactions that require light in order to occur.
occur on the thylakoid membranes
- photolysis of water
- photophosphorylation
What are light independent reactions?
reactions that do not req light, occur in stroma
- e.g carboxylation (carbon fixation)
- calvin cycle
- synthesis of carbohydrates
How is hydrogen and oxygen obtained?
- splitting water molecules into hydrogen and oxygen (byproduct)-> when light is available -> energy req
- twelve molecules split for 1 glucose molecule
organisms carrying out photosynthesis?
- plants
- algae
- cyanobacteria
bubbles underwater produced (o2) rise to surface
terrestrial organisms diffuse o2 to atmosphere
Outline an experiment for chromatography
1) tear leaf to small pieces
2) grind leaf -> with sharp sand + propanone -> extract pigments
3) sample -> watch glass
4) evaporate to dryness -> hot air (blow dryer)
5) drops of propanone -> dissolve pigments
6) place a concentrated amount 10mm away from end of chroma paper
7) suspend strip -> tube -> base dips into solvent
8) remove -> when solvent nearly reached top -> pencil line, shows how far solvent moved (draw)
9) pigment identified by:
- colour
- Rf value
what does the rf value show?
- pigments travel -> different rates -> whether pigment is more attracted to the hydrophobic solvent -> or hydrophilic chromatography strip
rf value
Distance moved by spot/distance moved by solvent
general rule between wavelength and energy?
- shorter WL, higher energy a photon of light has
What happens when a photosynthetic pigment uses energy obtained from absorption of light?
- electron in molecule -> higher energy level jump
- conversion of energies
- “excited” electron -> can be passed on to other molecules
- energy carried by electrons -> ends up in glucose/other compounds
- specific wavelengths required -> have the amount of energy needed -> other wavelengths are reflected
What wavelengths does chlorophyll absorb effectively?
- red, blue
- green is mostly reflected, making it appear green
other pigments used in photosynthesis?
-xanthophyll, carotene
What is an absorption spectra?
- graph -> shows the absorbance of light by photosynthetic pigments (here chlorophyll) for all the wavelengths of light
action spectra
Action spectrum is a graph curve depicting the relative rates of photosynthesis for all wavelengths of light as a % maximum rate
What is a limiting factor?
- factor limiting rate at a particular time -> nearest to its minimum
temperature as a limiting factor
increases -> rate increases steeply until optimum temp -> past, and rate falls steeply
light intensity as a limiting factor
at low/medium, rate is directly proportional to intensity
at high, intensities -> rate plateaus
co2 concentration as a limiting factor
no photosynthesis -> low conc
low - high -> positive correlated (conc and rate)
very high -> plateaus
Design an investigation to deduce rate of photosynthesis…
- 1 limitant investigated (IV)
- suitable range should be chosen (lowest to where no longer limits)
- accurate method chosen for rate (DV) -> measure of o2 production per unit time
- all other factors are constant (CV)
How to measure temperature
- place pondweed -> thermostatically controlled water bath
- 5 - 45C (in 5/10 intervals)
- Controlling factor: set thermo at 25 and keep it there throughout experiment
How to measure light intensity
- move light source -> different distances -> measure intensity -> lux metre -> intensity = 1/distance^2
- range: 4,5,7,10,14 cm, + no light
- Controlling factor: keep light source at constant distance (5cm)
How to measure co2
- use pondweed, place in beaker
- start with boiling, cooled water -> add NaHCO3 -> increase co2 concentration
- range = 0-50 mmol dm^-3 -> in 10 mmol dm^-3 intervals
- Controlling: add enough -> give high concentration (50 mmol dm^-3)
What are carbon dioxide enrichment experiments?
- experiments to predict future rates of photosynthesis and plant growth
- co2 artificially increased - e.g greenhouses + free air carbon dioxide enrichment (FACE) experiments.
what are the FACE experiments (Free air carbon dioxide enrichment)
- circles of towers built -> co2 released
- co2 concentration monitored -> inside circles -> released upwind
- control plots are used -> air instead
- thus rise can predict consequences for plants + other stakeholders
- hypothesis that increased photosynthesis + plant growth -> moderate rises in co2 conc can be tested
What are photosystems?
- molecular arrays -> chlorophyll accessory pigments
- special chlorophylls in reaction centre -> pairs of excited electrons emitted
- in thylakoid membrane
What is a thylakoid?
- sac like vesicle
- flattened and arranged in stacks in chloroplasts for photosynthetic eukaryotes
Two types of photosystems
PS1:
- positioned -> thylakoid membrane exposed to stroma surrounding
PS2:
- not in contact with stroma
Functional units of Photosystems?
- reaction centre -> special chlorophyll -> emits e*
- antenna complexes -> harvest light energy -> funnel to reaction centre -> many pigment molecules -> precise arrangement
Advantages of a structured array of different types of pigment molecules in a photosystem?
- wider array -> larger no. pigment -> intercept more photons > supplies energy to reaction centre -> faster rate
- Wider range of wavelengths absorbed-> due to many different types of pigment
- energy transferred pigment to pigment -> excitation energy transfer (photoactivation) -> precise array in antenna complex - energy funnelled to reaction centre
What occurs during photolysis of water in PS II
- P680 (chlorophyll) in centre emits e* -> reduced P680 -> until it regains an e
- it is released after photolysis -> in OEC facing the space inside thylakoid
- OEC binds two water molecules and splits them -> 4e 4p and 1 o2
- electrons passed to -> p680
- protons released -> thylakoid space -> proton gradient across membrane
- o2 waste, diffuses out
What has the advent of oxygen generation by photolysis done for living organisms and geological processes?
- it initiated accumulation of o2 in atmosphere -> organisms respire aerobically -> oxidation of dissolved iron+other minerals in oceans occur
What is chemiosmosis?
- Production of ATP -> potential energy from the proton gradient from chains of electron carriers -> in thylakoid
- excited electrons pass ETC, releases energy -> pump protons, stroma to thylakoid space
How can electrons be supplied?
Cyclic Photophosphorylation
- process of producing ATP from ADP with light.
- pairs of excited electrons -> emitted (PSI) -> after passing ETC -> return
Non cyclic Photophosphorylation
- Pairs of excited (PSII) -> flow to PSI
What is Reduced NADP?
- useful product of LDR in thylakoids
- ferredoxin accepts electrons emitted from PSI
- binds to NADP reductase -> transfers electron it is carrying
-> reducing it
NADP + 2e- -> NADPH
how does NADP collide with ferredoxin?
- NADP is dissolved in stroma -> reduced -> molecular motion brings it to the enzyme’s active site.
- electrons emitted by PSI replaced by PSII -> passed to ETC
- Production of NADPH is accompanied by non-cyclic Photophosphorylation
What are thylakoids?
systems that consist of interacting and interdependent components
why is the thylakoid membrane a key component?
impermeable to protons -> gradient develops
encloses small vol of fluid - gradient develops rapidly
phospholipids, hold photosystems -> made of hydrophobic pigments
hold other components in correct relative positions (e.g above carriers, reductase, synthase)
What are thylakoid components dependent on?
- PS II supplying e*
- Electron carrier chain supplies energy
- proton pumps -> create -> gradient -> used by ATP synthase
- PS I supplies excited electrons to NADP reductase
- PS II resupplies PS I with electrons
- Water in thylakoid space -> supplies electrons to PS II
What is rubisco?
- enzyme responsible for catalysing the addition of carbon dioxide to Ribulose Biphosphate
- most abundant enzyme on Earth
What is carbon fixation?- light independent reaction
- the production of organic compounds from co2
Where does co2 come from?
- diffuses into stroma of chloroplast -> converted to more complex carbon compound, RuBP, is found in the stroma and reacts.
What happens within the carboxylation (COOH addition) reaction catalysed by rubisco?
- co2 combines -> ribulose bisphosphate (RuBP), pentose -> product, unstable six carbon compound -> splits to glycerate 3 phosphate
RuBP + CO2 -> 2 Glycerate-3-Phosphate (organic acid)
Limitations of Rubisco?
- high concentrations needed in stroma
- works slowly
- not effective in low co2 conc
How is triose phosphate formed?
- Glycerate 3 phosphate -> triose phosphate (3 carbon) by reduction in stroma
hydrogen needed -> from NADPH - energy supplied by ATP
Glycerate 3 phosphate + Reduce NADP + ATP -> TP + NADP + ADP
What is the Calvin cycle? (all carbon in compounds is fixed in calvin cycle, except glucose are traced back to an intermediate used)
- series of biochemical reactions to convert co2 to glucose
stages brief:
1) Co2 and RuBP, rubisco -> produce glycerate 3 phosphate (carbon fixation)
2) reduction, ATP and NADPH produced from LDR convert intermediate to triose phosphate (glyceraldehyde-3-phosphate)
3) some TP molecules -> regenerate RuBP -> cycle continues -> remaining converted to compounds -> source of energy + building blocks
What is required for the Calvin cycle to continue?
- regeneration of RuBP, 5/6 molecules of TP from light independent reaction used. 1 exits cycle
How is glucose produced?
- linking of two TP
- can be converted to other carbs e.g sucrose/starch
- pathways for aminos and fatty acids start with glycerate or TP or any intermediate from aerobic
Why are photosynthesis reactions interdependent?
- light independent reactions cannot happen without…
ATP, and NADPH from light dependent reactions.
Light dependent reactions require ADP and NADP -> independent reactions produce
