A Level CIE Biology: 13 Photosynthesis

what are chloroplasts

organelles in plant cells where photosynthesis occurs where each chloroplast is surrounded by double-membrane envelope [phospholipid bilayer]

what are chloroplasts filled with and what is the function

stroma, site of light-independent stage of photosynthesis

what is found in the stroma

system of membranes where the light-dependent stage of photosynthesis is carried out

what do the membranes in the stroma contain

pigments, enzymes, electron carriers required for light dependent reactions

where and what are thylakoids

in the membrane series, series of flattened, fluid filledsacs

what do thylakoids stack up to form

grana/granum

what are grana connected by and why

membranous channels called stroma lamellae. ensure stacks of sacs are connected but distanced from each other

what do the membranes of grana create

create a large surface area to increase the number of light-dependent reactions that can occur

why is the membrane system necessary

provides large number of pigment molecules in an arrangement that ensures as much light as necessary is captured

what 3 things the stroma also contain and why

• small 70s ribosomes - where proteins coded for by this loop of chloroplast dna are produced

• loop of dna - codes for some chloroplast proteins [other done by dna in plant cell nucleus ]

• starch grains - sugars formed during photosynthesis is stored as starch in here

chloroplast structure parts 7

• ribosomes

• chloroplast envelope - outer memb, inner memb

• starch grain

• stroma

• thylakoid

• granum

• grana

two stags of photosynthesis + where

• light-dependent stage (thylakoids)

• light-independent stage (stroma)

light dependent stage summary

• nadh produced when h+ combines w/ carrier molecule NADP using e- from photolysis of water

• atp produced in photophosphorylation (uses H+ gradient generated by photolysis of water

• energy from atp and h from nadph are passed from ld stage to li stage of photosynthesis

light independent stage summary

• energy and h from ld stage used

• takes place in calvin cycle

• complex organic mols produced including and not limited to carbs e.g. starch for storage, sucrose for translocation around plant and cellulose for cell walls

where does ld stage of photosyn occur

thylakoid membranes and thylakoid spaces

what do thylakoid membranes contain

pigments, enzymes and electron carriers

why and how is there a large sa

membranes of grana, to increase no. ld reactions

photosystems

pigment molecules arranged in light harvesting clusters known as photosystems

what does membrane system provide to ensure as much light as necessary is captured

large no. pigment mols

arrangement in photosystem

diff pigment mols arranged in funnel-like structures the thylakoid membrane (each pig mol passes energy down to next pig mol in cluster until reaches primary pigment reaction centre)

what is dissolved in stroma fluid

co2, sugars, enzymes and other mols

what is stroma

fluid that fills chloroplasts and surrounds thylakoids and site of li stage of photosyn

diff photosynthetic pigments within thylakoids absorb diff…

wl of light

chlorophylls name of pigment and colour of pigment

chlorophyll a and b, light green and dark green

pigment groups

chlorophylls, carotenoids

carotenoids name of pigment and colour of pigment

beta carotene and xanthophyll, orange and yellow

chlorophylls absorb what wl

blue-violet and red regions of light spectrum and reflect green light = plants green

carotenoids absorb what wl of light

blue-violet region of spectrum mainly

absorption spectrum

graph that shows absorbance of diff wl of light by particular pigment

action spectrum

graph that shows rate of photosyn at diff wl of light

when is rate of photosyn highest

at blue-violet and red regions of light spectrum as these are wl of light that plants can absorb

how is there a strong correlation between cumulative absorption spectra of all pigments and action spectrum

• both graphs have 2 main peaks at blue-violet region and red region of light spectrum

• both graphs have trough in green-yellow region of light spectrum

chromatography

experimental technique that is used to separate mixtures

chromatography process

• mixture dissolved in fluid/solvent called mobile phase and dissolved mixture passes through static material called stationary phase

• diff components within mix travel thru material at diff speeds

• diff components separate

• retardation factor (Rf) can be calc for each comp of mixture

Rf

distance travelled by comp/dist travelled by solvent

two most common techniques for separating photosynthetic pigments:

• paper chromatography - mix of pigments passed through paper/cellulose

• thin layer chromatography - mix of pigments passed through thin layer of adsorbent (e.g. silica gel) through whicb mix travels faster and separates more distinctly

what does rf value show

how far dissolved pigment travels through stationary phase (smaller rf = pigment less soluble and larger)

how can chromatography be used with a leaf

separate and identify chloroplast pigments that have been extracted from leaf as each pigment has unique rf value

rf values depend on solvent but generally: 4

• carotenoids have highest rf values close to 1

• chlorophyll b has must lower rf value

• chlorophyll a has rf value somewhere between carotenoids and chlor b

• small rf values indicate less soluble and larger pigment

during ld stage of photosyn

• light energy used to break down water (photolysis) to produce H+, e- and oxygen in thylakoid lumen

• proton gradient formed due to photolysis of water, resulting in high conc of H+ in thylakoid lumen

• e- travel through etc of proteins within memb

• nadph is produced when h+ ions in stroma nad e- from etc combine w/ carrier molecule nadp

• atp is produced during process known as photophosphorylation (adp+pi → atp) using proton gradient between thylakoid lumen and stroma to drive enzyme atp synthase

photophosphorylation of adp to atp can be ___ or ____ how

cyclic, non-cylic depending on pattern of e- flow in photosystem I or photosystem II or both

in cyclic photophosphorylation…

only photosystem I involved

in non-cyclic photophosphorylation…

both photosystem I and photosystem II are involved

photosystems

collections of photosynthetic pigments that absorb light energy and transfer the energy onto e-, each contains primary pigment

photosystem II primary pigment + location

beginning of etc where photolysis of water takes place. primary pigment p680 bc absorbs light at wl 680nm

photosystem I primary pigment and location

middle of etc, primary pigment p700 bc absorbs light at wl 700nm

energy carried by atp is used…

during li reactions of photosyn

what does cyclic photophosphorylation involve

photosystem I (PSI) only

cyclic psi photophosphorylation process

• light absorbed by psi located in thylakoid memb and passed to psi primary pigment p700

• e- in primary pig mol (chlorophyll) is excited to higher energy level and emitted from chlorophyll mol in photoactivation

• excited e- captured by e- acceptor and transported via chain of e- carriers (etc) before passing back to chlorophyll mol in psi

• as e- pass thru etc they provide energy to transport H+ from stroma to thylakoid lumen via proton pump

• buildup of protons in thylakoid lumen drives synthesis of atp from adp and an inorganic phosphate group (Pi) thru chemiosmosis

• then atp passes to li reactions

chemiosmosis

movement of chemicals/protons down conc grad, energy released from this can be used by atp synthase to synthesise atp

what does non cyclic photophosphorylation involve

psi and psii

where is light asborbed by psii

thylakoid memb and passed to psii primary pigment p680

non cyclic process (psii)

• e- in primary pig mol excited to higher energy level and is emitted from chlorophyll mol in photoactivation

• excited e- passed down chain of e- carriers (etc) before being passed on to psi

• during this atp is synthesised from adp and an inorganic phosphate group (Pi) by process of chemiosmosis

• atp then passes to li reactions

• psii contains water splitting enz called oxygen evolving complex which catalyses breakdown (photolysis) of water by light: 2H2O → 4H+ + 4e- + O2

• as excited e- leave primary pig of psii and are passed on to psi, they are replaced by e- from photolysis of water

non cyclic psi process

• at same time as photoactivation of e- in psii, e- in psi also undergo photoactivation

• excited e- from psi also pass along etec

• e- combine w/ h+ produced by photolysis of water and the carrier mol nadp to give nadph: 2H+ + 2e- + NADP → reduced NADP

• nadph then passes to li reactions to be used in synth of carbs

phosphorylation and chemiosmosis

• during photophosphorylation, energetic e- captured by e- acceptor in thylakoid memb

• energetic e- passed along chain of etc

• e- carriers are alternately reduced as they gain e- and then oxidised as they lose e- by passing to next carrier

• excited e- gradually release energy as they pass thru etc

• released energy usedd to actively transport h+ across thylakoid memb from stroma (fluid within chloroplasts) to thylakoid lumen

• proton pump transports protons across thylakoid memb from the stroma to thylakoid lumen

• creates a proton grad w high conc of protons in thylakoid lumen and low conc in stroma

• protons then return to stroma moving down proton conc grad by facil diffusion thru transmembrane atp synthase enz in chemiosmosis

• process provides energy needed to synthesise atp by adding an inorganic phosphate group Pi to ADP (ADP + Pi → ATP)

• whole process known as photophosphorylation as light provides the intial energy source of atp synth

• after being passed down etc de energised e- from psii are taken up by psi

what is passed from the ld stage to the li stage of photosyn

energy from atp and hydrogen from nadph

what is the eneryg and hydrogen used during

li reactions/calvin cycle to produce complex organic molecules including but not limited to carbs e.g. starch,sucrose, cellulose

why cant the calvin cycle continue indefinitely indarkness

inputs run out bc even tho photosyn doesnt in itself need energy from li and can take place in light or darkness, it requires inputs of atp and nadph from ld stage

3 main steps within calvin cycle

• rubisco catalyses the fixation of carbon dioxide by combination w molecule of ribulose bisphosphate (RuBP) a 5C compound to yield 2 mols of glycerate 3-phosphate (GP) a 3C compound

• gp reduced to triose phosphate (tp) in reaction involving nadph and atp

• RuBP is regenerated from tp in reactions that use ATP

how does carbon fixation occur

• co2 combines w/ 5c ribulose bisphosphate (rubp)

• enzyme aclled rubisco (ribulose bisphosphate carboxylase) catalyses reaction

• resulting 6c compound unstable and slpits into 2 3c compounds = glycerate 3-phosphate (gp)

• co2 has been fixed (removed from ext env and become part of organic matter of plant cell) 

• gp not carb but next step in calvin cycle converts it into one

how does reduction of gp occur

• energy from atp and h from nadph both produce during ld stage of photosyn are used to reduce gp to phosphorylated three-carbon 3c sugar known as triose phosphate (tp)

• 1/6 of those tp mols used to produce useful roganic molecules needed by plant:

  • tp can condense to become hexose phosphates 6c which can be used to make starch, sucrose or cellulose

  • tp can be converted to glycerol and glycerate 3-phosphates to fatty acids which join to form lipids for cell membranes

  • tp can be used in prod of aas for protein synth

regeneration of ribulose bisphophate

• 5/6 of tp mols used to regen RuBP

• requires atp

calvin cycle intermediates

• intermediate mols of calvin cycle such as gp and tp are used to produce other mols

• gp is used to produce some aas

• tp is used to produce

  • hexose phosphates (6c) whihc can be used to produce starch, succrose or cellulose

  • lipids for cell membs

  • aas for protein synth

5 factors plant for photosynthesis

• the presence of photosynthetic pigments

• a supply of co2

• a supply h2o

• light energy

• suitable temperature

if there is a shortage of factors…/below opt rate for plant

photosynthesis cannot occur at its maximum possible rate

3 main ext factors that affect the rate of photosyn (limiting factors of photosyn)

• light intensity

• co2 conc

• temp

when temp and co2 conc are constant what affects rop

changes in light intensity rate of

why does rop increases as light intens increases 

• greater light intensity, more eneryg supplied to plant so faster ld stage of photosyn occurs

• produces more atp and nadph for calvin cycle (li stage) which can also occur at greater rate

• then light intesity will become limiting factor 

what happens if light intesity continues to increase

• relationship above will no longer apply and rop will reach plateau

• li no longer limiting factor of photosyn so another factor will become the limitiing

• factosr which could be limitng rate when line on graph is horizontal include temp being too low or too high or not enough co2

why rop increases as co2 increases

• co2 one of the raw mats required for photosyn

• required for li stage of photosyn when co2 is combined w/ 5c RuBP

• the more co2 present the faster the calvin cycle can occur and faster overall rop

• continues until other factor required for photosyn rpevents rate of increasing further bc in short supply

• factors could be limiting the rate when line on graph horizontal include temp being too low/high or not enough light

as temp increases…

• rop increasess as reaction is controlled by enzymes

• this trend only continues up to a ceratin temp beyond which enz begin to denature and the rate of reaction decreases

why is calvin cycle a affected by temp

• temp has large effect on ror for metabolic reactions

• for photosyn, temp has little significant effect on ld reactions as these r driven by energy from light rather than k.e. of reacting mols

• however calvin cycle affected by temp as li reactions are enz controlled reactions e.g. rubisco catalyses the reaction between co2 and the 5c RuBP

why does dcpip and methylene blue change colour during photosyn 8

• ld reactions of photosyn take place in thylakoid memb and involve the release high energy e- from chlorophyll a mols

• mols picked up e- acceptors and then passed down etc

• if redox indicator present the indicator takes up the e- instead

• dcpip: oxidised (blue) → accepts e- → reduced (colorless)

• methylene blue: oxidised (blue) → accepts e- → reduced (colorless)

• may appear green bc chlorophyll green color

• rate at which redox indicator cahnges color from oxidised state to reduced can be used as measure of rop

• when light is at high intens or at more preferable light wls the rop of e- faster so roreduction of indicator is faster

Investigating the Rate of Photosynthesis: Redox Indicators method 4 + extra

• leaves crushed in liquid called isolation medium

  • produces concentrated leaf extract that contains suspension of intact and functional chloroplast

  • medium must have same water potential as leaf cells so chloroplasts dont shrivel/burst and contain buffer for constant pH

  • should also be ice cold to avoid damaging chlor and maintain memb structure

• small tubes set up w diff intensities or diff colours/wls of light shining

  • diff intensities must all be same wl

  • diff wl must all be smame intensity

• dcpip or methylene blue added to each tube as well as small vol of leaf extract

• time taken for redox indic to go colorless recorded = rop

how can effecft of limiting factors on rop be investigated with aquatic plants e.g. elodea or cabomba (pondweed)

• light intensity - change distance of source from plant (light intens = 1.d²)

• co2 conc - add diff qts of sodium hydrogencarbonate to water surrounding plant which dissolves to prod co2

• temp of solutoin surrounding plant - boiling tube w. submerged plant in water baths of diff temps

• while changing one of these factors during investigation as described below ensure other two remain constant e.g. light intens on rop needs glass tank between lamp and boiling tube containing pondweed to asborb heat from lamp so no temp change

Investigating the Rate of Photosynthesis: Aquatic Plants method

1. Ensure water is well aerated before by bubbling air through so oxygen gas given off by the plant during the investigation forms bubbles and does not dissolve in the water

2. Ensure that the plant has been well illuminated before starting the experiment

   • This will ensure that the plant contains all the enzymes required for photosynthesis and that any changes in rate are due to the independent variable rather than an increase in enzyme activity

3. Cut the stem of the pondweed cleanly just before placing it into the boiling tube

   • Cutting the stem at an angle provides a larger surface area from which bubbles can form

4. Set up the apparatus (as shown below) in a darkened room

   • This ensures that the lamp is the only light source and so allows light intensity to be controlled

   • Ensure that the pondweed is fully submerged in sodium hydrogencarbonate solution (1%); this ensures that the pondweed has a controlled supply of carbon dioxide for photosynthesis

5. Measure the volume of gas collected in the gas syringe in a set period of time, e.g. 5 minutes

6. Repeat step 5 at least twice more

7. Change the independent variable and repeat step 5 again

   • The independent variable could be the light intensity, carbon dioxide concentration or temperature depending on which limiting factor you are investigating

8. Record the results in a table and plot a graph of the volume of oxygen produced per minute against the independent variable