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

calvin cycle save my exams page