photosynthesis Flashcards
components of a chloroplast
starch grains
circular DNA
outer membrane
inner membrane
intergranal lamellae
70s ribosomes
stroma
grana
thylakoid discs
outer membrane vs inner membrane of chloroplast
inner membrane is more selective than outer membrane bc it has less transport proteins
intergranal lamellae function
connect grana
allow for sharing of resources
stroma contains what
enzymes involved in the light-independent reaction (Calvin cycle)
what are the thylakoids the site of
light dependent reaction
thylakoid discs contain what
photosynthetic pigments embedded in the thylakoid membrane
rough size of a chloroplast
10um
describe and explain how proteins are produced in a chloroplast
requires 70s ribosomes and circular DNA (encodes many proteins involved in p/s)
mRNA transcribed from a gene within the circular DNA using RNA polymerase
mRNA translated by a 70s ribosome to form a sequence of amino acids (primary structure)
what are the 2 types of photosynthetic pigments
primary pigments
accessory pigments
examples of primary pigment
chlorophyll a and b
what wavelengths of light does chlorophyll a absorb
wavelengths of light in the blue and red regions of the visible spectrum
examples of accessory pigments
chlorophyll b
carotenoids
xanthophylls
purpose of accessory pigments
increase the range of wavelengths of light that can be absorbed (so increase the rate of photosynthesis bc more energy can be captured from sunlight)
5 types of chlorophyll
chlorophyll a,b,c,d and bacteriochlorophyll
structure of chlorophyll molecule
hydrophobic till that inserts into the thylakoid membrane
porphyrin ring head that absorbs light
what makes chlorophyll a different to chlorophyll b
A converts light energy to chemical energy
what contains chlorophyll a vs b
all photosynthetic plants, algae and cyanobacteria contain chlorophyll a
only plants and green algae contain chlorophyll b, along w/ a few types of cyanobacteria
what wavelengths of light do carotenoids absorb
violet and blue-green
use of carotenoids (animals)
used as advertisements to attract animals, which can help the plant’s seeds
(e.g. red of tomato, yellow of corn seeds, orange of orange peel)
carotenoids function (p/s)
help capture light
get rid of excess light energy: when a leaf is exposed to full sun, it receives a huge amount of energy; if that energy is not handled properly, it can damage the photosynthetic machinery
carotenoids in chloroplasts help absorb the excess energy and dissipate it as heat
what does an absorption spectra show
the range of wavelengths of light that can be absorbed by certain pigments (shows individual pigments)w
what does an action spectra show?
the rate of photosynthesis at each wavelength of light (incorporates absorption of all pigments)
what is a photosystem
a light-harvesting complex embedded in the thylakoid membrane
components of a photosystem
accessory pigments
reaction centre
chlorophyll a
outline the importance of photosynthetic pigments in photosynthesis
primary pigments (e.g. chlorophyll a) absorb photons and excite electrons
accessory pigments channel light energy towards the primary pigment and increase the range of wavelengths of light that can be absorbed
this produces ATP and rescued NADP (NADPH), which can then be used in the light-independent stage
oxidation is?
loss of electrons
loss of hydrogen atoms
gain of oxygen
reduction is?
gain of electrons
gain of hydrogen atoms
loss of oxygen
what are the 2 stages of photosynthesis
light dependent stage
light independent stage
where does the light dependent stage of p/s take place
thylakoid membrane
where does light independent stage of p/s take place
stroma
first step of light-dependent stage of photosynthesis? ( in ps1 )
a photon of light is channelled through photosystem 2 towards the reaction centre
it hits chlorophyll, which excites electrons
water is split in photolysis by a water-splitting enzyme into H atoms and O2. each H atom splits into a proton (H+) and an electron
what happens to the hydrogen, oxygen and electrons after photolysis
the H+ are used in the synthesis of ATP and form proton gradient
the electrons are fed back in to the chlorophyll a of PS2 to reduce it so that another electron can be excited by a photon of light
the oxygen is used in aerobic respiration or diffuses out of the stomata
chemical equation for photolysis of water
H2O -> 2H+ + 2e- + 1/2O2
what happens to chlorophyll a when photon of light hits the photosystem?
it is oxidised
stages 2 and 3 of light-dependent stage of photosynthesis? (between PS 1 and 2)
excited electrons are accepted by an electron acceptor protein
electrons lose energy as they move through a chain of electron carrier proteins. this energy is transferred to protons, which pumps them from the storm, across the thylakoid membrane, into the thylakoid membrane, creating a steep proton gradient
stage 4 of light-dependent stage of photosynthesis? (moving to PS1)
H+ ions flow through ATP synthase channels down their electrochemical gradient (chemiosmosis)
they allow ADP and Pi to combine and form ATP (photophosphorylation)
electrons are accepted by photosystem 1
stage 5 of light-dependent stage of photosynthesis? ( leaving PS2 )
electrons are re-excited by another photon to a higher energy level
electrons pass through another chain of electron carrier proteins (energy lost transferred to H+ to continue being pumped from storm)
electrons recombine with H+ to form H atoms
H atoms are added to NADP to reduce it
chemical equation NADP reduction
NADP+ -> NADPH2
uses 2H and NADP reductase
what is ATP synthase
a transmembrane protein
ATP synthase function
H+ flow through ATP synthase by chemiosmosis and proton motive force
energy is converted from mechanical/kinetic to chemical
ADP + Pi -> ATP
what is photolysis
the splitting of water molecules using light energy
what is photolysis catalysed by?
a water-splitting enzyme/oxygen-evolving complex
describe cyclic photophosphorylation
electrons excited from PS1 are unable to be accepted by NADP, so return to PS1 via another chain of electron carriers
a steep proton gradient is maintained so ATP continues to be made
where does more cyclic phosphorylation occur
in cells with less chloroplasts (carry out less p/s) e.g. more cyclic than non-cyclic in guard cells compared to mesophyll
photosystems involved in cyclic vs non-cyclic phosphorylation
cyclic: PS1
non-cyclic: PS1 and PS2
does photolysis of water occur in cyclic and non-cyclic photophosphorylation?
cyclic: no
non-cyclic: yes
what is the electron donor in cyclic and non-cyclic photophosphorylation?
cyclic: PS1 (chlorophyll a)
non-cyclic: water
what is the final electron acceptor in cyclic and non-cyclic photophosphorylation?
cyclic: PS1
non-cyclic: NADP
products of cyclic and non-cyclic photophosphorylation?
cyclic: ATP
non-cyclic: NAPH + ATP +O2
what does the manganese ion at the centre of the oxygen-evolving complex act as?
cofactor
which pair of areas within a chloroplast shows the steepest pH gradient between them?
stroma and the space within the thylakoid membrane (thylakoid lumen)
what does the light-independent stage of p/s require?
products of the light-dependent reaction:
NADPH (provides reducing power)
ATP (releases energy when hydrolysed to ADP and Pi)
does the Calvin cycle require light
does not require photons to occur but still requires the products of the light dependent stage
input to Calvin cycle?
where does it come from?
CO2
enters plant through the stomata
what is rubisco
enzyme that catalyses the fixation go CO2 to RuBP
describe Calvin cycle
RuBP is carboxylsed (CO2 added), catalysed by enzyme RuBisCO
unstable 6-carbon intermediate formed
unstable compound splits and breaks down into 2 molecules of 3-carbon GP
GP is converted to TP (catalysed by ATP, which is hydrolysed into ADP and Pi, and NADPH, which is oxidised to produce NADP+)
1/6 of TP leave Calvin cycle
5/6 of TP are regenerated to form RuBp (requiring ATP, which is hydrolysed to produce ADP and Pi)
what does RuBP stand for
ribulose-1.5.-biphosphate
what does GP stand for
glycerate-3-phosphate
what does TP stand for
triode phosphate
what happens to the 1/6 of TP molecules which leaves the Calvin cycle
can be converted into glucose, starch, cellulose, amino acids, glycerol etc
this varies dependent on the metabolic requirements of the plant
how many turns of the Calvin cycle are necessary to make one molecule of glucose?
6
what is RuBisCO subject to competitive inhibition from?
oxygen formed from the photolysis of water in non-cyclic phosphorylation
explain the role of ATP in the light independent stage of photosynthesis
hydrolysed to release energy for conversion of GP to TP and regeneration of RuBP from TP
why are plants which only possess photosystem 1 unable to produce sugars
PS1 is only responsible for ATP production by cyclic phosphorylation
therefore NADP is NOT reduced so GP cannot be converted to TP
explain why the concentration of GP in chloroplasts increases as light intensity decreases at sunset
decreased light intensity so less photons to excite electrons from PS2
therefore less NADPH and ATP produced
therefore less conversion of GP to TP can take place
but CO2 still available so RuBP still carboxylated to GP
therefore GP conc increases
state and explain the effect of a decrease in CO2 conc on the cones of RuBP, GP and TP
less CO2 means less RuBP fixed to GP
therefore less GP produced
therefore less TP produced
RuBP accumulates so RuBP conc increases
4 factors affecting p/s rate
light intensity
CO2 concentration
temperature
water stress
how does light intensity affect rate of photosynthesis
- increasing light intensity increases photons so increases excitation of electrons, so more ATP formed, so more reduced NADP formed so increased rate of p/s
- LI no longer limiting (CO2 conc/temperature are now limiting)
how does CO2 conc affect rate of photosynthesis
1.increasing CO2 conc increases rate of carbon fixation by rubisco (RuBP to GP)
2. other factors become limiting e.g. temperature and light intensity)
how does temperature affect rate of photosynthesis
at lower temps, enzymes and substrates have limited KE so are unlikely to form ESCs therefore decreased frequency of collisions so v. low rate
as temp increases, enzymes and substrates gain KE, so frequency of successful collisions increases
active site starts to change shape (i.e. tertiary structure changes so denaturation)
what is water stress
occurs when there is insufficient water in the soil
abscisic acid (ABA) synthesised in the root and transported up the xylem in the transpiration stream
ABA acts on receptors on the guard cell plasma membranes so ions are pumped (AT) out of guard cells; water follows by osmosis down a WP gradient so guard cells become flaccid and stomata close
why does a leaf in the sun have a greater depth of palisade mesophyll than a leaf in the shade?
more photons available so more photosystems required to absorb the available light energy
how much of available light energy is used
around 0.5%
the remainder is reflected/transmitted
how does a shade leaf differ from a sun leaf
more air spaces in shade leaf so more CO2 available for Calvin cycle
shade leaf is thinner so photons have a shorter distance to travel
palisade mesophyll shallower in shade leaf
what is a compensation point
when rate of p/s is equal to rate of respiration
what is compensation point based on?
temperature and light intensity
what is equal at compensation point
rate of p/s and rest
net CO2 uptake and production
net sugar production and use
why does a shade plant have a lower compensation point
able to use more of the available light energy for p/s
explain changes in levels of GP and RuBP when light is switched off
no light= no photons to excite electrons from PS2
therefore no NADPH produced (and less ATP produced in LDR)
therefore no conversion of GP to TP
CO2 still available so GP accumulates as RuBP fixed to GP so GP levels increase
less TP means less regeneration of RuBP therefore RuBP levels decrease
photosynthesis is a type of what nutrition?
autotrophic (production os organic compounds from simple inorganic compounds using an external source of energy (light as photons))
what technique is used to separate pigments from a plant using solvents
chromatography
