Topic 8.2 Photosynthesis
Feb 24, 2016
Topic 8.2 Photosynthesis
Assessment Statements8.2.1 Draw and label a diagram showing the structure of a
chloroplast as seen in electron micrographs.8.2.2 State that photosynthesis consists of light-dependent and
light independent reactions.8.2.3 Explain the light-dependent reactions.8.2.4 Explain photophosphorylation in terms of chemiosmosis.8.2.5 Explain the light-independent reactions.8.2.6 Explain the relationship between the structure of the
chloroplast and its function.8.2.7 Explain the relationship between the action spectrum and
the absorption spectrum of photosynthetic pigments in green plants.
8.2.8 Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide.
Chloroplast StructureInternal membranes called thylakoids are the location of the light dependent reaction.
Stroma surrounds the thylakoids and inside the double membrane. This is the location of the light independent reaction that includes the Calvin cycle.
The stroma often contains starch grains and oil droplets both products of photosynthesis.
Structure FunctionThylakoid membranes
folded into granaSmall space inside
thylakoidsFluid filled stroma
LSA for absorption of light
Concentrates H+ ions
Compartmentalisation of enzymes for Calvin cycle
thylakoids
grana (sing. granum)
stroma
Stages of Photosynthesis
6CO2 + 6H2O → C6H12O6 + 6O2
In the light independent reaction (LIR) carbon is fixed using NADPH + ATP
Water is split in the light dependent reactions (LDR) to produce H+ ions which are used to generate ATP and NADPH
Used in respiration, stored as starch, converted to cellulose cell walls and other products
Light dependent reactionThylakoid membranes
Uses light - absorbed by chlorophyll
Uses H2O and produces O2
Coupled to reduction of ADP to ATP and coenzyme NADP+ to NADPH + H+
Involves photolysis and photophosphorylation
The Light Independent Reaction
Takes place in the stromaEnzymes involved
Temperature sensitive
Uses CO2 - carbon fixation produces glucose
Uses products of LDR - ATP and NADPH
Light has four functionsIt excites the electrons, which then allows
photophosphorylation to occur.
It excites the electrons, which then allows the reduction of NADP to NADPH.
It triggers the opening of the stomata so that CO2 can enter.
It initiates the photolysis of water.
Photophosphorylation
The addition of phosphate using light energy.e.g. ADP + Pi ATP
e.g. when light energy is used in the Z scheme to form ATP from ADP and phosphate.
PhotolysisThe splitting of water into H+, e– and oxygen
using light energy.
e- e- e- e-
Reduction
Reduction is loss of oxygen or gain of H+ or electrons (e-).
e.g. when NADP + H+ NADPH
NADP is reduced to NADPH by the addition of H+ and electrons.
The opposite of reduction is oxidation
Light dependent reaction
Chlorophyll molecules are arranged clusters called photosystems and embedded in the thylakoid membranes
Light Dependent Reaction
PhotoactivationWhen light hits the chlorophyll molecule the light
energy is transferred to electrons.
The electrons become excited, and break their bonds.
This is called photoactivation.
excited or activated state Electron
returns to ground state emitting a packet of energy
Light is absorbed and activates the electron
ATP Production
Light (680nm) strikes PSII
Excited electron leaves the chlorophyll molecule
Exciting an electron in the chlorophyll molecule
And is passed along a series of electron acceptors (ETC) to PSI
Electrons are replaced through photolysis
H+ produced by photolysis accumulate in the thylakoid space
Protons pass through ATP Synthase by chemiosmosis
This generates ATP
ChemiosmosisThe diffusion of ions across a partially permeable
membrane through ATP Synthase.
As electrons pass along the ETC energy is released.
This energy is used to pump protons across the thylakoid membrane into the thylakoid space.
As protons build up a gradient is created.
The flow of electrons from the thylakoid to the stroma generates ATP.
Production of NADPH
Light (700nm) strikes PSI
Electrons are re-excited and leave chlorophyll
Accepted by ferredoxin and passed down the ETC
Used to reduce NADP with a H+ ion
NADPH and ATP are passed to the LIR
Non-cyclic Phosphorylation
Non-cyclic phosphorylation makes ATP and NADPH (needed for the LIR)
Cyclic phosphorylation only makes ATP
You need more ATP than NADPH2 for the LIR
Route of electrons
First electron donor
Photosystems
Last electron acceptor
Products NADPH2 and ATP ATP only
returns to same moleculedoesn’t return to same molecule
water PSI
PSI and PSII PSI only
NADP PSI
Light Independent ReactionsTakes place in the stroma
Uses light energy trapped in the LDR (ATP and reduced NADP)
The process is controlled by enzymes Ribulose Bisphosphate Carboxylase (Rubisco)
Metabolic cycle
Involves carbon fixation
The Calvin Cycle - 3 stages
1 Carbon fixation
2 Reduction
3 Regeneration
CO2 is added to a 5 carbon compound called ribulose bisphosphate (RuBP)
RuBP splits to form glycerate 3 phosphate (G3P)
G3P is reduced to triose phosphate (TP)
⅙ TP molecules is used to make hexose bisphosphate ⅚ TP molecules are used to regenerate RuBP
CO2
ribulose bisphosphate(RuBP)
P P P
P
P P
P P
unstable 6C compound
glycerate 3 phosphate(G3P)
triose phosphate (TP)
hexose bisphosphate
NADPH2
NADP
ADP + Pi
ATP
carbon fixation
Reduction
Regeneration
ChlorophyllFound within chloroplastsAbsorb and capture lightMade up of a group of five pigments
Chlorophyll aChlorophyll bCarotenoids: xanthophyll and carotenePhaetophytin
Chlorophyll a is the most abundantProportions of other pigments accounts for varying
shades of green found between species of plants
Absorption Spectrum
Action Spectrum
PSII absorbs at 680nm
PSI absorbs at 700nm
Blue light is high energy and used in photosynthesis
Green light is reflected and not absorbed
Different pigments absorb different wavelengths of light, making photosynthesis more efficient. The greater the range of wavelengths of light that can be absorbed, the greater the light energy obtained.
Primary pigments excite electrons. e.g.chlorophyll a (NB there are different forms of chlorophyll a, e.g. 720 nm and 680 nm).
Accessory pigments channel electrons to primary pigment for photoexcitation. e.g.: chlorophyll b, carotene, xanthophyll.
Limiting FactorsThe rate of photosynthesis is affected by light intensity, carbon
dioxide concentration and temperature.
Under a given set of conditions only one factor will affect the rate of photosynthesis this factor is at its minimum and is called the limiting factor
The overall rate of photosynthesis is determined by the step that is proceeding most slowly (rate-limiting step).
Each factor e.g. light, temperature etc. can become the limiting factor in any on the rate-limiting steps.
(a) Draw a labelled diagram of the structure of a chloroplast as seen with an electron microscope (4)
Award [1] for each of the following clearly drawn and correctly labelled.
double/inner and outer membrane/envelope—shown as two concentric continuous lines close together;
granum/grana —shown as a stack of several disc-shaped subunits;
(intergranal) lamella — shown continuous with thylakoid membrane;
thylakoid — one of the flattened sacs;stroma;(70S) ribosomes/(circular) DNA / lipid globules / starch
granules /thylakoid space;
Effect on the concentration of TP, GP and RuBP
Factor Effect on TP
Effect on GP
Effect on RuBP
light intensity
carbon dioxide
concentration
temperature
Factor Effect on TP Effect on GP Effect on RuBP
Lightintensity
Decreasing light intensity means less ATP and reduced NADP, so less TP is made since ATP and reduced NADP are needed to make TP from GP.
Decreasing light intensity means more GP because RuBP can be converted to GP but without ATP and reduced NADP GP will not be used up to make TP.
Decreasing light intensity means less ATP and reduced NADP, so less RuBP because RuBP is still being used up to make GP but RuBP is not being regenerated as GP cannot be made into TP, which is needed to make RuBP.
Carbon dioxide concentration
As carbon dioxide increases TP increases. Because more CO2 is fixed, so more GP is made, so more TP.
As carbon dioxide increases GP increases. Because more CO2 is fixed, so more GP is made.
As carbon dioxide increases RuBP decreases. Because more CO2 is fixed, so more GP is made and more RuBP is used up.
Temperature As temperature increases TP increases. But at high temperatures TP will decrease because the enzyme RuBisCO denatures and less carbon dioxide fixed, so less GP will be made and so less TP is made.
As temperature increases GP Increases. But at high temperatures will decrease because the enzyme RuBisCO denatures and less carbon dioxide fixed, so less GP will be made and so less TP is made.
As temperature increases RuBP decreases because as the rate of enzyme action increases more RuBP is used up. When the RuBisCO denatures at high temperature less RuBP will be used up as CO2 is not fixed.
Outline the light-dependent reactions of photosynthesis (6)
(chlorophyll/antenna) in photosystem II absorbs light;absorbing light/photoactivation produces an excited/high
energy/free electron;electron passed along a series of carriers;reduction of NADP+ / generates NADPH + H+;absorption of light in photosystem II provides electron for
photosystem I;photolysis of water produces H+ / O2;called non-cyclic photophosphorylation;in cyclic photophosphorylation electron returns to chlorophyll;generates ATP by H+ pumped across thylakoid membrane / by chemiosmosis / through ATP synthetase/synthase;
Explain the effect of light intensity and temperature on the rate of photosynthesis.(8)
both light and temperature can be limiting factors;other factors can be limiting;graph showing increase and plateau with increasing light / description of this;graph showing increase and decrease with increasing temperature /description of
this; light:affects the light-dependent stage;at low intensities insufficient ATP;and insufficient NADPHH + H+ produced;this stops the Calvin cycle operating (at maximum rate); temperature:affects light-independent stage / Calvin cycle;temperature affects enzyme activity;less active at low temperatures / maximum rate at high temperatures;but will then be denatured (as temperature rises further);Award [5 max] if only one condition is discussed.