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.

ATP Synthase

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

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter39/cyclic_and_noncyclic_photophosphorylation.html

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

http://www.youtube.com/watch?v=Gm7U3G0I1IU

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.

Review

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.