3.2 PHOTOSYNTHESIS USES LIGHT ENERGY

Photosynthesis equation

6CO₂+ 6H₂O→ C₆H₁₂O₆ +6O₂

Distribution of chloroplasts (light trapping)

Main → Palisade mesophyll

Location → Below upper epidermis→ High light intensity

Maximises light absorption

Spongy mesophyll fewer

Chloroplasts movement why ?

Move and rotate within palisade cell depending on light intensity

Low→ Surface maximum absorption of light intensity

High→ Vertical against cell wall prevent over exposure pigment bleaching

Chloroplast adaptions

Large surface area - max absorption of light

Pigments in thylakoids single layer membrane surface - Maximise light absorption

Grana - large surface area for light dependent

Starch grains - store energy

Leaf structure (top→bottom)

Cuticle

Upper epidermis

Palisade mesophyll

Spongy mesophyll

Lower epidermis

Stoma

Leaf adaptions (light trapping)

Orientate perpendicular to light

Large surface area→ capture more light

Thin leaf→ light penetrate short co2 diffusion distance

Transparent upper epidermis - light penetrate

Palisade near top- lots chloroplasts

Air space spongy mesophyll - rapid gas diffusion

Stomata → co2 entry o2 exit

Chloroplasts as tranducer

Change energy from one form to another

Light photon energy → chemical energy (ATP)

• Light absorbed by pigments (in thylakoid membranes ) drive photosynthesis

Photosynthetic pigments

Absorb light energy at particular wavelengths of light

Different pigments absorb different wavelengths

Chlorophyll a, b carotene xanthophylls

Where are pigments found

Thylakoid membranes of chloroplasts

Advantage of having lots of pigments

Greater rage of wavelengths absorbed

More photons absorbed more products of light dependent made

Greater rate of photosynthesis

Chromatography of leaf pigments

Separate pigments to identify

Principle: Pigments move at different rates depending on solubility in solvent (higher Rf more soluble)

Rf Value

Distance moved by pigment / Distance moved by solvent

Compare calculated Rf with known to identify

Always <1

Light harvesting

pigments absorb light - antenna complex

• Accessory (broad spectrum)

• Chlorophyll a reaction centre - energy transferred maximise light capture efficiency

Reaction centre

2 Chlorophyll a pigments (primary pigments)

Excited chlorophyll emits high energy electrons → start light dependent

Photosystems

Photosystem 1 absorption peak 700nm (far red)

Photosystem 2 absorption peak 680nm (red)

Thomas Englemann - spirogyra

Determine wavelengths most used

Algae spirogyra suspension motile aerobic bacteria

Prism refract white light → rainbow`

• Wavelengths making most oxygen corespond

• Bacteria migrate towards region with highest oxygen concentration (blue and red) most photosynthetic activity

Absorption spectrum

Amount of light absorbed (by pigment) against different wavelengths

Action spectrum

Rate of photosynthesis at different wavelengths of light

Absorption and action close correlation

Suggest wavelengths absorbed by pigments used for photosynthesis

What prevents light escaping when absorbed

Proteins in antenna complex prevent light energy escaping

Light dependent photophosphorylation - Thylakoid membrane

Light absorbed Photosystem II (PSII) Electrons excited (gain energy) leave

Pass ETC Energy released → ATP (photophosphorylation)

Electrons lost from PSII are replaced by photolysis of water

• Water → O₂ + H⁺ + e⁻

Photophosphorylation

Addition of phosphate + ADP → ATP by light energy

Cyclic photophosphorylation

PSI absorbs light → electrons excited (chlorophyll a)

Electrons → electron acceptor → ETC

Energy released → proton gradient → ATP (chemiosmosis)

Electrons return to PSI (lower energy)

Non cyclic z scheme

• Light absorbed by PSII → electrons excited (chlorophyll a)

• Electrons → electron acceptors → ETC

• Energy → proton pumping (stroma → thylakoid) → ATP

• Photolysis of water replaces lost electrons

  • H₂O → 2H⁺ + 2e⁻ + ½O₂ (O₂ released)

• Light absorbed by PSI → electrons re-excited

• Electrons → electron acceptor → NADP

• NADP + 2e⁻ + H⁺ → NADPH

Photolysis of water

Water in thylakoid space split

H20 → 2H+ 2e- ½O2
electron removed replace lost by the chlorophyll photosystem 2

light is responsible only indirectly for splitting water.

Light independent - Stroma

Consume Co2 energy from ATP and reduced NADP and organic chemicals, (carbohydrates are produced)

Calvin cycle- light independent stage

• Uptake CO2 by 5c Ribulose bisphosphate - Rubisco → unstable 6c

• Splitting occur 2× 3c glycerate-3 phosphate

• ATP and reduced NADP (from light dependent used) reducing

• 2x Triose phosphate 3c (glucose→ starch)

• Regeneration ribulose phosphate 5c → ribulose bisphosphate (require ATP)

What is needed for the calvin cycle?

• 2 ATP ( glycerate3phosphate → triose phosphate ) (ribulose phosphate → ribulose bisphosphate)

• Reduced NADP ( glycerate 3 phosphate → triose phosphate)

What happens to ADP and NADP after the calvin cycle?

Return back to the light dependent stage (thylakoid membrane ) to reform ATP and reduced NADP

Product synthesis carb lipids proteins (calvin cycle products)

Carbohydrates

• Glucose (fructose bisphosphate) → starch (alpha) cellulose cell walls

Lipids

Acetyl coenzyme A (from Glycerate 3phosphate) → fatty acids → triglycerides

Proteins

GP → amino acids Nitrogen derived from NH4+ and NO3- absorbed by root hair cells active transport

Calvin 14Co2 experiment

Chlorella exposed 14CO2

Samples at time intervals

Hot ethanol stop enzymatic reactions

Radioactive compounds separated by chromatography

Dark large spot - more present GP TP carbon fixation

Interpreting autoradiographs

1. Describe -Spot size and darkness

2. Compare

5s: Mainly GP (early product) small TP

30s More GP and TP amino acids and sucrose

3. Explain: Gp before TP (Amino acids require GP) Sugars

4.Starch lipids proteins

Limiting factor definition

Factor limit rate of physical process by being short supplly

Carbon dioxide as limiting factor

Carbon source for calvin cycle

Low Co2 → RuBP cant fix Co2 → Less GP

TP and glucose slow

Light intensity as limiting factor

• Darkness light independent reactions can’t occur no oxygen evolved

• If light intensity higher than optimum, rate of photosynthesis decrease pigments damaged, wont absorb light efficiently

Low light → Less ATP and reduced NADP → Slow calvin cycle

Need to excite electrons

Light compensation point

Light intensity at which plant has no net gas exchange volume produced (respiration) released (photosynthesis) equal

Sun and shade plants

Shade plants

• Low LCP → photosynthesis efficiently under low light

Sun plants

• Higher LCP → need more light

Temp

Optimum around 25 after enzyme denature rate of photosynthesis decreases

Effect enzyme activity

• Rubisco and ATP synthase low temp too slow

Minerals

Inorganic ions

Macronutrients needed in substantial quantities (magnesium copper)

Micronutrients needed in tiny amounts (manganese copper)

Nitrogen

Absorbed root hair cells active transport as NO3-

In xylem as amino acids

Nucleic acids, amino acids, nucleotides

Deficiency

• Reduced growth of all organs

• Chlorosis yellowing

Magnesium

Absorbed ad Mg2+ → xylem

Need for chlorophyll (avoid chlorosis)

How many times does the calvin cycle need to occur to form one molecule of glucose?

6 times

1/6 carbon leave for carbohydrate

If inhibitor blocks electrons enter ETC after ps2 what happens ?

Stop electrons ps2→ ps1

Block nadp reduction

Cyclic only ps1 can occur as ps1 → ps1 carrier involved not affected

Calvin cycle in dark and light

• Initally Co2 and RuBO continues

• GP cant be converted to TP

• No ATP and reduced nadp only made in light dependent

• Less TP for regeneration and glucose

• Rate of co2 and RuBp decreas

Sweetness when photosynthesis rate higher than respiration

Higher rate of photosynthesis

More sugar is produced by photosynthesis than used in respiration

More sugar stored sweeter