Photosynthesis: Light, Photolysis, and ATP

• Photosynthesis is the process where light energy is transformed into chemical energy, vital for most life in ecosystems
• This chemical energy is then in turn used throughout ecosystems
• Photosynthesis converts carbon dioxide into glucose
• Oxygen is produced as a by-product during photosynthesis
• Chromatography can be used to separate pigments, employing Rf values for identification.

Light Absorption

• Photosynthetic pigments absorb different wavelengths of light
• Absorption and action spectra can be compared
• Limiting factors like light, carbon dioxide and temperature affect the rate of photosynthesis
• Carbon dioxide enrichment experiments predict future photosynthesis and plant growth rates
• Photosystems are pigment arrays that generate and emit excited electrons in photosynthetic membranes, which is more efficient that individual pigment molecules

Photolysis and ATP Production

• Photolysis replaces lost electrons in photosystem II and produces oxygen
• ATP is produced by chemiosmosis through both cyclic and non-cyclic photophosphorylation
• NADP is reduced by photosystem I
• Thylakoids facilitate photolysis, chemiosmosis, and NADP reduction

Carbon Fixation and Calvin Cycle

• Carbon fixation is carried out by Rubisco
• The Calvin Cycle is completed by generating triose phosphate using ATP and reduced NADP and by regenerating RuBP using ATP
• The products of photosynthesis are used to produce a variety of carbon compounds
• Without light, light-independent reactions cannot continue/ occur
• Photosynthesis produces carbon compounds from inorganic molecules, requires light energy, and releases oxygen.
• The chemical equation for photosynthesis is: 6CO2 + 12H₂O + Light → C6H12O6 + 6O2 + 6H₂O

Chloroplast Structure and Function

• The chloroplast is the site of photosynthesis
• Chlorophyll pigments absorb light energy in the chloroplast
• Carbon dioxide and water are converted into glucose and oxygen
• Photosynthesis involves light-dependent reactions where O₂ is produced (photolysis) and light-independent reactions where glucose is produced

Visible Light and Pigments

• Visible light is a range of electromagnetic waves detectable by the eyes
• Violet has the shortest wavelength and red has the longest wavelength
• Chlorophylls absorb other lights more and reflect green light the most
• Accessory pigments like carotenoids absorb other wavelengths
• Chlorophyll a absorbs orange/red and purple light
• Chlorophyll b absorbs yellow and blue light
• All 3 pigments do not absorb green light well.
• The highest rates of photosynthesis occur at blue and red wavelengths

Chromatography Practical

• Leaves are crushed using propanone/acetone to dissolve the pigment out
• A spot of pigment solution is placed at the bottom of the chromatography paper
• Dip the paper in solvent
• As the solvent passes through, it separates the pigments into different layers based on their travel rates
• The Rf value, which is the (distance moved by compound) / (distance moved by solvent), is calculated

Thin-Layer Chromatography

• Thin-layer chromatography uses silica gel, aluminium oxide, or cellulose as the stationary phase
• Compared to paper chromatography, it tends to give better results.

Limiting Factors

• A limiting factor restricts the rate of a reaction when present in a low amount.
• Limiting factors in photosynthesis include temperature, light intensity, and CO₂ concentration
• Design three methods using water plants that you could use to investigate how changing temperature, light intensity or CO₂ concentration affects the rate of photosynthesis.

Investigating Factors with Water Plants

• Use a water plant in all three experiments and count the oxygen bubbles, while controlling all other factors
• Change light intensity by varying the distance of a lamp
• Change the water temperature using a water bath
• Adjust the amount of sodium hydrogen carbonate added to the water to change CO₂ concentration

Carbon Dioxide Enrichment Experiments

• Enclosed greenhouses are a method for investigating increasing carbon dioxide concentrations and plant response
• In FACE (free-air carbon dioxide enrichment) experiments, the effects of elevated CO₂ on plants can be researched
• Enclosed greenhouses elevate CO2 levels by burning fossil fuels and control all other conditions, total biomass or yield of fruits/vegetables is measured under controlled conditions
• In FACE, CO2 is released around a 1-30m circular area, monitored by sensors
• It is very expensive to conduct these studies

Light-Dependent Stage: Photosystems

• Light splits water to form: Oxygen, ATP and Hydrogen ion (NADPH)
• Takes place in the thylakoids
• Light is used to generate ATP and reduced NADP
• Oxygen is produced

Light-Independent Stage: Calvin Cycle

• Takes place in the stroma
• ATP and reduced NADP are used to form carbon compounds (glucose) from CO2

Photosystems.

• Photosystems are molecular arrays of chlorophyll and accessory pigments within protein complexes

Photosystem Types

• Photosystem I is most sensitive to light wavelengths of 700 nm
• Photosystem II is most sensitive to light wavelengths of 680 nm

Photoactivation

• This is caused in the reaction centre
• It results to the emission of an excited electron
• The electron released by Photosystem II.

Advantage of Pigment Molecule Diversity

• Light energy can only be absorbed to photoactivate the central chlorophyll molecule with the use of a variety of pigment molecules

Electron Transport Chain

• The electron is passed along a series of proteins in the membrane
• As they are passed from protein to protein, they drop in energy level and release that energy to pump H+ from stroma into the intermembrane space of the thylakoids
• This process creates a concentration gradient

ATP Generation

• ATP generation occurs because the membrane is impermeable to H+ ions
• Sufficent energy is released to phosphorylate ADP to ATP when H+ gradient diffuse through the ATP synthase complex
• This is photophosphorylation

NADPH Production

• The electrons are then re-excited by light energy absorbed by PSI and passed along a series of proteins.
• The electrons from ferrodoxin + H+ ions convert NADP to reduced NADP
• This non-cyclic photophosphorylation
• The electrons don't return to the PSII

Cyclic Photophosphorylation

• Light causes the excitation of electrons from photosystem I
• Electrons move along the electron transport chain, pumping H+ generating ATP with ATP synthase
• This process returns electrons to PSI after moving along the ETC

Photosystem II Water Photolysis

• The release of an electron from photosystem II at the reaction centre, results in a powerful oxidising agent
• The photolysis of water: 2H2O → 4H+ + O2 + 4e- releases is done via Photosystem II
• The electrons replace those lost by PSII
• H+ are used for the concentration gradient
• O₂ is a waste product and is released.

Light-Independent Stages

• Happens in the stroma
• It requires ATP, CO₂ and H+ (from reduced NADPH)
• ATP is used for energy with CO2
• Rubisco catalyses the reaction
• Carboxylation is adding atmospheric carbon dioxide to ribulose bisphosphate (RuBP) and this is catalysed by rubisco
• Next reduction of glycerate-3-phosphate leads to one glycerate 3-phosphate (GP) is converted to triose phosphate with requires one ATP and one reduced NADP
• Regeneration of RuBP, we are able and This uses up the 12 triose phosphates molecules generated earlier
• It is important that the initial compound (RuBP)
• Uses triose phosphate to produce carbon compounds and it helps maintain the cycle.
• Triose phosphate produce carbon like glucose/starch, Amino acids, Fatty acids and DNA/RNA

Summary for All Students

• Photosynthesis transforms light energy into chemical energy
• Light-dependent reactions involve water splitting to produce hydrogen and oxygen
• Light-independent reactions use hydrogen to convert carbon dioxide into glucose
• Chromatography identifies photosynthetic pigments using absorption and action spectra
• Key limiting factors are carbon dioxide concentration, light intensity, and temperature
• Carbon dioxide enrichment experiments forecast the impact of increased CO₂ levels on plant growth

Summary HL only

• Photosystems contain pigments and proteins that absorb light, leading to electron emission
• Electrons pass along an electron transport chain (ETC), pumping protons into thylakoid space
• Protons flow back through ATP synthase, generating ATP by chemiosmosis, where water is split.
• First carbon is fixed by the enzyme Rubisco as it combines carbon dioxide with RuBP
• Majority of triose phosphate is used to regenerate RuBP for glucose formation
• Also used to build carbohydrates, lipids, amino acids, and nucleic acids
• Reactions of photosynthesis are interdependent, so one will not occur without the other happening.