8.3 PHOTOSYNTHESIS

Photosynthesis: Is a metabollic pathway. Carbon dioxide along with water is used to produce carbohydrates. Oxygen is released as a waste gas

                               sunlight

6CO2 + 6H2O _______________ C6H12O6 + 6O2

Photosynthesis occurs in a Chloroplast

• Stroma: Light independent reactions

70S ribosomes and naked DNA

• Thylakoid membranes: Light dependent reactions

Starch grains and oil droplets are products of photosynthesis and acts as chemical energy stores

 Conversion of light energy into a flow of excited electrons

The following processes take place:

• Photoactivation
• Photolysis
• Electron Transport Chain
• Chemiosmosis
• ATP synthesis
• NADP reduction

Two types photosystems are embedded in the Tylakoid membrane. Photosystem i and Photosystem 2. The difference between the two photosystems:

• Photosystem i is sesitive to light wavelengths of 700 nm
• Phtosystem ii is sesitive to light wavelengths of 680 nm

Photosystem: Clusters of hundreds of photosynthetic pigments that collect light energy (including wavelengths) and pass it to Chlorophyll at the reaction centre

Reaction Centre: A central Chlorophyll molecule

• Chlorophyll is part of a group of pigment molecules in photosytems

• Pigments in photosystems (chlorophyll) absorb photons causing electrons within the pigment to be raised to a higher energy level/become excited

• Excited electons are passed from pigment to pigment till the reach Chlorophll A in the reaction centre of the photosystem

• The electrons are then passed to electron acceptors in the Tylakoid membrane

• A photon of light is absorbed by a pigment in Photosystem ii and transferred to other pigment molecules until it reaches one of the Chlorophyll A molecules in the reaction centre

• The photon energy excites one electron within the Chlorophyll A so that it is raised to a higher energy state

• This electron is captured by the primary acceptor of the reaction centre

• Photosystem ii replaces the excited electron before any more photons are absorbed through the process of Photolysis

• Photolysis: Using light water is split by an enzyme to produce electons [given one by one to chlorophyll A in reaction centre] hydrogen ions [contribute to proton gradient] and an oxygen atom [waste gas]

• The excited electrons pass from the primary acceptor down an electron transport chain (though a chain of carriers) loosing energy at each exchange (as they pass each carrier)

• Enough energy is released to pump protons across the Thylakoid membrane from the stoma into the space or lumen inside Tylakoid creating a proton gradient

• ATP Synthase diffuses protons across membrane back into Stroma and uses the energy of the protons to produce ATP

Chemiosmosis: Generation of ATP using energy released by movement of hydrogen ions across membrane

Photophosphorylation: Production of ATP in Chloroplasts

Energy is needed - this energy is obtained by absorbing light

• At the end of the Electron Transport Chain electrons are passed to Photosystem i

• A pair of excited electrons is emitted from the reaction centre of Photosystem i and passes along a chain of electron acceptors

• At the end of the chain electrons are passed to NADP Reductase in membrane

• NADP is reduced to NADPH by accepting the two electrons from Photosystem 1 and two Hydrogen ions from Stoma

Light energy is used to split water, releasing H+ which can be used by ATP Synthase to produce ATP

NADP is reduced to NADPH and H+

Light Dependent Reaction Products: ATP and NADPH (used in light independent reactions)

Oxygen is released as a waste gas

The light-dependent reactions in photosynthesis take place on the thylakoid membranes. Explain the light-dependent reactions

• Pigments in Photosystem ii absorb light
• Photoactivation produces an excited electron
• Electons pass down Electron Transport Chain
• Protons are pumped into thylakoid space
• ATP produced (by light dependent reactions)
• ATP production by chemiosmosis/ATP Synthase
• Electrons from Photosystem ii passed to Photosystem i
• Photoactivation excites electrons in Photosystem i (to higher energy level)
• Production of NADPH using electrons from Photosystem i
• Electrons from Photolysis needed for Photosystem ii
• Oxygen from Photolysis is a waste product
• In cyclic photophosphorylation electrons from Photosystem i return to it

Occurs in Stroma

• Carbon Fixation

Carboxylation: CO2 combines with RuBP (5 carbon sugar) this is catalyzed by enzyme Rubisco

Product (6 carbon) is unstable and splits into 2 three carbon molecules (glycerate 3 phosphate)

First step coverts CO2 to organic compounds

• Reduction of Glycerate 3 Phosphate

Glycerate 3 Phosphate is convereted into a carbohydrate by reduction

Hydrogen from NADPH

Energy supplied by ATP

Product: Triose Phosphate

• Releasing a molecule of Triose Phosphate

Triose Phosphate can be convereted into many carbohydrates

(glucose produced by linking 2 triose phosphates)

Starch (storage form of carbs) created by linking many glucose phosphates

• Regeneratation of RuBP

An RuBP must be produced to replace what has been lost

Some Triose Phosphate are converted back to RuBP this requires ATP

For ever 6 molecules of Triose Phosphate 5 must be convereted back into RuBP

• RuBP a 5 carbon compound binds to an incoming carbon dioxide molecule in a process called carbon fixation. This fixation is catalysed by the enzyme Rubisco. The result is an unstable 6 carbon compound

• The unstable 6 carbon compound breaks down into two 3-carbon compounds called glycerate 3 phosphate

• The 3 carbon molecules of GP are acted upon by ATP and NADPH from the light dependent reacton to form two other 3 carbon molecules called Triose Phosphate. This is a reduction reaction

• The molecules of TP may go in either two directions. Some leave the cycle to become sugar phosphates that may become more complex carbohydrates. Most however continue in the cycle to reproduce the origniating compound of the cycle RuBP

• In order to regain RuBP molecules from TP the cycle uses ATP

Explain how the light-independent reactions of photosynthesis rely on light dependent reactions

• Light causes photoactivation/excitation of electrons
• This leads to the generation of both ATP and NADPH in the light dependent reactions
• The flow of electrons cause pumping of protons into thylakoid; ATP formation when protons pass back across thylakoid membrane. ATP needed to regenerate RuBP for light dependent reactions. The photo activated electons are passed to NADP reducing it to NADPH
• The light independent reaction fixes CO2 to make Glycerate 3 Phosphate; Glycerate 3 Phosphate becomes reduced to Triose Phosphate
• This reduction uses both ATP and NADPH