Light Dependent Stage Of Photosynthesis

The Lite-Dependent Phase

• Photosynthesis occurs in two stages: the lite-dependent stage, which takes place in the thylakoids, and the light-contained phase, which takes place in the stroma
• During the lite-dependent stage of photosynthesis:
  • Light energy is used to breakdown water (photolysis) to produce hydrogen ions, electrons and oxygen in the thylakoid lumen
  • A proton gradient is formed due to the photolysis of water resulting in a high concentration of hydrogen ions in the thylakoid lumen
  • Electrons travel through an electron transport concatenation of proteins within the membrane
  • Reduced NADP (NADPH) is produced when hydrogen ions in the stroma and electrons from the electron transport chain combine with the carrier molecule NADP
  • ATP is produced during a process known as photophosphorylation

Photophosphorylation & chemiosmosis

• Photophosphorylation is the name for the overall process of using light free energy and the electron transport chain to phosphorylate ADP to ATP
  • The light-dependent reaction is sometimes called 'photophosphorylation'
• During photophosphorylation, energetic (excited) electrons are passed along a concatenation of electron carriers (known every bit the electron transport concatenation)
• The electron carriers are alternately reduced (equally they gain an electron) and so oxidised (as they lose the electron by passing information technology to the next carrier)
• The excited electrons gradually release their energy equally they pass through the electron transport chain
• The released energy is used to actively transport protons (H⁺ ions) across the thylakoid membrane, from the stroma (the fluid within chloroplasts) to the thylakoid lumen (the space within thylakoids)
  • A 'proton pump' transports the protons beyond the thylakoid membrane, from the stroma to the thylakoid lumen
  • The energy for this active transport comes from the excited electrons moving through the electron transport chain
• This creates a proton gradient, with a high concentration of protons in the thylakoid lumen and a low concentration in the stroma
• Protons then return to the stroma (moving down the proton concentration gradient) by facilitated improvidence through transmembrane ATP synthase enzymes in a process known equally chemiosmosis
• This process provides the free energy needed to synthesise ATP by adding an inorganic phosphate group (P_i) to ADP (ADP + P_i → ATP)
• The whole process is known as photophosphorylation as light provides the initial energy source for ATP synthesis
• The photophosphorylation of ADP to ATP tin can be cyclic or non-cyclic, depending on the pattern of electron flow in photosystem I or photosystem II or both
  • In cyclic photophosphorylation, just photosystem I is involved
  • In non-cyclic photophosphorylation, both photosystem I and photosystem II are involved
• Photosystems are collections of photosynthetic pigments that absorb calorie-free free energy and transfer the free energy onto electrons, each photosystem contains a primary pigment
  • Photosystem II has a primary pigment that absorbs light at a wavelength of 680nm and is therefore chosen P680
  • Photosystem Two is at the offset of the electron ship chain and is where the photolysis of water takes place
  • Photosystem I has a master pigment that absorbs light at a wavelength of 700nm and is therefore called P700
  • Photosystem I is in the eye of the electron transport chain
  • The free energy carried by the ATP is then used during the light-independent reactions of photosynthesis

Circadian photophosphorylation

• Cyclic photophosphorylation involves photosystem one (PSI) only
• Light is captivated past photosystem 1 (located in the thylakoid membrane) and passed to the photosystem I primary pigment (P700)
• An electron in the principal pigment molecule (ie. the chlorophyll molecule) is excited to a higher energy level and is emitted from the chlorophyll molecule in a process known every bit photoactivation
• This excited electron is captured by an electron acceptor, transported via a chain of electron carriers known equally an electron send chain earlier being passed back to the chlorophyll molecule in photosystem ane (hence: cyclic)
• Equally electrons pass through the electron send chain they provide free energy to transport protons (H⁺) from the stroma to the thylakoid lumen via a proton pump
• A build-upward of protons in the thylakoid lumen can then be used to drive the synthesis of ATP from ADP and an inorganic phosphate group (P_i) past the process of chemiosmosis

Cyclic photophosphorylation

Non-cyclic photophosphorylation

• Lite is absorbed past photosystem 2 (located in the thylakoid membrane) and passed to the photosystem ii primary pigment (P680)
• Two electrons in the primary pigment molecule (ie. the chlorophyll molecule) are excited to a higher energy level and are emitted from the chlorophyll molecule in a process known as photoionisation
• Each excited electron is passed down a chain of electron carriers known as an electron transport concatenation, before being passed on to photosystem 1
• During this process chemiosmosis occurs:
  • The energy given past the electrons moving through the electron ship chain enables H­­⁺ ions (protons) to laissez passer from a low concentration in the stroma to a high concentration in the thylakoid lumen
  • The creation of this proton gradient across the membrane after drives the synthesis of ATP in photophosphorylation
• Photosystem 2 contains a water-splitting enzyme called the oxygen-evolving complex which catalyses the breakdown (photolysis) of h2o by light:

H₂O → 2H⁺ + 2e^- + ½O₂

• Equally the excited electrons go out the main pigment of photosystem 2 and are passed on to photosystem 1, they are replaced past electrons from the photolysis of water
• At the same time as photoactivation of electrons in photosystem 2, electrons in photosystem 1 (PSI) as well undergo photoionisation
• The excited electrons from photosystem 1 besides laissez passer along an electron ship chain, alternatively reducing and oxidising proteins as they are accepted so passed on
• These electrons combine with hydrogen ions (produced by the photolysis of water and transported out of the thylakoid lumen past ATP synthase) and the carrier molecule NADP to give reduced NADP:

2H⁺ + 2e^- + NADP →  reduced NADP

• The reduced NADP (NADPH) and then passes to the light-independent reactions to be used in the synthesis of carbohydrates
• The electrons lost by photosystem 1 are replaced by the de-energised electrons from photosystem 2

Light-dependent photophosphorylation leads to the product of ATP and NADP

Comparison of Circadian Photophosphorylation & Not-cyclic Photophosphorylation Tabular array

Exam Tip

Make sure you lot know the deviation between the two forms of photophosphorylation!Cyclic photophosphorylation differs from non-cyclic photophosphorylation in ii key means:

• Cyclic photophosphorylation just involves photosystem I (whereas non-circadian photophosphorylation involves photosystems I and II)
• Circadian photophosphorylation does not produce reduced NADP (whereas not-cyclic photophosphorylation does)

Light Dependent Stage Of Photosynthesis,

Source: https://www.savemyexams.co.uk/a-level/biology/ocr/17/revision-notes/5-communication-homeostasis--energy/5-6-photosynthesis/5-6-5-the-light-dependent-stage/

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