Thylakoids

• Plant cells contain?chloroplasts?which is the site of photosynthesis
• Chloroplasts are filled with a fluid known as the?stroma
• The?system of membranes?found in the stroma of the chloroplast consists of a series of flattened fluid-filled sacs known as?thylakoids
• In places, these thylakoids stack up to form structures known as?grana?(singular – granum)
• The?light-dependent stage?of photosynthesis occurs in the?thylakoid membranes?and the?thylakoid spaces?(the spaces inside the thylakoids)
• The thylakoid membranes contain the?pigments,?enzymes?and?electron carriers?required for the light-dependent reactions
  • These proteins and pigments make up an electron transport chain used to carry electrons through the membrane
  • Electron energy can be used to move H--+?ions across the membrane
  • Electrons are donated by water molecules and accepted by NADP molecules that form NADPH

   

• The membranes of the grana create a?large surface area?to?increase the number of light-dependent reactions?that can occur
• This membrane system provides a large number of pigment molecules in an arrangement that ensures?as much light as necessary is absorbed
• The pigment molecules are arranged in light-harvesting clusters known as?photosystems

Photosystems are funnel-like structures found in the thylakoid membrane. They are made from accessory pigments that absorb light energy and transfer it to the primary pigment reaction centre.

Photosystems

• Photosystems?are collections of photosynthetic pigments that absorb light energy
• In a photosystem, the different pigment molecules are arranged in?funnel-like structures?the thylakoid membrane
  • Each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the?primary pigment reaction centre

   

• Photosystem II?has a primary pigment that absorbs light at a wavelength of 680nm and is therefore called?P680
  • Photosystem II is at the beginning of the electron transport chain and is where the photolysis of water takes place

   

• Photosystem I?has a primary pigment that absorbs light at a wavelength of 700nm and is therefore called?P700
  • Photosystem I is in the middle of the electron transport chain

   

• When the light energy absorbed by the pigments is passed to the primary pigment,?chlorophyll, this causes the?photoionisation of chlorophyll
  • Photoionisation is when a chlorophyll pigment becomes positively charged as a result of losing two electrons when it absorbs light energy
  • The two electrons are lost because they are excited by the light energy and move to a higher energy level

   

The Light-Dependent Reaction

• Light energy is used to breakdown water in a reaction known as?photolysis;?this produces hydrogen ions, electrons, and oxygen in the?thylakoid lumen
• A?proton gradient?is formed as the?photolysis of water?results in a high concentration of hydrogen ions in the thylakoid lumen
• Electrons travel through an?electron transport chain?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?(ADP + Pi?→ ATP) using the proton gradient between the thylakoid lumen and stroma to drive the enzyme ATP synthase

Photosystem II

• Light is absorbed?by?photosystem II?(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 chain, before being passed on to photosystem I
• During this process?chemiosmosis?occurs:
  • The energy given by the electrons moving through the electron transport chain enables H--+?ions (protons) to pass from a low concentration in the stroma to a high concentration in the thylakoid lumen
  • The creation of this proton gradient across the membrane later drives the synthesis of ATP in?photophosphorylation

   

• Photosystem II contains a?water-splitting enzyme?called the?oxygen-evolving complex?which catalyses the breakdown (photolysis) of water by light:

H2O → 2H+?+ 2e-?+ ?O2

• As the excited electrons?leave?the primary pigment of photosystem II and are passed on to photosystem I, they are?replaced?by?electrons?from the?photolysis?of water

Photosystem I

• At the?same time?as photoactivation of electrons in photosystem II,?electrons in?photosystem I also undergo photoionisation
• The excited electrons from photosystem I also pass along an?electron transport chain, alternatively?reducing and oxidising proteins?as they are accepted then passed on
• These electrons?combine?with?hydrogen ions?(produced by the photolysis of water and transported out of the thylakoid lumen by ATP synthase) and the?carrier molecule NADP?to give?reduced NADP:

2H+?+ 2e-?+ NADP →?reduced NADP

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

Photophosphorylation & Chemiosmosis

• Photophosphorylation is the name for the overall process of using light 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 chain of electron carriers (known as the?electron transport chain)
• The electron carriers are alternately?reduced?(as they?gain?an electron) and then?oxidised?(as they?lose?the electron by passing it to the next carrier)
• The excited electrons?gradually release their energy?as 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 across 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 diffusion?through?transmembrane?ATP synthase?enzymes in a process known as?chemiosmosis
• This process provides the energy needed to?synthesise ATP?by adding an inorganic phosphate group (Pi) to ADP (ADP + Pi?→ ATP)
• The whole process is known as?photophosphorylation?as?light?provides the initial?energy source?for ATP synthesis

Light Dependent Photophosphorylation leads to the production of ATP and NADP