Light dependent

• Light dependent reaction on thylakoid membrane in chloroplast
• In the light-dependent reaction, chlorophyll in photosystem II absorbs light energy, exciting electrons to a higher energy level, releasing them from chlorophyll, a process known as photoionisation.
• Some energy from electrons released during photoionisation is conserved in the production of ATP and reduced NADP.
• In the production of ATP, electrons pass down the electron transfer chain (electron carriers) from photosystem II to photosystem I via redox reactions, losing energy at each step.
• The energy used to actively transport protons from the stroma into the thylakoid is created by electrons passing down the electron transfer chain.
• This energy is used to create a proton / electrochemical gradient across the thylakoid membrane, which is higher in the thylakoid than in the stroma.
• Protons move by facilitated diffusion down the electrochemical gradient into the stroma via the enzyme ATP synthase embedded in the thylakoid membrane.
• Energy from this allows ADP + Pi to form ATP, a process known as photophosphorylation.
• In the production of reduced NADP, electrons in photosystem I are excited and transferred to NADP, reducing NADP to form reduced NADP.
• Electrons replace those lost from chlorophyll.
• Products of the light dependent reaction

  • ATP go to light independent reaction

  • Reduced NADP go to light independent reaction

  • Oxygen leaves cell as a by-product or used in respiration
• The Light-Dependent Reaction

  • The light-dependent reaction is the first step of photosynthesis
  • This takes place in the Thylakoid membranes of chloroplasts and involves the green pigment chlorophyll
  • Chlorophyll is found in structures called photosystems that are embedded in the thylakoid membrane
  • Between the photosystems is a system of protein channels called electron carriers that make up the electron transport chain
  • In the light-dependent reaction, ATP, reduced NADP, and 02 are made by phosphorylation
• Non-cyclic phosphorylation forms ATP, reduced NADP, and 02.
• Light energy strikes photosystem 2 (PSIl) and is absorbed.
• The light energy excites electrons in the chlorophyll inPSII.
• These electrons, now at a higher energy level, move down the electron transport chain to photosystem 1 (PSI)
• As they move down the electron transport chain, they lose energy that is used to actively transport H+ ions into the thylakoid against their concentration gradient.
• The H+ ions flow down their concentration gradient through the ATP synthase enzyme channel which causes ADP and a phosphate group (Pi) to be combined to form ATP.
• The H+ ions bond to NADP to form reduced NADP (NADPH).
• Non-cyclic phosphorylation only forms ATP.
• Light strikes PSI.
• The electrons are moved in a loop from PSI to the electron carriers and back again.
• This process only forms small amounts of ATP.