photosynthesis

• describe photoionisation

  • light energy absorbed by chlorophyll molecules boost energy of electron pairs raising there energy level
  • electrons become excited
  • electron pair become so energetic they leave chlorophyll molecule
  • the chlorophyll molecule becomes ionised and also oxidised as it loses an electron pair
• what are the electrons that leave the chlorophyll taken up by ?


  via an electron carrier - which has been reduced as it has gained electrons
• outline chemiosmotic theory

  • The model states that energy from electrons passed through a chain of proteins in the membrane (the electron transport chain) is used to pump protons (hydrogen ions) up their concentration gradient into the intermembrane space
  • The hydrogens are then allowed to flow by facilitated diffusion through a channel in ATP synthase into the matrix
  • The energy of the hydrogens flowing down their concentration gradient is harnessed (a bit like water flowing through a hydroelectric damn) resulting in the phosphorylation of ADP into ATP by ATP synthase
• where do light dependant reactions occur ?

  Thylakoid membranes
• what does photolysis release ?

  • hydrogen ions
  • electrons
  • oxygen
• what process provides energy for ATP synthase to catalyse production of ATP ?

  Chemiosmosis
• what is chemiosmosis ?

  the diffusion of protons down the proton gradient through ATP synthase.
• how is a proton gradient maintained when ATP is being produced so ATP synthase has enough energy ?

  To maintain the proton gradient, protons are actively transported from the stroma to the thylakoid space. This ensures that there’s always a higher concentration of protons in the thylakoid space compared to the stroma.
• what is responsible for maintaining a proton gradient in the light dependant reaction ?

  active transport
• Describe the proton gradient across the thylakoid membrane in the light-dependent reaction of photosynthesis, and explain how it is maintained

  • higher concentration of protons in the thylakoid space compared to the stroma
  • To maintain this proton gradient, there is active transport of protons from the stroma into the thylakoid space
• Describe the process of photoionisation during the light-dependent reaction of photosynthesis

  light energy is absorbed by the chlorophyll electrons absorb light energy and become excited the electrons leave chlorophyll via an electron carrier the chlorophyll becomes oxidised and positively charged this process is known as photoionisation
• Describe the electron transport chain in the light-dependent reaction of photosynthesis

  after photoionisation , electrons move along the collection of protein complexes electron transfer chain via a series of oxidation and reduction reactions when reaching NADP the electrons react with the NADP and a H+ proton that is in the stroma NADP is reduced to NADPH which is a product of a light-dependant reaction
• Describe how electrons lost from chlorophyll during photoionisation are replaced

  • chlorophyll loses an electron pair during photoionisation making chlorophyll positively charged due to oxidation of chlorophyll
  • to replace the electron pair lost light splits water molecules in the thylakoid space
  • undergoing photolyis
  • water molecules break into electrons which replace the electrons lost during photoionisation
  • hydrogen protons which maintain a high concentration gradient of protons in the thylakoid space
  • oxygen which diffuses out or is used in respiration
• What is the first step in the light-dependent reaction of photosynthesis?

  Photoionisation occurs
• What happens during photoionisation in chlorophyll?

  Light energy excites electrons, oxidizing chlorophyll
• What replaces the lost electron pair in chlorophyll?

  Photolysis of water
• What happens to the electron carrier after it receives electrons?

  It becomes reduced
• What is the role of the electron carrier in the light-dependent reaction?

  It passes electrons through protein complexes
• What does NADP become after reacting with a hydrogen proton?

  It becomes reduced NADPH
• What is the remaining energy used for in the light-dependent reaction?

  Active transport of protons to thylakoid space
• What does maintaining the proton gradient allow to occur?

  Chemiosmosis
• Through which enzyme do protons diffuse back into the stroma?

  ATP synthase
• What does chemiosmosis supply energy for?

  Catalyzing the reaction of ADP and Pi
• What is the product formed when ADP and Pi react during chemiosmosis?

  ATP
• What are the main steps of the light-dependent reaction of photosynthesis?

  1. Photoionisation occurs in chlorophyll.
  2. Electrons are excited and oxidize chlorophyll.
  3. Lost electrons are replaced by photolysis of water.
  4. Electrons are carried through protein complexes.
  5. NADP is reduced to NADPH.
  6. Energy is used for active transport of protons.
  7. Proton gradient allows chemiosmosis.
  8. Protons diffuse back via ATP synthase.
  9. ATP is formed from ADP and Pi.
• How are Chloroplasts are Adapted for Photosynthesis ?

  • Adaptation 1 : The thylakoid membrane has a large surface area for molecules involved in the light-dependent reaction. This maximises the amount of ATP and NADPH that can be made at one time.
  • Adaptation 2 : Proteins in the grana hold the chlorophyll in such a way that the maximum amount of light can be absorbed at one time
  • Adaptation 3 : the thylakoid membranes contain ATP synthase for efficient ATP production. The membranes are also selectively permeable which allows them to establish and maintain a proton gradient
  • Adaptation 4: Chloroplasts contain both DNA and ribosomes. This means that proteins involved in the light-dependent reaction can be easily and quickly produced.
• Explain the role of water in the light-dependent reaction

  Photolysis of water provides electrons to replace the electrons lost from chlorophyll during photoionisation. In photolysis, light splits water into electrons, protons and oxygen. Protons help maintain the high concentration of protons in the thylakoid space, and oxygen diffuses out of the plant or is used in respiration
• Describe the reaction in the light-independent reaction of photosynthesis that involves carbon dioxide

  • In the light-independent reaction of photosynthesis, when carbon dioxide enters the stroma, it reacts with a molecule called ribulose bisphosphate (RuBP), which has five carbons.
  • This reaction is catalysed by an enzyme called rubisco and forms two molecules of glycerate 3-phosphate (GP), which have three carbons each.
• Describe the reaction involving glycerate 3-phosphate in the Calvin cycle. Include details about the fate of the products produced in this reaction

  In the Calvin cycle, glycerate 3-phosphate is converted into triose phosphate (TP), which has three carbons. This reaction requires reduced NADP and ATP from the light-dependent reaction.NADP, ADP and Pi​ produced in this reaction return to the thylakoid membrane to be used up in the light-dependent reaction to produce new molecules of reduced NADP and ATP.
• what are organic products that triose phosphate is converted into in the light-independent reaction of photosynthesis?

  Roughly 20% of the triose phosphate molecules made are used to produce a range of useful organic substances, such as glucose, amino acids and glycerol. The rest is converted back into RuBP.
• Describe fully the light-independent reaction of photosynthesis. Include examples of organic substances produced in the reaction in your answer

  carbon dioxide enters the stroma, reacting with ribulose bisphosphate (RuBP), reaction is catalysed by an enzyme called rubisco and forms two molecules of glycerate 3-phosphate (GP), which have three carbons each.Next, glycerate 3-phosphate is converted into triose phosphate (TP), which has three carbons. This reaction requires reduced NADP and ATP from the light-dependent reaction.NADP, ADP and Pi produced in this reaction return to the thylakoid membrane to be used up in the light-dependent reaction to produce new molecules of reduced NADP and ATP.

  organic substance which are formed from 20% of triose phosphate are ; glycerol, amino acids , glucose

  the remaining 80% reforms ribulose biphospate