topic 5

• what is chemiosmosis?

  The movement of ions across a partially permeable membrane, from an area of a high concentration to an area of low concentration
• describe the light-dependent reaction

  The light-dependent reaction of photosynthesis takes place in the thylakoid

  membrane. One important molecule produced in the light-dependent reaction is ATP, which is produced by an enzyme called ATP synthase. To catalyse the production of ATP, energy is provided to the enzyme by the diffusion of protons down a proton gradient via a process called chemiosmosis. The protons move from the thylakoid space into the stroma. This method of producing ATP is known as photophosphorylation.

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

  There is a 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.
• When light hits a leaf, chlorophyll absorbs the light. Light energy is transferred to electrons within chlorophyll, which are said to be in an excited state. After the electrons gain energy, they leave chlorophyll. Therefore, this molecule becomes positively charged as it is oxidised. This process is known as photoionisation.
• Describe the process of photoionisation during the light-dependent reaction of photosynthesis.

  When light hits a leaf, chlorophyll absorbs the light. Light energy is transferred to electrons within chlorophyll, which are said to be in an excited state. After the electrons gain energy, they leave chlorophyll. Therefore, chlorophyll becomes positively charged as it is oxidised. This process is known as photoionisation.

  
• what happens if the electron transfer chain grinds to a halt?

  protons can’t be actively transported into the thylakoid space which means there isn't a proton gradient so ATP cannot be produced
• what is NADP?

  A co-enzyme
• what do electrons at the end of the electron transfer chain react with

  NADP and a proton
• end of the electron transport chain in the light-dependent reaction of photosynthesis

  NADP reacts with an electron and a proton to form NADPH.

  NADP is reduced.

  
• Describe the electron transport chain in the light-dependent reaction of photosynthesis. Include details about what happens to electrons at the end of the chain in your answer.

  After photoionisation, electrons move through a collection of protein complexes and molecules known as the electron transfer chain in a series of oxidation-reduction reactions. Electrons at the end of this chain react with NADP and a proton to form reduced NADP, another product of the light-dependent reaction.
• Describe how electrons lost from chlorophyll during photoionisation are replaced.

  Once electrons in chlorophyll are lost during photoionisation, they are replaced by water in a reaction called photolysis. In this reaction, 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.
• what is another word for reduced NADP?

  NADPH
• Where do the light-dependent reactions of photosynthesis take place?

  In the thylakoid membrane of chloroplasts
• What absorbs light energy during the light-dependent reactions?

  Chlorophyll in a protein complex
• What happens to electrons when light hits chlorophyll?

  They become excited through photoionisation
• What replaces the electrons lost from chlorophyll?

  Electrons from the splitting of water
• What is the process called when water is split to replace electrons in chlorophyll?

  Photolysis
• How do electrons move along the thylakoid membrane during the light-dependent reactions?

  Through the electron transfer chain via oxidation-reduction reactions
• What is released during the oxidation-reduction reactions in the electron transfer chain?

  Energy
• What does the energy released in the electron transfer chain do?

  It actively transports protons into the thylakoid space
• What is maintained by the active transport of protons into the thylakoid space?

  The proton gradient
• What process occurs due to the proton gradient in the thylakoid space?

  Chemiosmosis
• What enzyme allows protons to diffuse back into the stroma?

  ATP synthase
• What reaction does ATP synthase catalyze during chemiosmosis?

  ADP + Pi to form ATP
• What do electrons at the end of the electron transfer chain react with to produce NADPH?

  NADP and a proton
• What is NADPH required for in photosynthesis?

  Light-independent reactions (LIR)
• 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.
• Which of the following are adaptations of the chloroplast for the light-dependent reaction?

  Chloroplasts contain both DNA and ribosomes.

  Chlorophyll is arranged in a way that maximises light absorption.

  Thylakoid membranes are selectively permeable.

  The thylakoid membrane has a large surface area.

  Thylakoid membranes contain ATP synthase.
• what is another name for the light-independent reaction?

  Calvin Cycle
• Describe the reaction in the light-independent reaction of photosynthesis that involves carbon dioxide.

  In the LDR of photosynthesis, when CO2 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.

  
• Which molecule(s) is/are products of the light-dependent reaction that are used in the light-independent reaction?

  ATP and reduced NADP
• 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 PiPi​ 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?

  Glucose, amino acids, glycerol and Ribulose bisphosphate
• The majority of triose phosphate molecules in the Calvin cycle are used to reform ribulose bisphosphate (RuBP). This reaction requires energy in the form of ATP.
• Describe fully the LIR of photosynthesis.

  In the LIR of photosynthesis, carbon dioxide enters the stroma, reacting with a molecule called ribulose bisphosphate (RuBP), which has 5 carbons. This reaction is catalysed by an enzyme called rubisco and forms 2 molecules of glycerate 3-phosphate (GP), which have 3 carbons each.Glycerate 3-phosphate is converted into triose phosphate (TP), which has 3 carbons. This reaction requires reduced NADP and ATP from the LDR NADP, ADP and Pi produced in this reaction return to the thylakoid membrane to be used up in the LDR to produce new molecules of reduced NADP and ATP.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 majority of triose phosphate, however, is used to reform ribulose bisphosphate, ensuring that the LIR can continue. This reaction requires energy in the form of ATP.
• what are the 3 factors affecting the rate of photosynthesis?

  light, carbon dioxide and temperature
• Which stage of photosynthesis is light used in?

  light-dependent reaction
• As the temperature increases, the rate of photosynthesis initially increases, and then decreases. What does temperature affect?

  enzymes in the light-independent reaction.

  molecules in the thylakoid membrane.

  at very high temperatures, proteins in the thylakoid membranes can get damaged, which prevents the electron transport chain from functioning properly. This means the light-dependent reaction also decreases, decreasing the rate of photosynthesis.
• Describe and explain how the structure of a chloroplast is related to its function (4)

  1. Thylakoids are flattened sacs which increase SA for photosynthesis
  2. Chlorophyll absorbs light for photosynthesis
  3. stroma contains enzymes for photosynthesis
  4. stroma contains DNA and ribosomes for protein synthesis
  5. stroma contains starch grains for energy storage
• describe and explain how the structure of a chloroplast is adapted for photosynthesis (6 marks)

  1. large SA in small volume of membrane - thylakoid
  2. which allows LDR of photosystems
  3. contains pigments
  4. which allows maximum absorption of light
  5. grana surrounded by stroma
  6. which allows products of the LDR to pass readily into the stroma for the LIR
  7. chloroplast contains its own DNA and ribosomes
  8. which allows chloroplast to make some of the enzymes it needs for photosynthesis
  9. thylakoids have small internal volume
  10. which maximises the proton gradient