Photosynthesis

• thylakoids are flattened sacs of membrane containing chlorophyll found in chloroplasts.

  they are the site of the light-dependent reaction for photosynthesis
• Grana are stacks of thylakoids found in the chloroplasts
• stroma?

  liquid found in chloroplasts that is the site for the light-independent reaction
• Order of photosynthesis:

  light-dependent reaction -> light-independent reaction

  the light dependent reaction produces molecules that are needed in the light independent reaction
• The light dependent reaction occurs between the thylakoid space and stroma where protons go from the former to the latter and are actively transported back using an electron chain reaction but with a different way to start it.

  (very similar to the last stage of aerobic respiration)
• the thylakoid membrane contains 2 photosystems:

  photosystem one is cyclic

  photosystem two is non-cyclic
• Chloroplasts are adapted to their function by:

  • Thylakoid membrane has a large surface area
  • membrane and ATP synthase and selectively permeable membrane for atp synthesis
  • Contains DNA and ribosomes to synthesis the required proteins like atp synthase
• Light-independent reactions occur in the stroma, they use carbon dioxide and produce glucose and oxygen
• Light-dependent Reaction

  1. Produces ATP + NADP

  2. Occurs between stroma and thylakoid space via thylakoid membrane

  3. Process is called photophosphorylation

  4. Proton gradient: Thylakoid space → Stroma

  5. NADP is in plants and helps with the ETC
• Photosystems

  Protein complexes embedded in the thylakoid membrane that contain chlorophyll and pigments to absorb light energy, which is then used to excite electrons in the chlorophyll
• Photosystem 2 (non-cyclic)

  1. Chlorophyll receives light

  2. Electrons within become excited

  3. Electrons leave chlorophyll leaving it positive

  4. Electrons go through transport proteins via oxidation-reduction reactions

  5. NADP takes electrons that have reached the end of the electrons transfer chain becoming reduced NADP
• Photolysis

  1. Water → protons + oxygen + electrons

  2. Electrons replenish the chlorophyl of photosynthesis site 1

  3. Protons help maintain steep concentration gradient in the thylakoid

  4. Oxygen either diffuses out or is used in respiration
• Photosynthesis site one (cyclic)

  1. Light excite the electrons in the chlorophyll and sends them to the beginning of the electron transfer chain

  2. Releases energy for the active transport of protons

  3. Electrons make it back to photosynthesis site one and do not result in the formation of NADPH

  4. Helps form ATP
• Chloroplast Adaptations

  • Thylakoid membrane has a large SA for molecules involved in photophosphorylation
  • Proteins in the grana hold proteins in a way to maximise the amount of sunlight that can be absorbed at a time
  • Thylakoid membranes contain ATP synthase for the synthesis of ATP
  • Thylakoid membranes are selectively permeable which allows them to establish a proton gradient
  • Chloroplasts have DNA and ribosomes to easily produce the necessary proteins for the light dependant reaction
• Light-independent Reaction/ Calvin cycle

  1. Carbon dioxide enters the stroma

  2. Ribulose bisphosphate + Carbon dioxide —(rubisco)→ Glycerate 3-phosphate

  3. Glycerate 3-phosphate + NADPH → Triose phosphate + NADP

  4. Energy is required and provided via ATP → ADP + Pi

  5. 80% of Triose phosphate is converted back into ribulose bisphosphate

  6. 20% of Triose phosphate is converted into Glucose, Amino acids, Glycerol
• The products the light-independent reaction needs from the light-dependent reaction is:

  • ATP
  • NADPH
• What is what is produced in the calvins cycle used for?

  glucose - respiration

  amino acids - nucleic acids, proteins

  glycerol - forming triglycerides
• Factors affecting photosynthesis:

  temperature - if too high then enzymes in the ETC in the light-dependent reaction and rubisco in the light-independent reaction will be denatured

  Light intensity - the greater the light intensity the more products that are produced from the light-dependent reaction which means more calvin cycles can occur so even though it is a light independent reaction it still is affected by light

  CO2 concentration - the more CO2 there is the faster the conversation of ribulose bisphosphate to glycerate 3-phosphate