Module 5.2.1- Photosynthesis

• Interrelationship between the process of photosynthesis and respiration


  Respiration reactants= Photosynthesis products

  Respiration products= Photosynthesis reactants

  Respiration- releases energy

  Photosynthesis- traps energy
• Chloroplast Structure- Network of membranes


  Network of membranes provide a large surface area to maximise the absorption of light in first stage in photosynthesis
• Chloroplast Structure- Outer membrane


  Separates intermembrane space from cytosol. Part of the chloroplast envelope.
• Chloroplast Structure- Lamellae


  A sheet like membrane found within a chloroplast. They are membranous channels which join grana together.
• Chloroplast Structure- Grana


  Thylakoids stacked together.
• Chloroplast Structure- Thylakoids


  Provides a large surface area for the photosynthetic pigments. Site of light-dependent reactions. Contains chlorophyll that is used to absorb light.
• Chloroplast Structure- Stroma


  Contains enzymes, light independent reactions take place here. Site of chemical reactions resulting in the formation of complex organic molecules.
• Chloroplast Structure- DNA


  Contains the genetic info that allows the chloroplast ribosomes to produce proteins needed for photosynthesis.
• What is the site of the light dependent reactions?

  thylakoids
• What is the site of the light independent reactions?

  stroma
• Chlorophyll a


  The primary pigment. Absorbs red and blue light reflecting green light. (pigment in plants that absorbs light energy)
• Name other types of pigments

  Chlorophyll b, xanthophylls, carotenoids (accessory pigments). They all absorb different wavelengths of light that chlorophyll a does not absorb.
• What is a light harvesting complex?

  chlorophyll b, xanthophyll's and carotenoids are embedded in the thylakoid membrane of chloroplast to form a light-harvesting system with other proteins.

  Absorbs different wavelengths of light energy and transfer this energy to the reaction centre.
• What is in the middle of the reaction center and what is its function.

  Chlorophyll a

  Reactions of photosynthesis take place here.
• Photosystem


  cluster of pigments and proteins found in thylakoids. Light harvesting system and reaction center collectively known as a photosystem.
• Summary of the light-dependent stage

  Light energy is absorbed

  ATP, NADP is reduced
• Light dependent stage- Non cyclic photophosphorylation


  Both Photosystems involved (PSII -> PSI)

  
• Non-cyclic photophosphorylation- stage 1

  1) The light absorbed excites electrons at the reaction centres of the photosystem
• Non-cyclic photophosphorylation- stage 2 after electrons are excited

  2) they are released from from reaction centre of PSII and are passed to an electron transport chain to produce ATP by chemiosmosis.
• How are electrons lost from the reaction centre during non-cyclic photophosphorylation replaced in PSII

  Using electrons from the photolysis of water
• Non-cyclic photophosphorylation- stage 3 after electrons are excited in PSI

  3) electrons are released from the reaction centre to another electron transport chain to produce more ATP by chemiosmosis
• How are electrons lost from the reaction centre during non-cyclic photophosphorylation replaced in PSI

  Using electrons from PSII
• Non-cyclic photophosphorylation- stage 4 as electrons leave the electron transport chain


  4) electrons leaving are accepted by coenzyme NADP to form NADPH
• Photolysis


  Using light, water is split to form oxygen, hydrogen ions, and electrons

  
• What happens to H+ ions formed from photolysis


  Protons are released into the lumen of the thylakoids, increasing proton concentration across the membrane. They come back down the gradient to form more ATP. Once back they combine with NADP and electron from PSI to form NADPH
• Cyclic-photophosphorylation


  electrons leaving PSI can be returned back to PSI instead of forming NADPH. No need for electrons from PSII when this happens

  
• Differences between cyclic photophosphorylation and non-cyclic photophosphorylation

  Non-cyclic photophosphorylation: ATP and NADPH produced. Both PSI AND PSII involved.

  Cyclic photophosphorylation: ATP ONLY produced. PSI ONLY involved.
• Light-independent stage


  Second stage of photosynthesis. Involves using ATP, reduced NADP (products of light dependent reactions) and carbon dioxide (raw material) to make organic molecules.
• Light-independent stage- Calvin cycle: how does carbon dioxide enter (raw material)


  Carbon dioxide enters the intercellular spaces within the spongy mesophyll of leaves by diffusion from the atmosphere through the stomata
• Calvin cycle- stage 1 carbon dioxide combines with...

  ... ribulose bisphosphate (RuBP-5C). Carbon dioxide is therefore FIXED.
• Carbon fixation


  The initial incorporation of carbon into organic compounds.
• Calvin cycle- stage 2 what enzyme catalyses the reaction with carbon dioxide and what is formed?

  2) RuBisCO catalyses this reaction to form an unstable 6-carbon intermediate.
• Calvin cycle- stage 3 upon forming the 6-carbon intermediate


  3) It immediately breaks down to for two 3-carbon glycerate-3-phosphate (GP) molecules
• Calvin cycle- stage 4 what are the 2 GP molecules converted into and how

  4) converted into 2 triose phosphate molecules using ATP and a hydrogen atom from NADPH. (products of light-dependent stage)
• Triose phosphate uses in Calvin cycle

  Starting point for the synthesis of carbohydrates, lipids, proteins and nucleic acids.
• Calvin cycle- stage 5 after triose phosphate is made

  5) 5 out of the 6 triose phosphate (or 10 out of 12 TP in a full cycle) are recycled to regenerate 6 RuBP molecules.
• Calvin cycle- Summary

  1) Carbon fixation (6C)2) Reduction (GP -> TP)3) Regeneration (TP -> RuBP)

  
• Limiting factor- light intensity

  light is needed as an energy source. As light intensity increases, ATP and NADPH are produced at a higher rate.
• Limiting factor- carbon dioxide concentration


  needed as a source of carbon, increasing rate of carbon dioxide increases rate of carbon fixation in the Calvin cycle and therefore the rate of TP production
• Limiting factor- temperature


  affects the rate of enzyme-controlled reactions

  stomata closure -> prevents CO2 diffusing in