Chapter 17

• The need for energy/ cellular respiration 


  • metabolic activities such as active transport, movement and anabolic reactions all require energy
  • cellular respiration converts chemical energy into ATP so it can be used
• Photosynthesis equation

  • 6CO2 + 6H2O -> C6H12O6 + 6O2
  • carbon dioxide + water -> glucose & oxygen
• Respiration equation

  
  

  • C6H12O6 + 6O2 -> 6CO2 + 6H20
  • glucose + oxygen - > water + carbon dioxide
• Purpose of photosynthesis

  trap energy
• Purpose of respiration

  release energy
• Inter-relationship between photosynthesis and respiration

  • in photosynthesis light provides energy needed to build organic molecules like glucose
  • this energy is used to form chemical bonds in ATP which are then broken to release energy needed to make bonds as glucose is formed
  • in respiration organic molecules (glucose) are broken down & energy released is used to synthesise ATP
  • ATP then used to supply energy needed to break bonds in the metabolic reactions of the cell
• What is chemiosmosis?

  process that ATP is produced in both photosynthesis & respiration - occuring in thylakoids of chloroplasts
• Chemiosmosis 


  • high energy electrons pass along an electron transport chain
  • this releases energy which is used to pump protons (H+ ions) across a membrane - creates a proton gradient
  • protons diffuse from area of high conc (inside thylakoid membrane) to area low conc (outside membrane)
  • proton gradient maintained as result of impermeability of the membrane to hydrogen ions
  • impermeability means protons have to move through channel proteins linked to ATP synthase
  • flow of protons through channels provides energy to ATP synthase enzyme - allows it to combine ADP & Pi to produce ATP

  
• Autotrophic organisms

  • can photosynthesise
  • e.g. plants & algae
• Heterotrophic organisms

  • obtain complex organic molecules by eating other organisms
  • e.g. animals
• Structure of chloroplasts

  • network of membranes provides large surface area to maximise absorption of light - essential in LDR
  • membranes form flattened sacs called thylakoids - stacked to form grana
  • grana joined lamellae
  • photosynthetic pigments are arranged in photosystems for maximum light absorption
  • fluid-filled matrix is called stroma - contains enzymes needed to catalyse reactions of LDR
• Label diagram of chloroplasts

  A) inner membrane

  B) outer membrane

  C) thylakoid

  D) granum

  E) stroma

  F) stroma lamella

  G) intermembrane space

  
• Chlorophyll a 


  • primary pigment
  • absorbs mainly red & blue light & reflects green light
  • presence of large quantities of chlorophyll is reason for green colour of plants
• Accessory photosynthetic pigments

  • chlorophyll b, xanthophylls, and carotenoids
  • absorb different wavelengths of light than chlorophyll a
  • diff combos of pigments are reason for diff shades & colour of leaves
• Define photosynthetic pigment

  organic molecule that absorbs some colour of light but not others & transfers the light energy to chemical energy
• Light harvesting system (antennae complex)

  • formed by accessory pigments
  • absorb (harvest) light energy of different wavelengths & transfer this energy to the reaction centre
  • chlorophyll a located in reaction centre - where reactions involved in photosynthesis take place
  • light harvesting system & reaction centre are collectively known as a photosystem

  
• Investigating photosynthetic pigments

  • pigments can be separated & identified using chromatography
  • mobile phase = solution containing mixture of pigments
  • stationary phase = thin layer of silica gel applied to glass
  • pigment molecules interact with the stationary phase to different extent & therefore move at different rates - results in them being separated
  • Rf = dist travelled by pigment/ dist travelled by solvent
• How are electrons excited?

  • electrons present in pigment molecules are excited by absorbing light from sun
  • high energy electrons are released when chemical bonds are broken in respiratory substrate molecules (e.g. glucose)
• Summary of light-dependent stage of photosynthesis

  • energy from sunlight is used to form ATP
  • hydrogen from water is used to reduce coenzyme NADP to reduced NADP

  
• Non-cyclic photophosphorylation

  • light energy is absorbed by chlorophyll at reaction centre of PSII - releases excited electrons
  • electrons pass along an electron transport chain & ATP is produced
  • at end of electron transport chain electrons pass into PSI
  • electrons lost from PSII are replaced by electrons from photolysis
  • PSI absorbs light - electrons become excited
  • more ATP is produced via a second electron transport chain
  • electrons lost from PSI are replaced by electrons that left PSII
  • electrons from PSI & hydrogen ions released from photolysis of H2O combine to produce reduced NADP

  
• Photolysis 


  • loss of electrons from PSII makes it unstable
  • PSII stimulates splitting/ photolysis of water into hydrogen ions, electrons, & oxyen
  • electrons then pass to reaction centre of PSII making it stable again - replace electrons lost from PSII & allows it to keep working
  • protons are released into thylakoids, increasing proton concentration across membrane - ATP is produced by chemiosmosis
  • once returned to stroma, hydrogen ions combine with NADP & an electron from PSI to form reduced NADP
  • process removes hydrogen ions from stroma - helps maintain proton gradient of thylakoid membrane

  
• Equation of photolysis reaction

  H2O -> 2H+ + 2e- + 1/2 O2
• Cyclic photophosphorylation 


  • electrons leaving electron transport chain after PSI can be returned to PSI instead of being used to form reduced NADP
  • means PSI can lead to production of ATP without any electrons being supplied from PSII
  • does not produce any reduced NADP

  
• Summary of light-independent stage of photosynthesis

  • takes place in the stroma
  • hydrogen from reduced NADP & carbon dioxide is used to build organic molecules such as glucose
  • ATP supplies the required energy

  
• Calvin cycle (LIR)


  • carbon dioxide enters intercellular spaces within spongy mesophyll by diffusion through the stomata
  • RuBisCO catalyses reaction between CO2 & a 5 carbon (5C) molecule called RuBP - carbon is fixed
  • produces an unstable 6C compound which breaks down into 2 3C GP molecules
  • each GP molecule is reduced to another 3C molecule, triose phosphate (TP), using a hydrogen atom from reduced NADP & ATP - both supplied from LDR
  • TP can be used to form lipids/ amino acids/ glucose/ nucleic acids
  • half of TP is recycled to regenerate RuBP so Calvin cycle continues

  
• What does the 'fixation' of carbon in the calvin cycle mean?

  as carbon dioxide combines with RuBP, carbon is incorporated into an organic molecule
• RuBisCO


  • not very efficient enzyme as it is inhibited competitively by oxygen in the air
  • means chloroplasts need to contain a lot of it to carry out photosynthesis at a sufficient rate to sustain life
• TP


  starting point for synthesis of many complex biological molecules, including carbohydrates, lipids, amino acids & nucleic acids
• Regeneration of RuBP

  • for one glucose molecule to be produced 6 CO2 molecules have to enter the Calvin cycle
  • results in 6 full turns of cycle - results in production of 12 TP molecules, 2 of which removed to make glucose
  • means 10 TP molecules are recycled to regenerate RuBP
  • 10 x TP (each made of 3C) = 30 carbons, which are rearranged to form 6 x RuBP - each containing 5C)
  • ATP supplies energy for reactions involved
• Due to RuBisCO what type of reaction is the calvin cycle and what does this mean?

  • enzyme-controlled reaction
  • needs optimum conditions