photosynthesis

• autotrophs can synthesise their won complex organic nutrients using an ionorganic carbon source
• chemoautotrophs - synthesise complex organic molecules using energy from exergonic reactions

  e.g., nitrifying bacteria
• photoautotrophs - organisms that use light energy to make their own complex organic molecules
• heterotrophs - digest complex organic molecules into simple soluble ones and absorb these
• compensation point is when photosynthesis and respiration occur at the same rate and there is no net gain or loss of carbohydrate or oxygen
• how a leaf is adapted to photosynthesis

  • upper epidermis is transparent to allow light to enter
  • palisade cells contain many chloroplasts
  • air spaces allow better diffusion
  • large sa to obtain the most energy
  • thin cell walls for light penetration
• function of chloroplast envelope - control entry and exit of ions into the cell
• function of grana - large surface area for chlorophyll and other pigments for photosystem 11, contain electron carriers and atp synthase
• function of inter-granal lamella - where photosystem 1 pigments are found
• function of stroma - provide site for light dependent reactions
• function of DNA - code for enzymes and pigments, chloroplast to reproduce by binary fission
• function of starch grains - does not affect water potential
• photosynthetic pigment - a substance that absorbs certain wavelengths of light and capture light energy for photosynthesis
• chlorophyll pigment

  • green
  • tails embed themselves in the thylakoid membrane
  • head contains magnesium
  • either a/b
• pigment molecules are arranged in clusters in the thylakoid membranes

  these are called photosystems

  each photosystem spans the membrane
• primary pigment - one chlorophyll molecule acts as a reaction centre for each photosystem
• accessory pigments - the remaining pigments absorb light and funnel the energy to the reaction centre. these act as a light harvesting system
• photosystem 1 found on intergranal lamellae
• photosystem 11 found in granal lamellae
• light dependent stage summary

  • in thylakoid membrane
  • Light energy absorbed by photosystems
  • water is split
  • atp produced
  • reduced NADP produced
  • oxygen released as waste product
• photolysis

  • photosystem 11 contains enzyme activated by light that splits water when activated
  • hydrogen ions reduce NADP
  • electrons replace those lost by oxidised chlorophyll in photosystem11
  • oxygen diffuses out and is used for respiration
  • $2water =$$4h^+$$+$$4e^-+$$02$
• photophosphorylation - the process of adding phosphate to a molecule using light energy

  1. light hits chlorophyll a and electrons become excited
  2. excited electrons are picked up by an electron carrier
  3. as the electrons lose energy this energy is used to pump protons into the thylakoid space
  4. protons move back into stroma down concentration gradient through ATP synthase
• cyclic photophosphorylation

  • photosystem 1 only
  • no reduced NADP therefore no sugar
  • electrons become excited and are passed down electron carriers back into the same chlorophyll molecule
• non-cyclic photophosphorylation

  • light hitting PS1 and PS11 cause electrons to be emitted from both primary pigments
  • electron from PS11 pass down electrons carrier generating ATP
  • electrons lost from PS1 are used together with h ions from the photolysis of water to reduced NADP
  • electrons from photolysis of water replace those lost b PS11
• Calvin cycle process

  1. carbon dioxide diffuses into the stroma
  2. combines with ribulose bisphosphate catalysed by rubisco
  3. carboxylation produced a 6 c compound which splits into two molecules of glycerate 3 phosphate
  4. each GP molecule is reduced to become triose phosphate
  5. atp and reduced nadp provide the hydrogen ions and phosphate needed for this
  6. 5/6th of the triose phosphate is used to regenerate rubp
  7. required atp
• products of trios phosphate

  • glycerol - lipids
  • hexose sugars - sucrose, fructose, cellulose, starch
  • glucose
• products of glycerate 3 phosphate

  • amino acids
  • fatty acids
• factors affecting the rate of photosynthesis

  • light intensity and wavelength - light required for photolysis of water and photophosphorylation and stomatal opening
  • carbon dioxide concentration - limiting factor as essential for Calvin cycle
  • temperature - above 25 degrees rubisco catalysing rubp with oxygen (photorespiration)
  • water stress - if cannot replace what is lost by transpiration abscisic acid builds up ad the stomata close
• limiting factors and the Calvin cycle

  • light intensity - the Calvin cycle is depedent on the atp and reduced NADP made from light dependent reactions
  • carbon dioxide concnetration - conversion of rubp to gp is slow
  • temperature - all stages are catalysed by enzymes