17) Energy for biological responses

• energy requiring processes:

  • anabolism
  • active transport
  • nerve transmission
  • endocytosis
  • exocytosis
  • transcription
• ATP

  • adenine
  • phosphate
  • ribose
• ATP+ H20 = ADP + P + energy
• ATP

  • not a good long term energy source, due to its relative instability of the phosphate bonds, so fats and carbohydrates are better for storage
  • ATP is a source of immediate energy
• ATP

  • small, moves easily in and out of organelles
  • soluble, energy requiring processes takes place in aqueous solutions
  • phosphate bonds have intermediate energy, the quantities of energy released are large enough to support cellular reactions but not so large that energy is wasted
  • easily regenerated
• The hydrolysis of one mole of ATP generates 30.5 kj/mol
• photosynthesis:

  6CO2 + 6H20 = C6H1202 + 6o2
• Light dependent reaction

  • energy from the sunlight is absorbed and ATP is formed
  • hydrogen from water is used to replace the co-enzyme NADP to reduced NADPH2
• Light independent reaction (calvin cycle)

  • hydrogen from reduced NADP and carbon dioxide are used to build organic molecules such as glucose
  • ATP from light dependent stage supplies energy
• autotrophs make their own food using carbon dioxide via photosynthesis
• heterotrophs assimilate energy by consuming plants and animals
• chloroplasts

  • double membrane - has pigments of chlorophyll for light absorption
  • stroma - site of many chemical reactions, as it contains enzymes and the correct pH level for light independent reaction
  • thylakoid - Has ATP synthase for photophosphorylation, which absorbs light
  • granum - flat membrane stacks increase SA:V ratio for a faster rate of absorption
  • lamella - connects thylakoids and provides structure and supports to the chloroplast
• respiration:

  6o2 + C6H12o6 = 6CO2 + 6H20 + energy
• Photosynthesis stages:

  • Capturing of light energy - pigments of chlorophyll.
  • Light dependent reaction - light energy converted into chemical energy.
  • Light independent reaction - sugars and other organic molecules are produced.
• Photosynthetic pigments:

  chlorophyll a - main pigment that absorbs red and blue light and reflects green light

  chlorophyll b - an accessory pigment mostly found with chlorophyll a in light-harvesting complex

  Xanthophylls and carotenoids - absorb different wavelengths than chlorophyll, broadening the spectrum of light that can drive photosynthesis.
• Light dependent stage:

  • takes place in thylakoid membrane
  • light energy absorbed by photosystems
  • water split by photolysis
  • excited electrons are passed along the electron transport chain
  • some ATP is produced by photophosphorylation
  • NADP is reduced
• LDS:

  1. light is absorbed & electrons get excited

  • light shines on photosystem 2
  • the absorbed energy leads to excitation of 2 electrons which then are accepted by the electron transport chain
• LDS:

  2. water is split into protons, electrons and oxygen

  • photosystem 2 contains an enzyme that split water in the presence of light - photolysis
  • the 2 electrons are used to replace the 2 excited electrons which have left chlorophyll in photosystem 2
• Photolysis - splitting of water by light
• photolysis equation:

  2H20 = O2 + (4H+) +(4e-)
• LDS:

  3. high energy electrons are moved along the electron transport chain

  • at each stage a small amount of energy is released
  • electron carriers are molecules which all contain iron
  • energy is used to pump protons (H+) across the thylakoid membrane from stroma into thylakoid space forming a proton pump
• LDS:

  4. Proton gradient is used to drive the production of ATP by ATP synthase

  • this is chemiosmosis
  • Production of ATP from ADP and phosphate using light is called photophosphorylation
• photophosphorylation - ATP formed using light energy
• two types of photophosphorylation:

  • cyclic
  • non-cyclic
• cyclic photophosphorylation:

  • uses only photosystem 1
  • excited electrons pass to an electron acceptor and back to the chlorophyll molecule
  • electron pass along electron transport carriers
  • electrons leaving return to photo stage 1
  • energy released is used to synthesise ATP
  • no NADP is formed/used
• non-cyclic photophosphorylation:

  • involves photosystem 1 and 2
  • light strikes photosystem 2, excites electrons and they pass along electron transport chain and energy released is used to synthesise ATP
  • light also strikes photosystem 1 and a pair of electrons are lost
  • electrons and protons produced by photolysis of water join NADP to become reduced NADP
• Light independent reaction

  • builds sugars from carbon dioxide and regenerates ribulose bisphosphate (RuBP)
  • takes place in the stroma of the chloroplast
• Light independent reaction

  1. fixation
  2. reduction
  3. regeneration
• Light independent reaction

  1. fixation

  • the RUBISCO enzyme catalyses the combination of ribulose bisphosphate (RuBP) and carbon dioxide
  • RuBP is a 5-carbon compounds which forms a transient (breaks down easily) molecule
  • The 6-carbon immediately splits into two molecules of glycerate-3-phosphate (GP)
• Light independent reaction - fixation stages as a flow chart

  RuBP (5c) + Co2 (1c) (catalysed by RUBISCO) = 6c = 3c (GP) + 3C (GP)
• Light independent reaction:

  2. reduction

  • glycerate-3-phosphate (GP) is then changed into triose phosphate (TP) by the addition of phosphate (from ATP) and two electrons (from reduced NAPD) from the light dependent reaction
• Light independent reaction:

  3. regeneration

  • 5 out of every 6 molecules of triose phosphate (TP) are recyled by phosphorylation, using ATP from the light dependent reaction and the rest are used to make glucose
• Light independent reaction

  • some glycerate-3-phosphate (GP) is used to make amino acids and fatty acids
  • glucose can be isomerised to form fructose
  • glucose and fructose can combine to form sucrose
  • pairs of triose phosphate (TP) can form sugars
• photosynthesis requires:

  photosynthetic pigment - absorb light energy

  co2 - provides carbon to make glucose

  water - provides electrons and hydrogen irons

  light energy - energy to split water into ATP and reduced NADP

  enzymes - internal
• limiting factor - factor in lowest supply and therefore limiting the rate of reaction
• Light intensity:

  • limits the light dependent reaction
  • less light = less excitation of chlorophyll and electrons
  • less electrons pass down the electron transport chain
• Light intensity:

  • limits the light dependent reaction and glucose-3-phosphate (GP) accumulates
  • ATP and reduced NADP are not produced, therefore GP cannot be reduced to triose phosphate (TP)
• Light intensity:

  low light intensity - limits LDS, so not much ATP and NADPH are produced. Slows LIR and photosynthesis slows.

  moderate light intensity - produced more ATP and NADPH in LDS, which regenerates RuBP in LIS quicker. More GP converted into TP.

  high light intensity - more light than needed, so temperature becomes limiting factor and rate plateaus
• carbon dioxide concentration:

  • co2 concentration limits light independent reaction
  • less co2 means there is less available to combine with RuBP
• carbon dioxide concentration:

  • less co2, means less RuBP converted to GP
  • RuBP accumulates and GP will decrease