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Ecosystem > Photosynthesis
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  • All chemical reactions in a cell → arranged into metabolic pathways
  • Intermediates in these pathways are called metabolites
  • Catabolic reactions (e.g. respiration) release energy
  • Anabolic reactions (e.g. photosynthesis) use up energy

Leaf Structure

  • Phototropism → growth towards light
  • Large leaf surface area → captures more light
  • Thin leaves → few cell layer → more light captured by chlorophyll
  • Leaf mosaic increases leaf exposure to light
  • Internal Structure
    • Cuctile → reduces H2O loss by evaporation
    • Palisade mesophyll → chloroplast can move around cell for max light absorption
    • Spongy mesophyll → allows diffusion of gases through leaves
    • Phloem sieve tube → removes products of photosynthesis


  • Thylakoid membrane: light-dependent → large surface area
    • Associated with chlorophyll, accessory pigments, ETC, enzymes
  • Grana: stacks of thylakoid membranes
    • Enclose hydrogen reservoir used in chemiosmosis
  • Lamella: tubular extensions forming a network between grana
  • Stroma: RuBP carboxylase catalyses light independent reaction
  • Starch granule → insoluble storage carbohydrate product of photosynthesis

Light Dependent Reaction

  • In Thylakoid Membrane / Granum
  • IMG 5-14-6

Chemiosmosis/Photophosphorylation produces ATP

//H+ from photolysis remain in thylakoid space

  • ETC releases energy
  • Used to pump H+ from stroma into thylakoid space
  • By active transport and electrochemical gradient
  • H+ conc in thylakoid space > stroma
  • H+ ions pass back from space between two mitochondrial membranes
  • Through pores which are associated with the enzyme ATP synthetase
  • Energy from the ETC will be used to produce ATP
  • Diffuse down conc gradient across thylakoid membrane
  • Produces ATP by photophosphorylation

Light Independent Reaction

  • In Stroma (Calvin Cycle)
  • IMG 5-14-6

Summary WITH IMG!!!!!!!!!!!!!!!

  • The light dependent reaction takes place in the chloroplast and is important to produce ATP and NADPH + H+. Both molecules and carbon dioxide (CO2) are needed in the light independent reaction (Calvin Cycle) to produce a hexose sugar, such as glucose, from RuBP.
  • Glucose is used to in the mitochondrion to produce the energy molecule ATP. NAD+ is also needed for respiration, but is not produced by photosynthesis.
  • PARP (Poly-ADP-Ribose-Polymerase) and PARG (Poly-ADP-Ribose-Glycohydrolase) are important if the plant is exposed to excessive stress factors (such as extreme temperatures). Note: knowledge about PARP and PARG is not required for your exam.
  • IMG
  • Picture 5-1 from Bayer research.
  • Genetic engineering improves crop yields,
  • Braving the drought [view article]

Energy Transfer

Ecological pyramids

  • Pyramids of numbers
    • Total number of organisms in a food chain at each trophic level
    • Highest number at the bottom (usually producers, then consumers)
    • Pyramid will be inverted if lots of small animals are feeding off one large plant
  • Pyramids of biomass
    • Total biomass of organisms in a food chain at each trophic level
    • Always pyramid shaped
    • Organisms multiplying rapidly may have biomass less than primary consumers
    • Dry mass is measured / H2O stores no energy and varies in different organisms
  • Pyramid of energy
    • Amount of energy transferred to each level of a food chain in an ecosystem
    • Always pyramid-shaped / no energy loss

Transfer of energy between trophic levels

Food chains and food webs

  • Energy is used to produce new cells / remains fixed in that organism
  • Energy is passed on to the next trophic level through feeding
  • Producers are photoautotrophs (plants)
    • Transduce light energy into chemical P.E. by forming new tissues and storing organic compounds (starch, glucose, lipids, proteins)
  • Consumers are herbivores, carnivores and omnivores
  • Decomposers are detritivores and saprophytes
    • Break down dead complex organic molecules into simple inorganic molecules
  • Food chains are feeding relationships and linked with each other to form complex food webs
    • Some organisms feed on different trophic levels / leaves and insects
    • Some organisms feed on different foods when they are larvae (leaves) and when they are adult (nectar produced by different flowers)

Energy transfer and efficiency

  • 2% of light energy is converted to chemical P.E. by photosynthesis
    • Rest is lost reflection from leaves / heat loss / not all wavelengths are utilised / light strikes non-photosynthetic structures
  • 10% of that are passed on along trophic levels
    • Rest is lost in respiration / as heat/faeces/urine
  • Chemical P.E. / generates heat / stores energy by forming organic matter (new cells)
    • Mammals are homeothermic / must maintain constant body temp
    • Warm environment / less energy maintains body temp / more organic matter stored / more transferred to consumer
    • Small organisms / large surface area:volume ratio / lose high amounts of energy
  • Carnivores fix organic matter more efficiently than herbivores
    • Herbivores feed on plants
    • Thus, take up cellulose and lignin / difficult to digest
    • Thus, more food passes through gut and is lost as faeces
  • Trout fix organic matter most efficiently, they are
    • Poikilotherms → must NOT maintain constant body temp
    • More energy is used to fix organic matter
    • Carnivores are harvested while they are still young and grow rapidly
    • Trout transfer most energy to consumer (human) in terms of food
  • [EXAM] Number of food chains is limited
    • Due to energy losses (at each trophic level)
    • In respiration/egestion/excretion/movement/as heat
    • (Too) little energy is left to sustain higher trophic levels/to be passed on