• The sun is the initial (first) source of energy for most food chains
  • Light energy?from the sun?is converted by producers into chemical energy?stored in the tissues of plants during the process of?photosynthesis
  • Chemical energy stored in the tissues of plants?passes to primary consumers?when they ingest plants, and?on to secondary consumers?when the primary consumers are themselves ingested
• There are a few unusual exceptions, such as food chains located in deep sea volcanic vents that rely on bacteria gaining energy from chemical processes; these are chemoautotrophic bacteria

• Photosynthesis?is the process of converting light energy into chemical energy
• The photosynthetic pigment chlorophyll?absorbs light energy?when sunlight lands on photosynthesising parts of a plant
• This light energy is used to power several processes which result in the?production of organic molecules,?or?carbon compounds, including:
  • Glucose, which is used in respiration or stored in plant cells in the form of?starch
  • Lipids
  • Amino acids
• Hydrolysing?these carbon compounds releases energy, so they can be said to contain?stored chemical energy
• These carbon compounds are used by the plant to build plant tissues, meaning that the energy stored in them is?stored within the tissues of the plant

• Chemical energy, stored in?carbon compounds?in plant tissues, is?passed to the primary consumer?when the plant is?ingested
  • The primary consumer?digests?the plant tissues and?absorbs?the carbon compounds containing?stored chemical energy
  • These carbon compounds can either be used to fuel?respiration?or to build up animal tissue, meaning that the?stored chemical energy is transferred?to the tissues of the primary consumer
• When the primary consumer is ingested, the?carbon compounds in its tissues,?along with their?stored chemical energy, pass to the?secondary consumer, and so on up the food chain
• When an organism dies, the chemical energy stored in carbon compounds in its tissues passes to?detritivores?and?saprotrophs
• In a food chain, the arrows represent the?transfer of energy, in the form of stored chemical energy in carbon compounds, from one?trophic level?to the next, by the process of?feeding

A simple food chain - the blue arrows show how the chemical energy originally stored by the producer is transferred to other organisms in the food chain

• The chemical energy stored in ingested carbon compounds can be?released by the process of?respiration
  • The carbon compound?glucose?is the fuel for respiration
  • Other carbon compounds such as lipids?can be converted into glucose?before being respired
• The energy released during respiration can be used by organisms to carry out the?functions of life
  • Metabolism - the enzyme-catalysed reactions taking place inside cells
  • Reproduction - the sexual or asexual production of offspring
  • Homeostasis - the maintenance of internal conditions within tolerable limits
  • Growth - increasing in size
    • Note that during growth some of the chemical energy in the?carbon compounds?ingested by an organism is?incorporated into the tissues?of the organism as it grows; this?stored chemical energy can be passed to the next trophic level?in the food chain
  • Response - sensing and responding to the environment
  • Excretion - disposal of metabolic waste
  • Nutrition - gaining energy and nutrients
• The process of respiration also?releases heat as a by-product

• The process of?respiration?in living organisms?releases heat as a by-product
  • This applies in?producers, consumers, detritivores, and?saprotrophs
• Living organisms?cannot convert the heat energy that is released as a by-product of respiration into any other form of energy
• This means that?heat energy is lost?from food chains at every trophic level, as well as during?decomposition
• This?heat energy?is ultimately?lost?to the environment
• Ecosystems have enough energy to be sustainable because energy is always entering food chains via producers as?they capture light energy from the sun