B13 energy + ecosystems

• organisms can be divided into three groups based on how they obtain energy and nutrients:

  • producers
  • consumers
  • saprobionts
• producers are photosynthetic organisms which manufacture organic substances using light energy, water, carbon dioxide and mineral ions
• consumers are organisms which obtain their energy by feeding on other organisms instead of using light energy directly
• saprobionts are organisms which decompose other organisms by breaking down the complex materials in dead organisms into simple ones, releasing minerals and elements in a form that can be absorbed by plants
• primary consumers eat producers

  secondary consumers eat primary consumers

  tertiary consumers eat secondary consumers
• a food chain is a relationship in which each level consumes the level below it, and is consumed by the level above it, except producers and apex predators
• each stage in a food chain is called a trophic level
• arrows in a food chain show the direction of energy flow, read them as ‘is eaten by’
• a food web is when many different food chains are linked together as they all occur in the same habitat so involve the same animals in different combinations
• biomass is the total mass of living material in a specific area at a given time, measured in dry mass per given area per given time
• for calorimetry, fresh mass is easy to access but hard to measure as it contains varying amounts of water, and to get an accurate reading you need to use dry mass or carbon, which means the organism must be dead
• the chemical energy store in dry mass can be estimated using calorimetry
• ecosystems rely on the Sun as their ultimate source of energy
• very little of the sun's energy is captured by producers and used to produce organic molecules because:

  • most is reflected back into space by clouds/dust or absorbed back into the atmosphere
  • not all wavelengths of light can be absorbed by photosynthetic pigments
  • light may not fall on a chlorophyll molecule
  • other factors such as carbon dioxide levels affect rate of photosynthesis
• GPP = gross primary production
• GPP is the total chemical energy store in plant biomass in a given area/volume, in a given time
• part of GPP is used by producers for respiration so is lost
• NPP = net primary production
• NPP is the total chemical energy store in plant biomass after respiratory losses to the environment have been taken into account
• the equation for production and loss in a ecosystem is:

  NPP = GPP - R

  NPP is net primary production

  GPP is gross primary production

  R is respiratory loss
• very little of the energy produced by producers is transferred when they are consumed
• very little of the energy produced by producers may be transferred when they are consumed by primary consumers, and further down the chain, because:

  • some parts of the organism are not consumed, such as bones
  • some parts of the organism are consumed but not digested so are lost in faeces
  • some of the energy is lost by excretion such as urine
  • some energy losses occur as heat loss from respiration and to the environment as much energy is required to maintain a stable body temperature
• the equation for the net production of consumers is:

  N = I - (F + R)

  N is net production

  I is chemical energy store in ingested food

  F is energy lost in faeces and urine

  R is energy lost in respiration
• most food chains only have 4 or 5 trophic levels because insufficient energy is available to support a large enough breeding population at a higher trophic level than this
• the total biomass of organisms in a specific area decreases as trophic levels increase
• the total amount of energy available at each level decreases as you move up the food chain
• all nutrient cycles have a simple basic sequence:

  • nutrient is taken up by producers
  • producer incorporates the nutrient into complex organic molecules
  • producer is eaten, passing the nutrient onto primary consumers
  • nutrient passes along the food chain
  • when producers and consumers die, their complex molecules are broken down by saprobiotic microorganisms which release the nutrient in its original state again
• nitrogen is required in living organisms because it is needed to form proteins, nucleic acids, and many nitrogenous compounds
• nitrogen is difficult to extract from the air, so it must be recycled from dead organisms and waste products, plants take up most of their nitrogen from nitrate ions in the soil, absorbing them by active transport into the roots
• the nitrogen cycle has four main steps:

  • ammonification
  • nitrification
  • denitrification
  • nitrogen fixation
• in the nitrogen cycle, ammonification:

  • saprobiontic microorganisms break down organic matter into ammonia
  • they use proteases to break down proteins into amino acids, then deaminases to remove amino groups
  • they use the decomposition products for respiration
• in the nitrogen cycle, nitrification:

  • nitrifying bacteria convert ammonia to nitrite ions then nitrate ions in an oxidation reaction
  • nitrate ions are the form most easily absorbed by plants
• in the nitrogen cycle, denitrification:

  • denitrifying bacteria convert nitrate ions to nitrogen gas
  • wasteful process, can be prevented by keeping soil well drained and aerated
• in the nitrogen cycle, nitrogen fixation:

  • nitrogen-fixing bacteria fix nitrogen gas into other compounds by reducing it to ammonia which dissolves into ammonium ions
  • free-living nitrogen fixing bacteria live in the soil without interacting with other organisms
  • mutualistic nitrogen-fixing bacteria live in the root nodules of leguminous plants, which they have a mutualistic relationship with as they both benefit each other
• phosphorus is required in living organisms because it is a component of ATP, phospholipids and nucleic acids
• the main source of phosphorus is mineral form from sedimentary rocks, which release phosphate ions into the soil where they are absorbed by active transport into the roots of plants
• the phosphorus cycle includes:

  • erosion causes ions in sedimentary rocks to be dissolved in soil + water
  • absorption of ions from soil + water by plants
  • feeding and digestion of ions in plants by animals
  • excretion of excess ions by animals causes ions to be dissolved in soil + water
  • excretion and decomposition of wastes and remains cause ions to be in guano, bones + shells
  • erosion of guano, bones + shells causes ions to be dissolved in soil + water
  • deposition of guano, bones + shells causes ions to be in sedimentary rocks
  • sedimentation of ions in soil + water causes ions to be in rocks
• mycorrhizae are associations between plant roots and fungi which important in facilitating the uptake of water and inorganic ions by plants
• mycorrhizae increase the surface area of roots and act as a sponge, holding water and minerals, this allows the plant to better resist drought and take up inorganic ions more easily
• fertilisers need to be added to the soil to replace the nitrates and phosphates lost by harvesting plants and removing livestock