energy and ecosystems

• • An ecosystem in a particular area includes:
  • All the living components (i.e. biotic factors such as organisms and their interactions)
  • All the non-living components (i.e. abiotic factors such as temperature and rainfall)
• • All ecosystems include (and depend on) primary producers
  • Primary producers are organisms that make their own glucose
  • For example, plants and algae produce glucose via photosynthesis
  • Even deep-sea ecosystems such as those around hydrothermal vents (where there is no light) depend on primary producers (in this case bacteria) that use a process known as chemosynthesis to make glucose from the chemicals released from these vents
• • In ecosystems where sunlight and water is available, the process of photosynthesis enables plants to synthesise organic compounds (glucose and other sugars) from carbon dioxide
  • In terrestrial (land-based) ecosystems, plants use CO2 from the atmosphere
  • In aquatic (water-based) ecosystems, plants use CO2 dissolved in the water
• • The process of photosynthesis transforms light energy into chemical energy held in biological molecules
  • The chemical energy in these biological molecules can then be used by other organisms within the community known as consumers (i.e. the organisms in higher trophic levels)
  • Primary consumers (herbivores or omnivores) feed on producers
  • Secondary consumers (carnivores or omnivores) feed on primary consumers
  • Tertiary consumers (carnivores or omnivores) feed on secondary consumers
• • The process of photosynthesis transfers light energy to chemical energy in biological molecules
  • In ecosystems where sunlight and water is available, the process of photosynthesis enables plants to synthesise organic compounds (glucose and other sugars) from carbon dioxide
  • Most of these sugars synthesised by plants are used by the plant as respiratory substrates
  • A respiratory substrate is a molecule (such as glucose) that can be used in respiration, to release energy for growth
• Sugars (not used in respiration)

  Used to make other groups of biological molecules
• Groups of biological molecules made from sugars

  • Starch
  • Cellulose
  • Lipids
  • Proteins
• Starch

  • Complex carbohydrate molecule formed from many glucose molecules
  • Acts as a short-term energy storage molecule
• Cellulose

  • Another complex carbohydrate molecule formed from many glucose molecules
  • Acts as a structural component of plant cell walls
• Lipids

  • Plant cells can convert sugars produced during photosynthesis into lipids
  • Lipids act as another type of (longer-term) energy storage molecule
• Proteins

  • Plant cells can combine sugars produced during photosynthesis with nitrates to make amino acids
  • Amino acids can then be used to produce proteins
• These different groups of biological molecules (all formed from the sugars synthesised by plants during photosynthesis) make up the biomass of the plants
• Biomass

  The mass of living material
• The biomass can also be thought of as the chemical energy that is stored within the plant
• The sugars synthesised during photosynthesis can be used in respiration or the creation of other biological molecules needed by plants#
• The biomass of an organism (or of a sample of tissue from an organism) is:

  • The mass of living material of the organism or tissue
  • The chemical energy that is stored within the organism or tissue
• • Biomass can be measured in terms of:
  • The dry mass of an organism or tissue (in a given area)
  • The mass of carbon that an organism or tissue contains
  • The mass of carbon that a sample (i.e. an organism or tissue) contains is generally taken to be 50% of the dry mass of the sample
• Dry mass 

  • The dry mass is the mass of the organism or tissue after all the water has been removed
  • The dry mass of a sample can be used to calculate the biomass of a total population of organisms or of a particular area. For example:
  • If the dry mass of one daffodil plant is found to be 0.1 kg, then the dry mass (i.e. the biomass) of 200 daffodils would be 20 kg (0.1 x 200 = 20)  
• Calorimetry 

  • Calorimetry can be used to estimate the chemical energy stored in dry biomass
  • This involves burning the sample of dry biomass in a piece of equipment known as a calorimeter
  • The burning sample heats a known volume of water
  • The change in temperature of the water provides an estimate of the chemical energy the sample contains
• • Dry mass is used to measure the mass of living material in a plant sample
  • The dry mass is the mass of the organism or tissue after all the water has been removed
  • To find the dry mass, the sample must first be dehydrated (dried out until it contains no more water)
  • A calorimeter can then be used to estimate the chemical energy stored within the dried plant sample
• Apparatus to dry plant

  • Heat-proof
  • Open-topped container
• Crucible

  Heat-proof, open-topped container used to dry plant samples
• Crucible


  • Able to withstand temperatures inside the oven
  • Open-topped to allow moisture to escape and evaporate
• Oven


  Used to slowly dry the sample
• Digital balance

  • Used for monitoring the mass of the plant sample as it dries out
  • Needs high precision to detect small changes in sample mass
• Calorimeter


  • Can be simple and inexpensive (easy for students to set up using classroom equipment)
  • Can be very precise, expensive pieces of apparatus known as bomb calorimeters (more commonly found in professional scientific laboratories)
• Finding the dry mass of a plant sample

  1. Weigh the crucible (heat-proof container) without the sample first

  2. Place the sample in the crucible

  3. Place the crucible in the oven

  4. Set the oven to a low temperature (if the temperature is too high the sample may burn, which would cause it to lose biomass)

  5. Remove and weigh the crucible (containing the sample) at regular intervals during the drying process

  6. Once the mass of the crucible (and sample) stops decreasing and becomes constant, the sample is fully dehydrated (all the water has been removed)

  7. From this final constant mass, subtract the original mass of the crucible (without the sample in it) to find the dry mass of the sample
• Finding the energy released by a sample of plant biomass:

  • To estimate the chemical energy stored in the dried sample, use a calorimeter
  • A calorimeter burns the dried sample and uses the energy released to heat a known volume of water
  • Measure the change in temperature of the water
  • This temperature change can be used to estimate the chemical energy stored within the sample
  • This energy is measured in joules (J) or kilojoules (kJ)
  • 1 joule is the energy needed to raise the temperature of 0.24 g of water by 1 °C
  • 1 kilojoule (kJ) is 1000 joules
• Gross primary production (GPP) can be defined as the amount of chemical energy stored in the carbohydrates within plants (during photosynthesis)
• Gross primary production can be expressed in:

  • Units of energy per unit area
• Gross primary productivity

  • The rate at which plants are able to store chemical energy via photosynthesis is referred to as gross primary productivity
  • Gross primary productivity is expressed using units of energy/mass per unit area per unit time
  • Time must be included as it is a rate
• Net primary production (NPP) refers to the amount of energy available to herbivores in the plant’s biomass after plant respiratory losses
• • The chemical energy that is leftover in a plant after respiratory loss is known as the net primary production (NPP)
  • Net primary production can therefore be calculated using the equation below:

  NPP = GPP - R
• • NPP is important because it represents the energy that is available to organisms at higher trophic levels in the ecosystem, such as primary consumers (herbivores and omnivores) and decomposers
• Net primary productivity

  • Net primary productivity is expressed using units of energy/mass per unit area per unit time
  • Time must be included as it is a rate
• Decomposers


  Organisms that break down dead plant and animal material to gain the chemical energy still stored in the dead matter
• Main groups of decomposers

  • Bacteria
  • Fungi
• How decomposers break down dead matter

  1. Secrete digestive enzymes onto the surface of the dead organism

  2. Enzymes break down the dead matter into small soluble food molecules

  3. Decomposers absorb the small food molecules
• The net primary production (i.e. the chemical energy) of producers is also available to decomposers
• Decomposers carry out a very important function in ecosystems