Energy and ecosystems

synthesis of organic compounds in ecosystems

• In ecosystems, organic compounds are synthesised by plants during photosynthesis

• Plants use the carbon present in carbon dioxide to produce organic molecules

  • In terrestrial ecosystems plants use CO₂ from the atmosphere

  • In aquatic ecosystems plants use dissolved CO₂

• Plants are described as the primary producers in an ecosystem because they produce their own organic molecules

• The organic compounds produced by plants are passed to other organisms in an ecosystem when herbivores consume plant tissue

photosynthetic producers

• Plants synthesise sugars during photosynthesis

• Most of the sugars produced are used by the plant as respiratory substrates

• Remaining sugars are used to make other groups of biological molecules; these molecules form plant biomass, e.g.:

  • starch: a complex carbohydrate molecule that functions as an energy storage molecule inside plant cells

  • cellulose: another complex carbohydrate that forms a structural component of plant cell walls

  • lipids: plant cells convert sugars into lipids, the functions of which include energy storage and formation of waxy cuticle on leaves

  • proteins: plant cells combine sugars with nitrates to make amino acids, which can then be used to produce proteins, e.g. membrane proteins and enzymes

biomass

• The term biomass can be defined as:

the mass of living material present in an organism or tissue sample

• Biomass can be measured in terms of:

  • the dry mass of an organism or tissue sample per given area; this is the mass of tissue after water has been removed

    • The water content of living tissue can vary, so dry mass is a more reliable measure

    • Units for dry mass are mass per unit area, e.g. kg m^-2

  • the mass of carbon present in an organism or tissue sample per given area: this is taken to be 50 % of the dry mass

calculating dry mass

The dry mass of a sample can be scaled up to calculate the biomass of a total population or area

measuring chemical energy

• The biomass of an organism is formed from organic molecules, so biomass is a measure of the chemical energy stored in an organism or tissue sample

• The chemical energy stored in dry biomass can be measured using calorimetry

determining dry mass

• To find the dry mass of a sample, it must be dried out until it contains no more water

• This can be accomplished using a crucible, oven and a digital balance as follows:

  1. set an oven to a low temperature

     • If the temperature is too high the sample may burn, which would cause it to lose biomass

  2. weigh the crucible and record its mass

  3. place the sample in the crucible and place the crucible in the oven

     • The crucible has no lid, allowing any moisture leaving the sample to evaporate

  4. remove and weigh the crucible and sample at frequent intervals during the drying process

  5. repeat step 4 until the mass of the crucible and sample stops decreasing

     • At this point the sample is fully dehydrated

  6. subtract the original mass of the crucible, calculated at step 2, from the mass determined in step 5 to find the dry mass of the sample

limitations of determining dry mass

• It can take a long time to fully dehydrate a plant sample to find its dry mass; the drying process could take several days for larger samples

• Precise equipment may not be available in a school laboratory

  • A very precise digital balance should be used to detect extremely small changes in mass

determining stored energy

• A calorimeter can be used to estimate the chemical energy stored within a dried plant sample as follows:

  1. use a measuring cylinder to measure a set volume of water into a copper beaker

  2. record the mass of the water

  3. record the starting temperature of the water using a thermometer

  4. record the starting mass of the dry sample

  5. place the sample in a crucible and place beneath the beaker of water

  6. set fire to the sample and allow it to burn until it has completely burned away

  7. record the final temperature of the water

  8. calculate the change in temperature of the water

  9. calculate the energy transferred per gram of the dry sample as follows:

Energy transferred per gram = mass of water x temp change x 4.2/ mass of sample

• 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

limitations of determining stored energy

• The more simple and basic the calorimeter, the less accurate the energy estimate will be

  • Heat energy from the burning sample may be transferred to the surrounding environment and not to the water

• A bomb calorimeter can give a highly accurate estimate, but is an expensive piece of equipment for schools

gross primary production

• Gross primary production (GPP) can be defined as:

the chemical energy stored in plant biomass, in a given area or volume

• Gross primary production in terrestrial plants can be expressed as:

  • energy per unit area, e.g. J m^–2

  • mass per unit area, e.g. g m^–2

    • biomass = stored chemical energy, so a measure of mass can be used to represent energy

• Gross primary production in aquatic environments can be expressed as:

  • energy per unit volume, kJ m^-3

  • mass per unit volume, e.g. kg m^-3

what is net primary production

• Net primary production (NPP) can be defined as

the chemical energy stored in plant biomass after respiratory losses to the environment have been taken into account

• Plants convert light energy to chemical energy in the form of glucose during photosynthesis; the total energy stored in this way is gross primary production (GPP)

• Some of the energy stored during GPP is lost to the environment, e.g. when waste heat is generated during respiration

• The energy that remains in the plant tissues after these energy losses is the NPP

  • NPP is important because it represents the energy that is available to consumers in the ecosystem

net primary production equation

NPP = GPP - R

• Where:

  • GPP = gross primary production

  • R = respiratory loss to the environment

• NPP, like GPP, is expressed as energy per unit area or volume, e.g.:

  • J m^–2

  • J m^–3

energy from net primary production

• The energy remaining in the tissues of plants after respiratory loss is known as net primary production (NPP)

• NPP is available for:

  • plant growth and reproduction

  • transfer to consumers at other trophic levels when plant biomass is eaten by herbivores or broken down by decomposers

net production of consumers

• Consumers are organisms that gain energy from the tissues of other organisms

• When a consumer ingests the tissues of another organism, the stored chemical energy is either:

  • transferred to the consumer's tissues, where it is stored as chemical energy

  • lost to the environment

• The energy that is transferred to the tissues of consumers is the net production of consumers

  • This is also known as secondary production

• nergy is lost in the form of faeces and urine because:

  • consumers are not able to digest all of the food they eat, so some is egested as faeces

    • E.g. consumers may not digest all of the cellulose in plant matter, or the fur and bones of animal prey

  • energy may be stored in the bonds of excess amino acids, which are converted into urea for excretion in the urine

• Energy is lost during respiration in the form of heat, which is radiated into the environment

equation of net production of consumers

N = I - (F + R)

• Where:

  • I = the chemical energy in ingested food

  • F = the chemical energy lost to the environment in faeces and urine

  • R = the respiratory losses to the environment

productivity

the rate of primary or secondary production

primary production

transfer of energy to the tissues of plants

secondary production

transfer of energy to the tissues of consumers; also known as net production of consumers

equation for net primary production

NPP = GPP - R

• Where:

  • NPP = net primary production, or productivity

  • GPP = gross primary production, or productivity

  • R = respiratory losses

simplifying food webs

• Farmers can simplify food webs to reduce energy lost to non-human food chains; this can be achieved by removing pests from crops

  • Pests feed on crops, reducing crop biomass and meaning that crop plants need to expend energy on herbivory defences rather than on growth

  • This reduces the NPP of crops and therefore the energy available to humans

• Pests can be removed from crops with the use of, e.g.:

  • chemical pesticides

  • biological pest control

reducing respiratory loss

• The net production of livestock can be increased by reducing respiratory losses; this maximises the energy available for biomass production

• Respiratory loss can be reduced by, e.g.:

  • restricting movement: keeping animals in pens reduces energy needed for muscle activity

  • keeping animals warm: heated indoor environments reduce energy used for temperature regulation

  • antibiotics: these may be given to healthy animals to prevent infection, reducing energy used by the immune system

• These practices result in higher energy outputs in less time, often at lower cost, but they do raise ethical concerns about animal welfare

calculating percentage yield

• Modern farming strategies aim to increase the yield of crops or livestock

  • Theoretical yield is the yield that is theoretically possible under ideal conditions

  • Actual yield is the yield that is actually produced

• Percentage yield can be calculated as follows:

% yield = (actual yield ÷ theoretical yield) × 100