Lecture 3 Energetics: Lecture Notes part b

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- Jes

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  • The first slide shows two different schematic representations of the earth's energy balance
    • One shows energy flux in terms of short wave and long wave radiation inputs and outputs to and from the earth's surface
    • The other shows the energy flux in terms of trophic levels.

The major theme of this lecture: abiotic and biotic energy fluxes are interconnected and that an understanding of both is important if you want to understand ecosystem energetics.


Laws of thermodynamics

First Law
Energy is neither created nor destroyed. It can be transformed, converted into one form or another.
Heat is a form of energy.
SI unit: Joules [ 1 J = 1 (kg * m2) / s2]

Second Law
Energy can't recycle - in each transformation some of its capacity to do work gets used up.
Energy flows from a a low-entropy state to a high-entropy state.

Second Law and Life
Entropy is a physical quality, can be measured as J/K. It's the inverse of the ability of energy to do work.
Also measures the degree of disorder in a chemical system: the more disorder, the more entropy.
IOW: energy released as respiration is dissipated into the environment and can no longer do work. It's less ordered (there's more entropy)
Sunlight is a low-entropy form of energy.
Life (chemical energy stored in organic molecules) is a mid-entropy form of energy (in comparison to low-entropy)
Heat (surrounding environment) and thermal radiation are high entropy
Living things have more ordered chemistry than their surroundings. The energy stored for growth and reproduction is in a more ordered state than the energy excreted from the organism.

Radiation, energy and heat

Electromagnetic Radiation
- the whole spectrum of light: UV, visible light, photosynthetic active radiation (PAR), Infrared radiation (IR), microwave radiation, thermal radiation
Visible Light: radiation from the sun that passes through the atmosphere
IR = long-wave radiation = thermal radiation
Below is a rather massive diagram of EMR, that gives a basic sense of size for each wavelength.

  • all objects and bodies that have a temperature emit long wave radiation
  • every object emits EMR
  • the distribution of wavelengths emitted is affected by the temperature of the object

e.g. The temperature of the sun is 6000K. It's peak emissions consist of photons with more shorter wavelengths than those of the earth, whose temperature is 298K. Therefore, we tend to say the the earth emits longwave radiation, while the sun emits shortwave radiation

  • at higher temperature, EM peaks at shorter wavelengths.

Sensible heat

  • "heat we can sense or feel"
  • kinetic energy of molecular motion in solids, liquids or gas
  • can be transferred between bodies of different T, from hot to cold, via conduction, convection, advection.
  • Heat is a form of energy

Latent heat

  • energy is required to change liquid or solid water into water vapour
  • during evaporation, sensible heat is converted to latent heat IOW: energy in the form of sensible heat is consumed by water and held in water vapour as latent heat
  • during condensation, latent heat is converted to sensible heat IOW: energy is released by the water molecules as heat

Energy transformations in living things

Photosynthesis: the process by which light energy is converted into chemical energy.
6CO2 + 6H2O + light energy —-> C6H12O6 + stored chemical energy + 6O2
Some energy is lost through respiration, the rest is used for growth and reproduction. Energy used for growth and reproduction is available to consumers.

Energy transformations in living things

Trophic Levels: could be described as levels of energy capture within the biotic system
1. primary producers (autotrophs)
2. consumers of producers (e.g. herbivores)
3. consumers of consumers of producers (e.g. predators)

Autotrophs: capture energy from inorganic sources, either solar or chemical (e.g. HCN, CO2)
Heterotrophs: Acquire energy from other organic sources, include herbivores, predators, detrivores, parasites, fungi …

  • the flow of energy connects organisms in a food web
  • energy flows in ONE direction (low —> high entropy)
  • 5 - 20% of energy acquired by one trophic level is passed onto the next trophic level. This creates a trophic pyramid.
  • production in each trophic level is expressed as a flux
    • flux of energy: kJ/m2Y
    • flux of carbon: g C/m2y

Bioenergetics and oxidation-reduction

Oxidation: the process of losing electron(s)
Reduction: the process of gaining electron(s)
A reducing agent: loses/donates electrons and is oxidized, reduces other compounds
An oxidizing agent: gains electrons/is reduced, oxidizes other compounds.

  • the atmosphere is oxidizing
  • chemical compounds used in life are reduced when they are storing energy
  • photosynthesis reduces carbon
  • respiration oxidizes carbon

Aerobic and anaerobic respiration

Aerobic respiration: breakdown of organic molecules in the presence or involving oxygen (O2)

  • Both plants and animals do this, as well as some microbes
  • Electron acceptor in this process is oxygen

Anaerobic respiration: breakdown of organic molecules in the absence of oxygen

  • Some microbes and animals do this (animals - including humans - as fermentation)
  • Since there is no oxygen, this process uses different electron acceptors, such as NO3, SO4, Fe, etc.
  • Another type of anaerobic metabolism is methanogenesis, where CO2 is reduced to methane. CO2 is a "last resort" electron acceptor

Redox reactions involving nitrogen

  • this was just touched on in class, as we'll go through the nitrogen cycle at another time.
  • the only point made is that nitrogen will cycle through the biotic and abiotic systems via redox reactions.

Ecological efficiencies and ecosystem energetics

Recall that energy flows only one way and cannot be reused. With that in mind:

  • Only a very small part of the energy in sunlight (max is 2%) is captured through photosynthesis and can be used in the food web
  • Limits on consumer and predator populations (higher levels in the trophic pyramid) are set by trophic efficiencies (5-20% normally)
  • Energy input into an ecosystem can come from within the system (autochthonous energy input) or outside the system (allochthonous energy input)
  • Ecosystems can also be autotrophic or heterotrophic.
  • When energy flows through a system, entropy must increase in the universe (2nd law of thermo). In an ecosystem, it is generally released as heat.

Leaf temperature and leaf energy budget

This was a picture of a leaf illustrating energy input and output for the leaf system.


  • SW radiation (UV light)
  • possibly sensible heat (if the ambient temperature is warmer than the leaf's temperature)

Outputs (or what the leaf emits):

  • reflected SW radiation
  • latent heat (H2O) - through transpiration (cooling mechanism)
  • LW radiation (in the form of IR radiation, comes from higher temperature of leaf compared to surroundings)
  • CH2O (chemical energy created through photosynthesis: at best, 2% of input energy from sunlight)
  • possibly sensible heat (if ambient temperature is less than leaf temperature)

Net radiation is given by the following equation: Rn = (1-ρ)*St + Ld - Lu,

  • St = shortwave radiation transmitted (down) through atmosphere
  • ρ = shortwave albedo
  • Ld = longwave downwelling, or LW energy that enters ecosystem from above
  • Lu = longwave upwelling, or LW energy that leaves ecosystem by going upwards, such as IR radiation in the previous example

Every ecosystem has an energy budget (a leaf, a desert, a tropical rainforest…), which is not static. The budget will tend to fluctuate throughout the day and throughout the year.

Global scale energy transformations

Key point in this section was that the atmosphere is heated from below, UV radiation hits the surface, and heats the ground/rocks/vegetation, which then emits thermal radiation (LW) that rises to heat the atmosphere. A bottom up approach!

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