Energy Flow in Ecosystems

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  1. Describe three main laws of physics important to natural systems
  2. Discuss the second laws of thermodynamics
  3. Apply the second law to life processes and the environment

Energy Flow

All of nature is driven by energy. How this energy flows through the environment is important for understanding ecolgy.

It is governed by three laws:

  • Conservation of Mass
  • Conservation of Energy (First Law of Thermodynamics)
  • Second Law of Thermodynamics

Conservation of Mass

Conservation of Mass

All mass is conserved - it is neither created nor destroyed.


force acting through a distance
the capacity (ability) to do work
energy transferred due to a temperature difference

Conservation of Energy

Conservation of Energy

All energy is conserved - it is neither created nor destroyed.

(or There is no free lunch)

For historical reasons this is also known as the First_Law_of_Thermodynamics


you need fuel to keep your car running
you need food to keep yourself running

An alternative way to define the Conservation of Energy is

The change in energy (U) is equal to work (W) plus heat (Q)

ΔU = W + Q

This implies that energy can only be transferred two ways: work and heat.

It also implies that heat and work should behave the same way, but they do not.

Second Law


  • How do we uncook rice?
  • How do we unpeel an orange?
  • A book easily falls to the floor, but takes much effort to get back up. Why?

First law says cooking and uncooking are exactly the same.

These questions lead to the second law of thermodynamics.

Second law of thermodynamics

No system can completely convert heat to work
All systems tend toward disorder

The measure of disorder is called the Entropy

The second law therefore says that the total entropy will always increase.

Important notes about the Second Law:

  • We can convert work entirely into heat.
  • Since heat cannot be entirely converted to work, we say heat is lower quality than work.
  • Whenever energy is converted from one form to another, some is lost as heat.


The ultimate source of energy for life processes is the SUN.

Plants convert the sun's energy (light) into food (energy reserve) by photosynthesis

Plants and animals then use respiration and other processes to breakdown the food releasing energy

This energy is then used to move, think, and other actions.

  • Each step above involves changing the form of energy. Therefore, by the second law of thermodynamics each step losses waste heat and the resulting energy is of lower quality.

Whenever energy is transformed through life processes some energy is lost as heat (waste heat). Actually in each step about 90% of the energy is lost as heat

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Web Resources

Thermodynamics - A wikieducator project on this topic