Introduction to Energetics

Because we lost a day of class to the Great Snow Day of 2012, I am trying to figure out what is the best way to make sure that we cover all of the important topics for my portion of the course. I think that it is important for you to have a brief introduction to energetics before we start talking about photosynthesis and cellular respiration, but I think that I can present the relevant information to you in this blog post.

In order to understand the two important energetic processes taking place in living organims (photosynthesis and cellular respiration) it is useful to understand some basics of energetics.From a physics perspective, energy is required to do work. Because this is a biology class, we will focus on biological work. The three main types of biological work are (1) active transport, (2) biosynthesis, and (3) movement. The key point for this class is to realize that organisms require energy to do the biological work required to keep them alive.

Energetic processes follow the laws of physics. The two most important laws of physics that relate to energy are the First and Second Laws of Thermodynamics.

First Law of Thermodynamics

The total amount of energy in the universe is constant. Energy can not be created and existing energy can not be destroyed. Energy can only undergo conversion from one form to another.

Biological relevance- No living organisms are capable of creating their own energy so they must get it from another source.

Second Law of Thermodynamics

Left to itself, any system undergoes energy conversion to less organized form. Each time this happens some energy becomes so disorganized that it is no longer available to do work.

Entropy is a measure of the amount of energy that is so disorganized so that it can no longer do work. A simpler way of stating the Second Law of Thermodynamics is that entropy increases over time.

What does it mean when energy becomes disorganized? Another term for "organized energy" is "concentrated energy". Energy is only able to do work when it is concentrated enough to power a particular process.

Apparent Problem

The Second Law of Thermodynamics states that entropy should increase over time, yet life contains highly concentrated energy. How can this be?? They key phrase in the definition is "left to itself". It turns out that energetically, the earth is not left to itself; the earth receives a constant input of energy from the sun and it is this energy that is used to fight entropy.

Light

Light energy from the sun reaches the earth. Light is part of the electromagnetic spectrum. Different portions of the electromagnetic spectrum vary in their wave lengths. Forms of electromagnetic energy with shorter wavelengths (e.g., x rays and gamma rays) contain more energy than forms of energy with longer wave lengths (e.g, radio waves). Interestingly, light falls within the middle of the spectrum with wavelengths from about 400 - 700 nm. Different wavelengths of light have different colors. Ranging from the longest to the shortest wavelengths the colors are red, orange, yellow, blue, green, indigo, violet (some people remember this using ROY G BIV).

As you might recall from your physics class, light has characteristics of waves and of particles. Light energy is "packaged" in units known as photons and the amount of energy in a photon depends of the wavelength of that light.

Fusion reactions on the sun convert nuclear energy in to electromagnetic energy. The electromagnetic energy travels through outer space until reaches the earth. Unfortunately, we,and all other organisms can not directly use light energy to do biological work. Instead light energy must be converted into potential energy stored in the chemical bonds of molecules. This potential (stored) energy can then be used to power biological work.

What Happens When a Photon of Light Hits a Molecule?

Three things can happen when a photon of light hits a molecule- (1) the light can be transmitted (passed through), (2) the light can be reflected, or (3) the photon of light can be absorbed.

When a molecule absorbs a photon of light energy, the electromagnetic energy of light excites an electron in the molecule to a higher energy level (thus, giving the electron potential energy). The excited electron almost immediately falls back to resting stage and the potential energy in the electron is converted into heat (a form of electromagnetic energy) which is released to the atmosphere.

Pigments

When we think of pigments, we think of color. What determines an objects color? The color of an object depends on the wavelengths of light that are reflected back to our eyes. Thus, when you see red you are seeing the red wavelengths that have been reflected from the object that you are looking at. What happens to the other wavelength? They have been absorbed.

Different molecules absorb and reflect different wavelengths of light. A pigment is defined as a molecule that absorbs particular wavelengths of light. What is important to remember is that the color of a pigment is the color of light it reflects.

Absorption Spectrum

An absorption spectrum is a graph that plots how much light energy is absorbed (y-axis, usually measured as intensity or as a percentage) versus the wavelength of the ligh (x-axis, measured in nm). Take a look at the absorption spectrum shown below. You can see that this pigment absorbs mostly green wavelengths and reflects the red and violet wavelengths. When the red and violet wavelengths reach your eye it would appear to you as purple.

Can you draw the absorption spectrum for a red, green and blue pigment?



Expected Learning Outcomes

By the end of the course a fully engaged student should be able to

- give examples of biological work
- list different forms of energy, give examples of the different forms, and give examples of energy conversions
- define the First and Second Laws of Thermodynamics and discuss why these laws are important for biologists
- discuss electromagnetic energy, including the wavelengths associated with different forms of electromagnetic energy and the relationship between wavelength and energy
- define a photon
- discuss the three things that can happen when a photon of light hits a molecule
- define a pigment
- draw and interpret an absorption spectrum

Further Reading

Electromagnetic radiation- http://www.eoearth.org/article/Electromagnetic_radiation