1. Classify an organism as a phototroph, chemotroph, autotroph or heterotroph based on their source of energy and source of carbon to build organic molecules.
2. Define the terms Gibb’s free energy, entropy and enthalpy.
Carbon-based molecules provide the structural and functional diversity needed by living organisms. But something besides being made of carbon must separate living organisms from nonliving things. This 19th century head, carved in a larger piece of coal is carbon-based but is not alive. In contrast, Stanley, the friendly French bulldog, is also carbon-based but well alive. Show a biscuit to the carved head and to Stanley, and you will immediately see the difference between the two. So what does characterize living organisms?
Aside from being carbon-based, living organisms are characterized by continuous biochemical reactions, and by continuous exchange of matter and energy with their surroundings.
What is the matter exchanged by living organisms?
What does it mean for energy to be exchanged?
These two questions will be addressed in this LESSON.
First, let’s try to figure out what type of energy and matter does an organism exchange with its surroundings?
Matter such as water, minerals, gases, and organic molecules are exchanged. An organism must also extract energy from its surroundings, in the form of sunlight or complex molecules found in food.
The energy extracted cannot be directly used by itself. This potential energy has to be converted. For example, the potential energy contained in food is extracted and converted into energy used by cells to do work through a process called catabolism. Work being, for example, movement during muscle contraction or the establishment of osmotic chemical or electrical gradients. This energy that is extracted and converted is also used to synthesize complex biomolecules from building blocks in a process called anabolism. Metabolism is the sum of catabolic and anabolic biochemical reactions.
Living organisms can be classified based on the source of energy and carbon they use. 
All the organisms on Earth use one of two sources of energy—sunlight or potential energy stored in food.
A Living organism using sunlight as a source of energy is called a phototroph.
A living organism extracting energy from chemical molecules is called a chemotroph.
And among chemotrophs you find lithotrophs using inorganic molecules as fuel, and organotrophs that extract energy from organic molecules.
To obtain carbon for building biomolecules autotrophs fix carbon dioxide, while heterotrophs rely on organic molecules.
Note that this metabolic classification is different from the most common phylogenetic classifications. For example, two species evolutionarily distant, such as the bacteria Escherichia coli and humans, are both classified as chemoorganotrophs.
As I mentioned earlier, life involves a constant flow of matter and energy among the living organisms.
The flow of matter is cyclical. Autotrophs, such as plants, generate organic molecules and oxygen that will be used by heterotrophs. In turn, heterotrophs produce water and carbon dioxide used by autotrophs.
In contrast, the flow of energy is irreversible. Most of the energy coming in to Earth is from the sun. Part of this energy is captured by photosynthetic organisms. Other autotrophs receive the energy from chemical compounds. In both cases, the energy is converted to chemical energy used by both autotrophs and heterotrophs. But ultimately, energy is lost at various stages as heat or thermal energy. This form of energy cannot be reused by living organisms, because they cannot convert heat into chemical energy. So what is energy? You can think of energy as the capacity to work.
Like any system in our universe, living organisms obey the laws of thermodynamics.
Two laws dictate the ways living organisms use their energy. The first law of thermodynamics states that the universe contains a constant amount of energy. Energy is not created nor destroyed, but it can change form. Imagine a chicken on the ground floor and trying to go one floor up.
Why not?
The chicken possesses a certain amount of potential energy, depending on its position relative to its surroundings. Its potential energy is lower at the bottom of the stairs and higher at the top. If the chicken wants to go up the stairs, energy will be consumed and transformed. If the chicken opts to take the stairs, it converts energy from food into chemical that is then converted to kinetic energy, allowing the chicken to go up the stairs.
Alternatively, if it is a lazy chicken that takes the elevator, the elevator’s electrical energy is converted to make energy called kinetic energy that drives the chicken upstairs. Once again, the chicken at the top of the stairs has a higher potential energy. But you should note that the change of potential energy is the same, whether the chicken walks or takes the elevator.
Similarly, the molecule possesses potential energy which is stored in chemical bonds. There may be differences in how molecule A is converted to molecule B, with a different energy potential. But the change of potential energy will remain the same, regardless of the path used to go from to A to B. The second law of thermodynamics states that energy is transferred in a way that increases the randomness of the universe. The randomness is also called entropy. A spontaneous increase of entropy can be illustrated by the unavoidable decay of a sand castle under the action of the winds or the rising tide. This decay is far more likely than the spontaneous organization of grains of sand into a sand castle. Increased entropy mostly occurs through the transformation of various forms of energy into thermal energy.
A Question: all the time our cells are synthesizing molecules, and that seems like it would decrease entropy. So aren’t we breaking the second law of thermodynamics all the time?
ALi: Anabolism does decrease entropy by assembling large molecules from building blocks. But it does not break the second law of thermodynamics. Remember, the second law of thermodynamics applies to the whole universe, not to an isolated system. The decreased entropy associated with anabolism does not override the overall entropic increase in the universe. In fact, the energy lost as heat during anabolic reactions contributes to the increase of the entropy in the universe. The Gibb’s free energy equation is a fundamental equation describing thermodynamics, and can be applied to biochemical reactions.
G = H – TS
This equation connects three thermodynamics parameters– the free energy, G, H, the enthalpy, and S, the entropy. This equation states that the free energy of a system equals the enthalpy minus the product between entropy and temperature.
So what is enthalpy?
Enthalpy is the total energy of your system. In the case of a biochemical reaction, it includes all the energy stored in the chemical bonds of all the molecules involved in the reaction, as well as the environment around the reaction.
What is the meaning of the temperature times entropy– TS?
We know that S, the entropy, is a quantitative expression for the randomness of the system.
T is the temperature expressed in degrees Kelvin. And T amplifies the entropy term of the equation, because raising the temperature intensifies random molecular motion, leading to an increased disorder.
Now what is free energy?
While the enthalpy is the total energy of the system, the free energy is the only portion of the energy that is available to do work.
How much energy is not available to do work?
The answer is T times S. Whenever the cells do work, some energy is wasted as heat which contributes to random motion and increased entropy.
It explains why the flow of energy among living organisms is irreversible.
So in this LESSON, you have learned how matter flows from one type of living organism to another. You have also learned that the flow of energy in living organisms obeys the laws of thermodynamics. In the next LESSON, you’ll learn that knowing the free energy of a molecule helps us to predict how a molecule will react when combined with another molecule or when put in a situation when there is a chance for this molecule to change energetic state.
ALI SUDAIS JAN 