Introduction

What do the hairs on your arm “standing” upright, movement of nutrients into the cell, and production of food by photosynthesis all have in common? These are processes that require energy input at the cellular level. When you think of where you obtain cellular energy, you may say from eating nutritious food. You would not be wrong in that answer, but in the next few chapters we will peel back the layers to understanding metabolism basics at the cellular level and on the organism level. We will explore the energy currency of the cell, the process from which cells are able to produce energy which is used to power the above-mentioned processes and many others, and discuss how you are able to use nutrients within food to produce energy at the cellular energy through a process called cellular respiration.

Cells are miniature factories inside of which many reactions take place every second. The collection of all these reactions is referred to as cellular metabolism. Two examples of reactions taking place within the cell are reactions that break large organic molecules into smaller subunits and those that build large organic molecules from their subunits. Think about some of the important organic macromolecules that we discussed in Module 2 (carbohydrates, lipids, proteins, and nucleic acids). Can you name the repeating units, or monomers, that compose these macromolecules? In this module, we will explore how the single units, monomers, are combined to produce the polymers mentioned above, and how macromolecules can then be broken down. Why is this important for the overall functioning of the cell/ organism? We will add to the “why” in the next as we will look specifically at the break down of the sugar molecule (the monomer of carbohydrates) and how this monomer can be broken down to release energy.

We will also explore enzymes: molecules that catalyze reactions within the cell. We will examine how enzymes are able to speed reactions within the cell, factors that influence enzyme function in the cell, and how enzymes are regulated. By the end of this module, you should have an understanding of the laws of thermodynamics, the idea of spontaneous and non-spontaneous reactions and how to determine the nature of a reaction within the cell. In addition, you should understand the energy currency of the cell. Here we will focus on ATP.

Next, we will look the process of energy production at the cellular level or cellular respiration .

Previously, you learned about an organelle called the mitochondria . This organelle is often called the “powerhouse” of the cell. Why? The mitochondria is where most of the energy (ATP) is produced in the cell. Since this is a building course, think about what you can bring to the discussion in this module from your previous work. In Module 3, The Cell, you were introduced to the mitochondria and the cell types where you would expect to see more or less of these organelles. What were they? You know that eukaryotes have membrane-bound organelles, under which the mitochondria falls. Some examples of organisms that have mitochondria, are plants and animals.

What about bacteria? Bacteria fall under the prokaryote classification. We know from Module 2 that these organisms do not contain membrane-bound organelles, but some bacteria do have the ability to undergo cellular respiration (the process that produces energy/ ATP). In fact the mitochondria is thought to have originated through an endosymbiosis event where a eukaryotic cell engulfed (the “endo” part of endosymbiosis) a bacteria that had the ability to produce energy and that bacteria was incorporated (the “symbiosis” part of endosymbiosis) into the overall functioning of the eukaryotic cell rather than being digested and broken down. This is called theTheory of Endosymbiosis and is a topic for a discussion board this module.

In this module, you will also focus on understanding the processes of cellular respiration and fermentation. Here, you will research how ATP is made. We will take each of the three steps that make up cellular respiration and break them down for a better understanding of the process on the whole. For each step, compare the specific way in which the ATP is produced. It is important that for each of the following steps: Glycolysis, the Citric Acid Cycle (or Kreb’s Cycle), and the Electron Transport Chain, you understand:

1. where in the cell each takes place
2. how much ATP is produced
3. the general process of each step
4. how steps one and two (glycolysis and the Citric Acid Cycle, also called the Kreb’s Cycle) facilitate the last.

Be familiar with the compartments of the mitochondria and understand where the phases mentioned above take place. This will require a review of the use of the electron carriers, where the electron carriers “take” and “drop off” electrons, and where the energy-rich electrons come from originally. A good way to think about this process on the whole is the stepwise breakdown of sugar to release energy. Other molecules can be used to produce ATP and we will explore alternative fuel sources for this process as well. We will compare fermentation and cellular respiration and, for the types of organisms that undergo both, determine which is more beneficial to the cell ¹ .