Cellular Metabolism

Notes on the Web - Unit Four- Part 4

Cellular Metabolism
Bruce G. Stewart

General Objectives and Study Guide

Your objectives for these Notes on the Web and associated readings and exercises are:

To state and 1st and 2nd laws of thermodynamics and explain how they relate to life;
To define metabolism and list and define the general categories of substances and reactions involved;
To define ATP and explain its importance in metabolism and in the accomplishment of work in living things:
To describe metabolic pathways and the function of enzymes in reactions;
To define photosynthesis and explain the role of pigments in its function;
To reproduce the summary reaction equations for photosynthesis and cellular respiration and to explain their significance in understanding life;
To describe the general function of two energy organelles, chloroplasts and mitochondria;
To diagram a summary of cellular respiration including glycolysis, the Kreb's Cycle, and the electron transport chain showing the overall yield of ATP, NADH, and FADH₂ and the input of O₂ and output of CO₂;
To define three types of fermentation and identify advantages and limitations of each;
To define catabolism and anabolism;
To know common terms and facts associated with the topic of cellular metabolism.

For General Biology (BIO 1114) Class only: Johnson and Losos (2008) Chapter 6, Chapter 7 Section 7.1, and Chapter 8
For General Zoology (ZOO 1114) Class only: Hickman, et al. (2004) Chapter 4

Living Things Obtain and Manage Energy and Nutrients

A rock can passively have its energy increased! When sunlight strikes a rock, the temperature of the rock and the total amount of thermal energy it possesses increases. However, a rock cannot control this process, nor can it use the energy to do work or to obtain nutrients.

Living things by contrast can obtain and control energy through processes like photosynthesis and metabolism. Photosynthesis is the conversion of visible sunlight energy into other stored or usable forms of energy. It can be briefly summarized by the following simple chemical equation:

6CO₂ + 6H₂O --> C₆H₁₂O₆ + 6O₂

Metabolism is the process of converting energy from a stored form (e.g. energy in a sugar molecule) to other forms. All living things perform metabolism, but for those that cannot photosynthesize it is the only way to get usable energy. One type of energy-releasing metabolism is called aerobic respiration. It is basically the opposite of photosynthesis and therefore requires energy-rich molecules like simple sugars and oxygen to work. It can be summarized as follows:

C₆H₁₂O₆ + 6O₂--> 6CO₂ + 6H₂O

The energy from photosynthesis and metabolism can be used to do work such as building molecules of the body structure and creating movement. To summarize, all forms of life show metabolic activity; they extract & transform energy from their environment and use it for manipulating materials in ways that assure their own maintenance, growth, development & reproduction.

Nutrients are substances that an organism takes in and uses for growth and maintenance. Animals, for example, must metabolism nutrients to get the energy necessary to live. However, they also must use the atoms and molecules of nutrients as raw material for building their own molecules that make up their bodies. Both energy and materials are needed by all organisms. The elements that make up nutrients can be recycled, but the energy used to construct them cannot.

Endergonic versus Exergonic Reactions. In class or via Internet you will learn about these two energy concepts. Basically, compounds can store energy in their formation and release it to do work later. The energy storing reacts are called endergonic while the energy releasing reactions are termed exergonic. These two types of reactions can be coupled to allow transfer of energy for various purposes.

Location of Metabolic Activity. Metabolism in its various forms occurs inside cells. For example, photosynthesis occurs in the chloroplasts of plant and photosynthetic protist cells. Digestion is different from metabolism. It is the process of physically processing foods and chemically breaking food molecules down into small compounds like simple sugars, amino acids, and fatty acids that can be absorbed into the cells. Within cells specific metabolic processes occur in specific organelles or in the cytoplasm. The first steps of aerobic metabolism of all eukaryotic cells occurs in the cytoplasm, and later steps occur inside the mitochondria. In our class and/or in your textbook studies and Internet discussion thread interactions we are going to focus on the following aspects of metabolism:

Glycolysis (the initial breakdown of glucose series of reactions in the cytoplasm)
Transition Reaction (the pyruvate to acetyl group reaction) (occurs in the mitochondrion)
Krebs Cycle (= Citric Acid Cycle) (a cycle of reactions that transfer energy to intermediate high energy compounds plus some ATP (occurs in the mitochondrion)
Electron Transport System (oxidative phosphorylation system that produces the ATP) (occurs in the membrane of the cristae in the mitochondrion)

There are many details to these processes. However, the following will help you focus on some important summary details:

For General Biology students, from you textbook study and learn Figure 7.9: Accounting of Energy Yield per Glucose Molecule Breakdown. You should be able to reproduce it from memory.
For General Zoology students, from you textbook study and learn Figure 4-15. You should be able to reproduce it from memory.

Reminder about Textbook Study

As with other topics, your textbooks have excellent presentations of the materials on cellular metabolism, including generous excellent illustrations. Check the general objectives above to make sure that you have covered all of the topics in the textbook readings.

The "Self Test" and other questions will be helpful for general biology students, although many more detailed questions will be included in the lecture exam. Similarly, study questions in the zoology textbook will be helpful review for general zoology students, but again, they are not comprehensive.

As with all materials throughout the semester, you will have opportunities to ask questions or ask that any relevant material from your assignments be discussed in class and/or in threaded discussions on Internet.

Related Links

As is typical for all of us, learning is reinforced when we view and study the same materials presented in more than one way. Thus, I offer you the following links to resources that are covered in this part of your Notes on the Web.

The following link has a summary "map" of the metabolism of glucose by aerobic respiration. You can click on different areas of the map to study sections in more detail.

Metabolism of Glucose: Glycolysis, Kreb's Cycle, and Electron Transport Chain.

Here is another helpful link:

http://www.jonmaber.demon.co.uk/glyintro/page01.htm

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