6. Biochemical Pathways—Cellular Respiration


Mutation Leads to Personal Energy Crisis

Genes You Inherit from Mom Can Be Harmful

Ten-year-old Latisha Franklin has suffered from her own personal energy crisis since she was four. Latisha has been diagnosed with an uncommon illness, an abnormality called mitochondrial encephalopathy, or MELAS. MELAS is the acronym for mitochondrial encephalopathy, Zactic acidosis, and stroke-like episodes. It is caused by mutations in the DNA found in her mitochondria, mDNA. Mitochondria manufacture proteins using their own DNA. Enzymes help to produce useful chemical bond energy for cells, ATP. mDNA differs from the chromosomes found in the nucleus. They are much smaller and circular. Any changes (mutations) in mDNA can have far-reaching effects on the body’s ability to control energy production.

Latisha has suffered encephalopathy in the form of epilepsylike seizures and migraine-like headaches. She has also had severe muscle pain caused by excess lactic acid in her muscles, and stroke-like symptoms leading to paralysis and confusion.

The mutations that cause MELAS and the chemical changes that occur in mitochondria have been identified; however, there is no cure. Medical professionals can only manage symptoms.

• In what molecular form do cells use chemical-bond energy?

• How are these energy-containing molecules generated by cells?

• Why would a strict vitamin regimen be helpful in managing Latisha’s symptoms?


ü  Background Check

Concepts you should already know to get the most out of this chapter:

•   Features of oxidation-reduction chemical reactions (chapter 2)

•   The structure of carbohydrates (chapter 3)

•   The structure and function of mitochondria and the types of cells in which they are located (chapter 4)

•   How enzymes work in conjunction with ATP, electron transport, and a proton pump (chapter 5)


6.1. Energy and Organisms


There are hundreds of different chemical reactions taking place within the cells of organisms. Many of these reactions are involved in providing energy for the cells. Organisms are classified into groups based on the kind of energy they use. Organisms that are able to use basic energy sources, such as sunlight, to make energy-containing organic molecules from inorganic raw materials are called autotrophs (auto = self; troph = feeding). There are also prokaryotic organisms that use inorganic chemical reactions as a source of energy to make larger organic molecules. This process is known as chemosynthesis. Therefore, there are at least two kinds of autotrophs: Those that use light are called photosynthetic autotrophs and those that use inorganic chemical reactions are called chemosynthetic autotrophs. All other organisms require organic molecules as food and are called heterotrophs (hetero = other; troph = feeding). Heterotrophs get their energy from the chemical bonds of food molecules, such as carbohydrates, fats, and proteins, which they must obtain from their surroundings.

Within eukaryotic cells, certain biochemical processes are carried out in specific organelles. Chloroplasts are the sites of photosynthesis, and mitochondria are the sites of most of the reactions of cellular respiration (figure 6.1). Because prokaryotic cells lack mitochondria and chloroplasts, they carry out photosynthesis and cellular respiration within the cytoplasm or on the inner surfaces of the cell membrane or on other special membranes. Table 6.1 provides a summary of the concepts just discussed and how they are related to one another.



FIGURE 6.1. Biochemical Pathways That Involve Energy Transformation

Photosynthesis and cellular respiration both involve a series of chemical reactions that control the flow of energy. Organisms that contain photosynthetic machinery are capable of using light, water, and carbon dioxide to produce organic molecules, such as sugars, proteins, lipids, and nucleic acids. Oxygen is also released as a result of photosynthesis. In aerobic cellular respiration, organic molecules and oxygen are used to provide the energy to sustain life. Carbon dioxide and water are also released during aerobic respiration.


TABLE 6.1. Summary of Biochemical Pathways, Energy Sources, and Kinds of Organisms


Autotroph or Heterotroph

Biochemical Pathways

Energy Source

Kinds of Organisms




Inorganic chemical reactions

Certain Bacteria and Archaea

There are many types of chemosynthesis.




Certain Bacteria and Archaea

Photosynthesis in Bacteria and Archaea differs from photosynthesis that takes place in the chloroplasts of eukaryotic organisms.

Eucarya—plants and algae

Photosynthesis takes place in chloroplasts.

Autotroph and heterotroph

Cellular respiration

Oxidation of large, organic molecules

Bacteria and Archaea

There are many forms of cellular respiration. Some organisms use aerobic celluar respiration; others use anaerobic cellular respiration. Cellular respiration in Bacteria and Archaea does not take place in mitochondria.




Eucarya—plants, animals, fungi, algae, protozoa

Most Eucarya use aerobic celluar respiration and it takes place in mitochondria.


This chapter will focus on the reactions involved in the processes of cellular respiration. In cellular respiration, organisms control the release of chemical-bond energy from large, organic molecules and use the energy for the many activities necessary to sustain life. All organisms, whether autotrophic or heterotrophic, must carry out cellular respiration if they are to survive. Because nearly all organisms use organic molecules as a source of energy, they must obtain these molecules from their environment or manufacture these organic molecules, which they will later break down. Thus, photosynthetic organisms produce food molecules, such as carbohydrates, for themselves as well as for all the other organisms that feed on them. There are many variations of cellular respiration. Some organisms require the presence of oxygen for these processes, called aerobic processes. Other organisms carry out a form of respiration that does not require oxygen; these processes are called anaerobic.



1. How do autotrophs and heterotrophs differ?

2. What is chemosynthesis?

3. How are respiration and photosynthesis related to autotrophs and heterotrophs?