Obligate aerobes require oxygen to make ATP. Obligate anaerobes cannot survive in the presence of oxygen, so they occupy only anoxic habitats. Facultative anaerobes make ATP with oxygen, but if oxygen levels become low, they can use fermentation.
Some bacteria, including those we employ to make yogurt, make ATP using lactic acid fermentation; the acid may help reduce competition from other bacteria. Muscle cells can continue to produce ATP when O2 runs low using lactic acid fermentation, but muscle fatigue and pain may result.
Red muscle fibers use mostly aerobic respiration to make ATP for endurance tasks; white muscle fibers use mostly lactic acid fermentation to make ATP quickly for short, intense activities. Human muscles contain a mixture of red and white fibers, but genetics may give sprinters more white fibers, and marathoners more red.
Both alcoholic and lactic acid fermentation pathways change pyruvate in order to continue producing ATP by glycolysis.
Ethanol produced by bacteria through alcoholic fermentation of corn (and perhaps other fuels in the near future) may provide a more renewable fuel for vehicles than the fossil fuels upon which we currently depend. We employ yeasts to help make bread through alcoholic fermentation; as they produce carbon dioxide, the bread dough rises. We employ anaerobic organisms to make beer and wine through alcoholic fermentation; the alcohol content is limited to 18% by volume because levels above that are toxic to these organisms.
Aerobic respiration is far more energy-efficient than anaerobic respiration. Aerobic processes produce up to 38 ATP per glucose. Anaerobic processes yield only 2 ATP per glucose.
Review Questions
Classify your own cells as obligate aerobes, obligate anaerobes, or facultative anaerobes, and explain your reasoning. (Although these terms usually apply to whole organisms, assume they can also apply to individual cells within your body).
Identify yourself as a “sprinter” or an “endurance runner” and predict the type of muscle fiber (red or white) which predominates in your body. Explain your reasoning.
Construct a chart which compares alcoholic to lactic acid fermentation, considering at least three different features.
Outline the process used to produce fuel from corn and explain why some consider this fuel “renewable” and preferable to fossil fuels. Research the pros and cons of this fuel.
Explain how fermentation is used to make bread.
If two species of bacteria – one using aerobic respiration and the other using anaerobic respiration - were competing for the same source of glucose in the same environment, which one would out-compete the other? Explain why.
Human cells cannot carry out alcoholic fermentation, yet we use it for many purposes. Analyze its importance to human life.
Explain why both types of fermentation must change pyruvic acid, even though no energy is gained in this conversion.
Indicate the maximum alcohol content of wine and beer, and explain the reason for this limit.
Construct a chart comparing aerobic to anaerobic respiration using at least 5 characteristics.
Further Reading / Supplemental Links
Martin Hoagland, Bert Dodson, and Judith Hauck, Exploring the Way Life Works: The Science of Biology. Jones and Bartlett Publishers, Inc., 2001. Chapter 3: “Energy – Light to Life,” pp. 87-138.
“Biology, Answering the Big Questions of Life/Fermentation student lab” and “Biology, Answering the Big Questions of Life/Fermentation.“ Wikibooks, last modified 13 March 2007. Available on the web at: http://en.wikibooks.org/wiki/Biology%2C_Answering_the_Big_Questions_of_Life/Fermentation
Diana C. Linden and Roberta Pollack, “Chart of Important metabolic products.” In Biology 130 Introduction to Cellular Biochemistry Lectures, Occidental College, last updated 21 October 2000. Available on the web at:
http://departments.oxy.edu/biology/bio130/lectures_2000/metabolic_products.htm
J. Stein Carter, 1996, “Cellular Respiration and Fermentation.” last modified on Tue 02 Nov 2004. Available on the web at:
http://biology.clc.uc.edu/courses/bio104/cellresp.htm
John Kyrk, “Glycolysis.” Cell Biology Animation, 28 April 2007. Available on the web at: http://www.johnkyrk.com/glycolysis.html.
M.J. Farabee, 1992, 1994, 1997, 1999, 2000, 2001, 2007. “Glycolysis, the Universal Process.” Biobook, Estrella Mountain Community College, last modified 2007. Available on the web at: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookGlyc.html
Vocabulary
aerobic
With oxygen, or living or occurring only in the presence of oxygen.
alcoholic fermentation
The process for making ATP in the absence of oxygen, by converting glucose to ethanol and carbon dioxide.
anaerobic
Without oxygen; living or occurring in the absence of oxygen.
facultative anaerobe
An organism which can respire aerobically when oxygen is present, but is also capable of fermentation when oxygen levels are low.
glycolysis
The process of “splitting glucose” - stage 1 of aerobic cellular respiration and also the basis of anaerobic respiration; splits glucose into two 3-carbon pyruvates, producing 2 (net) ATP.
lactic acid fermentation
The process for making ATP in the absence of oxygen by converting glucose to lactic acid.
obligate aerobe
An organism which requires oxygen for cellular respiration.
obligate anaerobe
An organism which uses anaerobic respiration, and dies in the presence of oxygen.
Points to Consider
Humans seem to harness anaerobic respiration much more than aerobic respiration to create useful products, such as foods or fuels. Use your understanding of the two processes to explain why this makes sense.
Some controversy exists over whether or not ethanol produced by fermentation of corn is an efficient and wise way to produce fuel. Can you think of some reasons, pro and/or con?
How might the wing muscles of birds which migrate long distances compare to those of birds which do not migrate? Why do you suppose human muscles are mixtures of red and white fibers, rather than specialized, as in many birds?
Chapter 6: Cell Division and Reproduction
Lesson 6.1: Chromosomes and the Cell Cycle
Lesson Objectives
Describe the properties of cell division in prokaryotes.
Describe cell division in eukaryotes. Explain the main differences between cell division in prokaryotic and eukaryotic cells.
Describe the basic properties of chromosomes.
Describe the key steps in the cell cycle.
Identify and describe the main processes in mitosis.
Describe how the cell cycle is controlled and define cancer.
Introduction
You are made of many different types of cells. Nerve cells, skin cells, muscle cells, and many more. These cells obviously have many different functions, yet they all develop from the first cell that makes you. So do they all have the same DNA? Are all the cells in your body genetically identical? How does the first cell of an organism know to become two cells, then four cells, and so on? What tells these cells what to do? Your body produces about 25 million genetically identical cells every second. These new cells are formed when older cells divide, a process called cell division or cell reproduction.
Cell division is the final step in the life of a cell, otherwise known as the cell cycle. Eukaryotic cells and prokaryotic cells complete this process by a number of different mechanisms. The cell cycle is a repeating series of events, during which the eukaryotic cell carries out its necessary functions, including metabolism, cellular growth, and division, resulting in two genetically identical daughter cells. To produce two genetically identical daughter cells, the chromosomes need to replicate and the nucleus and cytoplasm need to divide. These are key events in the life of a cell.
Cell Division in Prokaryotes
Prokaryotic organism
s reproduce asexually by binary fission, a process that produces identical offspring (Figure below). In asexual reproduction, a single parent produces genetically identical offspring. As prokaryotes do not have a nucleus, and have only one circular chromosome, they do not need to reproduce by the same mechanism as eukaryotic cells. Prokaryotic cell division is a much simpler process. In prokaryotic cell division, after the single chromosome is copied, the cell grows larger. Eventually the two chromosomes separate and move to opposite ends of the cell. Newly formed cell membrane then grows into the center of the cell, separating the two chromosomes, and forming two genetically identical daughter cells. The formation of two daughter cells is called cytokinesis.
Figure 6.1
Binary fission. In binary fission, the single chromosome is copied and eventually separates into two separate chromosomes, the cell grows larger, and two identical cells form by cytokinesis.
Under ideal conditions, reproduction in bacteria is extremely efficient, with some bacteria reproducing every 20 minutes. This makes bacteria an extremely effective tool for the molecular biologist. However, bacteria do not usually live in ideal conditions; otherwise, bacteria would grow and divide extremely rapidly, eventually covering the surface of Earth. Bacterial growth is limited by nutrients and water, predation, and by their own wastes.
Cell Division in Eukaryotes
Cell division in eukaryotic organisms is very different from that in prokaryotes, mainly because of the many chromosomes in the nuclei of eukaryotic cells. Cell division in eukaryotic organisms is necessary for development, growth, and repair. This cell division ensures that each resulting daughter cell receives a complete copy of the organism’s entire genome. Remember that all of an organism’s DNA must be present in each somatic, or body, cell. This DNA contains the information necessary for that cell to perform its functions, and to give that organism its traits. Therefore, prior to cell division, the eukaryotic cell’s complete genome must be copied, ensuring that each daughter cell receives a complete set of the genome.
The formation of gametes, an organism’s reproductive cells, such as sperm and egg cells, involves a completely different method of cell division. This cell division ensures that each gamete receives half the amount of an organism’s DNA.
DNA, Chromosomes, and Genes
As previously discussed (in the Foundations of Life Science chapter), DNA contains the information necessary to make proteins, direct a cell’s activities, and give an organism its traits. This information is organized into structural units scattered along the length of the DNA molecule. These units are known as genes. A gene contains the information necessary to encode an RNA molecule or a protein. A single DNA molecule contains hundreds to thousands of genes. Different cell types use the information in different genes to make different proteins. This process gives different cell types distinct activities. Thus, a liver cell will have many different proteins than a kidney cell, giving the two cells types distinct activities. When a cell is using the information within a gene, the segment of DNA containing that gene is unwound, exposing the double helix to the cell machinery needed to use that information.
Prior to cell division, the DNA must duplicate itself in a process called DNA replication. This ensures that each resulting cell receives a complete set of the organism’s genome. But how is the replicated DNA divided up evenly? What guarantees that each new cell will receive a complete set of DNA? It was the identification of chromosomes that allowed this process to be characterized. As a eukaryotic cell prepares to divide, the DNA and associated proteins (histones) coil into a structure, known as a chromosome (Figure below). The DNA copies itself prior to this process, so the chromosome that forms consists of two identical chromatids, known as sister chromatids, identical copies of DNA. The two chromatids are attached at a region called the centromere. The chromatids separate from each other when the nucleus divides just prior to cell division. Thus, each new cell that results after cell division will have the complete amount of genetic material, identical to the original, or parent, cell. In human cells, this amounts to 46 chromosomes. These chromosomes come in pairs (one from each pair inherited from each parent). So these 46 chromosomes are actually two sets of 23 chromosomes each. For an animation of how the DNA coils into a chromosome, see http://www.hhmi.org/biointeractive/media/DNAi_packaging_vo2-sm.mov.
Figure 6.2
A representation of a condensed eukaryotic chromosome, as seen after the DNA has been copied. The chromosome is made of two identical, or sister, chromatids held together by a centromere.
Each human somatic cell (a body cell, or every cell other than a gamete) normally has two sets of chromosomes, one set inherited from each parent. Each set contains 23 chromosomes, for a total of 46 chromosomes. Each chromosome differs in size, from over 250 million nucleotide pairs to less than 50 million nucleotide pairs. Each chromosome contains a specific set of genes, making each chromosome essential to survival.
Each pair of chromosomes consists of two chromosomes that are similar in size, shape, and genes. These pairs of chromosomes are known as homologous chromosomes, or homologues. Upon fertilization, a zygote is formed (Figure below). A zygote is the first cell of a new individual. In humans, a zygote contains 23 pairs (or two sets) of chromosomes. Any cell containing two sets of chromosomes is said to be diploid. The zygote forms from the fusion of two haploid gametes. A haploid cell contains one set of chromosomes. In humans, a haploid gamete contains 23 chromosomes. Biologists use the symbol n to represent one set of chromosomes, and 2n to represent two sets. In humans, each set of chromosomes contains 22 autosomes and 1 sex chromosome. Autosomes are chromosomes that are not directly involved in determining the sex of an individual. The sex chromosomes contain genes that determine the sex of an individual.
Figure 6.3
Upon fertilization a diploid zygote is formed. In humans, a zygote has 46 chromosomes, 23 inherited from each parent. The gametes, sperm and eggs, are haploid cells, with 23 chromosomes each.
Whereas autosomes are found as homologous pairs in somatic cells, sex chromosomes come in two different sizes, shapes, and contain different genes. In many organisms, including humans, the sex chromosomes are known as the X and Y chromosomes. The Y chromosome contains genes that cause male development. Therefore, any individual with a Y chromosome is male, and a male will have both an X and Y chromosome (XY). Females, without a Y chromosome, will have two X chromosomes (XX). As females have two X chromosomes, they must pass an X chromosome to all of their children. As males have both an X chromosome (inherited from their mother) and a Y chromosome, they can give either chromosome to their children. If a child inherits a Y from his father, he will be male; if a child inherits an X from her father, she will be female. It therefore is the male gamete that determines the sex of the offspring.
The Cell Cycle
Cell division in eukaryotic cells is much more complex than in prokaryotic cells because of the many chromosomes within the nucleus. Both the cytoplasm and the genetic material must be divided, ensuring that each resulting daughter cell receives 46 separate chromosomes. To ensure this, in addition to the cell performing its necessary functions, the DNA must be copied, as must many organelles, prior to cell division.
The life of a eukaryotic cell is a cycle, known as the cell cycle (Figure below). The cell cycle is a repeating series of cellular growth and division. The cell cycle has four phases: the first growth (G1) phase, the synthesis (S) phase, the second growth (G2) phase, and the mitosis phase. The cell spends the majority of the cycle in the first three phases of the cycle, collectively known as interphase. After cytokinesis, two genetically identical daughter cells are formed.
Figure 6.4
The Cell Cycle. The cell cycle depicts the life of an eukaryotic cell. The cell cycle has four phases: the first growth (G) phase, the synthesis (S) phase, the second growth (G) phase. The M phase includes mitosis (M), and cytokinesis (C). The cell spends the majority of the cycle in the first three phases (G, S, G)
of the cycle, collectively known as interphase. After cytokinesis, two genetically identical daughter cells are formed. has an excellent animation of the cell cycle.
The first growth (G1) phase: The cell spends most of its life in the G1 phase. During this phase, a cell undergoes rapid growth and the cell performs its routine functions. If a cell is not dividing, the cell remains in this phase.
The synthesis (S) phase: For two genetically identical daughter cells to be formed, the cell’s DNA must be copied or replicated. When the DNA is replicated, both strands of the double helix are used as templates to produce two new complementary strands. These new strands then hydrogen bond to the template strands and two double helices form.
The second growth (G2) phase is a shortened growth period in which many organelles are reproduced or manufactured. Parts necessary for cell division are made during G2.
Mitosis is the phase of nuclear division, in which one nucleus divides and becomes two nuclei. After mitosis is cytokinesis, in which the cytoplasm divides in half, producing two daughter cells, each containing a complete set of genetic material.
Mitosis
Mitosis is the division of the cell’s nucleus, the final step before two daughter cells are produced. The cell enters mitosis as it approaches its size limitations. Four distinct phases of mitosis have been recognized: prophase, metaphase, anaphase, and telophase, with each phase merging into the next one (Figure below).
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