Cell Theory College Prep Biology

Discovery of Cells 1665- English Scientist, Robert Hooke, discovered cells while looking at a thin slice of cork. Microscope only had low power therefore he couldn’t see any cell structures. He described the cells as tiny boxes or a honeycomb Cells came from Latin word Cella meaning a small room He thought that cells only existed in plants and fungi Made suggestion of Spontaneous Generation

Spontaneous Generation For much of history, people believed that animals could come from non-living sources. They thought: Frogs developed from falling drops of rain Mice arose from sweaty shirts Flies arose from decaying meat. This is called abiogenesis; also known as spontaneous generation

Francesco Redi (1668) In 1668, Francesco Redi, an Italian physician, did an experiment to disprove spontaneous generation. He put meat in jars and covered some of them with gauze. He observed these jars to see in which one(s) maggots would develop

Redi’s Experiment His hypothesis: If jars are covered containing raw meat, then maggots would not appear because flies would not lay eggs on the meat. He put meat in jars and covered some of them with gauze. He observed these jars to see in which one(s) maggots would develop.

Elements of a Controlled Experiment Dependent variable- variable that is measured in the experiment, responds to changes in the manipulated variable Independent variable- variable that is changed during the course of the experiment, otherwise known as manipulated variable Constant-variables in an experiment that do not change in order to observe the effect of the independent variable Control group- standard by which to measure change

Redi’s (1626-1697) Experiments Evidence against spontaneous generation: 1. Unsealed – maggots on meat 2. Sealed – no maggots on meat 3. Gauze – few maggots on gauze, none on meat

Redi part 2 – answer to critics fine mesh allows in air, but not flies flies laid eggs on top of mesh no maggots in meat

Redi’s Conclusions: “All living beings come from seeds of the plants or animals themselves” The results of this experiment disproved the idea of spontaneous generation for larger organisms, but people still thought microscopic organisms like algae or bacteria could arise that way.

Anton van Leuwenhoek 1673- Used a handmade microscope to observe pond scum & discovered single-celled organisms Microscopes went from 50x to 270x He called them “animalcules” He also observed blood cells from fish, birds, frogs, dogs, and humans Therefore, it was known that cells are found in animals as well as plants

Lazzaro Spallanzani (1768) Researched the theory of spontaneous generation Proved that microorganisms could be killed after an hour of boiling in broth Paved the way for Louis Pasteur

Louis Pasteur (1822-1895) French Chemist & Microbiologist known for discoveries in vaccination, microbial fermentation, & pasteurization Exposed broths to air that contained a filter to prevent all particles from passing through to the growth medium & even with no filters at all nothing grew. Organisms only grew when the flasks were broken open and allowed to enter into the broth. One of the last & most important experiments disproving spontaneous generation

Louis Pasteur’s Experiment

Development of Cell Theory 1838- German Botanist, Matthias Schleiden, concluded that all plant parts are made of cells 1839- German physiologist, Theodor Schwann, who was a close friend of Schleiden, stated that all animal tissues are composed of cells.

Development of Cell Theory 1858- Rudolf Virchow, German physician, after extensive study of cellular pathology, concluded that cells must arise from preexisting cells.

The Cell Theory Complete The 3 Basic Components of the Cell Theory 1. All organisms are composed of one or more cells. 2. The cell is the basic unit of life in all living things. 3. All cells are produced by the division of preexisting cells.

Exceptions to the Cell Theory 1. Is a virus alive? A virus is not a cell. A virus cannot reproduce without a host. A virus does not consume or produce energy. A virus does contain DNA and RNA and can pass on its genome. Viruses have adapted to the environment and evolved.

Exceptions to the Cell Theory 2. Is the cell really the smallest unit of life? 3. If all cells come from other cells, where did the first cell come from? MITOSIS

Cells are the structural and functional units of life Two distinct groups of cells exist 1. Prokaryotic cells Simple and small Do not contain a nucleus or membrane-bound organelles Bacteria are prokaryotic Regardless of the group studied, cells are enclosed by a membrane and use DNA as their genetic information. Student Misconceptions and Concerns 1. Many students enter our courses with a limited appreciation of the diversity of life. Ask any group of freshmen at the start of the semester to write down the first type of animal that comes to mind, and the most frequent response is a mammal. As the diversity of life is explored, the common heritage of biological organization can be less, and not more, apparent. The diverse forms, habits, and ecological interactions overwhelm our senses with striking distinctions. Emphasizing the diversity as well as the unifying aspects of life is necessary for a greater understanding of the rich evolutionary history of life on Earth. 2. We live in a world that is largely understood by what we can distinguish and identify with our naked senses. However, the diversity of life and the levels of biological organization extend well below the physical scale of our lives. For many students, appreciating the diversity of the microscopic world is abstract, nearly on par with an understanding of the workings of atoms and molecules. The ability to examine the microscopic details of the world of our students (the surface of potato chips, the structure of table salt and sugar, the details of a blade of grass) can be an important sensory extension that prepares the mind for greater comprehension of these minute biological details. Teaching Tips 1. Consider asking students to bring to class a page or two of some article about biology that appeared in the media in the last month. Alternatively, you could have each student e-mail a Web address of a recent biology-related news event to you. You might even have them e-mail relevant articles to you for each of the main topics you address throughout the semester. 2. The scientific organization Sigma Xi offers a free e-mail summary of the major science news articles appearing each weekday in major U.S. news media. The first paragraph or so of each article is included in the e-mail with a hyperlink to the rest of the article. The diverse topics are an excellent way to learn of general scientific announcements and reports. Typically, 5–10 articles are cited in each e-mail. To sign up for this free service, go to http://www.mediaresource.org/instruct.htm. 3. Help the class think through the diverse interactions between an organism and its environment. In class, select an organism and have the class develop a list of environmental components that interact with the organism. Items in this list will likely fall into living and nonliving categories. 4. Here is a simple way to contrast the relative size of prokaryotic and eukaryotic cells. Mitochondria and chloroplasts are thought to have evolved by endosymbiosis (see Chapter 16). Thus, mitochondria and chloroplasts are about the size of bacteria, contained within a plant cell. A figure of a plant cell therefore provides an immediate comparison of these sizes, not side-by-side, but one inside the other! 5. Examples of biological form and function relationships are nearly endless. Those immediately apparent to your students will be easiest to comprehend. Have your students examine (in photos or in specimens) the teeth of various vertebrates. The diet of these animals is implied by the shape of the teeth (sharp teeth in carnivorous cats and blunted molars in a rat). Sliding your tongue over your teeth reveals our omnivorous history, with sharp canine teeth for slicing flesh and flat rear molars well-suited for grinding plant material.

Types of Cells Continued 2. Eukaryotic cells Possess organelles separated by membranes Contain a nucleus Plants, animals, and fungi are eukaryotic

Microscopic Images