Chapter 1

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University of Toronto Scarborough
Biological Sciences
David Harrison

Cell Biology Chapter 1 Cell and molecular biology is a reductionist, it is based on the view that knowledge of the parts of the whole can explain the character of the whole. 1.1 Discovery of Cells microscope – an instrument that provides a magnified image of a tiny object. Robert Hooke was an English microscopist who first discovered the cell in 1665 by examining a cork to see why it holds liquids in a bottle down so well. Anton van Leeuwenhoek was the first to examine a drop of pond water under the microscope and observe animolecules that darted back and forth. Not until 1830 that widespread importance of cells was realized, in 1838 Matthias Schleiden concluded that despite differences in the structure of various tissues, plants were made of cells, and that the plant embryo arose from a single cell. Schwann invented the cell theory: - All organisms are composed of one or more cells. - The cell is the structural unit of life. - Cells can arise only by division from a pre-existing cell (1855, Rudolf Virchow) 1.2 Basic Properties of Cells The first culture of human cells was begun by George Gey in 1951. Cells were obtained from a tumour these cells are still used around the world today because they are so much simpler to study than cells situated within the body than cells grown in vitro (in culture outside the body). Scanned electron microscope – high powered microscope which allows people to examine the detailed surface of cells. Transmission electron microscope – has been used to reveal the detailed internal structures of a cells. Cells Are Highly Complex and Organized The more complex a structure, the greater the number of parts that must be in their proper place, the less tolerance of errors in the nature, and the more regulation or control that must be exerted to maintain the system. Cell activities are precise. Each type of cell has a consistent appearance in the electron microscope that is its organelles have a particular shape and location from one individual of species to another. Each type of organelle has a consistent composition of macromolecules, which are arranged in a predictable pattern. Epithelial cells that line the intestine are tightly connected to each other. Apical ends of the cells that face the intestinal channel have long processes (microvilli) That facilitate absorption of nutrients. Intestinal cells have a large number of mitochondria that provide the energy required to fuel various membrane transports. Cells Possess a Genetic Program and the Means to Use it Organisms are built according to information encoded in a collection of genes. Genes are more than storage lockers for information: they constitute the blueprints for constructing cellular structures, the directions for running cellular activities, and the program for making more of themselves. Molecular structure of genes allow for changes in genetic information. Cells Are Capable of Producing More of Themselves Cells reproduce by division, a process in which the contents of a mother cell are distributed into two daughter cells. Prior to division, genetic material is duplicated and both cells receive the same information. Cells Acquire and Utilize Energy Developing and maintaining complexity required the constant input of energy. All of life on Earth get energy from the sun in electromagnetic radiation. Cells Cary Out a Variety of Chemical Reactions Virtually all chemical changes that take place in a cell require enzymes – molecules that greatly increase rate at which a chemical reaction occurs. Sum total of the chemical reactions in a cell represents the cell’s metabolism. Cells Engage in Mechanical Activities Cells are always busy, materials being transported, structures assembled, cell moves. These types of activities are based on dynamic, mechanical changes within cells, many of which are intiated by changes in the shape of ‘motor’ proteins. Cells are Able to Respond to Stimuli Some cells respond to stimuli in obvious ways; a single celled protest moves away form an object in its path or moves toward a source of nutrients. Most cells are covered with receptors that interact with substances in the environment in highly specific ways. Cell’s receptors provide pathways through which external agents can evoke specific responses in target cells. Cells respond to specific stimuli by altering their metabolic activities, moving from one place to another or even committing suicide. Cells are Capable of self Regulation The importance of a cell’s regulatory mechanisms becomes most evident when they break down. Failure of a cell to correct a mistake when it duplicates DNA may result in debilitating mutation, or a breakdown in which the cells become cancerous. Hans Driesch found that he could completely separate the first two or four cells of a sea urchin embryo and each isolated cells would develop into a normal embryo. Information for product design resides in the nucleic acids, and the construction workers are the proteins. Each step of a process must occur spontaneously in such a way that the next step is automatically triggered. Each type of cellular activity requires a unique set of highly complex molecular tools and machines. Cells Evolve It is presumed that cells evolved from some type of precellular life form, which in turn evolved from nonliving organic materials that were present in the primordial seas. The evolution of cells can be studied by examining organisms that are alive today. 1.3 Two Fundamentally Different Classes of Cells Two basic classes of cells, prokaryotic and eukaryotic, distinguished by size and types of internal structures or organelles, they contain. The structurally simpler, prokaryotic cells include bacteria, whereas the more complex eukaryotic cells include protists fungi plants and animals. Compelling evidence of prokaryotic life has been obtained from rocks about 2.7 billions years of age. These rocks not only contain fossilized microbes, they contain complex organic molecules that are characteristic of particular type of prokaryotic organisms including cyanobacteria. Complex multicellular animals appear approximately 600 million years ago. There is evidence that simpler eukaryotic organisms were present on Earth more than one billion years earlier. Characteristics that Distinguish Prokaryotic and Eukaryotic cells Shared properties reflect the fact that eukaryotic cells almost certainly evolved from prokaryotic ancestors. Both types of cells share an identical genetic language, common set of metabolic pathways, and many common structural features. Both types of cells are bounded by plasma membranes that serve as a selectively permeable barrier between living and nonliving worlds. Both types of cells may be surrounded by a nonliving cell wall that protects the delicate life form within. Cell walls of the two groups have similar functions but very different chemical composition. Eukaryotic cells are much more complex both structurally and functionally. Genetic material of a prokaryotic cell is present in a nucleoid: a poorly demarcated region of the cell that lacks a boundary membrane to separate it from the surrounding cytoplasm. Eukaryotic cells possess a nucleus: a region bounded by a complex membranous structure called the nuclear enve
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