Just as plants and animals are alive, so too are cells. Life, in fact, is the most basic property of cells, and cells are the smallest units to exhibit this property. Unlike the parts of a cell, which simply deteriorate if isolated, whole cells can be removed from a plant or animal and cultured in laboratory where they will grow and reproduce for extended periods of time. If mistreated, they may die. Death can be considered one of the basic properties of life, because only living entity faces this prospect. Remarkably, cells within the body generally die " by their own hand"- the victims of an internal program that causes cells that are no longer needed or cells that pose a risk of becoming cancerous to eliminate themselves.
Cells Are Highly Complex and OrganizedComplexity is a property that is evident but different to describe. For the present, we can think of complexity in terms of order and consistency. 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 interactions of the parts, and the more regulation or control must be exerted to maintain the system. Cellular activities can be remarkably precise. DNA duplication, for example, occurs with an error rate of less than one mistake every ten million nucleotides incorporated-and most of these are quickly corrected by an elaborate repair mechanism that recognizes the defect.
Cells Are Able to Respond to StimuliSome cells respond to stimuli obvious ways; a single-celled protist for example, moves away from an object in its path or moves toward a source of nutrients. Cells within a multicellular plant or animal respond to stimuli less obviously. Most cells are covered with receptors that interact with the substances in the environment in highly specific ways. Cells possess receptors to hormones, growth factors, and extracellular materials, as well as to substances on the surface of other cells. A cell's receptor provide pathways through which external agents can evoke specific responses in target cells. Cells may respond to specific responses in target cells. Cells may respond to specific stimuli by altering their metabolic activities, moving from one place to another, or even committing suicide.
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Cells Possess a Genetic Program and the Means to Use ItOrganism are built according to information encoded in a collection of gens. The human genetic program contains enough information, in converted to words, to fill millions of pages of text. Remarkably, this vast amount of information is packaged into a set of chromosome that occupies the space of a cell nucleus-hundreds of times smaller than the dot on this i. Genes are more than storage lockers for information: they constitute blueprints for constructing cellular structures, the directions for running cellular activities, and the program for making more of themselves.
Cells Acquire and Utilize EnergyDeveloping and maintaining complexity requires the constant input of energy. Virtually all of the energy by life on the Earth's surface arrives in the form of electromagnetic radiation from the sun. The energy of light is trapped by light-absorbing pigments present in the membranes of the photosynthetic cells. Light energy is converted by photosynthesis into chemical energy that is stored in energy-rich carbohydrates, such as sucrose or starch. For most animal cells, energy arrives prepackaged, often in the form of the sugar glucose. In humans, glucose is released by the liver into the blood where it circulates through the body delivering chemical energy to all the cells. Once in a cell, the glucose is disassembled in such a way that its energy content can be stored in a readily available form (usually as ATP) that is later put to use in running all of the cell's myriad energy-requiring activities. Cells expend an enormous amount of energy simply breaking down and rebuilding the macromolecules and organelles of which they are made. This continual "turnover," as it is called, maintains the integrity of cell components in the face of inevitable wear and tear and enables the cell to respond rapidly to changing conditions.
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Cells Carry Out a Variety of Chemical Reactions |
Cells Engage in Mechanical Activies |
Cells function like miniaturized chemicals plants. Even the simplest bacterial cell is capable of hundreds of different chemical transformations, none of which occurs at any significant rate in the inanimate world. Virtually all chemical changes that take place in cells requires enzymes-molecules that greatly increase the rate at which a chemical reaction occurs. The sum total of the chemical reactions in a cell represents the cell's metabolism.
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Cells are sites of bustling activity. Materials are transported from place to place, structures are assembled and then rapidly disassembled, and, in many cases, the entire cell moves itself from one site to another. These types of activities are based on dynamic, mechanical changes within cells, many of which are initiated by changes in the shape of "motor" proteins. Motor proteins. are just one of the many types of molecular "machines" employed by cells to carry out mechanical activities.
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Cells Evolve
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Cells Are Capable of Self-Regulation |
How did cells arise? Of all the major questions posed by biologist, this question may be the least like ever to be answered. 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. Whereas the origin of cells is shrouded in near-total mystery, the evolution of cells can be studied by examining organism that are alive today. If you were to observe the features of a bacterial cell living in the human intestinal tract and a cell that is part of the lining of the tract, you would be struck by the differences between the two cells. Yet both have evolved from a common ancestral that lived more than three billion years ago. Those structures that are shared by these to distantly related cells, such as their similar plasma membrane and ribosomes, must have been present in the ancestral cell.
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In addition to requiring energy, maintaining a complex, ordered state requires constant regulation. The importance of cell's regulatory mechanism becomes most evident when they break down. For example, failure of a cell to correct a mistake when it duplicates its DNA may result in a debilitating mutation, or a breakdown in a cell's growth-control safeguards can transform the cell into a cancer cell with the capability of destroying the entire organism. We are gradually learning how a cell controls its activities, but much more is left to discover.
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Cells Are Capable of Producing More of Themselves |
Just an individual organisms are generated by reproduction, so too are individual cells. Cells reproduce by division, a process in which the contents of a "mother" cell are distributed into two "daughter" cells. Prior to division, the genetic material is faithfully duplicated, and each daughter cells receives a complete and equal share of genetic information. In most cases, the two daughter cells have approximately equal volume. In some cases, however, as occurs when a human oocyte undergoes division, one of the cells can retain nearly all of the cytoplasm, even though it receives only half of the genetic material.
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