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Characteristics of Life • Biology is the scientific study of life. • What defines life? Most biologists look for a set of generation characteristics.
Living organisms all show order; highly-organized structures in their cells, tissues, organs, or organ systems. • Example: The presence of tiny hair-like structures called cilia covering the entire surface of a single-celled paramecium.
The internal structure of cilia, whether from Paramecium or a human trachea, have a nearly identical internal order. 0.1 µm Cross section of cilium, as viewed with an electron microscope Cilia of trachea cells Cilia of Paramecium
In a population, individuals with favorable traits will survive and reproduce at higher rates. • These traits are called adaptations. A male Chilean stag beetle, Chiasognatusgranti.
Living organisms have the ability to respond to changes in their environment called stimuli.
Living organisms must maintain a specific range of internal conditions in spite of the outside changes. This balance is called homeostasis. Chickens pant to give off excess heat.
All life is able to reproduce, passing their genes along to healthy, fertile offspring. King snakes hatching.
Organisms grow and develop along a specific sequence based on the instructions present in their genes.
Each organism must have a source of energy. Monarch butterfly caterpillar.
Organization of Life • The biosphere is the part of Earth that supports life. • An ecosystem is a specific area within the biosphere with a common set of physical characteristics. • Climate, soil, landforms, etc.
The biological community is made of all populations living and interacting in one ecosystem. • A population includes all members of a species that live in the same community. • An organism is an individual living thing.
Organisms are made of organ systems and organs, structures with a specific vital function. • Each organ is made of a collection of tissues, which are groups of cells with similar structure and functions. • The cell is the smallest unit of an organism that still demonstrates all the basic characteristics of life. Moss is an organism made of stems and leaves (organs). Each leaf is made of different tissues, which then contains many cells.
Beginnings of Biology • Many of the original understandings of life came from philosophers who did not actually conduct experiments, rather using logic and reason.
Aristotle, an influential Greek philosopher, proposed that life could arise from non-living matter. “So with animals, some spring from parent animals according to their kind, whilst others grow spontaneously and not from kindred stock; and of these instances of spontaneous generation some come from putrefying earth or vegetable matter, as is the case with a number of insects, while others are spontaneously generated in the inside of animals out of the secretions of their several organs.“ —Aristotle, History of Animals, Book V, Part 1
Scientific Method • The idea of spontaneous generation was eventually discarded through the use of the scientific method. • Scientific methodology involves: • observing and asking questions, • forming hypotheses • conducting controlled experiments, • collecting and analyzing data • drawing conclusions.
Scientific investigations begin with observation, the act of noticing and describing events or processes in a careful, orderly way. • A hypothesis is an explanation for an observation based on prior knowledge. • Aristotle’s idea of spontaneous generation began with the observation that maggots would arise from meat or dead flesh, fleas from dust, tapeworms from other organisms, etc.
During Aristotle’s time, all scientific ideas were based on thought and logic, not experimentation. • Experiments test a hypothesis under controlled conditions. • Whenever possible, a hypothesis should be tested by an experiment in which the independent variable is changed. • The variable that is changed or affected is called the dependent variable, and will be measured. • Any other variables should be held unchanged or constant.
All experiments involve the collection of data. • If this includes numerical measurements (time, distance, etc), the data is quantitative. • If this includes non-numerical descriptive observations, the data is qualitative.
An Italian physician named Francisco Redi proposed an experiment to test spontaneous generation. • A flask containing raw meat was left in the open. This was the control group, since no new variable was being introduced.
Two additional flasks were set up, one completely sealed, and the other covered with gauze to allow air flow. • These were the experimental groupsthat were being exposed to the independent variable. • The dependent variable, the appearance of maggots, was recorded.
Only the unsealed flask grew maggots. Redi’sconclusion was that his hypothesis was correct -- maggots did not spontaneously generate, but grew from tiny eggs laid by flies. • Conclusions always indicate whether the original hypothesis is supported or rejected.
Sources of Error • Accounting for every single variable in a scientific study is nearly impossible. There are many factors that can cause error or otherwise cause an incorrect conclusion. • Probability helps to predict the likeliness of an experimental result occurring simply due to random chance. • The effects of random chance are countered with having a large sample size in the experiment.
Dr. Edward Jenner is famous for discovering a the first vaccine for small pox. • Initially, his results were questioned by the Royal Society of Medicine because he had tested the vaccine on a single patient, his gardener’s son. • The results could have been the result of random chance; perhaps the child was already immune?
The results were only accepted after he found several other parents who volunteered their children, including his own to increase the sample size.
Another major problem with experiments is when scientists or patients influence the data to produce a specific result. • This is called bias. • Bias is not always intentional, but must be accounted for in the design of an experiment. • A blind experiment prevents the experimental subjects from knowing whether they are in the control or experimental group. • Eliminates the “placebo effect”. • A double-blind experiment prevents both the scientists and subjects from knowing which is the control and experimental group.
This clip from the PBS NOVA episode Secrets of the Psychicshighlights the importance of randomizing, accounting for probability, and minimizing bias.
Depending on the results of the experiment, the hypothesis may be rejected or modified. • New experiments may be designed and conducted until some version of the hypothesis is supported.
Finally, the experimental results are published. This allows other scientists to review and critique the experimental design, data collection, and analysis. • This is called peer review. • If the results are accepted,the hypothesis becomespart of the currentscientific understanding.
Hypotheses should not be confused with theories, which are large, broad explanations composed of many hypotheses and experimental results. • A theory would not be rejected by a single experiment, but a hypothesis could be. • An example of a theory is the germ theory, which states that some diseases are the result of the presence and actions of microorganisms within the body. • A scientific law is a description of an observed phenomenon. • Laws do not explain the phenomenon, simply state it.
Major Fields of Biology • Cell biology studies cells; the smallest and simplest units of life.
Genetics is the study of the inheritance of characteristics through DNA in chromosomes.
Taxonomy names and classifies different species to indicate how closely related they are.
Evolution is the study of how the organisms on present-day Earth came to be; their ancestors and how they are adapted to their environment.