Themes in the Study of Life

Outline

1. Life is organized on many structural levels
2. Each level of biological organization has emergent properties
3. Cells are an organism's basic units of structure and function
4. The continuity of life is based on heritable information in the form of DNA
5. A feeling for organisms enriches the study of life
6. Structure and function are correlated at all levels of biological organization
7. Organisms are open systems that interact continuously with their environments
8. Diversity and unity are the dual faces of life on Earth
9. Evolution is the core theme of biology
10. Science as a process of inquiry often involves hypothetico-deductive thinking

Biology, the study of life, is a human endeavor resulting from an innate attraction to life in its diverse forms (E.O. Wilson's Biophilia).

The science of biology is enormous in scope.

It reaches across size scales from submicroscopic molecules to the global distribution of biological communities.

It encompasses life over huge spans of time from contemporary organisms to ancestral life forms stretching back to nearly four billion years.

As a science, biology is an ongoing process.

As a result of new research methods developed over the past few decades, there has been an information explosion.

Technological advances yield new information that may change the conceptual framework accepted by the majority of biologists.

With rapid information flow and new discoveries, biology is in a continuous state of flux. There are, however, enduring unifying themes that pervade the science of biology:

• A hierarchy of organization.
  
• Emergent properties.
• The cellular basis of life.
• Heritable information.
• A feeling for organisms.
• The correlation between structure and function.
• The interaction of organisms with their environment.
• Unity in diversity.
• Evolution: the core theme.
• Scientific process: the hypothetico-deductive method.

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I. Life is organized on many structural levels

A characteristic of life is a high degree of order. Biological organization is based on a hierarchy of structural levels, with each level building on the levels below it.

There are levels of organization beyond the individual organism:

Population = Localized group of organisms belonging to the same species.

Community = Populations of species living in the same area.

Ecosystem = An energy-processing system of community interactions that include abiotic environmental factors such as soil and water.

Biomes = Large scale communities classified by predominant vegetation type and distinctive combinations of plants and animals.

Biosphere = The sum of all the planet's ecosystems.

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II. Each level of biological organization has emergent properties

Emergent property = Property that emerges as a result of interactions between components.

With each step upward in the biological hierarchy, new properties emerge that were not present at the simpler organizational levels.

Life is difficult to define because it is associated with numerous emergent properties that reflect a hierarchy of structural organization.

Some of the emergent properties and processes associated with life are:

1. Order. Organisms are highly ordered, and other characteristics of life emerge from this complex organization.
2. Reproduction. Organisms reproduce; life comes only from life (biogenesis).
3. Growth and Development. Heritable programs stored in DNA direct the species-specific pattern of growth and development.
4. Energy Utilization. Organisms take in and transform energy to do work, including the maintenance of their ordered state.
5. Response to Environment. Organisms respond to stimuli from their environment.
6. Homeostasis. Organisms regulate their internal environment to maintain a steady-state, even in the face of a fluctuating external environment.
7. Evolutionary Adaptation. Life evolves in response to interactions between organisms and their environment.

Because properties of life emerge from complex organization, it is impossible to fully explain a higher level of order by breaking it into its parts.

Holism = The principle that a higher level of order cannot be meaningfully explained by examining component parts in isolation.

• An organism is a living whole greater than the sum of its parts.
• For example, a cell dismantled to its chemical ingredients is no longer a cell.

It is also difficult to analyze a complex process without taking it apart.

Reductionism = The principle that a complex system can be understood by studying its component parts.

• Has been a powerful strategy in biology.
• For example, Watson and Crick deduced the role of DNA in inheritance by studying its molecular structure.

The study of biology balances the reductionist strategy with the goal of understanding how the parts of cells, organisms and populations are functionally integrated.

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III. Cell's are an organism's basic units of structure and function

The cell is an organism's basic unit of structure and function.

• Lowest level of structure capable of performing all activities of life.
• All organisms are composed of cells.
• May exist singly as unicellular organisms or as subunits of multicellular organisms.
• The invention of the microscope led to the discovery of the cell and the formulation of the cell theory.
• Robert Hooke (1665) reported a description of his microscopic examination of cork. Hooke described tiny boxes which he called "cells" (really cell walls). The significance of this discovery was not recognized until 150 years later.
• Antonie van Leeuwenhok (1600's) used the microscope to observe living organisms such as microorganisms in pond water, blood cells and animal sperm cells.
• Matthias Schleiden and Theodor Schwann (1839) reasoned from their own microscopic studies and those of others, that all living things are made of cells. This formed the basis for the cell theory. The cell theory has since been modified to inc lude the idea that all cells come from preexisting cells.

Over the past 40 years, use of the electron microscope has revealed the complex ultrastructure of cells:

• Cells are bounded by plasma membranes that regulate passage of materials between the cell and its surroundings.
• All cells, at some stage, contain DNA.

Based on structural organization, there are two major kinds of cells: prokaryotic and eukaryotic.

Prokaryotic cell = Cell lacking membrane-bound organelles and a membrane-enclosed nucleus.

• Found only in the Kingdom Monera, the bacteria and blue-green algae.
• Generally much smaller than eukaryotic cells.
• Contains DNA that is not separated from the rest of the cell, as there is no membrane-bound nucleus.
• Lacks membrane-bound organelles.
• Almost all have tough external walls.

Eukaryotic cell = Cell with a membrane-enclosed nucleus and membrane-enclosed organelles.

• Found in protists, plants, fungi and animals.
• Subdivided by internal membranes into different functional compartments called organelles.
• Contains DNA that is segregated from the rest of the cell. DNA is organized with proteins into chromosomes that are located within the nucleus, the largest organelle of most cells.
• Cytoplasm surrounds the nucleus and contains various organelles of different functions.
• Some cells have a tough cell wall outside the plasma membrane (e.g. plant cells). Animal cells lack cell walls.

Though structurally different, eukaryotic and prokaryotic cells have many similarities, especially in their chemical processes.

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IV. The continuity of life is based on heritable information in the form of DNA

Biological instructions for an organism's complex structure and function are encoded in DNA.

• Each DNA molecule is made of four types of chemical building blocks called nucleotides.
• The linear sequence of these four nucleotides encode the precise information in a gene, the unit of inheritance from parent to offspring.
• An organism's complex structural organization is specified by an enormous amount of coded information.

Inheritance itself is based on:

• A complex mechanism for copying DNA.
• All forms of life use essentially the same genetic code.
• A particular nucleotide sequence provides the same information to one organism as it does to another.
• Differences among organisms reflect differences in nucleotide sequence.

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V. A feeling for organisms enriches the study of life

We may study life at structural levels below or above the organism, but it is organisms that first attract us. A feeling for the organisms we seek to understand enriches our study of life at all levels.

Barbara McClintock is a good example of a biologist whose feeling for the organism contributed to an important scientific discovery.

• McClintock investigated inheritance of kernel color in Indian corn and uncovered a type of genetic change not yet recognized by the scientific community in the 1940's and 50's.
• In McClintock's biography, A Feeling for the Organism by Evelyn Keller, the author highlights that McClintock's success was due to her familiarity with corn. McClintock knew the individual corn plants in her experiments and enjoyed these atta chments.
• Because she was familiar with the corn plants in her experiments, McClintock was able to recognize unexpected results from corn breeding experiments and deduce the existence of transposable genetic elements.
• The significance of McClintock's discovery was not recognized until researchers in tile 1970's found a similar phenomenon in bacteria. She later received due recognition with the Nobel Prize in 1983.

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VI. Structure and function are correlated at all levels of biological organization

There is a relationship between an organism's structure and how it works. Form fits function.

• Biological structure gives clues about what it does and how it works.
• Knowing a structure's function gives insights about its construction.
• This correlation is apparent at many levels of biological organization.

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VII. Organisms are open systems that interact continuously with their environments

Organisms interact with their environment, which includes other organisms as well as abiotic factors.

• Both organism and environment are affected by the interaction between them.
• Ecosystem dynamics include two major processes:
  1. Nutrient cycling.
  2. Energy flow.

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VIII. Diversity and unity are the dual faces of life on Earth

Biological diversity is enormous.

Estimates of total diversity range from 5 million to over 30 million species. About 1.5 million species have been identified and named, including approximately 260,000 plants, 50,000 vertebrates and 750,000 insects.

To make this diversity more comprehensible, biologists classify species into categories.

Taxonomy = Branch of biology concerned with naming and classifying organisms.

• Taxonomic groups are ranked into a hierarchy from the most to least inclusive category: kingdom, phylum, class, order, family, genus, species.
• The traditional classification scheme recognized five-kingdoms: Monera, Protista, Plantae, Fungi and Animalia.

• New taxonomic techniques, such as DNA comparisons between species, have resulted in reevaluation of kingdoms
• Some schemes include six, eight or more kingdoms
• Three new super-kingdom levels of classification (Domain) have been created; all kingdoms can be assigned to one of these

There is unity in the diversity of life forms at the lower levels of organization. Unity of life forms is evident in:

• A universal genetic code.
• Similar metabolic pathways (e.g. glycolysis).
• Similarities of cell structure (e.g. flagella of protozoans and mammalian sperm cells).

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IX. Evolution is the core theme of biology

Evolution is the one unifying biological theme.

• Life evolves. Species change over time and their history can be described as a branching tree of life.
• Species that are very similar share a common ancestor at a recent branch point on tile phylogenetic tree.
• Less closely related organisms share a more ancient common ancestor.
• All life is connected and can be traced back to primeval prokaryotes that existed more than three billion years ago.

In 1859, Charles Darwin published On the Origin of Species in which he made two major points:

1. Species change, and contemporary species arose from a succession of ancestors through a process of "descent with modification."
2. A mechanism of evolutionary change is natural selection.

Darwin synthesized the concept of natural selection based upon the following observations:

• Individuals in a population of any species vary in many inheritable traits.
• Populations have the potential to produce more offspring than will survive or than the environment can support.
• Individuals with traits best suited to the environment leave a larger number of offspring, which increases the proportion of inheritable variations in the next generation. This differential reproductive success is what Darwin called natural selectio n.

Organisms' adaptations to their environments are the products of natural selection.

• Natural selection does not create adaptations; it merely increases the frequency of inherited variants that arise by chance.
• Adaptations are the result of the editing process of natural selection. When exposed to specific environmental pressures, certain inheritable variations favor the reproductive success of some individuals over others.

Darwin proposed that cumulative changes in a population over long time spans could produce a new species from an ancestral one.

Descent with modification accounts for both the unity and diversity of life:

• Similarities between two species may be a reflection of their descent from a common ancestor.
• Differences between species may be the result of natural selection modifying the ancestral equipment in different environmental contexts.

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X. Science as a process of inquiry often involves hypothetico-deductive thinking

As the science of life, biology has the characteristics associated with science in general.

Science is a way of knowing. It is a human endeavor that emerges from our curiosity about ourselves, the world and the universe. Good scientists are people who:

• Ask questions about nature and believe those questions are answerable.
• Are curious, observant and passionate in their quest for discovery.
• Are creative, imaginative and intuitive.
• Are generally skeptics.

Scientific method = Process which outlines a series of steps used to answer questions.

• Is not a rigid procedure.
• Based on the conviction that natural phenomena have natural causes.
• Requires evidence to logically solve problems.

The key ingredient of the scientific process is the hypothetico-deductive method, which is an approach to problem-solving that involves:

• asking a question and formulating a tentative answer or hypothesis by inductive reasoning.
• using deductive reasoning to make predictions from the hypothesis and then testing the validity of those predictions.

Hypothesis = Educated guess proposed as a tentative answer to a specific question or problem.

Inductive reasoning = Making an inference from a set of specific observations to reach a general conclusion.

Deductive reasoning = Making an inference from general premises to specific consequences, which logically follow if the premises are true.

• Usually takes the form of If..then logic.
• In science, deductive reasoning usually involves predicting experimental results that are expected if the hypothesis is true.

Useful hypotheses have the following characteristics:

• Hypotheses are possible causes. Generalizations formed by induction are not necessarily hypotheses. Hypotheses should also be tentative explanations for observations or solutions to problems.
• Hypotheses reflect past experience with similar questions. Hypotheses are not just blind propositions, but are educated guesses based upon available evidence.
• Multiple hypotheses should be proposed whenever possible. The disadvantage of operating under only one hypothesis is that it might restrict the search for evidence in support of this hypothesis; scientists might bias their search, as well as n eglect to consider other possible solutions.
• Hypotheses must be testable via the hypothetico-deductive method. Predictions made from hypotheses must be testable by making observations or performing experiments. This limits the scope of questions that science can answer.
• Hypotheses can be eliminated but not confirmed with absolute certainty. If repeated experiments consistently disprove the predictions, then we can assume that the hypothesis is false. However, if repeated experimentation supports the deducti ons, we can only assume that the hypothesis may be true; accurate predictions can be made from false hypotheses. The more deductions that are tested and supported, the more confident we can be that the hypothesis is true.

Another feature of the scientific process is the controlled experiment which includes control and experimental groups.

Control group = In a controlled experiment, the group in which all variables are held constant.

• Controls are a necessary basis for comparison with the experimental group, which has been exposed to a single treatment variable.
• Allows conclusions to be made about the effect of experimental manipulation.
• Setting up the best controls is a key element of good experimental design.

Variable = Condition of an experiment that is subject to change and that may influence an experiment's outcome.

Experimental group = In a controlled experiment, the group in which one factor or treatment is varied.

• Build on prior scientific knowledge.
• Try to replicate the observations and experiments of others to check on their conclusions.
• Share information through publications. seminars, meetings and personal communication.

What really advances science is not just an accumulation of facts, but a new concept that collectively explains observations that previously seemed to be unrelated.

• Newton, Darwin and Einstein stand out in the history of science because they synthesized ideas with great explanatory power.
• Scientific theories are comprehensive conceptual frameworks which are well supported by evidence and are widely accepted by the scientific community.

Biology is a multi-disciplinary science that requires knowledge of chemistry, physics and mathematics.

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