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Date posted: April 13, 2012

Life is the invisible, substantial, intelligent, individual, co-ordinating power and cause directing and controlling the forces involved in the production and activity of any organism possessing individuality.

The Genius of Homeopathy Lectures and Essays on Homeopathic Philosophy
by Dr Stuart M. Close

Life is a condition that distinguishes organisms from inorganic objects and dead organisms, being manifested by growth through metabolism, reproduction, and the power of adaptation to environment through changes originating internally. A diverse array of living organisms can be found in the biosphere on Earth. Properties common to these organisms – plants, animals, fungi, protists, archaea and bacteria – are a carbon and water-based cellular form with complex organization and genetic information. They undergo metabolism, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations.

Conventional definition: Often scientists say that life is a characteristic of organisms that exhibit the following phenomena:

1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.

2. Organization: Being composed of one or more cells, which are the basic units of life.

3. Metabolism: Consumption of energy by converting nonliving material into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.

4. Growth: Maintenance of a higher rate of synthesis than catalysis. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.

5. Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism’s heredity as well as the composition of metabolized substances, and external factors present.

6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun or an animal chasing its prey.

7.  Reproduction: The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.

When organic vitality is exhausted, or is withdrawn, his transient material organism dies, yields to chemical laws and is dissolved into its elements, while his substantial, spiritual organism continues its existence in a higher realm.

Health is that balanced condition of the living organism in which the integral, harmonious performance of the vital functions tends to the preservation of the organism and the normal development of the individual.

In health the vital force rules supreme, unhampered by any morbific influence. It harmonizes the vital processes, feeling and function is normal, there is a feeling of well   being, the mind looks out, the senses are alert, the sense perceptions are clear and normal.

Disease is an abnormal vital process, a changed condition of life, which is inimical to the true development of the individual and tends to organic dissolution.

Vital phenomena in health and disease are caused by the reaction of the vital substantial power or principle of the organism to various external stimuli. So long as a healthy man lives normally in a favorable environment he moves, feels, thinks, acts and reacts in an orderly manner. If he violates the laws of life, or becomes the victim of an unfavorable environment, disorder takes the place of order, disease destroys ease, he suffers and his body deteriorates.

Agents, material or immaterial, which modify health or cause disease, act solely by virtue of their own substantial, entitative existence and the co-existence of the vital substance, which reacts in the living organism to every impression made from within or without. The dead body reacts only to physical and chemical agents, under the action of which it is reduced to its chemical elements and dissipated as a material organism.

All reactions to stimuli by which the functions and activities; of the living body are carried on, originate in the primitive life substance at the point where it becomes materialized as cells and protoplasmic substance.

Agents from without which affect the living body to produce changes and modifications of its functions and sensations, act upon the protoplasm through the medium of the brain and nervous system. Food, drink, heat, light, air, electricity and drugs, as well as mental stimuli, all act primarily upon the living substance as materialized in the cells of the central nervous system, calling forth the reactions which are represented by functions and sensations.

“Power resides at the center, and from the center of power, force flows.”

The phenomena of life, as manifested in growth, nutrition, repair, secretion, excretion, self-recognition, self-preservation and reproduction, all take their direction from an originating center. From the lowest cell to the highest and most complex organism, this principle holds true. Cell wall and protoplasmic contents develop from the central nucleus, and that from the centrosome, which is regarded as the “center of force” in the cell. All fluids.. tissues and organs develop from the cell from within outwards, from center to circumference.

Organic control is from the center. In the completely developed human organism. vital action is controlled from the central nervous system. The activities of the cell are controlled from, the centrosome, which may be called the brain of the cell.

The central nervous system may be compared to a dynamo. As a dynamo is a machine, driven by steam or some other force, which, through the agency of electro-magnetic induction from a surrounding magnetic field, converts into electrical energy in the form. of current the -mechanical energy expended upon it, so the central nervous system is a machine driven by chemical force derived from food which, through the agency of electro-vital induction from a surrounding vital field, converts into vital energy, in the form of nerve current or impulses, the chemico-physical energy expended upon it.

As an electrical transportation system depends for its working force upon the dynamo located in its central power station, so the human body depends for the force necessary to carry on its operations upon the central power station, located in the central nervous system.

Any disturbance of conditions at the central power station is immediately manifested externally at some point in the system; and any injury to or break in the external system is immediately reflected back to the central station.

In health and disease it is the same, both being essentially merely conditions of life in the living organism, convertible each into the other. In each condition the modifying agent or factor acts primarily upon the internal life principle, which is the living substance of the organism. This reacts and produces external phenomena through the medium of the brain and nervous system which, extends to every part of the body. Food or poison, toxins or antitoxins, therapeutic agents or pathogenic micro-organisms, all act upon and by virtue of Be existence of the reacting life principle or living substance of the organism.

The living environment
Living organisms are made of the same components as all other matter, involve the same kind of transformations of energy, and move using the same basic kinds of forces. The Physical Setting, apply to life as well as to stars, raindrops, and television sets. But living organisms also have characteristics that can be understood best through the application of other principles.

Diversity of life
There are millions of different types of individual organisms that inhabit the earth at any one time—some very similar to each other, some very different. Biologists classify organisms into a hierarchy of groups and subgroups on the basis of similarities and differences in their structure and behavior. One of the most general distinctions among organisms is between plants, which get their energy directly from sunlight, and animals, which consume the energy-rich foods initially synthesized by plants. But not all organisms are clearly one or the other. For example, there are single-celled organisms without organized nuclei (bacteria) that are classified as a distinct group.

Animals and plants have a great variety of body plans, with different overall structures and arrangements of internal parts to perform the basic operations of making or finding food, deriving energy and materials from it, synthesizing new materials, and reproducing. When scientists classify organisms, they consider details of anatomy to be more relevant than behavior or general appearance. For example, because of such features as milk-producing glands and brain structure, whales and bats are classified as being more nearly alike than are whales and fish or bats and birds. At different degrees of relatedness, dogs are classified with fish as having backbones, with cows as having hair, and with cats as being meat-eaters.

For sexually reproducing organisms, a species comprises all organisms that can mate with one another to produce fertile offspring. The definition of species is not precise, however; at the boundaries it may be difficult to decide on the exact classification of a particular organism. Indeed, classification systems are not part of nature. Rather, they are frameworks created by biologists for describing the vast diversity of organisms, suggesting relationships among living things, and framing research questions.

The variety of the earth’s life forms is apparent not only from the study of anatomical and behavioral similarities and differences among organisms but also from the study of similarities and differences among their molecules. The most complex molecules built up in living organisms are chains of smaller molecules. The various kinds of small molecules are much the same in all life forms, but the specific sequences of components that make up the very complex molecules are characteristic of a given species. For example, DNA molecules are long chains linking just four kinds of smaller molecules, whose precise sequence encodes genetic information. The closeness or remoteness of the relationship between organisms can be inferred from the extent to which their DNA sequences are similar. The relatedness of organisms inferred from similarity in their molecular structure closely matches the classification based on anatomical similarities.

The preservation of a diversity of species is important to human beings. We depend on two food webs to obtain the energy and materials necessary for life. One starts with microscopic ocean plants and seaweed and includes animals that feed on them and animals that feed on those animals. The other one begins with land plants and includes animals that feed on them, and so forth. The elaborate interdependencies among species serve to stabilize these food webs. Minor disruptions in a particular location tend to lead to changes that eventually restore the system. But large disturbances of living populations or their environments may result in irreversible changes in the food webs. Maintaining diversity increases the likelihood that some varieties will have characteristics suitable to survival under changed conditions.

Heredity
One long-familiar observation is that offspring are very much like their parents but still show some variation: Offspring differ somewhat from their parents and from one another. Over many generations, these differences can accumulate, so organisms can be very different in appearance and behavior from their distant ancestors. For example, people have bred their domestic animals and plants to select desirable characteristics; the results are modern varieties of dogs, cats, cattle, fowl, fruits, and grains that are perceptibly different from their forebears. Changes have also been observed—in grains, for example—that are extensive enough to produce new species. In fact, some branches of descendants of the same parent species are so different from others that they can no longer breed with one another.

Instructions for development are passed from parents to offspring in thousands of discrete genes, each of which is now known to be a segment of a molecule of DNA. Offspring of asexual organisms (clones) inherit all of the parent’s genes. In sexual reproduction of plants and animals, a specialized cell from a female fuses with a specialized cell from a male. Each of these sex cells contains an unpredictable half of the parent’s genetic information. When a particular male cell fuses with a particular female cell during fertilization, they form a cell with one complete set of paired genetic information, a combination of one half-set from each parent. As the fertilized cell multiplies to form an embryo, and eventually a seed or mature individual, the combined sets are replicated in each new cell.

The sorting and combination of genes in sexual reproduction results in a great variety of gene combinations in the offspring of two parents. There are millions of different possible combinations of genes in the half apportioned into each separate sex cell, and there are also millions of possible combinations of each of those particular female and male sex cells.

However, new mixes of genes are not the only source of variation in the characteristics of organisms. Although genetic instructions may be passed down virtually unchanged for many thousands of generations, occasionally some of the information in a cell’s DNA is altered. Deletions, insertions, or substitutions of DNA segments may occur spontaneously through random errors in copying, or may be induced by chemicals or radiation. If a mutated gene is in an organism’s sex cell, copies of it may be passed down to offspring, becoming part of all their cells and perhaps giving the offspring new or modified characteristics. Some of these changed characteristics may turn out to increase the ability of the organisms that have it to thrive and reproduce, some may reduce that ability, and some may have no appreciable effect.

Cells
All self-replicating life forms are composed of cells—from single-celled bacteria to elephants, with their trillions of cells. Although a few giant cells, such as hens’ eggs, can be seen with the naked eye, most cells are microscopic. It is at the cell level that many of the basic functions of organisms are carried out: protein synthesis, extraction of energy from nutrients, replication, and so forth.

All living cells have similar types of complex molecules that are involved in these basic activities of life. These molecules interact in a soup, about 2/3 water, surrounded by a membrane that controls what can enter and leave. In more complex cells, some of the common types of molecules are organized into structures that perform the same basic functions more efficiently. In particular, a nucleus encloses the DNA and a protein skeleton helps to organize operations. In addition to the basic cellular functions common to all cells, most cells in multicelled organisms perform some special functions that others do not. For example, gland cells secrete hormones, muscle cells contract, and nerve cells conduct electrical signals.

Cell molecules are composed of atoms of a small number of elements—mainly carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur. Carbon atoms, because of their small size and four available bonding electrons, can join to other carbon atoms in chains and rings to form large and complex molecules. Most of the molecular interactions in cells occur in water solution and require a fairly narrow range of temperature and acidity. At low temperatures the reactions go too slowly, whereas high temperatures or extremes of acidity can irreversibly damage the structure of protein molecules. Even small changes in acidity can alter the molecules and how they interact. Both single cells and multicellular organisms have molecules that help to keep the cells’ acidity within the necessary range.

The work of the cell is carried out by the many different types of molecules it assembles, mostly proteins. Protein molecules are long, usually folded chains made from 20 different kinds of amino acid molecules. The function of each protein depends on its specific sequence of amino acids and the shape the chain takes as a consequence of attractions between the chain’s parts. Some of the assembled molecules assist in replicating genetic information, repairing cell structures, helping other molecules to get in or out of the cell, and generally in catalyzing and regulating molecular interactions. In specialized cells, other protein molecules may carry oxygen, effect contraction, respond to outside stimuli, or provide material for hair, nails, and other body structures. In still other cells, assembled molecules may be exported to serve as hormones, antibodies, or digestive enzymes.

The genetic information encoded in DNA molecules provides instructions for assembling protein molecules. This code is virtually the same for all life forms. Thus, for example, if a gene from a human cell is placed in a bacterium, the chemical machinery of the bacterium will follow the gene’s instructions and produce the same protein that would be produced in human cells. A change in even a single atom in the DNA molecule, which may be induced by chemicals or radiation, can therefore change the protein that is produced. Such a mutation of a DNA segment may not make much difference, may fatally disrupt the operation of the cell, or may change the successful operation of the cell in a significant way (for example, it may foster uncontrolled replication, as in cancer).

All the cells of an organism are descendants of the single fertilized egg cell and have the same DNA information. As successive generations of cells form by division, small differences in their immediate environments cause them to develop slightly differently, by activating or inactivating different parts of the DNA information. Later generations of cells differ still further and eventually mature into cells as different as gland, muscle, and nerve cells.

Complex interactions among the myriad kinds of molecules in the cell may give rise to distinct cycles of activities, such as growth and division. Control of cell processes comes also from without: Cell behavior may be influenced by molecules from other parts of the organism or from other organisms (for example, hormones and neurotransmitters) that attach to or pass through the cell membrane and affect the rates of reaction among cell constituents.

Interdependence of life
Every species is linked, directly or indirectly, with a multitude of others in an ecosystem. Plants provide food, shelter, and nesting sites for other organisms. For their part, many plants depend upon animals for help in reproduction (bees pollinate flowers, for instance) and for certain nutrients (such as minerals in animal waste products). All animals are part of food webs that include plants and animals of other species (and sometimes the same species). The predator/prey relationship is common, with its offensive tools for predators—teeth, beaks, claws, venom, etc.—and its defensive tools for prey—camouflage to hide, speed to escape, shields or spines to ward off, irritating substances to repel. Some species come to depend very closely on others (for instance, pandas or koalas can eat only certain species of trees). Some species have become so adapted to each other that neither could survive without the other (for example, the wasps that nest only in figs and are the only insect that can pollinate them).

There are also other relationships between organisms. Parasites get nourishment from their host organisms, sometimes with bad consequences for the hosts. Scavengers and decomposers feed only on dead animals and plants. And some organisms have mutually beneficial relationships—for example, the bees that sip nectar from flowers and incidentally carry pollen from one flower to the next, or the bacteria that live in our intestines and incidentally synthesize some vitamins and protect the intestinal lining from germs.

But the interaction of living organisms does not take place on a passive environmental stage. Ecosystems are shaped by the nonliving environment of land and water—solar radiation, rainfall, mineral concentrations, temperature, and topography. The world contains a wide diversity of physical conditions, which creates a wide variety of environments: freshwater and oceanic, forest, desert, grassland, tundra, mountain, and many others. In all these environments, organisms use vital earth resources, each seeking its share in specific ways that are limited by other organisms. In every part of the habitable environment, different organisms vie for food, space, light, heat, water, air, and shelter. The linked and fluctuating interactions of life forms and environment compose a total ecosystem; understanding any one part of it well requires knowledge of how that part interacts with the others.

The interdependence of organisms in an ecosystem often results in approximate stability over hundreds or thousands of years. As one species proliferates, it is held in check by one or more environmental factors: depletion of food or nesting sites, increased loss to predators, or invasion by parasites. If a natural disaster such as flood or fire occurs, the damaged ecosystem is likely to recover in a succession of stages that eventually results in a system similar to the original one.

Like many complex systems, ecosystems tend to show cyclic fluctuations around a state of approximate equilibrium. In the long run, however, ecosystems inevitably change when climate changes or when very different new species appear as a result of migration or evolution (or are introduced deliberately or inadvertently by humans).

Flow of matter and energy
However complex the workings of living organisms, they share with all other natural systems the same physical principles of the conservation and transformation of matter and energy. Over long spans of time, matter and energy are transformed among living things, and between them and the physical environment. In these grand-scale cycles, the total amount of matter and energy remains constant, even though their form and location undergo continual change.

Almost all life on earth is ultimately maintained by transformations of energy from the sun. Plants capture the sun’s energy and use it to synthesize complex, energy-rich molecules (chiefly sugars) from molecules of carbon dioxide and water. These synthesized molecules then serve, directly or indirectly, as the source of energy for the plants themselves and ultimately for all animals and decomposer organisms (such as bacteria and fungi). This is the food web: The organisms that consume the plants derive energy and materials from breaking down the plant molecules, use them to synthesize their own structures, and then are themselves consumed by other organisms. At each stage in the food web, some energy is stored in newly synthesized structures and some is dissipated into the environment as heat produced by the energy-releasing chemical processes in cells. A similar energy cycle begins in the oceans with the capture of the sun’s energy by tiny, plant-like organisms. Each successive stage in a food web captures only a small fraction of the energy content of organisms it feeds on.

The elements that make up the molecules of living things are continually recycled. Chief among these elements are carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, calcium, sodium, potassium, and iron. These and other elements, mostly occurring in energy-rich molecules, are passed along the food web and eventually are recycled by decomposers back to mineral nutrients usable by plants. Although there often may be local excesses and deficits, the situation over the whole earth is that organisms are dying and decaying at about the same rate as that at which new life is being synthesized. That is, the total living biomass stays roughly constant, there is a cyclic flow of materials from old to new life, and there is an irreversible flow of energy from captured sunlight into dissipated heat.

An important interruption in the usual flow of energy apparently occurred millions of years ago when the growth of land plants and marine organisms exceeded the ability of decomposers to recycle them. The accumulating layers of energy-rich organic material were gradually turned into coal and oil by the pressure of the overlying earth. The energy stored in their molecular structure we can now release by burning, and our modern civilization depends on immense amounts of energy from such fossil fuels recovered from the earth. By burning fossil fuels, we are finally passing most of the stored energy on to the environment as heat. We are also passing back to the atmosphere—in a relatively very short time—large amounts of carbon dioxide that had been removed from it slowly over millions of years.

The amount of life any environment can sustain is limited by its most basic resources: the inflow of energy, minerals, and water. Sustained productivity of an ecosystem requires sufficient energy for new products that are synthesized (such as trees and crops) and also for recycling completely the residue of the old (dead leaves, human sewage, etc.). When human technology intrudes, materials may accumulate as waste that is not recycled. When the inflow of resources is insufficient, there is accelerated soil leaching, desertification, or depletion of mineral reserves.

Evolution of life
The earth’s present-day life forms appear to have evolved from common ancestors reaching back to the simplest one-cell organisms almost four billion years ago. Modern ideas of evolution provide a scientific explanation for three main sets of observable facts about life on earth: the enormous number of different life forms we see about us, the systematic similarities in anatomy and molecular chemistry we see within that diversity, and the sequence of changes in fossils found in successive layers of rock that have been formed over more than a billion years.

Since the beginning of the fossil record, many new life forms have appeared, and most old forms have disappeared. The many traceable sequences of changing anatomical forms, inferred from ages of rock layers, convince scientists that the accumulation of differences from one generation to the next has led eventually to species as different from one another as bacteria are from elephants. The molecular evidence substantiates the anatomical evidence from fossils and provides additional detail about the sequence in which various lines of descent branched off from one another.

Although details of the history of life on earth are still being pieced together from the combined geological, anatomical, and molecular evidence, the main features of that history are generally agreed upon. At the very beginning, simple molecules may have formed complex molecules that eventually formed into cells capable of self-replication. Life on earth has existed for three billion years. Prior to that, simple molecules may have formed complex organic molecules that eventually formed into cells capable of self-replication. During the first two billion years of life, only microorganisms existed—some of them apparently quite similar to bacteria and algae that exist today. With the development of cells with nuclei about a billion years ago, there was a great increase in the rate of evolution of increasingly complex, multicelled organisms. The rate of evolution of new species has been uneven since then, perhaps reflecting the varying rates of change in the physical environment.

A central concept of the theory of evolution is natural selection, which arises from three well-established observations: (1) There is some variation in heritable characteristics within every species of organism, (2) some of these characteristics will give individuals an advantage over others in surviving to maturity and reproducing, and (3) those individuals will be likely to have more offspring, which will themselves be more likely than others to survive and reproduce. The likely result is that over successive generations, the proportion of individuals that have inherited advantage-giving characteristics will tend to increase.

Selectable characteristics can include details of biochemistry, such as the molecular structure of hormones or digestive enzymes, and anatomical features that are ultimately produced in the development of the organism, such as bone size or fur length. They can also include more subtle features determined by anatomy, such as acuity of vision or pumping efficiency of the heart. By biochemical or anatomical means, selectable characteristics may also influence behavior, such as weaving a certain shape of web, preferring certain characteristics in a mate, or being disposed to care for offspring.

New heritable characteristics can result from new combinations of parents’ genes or from mutations of them. Except for mutation of the DNA in an organism’s sex cells, the characteristics that result from occurrences during the organism’s lifetime cannot be biologically passed on to the next generation. Thus, for example, changes in an individual caused by use or disuse of a structure or function, or by changes in its environment, cannot be promulgated by natural selection.

By its very nature, natural selection is likely to lead to organisms with characteristics that are well adapted to survival in particular environments. Yet chance alone, especially in small populations, can result in the spread of inherited characteristics that have no inherent survival or reproductive advantage or disadvantage. Moreover, when an environment changes (in this sense, other organisms are also part of the environment), the advantage or disadvantage of characteristics can change. So natural selection does not necessarily result in long-term progress in a set direction. Evolution builds on what already exists, so the more variety that already exists, the more there can be.

The continuing operation of natural selection on new characteristics and in changing environments, over and over again for millions of years, has produced a succession of diverse new species. Evolution is not a ladder in which the lower forms are all replaced by superior forms, with humans finally emerging at the top as the most advanced species. Rather, it is like a bush: Many branches emerged long ago; some of those branches have died out; some have survived with apparently little or no change over time; and some have repeatedly branched, sometimes giving rise to more complex organisms.

The modern concept of evolution provides a unifying principle for understanding the history of life on earth, relationships among all living things, and the dependence of life on the physical environment. While it is still far from clear how evolution works in every detail, the concept is so well established that it provides a framework for organizing most of biological knowledge into a coherent picture.

Biotic vs. Abiotic
Organisms with similar needs may compete with one another for resources, including food, space, water, air, and shelter. In any particular environment, the growth and survival of organisms depend on the physical conditions including light intensity, temperature range, mineral availability, soil type, and pH.   Physical or non-living factors such as these which influence living things are called abiotic factors.   Living factors which influence living things are called biotic factors.   Some examples of biotic factors include disease and predation.

Energy Flow
Energy flows through ecosystems in one direction, typically from the Sun, through photosynthetic organisms or producers, to herbivores to carnivores and decomposers.   The chemical elements that make up the molecules of living things pass through food webs and are combined and recombined in different ways. At each link in a food web, some energy is stored in newly made structures but much energy is lost into the environment as heat. Continual input of energy from sunlight is required to keep this process going.   Energy pyramids are often used to show the flow of energy in ecosystems.

Material Cycles
The atoms and molecules on the Earth cycle among the living and nonliving components of the biosphere. Carbon dioxide and water molecules used in photosynthesis to form energy-rich organic compounds are returned to the environment when the energy in these compounds is eventually released by cells through the processes of cell respiration and other life activities.   The number of organisms any environment can support is called its carrying capacity.  The carrying capacity of an environment is limited by the available energy, water, oxygen, and minerals, and by the ability of ecosystems to recycle the remains of dead organisms through the activities of bacteria and fungi.  Living organisms have the capacity to produce populations of unlimited size, but available resources in their environments are finite. This restricts the growth of populations and produces competition between organisms.

Organism Relationships
Organisms interactions may be competitive or beneficial.  Organisms may interact with one another in several ways.  Some of these relationships include producer/consumer, predator/prey, or parasite/host relationships. Other organisms interactions include those in which one organism may cause disease in, scavenge, or decompose another.

Biodiversity
Due to evolution, there is a great number of different organisms which fill many different roles in ecosystems.  The number of different organisms in an ecosystem is called biodiversity.  Increased biodiversity increases the stability of the ecosystem.   Biodiversity also ensures the availability of diverse genetic material that may lead to future discoveries with significant value to humans.  As diversity is lost, potential sources of these materials for these discoveries may be lost with it.  A great diversity of species provides for variations which increase the chance that at least some living things will survive in the face of large changes in the environment.

Ecological Succession
The environment may be changed greatly through the activities of organisms, including humans, or when climate changes. Although sometimes these changes occur quickly, in most cases species gradually replace others, resulting in long term changes in ecosystems.    These changes in an ecosystem over time are called ecological succession.   Ecosystems may reach a point of stability that can last for hundreds or thousands of years.  If a disaster occurs, the damaged ecosystem is likely to recover in stages that eventually result in a stable system similar to the original one. 

Biochemical Processes
Almost all life on Earth ultimately depends upon the Sun for its energy.   The process of photosynthesis converts the Sun’s energy to sugars which living things may use as an energy source.   These sugars are converted to a form living things can use by a process called respiration.

Thousands of chemical reactions occur in living things.   These reactions are aided by compounds called enzymes.   Enzymes and some other kinds of molecules have specific shapes which allow them to function.

Disease
Homeostasis in an organism is constantly threatened.  Failure to respond effectively can result in disease or death.  Disease is a disturbance of homeostasis or steady state within an organism.   Many organisms, such as viruses, bacteria, fungi, and parasites may cause disease.   Disease also results from factors which are not living organisms.

The immune response is the defensive reaction of the body to foreign substances or organisms.  The immune system also protects against some cancer cells which may arise in the body.

Feedback Mechanisms
Dynamic equilibrium or homeostasis results from the ability of organisms to detect and respond to stimuli.   Feedback mechanisms are specific ways which have evolved in different living things to respond to internal or external environmental changes and maintain homeostasis.   A feedback mechanism is a process where the level of one substance or activity of an organ or structure influences another substance or structure in some manner. 

Living VS. Non-Living
Complex organisms, such as humans, require many systems for their life processes.   Less complex living things may lack the complex systems of more complex organisms, but they still carry on the basic life activities.   While non-living things may carry on some of these life processes, they do not carry on all of them, or these activities do not interact in a manner allowing the non-living thing to reproduce itself.

The components of living things in humans and other organisms, from organ systems to cell organelles, interact to maintain a balanced internal environment. This balanced internal environment is called dynamic equilibrium or homeostasis.  To successfully accomplish this, organisms possess many control mechanisms that detect internal changes and correct them to restore the internal balance of the organism.   If an organism fails to maintain homeostasis, this may result in disease or death.   Non-living things possess few control mechanisms to maintain homeostasis.

Population Diversity
The greater the diversity, or number of different species of organisms in an ecosystem, the ecosystem is more stable and likely to last.  These ecosystems contain many different kinds of organisms carrying on a variety of different nutritional modes.  Organisms can be categorized by the function they serve in an ecosystem. Each species in an ecosystem has a role for which it is best suited. In general, no two species have the same role in an ecosystem.  This allows different species to coexist successfully and helps maintain the stability of the ecosystem.   Ecosystems are stable due to the interactions among the many different populations. These interactions contribute to the overall maintenance and continued existence of the ecosystem.

Organizational Levels
Important levels of organization for structure and function of living things include cells, tissues, organs, organ systems, and whole organisms. The organs and systems of the body help to provide all the cells with their basic needs to carry on the life functions. The cells of the body are of different kinds and are grouped in ways that help their function.

All living things are composed of one or more cells, each capable of carrying out the life functions.   The organelles present in single-celled organisms often act in the same manner as the tissues and systems found in many celled organisms.  Single-celled organisms perform all of the life processes needed to maintain homeostasis, by using specialized cell organelles.

Cell Structure
Cells have particular structures or organelles that perform specific jobs. These structures perform the life activities within the cell. Just as body systems are coordinated and work together in complex organisms, the cells making up those systems must also be coordinated and organized in a cooperative manner so they can function efficiently together.

Inside the cell a variety of cell organelles, formed from many different molecules, carry out the transport of materials, energy capture and release, protein building, waste disposal, and information storage. Each cell is covered by a membrane that performs a number of important functions for the cell as well.

Life Functions
Humans and many other organisms require multiple systems for digestion, respiration, reproduction, circulation, excretion, movement, coordination, and immunity. The systems collectively perform the life processes.

Once nutrients enter a cell, the cell will use those raw materials for energy or as building blocks in the synthesis of compounds necessary for life.  The energy we initially obtain must must be changed into a form cells can use. A type of protein called an enzyme allows for these changes to occur within the cell.

Cellular Communication
Neurotransmitters and hormones allow communication between nerve cells and other body cells as well.  If nerve or hormone signals are changed, this disrupts communication between cells and will adversely effect organism homeostasis.   Additionally, the DNA molecule contains the instructions that direct the cell’s behavior through the synthesis of proteins.

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