Biology is the Scientific Study of Life!

Clickable Index of Biology
Cellular Structure

Congratulations on choosing one of the finest majors offered at our university. Biology is a fascinating field of study and can lead to a wide range of careers. It requires that you take some difficult courses. I hope you are registered for Chemistry, Math, and College Writing or Computer Science. These provide you with experience in field related to Biology. They are essential to your progress. Our major requires four semesters of Chemistry, and Chem 1 and Organic 1 are only offered in fall you should be in either the one or the other! For Math, your placement exams or previous coursework determines what you should be taking. But ultimately you need to finish Calculus 1 and Statistics. It may take you several semesters to accomplish this based on your Math skills. BE SURE you are NOT registered for "Math for Liberal Arts" or "Number Systems" or any other lower-level math class as these do NOT lead to Calculus!

OK, so what is Biology? As is true of almost all of our words in science, this word can be dissected. The first half of the word, Bio-, means "life" and the second half, -logy, means "study of." So biology is the study of life. Of course, based on our last lecture, we might add that biology is the scientific study of life!

OK, that was easy, but it begs a much deeper question: What is life?. Now that is a far more difficult question. A definition for "life" is not so easy to provide. But there are some properties that all living things share. And it is these properties that we focus on today! What are the properties of life?

Properties of Life

The individual living thing is called an organism and this is the focus of our course. So to be considered truly alive, each bacterium, archaeon, protist, plant, fungus, or animal must show the complete list of properties of life. Some of these organisms consist of just one single cell, and so these properties of life, must exist at the cell level too! This becomes quite obvious when you realize that a complex animal, such as a human, begins as a single cell, when a sperm and egg join together in syngamy to make a zygote. So your zygote contained all the genetic information to make every part of you, and thus must also possess the properties of life.

Properties of Life
Cellular Structure

The properties of life we are about to consider, are present in all living organisms. Some organisms may show certain of these properties more obviously than others, but all of these properties should be present in some measurable way for a thing to be considered a living organism. However, there are many non-living things that demonstrate some of these properties of life at least to some limited extent. So life is really a continuum...showing a range found in each of these properties...and some non-living entities are more "lively" than others. As we shall see, a computer is more lively than a book, for example. But neither of them have enough abundance of each of these properties to be considered a living organism. By similar application of definition, however, a virus is NOT a living organism (at least when considered independently of its host)!

Cellular Structure

Living organisms consist of one or more cells. The body has both structure and function. The cell is both a unit of structure and a unit of function. Among living organisms there are small single-celled organisms that are bacteria, archaea, protists, plants, fungi, and animals. But there are also species in each of these major groups of organisms that are multicellular...some consisting of billions of cells!

As a unit of structure, the cell has a boundary layer which might include a more-or-less rigid cell wall, and a more-or-less flexible cell membrane. The boundary is responsible for gas, water, mineral, nutrient, and waste exchange. Inside the cell are structures involved in information storage and processing with the information encoded in one or more molecules of DNA (deoxyribonucleic acid). The rest of the cell structure is involved in metabolism, movement, growth, and so on.

The cell then, being a property of living organisms, is itself a unit of life. It must be able to demonstrate all of these properties of life because, for many organisms, a single cell is the complete living organism!

Shown above are different kinds of micrographs of cells of a wide range of species. Perhaps you will notice some unity in this diversity of images!
A: a light micrograph of a magnetotactic bacterium (
B: a scanning electron micrograph of an archaeon (
C. a transmission electron micrograph of a protist (
D. a light micrograph a primitive plant cell (
E. an x-ray tomograph of a fungal cell (
F. a light micrograph of two animal cells (


Every cell carries out a range of biochemical processes. They exchange gases, take in nutrients and minerals, they process energy directly or indirectly, they process matter, they maintain their internal condition and modify their local environment, they produce waste, and they generate heat. Maintaining the proper levels of temperature, pH, water, and other chemicals can generally be called homeostasis.

Homeostasis is accomplished by a range of kinds of metabolic processes. Some of these are processes assemble materials and build up the organism. These are called anabolic metabolic processes. Certainly photosynthesis would qualify as a example of anabolism:

Photosynthesis:  CO2 + H2O light energy
O2 + CH2O

A plant cell takes in atmospheric carbon dioxide gas and soil water. It traps light energy with a chlorophyll molecule inside the cell. It uses the light energy to split the water molecule to release oxygen gas back to the atmosphere, and to reduce the carbon dioxide gas into a carbohydrate. The carbohydrates are polymerized into sugars, starches, or even cellulose polymers! So this process can be used to build up reserves and make cell walls as the cells grow or divide. This is definitely an anabolic process.

Metabolism is not always anabolic, however! There are many examples of catabolism as well. These metabolic processes break down rather than build up materials. An example of catabolic metabolism would be respiration:

Respiration:  O2 + CH2O ------------------> CO2 + H2O + chemical energy

In many ways photosynthesis and respiration are compensating metabolic pathways... in a plant cell photosynthetic productivity must exceed respiration for enough time each day to make up for respiration losses at night. If an organism has sufficient anabolic productivity it can build up its reserves and perhaps reproduce! However, if an organism has too much catabolism, it will use up its reserves, fail to reproduce, and perhaps even die. Healthy human weight management involves the balance between anabolism and catabolism.


The living organism uses its metabolism for more than just maintaining static conditions! Obviously one property of life is growth. The cell just after cell division is small and must grow to its mature size. While the vast majority of cells are microscopic, they nevertheless have grown to achieve their mature size. Some kinds of cells grow extensively and become macroscopic. Nerve cells of large animals may have a length measured in meters! The same is true of some plant fiber cells.

Growth can be defined as an irreversible change in size. Growth can be observed at the cell level as a permanent change in cell size. For multicellular organisms, growth can also be achieved by adding more cells to the organism's body. So growth can have both cell expansion and cell proliferation components.

It is important to notice that metabolism drives the growth of cells and bodies, but it can also cause other size changes that are not defined as growth. You may have observed the flexible body of a mature octopus flatten and spread out its mass to enter a thin crevice. These changes in size are not permanent, and therefore are not growth...when the octopus come out of the crevice, the mature size and shape is restored. Similarly when you contract a muscle, it gives an appearance of growth, but while it is getting larger in diameter, it is also shortening in length. When you relax the muscle it goes back to its original muscle contraction in not growth. However, it could be argued that body building does build muscle in more-permanent ways. You have probably read about people using anabolic steroids to build up their muscles more quickly in body building. Famous examples in baseball have been in the news on a regular basis.


A further property of life is movement. Indeed our discussion of muscle contraction reminds us that the shortening of a muscle moves bones that are hinged (or otherwise articulated) in the skeleton. Thus your biceps muscle contracts and flexes your arm; your triceps muscle contracts and extends your arm. These kinds of movements are quite visible and measurable. Movement is one of the features for distinguishing one group of organisms as animals.

So do organisms without this kind of movement lack this property of life? The answer to this question is that movement has many forms and movement of limbs of a multicellular body is just one of those forms. Moreover limb movement is observed in some organisms that are not animals! A plant can orient its leaves so that they minimally overlap and shade each other. The movements may be very slow but real. A growing sunflower tracks the movement of the sun with its flowers. A bean plant lifts its leaves each morning and tilts them down each day. Even one-celled organisms can move part of their cell to locomote. Some of the cells pictured above show flagella that undulate, spin, or rotate to pull or push the cell through an aqueous environment.

Some of the cells we have seen above, lack any limbs to move or to locomote. Nevertheless all of these cells will demonstrate just happens to occur inside the cell. Inside cells are various fluid compartments, and to move materials from one part of the cell to another efficiently, cells show cyclosis (also known as cytoplasmic streaming). The fluids and structures inside the cells move in more-or-less organized ways inside the cytoplasm. As for all forms of movement, cyclosis is driven by expending metabolic energy.


A fact of life, of course, is that it does not extend forever for an organism...eventually the organism will die. So to maintain the species, the organism must replace itself by reproduction. And, just as there are multiple kind of metabolism, growth, and movement, there are multiple kinds of reproduction.

For single-celled organisms, countless generations of individuals can be made by simple cell division. In prokaryotic bacteria and archaea, this cell division is called binary fission. In the eukaryotic cells of protists, plants, fungi, and animals this cell division process is called mitosis instead. Either way, the process results in a clone of genetically identical organisms. This is a form of asexual reproduction.

Many organisms also are able to reproduce sexually. Ancient organisms produced sexual cells that could join together. These cells are called gametes and their joining process is called syngamy (literally the union of gametes). Syngamy has been crudely called fertilization in the ancient world... but I will not use that term in this course, because it is both confusing and erroneous. Its continued use in biology books is an example of sticking to old traditions to serve tradition only.

Of course syngamy is most effective in producing new combinations of genes if gametes do not fuse to each other indiscriminately. So sexual differentiation of the gametes evolved early in the tree of life. Male gametes are generally defined as the smaller (see panel F in the figure above) of the two sexes of gamete, and if the gametes are of similar size, the male gametes are the motile ones. Female gametes are generally larger and likely sessile.

The product of syngamy, the zygote, obviously has twice as much DNA as either of the gametes. Again, tradition has used the term "fertilized egg" to name the zygote...but it is neither fertilized nor is it any longer an egg. So we will use only zygote to name the product of syngamy. Because the zygote has twice as much DNA as either gamete, sexual reproduction also generally involves a compensating process. The best known of these processes is meiosis. Meiosis is a special form of cell division that reduces the amount of DNA back to the level found in the gametes. So if syngamy is yin, then meiosis is yang. The zygote may undergo meiosis immediately, or might divide mitotically to form a whole body of diploid cells before maturing...delaying meiosis until reproductive maturity.

Responds to Stimuli

Living organisms respond to stimuli. So if we were to encounter an alien, one of our first ideas would be to poke at it to see if it responds to our touch. Responding to environmental stimuli in a prompt way is a hallmark of (at least some) living organisms.

Bacteria locomote to magnetic fields to bury themselves in sediments at shallow depth. Archaea convert available chemistry to methane. Protists ingest bacteria swimming nearby. Plants orient their leaves to the sun. Fungi detect environmental waste and produce digestive enzymes. A predatory animal observes its prey, stalks it, and then kills it for food. All of these examples are responses to environmental stimuli to help maximize the chances for the organism to grow, reproduce, or just continue to live.

The stimuli that organisms respond to have evolved in ways that might not be expected at first. For example plants use the same colors of light for photosynthesis that humans can see. But a plant also produces pigments that make ultraviolet patterns on their flowers that are invisible to us...but are visible to pollinating animals that see colors of light beyond our limited human vision. Plants determine when to flower based upon detecting far-red light (730 nm) beyond our vision.

On the other hand, while predators and prey have evolved really great ways to hear each other's approach, since an plant or a fungus lacks both stalk and escape locomotion possibilities, the ability to hear has not evolved. A very few plants have evolved to respond to touch of an herbivore, however, and sounds of high decibels can initiate those escape responses. Perhaps you know of the sensitive plant, Mimosa pudica. So while plants respond to vibration and touch, they exhibit no responses to different kinds of music that humans enjoy so thoroughly.

Of course, all of these responses described here are short-term responses to changes in the environment during the lifetime of the individual. It is also true that as a species, organisms must be able to respond to long-term changes in the environment. Failing to respond to long-term changes, could mean that a species would go extinct.


The ability of a species to respond with changes spanning generations to long-term changes in the environment or among other organisms in the environment is a critical property of life.

The earth environment is pretty stable, but there are long-term changes that naturally happen (ice ages, for example) and these must be accommodated if a species is to continue. As glaciers advance, species need to be able to migrate. As they migrate into new areas they face different environmental factors including competition, predation, and different food supplies, etc. So for a species to avoid extinction, it must be able to evolve.

Evolution is a very long-term we cannot expect to see it happen in one species during one generation time of that species. However, through differential reproductive success over many generations, a species does change and those changes are called evolution. Over many dozens of generations of an organism, we might observe a genetic change that proliferates through the population. In many thousands of generations the accumulated genetic changes might be significant enough to block reproduction with others of its kind. When evolution produces a population that can no longer breed with others of its species...but only within its own population, that event is called speciation. This is a most interesting evolutionary event as it has produced a new species.

To observe evolution within the time-span of an individual human life, we have to examine organisms that reproduce much more rapidly. In biology the selection of an organism for the purpose of our scientific goals is a critical choice. We have observed the evolution of antibiotic resistance genes in bacteria routinely exposed to specific antibiotics over many bacterial generations. MRSA (Methacillin Resistant Staphylococcus aureus) is a very pertinent example requiring us to resort to additional antibiotics to avoid human death.

For a longer-term example, within the span of the species Homo sapiens, we have domesticated and artificially selected changes in about five species of wolves to produce hundreds of very different dog breeds. As I watch our chihuahua/cocker spaniel mix dog do her tricks to obtain a treat, I realize how much we have changed the wild wolf! Other evolutionary events directed by humans include corn and bread wheat. These species do not exist in the wild. Wheat was created by prehistoric humans who selected among three species of grasses being cultivated together. Bread wheat thus has three complete genomes and thus is one of the original genetically-modified-organisms (GMOs). Today many people are getting angry about GMOs...but what few realize is that most of our foods have already been genetically modified by humans. Certainly the ancient techniques were primitive...they were field rather than laboratory based...but they were very effective if slow. Modern laboratory techniques can be far faster and much more directed and specific.

As we examine the genomes of the living organisms on our planet, their hereditary relationships are becoming clearer. The patterns of evolution are evidenced in the genetic sequences in DNA. As we shall see, the tree of life...the diversity of living organisms...derives from what appears to be only one source. The living organisms are marked by shared genes indicating their evolution from just one common ancestor. And so, as we proceed to look at the living organisms on our planet, we hopefully realize that we are just looking at our own relatives. In many ways then, we are ALL genetically modified organisms!


Biology: the Scientific Study of Life

Properties of Life

This page © Ross E. Koning 1994.

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