How to Define Life -points to ponder for comprehensive questions on final exam

Living organisms are fascinating to most people. Most of us are fascinated by large, warm fuzzies (raccoons, horses, pandas) while others find delight in small wonders (small animals and plant-like algae in pond water). Whatever your taste in creature fascination, the following list of attributes apply to all organisms.  The attributes below are common to all life and reflect the basic needs of acquiring energy, reproducing, and exchanging material with the environment to sustain these and other activities.

1. Living Organisms Reproduce. There are two means of reproduction, asexual and sexual. Asexual reproduction is a common and widespread phenomenon characterizing most fungi and protists and many plants and animals. Multi-cellular organisms reproduce asexually through fragmentation of the parent body & subsequent mitotic growth of the fragment that recreates a new individual that is a genetic clone of the original (recall plant cuttings from home).  The cloned cells originating from a single parent differentiate to form the tissues and organs of the new individual.  For single-celled life, cell division alone (mitosis) is the means of asexual reproduction--one cell divides into two and each goes its separate way becoming a new individual, no, or little, cell differentiation is required.  Sexual Reproduction is a bit more complex than asexual reproduction.  Sexual reproduction requires the formation of a new single-celled stage, the zygote. Two single-celled gametes unite in the process of fertilization to form the zygote. Prior to the next generation of sexually produced offspring, meiosis is required to reduce the chromosome number from the diploid to the haploid state. Thus, sexual reproduction consists of two required events: 1) meiosis, and 2) fusion of sex cells or gametes, e.g. egg & sperm, through an event called fertilization.  Chapters 7 and 8 explore reproduction and inheritance.

2.  Living Organisms are Composed of Chemicals.  Life is formed from carbon based chemicals, i.e. organic chemicals.  For example, the DNA of your genes and the amino acids of your muscle proteins are all carbon based molecules.  The chemicals that comprise life are basically the same between different organisms. Because these chemicals are dominated by Carbon, they are called organic chemicals. The same organic structure of DNA is common to all life and the same can be said of many carbohydrates, lipids, and proteins.  Chapter 2 explores these and other organic molecules in more detail.

3. Living Organisms Acquire Materials and Energy.   All organisms are made of matter, i.e. chemicals, and energy is required to assimilate these chemicals in the form of the individual.  Acquiring energy and materials is accomplished through many series of energy transformations as life transfers energy from one form to another. The chemical reactions that accomplish these energy transformations are collectively referred to as metabolism - chemical reactions within an organism that transfers energy, ex. cellular respiration (ATP production) & photosynthesis (carbohydrate production from carbon dioxide and light energy; i.e., organic molecules are formed from  carbon dioxide under the power of light).  Chapters 4, 5, & 6 explore these topics in more detail.

4. Living Organisms Sense & Respond to Environmental Stimuli. The environmental stimulus may come from outside the body (ex. touch a hot iron and you respond with quick withdrawal) or from inside the body (ex. blood sugar levels rise and insulin, which lowers blood sugar, is released in response). The state of a stable internal environment being maintained by various mechanisms is called homeostasis. 

5. Living Organisms Have the Capacity to Mutate and genetic mutations are the fundamental basis of evolutionary change as driven by natural selection - thanks to the slightly error prone DNA replication process, gene mutations are inevitable. Whether the resulting allele produced by a mutation becomes more frequent through differential reproductive success (natural selection) is situational.  Our coverage of Evolution explores this topic in more detail.  Of note, ancient mutations have lead to the formation of two distinct cell types among living organisms, prokaryotic cells and eukaryotic cells.  Prokaryotic cells not only lack a nucleus but they also belong only to bacteria.  Eukaryotic cells typically have a nucleus and characterize all life except bacteria. 

6. Life exhibits organization at many levels [see Fig. 1.1]. An underlying theme in the study of biology flows from this understanding, that we can learn more about the whole by reducing it to its component parts.  This idea is known as reductionism.  Yet, we acknowledge the existence of emergent properties - properties emerge at higher levels of organization that cannot be predicted based on knowledge of the parts alone.

subatomic part. -> atom-> molecule->cell parts->tissues->organs-->organ systems---->individuals---> populations--->communities--->ecosystems