WHAT IS LIFE? It is difficult to come up with a single definition of life. Chapter 1

All living things exhibit five characteristics in combination. We currently distinguish living from nonliving based upon five characteristics. Living organisms must possess all five characteristics. Inanimate objects may possess one or more of these characteristics but not all.

A. Characteristics of Life 1. Organization chemical (atom -> molecule -> macromolecule) organelle cell* tissue organ organ system multicellular organism All living things exhibit organization: Chemical level - atoms combine to make molecules; molecules combine to make macromolecules. Organelle (a membrane bound structure within a complex cell) level. * Cellular level - smallest level of organization that exhibits all characteristics of life. This is the highest level of organization exhibited by some organisms (Ameba, Paramecium, Euglena, bacteria, algae). Tissue level - a collection of cells that perform a common function. Organ level - a collection of tissues that perform a common function. Organ system level - a collection of organs that perform a common function. Multicellular organism - a living individual composed of many organ systems.

Each level of biological organization exhibits emergent properties. Ex. Capillaries transport blood (property not exhibited by individual endothelial cells). Emergent property - property not present at lower levels. Endothelial cells must aggregate in a specific way for them to transport blood.

2. Energy Use & Metabolism Metabolism - biochemical reactions that acquire & use energy. Why do organisms need energy? to combat entropy to build new structures to repair/break down old structures to reproduce How do organisms obtain energy? Organisms take in energy and transform it (by metabolism) to do many kinds of work Entropy = the natural tendency of matter to be disorganized.

Organisms obtain energy: * from the nonliving environment (sunlight or chemicals) - these organisms are called producers or autotrophs. * by eating other organisms - these organisms are called consumers or heterotrophs. * by eating dead organisms or their products [feces] - these organisms are called decomposers.

3. Maintenance of Homeostasis Homeostasis - the ability of an organism to maintain its internal environment despite conditions in the external environment. Ex. Human body temperature is ~98.6ºF if body temperature rises, you sweat. if body temperature lowers, you shiver. Inability to maintain homeostasis leads to illness or death of the organism. Ex. High body temperatures inactivate enzymes, so chemical reactions vital for life cannot occur.

4. Reproduction, Growth & Development Asexual reproduction - involves a single parent; progeny are genetically identical to the parent. Sexual reproduction - involves 2 parents; progeny are genetically diverse. Is it essential for an individual to reproduce? No, but it is essential if a population is to survive for more than one generation.

5. Irritability & Adaptation Irritability - immediate response to a stimulus. Irritability can be essential for survival (Venus flytrap captures a meal).

Over time, adaptations are modified by natural selection. Adaptation - an inherited behavior or characteristic that enables an organism to survive & reproduce. Adaptations develop over time. Examples: Camouflage is an adaptation to acquire food or escape predation. Picture of Adder snake. Leaf curling is an adaptation to minimize wind damage. Over time, adaptations are modified by natural selection.

Natural Selection - the enhanced survival & reproductive success of individuals whose inherited traits better adapt them to a particular environment. Male bird of paradise - males that are most attractive to females will contribute more offspring to succeeding generations. Thus, they come to make up more of the population over time. As the population changes, evolution occurs. Cactus - well adapted to it’s desert environment. Stem is modified to store water, while spines (modified leaves) protect it against being eaten. Fewer than 1% of the species that have ever existed on earth are alive today.

B. Biodiversity Life on earth is diverse, yet similar. Taxonomists place organisms into groups based upon evolutionary relationships. Broadest, most inclusive group (taxon) is the domain. Domain  Kingdom  Phylum or Division  Class  Order  Family  Genus  Species Genus & species refer to the organism’s binomial (name). Biodiversity - the different types of organisms on earth. Similarity of life - all are composed of cells & have DNA as their genetic material. This suggests that all life evolved from a common ancestor.

Bacteria - unicellular prokaryotes Archaea - unicellular prokaryotes The Three Domains: Bacteria - unicellular prokaryotes Archaea - unicellular prokaryotes Eukarya - eukaryotes Kingdom Protista Kingdom Plantae Kingdom Fungi Kingdom Animalia Bacteria & Archaea: Prokaryotes lack nuclei & membrane-bound organelles. Some bacteria & archaea are autotrophic, while others are heterotrophic; have distinctive cell walls. Bacteria & Archaea differ from each other at the molecular level. Eukarya: Eukaryotes possess nuclei & membrane-bound organelles. Most are multicellular. Kingdom Protista - most are unicellular; some have cell walls; some are autotrophic, while others are heterotrophic. Kingdom Plantae - multicellular; have cell walls of cellulose; usually autotrophic; have complex organ systems. Kingdom Fungi - most multicellular; have cell walls of chitin; are heterotrophic (by absorption); have tissues. Kingdom Animalia - multicellular; no cell walls; are heterotrophic (by ingestion); have complex organ systems.

Human classification scheme: Domain Eukarya Kingdom Animalia Phylum Chordata Class Mammalia Order Primates Family Hominidae Genus & species Homo sapiens

C. The Study of Life Scientists study life by using the scientific method. Scientific method - a systematic approach to interpreting observations; involves observing, questioning, reasoning, predicting, testing, interpreting, concluding & posing further questions. Hypothesis - a prediction based on previous observations or knowledge. Experiment is done to test hypothesis. It can disprove a hypothesis but can never prove it. Well designed experiments compare an experimental group to a control group. Both groups are treated identically with exception of a single factor, or variable. Note: a large sample size helps ensure meaningful results. Interpretation of data has pitfalls. Experimental evidence may lead to multiple interpretations or misinterpretations.

What is difference between hypothesis, theory & law? Hypothesis - “an educated guess”; a tentative explanation of phenomena. Theory - a widely accepted explanation of natural phenomena; has stood up to thorough & continual testing. Law - a statement of what always occurs under certain conditions. Theory - Einstein’s theory of relativity; Darwin’s theory of evolution. Laws - Mendle’s laws of independent assortment & segretation.