Foundations of Life: Characteristics, Classification, and Evolution

Characteristics of Life

Defining Life: What Does It Mean to Be Alive?

Biologists use a set of common features to distinguish living organisms from non-living matter. These characteristics help define what it means to be alive and are used to classify and study life forms.

• Organized Structure: All living things are composed of one or more cells, which are considered the basic units of life. Cells have complex, organized structures that carry out specific functions.

• Regulation or Homeostasis: Living organisms maintain a stable internal environment despite changes in the external environment. This process is called homeostasis.

• Metabolism (Use of Energy): Life requires energy to carry out processes such as growth, repair, and reproduction. Organisms obtain energy from their environment and convert it into usable forms.

• Growth, Reproduction, and Death: Living things grow, reproduce to create new individuals, and eventually die.

• Response to Environment: Organisms can sense and respond to stimuli in their environment, such as light, temperature, or chemicals.

• Evolutionary Adaptation: Populations of living organisms evolve over generations through changes in genetic information, allowing them to adapt to their environments.

Viruses: Are They Alive?

Debate: The Status of Viruses

Viruses are submicroscopic infectious agents that contain genetic material (DNA or RNA) and can evolve over time. However, they lack many features of living organisms and require a host cell to reproduce.

• Traits Supporting Life: Viruses have genetic material and can evolve.

• Traits Opposing Life: Viruses cannot reproduce independently, do not have cellular structures, cannot maintain homeostasis, and do not metabolize energy on their own.

• Conclusion: Most biologists do not consider viruses to be alive because they do not meet all the criteria for life.

Example: The influenza virus can only replicate inside the cells of a host organism, using the host's machinery to produce new virus particles.

Scientific Inquiry and the Nature of Science

How Science Works

Science is a systematic way of understanding the natural world through observation, experimentation, and reasoning. It relies on testable explanations and predictions.

• Inquiry: The process of asking questions and seeking answers through evidence and experimentation.

• Hypotheses vs. Theories: A hypothesis is a testable, tentative explanation for an observation. A theory is a well-substantiated explanation that is supported by a large body of evidence.

• Limitations of Science: Science cannot answer questions about supernatural phenomena or subjective values; it is limited to natural explanations based on evidence.

Classification of Life

Broad Categories: Prokaryotes and Eukaryotes

All living organisms are classified into two main cell types based on their cellular structure.

• Prokaryotes: Organisms without a membrane-bound nucleus. Includes Bacteria and Archaea.

• Eukaryotes: Organisms with a membrane-bound nucleus and other organelles. Includes Animals, Plants, Fungi, and Protists.

The Three Domains of Life

Modern classification divides life into three domains based on genetic and cellular differences.

Domain	Cell Type	Examples
Bacteria	Prokaryotic	Escherichia coli, Cyanobacteria
Archaea	Prokaryotic	Thermophiles, Halophiles
Eukarya	Eukaryotic	Animals, Plants, Fungi, Protists

Unity and Diversity of Life

Unity: Shared Features

Despite the diversity of life, all organisms share certain fundamental characteristics, such as the universal genetic code (DNA), cellular structure, and similar metabolic pathways.

• Universal Genetic Code: All living things use DNA as their genetic material.

• Cellular Organization: All organisms are made of cells, the basic units of structure and function.

Diversity: Adaptation and Evolution

Life on Earth is incredibly diverse, with millions of species adapted to different environments. This diversity arises from evolutionary processes.

• Adaptation: Organisms develop traits that help them survive and reproduce in their environments.

• Speciation: The formation of new species through evolutionary processes, often due to isolation and environmental differences.

Evolution by Natural Selection

Darwin's Theory

Charles Darwin proposed that evolution occurs through natural selection, a process where individuals with advantageous traits are more likely to survive and reproduce.

• Variation: Individuals in a population vary in their traits, often due to mutations.

• Heritability: Traits can be passed from parents to offspring.

• Selection: Some individuals survive and reproduce better than others because of their traits.

• Adaptation: Over time, beneficial traits become more common in the population.

Example: Darwin's finches on the Galápagos Islands evolved different beak shapes to exploit different food sources, leading to speciation.

Chemical Foundations of Life

Essential Elements for Life

Life is based on a small set of chemical elements that are abundant and have properties suitable for building complex molecules.

• Major Elements: Oxygen (O), Carbon (C), Hydrogen (H), and Nitrogen (N) make up about 96% of the human body mass.

• Other Essential Elements: Calcium (Ca), Phosphorus (P), Potassium (K), Sulfur (S), Sodium (Na), Chlorine (Cl), and Magnesium (Mg).

• Trace Elements: Elements required in very small amounts, such as Iron (Fe), Iodine (I), and Zinc (Zn).

Element	Approximate % of Body Mass	Role
Oxygen (O)	65%	Component of water, needed for cellular respiration
Carbon (C)	18.5%	Backbone of organic molecules
Hydrogen (H)	9.5%	Component of water and organic molecules
Nitrogen (N)	3.3%	Component of proteins and nucleic acids
Calcium (Ca)	1.5%	Bone structure, signaling
Phosphorus (P)	1.0%	Component of nucleic acids, ATP
Potassium (K)	0.4%	Nerve function
Sulfur (S)	0.3%	Component of proteins
Sodium (Na)	0.2%	Nerve function
Chlorine (Cl)	0.2%	Water balance
Magnesium (Mg)	0.1%	Enzyme function
Trace Elements	<0.01%	Various specialized roles

Deficiency and Excess: Both a lack and an excess of certain elements can cause health problems. For example, iodine deficiency can cause goiter, while selenium excess can be toxic.

Summary Table: Characteristics of Life vs. Viruses

Characteristic	Living Organisms	Viruses
Cellular Structure	Yes	No
Homeostasis	Yes	No
Metabolism	Yes	No
Growth	Yes	No
Reproduction	Yes (independent)	Only in host cell
Response to Environment	Yes	Limited
Evolution	Yes	Yes