Characteristics of Life

Defining Features of Living Organisms

Biochemistry begins with understanding what distinguishes living organisms from nonliving matter. There are eight characteristics shared by all living organisms:

• Composed of cells: The cell is the basic and most fundamental unit of life.

• Order: Organisms are not random; they have complex and ordered structures built from molecules and cells.

• Response to stimuli: Organisms can respond to environmental changes or signals.

• Homeostasis: Mechanisms for maintaining internal stability, such as temperature or chemical balance.

• Reproduce: The capacity to produce more life, either sexually or asexually.

• Metabolism: All living things carry out chemical reactions to obtain and use energy.

• Heredity: All life contains DNA, the molecule that stores genetic information and passes it to offspring.

• Evolve: DNA mutations over time lead to changes in populations, allowing adaptation to changing environments.

Example: Humans, plants, and bacteria all share these eight characteristics, though they may express them differently.

Viruses and Life

Viruses are not considered alive because they do not possess all eight characteristics. For example, viruses:

• Contain hereditary information (DNA or RNA)

• Cannot reproduce independently; they require a host cell

• Are not composed of cells

• Do not metabolize or maintain homeostasis

Practice Question: Which of the following is not a characteristic of life?

• Ability to metabolize oxygen (not universal to all life)

Practice Question: Which statement about viruses is false?

• "Viruses are composed of cells" is false; viruses are acellular.

Biological Levels of Organization

Hierarchy from Molecules to Organisms

All living organisms contain a biological hierarchy of organization, which is essential for understanding biochemistry. This hierarchy moves from the largest structures to the smallest:

• Multicellular Organism: Complete living entity (e.g., human, plant)

• Organ Systems: Groups of organs working together (e.g., digestive system)

• Organs: Structures with specific functions (e.g., heart, brain)

• Tissues: Groups of similar cells performing a function (e.g., muscle tissue)

• Cells: Basic unit of life (e.g., neuron, erythrocyte)

• Organelles: Specialized structures within cells (e.g., mitochondria, nucleus)

• Macromolecules: Large molecules essential for life (e.g., proteins, nucleic acids, polysaccharides)

• Molecules: Chemical compounds (e.g., water, glucose)

• Atoms: Fundamental units of matter (e.g., carbon, hydrogen)

Example: The protein hemoglobin is a macromolecule found in red blood cells (cells), which are part of blood (tissue), within the circulatory system (organ system) of a human (organism).

Biochemical Unity of Life

Molecular Similarity Across Organisms

All living organisms are remarkably similar at the molecular/atomic levels of life. This concept is foundational in biochemistry, as it allows scientists to study basic processes in simple organisms and apply the knowledge to more complex life forms.

• Biochemical Unity: The same types of macromolecules (proteins, nucleic acids, lipids, carbohydrates) are found in all living things.

• Application: Model organisms (e.g., Escherichia coli, yeast) are used to study fundamental biochemical processes.

Example: The genetic code is nearly universal among all living organisms, supporting the idea of biochemical unity.

Additional info: The study of biochemistry often begins with these foundational concepts, which are essential for understanding more advanced topics such as metabolism, molecular genetics, and cellular signaling.