Unit 2 Lesson 1 — Characteristics of Living Things

Cell

The basic structural and functional unit of life; the smallest entity that can perform all life processes independently, and the building block of multicellular organisms.

~30 trillion human cells

Approximate current estimate of the number of human cells in a typical adult human body (transcript figure).

~38 trillion bacteria

Approximate estimate of single-celled bacteria living in and on the typical adult human, highlighting the body’s microbiome.

Living things are made of cells

Principle that an organism can be unicellular (single cell) or multicellular (many cells), and all life is cellular in organization.

Response to stimuli

The ability of organisms to detect and react to changes in internal or external environments (e.g., pulling your hand away from a hot surface).

Adaptation

Heritable trait developed through generations that improves survival or reproduction in a specific environment (e.g., hibernation in bears as seasonal adaptation).

Growth

Biological increase in size and/or cell number (in multicellular organisms growth typically involves cell division producing more cells).

Development

Progressive changes in form and function over an organism’s lifetime; in multicellular organisms this includes cell specialization.

Metabolism

Sum of all chemical reactions that obtain, convert, store, and use energy in organisms; includes catabolic (breakdown) and anabolic (building) pathways.

Anabolism

Energy-requiring metabolic processes that build complex molecules (e.g., protein synthesis from amino acids).

Catabolism

Energy-releasing metabolic processes that break down complex molecules (e.g., cellular respiration breaking glucose into CO₂ and H₂O).

Homeostasis

Active regulation of internal conditions (temperature, pH, fluid balance, etc.) necessary for cells and enzymes to function properly.

Reproduction

Biological process producing new individuals either sexually (genetic mixing) or asexually (clonal division).