While estimates of total biodiversity vary significantly, the number of formally described and cataloged species is generally cited in scientific literature as being approximately 1.4 to 1.5 million. This figure represents the cumulative effort of taxonomists over centuries to document the Earth's biological diversity, though millions more remain undiscovered.

Chemistry is the branch of science that investigates the properties, composition, and structure of matter, as well as the changes matter undergoes during chemical reactions. It provides the foundational principles for understanding biological processes at the molecular level.

Biochemistry is the branch of science that explores the chemical processes within and related to living organisms. It is a laboratory-based science that brings together biology and chemistry. By using chemical knowledge and techniques, biochemists can understand and solve biological problems, such as metabolic pathways and molecular interactions.

A hypothesis is a tentative, testable explanation for a phenomenon observed in the natural world. It serves as the starting point for further scientific investigation, allowing researchers to conduct experiments or gather data to support or refute the proposed idea. Unlike a theory, which is a well-substantiated explanation, a hypothesis is preliminary and requires empirical validation through the scientific method.

Thermodynamics is the branch of physical science that deals with the relationships between heat, work, temperature, and energy. It governs how energy is transferred within systems and how it affects the physical properties of matter. In biological systems, the laws of thermodynamics are essential for understanding metabolic processes, energy conversion, and the maintenance of life.

Biogeography is the branch of biology that deals with the geographical distribution of plants and animals. It examines the patterns of species distribution across space and time, helping scientists understand how environmental factors and evolutionary history influence where organisms live on Earth.

In biological organization, cells are the basic structural and functional units of life. When cells of similar structure and origin aggregate to perform a specialized function, they form a tissue. Examples include epithelial, connective, muscular, and nervous tissues in animals. This level of organization is essential for the complex physiological processes required by multicellular organisms.

Biological organization follows a hierarchical structure where cells group together to form tissues, and various tissues integrate to perform specific physiological functions, thereby forming an organ. Organs subsequently work together to form organ systems, which maintain the overall life processes of the organism.

In the hierarchy of biological organization, an organ system is defined as a group of organs that cooperate to perform specific, integrated physiological processes necessary for the survival of the organism. Examples include the digestive, circulatory, and respiratory systems, each composed of distinct organs working in concert to maintain homeostasis.

Isotopes are variants of a particular chemical element that have the same number of protons (atomic number) but a different number of neutrons, resulting in different atomic masses. This is a fundamental concept in chemistry and biological tracer studies.