What Is Biology?

The Science of Life

Biology is the scientific study of life and living organisms. It seeks to understand the fundamental processes that define living systems, from the molecular to the ecosystem level. Scientists and philosophers have proposed several definitions of life, but all emphasize the ability to sustain and replicate, respond to the environment, and evolve over time.

• Life is a self-sustained chemical system capable of undergoing Darwinian evolution.

• Life is a self-replicating, evolving system based on organic chemistry.

• Life distinguishes a vital and functional being from a dead body.

Properties of Life

Essential Characteristics

All living things share a set of fundamental properties that distinguish them from non-living matter. These properties are supported by specific biomolecules and cellular structures.

• Membrane delimited: Living things are separated from their environment by a membrane.

• Cellular organization: All life is made up of cells.

• Information storage and transmission: Life stores genetic information and follows instructions (e.g., DNA).

• Energy transformation: Life stores and uses energy to perform work.

• Self-replication: Life can reproduce itself.

• Response to environment: Life controls its functions and responds to external stimuli.

• Evolution: Life evolves over generations.

Example: Mature Red Blood Cell

Mature red blood cells in mammals lack DNA, mitochondria, and the ability to divide, yet they are derived from living cells and perform essential functions in the body.

Example: Mule

A mule is a hybrid animal that cannot reproduce, raising questions about the definition of life based solely on the ability to replicate.

Main Biomolecules Supporting Life

Types and Functions

Four major classes of biomolecules are essential for life, each supporting different properties of living systems:

• Lipids: Form membranes, store energy, and provide insulation.

• Carbohydrates: Serve as energy sources and structural components.

• Nucleic Acids: Store and transmit genetic information (DNA, RNA).

• Proteins: Catalyze reactions, provide structure, and regulate processes.

Nucleic Acids: Information Storage and Transmission

Nucleic acids, such as DNA and RNA, are responsible for storing and transmitting genetic information. DNA encodes instructions for building proteins, while various forms of RNA (mRNA, tRNA, rRNA) are involved in protein synthesis.

Lipids: Membrane Structure and Function

Lipids are hydrophobic molecules that form the selectively permeable barriers of cell membranes. Their structure allows for compartmentalization, energy storage, and insulation.

Carbohydrates and Proteins: Energy and Function

Carbohydrates are broken down to provide energy, while proteins perform a wide variety of cellular functions, including catalysis, signaling, and structural support.

• Energy from sunlight or organic compounds is converted into ATP, the cell's energy currency.

Cells: The Basic Unit of Life

Definition and Essential Features

The cell is the smallest unit that meets all the properties of life. All organisms are composed of one or more cells, which carry out the essential functions of life.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two main types based on their structural features:

Are Viruses Alive?

Debate on the Definition of Life

Viruses exhibit some but not all properties of life. They contain genetic material and can evolve, but they lack cellular structure, do not store energy, and require a host cell to replicate. Thus, whether viruses are considered alive depends on the criteria used.

Summary Table: Properties of Life and Biomolecules

Class Recap/Objectives

• Practice designing experimental controls

• Understand that the definition of "living" can vary

• Recognize the course definition of life

• Identify biomolecules and their roles in supporting life

• Know that cells are the units of life

• Distinguish between types of cells and compare their features