Introduction to Biology

What Is Biology?

Biology is the scientific study of life and living organisms. It seeks to understand the structure, function, growth, origin, evolution, and distribution of living things. Scientists and philosophers have proposed several definitions of life, but all emphasize the unique properties that distinguish living systems from non-living matter.

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

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

• Life is the quality that distinguishes a vital and functional being from a dead body.

Fundamental Properties of Life

All living things share several essential properties that define life:

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

• Cellular organization: All life is made up of cells, the basic units of structure and function.

• Information storage and transmission: Life stores and follows genetic instructions, primarily in the form of DNA.

• Energy utilization: Living organisms store and use energy to carry out cellular processes.

• Self-replication: Life can reproduce itself, passing on genetic information to offspring.

• Response to environment: Living things control their functions and respond to environmental changes.

• Evolution: Populations of living organisms evolve over generations.

Which Properties of Life Fit?

Not all cells or organisms exhibit every property at all times. For example, mature red blood cells in mammals lack DNA and mitochondria, so they do not divide or replicate.

Some organisms, such as mules, are unable to reproduce but are still considered alive because they meet other criteria of life.

Biomolecules and the Properties of Life

Main Biomolecules Supporting Life

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

• 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 biochemical 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.

Self-Replication

All biomolecules facilitate the process of self-replication, allowing life to reproduce and pass on genetic information to the next generation.

Membrane Delimitation: The Role of Lipids

Lipids are crucial for forming cell membranes, which provide a selectively permeable barrier that separates the cell from its environment. This compartmentalization is essential for maintaining the internal conditions necessary for life.

Energy Storage and Utilization

Living organisms store energy in chemical bonds and convert it into usable forms, such as ATP (adenosine triphosphate), to power cellular activities. Energy can be derived from sunlight (photosynthesis) or organic compounds (cellular respiration).

Monomers to Polymers

Biomolecules are often built from smaller units (monomers) that join to form larger structures (polymers):

• Carbohydrates: Monosaccharides (e.g., glucose) form polysaccharides (e.g., glycogen).

• Amino acids: Link together to form proteins (polypeptides).

• Nucleotides: Form nucleic acids (DNA, RNA).

• Fatty acids and glycerol: Form lipids (triglycerides, phospholipids).

Proteins: Control and Response

Proteins are responsible for controlling cellular functions and responding to environmental signals. They act as enzymes, receptors, transporters, and structural components.

Evolution

Evolution is the change in the heritable characteristics of biological populations over successive generations. It is a fundamental property of life, driven by genetic variation and natural selection.

The Cell: 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 necessary for survival and reproduction.

Types of Cells

Cells are classified into two main types: prokaryotic and eukaryotic.

Prokaryotic Cells (Bacteria and Archaea)

• No nucleus (no nuclear membrane)

• Circular DNA

• Small size (0.1–5 microns)

• Often lack membrane-bound organelles

• Fast replication

Eukaryotic Cells (Protozoa, Fungi, Plants, Animals)

• Have a nucleus and membrane-bound organelles

• Linear DNA (chromosomes)

• Larger size (1–100 microns)

Are Viruses Alive?

Viruses exhibit some but not all properties of life. They can store and follow genetic instructions, evolve, and respond to their environment, but they lack independent metabolism and cannot replicate without a host cell. Thus, whether viruses are considered alive is debated.

Class Recap and Objectives

• Practice designing experimental controls

• Understand that the definition of "living" can vary

• Recognize the course definition of life

• Identify the four main biomolecules and their roles in supporting life

• Know that cells are the fundamental units of life

• Distinguish between types of cells and compare their features'