Introduction to Biology

The Science of Life

Biology is the scientific study of life, encompassing the structure, function, growth, origin, evolution, and distribution of living organisms. Scientists and philosophers have defined life in various ways, but common themes include self-sustained chemical systems, the ability to undergo Darwinian evolution, and the presence of organic chemistry.

• Life is a self-sustained chemical system capable of undergoing Darwinian evolution (Joyce 1995, NASA definition).

• Life is a self-replicating, evolving system based on organic chemistry (Page 2001).

• Life distinguishes a vital and functional being from a dead body (medical definition).

Fundamental Properties of Life

Essential Characteristics

Living organisms share several fundamental properties that distinguish them from non-living matter. These properties are facilitated by biomolecules and are summarized below:

• Membrane delimited: All living things are surrounded by membranes that compartmentalize their internal environment.

• Cellular organization: Life is made up of cells, the basic unit of structure and function.

• Information storage and transmission: Life stores instructions (genetic information) and follows them.

• Energy transformation: Life stores energy and uses it for cellular processes.

• Self-replication: Life can reproduce itself.

• Control and response: Life controls its functions and responds to environmental stimuli.

• Evolution: Life evolves over generations through changes in heritable characteristics.

Properties of Life: Examples and Exceptions

• Cellular organization: Most cells contain DNA, divide, and have mitochondria. Example: Mature red blood cells lack DNA, do not divide, and lack mitochondria, yet are still considered cells.

• Membrane delimited: All cells are surrounded by a membrane.

• Energy transformation: Cells transform energy, often storing it as ATP.

• Evolution and replication: Life evolves and replicates, but some hybrids (e.g., mules) are not capable of reproduction.

Biomolecules Supporting Life

Main Classes of Biomolecules

Four major types of biomolecules are essential for life, each supporting specific properties:

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

• Carbohydrates: Store and provide energy.

• Nucleic Acids: Store and transmit genetic information.

• Proteins: Facilitate cellular functions and responses.

Nucleic Acids: Information Storage and Transmission

Nucleic acids, such as DNA and RNA, are responsible for storing and transmitting genetic information.

• DNA: Stores genetic instructions for cellular processes.

• mRNA: Transmits information from DNA to ribosomes for protein synthesis.

• tRNA and rRNA: Facilitate translation and protein assembly.

Self-Replication

All biomolecules contribute to the self-replication of cells, ensuring continuity of life.

Membrane Delimitation: Role of Lipids

Lipids are crucial for forming cell membranes, which act as selectively permeable barriers.

• Compartmentation: Membranes allow cells to maintain distinct internal environments.

• Energy and insulation: Lipids can also serve as energy sources and provide insulation.

Energy Storage and Use

Cells convert energy from sunlight or organic compounds into ATP, a nucleic acid used for cellular work.

• ATP: The primary energy currency of the cell.

• Energy sources: Sunlight, carbohydrates, lipids, and proteins.

Monomers to Polymers

Biomolecules are often built from monomers into polymers:

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

• Amino acids: Monomers that form proteins.

Control and Response: Role of Proteins

Proteins are responsible for controlling cellular functions and responding to environmental signals.

Evolution

Evolution is defined as the change in heritable characteristics of biological populations over successive generations.

Cells: The Basic Unit of Life

Cellular Organization

The cell is the smallest unit that meets all the properties of life. All organisms are composed of either a single cell or multiple cells.

Types of Cells

Cells are classified into two main types:

• Prokaryotic Cells:

  • No nucleus (no nuclear membrane)

  • Circular DNA

  • Small size (0.1-5 microns)

  • Often lack membrane-bound organelles

  • Fast replication

  

• Eukaryotic Cells:

  • 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.

• Membrane delimited: Sometimes, viruses need to enter a cell to function.

• Information storage and transmission: Yes, viruses store and follow instructions.

• Energy storage: No, viruses do not store energy.

• Energy use: Yes, viruses use energy from host cells.

• Self-replication: Uncertain; viruses require host cells to replicate.

• Control and response: Yes, viruses respond to their environment.

• Evolution: Yes, viruses evolve.

Class Recap and Objectives

• Practice designing controls in experiments.

• Understand that the definition of "living" can vary.

• Set up a course definition of life.

• Introduce biomolecules and their roles in supporting life: lipids, nucleic acids, proteins, carbohydrates.

• Recognize that cells are the units of life.

• Know types of cells and compare their features.

Summary Table: Properties of Life and Biomolecules

Additional info:

• Definitions and examples were expanded for clarity.

• Table was inferred from repeated key points in the notes.