Fundamental Properties of Life and Cellular Organization

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 most definitions emphasize self-sustained chemical systems capable of Darwinian evolution, self-replication, and 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).

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 as follows:

• 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 and follows instructions, primarily through nucleic acids.

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

• Self-replication: Life is capable of reproducing itself.

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

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

Examples and Exceptions

• Mature Red Blood Cells: These cells lack DNA, mitochondria, and the ability to divide, yet they are derived from living cells and perform essential functions.

• Mule: An example of an organism that cannot reproduce, yet is alive.

Biomolecules Supporting Life

Main Classes of Biomolecules

The properties of life are supported by four main classes of biomolecules:

• Lipids: Provide membrane structure, energy storage, and insulation.

• Carbohydrates: Serve as energy sources and structural components.

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

• Proteins: Facilitate cellular functions, catalyze reactions, and respond to environmental changes.

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 life, while RNA plays roles in protein synthesis and regulation.

• DNA: Double-stranded molecule storing genetic information.

• mRNA: Messenger RNA carries genetic instructions from DNA to ribosomes.

• tRNA: Transfer RNA brings amino acids to ribosomes during protein synthesis.

• rRNA: Ribosomal RNA forms the core of ribosome structure and function.

Self-Replication

All biomolecules facilitate the process of self-replication, which is essential for life. Cells replicate their DNA and divide to produce new cells.

Membrane Delimitation: Role of Lipids

Lipids form the selectively permeable barriers of cell membranes, enabling compartmentation and protection. Their hydrophobic and hydrophilic properties make them ideal for membrane formation.

• Phospholipids: Major component of cell membranes, forming bilayers.

• Energy source: Lipids can be metabolized for energy.

• Insulation: Lipids help maintain temperature and protect cells.

Energy Storage and Use

Life stores energy in molecules such as carbohydrates, lipids, and proteins, and converts it into ATP (adenosine triphosphate), which is used by cells to perform work. Energy can be derived from sunlight (photosynthesis) or organic compounds (cellular respiration).

• ATP: The universal energy currency of cells.

• Photosynthesis: Converts sunlight into chemical energy.

• Cellular respiration: Converts organic molecules into ATP.

Monomers to Polymers

Biomolecules are often built from monomers into polymers.

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

• Amino acids: Monomers that form proteins (polypeptides).

Control and Response: Role of Proteins

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

Evolution

Evolution is the change in heritable characteristics of biological populations over successive generations. It is a fundamental property of life, enabling adaptation and diversity.

• Natural selection: Drives evolution by favoring advantageous traits.

• Mutation: Introduces genetic variation.

Cellular Organization

The Cell: The Basic Unit of Life

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.

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles; include bacteria and archaea.

• Eukaryotic cells: Have a nucleus and membrane-bound organelles; include protozoa, fungi, plants, and animals.

Types of Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No	Yes
DNA Structure	Circular	Linear (chromosomes)
Size	0.1-5 microns	1-100 microns
Organelles	Rarely membrane-bound	Membrane-bound
Replication	Fast	Slower

Are Viruses Alive?

Viruses exhibit some properties of life, such as storing and following instructions, evolving, and responding to the environment, but lack others, such as independent energy storage and self-replication outside a host cell.

• Membrane delimited: Sometimes, viruses require entry into a cell.

• Information storage: Yes, viruses have genetic material.

• Energy use: Viruses do not store energy but can use host energy.

• Self-replication: Only within host cells.

• Evolution: Yes, viruses evolve.

Class Recap/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.

• Recognize cells as the units of life.

• Know types of cells and compare their features.