Introduction to Biology

What Is Biology?

Biology is the scientific study of life and living organisms. It seeks to understand how life works, from the molecular to the ecosystem level. Scientists and philosophers have defined life in various ways, but most definitions emphasize self-sustained chemical systems, evolution, and the ability to replicate and respond to the environment.

• 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

All living things share several essential properties that distinguish them from non-living matter:

• Membrane delimited: Life is separated from its environment by a membrane.

• Cellular organization: Life is, or is made up of, cells.

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

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

• 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 lack DNA, do not divide, and have no mitochondria, illustrating exceptions to some properties of life.

Example: Mule

A mule is an example of an organism that cannot replicate (is sterile), showing that not all living things meet every property perfectly.

Biomolecules: The Chemical Basis of Life

Main Biomolecules Supporting Life

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

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

• Carbohydrates: Store energy, provide structural support.

• Nucleic Acids: Store and transmit genetic information.

• Proteins: Catalyze reactions, provide structure, facilitate response to environment.

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: Stores genetic information.

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

• tRNA: Transfer RNA, brings amino acids to ribosomes.

• rRNA: Ribosomal RNA, forms part of the ribosome.

Self-Replication

All biomolecules facilitate self-replication, a fundamental property of life. Cells replicate their DNA and divide to produce new cells.

Cell Membrane: Lipids and Compartmentation

Lipids form the cell membrane, creating a selectively permeable barrier that separates the cell from its environment. This compartmentation is essential for maintaining internal conditions and supporting cellular functions.

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

• Phospholipids: Key component of membranes, with hydrophilic heads and hydrophobic tails.

Energy Storage and Use

Cells store energy in molecules like carbohydrates, lipids, and proteins, and convert it into ATP (adenosine triphosphate), which is used to perform cellular work. Energy can be derived from sunlight (photosynthesis) or organic compounds (cellular respiration).

• ATP: The primary energy currency of the cell.

• Photosynthesis: Converts sunlight into chemical energy.

• Cellular respiration: Converts organic molecules into ATP.

Monomers and 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: Monomers that form proteins (polypeptides).

Cell Components and Types

Cells: The Unit of Life

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

Types of Cells

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

Prokaryotic Cells (Archaea and Bacteria)

• No nucleus: DNA is not enclosed in a membrane.

• Circular DNA: Genetic material is circular.

• Small size: 0.1–5 microns.

• No membrane-bound organelles: Simpler internal structure.

• Fast replication: Rapid cell division.

Eukaryotic Cells (Protozoa, Fungi, Plants, Animals)

• Nucleus: DNA is enclosed in a membrane-bound nucleus.

• Linear DNA: Organized into chromosomes.

• Size: 1–100 microns.

• Membrane-bound organelles: Complex internal structure.

Are Viruses Alive?

Viruses exhibit some properties of life, such as storing and following instructions, evolving, and responding to the environment. However, they lack independent energy storage and self-replication outside a host cell, making their classification as living or non-living debated.

Recap and Objectives

• Practice designing controls in experiments.

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

• Set up a course definition of life based on fundamental properties.

• Introduce biomolecules and their roles in supporting life.

• Recognize cells as the units of life.

• Know types of cells and compare their features.

Summary Table: Properties of Life and Biomolecules

Additional info: This study guide expands on brief lecture notes to provide context and examples for each property and biomolecule, ensuring completeness for exam preparation.