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 defined life in various ways, but common themes include the ability to evolve, self-replicate, and maintain organized structures through chemical processes.

• Definition (NASA): Life is a self-sustained chemical system capable of undergoing Darwinian evolution.

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

• Medical definition: The quality that distinguishes a vital and functional being from a dead body.

Properties of Life

Fundamental Characteristics

All living things share several key properties that distinguish them from non-living matter. These properties are supported by specific biomolecules and cellular structures.

• Membrane delimited: Life is separated from the environment by a selectively permeable membrane.

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

• Information storage and transmission: Life stores genetic information (usually in DNA) and uses it to direct cellular activities.

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

• Self-replication: Life can reproduce itself.

• Response to environment: Life can sense and respond to environmental changes.

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

Examples and Exceptions

• Mature Red Blood Cell: Lacks DNA, mitochondria, and cannot divide, but is still considered alive as part of a multicellular organism.

• Mule: A hybrid animal that is sterile and cannot reproduce, yet is alive.

Biomolecules Supporting Life

Main Classes of Biomolecules

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

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

• Carbohydrates: Provide energy and structural support.

• 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 holds the instructions for building proteins, while RNA helps translate these instructions into cellular functions.

Self-Replication

All biomolecules contribute to 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 create a selectively permeable barrier between the cell and its environment. This compartmentalization is essential for maintaining the internal conditions necessary for life.

• Phospholipids: The main component of cell membranes, with hydrophilic heads and hydrophobic tails.

• Functions: Compartmentalization, energy storage, insulation.

Energy Storage and Use

Cells store energy in chemical bonds and convert it into ATP (adenosine triphosphate), which is used to power cellular processes. Energy can be derived from sunlight (photosynthesis) or organic compounds (cellular respiration).

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).

• Proteins: Amino acids form polypeptides and proteins.

Control and Response: The Role of Proteins

Proteins act as enzymes, receptors, and structural components, allowing cells to control their internal functions and respond to environmental signals.

Evolution

Evolution is the change in 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 based on their structure:

• Prokaryotic Cells: Include Bacteria and Archaea. They lack a nucleus and membrane-bound organelles, have circular DNA, are generally small (0.1–5 microns), and replicate quickly.

• Eukaryotic Cells: Include Protozoa, Fungi, Plants, and Animals. They have a nucleus, membrane-bound organelles, linear DNA (chromosomes), and are larger (1–100 microns).

Are Viruses Alive?

Viruses exhibit some but not all properties of life. They can store and transmit information, evolve, and respond to their environment, but they lack independent metabolism and cannot self-replicate without a host cell. Thus, their status as living or non-living is debated.

Summary Table: Properties of Life and Biomolecules

Class 2 Recap/Objectives

• Practice designing controls in experiments

• 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