Chapter 1: The Character of Living Organisms – Characteristics and Classification of Life

Chapter 1: The Character of Living Organisms

Characteristics of Living Organisms

Living organisms are defined by a set of fundamental characteristics that distinguish them from non-living matter. These characteristics are universal among all forms of life and provide a framework for understanding biological systems.

• Molecular Composition: Living things have unique molecular compositions, primarily consisting of proteins, carbohydrates, lipids, and nucleic acids. These biomolecules are essential for structure and function.

• Energy and Raw Materials: All living organisms require energy (the capacity to do work) and raw materials (such as oxygen, water, carbon, amino acids, and sugars) to sustain life and carry out metabolism.

• Cellular Organization: Living things are composed of one or more cells, which are the smallest units exhibiting all characteristics of life. Cells arise from pre-existing cells through division.

• Homeostasis: Organisms maintain a stable internal environment, a state known as homeostasis. This is necessary for proper cellular function and survival.

• Response to Environment: Living things can sense and respond to changes in their external environment. For example, humans blink in bright light, and plants turn their leaves toward the sun.

• Growth and Reproduction: Living organisms grow (increase in size) and reproduce (produce new individuals). Growth in living things is due to cell division and enlargement, while reproduction ensures the continuation of the species.

• Evolution: Populations of living organisms evolve over time through inheritable changes, leading to adaptation and diversity. For example, bacteria can evolve resistance to antibiotics.

Key Biomolecules in Living Organisms

Living organisms are composed of specific types of molecules not typically found in non-living things. These include:

• Proteins: Polymers of amino acids that perform structural, enzymatic, and regulatory functions.

• Carbohydrates: Sugars and polysaccharides used for energy storage and structural support.

• Lipids: Fats, oils, and phospholipids important for energy storage and membrane structure.

• Nucleic Acids: DNA and RNA, which store and transmit genetic information.

Metabolism: Energy and Raw Materials

Metabolism encompasses all chemical reactions occurring within cells. These reactions require both energy and raw materials:

• Energy: Defined as the capacity to do work. Organisms obtain energy from their environment (e.g., sunlight, food).

• Raw Materials: Essential chemicals such as oxygen, water, carbon, amino acids, and sugars are required for metabolic processes.

• Metabolism: The sum of all chemical reactions in cells, including catabolic (breaking down molecules) and anabolic (building molecules) pathways.

Cellular Organization

Cells are the fundamental units of life. All living things are composed of cells, which can be:

• Unicellular: Consisting of a single cell (e.g., bacteria, some protists).

• Multicellular: Composed of many cells with specialized functions (e.g., plants, animals, fungi).

• Cell Theory: All cells arise from pre-existing cells through cell division.

Homeostasis

Homeostasis is the maintenance of a stable internal environment despite external changes. This balance is crucial for the proper functioning of cells and the organism as a whole.

• Definition: A dynamic state of equilibrium in the body, maintained by regulatory mechanisms.

• Examples:

  • Shivering to generate heat when cold.

  • Reflexes and reabsorption of water by the kidneys to maintain fluid balance.

Response to the Environment

Living organisms detect and respond to stimuli in their environment, which is essential for survival and adaptation.

• Examples:

  • Humans shielding their eyes from bright sunlight.

  • Plants turning their leaves toward the sun (phototropism); roots growing toward water sources (hydrotropism).

Growth and Reproduction

Growth and reproduction are essential processes for the continuation of life.

• Growth: Increase in size and number of cells.

• Reproduction: The process by which organisms produce new individuals. This can be asexual (one parent) or sexual (two parents).

• Comparison: Non-living things may increase in size (e.g., crystals grow), but they do not reproduce or pass on genetic information.

Evolution

Evolution is the process by which populations of organisms change over generations through inheritable genetic variations.

• Population: A group of individuals of the same species living in a specific area and interacting with each other.

• Evolution: Inheritable changes in living organisms over time, leading to adaptation and diversity.

• Example: Bacteria evolving resistance to antibiotics, such as the breakdown of the β-lactam ring in β-lactam antibiotics, rendering them ineffective.

Classification of Living Things

Living organisms are classified based on shared characteristics. The broadest classification divides life into three domains:

Domain	Key Features	Examples
Bacteria	Prokaryotic, unicellular, diverse metabolic types	Escherichia coli, Streptococcus
Archaea	Prokaryotic, unicellular, often extremophiles	Halobacterium, Methanogens
Eukarya	Eukaryotic, unicellular or multicellular, includes four kingdoms	Protista, Animalia, Fungi, Plantae

• Kingdoms of Eukarya:

  • Protista: Mostly unicellular or simple multicellular; eukaryotic (e.g., protozoans, algae).

  • Animalia: Multicellular; eukaryotic; heterotrophic (e.g., animals).

  • Fungi: Multicellular (except yeasts); eukaryotic; decomposers (e.g., molds, mushrooms).

  • Plantae: Multicellular; eukaryotic; photosynthetic (e.g., plants).

Levels of Biological Organization

Biology studies life at multiple levels of organization, from molecules to the biosphere:

• Molecules

• Cells

• Tissues

• Organs

• Organ systems

• Organisms

• Populations

• Communities

• Ecosystems

• Biosphere

Classification of Humans

Humans are classified within the domain Eukarya and have a specific taxonomic hierarchy:

Taxonomic Rank	Classification
Domain	Eukarya
Kingdom	Animalia
Phylum	Chordata
Class	Mammalia
Order	Primates
Family	Hominidae
Genus	Homo
Species	Homo sapiens

Defining Features of Humans

• Ability to stand upright and walk on two legs (bipedalism)

• Opposable thumbs for grasping objects

• Large brain relative to body size

• Complex language (spoken and written)

Example: Evolution of Antibiotic Resistance in Bacteria

One of the most significant examples of evolution in action is the development of antibiotic resistance in bacteria. For instance, some bacteria can break the β-lactam ring of β-lactam antibiotics, rendering the drug ineffective. This is an example of natural selection and adaptation.

Additional info: The β-lactam ring is a four-membered lactam (a cyclic amide) that is essential for the antibiotic activity of penicillins and cephalosporins. Bacteria that produce β-lactamase enzymes can hydrolyze this ring, leading to antibiotic resistance.