Characteristics of Life

The Eight Characteristics of Living Things

All living organisms share a set of fundamental characteristics that distinguish them from non-living matter. These characteristics are used by biologists to define life and to differentiate living things from non-living entities such as viruses.

• Cellular Organization: All living things are composed of one or more cells, which are the basic units of life.

• Organization: Living organisms exhibit ordered structures, where smaller components build up to form complex systems.

• Response to Stimuli: Organisms can detect and respond to changes in their environment, such as light, temperature, or chemicals.

• Homeostasis: The ability to maintain stable internal conditions (e.g., body temperature, pH) despite external changes. Example: Humans sweat to cool down when overheated.

• Reproduction: The process of producing new organisms. This can be asexual (one parent) or sexual (two parents).

• Energy Use: Living things acquire and utilize energy from their environment, such as food or sunlight, to power cellular processes.

• Genetic Information: All living things contain DNA, which stores hereditary information and instructions for growth and function.

• Evolution: Populations of organisms change genetically over time, allowing adaptation to their environment.

Note: Viruses are generally not considered alive because they lack several key characteristics, especially independent reproduction and cellular structure.

Biological Levels of Organization

Hierarchy from Atoms to Biosphere

Biological systems are organized in a hierarchy of structural levels, from the smallest chemical units to the entire biosphere.

• Atom → Molecule → Organelle → Cell

• Tissue → Organ → Organ System → Organism

• Population → Community → Ecosystem → Biosphere

Example: Muscle cell (cell) → muscle tissue → heart (organ) → circulatory system (organ system) → human (organism).

Experimental Variables

Designing Biological Experiments

Understanding variables is essential for designing and interpreting scientific experiments.

• Independent Variable (IV): The factor that the researcher changes or controls. It is usually plotted on the x-axis of a graph.

• Dependent Variable (DV): The outcome that is measured in the experiment. It is usually plotted on the y-axis.

Example: In an experiment testing the effect of water on plant growth, the IV is the amount of water given, and the DV is the measured plant growth.

Monomers, Polymers, and Biological Reactions

Building and Breaking Biological Molecules

Many biological molecules are polymers, made by linking smaller units called monomers. Two key types of reactions are involved in their synthesis and breakdown:

• Monomer: A single building block molecule (e.g., glucose, amino acid).

• Polymer: A long chain composed of many monomers (e.g., starch, protein).

• Dehydration Synthesis: A chemical reaction that builds polymers by joining monomers with covalent bonds, releasing water as a byproduct.

  • Equation:

• Hydrolysis: A chemical reaction that breaks polymers into monomers by adding water to split covalent bonds.

  • Equation:

The Four Major Biological Macromolecules

Overview of Structure, Function, and Examples

Living organisms are built from four main classes of macromolecules: carbohydrates, proteins, lipids, and nucleic acids. Each has unique structures, building blocks, and biological roles.

Details of Each Macromolecule

Carbohydrates

• Monomer: Monosaccharide (e.g., glucose)

• Polymer: Polysaccharide (e.g., starch, cellulose)

• Bond: Glycosidic bond (formed by dehydration synthesis)

• Main Functions: Short-term energy storage, structural support in plants and animals

• Examples: Starch (plants), glycogen (animals), cellulose (plant cell walls), chitin (exoskeletons)

Proteins

• Monomer: Amino acid (20 different types, each with a unique R group)

• Polymer: Polypeptide or protein

• Bond: Peptide bond

• Key Concept: The three-dimensional structure of a protein determines its function. Denaturation (loss of structure) leads to loss of function.

• Examples/Functions: Enzymes (catalyze reactions), transport proteins (e.g., hemoglobin), structural proteins, signaling molecules (hormones, receptors)

Lipids

• Monomer/Polymer: Lipids do not have true monomers or polymers, but are often built from fatty acids and glycerol.

• Defining Feature: Hydrophobic (insoluble in water); some are amphipathic (contain both hydrophilic and hydrophobic regions).

• Main Functions: Long-term energy storage, membrane structure, hormones, waterproofing/protection

• Examples: Triglycerides (fats/oils), phospholipids (cell membranes), steroids (cholesterol, hormones), waxes

Nucleic Acids

• Monomer: Nucleotide (composed of a phosphate group, pentose sugar, and nitrogenous base)

• Polymer: DNA or RNA

• Bond: Phosphodiester bond (forms the sugar-phosphate backbone)

• Main Function: Store and transmit genetic information

• DNA vs. RNA:

  • DNA contains deoxyribose sugar and the base thymine (T); usually double-stranded with antiparallel strands.

  • RNA contains ribose sugar and the base uracil (U); usually single-stranded.