Characteristics of Life and Cellular Organization

Posted on Mar 5, 2025 in Biology

Characteristics of Life

Key characteristics define living organisms:

  • Movement: Ability to change position or move internally.
  • Responsiveness: Reaction to internal and external changes.
  • Growth: Increase in size without changing shape.
  • Reproduction: Production of new organisms and new cells.
  • Respiration: Obtaining oxygen, removing carbon dioxide, and releasing energy from foods.
  • Digestion: Breakdown of food substances into simpler forms.
  • Absorption: Movement of substances through membranes and into body fluids.
  • Circulation: Movement of substances in body fluids.
  • Assimilation: Changing absorbed substances into chemically different forms.
  • Excretion: Removal of wastes produced by metabolic reactions.

These characteristics can be used to differentiate a living organism from a non-living structure. A non-living structure will typically lack most, if not all, of these characteristics.

Levels of Organization in Living Systems

Living systems exhibit a hierarchical organization:

  1. Atoms: The basic building blocks of matter.
  2. Molecules: Combinations of atoms.
  3. Organelles: “Little organs” within cells, each with specific functions, enabling cells to operate.
  4. Cells: The fundamental units of life.
  5. Tissues: Groups of similar cells performing specific functions (e.g., skin, muscle, fat).
  6. Organs: Structures composed of different tissues working together.
  7. Organ Systems: Groups of organs that cooperate to accomplish a common purpose.
  8. Organism: A complete individual.

This organization is crucial for the survival of multicellular organisms because it allows for specialization and division of labor. Different cells and tissues can perform specific tasks, increasing efficiency and complexity.

Critical Factors for Maintaining Human Life

Several factors are essential for human life:

  1. Water: Essential for metabolic processes.
  2. Nutrients: Provide energy and building blocks for growth and repair.
  3. Oxygen: Required for cellular respiration.
  4. Heat: Necessary for metabolic reactions to occur at an appropriate rate. Higher temperatures generally increase enzyme activity (within limits).
  5. Pressure:
    • Atmospheric pressure: Important for breathing. Optimal at sea level.
    • Blood pressure: Crucial for blood circulation. The body functions best at a lower blood pressure (around 120/80). High blood pressure can cause capillaries to burst, while low blood pressure can lead to fainting.

Both the quality and quantity of these factors are critical. Imbalances (too much or too little) can be detrimental to health.

Metabolism

Metabolism refers to all the chemical processes that occur within a living organism to maintain life. All physiological functions are based on metabolic activity. Without metabolism, other bodily functions would cease. Heat is required for metabolism, and enzymes are crucial catalysts during metabolic processes.

Cellular Components

Protoplasm, Nucleoplasm, and Cytoplasm

  • Protoplasm: The colorless material comprising the living part of a cell, including the cytoplasm, nucleus, and other organelles. It is surrounded by a plasma membrane.
  • Nucleoplasm: A highly viscous liquid that surrounds the chromosomes and nucleoli within the nucleus. It contains substances like nucleotides (for DNA replication) and enzymes (for nuclear activities). It contains the genetic material (DNA).
  • Cytoplasm: The jelly-like material that makes up much of a cell inside the cell membrane. It contains the nucleus. Enzymes are active here, and protein synthesis, as well as carbohydrate and lipid metabolism, occur. It is approximately 52% cytosol (water), and it holds all the organelles (permanent structures).

Nucleoplasm is located *inside* the cytoplasm.

Organelles vs. Inclusions/Vesicles

  • Cell Inclusion: A non-living, temporary substance, such as pigment granules, fat droplets, or nutritive substances.
  • Cell Vesicles: Temporary structures that come and go, created for specific functions like transport, absorption, and excretion.

The Cell Theory

The cell theory has three main components:

  1. The cell is the basic unit of life.
  2. All living things are made up of one or more cells (tissues).
  3. All cells come from preexisting cells.

If a virus were to be considered a living organism, the cell theory might need modification. A virus can replicate itself, similar to a cell, but it is not a complete cell. It requires a host cell to reproduce and cannot function independently.