Levels of Organization in Organisms

Levels of Organization

• Organisms can be unicellular or multicellular.

  • Unicellular organisms:

    • Consist of a single cell.
    • Lack organ systems or organs.
    • All functions for survival occur within the single cell.

  • Multicellular organisms:

    • Composed of multiple cells.
    • Organization from smallest to largest: cell, tissue, organ, organ system, organism.
    • Structures smaller than cells include atoms, molecules, and organelles.

Cells

• Basic functional and structural unit of life.

  • Smallest form considered 'alive'.
  • Capable of performing metabolic functions.

Tissues

• Histology: The study of tissues.

• Tissues are groups of cells with similar structure and function.

• Focus on animal tissues; plant tissues discussed elsewhere.

• Four main types of animal tissue:

  • Epithelial

  • Connective

  • Muscle

  • Nervous

Epithelial Tissue

• Covers the outside of the body and lines organs and glands.

• Cells are usually close-packed.

• Functions:

  • Protection against mechanical injury.

  • Barrier against infection and fluid loss.

• Arrangements of tissue:

  • Stratified: More than one layer of tissue.

    • Protects against desiccation and mechanical injury.

    • Example: stratified squamous epithelium in the mouth.

  • Simple: One layer of tissue.

  • Pseudostratified and transitional: Not discussed in this course.

• Classifications based on cell shape:

  • Squamous: Flat, scale-like cells; 'fried egg' appearance.

  • Columnar: Rectangular cells with nucleus at the base.

  • Cuboidal: Square cells with nucleus in the center.

Connective Tissue

• Main function: Provide support and connect body parts.

• Cells are sparsely located.

• Types:

  • Loose connective tissue

  • Fibrous connective tissue

  • Adipose (fatty) tissue

  • Blood

  • Bone

  • Cartilage

Muscle Tissue

• Responsible for contractions.

• Single cells are called fibers.

• Three types:

  • Skeletal muscle: Under voluntary control; attached to the skeleton for movement (e.g., biceps brachii, pectoralis muscles).

  • Cardiac muscle: Found in the heart; responsible for heartbeat; involuntary control.

  • Smooth muscle: Located in internal organs; responsible for unconscious contractions (e.g., digestive system).

Nervous Tissue

• Function: Transmit electrical impulses throughout the body.

• Found in the brain, spinal cord, and nerves.

• Single functional unit: Neuron.

  • Electrical impulse transmission from dendrite to axon is essential.

  • Axon terminals pass messages to muscles, causing contraction.

Organs

• Composed of several tissue types.

• Example: Stomach contains;

  • Mucosa: Epithelial layer lining the lumen.

  • Submucosa: Connective tissue with blood vessels and nerves.

  • Muscularis: Smooth muscle tissue.

  • Serosa: Thin layer of connective and epithelial tissue.

Organ Systems

• Combination of organs working together.

• Example: Cardiovascular system includes the heart and blood vessels, distributing oxygen and nutrients.

• All organ systems are interrelated.

• Example: Digestive system breaks down macromolecules; circulatory system absorbs and delivers smaller molecules.

Organism

• Organ systems working together to form an organism.