Cell Structure and Function

Plasma Membrane

• The thin outermost membrane layer of the cell.
• Function:
  • Selectively allows substances to pass into and out of the cell.
  • Maintains the integrity of the cell by keeping the cell and its contents separate and distinct from the surrounding environment.

Structure of the Plasma Membrane

• The molecular structure is called the fluid mosaic model.
• Formulated in 1972 by Singer and Nicholson.
• 'Fluid' describes the movement of proteins and lipids within their layer.
• 'Mosaic' describes the scattered order of arrangement of the proteins.
• According to the fluid mosaic model:
  1. The membrane is a fluid double layer or bilayer composed mainly of phospholipids and proteins.
  2. Phospholipids have one electrically charged head and an uncharged tail.

Cytoplasm

• The fluid-filled portion outside the nucleus and within the plasma membrane.
• Discovered in 1862 by Kolliker.
• Composed of two phases:
  1. Aqueous phase:
     • Consists of fluid cytosol.
     • Cytosol is mostly water (70-80%) and contains organic compounds such as proteins, lipids, complex carbohydrates, nucleic acids, amino acids, vitamins, and electrolytes.
  2. Particulate phase:
     • Consists of defined structures including lipid droplets, inclusions, and organelles.
• Contains cytoskeleton.

Organelles

• Subunits of a cell with specific structure and function(s).
• Membrane-bound structures with a specific function or functions.