Biological Membranes

Biological Membranes: Introduction

  • Biological membranes are:
    • Thin.
    • Structurally bland.
    • Straightforward to describe.
    • Define borders of cells, tissues, and organelles.
    • Carry out biological functions within cells.
    • A source of scientific inquiry.

Plasma Membrane

  • The most commonly tested biological membrane on the MCAT is the plasma membrane.
  • Functions of the plasma membrane:
    • Signaling.
    • Entry of nutrients.
    • Expulsion of waste.
    • Cell recognition.
    • Transport of materials between tissues.
    • Electronic functions.
  • Components of cell membranes:
    • Phospholipids (stretchy and flexible).
    • Cholesterol and proteins (stabilizing molecules).

Fluid Mosaic Model

  • The cell plasma membrane is described as a semipermeable phospholipid bilayer.
  • Function:
    • Acts as a semipermeable barrier, selecting which particles can enter and leave the cell.
    • Selectivity is mediated by:
      • Channels and carriers.
      • The membrane itself.
  • Composition:
    • Two layers of phospholipids.
    • Allows fat-soluble compounds to cross easily.
    • Requires larger and water-soluble compounds to seek alternative entry.
  • The theory underlying the structure and function of the cell membrane is the fluid mosaic model.

General Membrane Structure and Function

  • The phospholipid bilayer also includes proteins and distinct signaling areas within lipid rafts.
  • Carbohydrates associated with membrane-bound proteins create a glycoprotein coat.
  • The cell walls of plants, bacteria, and fungi contain higher levels of carbohydrates.
  • Main function: Protect the interior of the cell from the external environment.
  • Selectively regulate traffic into and out of the cell.
  • Involved in intracellular and intercellular communication and transport.
  • Cell membranes contain proteins embedded within the lipid bilayer that act as cellular receptors during signal transduction.
  • These proteins play an important role in regulating and maintaining overall cellular activity.

Membrane Dynamics

  • The cell membrane functions as a stable semi-solid barrier between the cytoplasm and the environment but is in a constant state of flux on the molecular level.
  • Phospholipids move rapidly in the plane of the membrane through simple diffusion.
  • Lipid rafts are collections of similar lipids with or without associated proteins that serve as attachment points for other biomolecules.
    • These rafts often serve roles in signaling.
  • Both lipid rafts and proteins also travel within the plane of the membrane, but more slowly.
  • Lipids can also move between the membrane layers, but this is energetically unfavorable because the polar head group of the phospholipid must be forced through the nonpolar tail region in the interior of the membrane.
  • Specialized enzymes called flippases assist in the transition or flip between layers.
  • Dynamic changes in concentrations of various membrane proteins are mediated by gene regulation, endocytotic activity, and protein insertion.
  • Many cells, particularly those involved in biosignaling processes, can up- or downregulate the number of specific cellular receptors on their surface in order to meet cellular requirements.