Cell Membranes and Transport: Structure, Function, and Mechanisms

Cells: Structure and Membrane Transport

Learning Objectives

• Describe the location and relationship of intracellular and extracellular fluids.

• Explain the basic structure of the plasma membrane.

• Identify and describe the three main types of cell-to-cell junctions: tight junctions, desmosomes, and gap junctions.

• Summarize the processes involved in cell-to-cell signaling and signal reception.

Extracellular and Intracellular Materials

Extracellular Materials

Substances found outside cells are collectively known as extracellular materials. These play crucial roles in tissue structure and function.

• Extracellular fluids (ECFs): Body fluids that bathe and surround cells.

  • Interstitial fluid: Bathes and surrounds tissue cells.

  • Cerebrospinal fluid: Surrounds nervous system organs.

• Cellular secretions: Substances such as saliva, mucus, and gastric fluids.

• Extracellular matrix: A network of proteins and polysaccharides that acts as a glue to hold cells together.

Intracellular Fluid (ICF)

Intracellular fluid is the fluid contained within cells, providing the medium for cellular processes.

Plasma Membrane Structure and Function

Overview of the Plasma Membrane

The plasma membrane (also called the cell membrane) is a dynamic barrier that separates the intracellular fluid (ICF) from the extracellular fluid (ECF). It regulates what enters and exits the cell, maintaining homeostasis.

• Acts as a selective barrier.

• Controls cellular activity by regulating the movement of substances.

Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a flexible, moving structure composed of various molecules.

• Phospholipid bilayer: The fundamental structure, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Cholesterol: Interspersed within the bilayer, providing stability and fluidity.

• Membrane proteins: Embedded within or attached to the bilayer, performing specialized functions.

• Glycocalyx: Surface sugars important for cell recognition and protection.

• Cell junctions: Structures that connect adjacent cells.

Phospholipids: Amphipathic Molecules

• Phospholipids are the most abundant lipids in membranes.

• They are amphipathic, containing both hydrophobic (nonpolar) and hydrophilic (polar) regions.

• The bilayer forms a stable boundary between two aqueous compartments (inside and outside the cell).

Membrane Proteins

Membrane proteins are essential for communication and transport across the plasma membrane. They account for about half the membrane's mass and have specialized functions.

• Integral proteins: Span the membrane and are involved in transport and signaling.

• Peripheral proteins: Attached to the membrane surface, often involved in signaling or maintaining cell shape.

Cell-to-Cell Junctions

Tight Junctions

Tight junctions are formed when integral proteins on adjacent cells fuse, creating an impermeable seal that encircles the cell. This prevents fluids and most molecules from moving between cells.

• Important in tissues where leakage must be prevented (e.g., intestinal lining).

Desmosomes

Desmosomes (anchoring junctions) are rivet-like structures formed by linker proteins (cadherins) that interlock neighboring cells. They are anchored to plaques on the inside of the plasma membrane, providing mechanical stability and preventing cells from tearing apart.

• Common in tissues subject to mechanical stress (e.g., skin, heart muscle).

Gap Junctions

Gap junctions are formed by transmembrane proteins (connexons) that create channels between adjacent cells. These allow ions and small molecules to pass directly from cell to cell, facilitating communication and coordination.

• Essential in cardiac and smooth muscle for synchronized contraction.

Summary Table: Types of Cell Junctions

Junction Type	Main Structure	Function	Example Location
Tight Junction	Fused integral proteins	Prevents passage between cells	Intestinal epithelium
Desmosome	Cadherin linker proteins, plaques	Anchors cells, resists tearing	Skin, heart muscle
Gap Junction	Connexon channels	Allows communication, ion passage	Cardiac muscle

Additional info:

• Cell-to-cell signaling involves receptor proteins in the plasma membrane that detect and respond to chemical signals from other cells. The three major processes in signal reception are: reception (signal detection), transduction (signal relay), and response (cellular change).

• These notes are based on slides from a college-level Anatomy & Physiology course, focusing on cell structure and membrane function.