Plasma Membrane

Overview of Fluids Surrounding and Within Cells

  • The discussion revolves around the fluids that are either in or surrounding the cell within tissues.

Intracellular Fluid (ICF)

  • Definition: Fluid contained within the cell.

    • Also known as Cytosol: The liquid part of the cytoplasm excluding organelles.

  • Abbreviation: ICF (Intracellular Fluid).

Extracellular Fluid (ECF)

  • Definition: Fluid located outside the cells.

    • Abbreviation: ECF (Extracellular Fluid).

  • Components:

    • Interstitial Fluid: The fluid that immediately surrounds the cells within tissues.

    • Origin of the term: "Inter" means around, and "sticial" refers to tissue.

    • Plasma: The fluid component of blood that moves through blood vessels.

Fluid Compartment Comparison

  • Intracellular Fluid (ICF) vs. Extracellular Fluid (ECF):

    • ICF: Fluid inside the cell, essential for cellular functions.

    • ECF: Comprised of interstitial fluid surrounding cells and plasma within blood vessels.

Importance for Homeostasis

  • Homeostasis: The regulation of the internal environment surrounding cells.

    • Key factors regulated include:

    • Nutrients

    • Electrolytes

    • pH levels

    • Proper balance is critical for cell survival.

Plasma Membrane Structure and Functions

  • Definition: The plasma membrane separates ICF from ECF, acting as a barrier.

  • Key Functions:

    • Regulation: Controls what enters and exits the cell.

    • Communication: Facilitates interaction between cells.

    • Example: Hormones or neurotransmitters bind to receptors on the membrane, triggering cellular responses.

  • Composition:

    • Thin membrane composed primarily of two layers of phospholipids.

    • Phospholipid bilayer thickness: Approximately two phospholipid molecules thick, measured in nanometers to microns.

  • Fluid Mosaic Model:

    • Description: The membrane behaves like a fluid, with lipids and proteins that move around, creating a dynamic environment.

    • Components include:

    • Lipids (primarily phospholipids)

    • Cholesterol

    • Carbohydrates (as glycoproteins and glycolipids)

    • Proteins

Phospholipids

  • Structure:

    • Hydrophilic Heads: Polar regions facing the intra- and extracellular environments.

    • Hydrophobic Tails: Nonpolar regions facing each other within the bilayer.

    • Approximately 90% of the membrane's structure consists of these tails, impacting permeability.

Cholesterol

  • Role: About 20% of plasma membrane composition; acts as a temperature buffer.

    • Functionality: Helps maintain membrane integrity by preventing tight packing at low temperatures and maintaining fluidity at high temperatures.

    • Significance for cold-blooded vs. warm-blooded animals regarding temperature fluctuations.

Carbohydrates in the Cell Membrane

  • Types:

    • Glycoproteins: Proteins with carbohydrate chains attached.

    • Glycolipids: Carbohydrate chains attached to lipid head groups.

  • Function: These molecules serve as signaling flags for the immune system, identifying cells uniquely to prevent rejection during organ transplants.

Membrane Proteins

  • Major second component of the plasma membrane, carrying out various functions. Functions include:

    • Transport across the membrane.

    • Receptor action for signaling.

    • Enzyme activity.

  • Categories:

    • Integral (Transmembrane) Proteins: Cross the entire membrane and link intracellular and extracellular spaces.

    • Peripheral Proteins: Located either internally or externally without traversing the membrane.

Permeability of the Plasma Membrane

  • Definition: Refers to the ability of substances to cross the membrane.

  • Key Characteristics:

    • Selective/Semi-Permeability: Only allowing certain molecules through while restricting others.

    • Example: An analogy of a bottle's opening (freely permeable) versus its walls (not permeable).

  • Factors Affecting Permeability:

    • Charge of Molecule: Ions (e.g., Na extsuperscript{+}, K extsuperscript{+}, Cl extsuperscript{-}, Ca extsuperscript{2+}) cannot pass through without a transporter due to the hydrophobic nature of the middle layer.

    • Molecule Size: Smaller molecules can penetrate more easily than larger ones.

    • Example: Urea (small) vs. triglycerides (large).

    • Lipid Solubility: Nonpolar molecules diffuse through more readily than polar molecules.

    • Notable Exception: Water (H extsubscript{2}O) can penetrate despite being polar due to its small size.