Cell/Plasma Membrane

Structure and Functions of the Cell Membrane

The cell membrane (also called the plasma membrane) is a dynamic structure that separates the interior of the cell from its external environment. It plays a crucial role in maintaining cellular integrity and regulating interactions with the surroundings.

• Physical barrier: Separates intracellular and extracellular environments.

• Regulates transport: Controls influx and efflux of substances via selective permeability.

• Communication: Contains a variety of receptors for signal transduction.

• Structural support: Provides anchoring points for the cytoskeleton.

• Junctions: Forms connections between cells or between cells and the extracellular matrix.

Phospholipid Structure and Amphipathic Properties

Phospholipids are the major component of the cell membrane, giving it its unique properties.

• Amphipathic nature: Each phospholipid has a polar (hydrophilic) head containing phosphate and a nonpolar (hydrophobic) tail made of fatty acids.

• This arrangement forms a bilayer, with hydrophobic tails facing inward and hydrophilic heads facing outward, creating a semi-permeable barrier.

Example: The phospholipid bilayer prevents free passage of ions and polar molecules, requiring specialized transport mechanisms.

Fluid Compartments in the Body

Types of Body Fluids

The body is divided into distinct fluid compartments, each with specific ionic compositions and functions.

• Intracellular Fluid (ICF): Fluid within cells.

• Extracellular Fluid (ECF): Fluid outside cells, further subdivided into:

  • Interstitial Fluid: Surrounds tissue cells.

  • Plasma: Fluid component of blood.

  • Cerebrospinal Fluid: Surrounds brain and spinal cord.

• Compartments have different ionic compositions but similar osmotic concentrations.

• Exchange of substances occurs between compartments, regulated by membranes.

Major Ions in Body Fluids

Body fluids contain various cations (positively charged ions) and anions (negatively charged ions) that are distributed differently between compartments.

Example: The high concentration of K+ inside cells and Na+ outside cells is essential for nerve impulse transmission.

Cell Junctions

Types of Cell-Cell Junctions

Cell junctions are specialized structures that connect cells to each other or to the extracellular matrix, providing structural integrity and communication.

• Tight junctions: Form permeability barriers, preventing passage of substances between cells.

• Desmosomes: Anchoring junctions that prevent cells from being torn apart; found in tissues subject to mechanical stress (e.g., skin).

• Gap junctions: Allow direct communication between adjacent cells via interlocking membrane proteins.

Example: Gap junctions are abundant in cardiac muscle, allowing rapid spread of electrical signals.

Cell-Matrix Junctions

These junctions anchor cells to the underlying extracellular matrix, providing stability and signaling.

• Hemidesmosomes: Connect cytoskeleton fibers to matrix proteins, anchoring cells to the basement membrane.

Membrane Transport Mechanisms

Passive Transport

Passive transport does not require energy and relies on concentration gradients.

• Simple diffusion: Movement of molecules from high to low concentration.

• Facilitated diffusion: Uses channels or carriers for transport of specific molecules.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradients.

• Primary active transport: Direct use of ATP (e.g., Na+/K+ pump).

• Secondary active transport: Uses energy from ion gradients (e.g., symporters, antiporters).

• Vesicular transport: Includes endocytosis (receptor-mediated, phagocytosis), exocytosis, and transcytosis.

Equation:

Where is the flux, is the diffusion coefficient, and is the concentration gradient.

Membrane Receptors

Types of Membrane Receptors

Membrane receptors are proteins that bind specific ligands and initiate cellular responses.

• Chemical (ligand)-gated ion channels: Open or close in response to binding of a chemical messenger.

• Enzyme-linked receptors: Possess intrinsic enzymatic activity (e.g., kinase activity) upon ligand binding.

• G protein-coupled receptors (GPCRs): Activate intracellular signaling pathways via second messengers.

Example: GPCRs are involved in hormone and neurotransmitter signaling.

Receptor-Mediated Endocytosis

This process allows cells to selectively internalize molecules by binding them to specific receptors, forming a coated pit and vesicle.

• Advantage: Greater specificity and efficiency compared to pinocytosis.

Types of Ion Channels

Classification of Ion Channels

Ion channels are membrane proteins that allow ions to pass through the membrane, influencing membrane potential and cell function.

• Leakage (passive) channels: Always open, allowing ions to move down their gradients.

• Chemical (ligand)-gated channels: Open or close in response to chemical signals.

• Voltage-gated channels: Open or close in response to changes in membrane potential.

Example: Voltage-gated Na+ channels are essential for action potential generation in neurons.

Membrane Potential Changes

Movement of cations or anions across the membrane alters the membrane potential, which is the electrical difference across the cell membrane.

• Depolarization: Membrane potential becomes less negative (e.g., influx of Na+).

• Hyperpolarization: Membrane potential becomes more negative (e.g., efflux of K+ or influx of Cl-).

Equation:

Where is the membrane potential.

Summary Table: Cell Junctions

Additional info:

• Membrane channels are required for ions to enter/exit the cell due to the hydrophobic nature of the lipid bilayer, which prevents free passage of charged particles.

• Receptor-mediated endocytosis is more efficient and selective than pinocytosis, allowing cells to concentrate specific molecules from the extracellular fluid.