Cell membranes are the gatekeepers of life, controlling what enters and exits cells. They're made of a phospholipid bilayer with embedded proteins, acting as a selective barrier. This structure allows for essential functions like transport and signaling.

Transport across cell membranes occurs through various mechanisms. Passive transport, like diffusion, requires no energy, while active transport uses ATP to move substances against concentration gradients. Understanding these processes is crucial for grasping cellular homeostasis and function.

Cell Membrane Structure and Function

Components of cell membrane

• Phospholipid bilayer consists of amphipathic molecules with hydrophilic heads and hydrophobic tails that form a barrier between intracellular and extracellular environments and provides fluidity and flexibility to the membrane (fluid mosaic model)
• Integral proteins embedded within the phospholipid bilayer include channel proteins, carrier proteins, and receptor proteins that facilitate transport of molecules (ions, glucose) and cell signaling
• Peripheral proteins attached to the surface of the membrane or to integral proteins serve as enzymes, structural elements, or anchors for the cytoskeleton
• Cholesterol helps maintain membrane fluidity and stability and regulates membrane permeability
• Glycocalyx is a layer of carbohydrates attached to membrane proteins and lipids that provides cell recognition and adhesion
• Lipid rafts are specialized membrane microdomains enriched in cholesterol and sphingolipids that play a role in signal transduction and protein trafficking

Structure and selective permeability

• Hydrophobic core of the phospholipid bilayer prevents passage of polar and charged molecules (ions, amino acids) but allows diffusion of small, nonpolar molecules (oxygen, carbon dioxide)
• Size and charge of molecules affect their ability to pass through the membrane, with small, uncharged molecules passing more easily than large, charged, or polar molecules
• Membrane proteins, including channel proteins and carrier proteins, facilitate the passage of specific molecules and maintain concentration gradients and cellular homeostasis
• Membrane fluidity is influenced by temperature, cholesterol content, and fatty acid composition, affecting the mobility of membrane components and their functions

Transport Mechanisms Across the Cell Membrane

Diffusion through lipid bilayer

• Freely diffusible materials include small, nonpolar molecules (oxygen, carbon dioxide, steroid hormones) and lipid-soluble molecules
• Materials requiring assistance include ions (Na+, K+, Ca2+, Cl-), large, polar molecules (glucose, amino acids), and charged molecules (proteins, nucleic acids)

Passive vs active transport mechanisms

• Passive transport
  1. Diffusion involves movement of molecules from high to low concentration, requires no energy input, and includes examples like oxygen and carbon dioxide exchange and water movement (osmosis)
  2. Facilitated diffusion involves movement of specific molecules (glucose via GLUT proteins, ions through channels) through channel or carrier proteins down the concentration gradient
• Active transport
  1. Primary active transport directly uses ATP to power the transport of molecules against the concentration gradient, such as the sodium-potassium pump (Na+/K+ ATPase)
  2. Secondary active transport uses the electrochemical gradient created by primary active transport and includes examples like sodium-glucose cotransport (SGLT) and calcium-sodium exchanger (NCX)
• Vesicular transport
  • Endocytosis involves uptake of extracellular materials by membrane invagination, including:
    • Phagocytosis: uptake of large particles (bacteria, cell debris)
    • Pinocytosis: uptake of fluids and solutes
  • Exocytosis involves release of intracellular materials by vesicle fusion with the membrane, such as neurotransmitter release at synapses

Membrane Electrical Properties

• Membrane potential is the electrical potential difference across the cell membrane, resulting from the unequal distribution of ions
• Ion channels are specialized membrane proteins that allow the selective passage of specific ions, contributing to the establishment and maintenance of membrane potential