CHAPTER 5

5.1 Components and Structure

The plasma membrane defines the cell’s borders, helps maintain cellular integrity, and is selectively permeable. The fluid mosaic model describes the membrane as a dynamic, two‑dimensional mosaic of phospholipids, cholesterol, proteins, and carbohydrates. The principal components are phospholipids (form the bilayer), cholesterol (embedded within the bilayer), proteins (integral and peripheral), and carbohydrates attached to lipids or proteins (glycolipids and glycoproteins) that form the glycocalyx on the exterior surface. The membrane is about 5–10 nm thick and can adapt its organization and composition to meet cellular needs. In a typical human cell, the mass distribution is roughly protein ≈ 50%, lipids ≈ 40%, and carbohydrates ≈ 10%, though these proportions vary widely among cell types (e.g., myelin, mitochondria, red blood cells). The glycocalyx provides cell‑to‑cell recognition, self/non‑self discrimination, and mediates interactions with the extracellular environment; it also can influence viral entry by presenting specific surface patterns. Integral proteins span the membrane and perform transport, signaling, and catalytic roles, while peripheral proteins reside on the membrane’s surfaces and may anchor the cytoskeleton or act as enzymes. Phospholipids are amphiphilic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer that separates intracellular from extracellular fluids. Carbohydrates extend from the exterior surface as glycoproteins and glycolipids, contributing to recognition and adhesion.

5.2 Passive Transport

Plasma membranes are selectively permeable, allowing some substances to cross freely while others require assistance or energy. Nonpolar, lipid‑soluble molecules diffuse readily through the lipid core, whereas polar molecules and ions require channels or carrier proteins to cross. Diffusion is a passive process that moves substances from high to low concentration and does not consume cellular energy. Several factors influence diffusion rate: the extent of the concentration gradient, molecular mass, temperature, solvent density, available surface area, membrane thickness, and distance to travel. Facilitated diffusion uses membrane channels or carrier proteins to move polar or charged solutes down their gradients without ATP. Channel proteins form hydrophilic pathways; aquaporins specifically accelerate water passage. Carrier proteins bind the diffusing substance and undergo conformational changes, which can be slower and subject to saturation. Some channels are gated, opening or closing in response to stimuli. Osmosis is the diffusion of water across a semipermeable membrane, aided by aquaporins, and is governed by solute concentrations on either side of the membrane. Tonicity describes how an extracellular solution affects cell volume via osmosis and is tied to osmolarity, the total solute particle concentration. Hypotonic solutions have lower osmolarity outside the cell, causing water entry; hypertonic solutions have higher osmolarity outside, causing water exit; isotonic solutions have equal osmolarity and no net water movement. Osmoregulation enables organisms to manage water balance; plants exploit turgor pressure to maintain rigidity, while some protists use contractile vacuoles to expel excess water. Facilitated diffusion of ions and large polar molecules can be rapid via channels, or slower via carrier proteins, and both can be saturated when all transporters are occupied.

5.3 Active Transport

Active transport moves substances against their electrochemical gradient and requires energy, typically from ATP. An electrochemical gradient combines a chemical concentration gradient with an electrical gradient across the membrane. Primary active transport directly uses ATP to transport ions, altering both concentration and charge. Secondary active transport uses the energy stored in the electrochemical gradient created by the primary pump to move other substances, often piggybacking on the gradient to move solutes against their own gradients. Carrier proteins drive these processes and come in three main types: uniport (one substance in one direction), symport (two substances in the same direction), and antiport (two substances in opposite directions). Classic examples include Na⁺/K⁺ ATPase (sodium‑potassium pump), Ca²⁺ ATPase, and H⁺ pumps. The Na⁺/K⁺ ATPase maintains the Na⁺ and K⁺ gradients and creates a net negative interior, making it an electrogenic pump. Its typical cycle involves binding three Na⁺ ions on the intracellular side, ATP‑dependent phosphorylation and conformational change that releases Na⁺ outside, binding of two K⁺ ions on the outer side, dephosphorylation, and return to the original conformation with K⁺ released inside. This activity sustains the electrochemical gradient that supports secondary transport processes, such as the uptake of amino acids and glucose via cotransporters. Other pumps include Ca²⁺ ATPase and H⁺ ATPase, contributing to ion homeostasis and membrane potential.

5.4 Bulk Transport

Bulk transport moves large particles and volumes via vesicular mechanisms. Endocytosis internalizes material and includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor‑mediated endocytosis, which is highly selective. Phagocytosis involves membrane invagination around large particles, aided by clathrin during the formation of a coated pit, vesicle formation, fusion with lysosomes, and eventual digestion in endosomes. Pinocytosis ingests extracellular fluid and small solutes via vesicles that are generally smaller and may not fuse with lysosomes. Receptor‑mediated endocytosis concentrates specific targets by using receptors and clathrin to form coated pits, enabling selective uptake of molecules such as LDL; defects in this pathway can cause disease. Exocytosis is the reverse process, in which vesicles fuse with the plasma membrane to release contents into the extracellular space, as with neurotransmitter secretion. Bulk transport enables secretion, membrane turnover, and intercellular communication, and underlies many physiological processes.