5.1 Components and Structure

Cell membranes serve as the boundary between a cell's interior and the outside environment, controlling what gets in and out while also enabling communication between cells. Understanding membrane structure is foundational because it explains how cells maintain homeostasis, respond to signals, and adapt to changing conditions.

Cell Membrane Structure and Function

Fluid mosaic model of membranes

The fluid mosaic model describes the cell membrane as a phospholipid bilayer studded with a variety of proteins. "Fluid" refers to the fact that components can move laterally within the membrane. "Mosaic" refers to the patchwork of different proteins scattered throughout.

• Phospholipids are the main structural component of the membrane
  • Each phospholipid is amphipathic: it has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) fatty acid tails
  • In the bilayer, the hydrophobic tails face inward toward each other, forming a water-resistant interior
  • The hydrophilic heads face outward on both sides, contacting the aqueous cytoplasm on one side and the extracellular fluid on the other
• Proteins embedded in or attached to the bilayer give the membrane its mosaic quality
  • Integral (transmembrane) proteins span the entire bilayer and often function as channels, transporters, or receptors (e.g., glycophorin in red blood cells)
  • Peripheral proteins sit on the membrane surface, attached by weak interactions, and can be removed without disrupting the bilayer (e.g., cytochrome c on the inner mitochondrial membrane)
• Lateral movement of phospholipids and proteins within the plane of the membrane is constant
  • This fluidity allows the membrane to reorganize in response to cellular needs, such as during cell signaling or when a cell changes shape
  • Phospholipids swap places with their neighbors millions of times per second, though they rarely flip from one layer to the other (a process called "flip-flop")

Components of membrane structure

Phospholipids form the selectively permeable barrier that is the membrane's foundation. Because the interior of the bilayer is hydrophobic, it blocks most large, polar, or charged molecules from freely crossing. Small nonpolar molecules (like $O_2$ and $CO_2$) pass through easily, while ions and glucose cannot cross without help.

Proteins carry out most of the membrane's specific functions:

• Transport proteins move molecules across the membrane
  • Channel proteins form hydrophilic pores that allow specific ions or small molecules through by passive transport (e.g., aquaporins shuttle water molecules)
  • Carrier proteins bind to a molecule and change shape to move it across (e.g., the $Na^+/K^+$ ATPase actively pumps sodium out and potassium in)
• Receptor proteins bind to specific signaling molecules (ligands) on the extracellular side, triggering a response inside the cell (e.g., the insulin receptor activates glucose uptake)
• Enzymatic proteins catalyze reactions at the membrane surface (e.g., adenylate cyclase converts ATP to cAMP during signal transduction)
• Cell adhesion proteins anchor cells to neighboring cells or to the extracellular matrix (e.g., integrins connect the cytoskeleton to external structures)

Carbohydrates are found exclusively on the extracellular surface, attached to proteins (forming glycoproteins) or to lipids (forming glycolipids).

• They function in cell recognition and cell-cell communication. For example, blood group antigens (A, B, O) are carbohydrate markers on red blood cells that determine blood type.
• They can serve as binding sites for pathogens; the influenza virus, for instance, attaches to specific carbohydrate receptors on host cells.
• Together, these carbohydrates form the glycocalyx, a sugary coat that protects the cell surface, provides lubrication, and helps prevent unwanted cell clumping.

Factors affecting membrane fluidity

Membrane fluidity refers to how easily membrane components move laterally. Too rigid and the membrane can't function; too fluid and it loses structural integrity. Three major factors control fluidity:

Temperature

• Higher temperatures increase kinetic energy, causing phospholipids to move more rapidly and increasing fluidity. Bacteria living in hot springs, for example, have membranes adapted to remain stable at extreme heat.
• Lower temperatures slow phospholipid movement, causing tails to pack more tightly and reducing fluidity. Winter wheat adjusts its membrane composition as temperatures drop to avoid becoming too rigid.

Cholesterol

• Cholesterol molecules wedge between phospholipids in animal cell membranes and act as a fluidity buffer.
• At high temperatures, cholesterol restrains phospholipid movement, preventing the membrane from becoming too fluid.
• At low temperatures, cholesterol disrupts tight packing of phospholipid tails, preventing the membrane from becoming too rigid. Antarctic fish rely on this effect to keep their membranes functional in near-freezing water.

Fatty acid saturation

• Saturated fatty acid tails have no double bonds, so they're straight and pack tightly together. This decreases fluidity. (Think of butter, which is solid at room temperature due to saturated fats.)
• Unsaturated fatty acid tails contain one or more double bonds that introduce kinks, preventing tight packing and increasing fluidity. (Think of olive oil, which stays liquid at room temperature due to unsaturated fats.)

Organisms actively adjust these factors to maintain optimal fluidity. Cold-adapted bacteria, for instance, increase the proportion of unsaturated fatty acids in their membranes when temperatures drop. This kind of adjustment matters because proper fluidity is required for membrane proteins to function correctly, whether they're transporting molecules, relaying signals, or catalyzing reactions.

Membrane dynamics and specialized structures

• Membrane permeability depends on the specific mix of lipids and proteins present. Changing the composition (more cholesterol, different proteins) changes what can cross.
• Lipid rafts are small, specialized regions of the membrane enriched in cholesterol and sphingolipids. These microdomains are thicker and less fluid than the surrounding membrane, and they help organize signaling molecules and receptors into functional clusters.
• Membrane potential is the difference in electrical charge across the membrane (typically negative on the inside relative to the outside). This charge difference, maintained largely by ion pumps and channels, is essential for processes like nerve impulse transmission and ATP production.