Cellular Membrane Structure and Function

• The plasma membrane is the boundary separating a living cell from its surroundings.
• The plasma membrane has selective permeability, allowing some substances to cross more easily than others.
• Small molecules cross the cell membrane using passive transport, not requiring energy, but may need transport proteins.
• Small molecules use active transport, which requires both energy and a transport protein.
• Large molecules use bulk transport, specifically exocytosis or endocytosis, to travel in and out.
• Cellular membranes consist of lipids, proteins, and some carbohydrates

Membrane Composition

• Membranes are primarily composed of phospholipids.
• Phospholipids are amphipathic molecules, containing hydrophobic ("water-fearing") and hydrophilic ("water-loving") regions.
• Phospholipids form a bilayer where hydrophobic tails are inside, and hydrophilic heads are exposed to water.
• Most membrane proteins are amphipathic.
• The hydrophilic regions of proteins are oriented toward the cytosol and extracellular fluid.
• The hydrophobic regions of proteins are embedded in the bilayer.
• The fluid mosaic model describes the membrane as a mosaic of protein molecules bobbing in a fluid phospholipid bilayer.
• Proteins are not randomly distributed, and form groups to carry out specific functions.

Membrane Fluidity

• Membranes are held together mainly by weak hydrophobic interactions.
• Lipids and some proteins can move sideways within the membrane.
• Lipids rarely flip-flop across the membrane from one phospholipid layer to the other.
• Membranes switch from a fluid to a solid state as temperatures cool.
• The temperature at which a membrane solidifies relies on lipid types
• Membranes with rich unsaturated fatty acids are more fluid than those rich in saturated fatty acids.
• Membranes must be fluid for proper function.
• Cholesterol is a membrane component in animal cells that has variable effects on membrane fluidity.
• At warm temperatures (37°C), cholesterol restrains the movement of phospholipids.
• At cool temperatures, cholesterol maintains fluidity by preventing tight packing.
• Plants use steroid lipids to buffer membrane fluidity.
• Fluidity affects both permeability and movement of transport proteins.
• Membranes that are overly fluid may not be able to support protein function.
• Organisms at extreme temperatures have adaptive differences in membrane lipid composition.

Differences in Lipid Composition

• Membrane lipid composition adapts to environmental conditions in many species.
• Cell membranes have a high proportion of unsaturated hydrocarbon tails in fish that live in extreme cold.
• Organisms living in variable temperature conditions can change lipid composition according to temperature.
• Winter wheat increases the percentage of unsaturated phospholipids in autumn to prevent membrane solidification during winter.

Membrane Proteins and Functions

• A membrane is a collage of different proteins, often clustered in groups, within the lipid bilayer.
• Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane's functions.
• The protein composition of membranes varies among cells within an organism, and among intracellular membranes within a cell.
• Peripheral proteins are bound to the surface of the membrane.
• Integral proteins penetrate the hydrophobic core.
• Transmembrane proteins are integral proteins that span the membrane.
• Hydrophobic regions of an integral protein consist of nonpolar amino acids, coiled into α helices.
• Some membrane proteins are held in place by attachment to the cytoskeleton inside the cell while others are attached outside the cell.
• Integrins attach to fibers of the extracellular matrix.
• Cell-surface membrane proteins carry out several functions:
  • Transport
  • Enzymatic activity
  • Signal transduction
  • Cell-cell recognition
  • Intercellular joining
  • Attachment to the cytoskeleton and extracellular matrix (ECM)
• Cell-surface proteins are important in medicine.
• HIV enters immune cells by binding to cell-surface protein CD4 and a "co-receptor" CCR5.
• Those lacking CCR5 are immune to HIV infection.
• Drugs are in development to mask CCR5 and block HIV entrance in nonimmune individuals.

Cell-Cell Recognition

• Cells recognize each other by binding to molecules on the surface of the membrane.
• Many of these surface molecules are bonded to short, branched chains of carbohydrates.
  • Glycolipids are carbohydrates bonded to lipids.
  • Glycoproteins are carbohydrates bonded to proteins.
• The diversity of surface carbohydrates enables markers for cell identification.