Introduction

• Lipids and proteins are the primary ingredients of cellular membranes even though carbohydrates play an important role

  • The most abundant lipids are phospholipids

  • A phospholipid is an amphipathic molecule meaning that it has both a hydrophobic and a hydrophilic region

• The presence of a phospholipid bilayer represents a boundary between 2 aqueous compartments because of the molecular arrangement

  • The molecular structure shelters the hydrophobic tails of the phospholipids from water while it exposes the hydrophobic heads to the water

• Most membrane proteins are amphipathic

  • The proteins can stay in the phospholipid bilayer with their hydrophilic heads extending outwards

    • This structure maximizes the contact between the hydrophilic parts of the protein and the water in the cytosol

    • The interpretation of the structure also provides its hydrophobic parts to function in a nonaqueous environment

  • In a fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids

    • The proteins are not distributed evenly throughout the membrane

The Fluidity of Membranes

• A membrane is held together by hydrophobic interactions, which are weaker than covalent bonds

• Most of the lipids and some of the proteins can shift sideways in the plane of the membrane

  • This sideways movement of phospholipids is rapid

  • Proteins are much larger than phospholipids and move more slowly through the membrane

    • Some membrane proteins move in a highly directed manner along the cytoskeletal fibers by motor proteins

    • Other proteins simply drift or hang out in the membrane

• A membrane remains fluid as temperature decreases until the phospholipids come closely packed together

  • The membrane solidifies based on the type of lipid(s) it's made of:

    • As the temperature decreases, the membrane remains fluid but only if there is a massive amount of phospholipids with unsaturated hydrocarbon tails

      • Because of the location of the double bond(s) in the unsaturated hydrocarbon tail, the tail cannot be packed as closely as saturated hydrocarbon tails

      • This looseness between the gaps of the unsaturated hydrocarbon tail causes the membrane to have more fluidity

  • Cholesterol is wedged between phospholipids in the plasma membranes of animal cells

    • At high temperatures (37°), cholesterol makes the membranes less fluid by limiting phospholipid movement

      • Limiting the movement causes a low temperature required for the membrane to solidify

      • Cholesterol helps membranes resist changes when the temperature changes

• Membranes must be fluid to work properly

  • When a membrane solidifies, it's permeability changes and the enzymatic proteins become more inactive

  • Membranes that are too fluid cannot support protein function

Evolution of Differences in Membrane Lipid Composition

• The ability to change lipid composition in cell membranes has evolved in organisms that live where temperatures vary often

  • The percentage of unsaturated phospholipids increase in plants, living in extreme cold, which keep the membranes from solidifying in winter

• Natural selection has favored organisms whose membranes have a sufficient amount of lipids in order to ensure fluidity for the environment

Membrane Proteins and their Functions

• A membrane is a collage of different proteins embedded in the fluid matrix of a lipid bilayer

  • Phospholipids form in the main fabric of the membrane, while proteins determine most of the membrane’s functions

  • Different types of cells contain different sets of membrane proteins

    • The various membranes within each cell contain a unique assortment of proteins

• There are two major populations of membrane proteins: integral and peripheral proteins

  • Integral proteins - penetrate the hydrophobic interior of the lipid bilayer

    • The majority are transmembrane proteins, which span the membrane, and the other integral proteins extend only partway into the hydrophobic interior

    • The hydrophobic regions of an integral protein consist of one or more stretches of  nonpolar amino acids

    • The hydrophilic parts of the molecule are exposed to the aqueous solution on either side of the membrane

  • Peripheral proteins - not embedded in the lipid bilayer, but are loosely bound to the surface of the membrane and exposed to parts of integral proteins

• On the cytoplasmic side of the plasma membrane, some proteins are held in place by attachment to the cytoskeleton

  • On the extracellular side, certain proteins may be attached to fibers of the extracellular matrix

• 6 major functions performed by proteins of the plasma membrane:

  • Transport

    • A protein may provide a hydrophilic channel across the membrane that is selective for a particular solute

    • Other proteins transfer a substance from one side to another by changing shape (structure)

      • Some of these proteins use ATP as an energy source to pump substances across the membrane

  • Enzymatic activity

    • A protein built into the membrane may be an enzyme with its active site exposed to substances in the neighboring solution

    • Sometimes, several enzymes in a membrane form a team that carry out sequential steps of a metabolic pathway

  • Signal transduction

    • A membrane protein may have a specific shape that is able to fit the shape of a chemical messenger like a hormone

    • The external messenger (signaling molecule) may cause the protein to change shape, allowing it to pass on the message to the inside of the cell

  • Cell-cell recognition

    • Some glycoproteins act as identification tags that are recognized by membrane proteins of other cells

    • This type of cell-to-cell binding is usually short-lived compared with that shown in intercellular joining

  • Intercellular joining

    • Membrane proteins of neighboring cells may hook together in various kinds of junctions (gap or tight junctions)

    • This type of bonding is more long-lasting than cell-cell recognition

  • Attachment to the cytoskeleton and extracellular matrix (ECM)

    • Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins

      • This function helps to stabilize the location of certain membrane proteins

    • Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes

The Role of Membrane Carbohydrates in Cell-Cell Recognition

• Cell-to-cell recognition - a cell’s ability to distinguish one type of neighboring cell from another

  • Cells recognize other cells by binding to molecules on the extracellular surface of the plasma membrane

• Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units

  • Glycolipids - a lipid with one or more covalently attached carbohydrates

  • Glycoproteins - a protein with one or more covalently attached carbohydrates

    • Most membrane carbohydrates are bonded to proteins

  • The carbohydrates on the extracellular side of the plasma membrane vary

    • The diversity of the molecules and their location based on the cell’s surface enable the membrane to function as markers that differentiate one cell from another

Synthesis and Sidedness of Membranes

• Membranes have distinct inside and outside faces

  • The 2 lipid layers may differ in lipid concentration and each protein has directional orientation in the membrane

  • As the membrane is being built by the ER and Golgi body, the asymmetric arrangement of proteins, lipids, and carbohydrates is determined