Key Concepts Plasma membranes are made up of selectively permeable bilayers of phospholipids. Phospholipids are amphipathic lipid molecules – they have hydrophobic and hydrophilic regions. Ions and molecules diffuse spontaneously from regions of higher concentration to regions of lower concentration. Movement of water across a plasma membrane is called osmosis. In cells, membrane proteins are responsible for the passage of insoluble substances that can’t cross the membrane on their own.

The Importance of Membranes The plasma membrane, or cell membrane, separates life from nonlife. The plasma membrane separates the cell’s interior from the external environment. Membranes function to: Keep damaging materials out of the cell Allow entry of materials needed by the cell Facilitate the chemical reactions necessary for life

Lipids: What Is a Lipid? Lipids are carbon-containing compounds that are found in organisms and that are largely nonpolar and hydrophobic. Hydrocarbons are nonpolar molecules that contain only carbon and hydrogen. Lipids do not dissolve in water because they have a major hydrocarbon component called a fatty acid. A fatty acid is a hydrocarbon chain bonded to a carboxyl (—COOH) functional group. Fatty acids and isoprene are the key building blocks of lipids.

Three Types of Lipids Found in Cells Lipid structure varies widely. The three most important types of lipids found in cells: Fats are composed of three fatty acids linked to glycerol. Also called triacylglycerols or triglycerides Steroids are a family of lipids with a distinctive four-ring structure. Cholesterol is an important steroid in mammals. Phospholipids consist of a glycerol linked to a phosphate group (PO42–) and to either two chains of isoprene or two fatty acids.

The Structure of Membrane Lipids Membrane-forming lipids contain both a polar, hydrophilic region and a nonpolar, hydrophobic region. Phospholipids are amphipathic: The “head” region, consisting of a glycerol, a phosphate, and a charged group, contains highly polar covalent bonds. The “tail” region is comprised of two nonpolar fatty acid or isoprene chains. When placed in solution, the phospholipid heads interact with water while the tails do not, allowing these lipids to form membranes.

Phospholipids and Water Phospholipids do not dissolve when they are placed in water. Water molecules interact with the hydrophilic heads but not with the hydrophobic tails. This drives the hydrophobic tails together. Upon contact with water phospholipids form either: Micelles Heads face the water and tails face each other. Phospholipid bilayers (lipid bilayers)

Phospholipid Bilayers Phospholipid bilayers form when two sheets of phospholipid molecules align. The hydrophilic heads in each layer face a surrounding solution, while the hydrophobic tails face one another inside the bilayer. Phospholipid bilayers form spontaneously, with no outside input of energy required.

Selective Permeability of Lipid Bilayers The permeability of a structure is its tendency to allow a given substance to pass across it. Phospholipid bilayers have selective permeability. Small or nonpolar molecules move across phospholipid bilayers quickly. Charged or large polar substances cross slowly, if at all.

Many Factors Affect Membrane Permeability Many factors influence the behavior of the membrane: Number of double bonds between the carbons in the phospholipid’s hydrophobic tail Length of the tail Number of cholesterol molecules in the membrane Temperature

Bond Saturation and Membrane Permeability Double bonds between carbons in a hydrocarbon chain can cause a “kink” in the hydrocarbon chain, preventing the close packing of hydrocarbon tails, and reducing hydrophobic interactions. Unsaturated hydrocarbon chains have at least one double bond. Hydrocarbon chains without double bonds are termed saturated. Saturated fats have more chemical energy than unsaturated fats. Membranes with unsaturated phospholipid tails are much more permeable than those formed by phospholipids with saturated tails.

Other Factors That Affect Permeability Hydrophobic interactions become stronger as saturated hydrocarbon tails increase in length. Membranes containing phospholipids with longer tails have reduced permeability. Adding cholesterol to membranes increases the density of the hydrophobic section. Cholesterol decreases membrane permeability. Membrane fluidity decreases with temperature because molecules in the bilayer move more slowly. Decreased membrane fluidity causes decreased permeability.

Fluidity of the Membrane Individual phospholipids can move laterally throughout the lipid bilayer. They rarely flip between layers. How quickly molecules move within and across membranes is a function of temperature and the structure of the hydrocarbon tails in the bilayer.

Solute Movement across Lipid Bilayers Materials can move across the cell membrane in different ways. Passive transport does not require an input of energy. Active transport requires energy to move substances across the membrane. Small molecules and ions in solution are called solutes, have thermal energy, and are in constant, random motion. This random movement is called diffusion. Diffusion is a form of passive transport.

Diffusion along a Concentration Gradient A difference in solute concentrations creates a concentration gradient. Molecules and ions move randomly when a concentration gradient exists, but there is a net movement from high- concentration regions to low-concentration regions. Diffusion along a concentration gradient increases entropy and is thus spontaneous. Equilibrium is established once the molecules or ions are randomly distributed throughout a solution. Molecules are still moving randomly but there is no more net movement.

Osmosis Water moves quickly across lipid bilayers. The movement of water is a special case of diffusion called osmosis. Water moves from regions of low solute concentration to regions of high solute concentration. This movement dilutes the higher concentration, thus equalizing the concentration on both sides of the bilayer. Osmosis only occurs across a selectively permeable membrane.

Osmosis and Relative Solute Concentration The concentration of a solution outside a cell may differ from the concentration inside the cell. An outside solution with a higher concentration is said to be hypertonic to the inside of a cell. A solution with a lower concentration is hypotonic to the cell. If solute concentrations are equal on the outside and inside of a cell, solutions are isotonic to each other.

Osmosis in Hypertonic, Hypotonic, and Isotonic Solutions In a hypertonic solution, water will move out of the cell by osmosis and the cell will shrink. In a hypotonic solution, water will move into the cell by osmosis and the cell will swell. In an isotonic solution, there will be no net water movement and the cell size will remain the same.

The Fluid-Mosaic Model of Membrane Structure Although phospholipids provide the basic membrane structure, plasma membranes contain as much protein as phospholipids. The fluid-mosaic model of membrane structure suggests that some proteins are inserted into the lipid bilayer, making the membrane a fluid, dynamic mosaic of phospholipids and proteins.

Membrane Proteins Integral proteins are amphipathic and so can span a membrane, with segments facing both its interior and exterior surfaces. Integral proteins that span the membrane are called transmembrane proteins. These proteins are involved in the transport of selected ions and molecules across the plasma membrane. Transmembrane proteins can therefore affect membrane permeability. Peripheral proteins are found only on one side of the membrane. Often attached to integral proteins

Membrane Proteins Affect Ions and Molecules The transmembrane proteins that transport molecules are called transport proteins. There are three broad classes of transport proteins, each of which affects membrane permeability: Channels Carrier proteins or transporters Pumps

Ion Channels and the Electrochemical Gradient Ion channels are specialized membrane proteins. Ion channels circumvent the plasma membrane’s impermeability to small, charged compounds. When ions build up on one side of a plasma membrane, they establish both a concentration gradient and a charge gradient, collectively called the electrochemical gradient. Ions diffuse through channels down their electrochemical gradients. This passive transport decreases the charge and concentration differences between the cell’s exterior and interior.

Facilitated Diffusion via Channel Proteins Cells have many different types of channel proteins in their membranes, each featuring a structure that allows it to admit a particular type of ion or small molecule. These channels are responsible for facilitated diffusion: the passive transport of substances that would not otherwise cross the membrane.

Facilitated Diffusion via Carrier Proteins Facilitated diffusion can occur through channels or through carrier proteins, or transporters, which change shape during the transport process. Facilitated diffusion by transporters occurs only down a concentration gradient, reducing differences between solutions. Glucose is a building block for important macromolecules and a major energy source, but lipid bilayers are only moderately permeable to glucose. A glucose transporter named GLUT-1 increases membrane permeability to glucose.

Active Transport by Pumps Cells can transport molecules or ions against an electrochemical gradient. This process requires energy in the form of ATP and is called active transport. Pumps are membrane proteins that provide active transport of molecules across the membrane. For example, the sodium-potassium pump, Na+/K+-ATPase, uses ATP to transport Na+ and K+ against their concentration gradients.

Secondary Active Transport In addition to moving materials against their concentration gradients, pumps set up electrochemical gradients. These gradients make it possible for cells to engage in secondary active transport, or cotransport. The gradient provides the potential energy required to power the movement of a different molecule against its particular gradient.

Summary of Membrane Transport There are three mechanisms of membrane transport: Diffusion Facilitated diffusion Active transport Diffusion and facilitated diffusion are forms of passive transport and thus move materials down their concentration gradient and do not require an input of energy. Active transport moves materials against their concentration gradient and requires energy provided by ATP or an electrochemical gradient.

Plasma Membrane and the Intracellular Environment The selective permeability of the lipid bilayer and the specificity of the proteins involved in passive transport and active transport enable cells to create an internal environment that is much different from the external one.