Overview: Life at the Edge

           The plasma membrane is the boundary that separates the living cell from its surroundings

           The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

 

Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins

           Phospholipids are the most abundant lipid in the plasma membrane

           Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

           The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

Membrane Models: Scientific Inquiry

           Membranes have been chemically analyzed and found to be made of proteins and lipids

           Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer

           In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins

           Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions

           In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

           Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

           Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer

The Fluidity of Membranes

           Phospholipids in the plasma membrane can move within the bilayer

           Most of the lipids, and some proteins, drift laterally

           Rarely does a molecule flip-flop transversely across the membrane

           As temperatures cool, membranes switch from a fluid state to a solid state

           The temperature at which a membrane solidifies depends on the types of lipids

           Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

           Membranes must be fluid to work properly; they are usually about as fluid as salad oil

           The steroid cholesterol has different effects on membrane fluidity at different temperatures

           At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids

           At cool temperatures, it maintains fluidity by preventing tight packing

Evolution of Differences in Membrane Lipid Composition

           Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions

           Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary

Membrane Proteins and Their Functions

           A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer

           Proteins determine most of the membrane’s specific functions

           Peripheral proteins are bound to the surface of the membrane

           Integral proteins penetrate the hydrophobic core

           Integral proteins that span the membrane are called transmembrane proteins

           The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

           Six major functions of membrane proteins

         Transport

         Enzymatic activity

         Signal transduction

         Cell-cell recognition

         Intercellular joining

         Attachment to the cytoskeleton and extracellular matrix (ECM)

The Role of Membrane Carbohydrates in Cell-Cell Recognition

           Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane

           Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

           Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Synthesis and Sidedness of Membranes

           Membranes have distinct inside and outside faces

           The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus

 

Concept 7.2: Membrane structure results in selective permeability

           A cell must exchange materials with its surroundings, a process controlled by the plasma membrane

           Plasma membranes are selectively permeable, regulating the cell’s molecular traffic

The Permeability of the Lipid Bilayer

           Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly

           Polar molecules, such as sugars, do not cross the membrane easily

Transport Proteins

           Transport proteins allow passage of hydrophilic substances across the membrane

           Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel

           Channel proteins called aquaporins facilitate the passage of water

           Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane

           A transport protein is specific for the substance it moves

 

Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment

           Diffusion is the tendency for molecules to spread out evenly into the available space

           Although each molecule moves randomly, diffusion of a population of molecules may be directional

           At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other

           Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases

           No work must be done to move substances down the concentration gradient

           The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen

Effects of Osmosis on Water Balance

           Osmosis is the diffusion of water across a selectively permeable membrane

           Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides

Water Balance of Cells Without Walls

           Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water

           Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

           Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water

           Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

           Hypertonic or hypotonic environments create osmotic problems for organisms

           Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments

           The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Water Balance of Cells with Walls

           Cell walls help maintain water balance

           A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)

           If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

           In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

Facilitated Diffusion: Passive Transport Aided by Proteins

           In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane

           Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane

           Channel proteins include

         Aquaporins, for facilitated diffusion of water

         Ion channels that open or close in response to a stimulus (gated channels)

           Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

 

           Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria

 

Concept 7.4: Active transport uses energy to move solutes against their gradients

           Facilitated diffusion is still passive because the solute moves down its concentration gradient, and the transport requires no energy

           Some transport proteins, however, can move solutes against their concentration gradients

The Need for Energy in Active Transport

           Active transport moves substances against their concentration gradients

           Active transport requires energy, usually in the form of ATP

           Active transport is performed by specific proteins embedded in the membranes

           Active transport allows cells to maintain concentration gradients that differ from their surroundings

           The sodium-potassium pump is one type of active transport system

How Ion Pumps Maintain Membrane Potential

           Membrane potential is the voltage difference across a membrane

           Voltage is created by differences in the distribution of positive and negative ions across a membrane

           Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane

         A chemical force (the ion’s concentration gradient)

         An electrical force (the effect of the membrane potential on the ion’s movement)

           An electrogenic pump is a transport protein that generates voltage across a membrane

           The sodium-potassium pump is the major electrogenic pump of animal cells

           The main electrogenic pump of plants, fungi, and bacteria is a proton pump

           Electrogenic pumps help store energy that can be used for cellular work

Cotransport: Coupled Transport by a Membrane Protein

           Cotransport occurs when active transport of a solute indirectly drives transport of other solutes

           Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

 

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

           Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins

           Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles

           Bulk transport requires energy

Exocytosis

           In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents

           Many secretory cells use exocytosis to export their products

Endocytosis

           In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane

           Endocytosis is a reversal of exocytosis, involving different proteins

           There are three types of endocytosis

         Phagocytosis (“cellular eating”)

         Pinocytosis (“cellular drinking”)

         Receptor-mediated endocytosis

           In phagocytosis a cell engulfs a particle in a vacuole

           The vacuole fuses with a lysosome to digest the particle

           In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles

           In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation

           A ligand is any molecule that binds specifically to a receptor site of another molecule