• Prokaryotes, which represent the simplest and smallest cells, are about 1–2 m in length

  • Are surrounded by a plasma membrane but contain no internal membrane-limited subcompartments

• In the larger cells of eukaryotes the rates of chemical reactions would be limited by the diffusion of small molecules if a cell were not partitioned into smaller subcompartments termed organelles

  • Each organelle is surrounded by one or more biomembranes

  • Each type of organelle contains a unique complement of proteins—some embedded in its membrane(s), others in its aqueous interior space, or lumen

• The cytoplasm is the part of the cell outside the largest organelle, the nucleus

• The cytosol, the aqueous part of the cytoplasm outside all of the organelles, also contains its own distinctive proteins

• The structural basis of the unique design of each cell type lies in the cytoskeleton, a dense network of three classes of protein filaments that permeate the cytosol and mechanically support cellular membranes

5.1 - Biomembranes: Lipid Composition and Structural Organization

• Phospholipids of the composition present in cells spontaneously form sheetlike phospholipid bilayers, which are two molecules thick

• The hydrocarbon chains of the phospholipids in each layer, or leaflet, form a hydrophobic core that is 3–4 nm thick in most biomembranes

• A cross-section of all single membranes stained with osmium tetroxide looks like a railroad track: two thin dark lines between them

• The lipid bilayer has two important properties

  • First, the hydrophobic core is an impermeable barrier that prevents the diffusion of water-soluble (hydrophilic) solutes across the membrane

  • The second property of the bilayer is its stability, it is maintained by hydrophobic and Van der Waals interactions between the lipid chains

• Natural membranes from different cell types exhibit a variety of shapes, which complement a cell’s function

• All cellular membranes enclose an entire cell or an internal compartment, they have an internal face (the surface oriented toward the interior of the compartment) and an external face (the surface presented to the environment)

• The surfaces of a cellular membrane are designated as the cytosolic face and the exoplasmic face

• This nomenclature is useful in highlighting the topological equivalence of the faces in different membranes

• Three organelles—the nucleus, mitochondrion, and chloroplast—are surrounded by two membranes; the exoplasmic surface of each membrane faces the space between the two membranes

Three Classes of Lipids Are Found in Biomembranes

• A typical biomembrane is assembled from phosphoglycerides, sphingolipids, and steroids

• All three classes of lipids are amphipathic molecules having a polar (hydrophilic) head group and hydrophobic tail

• Phosphoglycerides, the most abundant class of lipids in most membranes, are derivatives of glycerol 3-phosphat

  • Two fatty acyl chains may differ in the number of carbons that they contain (commonly 16 or 18) and their degree of saturation (0, 1, or 2 double bonds)

  • In phosphatidylcholines, the most abundant phospholipids in the plasma membrane, the head group consists of choline, a positively charged alcohol, esterified to the negatively charged phosphate

  • In other phosphoglycerides, an OH-containing molecule such as ethanolamine, serine, and the sugar derivative inositol is linked to the phosphate group

  • The plasmalogens are a group of phosphoglycerides that contain one fatty acyl chain, attached to glycerol by an ester linkage, and one long hydrocarbon chain, attached to glycerol by an ether linkage (C---O---C)

• The second class of membrane lipid is the sphingolipids

  • All of these compounds are derived from sphingosine, amino alcohol with a long hydrocarbon chain

  • Contain a long-chain fatty acid attached to the sphingosine amino group

• Cholesterol and its derivatives constitute the third important class of membrane lipids, the steroids

  • The basic structure of steroids is a four-ring hydrocarbon

  • Cholesterol, the major steroidal constituent of animal tissues, has a hydroxyl substituent on one ring

  • Although cholesterol is almost entirely hydrocarbon in composition, it is amphipathic because its hydroxyl group can interact with water

Most Lipids and Many Proteins Are Laterally Mobile in Biomembranes ‘

• In the two-dimensional plane of a bilayer, thermal motion permits lipid molecules to rotate freely around their long axes and to diffuse laterally within each leaflet.

• In both natural and artificial membranes, a typical lipid molecule exchanges place with its neighbors in a leaflet about 107 times per second and diffuse several micrometers per second at 37oC

• The lateral movements of specific plasma membrane proteins and lipids can be quantified by a technique called fluorescence recovery after photobleaching (FRAP)

• The results of FRAP studies with fluorescence-labeled phospholipids have shown that, in fibroblast plasma membranes

  • All the phospholipids are freely mobile over distances of about 0.5 m, but most cannot diffuse over much longer distances

Lipid Composition Influences the Physical Properties of Membranes

• A typical cell contains myriad types of membranes, each with unique properties bestowed by its particular mix of lipids and proteins.

• Differences in lipid composition may also correspond to the specialization of membrane function

• In these polarized cells, the ratio of sphingolipid to phosphoglyceride to cholesterol in the basolateral membrane is 0.5:1.5:1

  • Roughly equivalent to that in the plasma membrane of a typical unpolarized cell subjected to mild stress

• The ability of lipids to diffuse laterally in a bilayer indicates that it can act as a fluid

• The degree of bilayer fluidity depends on the lipid composition, structure of the phospholipid hydrophobic tails, and temperature

• At usual physiologic temperatures, the hydrophobic interior of natural membranes generally has a low viscosity and a fluidlike, rather than gel-like, consistency

• The lipid composition of a bilayer is its local curvature, which depends on the relative sizes of the polar head groups and nonpolar tails of its constituent phospholipids

• Lipids with long tails and large head groups are cylindrical in shape; those with small head groups are cone-shaped

Membrane Lipids Are Usually Distributed Unequally in the Exoplasmic and Cytosolic Leaflets

• A characteristic of all membranes is an asymmetry in lipid composition across the bilayer

• Although most phospholipids are present in both membrane leaflets, they are commonly more abundant in one or the other leaflet

• The relative abundance of a particular phospholipid in the two leaflets of a plasma membrane can be determined on the basis of its susceptibility to hydrolysis by phospholipases

  • Enzymes that cleave various bonds in the hydrophilic ends of phospholipids

• In pure bilayers, phospholipids do not spontaneously migrate, or flip-flop, from one leaflet to the other

• Energetically, such flip-flopping is extremely unfavorable because it entails movement of the polar phospholipid head group through the hydrophobic interior of the membrane

• The preferential location of lipids to one face of the bilayer is necessary for a variety of membrane-based functions

• Phosphatidylserine also is normally most abundant in the cytosolic leaflet of the plasma membrane

• In the initial stages of platelet stimulation by serum, phosphatidylserine is briefly translocated to the exoplasmic face

  • Presumably by a flippase enzyme, where it activates enzymes participating in blood clotting

Cholesterol and Sphingolipids Cluster with Specific Proteins in Membrane Microdomains

• The results of recent studies have challenged the long-held belief that lipids are randomly mixed in each leaflet of a bi-layer

  • Lipids may be organized within the leaflets was the discovery that the residues remaining after the extraction of plasma membranes with detergents contain two lipids: cholesterol and sphingomyelin

  • Because these two lipids are found in more ordered, less fluid bilayers, researchers hypothesized that they form microdomains termed lipid rafts

  • Surrounded by other more fluid phospholipids that are easily extracted by detergents

• Biochemical and microscopic evidence supports the existence of lipid rafts in natural membranes

5.2 - Biomembranes: Protein Components and Basic Functions

• Membrane proteins are defined by their location within or at the surface of a phospholipid bilayer

• Every biological membrane has the same basic bilayer structure, the proteins associated with a particular membrane are responsible for its distinctive activities

• The density and complement of proteins associated with biomembranes vary, depending on cell type and subcellular location

• The lipid bilayer presents a unique two-dimensional hydrophobic environment for membrane proteins

• Some proteins are buried within the lipid-rich bilayer; other proteins are associated with the exoplasmic or cytosolic leaflet of the bilayer

• Domains within the plasma membrane, particularly those that form channels and pores, move molecules in and out of cells

• Domains lying along the cytosolic face of the plasma membrane have a wide range of functions

  • From anchoring cytoskeletal proteins to the membrane to trigger intracellular signaling pathways

Proteins Interact with Membranes in Three Different Ways

• Membrane proteins can be classified into three categories—integral, lipid-anchored, and peripheral—on the basis of the nature of the membrane–protein interactions

• Integral membrane proteins, also called transmembrane proteins, span a phospholipid bilayer and are built of three segments

  • Domains resemble other water-soluble proteins in their amino acid composition and structure

  • In all transmembrane proteins examined to date, the membrane-spanning domains consist of one or more helices or of multiple B strands

• Lipid-anchored membrane proteins are bound covalently to one or more lipid molecules

  • The hydrophobic carbon chain of the attached lipid is embedded in one leaflet of the membrane and anchors the protein to the membrane

• Peripheral membrane proteins do not interact with the hydrophobic core of the phospholipid bilayer

• They are usually bound to the membrane indirectly by interactions with integral membrane proteins or directly by interactions with lipid head groups

Membrane-Embedded ox Helices are the Primary Secondary Structures in Most Transmembrane Proteins

• Soluble proteins exhibit hundreds of distinct localized folded structures or motifs

• The repertoire of folded structures in integral membrane proteins is quite limited, with the hydrophobic ox helix predominating

• Glycophorin A: the major protein in the erythrocyte plasma membrane, is a representative single-pass transmembrane protein, which contains only one membrane-spanning B helix

• A large and important family of integral proteins is defined by the presence of seven membrane-spanning ox helices

• Among the more than 150 such “seven spanning” multipass proteins that have been identified are the G protein-coupled receptors

• In the high-resolution structure of bacteriorhodopsin now available, the positions of all the individual amino acids, retinal, and the surrounding lipids are determined

• Ion channels compose a second large and important family of multipass transmembrane proteins

  • As revealed by the crystal structure of a resting K channel, ion channels are typically tetrameric proteins

• Polar and hydrophobic residues lining the center of the bundle form a channel in the membrane, but as with bacteriorhodopsin virtually all of the amino acids on the exterior of the membrane-spanning domain are hydrophobic

Multiple B Strands in Porins Form Membrane-Spanning “Barrels”

• The porins are a class of transmembrane proteins whose structure differs radically from that of other integral proteins

• Several types of porin are found in the outer membrane of gram-negative bacteria such as E. coli and in the outer membranes of mitochondria and chloroplasts

• The porins in the outer membrane of an E. coli cell provide channels for the passage of disaccharides and other small molecules as well as phosphate

• The amino acid sequences of porins are predominantly polar and contain no long hydrophobic segments typical of integral proteins with ox-helical membrane-spanning domains

• In a porin monomer, the outward-facing side groups on each of the B strands are hydrophobic and form a nonpolar ribbonlike band that encircles the outside of the barrel

Covalently Attached Hydrocarbon Chains Anchor Some Proteins to Membranes

• In eukaryotic cells, several types of covalently attached lipids anchor some proteins to one or the other leaflet of the plasma membrane and certain other cellular membranes

• A group of cytosolic proteins is anchored to the cytosolic face of a membrane by a fatty acyl group (e.g., myristate or palmitate) attached to the N-terminal glycine residue

• Retention of such proteins at the membrane by the N-terminal acyl anchor may play an important role in a membrane-associated function

• The second group of cytosolic proteins is anchored to membranes by an unsaturated fatty acyl group attached to a cysteine residue at or near the C-terminus

• Sometimes, a second geranylgeranyl group or a palmitate group is linked to a nearby cysteine residue

• Some cell-surface proteins and heavily glycosylated proteoglycans of the extracellular matrix are bound to the exoplasmic face of the plasma membrane by a third type of anchor group, glycosylphosphatidylinositol (GPI)

• Treatment of cells with phospholipase C releases GPI-anchored proteins such as Thy-1 and placental alkaline phosphatase (PLAP) from the cell surface

• Although PLAP and other GPI-anchored proteins lie in the opposite membrane leaflet from acyl-anchored proteins, both types of membrane proteins are concentrated in lipid rafts

All Transmembrane Proteins and Glycolipids Are Asymmetrically Oriented in the Bilayer

• Lipid-anchored proteins are just one example of membrane proteins that are asymmetrically located with respect to the faces of cellular membranes

• Each type of transmembrane protein also has a specific orientation with respect to the membrane faces

• Many transmembrane proteins contain carbohydrate chains covalently linked to serine, threonine, or asparagine side chains of the polypeptide

• Such transmembrane glycoproteins are always oriented so that the carbohydrate chains are in the exoplasmic domain

Interactions with the Cytoskeleton Impede the Mobility of Integral Membrane Proteins

• From 30 to 90 percent of all integral proteins in the plasma membrane are freely mobile, depending on the cell type

• The lateral diffusion rate of a mobile protein in a pure phospholipid bilayer or isolated plasma membrane is similar to that of lipids

• These findings suggest that the mobility of integral proteins in the plasma membrane of living cells is restricted by interactions with the rigid submembrane cytoskeleton

• In regard to mobile proteins, such interactions are broken and remade as the proteins diffuse laterally in the plasma membrane, slowing down their rate of diffusion

Lipid-Binding Motifs Help Target Peripheral Proteins to the Membrane

• Analyses of genome sequences have revealed several widely distributed lipid-binding motifs in proteins

• The phospholipases are representative of those water-soluble enzymes that associate with the polar head groups of membrane phospholipids to carry out their catalytic functions

• Binding induces a small conformational change in phospholipase A2 that fixes the protein to the phospholipid heads and opens the hydrophobic channel

• As a phospholipid molecule diffuses from the bilayer into the channel, the enzyme-bound Ca2+ binds to the phosphate in the head group, thereby positioning the ester bond to be cleaved next to the catalytic site

The Plasma Membrane Has Many Common Functions in All Cells

• In all cells, the plasma membrane acts as a permeability barrier that prevents the entry of unwanted materials from the extracellular milieu and the exit of needed metabolites

• Specific membrane transport proteins in the plasma membrane permit the passage of nutrients into the cell and metabolic wastes out of it

  • Others function to maintain the proper ionic composition and pH (≈7.2) of the cytosol

• The plasma membrane is highly permeable to water but poorly permeable to salts and small molecules such as sugars and amino acids

• Owing to osmosis, water moves across such a semipermeable membrane from a solution of low solute (high water) concentration to one of high solute (low water) concentration

  • Until the total solute concentrations and thus the water concentrations on both sides are equal

• When most animal cells are placed in an isotonic solution (i.e., one with a total concentration of solutes equal to that of the cell interior), there is no net movement of water into or out of cells

• Unlike animal cells, bacterial, fungal, and plant cells are surrounded by a rigid cell wall and lack the extracellular matrix found in animal tissues

• The plasma membrane is intimately engaged in the assembly of cell walls, which in plants are built primarily of cellulose

• The plasma membrane has other crucial roles in multicellular organisms

• Few of the cells in multicellular plants and animals exist as isolated entities; rather, groups of cells with related specializations combine to form tissues

• The plasma membranes of many types of eukaryotic cells also contain receptor proteins that bind specific signaling molecules leading to various cellular responses

5.3 - Organelles of the Eukaryotic Cell

Endosomes Take Up Soluble Macromolecules from the Cell Exterior

• Transport proteins in the plasma membrane mediate the movement of ions and small molecules across the lipid bilayer

• Proteins and some other soluble macromolecules in the extracellular milieu are internalized by endocytosis

• Some membrane proteins are recycled back to the plasma membrane; other membrane proteins are transported to a late endosome where further sorting takes place

Lysosomes Are Acidic Organelles That Contain a Battery of Degradatuve Enzymes

• Lysosomes are a great example of the ability of intracellular membranes to form closed compartments in with the composition of the lumen differs substantially from that t=of the surrounding cytosol

• An aged organelle is degraded in a lysosome is called autophagy (“eating oneself”)

• Materials taken into a cell by endocytosis or phagocytosis also may be degraded in lysosomes

• Lysosomes contain a group of enzymes that degrade polymers into their monomeric subunits

• All the lysosomal enzymes work most efficiently at acid pH values and collectively are termed acid hydrolases

• Two types of transport proteins in the lysosomal membrane work together to pump H+ and Cl- ions (HCl) from the cytosol across the membrane, thereby acidifying the lumen

• The acid pH helps to denature proteins, making them accessible to the action of the lysosomal hydrolases, which themselves are resistant to acid denaturation

• Lysosomes vary in size and shape, and several hundred may be present in a typical animal cell

  • In effect, they function as sites where various materials to be degraded collect

  • Primary lysosomes are roughly spherical and do not contain obvious particulate or membrane debris

  • Secondary lysosomes, which are larger and irregularly shaped, appear to result from the fusion of primary lysosomes with other membrane-bounded organelles and vesicles

Peroxisomes Degrade Fatty Acids and Toxic Compounds

• All animal cells (except erythrocytes) and many plant cells contain peroxisomes

• Peroxisomes contain several oxidases—enzymes that use molecular oxygen to oxidize organic substances

  • In the process of forming hydrogen peroxide (H2O2), a corrosive substance

• The energy released during peroxisomal oxidation is converted into heat, and the acetyl groups are transported into the cytosol

  • Where they are used in the synthesis of cholesterol and other metabolites

• In most eukaryotic cells, the peroxisome is the principal organelle in which fatty acids are oxidized, thereby generating precursors for important biosynthetic pathways

The Endoplasmic Reticulum Is a Network of Interconnected Internal Membranes

• The largest membrane in a eukaryotic cell encloses the endoplasmic reticulum (ER)—an extensive network of closed, flattened membrane-bounded sacs called cisternae

• The smooth endoplasmic reticulum is smooth because it lacks ribosomes

• In contrast, the cytosolic face of the rough endoplasmic reticulum is studded with ribosomes

• The Smooth Endoplasmic Reticulum: The synthesis of fatty acids and phospholipids takes place in the smooth ER

  • Many cells have very little smooth ER, this organelle is abundant in hepatocytes

• The Rough Endoplasmic Reticulum: Ribosomes bound to the rough ER synthesize certain membrane and organelle proteins and virtually all proteins to be secreted from the cell

  • A ribosome that fabricates such a protein is bound to the rough ER by the nascent polypeptide chain of the protein

  • Newly-made membrane proteins remain associated with the rough ER membrane and proteins to be secreted accumulate in the lumen of the organelle

• All eukaryotic cells contain a discernible amount of rough ER because it is needed for the synthesis of plasma-membrane proteins and proteins of the extracellular matrix

The Golgi Complex Processes and Sorts Secreted and Membrane Proteins

• Several minutes after proteins are synthesized in the rough ER, most of them leave the organelle within small membrane-bounded transport vesicles

• These vesicles, which bud from regions of the rough ER not coated with ribosomes carry the proteins to another membrane-limited organelle, the Golgi complex

• The stack of Golgi cisternae has three defined regions—the cis, the medial, and the trans

• Transport vesicles from the rough ER fuse with the cis region of the Golgi complex, where they deposit their protein contents

• After proteins to be secreted and membrane proteins are modified in the Golgi complex, they are transported out of the complex by the second set of vesicles

  • Which seem to bud from the trans side of the Golgi complex