Cell Membranes and Organelles PDF

CELL MEMBRANES AND ORGANELLES Dr. Selma Yılmaz Department of Medical Biology What are the cells? Figure 1. Transport proteins in the cell membrane  Cells are small A plasma membrane is permeable to specific...

CELL MEMBRANES AND ORGANELLES Dr. Selma Yılmaz Department of Medical Biology What are the cells? Figure 1. Transport proteins in the cell membrane  Cells are small A plasma membrane is permeable to specific molecules that a cell needs. Transport proteins membrane-bounded in the cell membrane allow for selective passage of specific molecules from the external Compartments filled environment. Each transport protein is specific with to a certian molecule (indicated by matching colors). © 2010 Nature Education a concentrated aqueous solution of chemicals. TWO GENERAL TYPES OF CELLS: A. Prokaryotic ("before nucleus") cells: Found in prokaryotes. B. Eukaryotic ("true nucleus"): Found in eukaryotes. Characteristics of Prokaryotic & Eukaryotic Cells All cells have: 1. Cell or plasma membrane (separates the cell from the outer environment) 2. Genetic material (DNA) 3. Cytoplasm.  WHAT MAJOR COMPONENTS Fgur 2. The composition of a bacterial cell. Most of a cell is water (70%). The remaining 30% contains varying proportions of structural and functional molecules. DO CELLS HAVE? © 2010 Nature Education Water (H2O) is the most abundant molecule in cells (70% or more of total cell mass). Cells are composed of water, inorganic ions, and carbon-containing Organic molecules. Nucleic acids are the molecules that contain and help express a cell's genetic code. There are two major classes of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Proteins are a second type of intracellular organic molecule. These substances are made from chains of smaller molecules called amino acids, and they serve a variety of functions in the cell, both catalytic and structural. For example, proteins called enzymes convert cellular molecules (whether proteins, carbohydrates, lipids, or nucleic acids) into other forms that might help a cell meet its energy needs, build support structures, or pump out wastes. Carbohydrates, the starches and sugars in cells, are another important type of organic molecule. Simple carbohydrates are used for the cell's immediate energy demands, whereas complex carbohydrates serve as intracellular energy stores. Complex carbohydrates are also found on a cell's surface, where they play a crucial role in cell recognition. Finally, lipids or fat molecules are components of cell membranes — both the plasma membrane and various intracellular membranes. They are also involved in energy storage, as well as relaying signals within cells and from the bloodstream to a cell's interior. -Some cells also feature orderly arrangements of molecules called organelles. -One example is the mitochondrion in animals (chloroplasts in plants) — commonly known as the cell's "power plant" — which is the organelle that holds and maintains the machinery involved in energy-producing chemical reactions.  Plasma veya Hücre Membranı Eukaryotic cells Only a single membrane (the plasma membrane) surrounds the cell, but the interior contains many membrane-limited compartments, or organelles. Ultra structure of eubacterial cells Many prokaryotic cell membranes are similar to eukaryotic cell membranes. Prokaryotic cell lack membrane-bound organelles and internal membranes.  PLASMA OR CELL MEMBRANE  Plasma veya hücre membrane encloses the cytoplasm of any cell. Major functions of plasma membrane : 1. Protection: encloses the cytoplasm of any cell and defines the exterior of the cell; protects the cell and may be involved in cell movement, cell secretion and transmitting the impulses. 2. Regulation of passage of materials: transports and regulates what comes in and what goes out of the cell: controls the movement of molecules between the cytosol and the extracellular medium. It is selectively permeabl. Passage of materials across the membrane occurs by passive transport (e.g. Osmosis, simple difussion), facilitated diffusion (e.g. İon transport through ion channels), and active transport (e.g. Na-K pump), bulk transport (by exocytosis and receptor mediated endocytosis, phagocytosis) 3. Maintenance of differential distribution of ions: regulates and maintains the differential distribution of ions inside and outside of the cell (e.g. membrane potential) 4. Response to the environment: Responds to the changes in its environment with the help of receptor proteins. These proteins receive chemical messages from other cells and thus help in cell recognition, responding to hormones, growth factors and neuro transmitters. 5. Communication with the neighbouring cells: Maintains structural and chemical relationships with the neighbouring cells by the help of certain glikoproteins.  PLASMA OR CELL MEMBRANE Structure  Many prokaryotic cell membranes are similar to eukaryotic cell membranes.  Its structure is referred to as the Fluid Mosaic Model, because the structure behaves more like a fluid than a solid.  According to the fluid mosaic model, the membrane is viewed as a two-dimensional mosaic of phospholipid and protein molecules. Plasma or Cell Membrane Contains: Membrane Lipids: The Primary Membrane Lipids are Phospholipids (phospholipid bilayer: hydrophobic fatty acid tails & hydrophilic phosphate heads-), Glycolipids (glycosphingolipids) and Cholesterol. Membrane Proteins: (proteins float in the fluid lipid bilayer) Integral, peripheral, lipid-anchored, and glycoproteins.  PLASMA OR CELL MEMBRANE Differences are between prokaryotic and eukaryotic cell membrane: 1. In eukaryotic membranes, proteins involved in electron transport chain (ETC) and photosynthesis are not found in cell membrane, but are found in cytoplasmic organelles (mitochondria and chloroplast respectively). ETC is situated in prokaryotic cell membrane. 2. Eukaryotic cell membrane contains cholesterol (in prokaryotes, only mycoplasmas have cholesterol in their cell membrane).  BIOLOGIC MEMBRANES (BIOMEMBRANES) IN EUKARYOTIC CELL  The eukaryotic cell is separated from the external environment by the plasma membrane and organized into membrane-limited internal compartments (organelles and vesicles).  The total surface area of internal membranes far exceeds that of the plasma membrane Schematic overview of the major components of eukaryotic cell architecture  THE FACES OF CELLULAR MEMBRANES All cellular membranes have a cytosolic (an internal face, the side oriented toward the interior of the compartment) and an external face (the side presented to the environment). Three organelles—the nucleus, mitochondrion, and chloroplast—are enclosed by two membranes separated by a small intermembrane space. THE FACES OF CELLULAR MEMBRANES Eukaryotic Plasma Membrane  All biological membranes have the same basic phospholipid bilayer structure.  The watery interior of cells is surrounded by the plasma membrane, a two-layered shell of phospholipids.  Cholesterol (red) and various proteins (not shown) are embedded in the bilayer.  Like other membrane lipids, the steroid cholesterol is amphipathic: Spontaneously form either planar bilayers (micelle)or liposomes when dispersed in aqueous solutions. Amphipathic molecules form micelle or liposomes.  The bilayer structure of biomembranes Electron micrograph of a thin section through an erythrocyte membrane. THE PRIMARY MEMBRANE LIPIDS 1. Phospholipids a. Phosphoglycerides b. Sphingolipids 2. Glycolipids (Glycosphingolipids) 3. Cholesterol: All biological membranes also contains cholesterol and other steroids. PHOSPHOLIPIDS  Membrane lipids are composed primarily of phospholipid Molecules.  Phospholipids are polar, ionic compounds.  All biological membranes have the same basic phospholipid bilayer structure.  Phospholipids consist of one or two long fatty acids (hydrophobic hydrocarbon chains) linked to a hydrophilic head group. In a symmetrical two-layer phospholipid structure: Hydrophilic phosphate heads. The polar head groups of phospholipids are exposed to the aqueous medium. Hydrophobic fatty acid tails. The nonpolar hydrocarbon chains of the fatty acids are in the center. Phospholipids: Phosphoglycerides (Gliserofosfolipids) Sphingolipids Phospholipids GLYCEROPHOSPHOLIPIDS (PHOSPHOGLYCERIDES) Glycerol+2 fatty acyl chains+phosphate group+polar baş  Glycerophospholipids (Phosphoglycerides), the most abundant class of lipids in most membranes.  They contain a phosphate group. They have a glycerol backbone.  They are composed of an alcohol attached by a phosphodiester bridge to diacylglycerol (two fatty acyl chains attached to glycerol).  A typical phosphoglyceride molecule consists of two fatty acyl chains, a glycerol, and a polar head group attached to the phosphate group.  The 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).  A phosphogyceride is classified according to the nature of its head group. For example, a choline alcohol attached by a phosphodiester bridge to diacylglycerol produces phosphatidylcholine.  GLYCEROPHOSPHOLIPIDS (PHOSPHOGLYCERIDES) Gliserofosfolipid Phosphatidylcholine (PC) Phosphatidylethanolamine (PE) Phosphatidylserine (PS) Phosphatidylinositol (PI) SPHINGOLIPIDS Sphingosine+1 fatty acyl chain+phosphate+polar head  Sphingolipids are a class of lipids, contain a phosphate group.  Instead of a glycerol backbone, they contain sphingosine, an amino alcohol with a long unsaturated hydrocarbon chain.  Sphingolipids are composed of an alcohol attached by a phosphodiester bridge to a fatty amine, sphingosine.  In sphingolipids, a single fatty acid (hydrophobic hydrocarbon chain) is joined to sphingosine.  instead of a glycerol backbone, they contain sphingosine, an amino alcohol with a long unsaturated hydrocarbon chain.  Sphingomyelin (SM), a phospholipid that lacks a glycerol backbone, contains sphingosine, and phosphocholine,or phosphoethanolamine. Sphingomyelin is found mainly in the membranous myelin shetah that surrounds some nerve cell axons.  Sphingomyelin (SM), a phospholipid that lacks a glycerol backbone, contains sphingosine, and phosphocholine,or phosphoethanolamine. Sphingomyelin is found mainly in the membranous myelin shetah that surrounds some nerve cell axons. GLYCOLIPIDS Glycerol+2 fatty acyl chains+sugar veya Sphingosine+1 fatty acyl chain+sugar  Carbohydrates are found in many membranes.  They covalently bound to lipids as constituents of glycolipids.  They are generally found on the extracellular face of eukaryotic cellular membranes.  Glycolipids contain a sugar unit instead of a phosphate group.  Glycolipids consist of a single, long fatty acid (hydrophobic hydrocarbon chains) linked to a hydrophilic head group.  Glycolipids, in which a carbohydrate chain is attached to the glycerol or sphingosine backbone.  A ceramide, is composed of sphingosine and a fatty acid, a sphingosine attached to a long-chain fatty acid.  Ceramides are a family of waxy lipid molecules. TWO CLASSES OF GLYCOLIPIDS: Gangliosides (a multiple complex carbohydrate chains+ sphingolipid) and Cerebrosides (glucose or galactose + sphingolipid).  The simplest glycolipid, glucosylcerebroside, contains a single glucose unit attached to a ceramide.  Functions of Bound Carbohydrates to Lipids:  Increase the hydrophilic character of lipids and proteins.  Maintain stability of the membrane.  Facilitate cell–cell interactions.  Glycolipids and glycoproteins are involved in human blood- group antigens.  Glycolipids acts as receptors for viruses and other pathogens to enter cells.  Sphingolipids/Glycolipids Sphingomyelins, SM, Glucosylcerebroside, GlcCer CHOLESTEROL  Cholesterol and its derivatives constitute The third important class of membrane lipids, the steroids.  The basic structure of steroids is the 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.  Cholesterol is especially abundant in the plasma membranes of mammalian cells but is absent from most prokaryotic cells.  As much as 30–50 percent of the lipids in plant plasma membranes consist of certain steroids unique to plants.  Cholesterol is required for Membrane synthesis and Cholesterol metabolism (e.g. Bile acids synthesis and Steroid Synthesis).  Cholesterol is Required for Membrane Synthesis. Membranes: Cholesterol’s hydroxyl group can interact with water: membrane-membrane or protein-membrane interactions Cholesterol is also Required for Cholesterol Metabolism Cholesterol Metabolism.  Fluid Mosaic Model of Membranes: Membrane Fluidity  Phospholipids and sphingolipids are asymmetrically distributed in the two leaflets of the bilayer (Plasma membrane phospholipid asymmetry).  Cholesterol is fairly evenly distributed in both leaflets  Cholesterol is important in maintaining the fluidity of natural membranes and essential for normal cell growth and reproduction.  Natural biomembranes generally have a fluidlike consistency.  In general, membrane fluidity:  is decreased by sphingolipids and cholesterol and  is increased by phosphoglycerides  As a phospholipid bilayer is heated, it undergoes a phase transition from a gel-like to a more fluid state over a short temperature range  The lipid composition of a membrane also influences its thickness and curvature.  The lipid composition of a membrane also influences its thickness and curvature. Sphingomyelin (SM) Natural membranes generally Phosphatidylcholine (PC) has a low viscosity and a fluidlike consistency.  Fluid Mosaic Model of Membranes: Membrane Fluidity  The Phospholipid Composition Differs in Two Membrane Leaflets:  Most kinds of phospholipid, as well as cholesterol, are generally present in both membrane leaflets, although they are often more abundant in one or the other.  Most lipids and integral proteins are laterally mobile in biomembranes: Absolute asymmetry in protein orientation confers different properties on the two membrane faces  Proteins have never been observed to flip-flop across a membrane; Such movement would be energetically unfavorable.  In pure phospholipid bilayers phospholipids do not migrate, or flip-flop, from one leaflet of the membrane to the other.  In some natural membranes, however, phospholipids, occasionally do so (flip-flop), catalyzed by certain membrane proteins called flippases. Most lipids and integral proteins are laterally mobile in biomembranes.  In pure phospholipid bilayers phospholipids do not migrate, or flip-flop, from one leaflet of the membrane to the other.  In some natural membranes, however, phospholipids, occasionally do so (flip-flop), catalyzed by certain membrane proteins called flippases.  Lipid Rafts  Lipid rafts are microdomains containing cholesterol, sphingolipids, and certain membrane proteins that form in the plane of the bilayer.  These microdomains are sites for signaling across the plasma membrane.  Plasma Membrane Proteins The presence of specific sets of membrane proteins permits each type of membrane to carry out distinctive functions. Membrane Proteins 1. Membrane Proteins: (proteins float in the fluid lipid bilayer) a) Integral proteins - inserted in the bilayer; mainly involved in transport. 1.) carrier proteins - bind to specific substances & transport them across the cell membrane. 2.) channel proteins - proteins with a channel through which small, water soluble substances move across the cell membrane. b) Peripheral proteins - usually attached to membrane surface; some are enzymes; some are involved in the electron transport chain and/or photosynthesis (we’ll talk about these processes in the metabolism chapter); others are involved in the changes in cell shape that occur during cell division. c) Lipid-anchor proteins: three different type of proteins including prenylated proteins which are particularly important for eukaryotic cell growth, differentiation and morphology. 2. Glycoproteins EUKARYOTİC CELL PLASMA MEMBRANE PROTEINS: ıntegral, peripheral, lipid-anchored Hydrophobic Fatty acyl side chains MAJOR FUNCTIONS OF THE PLASMA MEMBRANE PROTEİNS  Plasma membranes transport nutrients into and metabolic wastes out of the cell and maintain the proper ionic composition, pH and osmotic pressure of the cytosol.  Specific transport proteins (e.g.integral proteins such as pumps, transporters, and ion channels) inserted in cellular membranes carry out these functions of plasma membranes..  Specific transport proteins move small molecules and ions into or out of the cell and its organelles and transport permit the passage of certain small molecules (e.g., simple sugars, amino acids, vitamins) and ions such as sodium ions) but not others. PLASMA MEMBRANE PROTEİNS SPECIFIC FUNCTIONS Membran proteinler also Anchor the membrane to intracellular cytoskeletal fibers and the extracellular matrix or to the cell wall (bacterial and plant cells); Act in the interactions (e.g. Reactions of plasma membrane enzymes); Act in the communication between cells (e.g. Plasma membrane receptor proteins bind specific signaling molecules such as hormones); Are critical for cell development and proper functioning of multicellular tissues.  Membranes Specific Functions Enzymes bound to the plasma membrane catalyze reactions that would occur with difficulty in an aqueous solutions. A. INTEGRAL (TRANSMEMBRANE) PROTEİNS – Inserted in the bilayer  Most are glycosylated with a complex branched sugar group attached to one or several amino acid side chains. Glycoproteins and glycolipids are found exclusively in the exoplasmic leaflet. Both glycoproteins and glycolipids are especially abundant in the plasma membranes of eukaryotic cells; they are absent from the inner mitochondrial membrane, chloroplast lamellae, and several other intracellular membranes.  All integral proteins and glycolipids bind asymmetrically to the lipid bilayer with respect to the cytosolic and exoplasmic faces. Such asymmetry is an essential aspect of the structure and function of biological membranes.  Integral proteins are mainly involved in transport.  Integral (Transmembrane) Proteins Are Mainly İnvolved İn Transport: 1. Transporter (carrier) proteins - bind to specific substances and transport them across the cell membrane (e.g. sugar transporters). Pumps – carrier proteins move molecules from low concentration into high concentration (against the concentration gradient, e.g. Sodium- potassium ATP ase pump) 2. channel proteins - proteins with a channel through which small,water soluble substances move across the cell membrane. Glikoporin A: A typical transmembrane protein  amino acids with hydrophobic (uncharged) side chains (red spheres)  the positively charged residues (blue spheres) anchor glycophorin in the membrane by binding negatively charged phospholipid head groups B. PERİPHERAL PROTEİNS - usually attached to membrane surface  Some are enzymes; some are involved in the electron transport chain and/or photosynthesis  Others are involved in the changes in cell shape that occur during cell division.  Peripheral membrane proteins do not interact with the hydrophobic core of the phospholipid bilayer; Are usually bound to the membrane indirectly by interactions with integral membrane proteins or directly by interactions with lipid head groups.  Peripheral proteins are localized to either the cytosolic or the exoplasmic face of the plasma membrane. A peripheral protein: Phospholipase C (PLC) Calcium signaling. G-protein-coupled receptors (GPCRs) signal to PLC-β via activation of heterotrimeric G proteins. PLCs transform Phosphatidylinositol 4,5- bisphosphate (PtdIns(4,5)P2 (PIP2)-) to Diacylglycerol (DAG) and inositol (1,4,5)-triphosphate [Ins(1,4,5)P3] (Adapted from Trends in Biochemical Science-2005) C. LİPİD-ANCHORED PROTEİNS - Lipid-anchored proteinler. are bound covalently to one or Anchoring of plasma-membrane more lipid molecules. proteins to the bilayer by The hydrophobic carbon chain covalently linked hydrocarbon groups of the attached lipid is embedded in one leaflet of the membrane and anchors the protein to the membrane. The polypeptide chain itself does not enter the phospholipid bilayer. Three different type of proteins including prenylated proteins which are particularly important for eukaryotic cell growth, differentiation and morphology. GLYCOPROTEINS  Carbohydrates are found in many membranes Glycoproteins/Glycolipids is  They covalently bound either to involved in Human ABO blood- proteins as constituents of glycoproteins group antigens or to lipids as constituents of glycoproteins or glycolipids.  Glycoproteins are generally found on the extracellular face of eukaryotic cellular membranes. Bound Carbohydrates to Proteins: Increase the hydrophilic character of lipids and proteins Maintain stability of the membrane Facilitate cell–cell interactions Glycolipids and glycoproteins are involved in human blood-group antigens  CELL MEMBRANE TRANSPORT A. Transport of water 1. Osmosis (often included in passive transport) B. Transport of ions and small molecules 2.Passive transport a. Simple diffusion b. Facilitated diffusion 3. Active transport C. Transport of large molecules (Bulk transport) a. Phagocytosis b. Endocytosis - Pinocytosis - Receptor mediated endocytosis)  CELL MEMBRANE TRANSPORT PASSIVE TRANSPORT 1. Selective permeability of the plasma membrane allows certain materials only to enter and exit cells. 2. Simple diffusion:Transport of ions and small molecules: Simple Diffusion of particles is powered by movement of the particles within a solution and results in a distribution of particles that move from their area of higher concentration to an area of lower concentration. Many molecules enter (or exit) cells by moving from an area of high concentration to low concentration. 3. The plasma membrane is always a barrier to access the cytoplasm and entry into cells requires membrane proteins. 4. Osmosis: Water enters and exits cells through osmosis and requires water channel proteins. 5. Facilitated diffusion: Transport of ions and small molecules: Ion channels facilitate movement specific ions into or out of cells, along the concentration gradient of the ions. Transport proteins catalyze the movement of specific solutes across plasma membranes by a process called passive transport or catalyzed transport (and sometimes called facilitated diffusion). Channels and transport proteins bind their solute with a certain affinity and catalyze the movement of their solute across the membrane in a saturable manner. Selective Permeability of the Plasma Membrane 1. Selective permeability of the plasma membrane allows certain materials only to enter and exit cells. SIMPLE DIFFUSION OF PARTICLES: Transport of ions and small molecules 2. Simple Diffusion of particles is powered by movement of the particles within a solution and results in a distribution of particles that move from their area of higher concentration to an area of lower concentration. Many molecules enter (or exit) cells by moving from an area of high concentration to low concentration. Particles move from their area of higher concentration to an area of lower concentration. Amphipathic Phospholipids As Barriers To Membrane Diffusion. 3. The plasma membrane is always a barrier to access the cytoplasm and entry into cells requires membrane proteins. Osmosis 4. Water enters and exits cells through osmosis and requires water channel proteins. In osmosis water crosses a semi- permeable membrane from an areaof higher water concentration to an area of lower water concentration. 5. MEMBRANE PROTEİNS FACİLİTATE TRANSPORT: PASSİVE TRANSPORT (FACİLİTATED DİFFUSİON) Individual membrane proteins (transmembrane proteins such as ion channels, transporters, and pumps) specifically bind to certain ligands (e.g., to chloride, to glucose, or to sodium and potassium) and facilitate their movement across the plasma membrane. Transmembrane proteins embedded within the plasma Membrane facilitate transport of molecules across the plasma membrane. Ion channels Ion channels facilitate movement specific ions into or out of cells, along the concentration gradient of the ions. The channel protein is inserted into the membrane such that hydrophilic amino acid residues of its structure form a path for the ion to enter the cell protected from the hydrophobic core of the phospholipids. Transport proteins Glucose Transporter Transport proteins catalyze the. movement of specific solutes across plasma membranes by a process called passive transport or catalyzed transport (and sometimes called facilitated diffusion). Channels and transport proteins bind their solute with a certain affinity and catalyze the movement of their solute across the membrane in a saturable manner. Glucose Transporter Glucose, an example of a molecule transported into cells by a specific transport protein, binds to its transporter with a certain affinity, Km. CELL MEMBRANE TRANSPORT: ACTIVE TRANSPORT Active transport involves the movement of ions or molecules against their concentration gradient in an energy-dependent process. 1. Primary active transport: requires the direct hydrolysis of ATP by the primary active transport protein.  Four classes of ATPases function in primary active transport.  P-class transporters move ions against their gradients, F-class and V-class transporters pump protons against their concentration gradients, and ABC- class transporters move drugs, ions, and xenobiotics against their gradients. 2. Secondary active transport: The transport of one solute is coupled to and dependent upon the transport of another solute, this process is described as cotransport. Transport proteins that function in secondary transport do not possess ATPase activity. Instead, they depend indirectly on ATP hydrolysis since it is required for the primary active transport that establishes the ion gradients that power secondary active transport. Cotransporters can be symporters or Antiporters. Symporters: are secondary active transporters that move substrates in the same direction across plasma membranes. Antiporters: cotransport molecules in opposite directions across plasma membranes. PRIMARY ACTIVE TRANSPORT Active transport involves the movement of ions or molecules against their concentration gradient in an energy-dependent process. 1. Primary active transport: requires the direct hydrolysis of ATP by the primary active transport protein. Four classes of ATPases function in primary active transport: P-class transporters move ions against their gradients, F-class and V- class transporters pump protons against their concentration gradients, and ABC- class transporters move drugs, ions, and xenobiotics against their gradients. PRIMARY ACTIVE TRANSPORT Four classes of primary active transporters. The sodium-potassium ATPase pump. ABC-class transporters have ATPbinding cassettes. SECONDARY ACTIVE TRANSPORT 2. Secondary active transport: The transport of one solute is coupled to and dependent upon the transport of another solute, this process is described as cotransport. Transport proteins that function in secondary transport do not possess ATPase activity. Instead, they depend indirectly on ATP hydrolysis since it is required for the primary active transport that establishes the ion gradients that power secondary active transport. Cotransport of substrates in secondary active transport. A substrate is moved against its concentration gradient by the power (enerji) of the ion gradient that moves from high to low concentration. SECONDARY ACTIVE TRANSPORT Cotransporters can be symporters or Antiporters. Symport involves cotransport of Antiport involves cotransport of substrates in the same direction substrates in opposite directions across membranes. across membranes. PRIMER AND SECONDARY ACTIVE TRANSPORT Cotransporters can be symporters or Antiporters. Cotransport (also known as symport) and exchange (also known as antiport).  2. Cell membranes and organelles-Review Questions What is the cell membrane composed of? Basic structure of a cell membrane bilayer? How phospholipids are positioned in a symetrical phospholipid bilayer? What is a Fluid mosaic model of cell membranes? Function of the cell membranes? Main functions and specific functions? What are the main membrane lipids? What is the differences between fosfogliserides ,sphingolipids and glycolipids? What are the examples of gliserophosholipids,sphingolipids and glycolipids? What is the cholesterol and its function in the cell membranes? Why is the cholesterol is an amphipathic molecule? How cholesterol interacts with water? How cholesterol and other phospholipids affect the membrane fluidity, thickness and curvature? Which of the prokaryotes do have cholesterol in its cell membrane and not have a cell wall?  2. Cell membranes and organelles-Review Questions What are the membrane proteins? How are they placed in a cell membrane? Functions of membrane proteins? Main functions of of integral and peripheral proteins? What are the function of carbohydrate groups of glycoproteins or glycolipids? How is the asymmetrical distribution of many membrane lipids and proteins? What is the importance of the cell membrane asymetrical distribution of many membrane lipids and proteins? Can membrane phosholipids and membrane proteins flip-flop? Can membrane proteins laterally move? What are the functions of membrane proteins in the membrane transport, in passive and active transport? For example: Transporters, ion channels, in the passive transport and ATPases pumps in active transport. What type of proteins synthesized on the bound or free ribosomes (where to go-trafficking )When Ribosomes bind to endoplasmic reticulum and when they remain free? What are the two types of secretions of proteins from the cells?