Chapter 4 General Features of Cells PDF

CHAPTER 4 GENERAL FEATURES OF CELLS 0  Cell theory 1. All living things are composed of one or more cells 2. Cells are the smallest units of living organisms 3. New cells come only from pre-existing cells by cell division...

CHAPTER 4 GENERAL FEATURES OF CELLS 0  Cell theory 1. All living things are composed of one or more cells 2. Cells are the smallest units of living organisms 3. New cells come only from pre-existing cells by cell division 0  Microscopy Most cells are so small that they cannot be seen with naked eye. The microscope is a magnification tool that enables researchers to structure and function of cells. Magnification, resolution and contrast are the three important parameters in microscopy. Magnification Loading… Ratio between the size of an image produced by a microscope and its actual size Resolution Ability to observe two adjacent objects as distinct from one another Contrast Ability to visualize a particular cell structure may depend on how different it looks from an adjacent structure. The contrast can be enhanced using dyes 0  Microscopy Microscope are categorized into two groups based on the source of illumination Light microscope Uses light for illumination Resolution 0.2 µm Electron microscope Uses an electron beam Resolution 2 nm 0  Cell types Based on cell structure, there are two types of cells 1. Prokaryotic Loading… (bacteria and archaea) 2. Eukaryotic 0  Prokaryotic cells Prokaryotes [grk. Pro, before, and karyon, nucleus] Simple cell structure Lack a membrane-enclosed nucleus Much smaller than eukaryotic cells Present in great numbers in the air, in bodies of water, in the soil, and even in the human organs Two categories: bacteria and archaea Both small Bacteria are abundant, most not harmful Archaea are less common, often found in extreme environments 0  Prokaryotic cell structure All prokaryotic Additional in cells contain: some 1. Cell wall 1. Capsule 2. Cell membrane 2. Plasmids 3. Cytoplasm 3. Flagella 4. DNA 4. Pili 5. Ribosomes 5. Membrane infolding 0  Typical bacterial cell Cell envelope Cytoplas Appendage 0 m s Structure of a Prokaryotic Cell: Cell Envelope Cell envelope includes plasma membrane, cell wall and glycocalyx (layer of polysaccharides outside cell wall). Plasma membrane: A phospholipids' bilayer much like the plasma membrane of eukaryotic cells. Has an important role in regulating the entrance and the exit of substances into the cytoplasm. Cell wall: Composed of a complex molecule peptidoglycan (amino disaccharides and peptide fragments). The cell wall maintains the overall shape of a bacterial cell (coccus, bacillus and spiral). Mycoplasma are bacteria that have no cell wall and therefore have no definite shape. It is above plasma membrane. Based on cell wall bacteria are either gram positive or gram negative Glycocalyx: Layer of polysaccharide lying outside the cell wall. Glycocalyx aids against drying out by traping water and help bacteria to resist a host’s immune system. It allows the bacterium to attach itself to inert surfaces (like teeth or rocks), eukaryotes (e.g. streptococcus pneumoniae attaches itself to lung cells), or other bacteria (their glycocalyxes can fuse to envelop the colony). Secreted from inside of the cell and forms a layer outside. It is viscous. Structure of a Prokaryotic Cell: Cell Envelope Glycocalyx or Capsule: 1.When glycocalyx firmly attached then is it called capsule else it is a slime layer 2.Well organized layer of polysaccharide. 3.Protects the bacterial cell and is often associated with pathogenic bacteria because it serves as a barrier against phagocytosis by white blood cells. 4.Made from starch or glycolipid 5.Protect bacteria from drying out which is called desiccation 6.Stop detection from immune system. Protect from viruses. 7.Also has adhesion properties Prokaryotic cell structure… No nucleus Nucleoid floating in cytoplasm One chromosome which is circular. Loading… No histone proteins with DNA. DNA is called naked DNA because there is no histone protein associated with DNA 0 Prokaryotic plasmid Smaller circular DNA Contains resistance genes, which can be transferred to other organisms Non-essential genes Replication is independent of chromosomal DNA. 0 Ribosome in prokaryotes Smaller than eukaryotic ribosome 70 S 30 s and 50 s subunits Make proteins 0 0  Structure of a Prokaryotic Cell: Cytoplasm Cytoplasm is semi-fluid solution encapsulated by the plasma membrane. Contains all sort of enzymes required for bacterial metabolism. Single chromosome located in gel-like region called nucleoid. Thousands of ribosomes involved in protein synthesis. Inclusion Bodies which are Storage granules of various substances. Some are nutrients that can be broken down when needed.  Structure of a Prokaryotic Cell: Appendages Bacteria may have the following appendages: Flagella: Responsible for most types of bacterial motility. Flagella are long appendages which rotate by means of a "motor" located just under the plasma membrane. Bacteria may have one, a few, or many flagella in different positions on the cell. Fimbriae: Small fibers that sprout from the cell surface. Not involved in the motility. Help bacteria to attach to a surface. Sex Pili: Rigid tubular structure used by bacteria to pass DNA from a cell to cell. Bacteria reproduce asexually by binary fission, but they can exchange DNA through the sex pili. 0  Eukaryotic cells DNA housed inside nucleus Eukaryotic cells exhibit compartmentalization into organelles: Organelle is subcellular structure or membrane-bounded compartment with its own unique structure and function Shape, size, and organization of cells vary considerably among different species and even among different cell types of the same species 0  The Proteome Determines the Characteristics of a Cell How does a single organism produce different types of cells? Identical DNA in different cells but different proteomes The proteome of a cell determines its structure and function Gene regulation, amount of protein, amino acid sequence of a particular protein and protein modification can influence a cell’s proteome Proteomes in healthy cells are different from the proteomes of cancerous cells Proteome is a set of expressed proteins in a given type of cell or organism, at a given time, under defined conditions. Cytosol Region of a eukaryotic cell that is outside the cell organelles but inside the plasma membrane Cytoplasm includes everything inside the plasma membrane.Cytosol, the endomembrane system and the semiautonomous organelles Metabolism Cytosol is central coordinating region for many metabolic activities of eukaryotic cells Catabolism: breakdown of a molecule into smaller components Anabolism: synthesis of cellular molecules and macromolecules 0  Cytoskeleton Serves as internal skeleton that maintains cell shape (construction and organization) and assists in movement of its parts. Strengthen cell. Holds organelles in place. Contains three types of elements: Microtubules: -Long cylindrical structures composed of polymers of alpha and beta tubulin. -Alpha and beta form dimer and they pair with each other to form a sheet like structure which is further folded into a cylinder shape structure. - They have polar structure with a plus end and minus end. - A single microtubule can oscillate between growing and shortening phases: Dynamic instability. - They play key roles in:. Intracellular transport (associated with dyneins and kinesins, they transport organelles like mitochondria or vesicle).. The axoneme of cilia and flagella.. The mitotic spindle.. Synthesis of the cell wall in plants. Intermediate filaments: -Tend to be more stable than microtubules and actin filaments, which readily polymerize and depolymerize -Function in the maintenance of cell-shape and rigidity by bearing tension -Found in cytoplasm, in nucleus and also outside cell so binding cells together. -Keratin family proteins Actin filaments (also known microfilaments): -composed of two intertwined actin chains -actin filament - Actin filaments support the plasma membrane and provide strength and shape to the cell. - Participate in some cell-to-cell or cell-to- matrix junctions. - Phagocytosis - Tensile strength 0 Cytoskeleton function Maintaining cell shape Microfilament: cytoplasmic streaming. Circulating cytosol so that all organelles receive cytoplasm. Cytokinesis of animal cells. Helps in cell division. Microtubule Segregation of chromosome during cell division anaphase Interact with centromere for separating sister chromatid Helps in motility. Are part of flagella, cilia. Intermediate filament: Helps in forming stable tissues. As it also appear outside the cell, it helps in cell to cell interaction 0  Motor Proteins Category of cellular proteins that use ATP as a source of energy to promote movement Consist of three domains called the head, hinge, and tail Walking analogy Ground is a cytoskeletal filament, your leg is the head of the motor protein, and your hip is the hinge Three different kinds of movements:. Motor protein moves the cargo from one location to another (kinesin). Motor protein can remain in place and cause the filament to move (myosin). Motor protein attempting to walk (both the motor protein and filament restricted in their movement) exerts a force that causes the filament to bend (dynein) Convert chemical energy into mechanical energy by hydrolysis of ATP https://www.youtube.com/watch?v=y-uuk4Pr2i8 0  Motor Proteins: Flagella and cilia. Flagella usually longer than cilia and present singly or in pairs. Cilia are often shorter than flagella and tend to cover all or part of the surface of a cell. Share the same internal structure. Microtubules, dynein, and axoneme. Microtubules form an arrangement called a 9 + 2 array. Movement involves the propagation of a bend, which begins at the base of the structure and proceeds toward the tip 0  Nucleus Contains chromatin in semi-fluid called nucleoplasm. Chromatin undergo condensation to form chromosomes just before the cell divides Chromatin contains DNA, diverse proteins and some RNA. -Separated from cytoplasm by double-membrane (outer and inner membrane) called nuclear envelope. - Pores in nuclear membrane allows transport of protein, RNA to cytoplasm -Primary function involves the protection, organization, and expression of the genetic material -Ribosome assembly occurs in the nucleolus -Nucleoplasm which is fluid inside nucleus 0  Endomembrane system Network of membranes enclosing the nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles Also includes plasma membrane May be directly connected to each other or pass materials via vesicles Restrict enzymatic reactions to specific compartments within cell. 0 Endomembrane system:Nuclear envelope -Double-membrane structure enclosing nucleus -Outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum membrane - Nuclear pores provide passageways permitting passage in and out of the nucleus - Materials within the nucleus are not part of the endomembrane system 0  Endoplasmic reticulum Network of membranes that form flattened, fluid-filled tubules or cisternae ER membrane encloses a single compartment called the ER lumen Physically continuous with the outer membrane of the nuclear envelope. Rough endoplasmic reticulum (rough ER) Is studded with ribosomes on the side of the membrane that faces the cytoplasm. Involved in protein synthesis and sorting. Rough ER modifies proteins after they have entered the ER lumen. ER enzymes add carbohydrates chain to protein. Other enzymes assist the folding process that result in the final shape of the protein. Smooth endoplasmic reticulum (smooth ER). Is continuous with rough ER. No attached ribosomes.. Detoxification, carbohydrate metabolism, calcium balance, synthesis and modification of lipids 0 Endoplasmic reticulum Loading… 0  Golgi apparatus Also called the Golgi body, Golgi complex, or simply Golgi Stack of flattened, membrane-bounded compartments, which are not continuous with the ER. Roles in post translational modification In animal cells, the inner face is directed toward the ER, and the outer face is directed toward the plasma membrane. Modifies proteins and lipids and packages them in vesicles. Vesicles transport materials between stacks Three overlapping functions: Secretion, processing, and protein sorting 0  Lysosomes. Membrane-bounded vesicles produced by the Golgi apparatus.. They have a very low pH and contain a powerful hydrolytic digestive enzymes (acid hydrolases) to break down proteins, carbohydrates, nucleic acids and lipids.. They digest food particles, and engulfed viruses or bacteria through endocytosis.. They digest excess or worn out organelles througn endocytosis “autophagy”.. The membrane surrounding a lysosome prevents the digestive enzymes inside from destroying the cell. Involved in autophagy (self death) 0  Vacuoles Functions of vacuoles are extremely varied, and they differ among cell types and even environmental conditions Central vacuoles in plants for storage and support Contractile vacuoles in protists for expelling excess water Phagocytic vacuoles in protists and white blood cells for degradation 0  Peroxisomes Relatively small organelles found in all eukaryotic cells General function to catalyze certain chemical reactions, typically those that break down molecules by removing hydrogen or adding oxygen Long fatty acid chains β-oxidation Reaction by-product is hydrogen peroxide (H2O2): Short & medium fatty acid chains [RH2 + O2 R + H2O2] Hydrogen peroxide is immediately broken Mitochondria down to water and oxygen by another peroxysomal enzyme called catalase. Enzymes in peroxisome are cell-specific. CO2 + H2O In liver peroxisome produce bile salts from cholesterol and others break down fats 0  Plasma membrane - Boundary between the cell and the extracellular environment - Membrane transport in and out of cell. Selectively permeable - Cell signaling using receptors - Cell adhesion 0  Semiautonomous organelles Semiautonomous because they divide by fission to produce more of themselves Somewhat independent Genetic material, synthesize some proteins, divide independently of cell Not entirely autonomous. Do depend on the cell for raw materials and most of their proteins Mitochondria, chloroplasts, 0 Involved in cellular respiration. Mitochondria Carbohydrate + oxygen carbon dioxide + water + energy (ATP)Adenosine triphosphate (ATP) is used for all energy-requiring processes in cells. Mitochondria produce most of ATP utilized by the cell. Mitochondria have two membranes, the outer and the inner membrane. The inner membrane that encloses matrix is called Cristae. The Matrix contains enzymes that break down carbohydrates and other nutrient molecules. Also involved in the synthesis, modification, and breakdown of several types of cellular molecules 0  Mitochondria and chloroplasts Two traits similar to bacteria 1. Contain DNA separate from the nuclear genome Mitochondrial and chloroplast genome. Single small circular double stranded chromosome.. Similar to bacterial chromosomes 2. Reproduce via binary fission (splitting in two) Like bacteria 0  Chloroplasts Photosynthesis: capture light energy and use some of that energy to synthesize organic molecules such as glucose: Solar energy + Carbon dioxide + water carbohydrate + oxygen Found in nearly all species of plants and algae. Chloroplasts are green due to the green pigment chlorophyll. Chloroplast have an outer and an inner membrane separated by a small space. The material within the chloroplast is called the stroma which contains a concentrated mixture of enzymes. Within the stroma are stacks of thylakoids, the sub-organelles which are the site of photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum). 0  CHAPTER 5: MEMBRANE STRUCTURE AND TRANSPORT 5.1 Membrane Structure 5.2 Synthesis of Membrane components in Eukaryotic cells 5.3Membrane Transport 0  Biological Membranes Biological Membranes (Cellular membranes or Biomembranes) Basic framework of the membrane is the phospholipid bilayer Phospholipids are amphipathic molecules. Hydrophobic region faces in. Hydrophilic region faces out Membranes also contain proteins and carbohydrates Relative amount of each vary 0 Lipids: Fats Loading… Lipids: Phospholipids Phospholipids: membrane components, contains phosphate group. Instead of third fatty acid attached to glycerol as in fat, there is a polar phosphate group. Amphipathic molecule Hydrophilic heads (phosphate region) hydrophobic tails ( Fatty acid chains) Arrange themselves so polar heads are adjacent to water. Bulk of cell plasma membrane consists of phospholipid bilayer. 0  Fluid-mosaic model Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules Membrane exhibits properties that resemble a fluid because lipids and proteins can move relative to each other within the membrane Loading… 0  Membrane is fluid Constituents of the membrane such as phospholipid and proteins are in motion. They move in lateral direction in a leaflet Some proteins are attached to other structure in either side of the membrane and those proteins are relatively immobile Phospholipids can also flip-flop (transverse diffusion), though this type of movement is less quickly and requires energy. 0  Proteins bound to membranes Although the phospholipid bilayer forms the basic foundation of the cellular membrane, the protein component carries out most other function. Proteins can bind to the membrane in three different ways. Integral membrane proteins (Intrinsic membrane proteins) 1. Transmembrane proteins One or more regions that are physically embedded in the hydrophobic region of the phospholipid bilayer 2. Lipid anchors Covalent attachment of a lipid to an amino acid side chain within a protein Peripheral membrane proteins (Extrinsic membrane proteins) Non-covalently bound to regions of integral membrane proteins that project out from the membrane, or they are bound to the polar head groups of phospholipids 0  ~25% of All Genes Encode Membrane Proteins Membranes are important biologically and medically Computer programs can be used to predict the number of membrane proteins 20–30% of all genes may encode membrane proteins; this is found throughout all domains of life including archaea, bacteria, and eukaryotes Function of many genes is still unknown, study may provide better understanding and better treatments Membranes are semifluid Rot atio Biomembranes exhibit properties of fluidity. nal mo Fluidity: individual molecules remain in close association ve yet have the ability to readily move within the membrane. me nt Lat Biomembranes are Semifluid: most lipids can rotate eral freely around their long axes and move laterally within mo the membrane leaflet. (two-dimensional motion). ve me Rotational and lateral movement keep the fatty acid tails nt within the hydrophobic interior. Such movement does not need energy. At 37 C lipid molecules exchanges places with its neighbors about 107 times/sec. In contrast to rotational and lateral movements, the “Flipflop” of lipids from one leaflet to the opposite leaflet does not occur spontaneously. The transport of lipids from one leaflet to another requires the action of the enzyme flippase, which uses energy from the hydrolysis of ATP to flip a lipid from one leaflet to another 0  Plasma membrane fluidity Plasma membranes are fluid in nature They can be determine by the movement of molecules such as phospholipid and proteins. more fluid means more movement of plasma membrane molecules and less movement will make the membrane rigid. In more fluid plasma membrane, attraction between molecules are not very strong compared to rigid plasma membrane. Rigid structure will have higher melting temperature. 0  Factors affecting fluidity The biochemical properties of the phspholipids have a profound effect on the fluidity of the phospholipid bilayer. 1. Length of fatty acyl tails: range from 14 to 24 carbon atoms Shorter acyl tails are less likely to interact, which makes the membrane more fluid Loading… fluid rigid 0  Factors affecting fluidity 2. Presence of double bonds in the acyl tails (unsaturated): Double bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid 0  Factors affecting fluidity 3. Presence of cholesterol (short and rigid molecule produced by animal cells): Cholesterol tends to stabilize membranes Effects depend on temperature. At higher temperature (mammals), cholesterol makes the membrane less fluid. At lower temperature, cholesterol makes the membrane more fluid and prevents it from freezing. 0  Experiments on lateral transport Frye and Edidin conducted an experiment verifying the lateral movement of membrane proteins Mouse and human cells were fused Temperature treatment at 0°C or 37°C Mouse membrane protein H-2 fluorescently labeled At 0°C cells the label stays on mouse side At 37°C cells label moves over entire cell 0  Not all integral membrane proteins can move Depending on the cell type, 10–70% of membrane proteins may be restricted in their movement Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally Also, membrane proteins may be attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix 0  Glycosylation Glycosylation refers to the process of covalently attaching a carbohydrate to a protein or lipid Glycolipid: carbohydrate attached to lipid Glycoprotein: carbohydrate attached to protein Glycosylation has functional consequences: - Carbohydrates attached to lipids and proteins can serve as recognition signals for other cellular proteins. Example: glycoproteins carrying a mannose-6- phosphate sugar a recognized as protein to be targeted to the lysosome. - Membrane glycoproteins and glycolipids often play a role in cell surface recognition - Carbohydrates have a protective effect. Cell coat or glycocalyx - carbohydrate-rich zone on the cell surface shielding cell from mechanical and physical damage. 0  Membrane structure can be viewed with an electron microscope Transmission electron microscopy (TEM), uses a biological sample that is thin sectioned and stained with heavy metal dyes such as osmium tetroxide. Osmium tetroxide binds tightly to the polar head groups of phospholipids, but it does not bind well to the fatty acyl chains 0  Transmembrane are first inserted into the ER membrane If a sequence within the polypeptyide contains a stretch of 20 amino acids that are mostly hydrophobic, this region will become a transmembrane segment. After the ER signal sequence is cleaved, this will create , a membrane protein with a single transmembrane segment. Some polypeptide may contain two or more transmembrane segment. Each time a polypeptide sequence contains a stretch of 20 hydrophobic a.a., an additional transmembrane segment is synthesized into the membrane.  Glycosylation Glycosylation is the attachment of a carbohydrate to a protein producing a glycoprotein. Carbohydrates may also be attached to lipids by glycosylation. In proteins, glycosylation may aid in protein folding, protect a protein from extracellular factors that could harm its structure. Glycosilation may also play a role in protein sorting. Example: protein destined to the lysosome have attached carbohydrate that serves as a sorting signal. Two forms of glycosylation occurs in eukaryotes: 1. N-linked glycosylation 2. O-linked glycosilation 0  glycosylation Protein modification happens during the process of protein synthesis – post translational. Asp-X-Thr-Ser consensus sequence N linked glycosylation: starts in ER and continues in Golgi, sugar chain is composed of 10-12 sugar residues O linked glycosylation: happens in Golgi Contains short chain of sugar residue (1-4 residues) Pre-formed oligosaccharide chain (Glc3Man9GlcNAc2) is present in ER. This chain is formed in cytoplasmic side as well as in ER side. They are attached to anchor (chain of multi isoprene units called as Dolichol phosphate) which are present in ER. 0 glycosylation N-Acetylglucosamine (GlcNAc) 0  Plasma Membrane Permeability The plasma membrane is a selectively permeable barrier between the cell and its external environment. Some substances can move across and some cannot. The PM structure ensures: Essential molecules (glucose and a.a.) enter a cell; Metabolic intermediates remain in the cell; Waste products exit. The phospholipid bilayer is a barrier to the diffusion of hydrophilic substances Hydrophobic interior makes formidable barrier to the movement of ions and hydrophilic molecules. 0 Diffusion: Process of diffusion. Diffusion represents the movement of molecules from a higher to a lower concentration until equilibrium is reached. The molecules become equally distributed.. Down concentration gradient. A solution contains both a solute (solid) and a solvent (liquid).. Solute movement from higher conc. to lower conc.. Passive diffusion occurs without transport protein. Solutes vary in their rates of penetration Diffusion is the process by which oxygen enter cells and the carbon dioxide exit cells 0  Cells maintain gradients across their membranes Diffusible molecules. Phospholipid bilayers are quite impermeable to ions and most hydrophylic molecules.. Living cells have the ability to maintain relatively constant internal environment that is distinctively different from their external environment.. Involves the establishment of gradients of solutes across the PM and organellar membrane. Transmembrane gradient Concentration of a solute is higher on one side of a membrane than the other Non-diffusible Ion electrochemical gradient molecules Gradients involving ions have two components electrical and chemical gradient 0  Passive Transport Passive transport refers to the diffusion of a solute across a membrane in a process that does not require an input of energy (ATP). Passive transport can occur in two ways: Passive diffusion: no carrier proteins required; Facilitated diffusion: requires carrier proteins. In passive transport, molecules follow concentration gradient (direction toward lower concentration). Active Transport: Requires both carrier protein and ATP. 0  The process of Osmosis Osmosis: Diffusion of water across a differentially (selectively) permeable membrane due to concentration differences. Diffusion of water across the membranes balances solute concentration. Diffusion always occurs from higher to lower concentration. The tendency of water to move into any cell creates an osmotic pressure. Osmotic pressure is defined as the hydrostatic pressure required to stop the net flow of water across a membrane due to osmosis.  The process of Osmosis: Tonicity Isotonic Solute and water concentrations both inside and outside the membrane are equal (No water movement in any direction). Hypertonic - Solution with a higher concentration of solute than the solution on the other side of the membrane. - Cells placed in a hypertonic solution will shrink as the water leave the cells. -Crenation refers to the shrinking of the cytoplasm due to osmosis. Hypotonic - Solution with a lower concentration of solute than the solution on the other side of the membrane. - Cells placed in a hypotonic solution will swell as the water enters, this which will lead to cell lysis. 0  Osmosis in animal cells Animal cells must maintain a balance between extracellular and intracellular solute concentrations to maintain their size and shape Crenation is the shrinking in a hypertonic solution 0  Osmosis in plant cells A cell wall prevents major changes in cell size In plant cell, osmotic pressure is called turgor pressure or cell turgor. Turgor pressure: pushes plasma membrane against cell wall - Maintains shape and sizeLoading… Plasmolysis: plants wilt because water leaves plant cells 0  Transport proteins Transport proteins enable biological membranes to be selectively permeable There are two classes of transport proteins: channels and transporters. Channels: Transmembrane proteins that form an open passageway for the facilitated diffusion of ions or molecules across the membrane. Solutes move directly through a channel to get to the other side. Example: Aquaporins is a channel that allows the movement of water accross the membrane. Most channels are gated, can open or close.. Ligand-gated. Intracellular regulatory proteins. Phosphorylation. Voltage-gated. Mechanosensitive channel 0  Transporters (carriers): Also, involved in the passage of molecules through the membrane, by combining with a substance and helping it cross the membrane. Bind their solute in a hydrophilic pocket and undergo a conformational change that switch the exposure of the pocket to the other side of the membrane Transporters provide the principal pathway for the uptake of organic molecules, such as sugars, amino acids, and nucleotides Key role in export by releasing of waste products from the cell. Transporter types 1. Uniporter: single molecule or ion. glucose transporter (GLUT). 2. Symporter/ cotransporter: 2 or more ions or molecules transported in same direction. Na+/glucose cotransporter (SGLT1) 3. Antiporter: 2 or more ions or molecules transported in opposite directions 0 Na+/Ca+ antiporter 4. Pump: Transporter that directly couples its conformational changes to an energy source, such as ATP hydrolysis. A common category of pumps found in all living cells are ATP-driven pumps which have a binding site for ATP. Energy released from ATP hydrolysis can be used to pump solute against a gradient. Pumps can be uniporters, symporters, or Antiporters. Active transport 0  Active transport Active transport is a movement of a solute across a membrane against its concentration gradient by combining with carrier proteins. Energetically unfavorable and requires the input of energy Primary active transport: Directly involves the functioning of pumps that directly use energy to transport solute against a gradient. Secondary active transport: Use pre-existing gradient to drive transport of solute. H+/sucrose symporter can utilize an H+ electrochemical gradient established by an ion pump , to move sucrose against its concentration gradient. 0  ATP-Driven Ion Pumps Generate Ion Electrochemical Gradients - Na+/K+-ATPase pump actively transport Na+ and K+ against their gradient by using the energy from ATP hydrolysis. - For each ATP hydrolyzed, the Na+/K+-ATPase pump function as an antiporter that pumps 3 Na+ outside (exported) the cell and 2 K+ into (imported) cell. - Na+/K+-ATPase pump is also called electrogenic pump because for each cycle of pumping results in the net export of one positive charge and alsoproduce an electrical gradient across the membrane. Antiporter Electrogenic pump- export 1 net positive charge 0 Membrane-Assisted Transport: Exocytosis Large marcomolecules (i.e. polypeptides, polysaccharides or polynucleotides) which are too large to be transported by carrier proteins, are transported into or out of the cell by vesicle formation. Exocytosis: process by which material inside the cell, which packaged into vesicles, is excreted into the extracellular environment. During exocytosis, vesicles fuse with PM as secretion occurs. Often, these vesicles have been produced by Golgi apparatus and contains proteins. During exocytosis, the membrane of the vesicle becomes a part of the PM, which is thereby enlarged. Examples: Insulin is released by pancreatic cells, upon a rise in blood sugar, by exocytosis. Membrane-Assisted Transport:Endocytosis Endocytosis: Cells take in substances by vesicle formation. During endocytosis, the PM invaginates, or folds inward, to form vesicle that brings substances into the cell. Endocytosis occurs in one of the three ways: Phagocytosis: - Process specific for large, solid material (food particle or another cell). - Certain types of human white blood cells are able to engulf debris such as worn out blood cells or bacteria. When an endocytic vesicle fuses with a lysosome, digestion occurs. Pinocytosis: - Occurs when vesicles form around a liquid or small, solid particles. Receptor-Mediated: - Specific and selective form of pinocytosis using a receptor protein shaped in such a way that a specific molecule such as a vitamin, peptide hormone or lipoprotein can bind to it.