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5/22/2017 Yust-Averett CELL BIOLOGY 1 Part II: Cell Membrane and Cellular Transport 5/22/2017 Yust-Averett 2 5/22/2017 Yust-Averett 3 CELLULAR MEMBRANE Cell membrane called plasma membrane Separates cell from its surroundings Yust-Averett 5/22/2017 Also 8 nm thick Need 8,000 to equal thickness of a piece of paper Supports the cell Protects the cell Controls what enters and exits the cell Selectively permeable Different membranes with different functions differ in their chemical composition and structure 4 CELL MEMBRANE Mostly made of proteins and lipids are used for transport Lipids are mostly phospholipids Creates a bilayer Molecular arrangement shelters the hydrophobic tails from water while exposing the hydrophilic heads to water Some Yust-Averett 5/22/2017 Proteins lipids are cholesterol molecules Help strengthen the cell membrane Also contains carbohydrates Used for cell to cell recognition Carbohydrates can attach to lipids creating glycolipids or attach to proteins creating glycoproteins 5 FLUID MOSAIC MODEL mosaic model refers to the nature of cell membranes 5/22/2017 Fluid Yust-Averett Fluid structure with a ‘mosaic’ of various molecules embedded in or attached to the bilayer of phospholipids. Mosaic - a pattern made of numerous small pieces fitted together 6 MEMBRANES ARE FLUID are not molecules locked rigidly in place 5/22/2017 Membranes Yust-Averett The lipids and proteins can shift laterally This happens frequently and rapidly Phospholipids switch places 107 times per second Proteins are larger and move more slowly but some do shift their positions 7 TEMPERATURE AND MEMBRANES Membranes must be fluid to work 5/22/2017 properly They Yust-Averett are usually about as fluid as salad oil Temperature can affect fluidity Fluidity will decrease as temperature decreases until phospholipids become a closely packed solid Cholesterol molecules buffer the effect of temperature changes Restrains the movement of phospholipids but also prevents them from packing together too tightly. 8 5/22/2017 Yust-Averett 9 MEMBRANE PROTEINS AND THEIR FUNCTIONS Proteins determine the membrane’s function Different types of cells contain different types of membrane proteins. Yust-Averett membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer 5/22/2017 A More than 50 different types of proteins in just RBC! 10 TWO TYPES OF MEMBRANE PROTEINS proteins are associated with a cell membrane. Integral proteins Embedded in hydrophobic core of bilayer Many are transmembrane proteins Span entire membrane Others are only partially embedded in core Peripheral Yust-Averett They are very specific to what they do with which molecules they react. 5/22/2017 Membrane proteins Not embedded in lipid bilayer at all Loosely bound to surface of membrane 11 FUNCTIONS OF MEMBRANE PROTEINS Yust-Averett Enzymatic activity – a protein built into the membrane that is an enzyme with its active site exposed to substances outside the cell. Several enzymes in a membrane may be grouped together to take part in a metabolic pathway. 5/22/2017 Transport – a protein that spans the membrane may provide a channel across the membrane that is selective for the transport of certain molecules. Channel proteins are involved in passive transport. Carrier proteins may be involved in passive or active transport; some require energy, some do not. Signal transduction – a membrane protein may act as a receptor and have a binding site that fits a chemical messenger (ligand) like a hormone. The protein can relay the message to the inside of the cell 12 without the chemical messenger actually having to enter the cell. FUNCTIONS OF MEMBRANE PROTEINS Attachment to the cytoskeleton and extracellular matrix – elements of the cytoskeleton may be bound to membrane proteins to help maintain cell shape and stabilize the location of certain membrane proteins. Yust-Averett Intercellular joining – membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. 5/22/2017 Cell to cell recognition – some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells 13 CELL – CELL RECOGNITION Needed for organism to function Yust-Averett ability to distinguish one type of neighboring cell from another 5/22/2017 Cell’s Ex: sorting of cells into tissues and organs in an animal embryo Basis for rejection of foreign cells by immune system (including those of transplanted organs) A, B, AB, and O blood types are designated by the carbohydrates on the surface of the RBC 14 SELECTIVE PERMEABILITY Cells Nutrients enter the cell Wastes leave the cell Yust-Averett Cells regulate transport of materials across the membrane in both directions 5/22/2017 maintain homeostasis by having selectively permeable cell membranes Large molecules require vesicles Small, hydrophobic (nonpolar) molecules can pass through the membrane rapidly. Ex: hydrocarbons Small, polar molecules require help to cross the membrane - transport proteins Water, sugar 15 TRANSPORT ACROSS THE CELL MEMBRANE Two Simple Diffusion Transport Dialysis and Osmosis Yust-Averett Passive 5/22/2017 types of movement occur across a cell membrane: Facilitated Diffusion Channel proteins Carrier proteins Active Protein pumps Transport Type of carrier protein Endocytosis and exocytosis 16 PASSIVE TRANSPORT Transport Does Yust-Averett not require energy Moves molecules down their concentration gradient from an area of high concentration to an area of low concentration Includes: 5/22/2017 Passive (Simple) Diffusion Facilitated Diffusion 17 PASSIVE TRANSPORT Substance Concentration gradient – a region along which the density of a chemical substance increases or decreases Dynamic equilibrium - occurs when both solutions have equal concentration and as many molecules are crossing the membrane in one direction as in the other Yust-Averett will diffuse down its concentration gradient until dynamic equilibrium is reached. 5/22/2017 Diffusion 18 PASSIVE TRANSPORT Dialysis – the passive movement of particles across a semi-permeable membrane from an area of high concentration to an area of low concentration Osmosis – the passive movement of water across a semi-permeable membrane from an area of high concentration to an area of low concentration Water can diffuse through the cell membrane but since it is polar this will happen very slowly – too slowly to support life. Facilitated diffusion through aquaporins allows water to move quickly across a cell membrane. Yust-Averett This occurs with nonpolar and very small uncharged polar molecules. 5/22/2017 19 PASSIVE TRANSPORT Facilitated diffusion Still being studied Most transport proteins are very specific; transport some substances but not others. Yust-Averett used by many polar molecules and ions to cross a membrane. Uses transport proteins 5/22/2017 Process 20 PASSIVE TRANSPORT Facilitated channels 5/22/2017 Protein diffusion involves: Provide a corridor that allows a specific molecule or ion to cross the membrane Yust-Averett Aquaporins Ion channels Move ions such as Na+, Gated channels Type of ion channel Open/close in response to electrical or chemical stimulus Carrier proteins Change shape in process of transporting molecules across membrane Example: glucose 21 ACTIVE TRANSPORT Active Transport 5/22/2017 Requires energy supplied by ATP Moves molecules against their Protein pumps (type of carrier proteins) Example: sodium-potassium pump Pumps sodium out of the cell and potassium into the cell Example: proton pump used during chemiosmosis Endocytosis Exocytosis Yust-Averett concentration gradient from an area of low concentration to an area of high concentration Include: 22 TRANSPORT PROTEINS Transport proteins (2 types) 5/22/2017 Allows polar substances to pass through the membrane quickly Specific for the substance it moves Example: glucose enters RBC by specific carrier proteins Channel proteins (protein channels) Yust-Averett Have hydrophilic channel that can be used as a tunnel through the membrane Example: Aquaporins Facilitate the passage of water Carrier proteins (passive or active) 23 Hold onto substances and change shape in a way that shuttles them across the membrane 5/22/2017 Yust-Averett 24 TRANSPORT ACROSS THE CELL MEMBRANE types of movement occur across a cell membrane: Simple Diffusion Transport Yust-Averett Passive 5/22/2017 Two Dialysis and Osmosis Facilitated Diffusion Channel proteins Carrier proteins Active Transport Protein pumps Endocytosis and exocytosis 25 PASSIVE TRANSPORT 5/22/2017 Dialysis Yust-Averett 26 PASSIVE TRANSPORT Osmosis Yust-Averett Water diffuses across the membrane from the region of higher water concentration to that of lower water concentration until the solute concentrations on both sides of the membrane are equal. Water moving into a cell by osmosis creates pressure which is called osmotic pressure 5/22/2017 27 SOLUTIONS Solids that are dissolved in a liquid Liquids that dissolve the solids Usually present in greater amounts Water is considered the universal solvent Yust-Averett Solvents 5/22/2017 Solutes 28 TYPES OF SOLUTIONS The 5/22/2017 Yust-Averett concentration of all molecules dissolved in a solution (the solutes) is called the osmotic concentration of the solution. Three terms are used to compare the osmotic concentrations of two solutions: Isotonic Hypertonic Hypotonic 29 TYPES OF SOLUTIONS Isotonic Equal osmotic concentration If a cell is immersed in an isotonic environment there will be no net movement of water across the plasma membrane. Water continues to flow across the membrane, but at the same rate in both directions. Yust-Averett with equal solute concentrations 5/22/2017 Solutions Dynamic equilibrium 30 TYPES OF SOLUTIONS cells need osmotic pressure to help maintain structure so they become flaccid (limp) in isotonic conditions. Yust-Averett animal cell fares best in an isotonic environment 5/22/2017 An Plant 31 TYPES OF SOLUTIONS Hypertonic Greater osmotic concentration If a cell is placed in a hypertonic solution the net movement of water will be out of the cell and the cell will shrink because osmosis moves water from an area where there is more water to an area where there is less water. Example: increase in salinity Yust-Averett with more solutes, less water 5/22/2017 Solution 32 TYPES OF SOLUTIONS cells, with cell walls, will become plasmolyzed (shrunken/shriveled, cell membrane pulls away from the cell wall) due to lack of osmotic pressure. Yust-Averett cells placed in a hypertonic solution will crenate (shrink/shrivel) as water moves out of the cell 5/22/2017 Animal Plant 33 TYPES OF SOLUTIONS Solution Yust-Averett with less solutes, more water If a cell is placed in a hypotonic solution the net movement of water will be into the cell and the cell will swell and burst because osmosis moves water from an area where there is more water to an area where there is less water 5/22/2017 Hypotonic 34 TYPES OF SOLUTIONS Yust-Averett cells can by lysed (or burst) due to osmotic pressure. 5/22/2017 Animal Plant cells can be turgid, or full of water but not lysed due to their cell walls. This is healthy for plants 35 TYPES OF SOLUTIONS • ANIMAL CELL – fares best in an isotonic environment… will lyse in hypotonic solution or crenate in hypertonic solution. • PLANT CELL – turgid and generally fares best in a hypotonic environment….tendency for water uptake balanced by the elastic cell wall pushing back on the cell. Become plasmolyzed if placed in hypertonic solution. • ARROW INDICATES WATER MOVEMENT WHEN CELL IS FIRST PLACED IN THE SOLUTIONS!!! 36 OSMOREGULATION Organisms control of water balance. Yust-Averett 5/22/2017 without cell walls living in hypertonic or hypotonic environments must have adaptations for osmoregulation. Example: Paramecium live in pond water which is hypotonic to the cell Paramecium doesn’t burst because it is equipped with a contractile vacuole, an organelle that functions as a bilge pump to force water out of the cell as fast as it enters 37 by osmosis. OSMOREGULATION 5/22/2017 Yust-Averett 38 5/22/2017 Yust-Averett 39 TRANSPORT ACROSS THE CELL MEMBRANE types of movement occur across a cell membrane: Simple Diffusion Transport Yust-Averett Passive 5/22/2017 Two Dialysis and Osmosis Facilitated Diffusion Channel proteins Carrier proteins Active Transport Protein pumps Endocytosis and exocytosis 40 ACTIVE TRANSPORT Active Transport 5/22/2017 Requires energy supplied by ATP Moves molecules against their Protein pumps (type of carrier proteins) Example: sodium-potassium pump Pumps sodium out of the cell and potassium into the cell Example: proton pump used during chemiosmosis Endocytosis Exocytosis Yust-Averett concentration gradient from an area of low concentration to an area of high concentration Include: 41 ACTIVE TRANSPORT Sodium-potassium pump in animal cells Uses energy from ATP This causes transport protein to change shape in a manner that moves a solute across the membrane. pump oscillates between two conformational states in a pumping cycle that translocates three Na+ ions out of the cell for every two K+ ions pumped into the cell. Yust-Averett Phosphate group is transferred directly to the transport protein 5/22/2017 Found The 42 SODIUM – POTASSIUM PUMP 5/22/2017 Yust-Averett 43 SODIUM – POTASSIUM PUMP 5/22/2017 Yust-Averett 44 ACTIVE TRANSPORT Moving Examples: proteins, polysaccharides, viruses, – too large to pass through membrane using channels or carrier proteins exocytosis and endocytosis Exocytosis Yust-Averett Includes 5/22/2017 LARGE molecules across the membrane Vesicle from cytoplasm fuses with membrane and materials are pushed out. Endocytosis Vesicle from extracellular area fuses with membrane and materials are brought in Three types of endocytosis Phagocytosis Pinocytosis Receptor-mediated endocytosis 45 ACTIVE TRANSPORT Exocytosis Yust-Averett of active transport Vesicles fuse with membrane to secrete materials out of the cell 5/22/2017 Type A transport vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the cell membrane When the vesicle membrane and the cell membrane come into contact, the lipid molecules of the two bilayers rearrange themselves so that the two membranes fuse. The contents of the vesicle than spill to the 46 outside of the cell and the vesicle membrane becomes part of the cell membrane EXOCYTOSIS 5/22/2017 Yust-Averett 47 ACTIVE TRANSPORT Endocytosis Yust-Averett of active transport Vesicle from extracellular area fuses with membrane and materials are brought in 5/22/2017 Type A small area of the cell membrane sinks inward to form a pocket As the pocket deepens, it pinches in, forming a vesicle containing material that had been outside the cell 48 ENDOCYTOSIS Phagocytosis eating” - particles taken in Yust-Averett In phagocytosis, a cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac that can be large enough to be considered a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. 5/22/2017 “cellular 49 ENDOCYTOSIS Pinocytosis drinking” – fluid taken in Yust-Averett In pinocytosis, the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplets. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. 5/22/2017 “cellular 50 RECEPTOR-MEDIATED ENDOCYTOSIS 5/22/2017 Yust-Averett This enables the cell to acquire bulk quantities of specific substance, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. The specific substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. 51 Example: cholesterol TRANSPORT REVIEW 5/22/2017 Yust-Averett 52