11232020 YustAverett CELL BIOLOGY 1 Part II Cell

11/23/2020 Yust-Averett CELL BIOLOGY 1 Part II: Cell Membrane and Cellular Transport

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CELLULAR MEMBRANE Cell membrane called plasma membrane Separates cell from its surroundings Yust-Averett 11/23/2020 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 Creates a bilayer Yust-Averett 11/23/2020 Proteins are used for transport Lipids are mostly phospholipids Molecular arrangement shelters the hydrophobic tails from water while exposing the hydrophilic heads to water Some 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 11/23/2020 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 11/23/2020 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

MEMBRANES Membranes must be fluid to work salad oil Yust-Averett They are usually about as fluid as Temperature can affect fluidity 11/23/2020 properly 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

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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 11/23/2020 A More than 50 different types of proteins in just RBC! 10

TWO TYPES OF MEMBRANE PROTEINS proteins are a cell membrane. associated with 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. 11/23/2020 Membrane proteins Not embedded in lipid bilayer at all Loosely bound to surface of membrane 11

PROTEINS Transport 11/23/2020 – 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. 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. 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 without the chemical messenger actually having to enter the cell. 12

PROTEINS Cell joining – membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. Yust-Averett Intercellular 11/23/2020 to cell recognition – some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells 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. 13

CELL – CELL RECOGNITION ability to distinguish one type of neighboring cell from another for organism to function 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) Yust-Averett Needed 11/23/2020 Cell’s 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 11/23/2020 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 types of movement occur across a cell membrane: Simple Diffusion Transport Yust-Averett Passive 11/23/2020 Two Dialysis and Osmosis 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: 11/23/2020 Passive (Simple) Diffusion Facilitated Diffusion 17

PASSIVE TRANSPORT Substance Yust-Averett will diffuse down its concentration gradient until dynamic equilibrium is reached. 11/23/2020 Diffusion 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 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 occurs with nonpolar and very small uncharged polar molecules. Yust-Averett Osmosis – the passive movement of water across a semi-permeable membrane from an area of high concentration to an area of low concentration 11/23/2020 This 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. 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 11/23/2020 Process 20

PASSIVE TRANSPORT Facilitated channels 11/23/2020 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 11/23/2020 Requires energy supplied by ATP Moves molecules against their concentration Yust-Averett gradient from an area of low concentration to an area of high concentration Include: 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 22

TRANSPORT PROTEINS Transport proteins (2 types) Channel glucose enters RBC by specific carrier proteins Yust-Averett Example: through the 11/23/2020 Allows polar substances to pass membrane quickly Specific for the substance it moves proteins (protein channels) Have hydrophilic channel that can be used as a tunnel through the membrane Example: Aquaporins Facilitate Carrier the passage of water proteins (passive or active) Hold onto substances and change shape in a way that shuttles them across the membrane 23

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TRANSPORT ACROSS THE CELL MEMBRANE types of movement occur across a cell membrane: Simple Diffusion Yust-Averett Passive Transport 11/23/2020 Two Dialysis and Osmosis Facilitated Diffusion Channel proteins Carrier proteins Active Transport Protein pumps Endocytosis and exocytosis 25

PASSIVE TRANSPORT 11/23/2020 Dialysis Yust-Averett 26

PASSIVE TRANSPORT Osmosis Yust-Averett 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 11/23/2020 Water 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 11/23/2020 Solutes 28

TYPES OF SOLUTIONS 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: 11/23/2020 The 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. Yust-Averett with equal solute concentrations 11/23/2020 Solutions Water continues to flow across the membrane, but at the same rate in both directions. Dynamic equilibrium 30

TYPES OF SOLUTIONS animal cell fares best in an isotonic environment cells need osmotic pressure to help maintain structure so they become flaccid (limp) in isotonic conditions. Yust-Averett Plant 11/23/2020 An 31

TYPES OF SOLUTIONS Solution 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 11/23/2020 Hypertonic 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 11/23/2020 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 11/23/2020 Hypotonic 34

TYPES OF SOLUTIONS Yust-Averett cells can by lysed (or burst) due to osmotic pressure. 11/23/2020 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. Example: Yust-Averett 11/23/2020 without cell walls living in hypertonic or hypotonic environments must have adaptations for osmoregulation. 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 by osmosis. 37

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TRANSPORT ACROSS THE CELL MEMBRANE types of movement occur across a cell membrane: Simple Diffusion Transport Yust-Averett Passive 11/23/2020 Two Dialysis and Osmosis Facilitated Diffusion Channel proteins Carrier proteins Active Transport Protein pumps Endocytosis and exocytosis 40

ACTIVE TRANSPORT Active Transport 11/23/2020 Requires energy supplied by ATP Moves molecules against their concentration Yust-Averett gradient from an area of low concentration to an area of high concentration Include: 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 41

ACTIVE TRANSPORT Sodium-potassium pump in animal cells Uses energy from ATP Yust-Averett Phosphate group is transferred directly to the transport protein 11/23/2020 Found This causes transport protein to change shape in a manner that moves a solute across the membrane. The 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. 42

SODIUM – POTASSIUM PUMP 11/23/2020 Yust-Averett 43

SODIUM – POTASSIUM PUMP 11/23/2020 Yust-Averett 44

ACTIVE TRANSPORT Moving Examples: proteins, polysaccharides, viruses, – too large to pass through membrane using channels or carrier proteins Exocytosis exocytosis and endocytosis Yust-Averett Includes 11/23/2020 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 45 Pinocytosis Receptor-mediated endocytosis

ACTIVE TRANSPORT Exocytosis Yust-Averett of active transport Vesicles fuse with membrane to secrete materials out of the cell 11/23/2020 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 outside of 46 the cell and the vesicle membrane becomes part of the cell membrane

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ACTIVE TRANSPORT Endocytosis 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 Yust-Averett of active transport Vesicle from extracellular area fuses with membrane and materials are brought in 11/23/2020 Type 48

ENDOCYTOSIS Phagocytosis In eating” - particles taken in 11/23/2020 “cellular Yust-Averett 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. 49

ENDOCYTOSIS Pinocytosis In drinking” – fluid taken in Yust-Averett 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. 11/23/2020 “cellular 50

ENDOCYTOSIS 11/23/2020 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. Yust-Averett Example: cholesterol 51

TRANSPORT REVIEW 11/23/2020 Yust-Averett 52