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CHAPTER 3 A TOUR OF THE CELL
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Concept 3.3: CELL MEMBRANE
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MEMBRANE STRUCTURE Membranes keep eukaryotic cells organized Partition the cell Control the movement of substances into and out of the cell Help maintain chemical environment for the cell
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MEMBRANE STRUCTURE Membranes are mainly composed of 2 layers of phospholipid molecules with protein molecules embedded in them Also contain small amounts of cholesterol and carbohydrate molecules
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PHOSPHOLIPIDS Similar to lipid molecules, but have only two fatty acids instead of three Two fatty acids at one end of molecule— tails—are hydrophobic Other end—head—is hydrophilic; has phosphate group attached
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Membrane Phospholipids
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PHOSPHOLIPID STRUCTURE ALLOWS MEMBRANES TO FORM BOUNDARIES Plasma membrane separates watery inside of cell from watery environment outside the cell Phospholipids form a two layer “sandwich” of molecules—phospholipid bilayer—at boundaries that surround the cell or organelle
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MEMBRANE PROTEINS Two main types —Fig. 3.4; p. 80: 1. Intracellular Receptors- Inside the Cell 2. Membrane Receptors – Located in the cell membrane.
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MEMBRANE PROTEINS Perform most specific membrane functions 1. Enzyme activity 2. Cell-to-cell recognition 3. Cell signaling 4. Transport of materials
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Concept 3.4: Diffusion and Osmosis
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DIFFUSION Diffusion: net movement of particles from an area of greater concentration to an area of lesser concentration Permeable membranes: membranes that particles to freely pass through them until the concentration of particles is equal on both sides of the membrane—balance or equilibrium
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Example of Diffusion
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PASSIVE TRANSPORT Selectively permeable membranes: allow some substances to cross more easily than others; block some completely Cell membranes are selectively permeable; O 2 and CO 2 pass freely; H 2 O must pass through protein channels; other molecules pass only under specific conditions
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PASSIVE TRANSPORT Passive transport: diffusion of a substance across a membrane with no energy expenditure by the cell
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Simple Diffusion
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TYPES OF PASSIVE TRANSPORT Simple diffusion: passage of substance directly through the membrane—O 2, CO 2 Facilitated diffusion: substances pass through a channel in a transport protein in the membrane—sugars, H 2 O, some ions
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Passive Transport
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TYPES OF PASSIVE TRANSPORT
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OSMOSIS Passive transport of water is so common that it is given a special name Osmosis: passive transport of H 2 O across a selectively permeable membrane Aqueous (water is the solvent) solutions are classified into 3 categories called osmotic solutions
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Movement of Water
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TYPES OF OSMOTIC SOLUTIONS Hypertonic Solution: a solution whose solute concentration is higher than the solute concentration inside the cell - Animal cells will lose water and shrivel when placed in a hypertonic solution— water moves out of the cell
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Cells placed in a hypertonic solution
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TYPES OF OSMOTIC SOLUTIONS Hypotonic Solution: a solution whose solute concentration is lower than the solute concentration inside the cell - Animal cells will swell and possibly burst when placed in a hypotonic solution—water moves into the cell
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Cell placed in a hypotonic solution
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TYPES OF OSMOTIC SOLUTIONS Isotonic Solution: a solution whose solute concentration is equal to the solute concentration inside the cell - Animal cells retain their shape when placed in isotonic solutions—no net gain or loss of water inside the cell
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Isotonic solution Water molecules move in and out of the cell at the same rate, and the cell retains it’s normal shape.
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BEHAVIOR OF RED BLOOD CELLS IN DIFFERENT OSMOTIC SOLUTIONS
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OSMOSIS & PLANT CELLS Hypotonic environment: plant cells are firmest and healthiest - Cell wall prevents cell membrane from bursting Isotonic environment: non-woody plants wilt Hypertonic environment: membrane pulls away from cell wall—kills the cell
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PLANT & ANIMAL CELLS IN DIFFERENT OSMOTIC SOLUTIONS
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Concept 3.5: Active Transport, Endocytosis, and Exocytosis
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ACTIVE TRANSPORT Substance is moved from area of lower concentration to an area of higher concentration Requires the cell to expend energy
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ACTIVE TRANSPORT Plays major role in maintaining cell’s chemical environment Two major types: 1) solute pumping 2) large molecule transport
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SOLUTE PUMPING Specific membrane transport protein pumps a solute across the membrane Energy is used to move solute from area of lower concentration to area of higher concentration Solutes moved into cells and out of cells
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SOLUTE PUMPING
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LARGE MOLECULE TRANSPORT Some particles are too large to move through the membrane Must be packaged in membrane sacs called vesicles Particle fuses with plasma membrane and is moved either into (endocytosis) or out of (exocytosis) the cell Both processes require energy
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TYPES OF ENDOCYTOSIS Phagocytosis: cell membrane engulfs and takes in particle of solid material; “cell eating” Pinocytosis: cell membranes engulfs and takes in small droplet of liquid; “cell drinking”
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EXOCYTOSIS Product of cell is exported Requires energy
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ENDOCYTOSIS Material is brought into the cell Requires energy
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ENDOCYTOSIS & EXOCYTOSIS
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PASSIVE vs. ACTIVE TRANSPORT
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