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1 Cell Membrane: Structure and Function 6.2-6.3 The cell membrane is the gateway into the cell.

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2 Key words you should know: Phospholipids SolutionPinocytosis PolarSolute HydrophilicSolventExocytosis HydrophobicPartially permeablePhospholipid bilayer Fluid mosaic modelSurface area GlycoproteinsTurgid PlasmolysisPassive transport CholesterolPlasmolysed ProteinsSelectively permeable Transport proteinsActive transport EnzymesCarrier protein Receptor moleculesBulk transport DiffusionEndocytosis Concentration gradientPhagocytosis Facilitated diffusionPhagocytes OsmosisCrenated

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3 Cell Membrane Structure All living cells are surrounded by a membrane. A cell membrane is also known as plasma membrane. To understand the function of anything in biology, you must study the structure first! Nerve cell Cell membrane { } cell membrane Gap between cells Cell Membranes from Opposing Neurons (TEM x436,740).

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4 Functions of the Plasma Membrane Protective barrier Regulate transport in & out of cell Regulate transport in & out of cell Allow cell-cell recognition Allow cell-cell recognition Provide anchoring sites for filaments of cytoskeleton Provide anchoring sites for filaments of cytoskeleton Provide a binding site for enzymes and hormones Provide a binding site for enzymes and hormones Interlocking surfaces bind cells together (junctions) Interlocking surfaces bind cells together (junctions) Contains the cytoplasm (fluid in cell) Contains the cytoplasm (fluid in cell)

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5 Phospholipids Cholesterol Proteins Proteins (peripheral and integral) Carbohydrates Structure of Cell Membrane

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6 PHOSPHOLIPID REVIEW Phospholipids are important structural components of cell membranes. Phospholipids are modified so that a phosphate group (PO4-) replaces one of the three fatty acids normally found on a lipid. phosphate group The addition of this group makes a polar "head" and two nonpolar "tails".

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7 HYDROPHILIC HEAD At one end of the phospholipid is a phosphate group and several double bonded oxygens. The electrons at this end of the molecule are not shared equally. This end of the molecule has a charge and is attracted to water. It is POLAR HYDROPHOBIC TAILS The two long chains coming off of the bottom of this molecule are made up of carbon and hydrogen. Because both of these elements share their electrons evenly these chains have no charge. They are NON POLAR. Molecules with no charge are not attracted to water; as a result water molecules tend to push them out of the way as they are attracted to each other. This causes molecules with no charge not to dissolve in water.

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8 Phospholipids can form: BILAYERS -2 layers of phospholipids with hydrophobic tails protected inside by the hydrophilic heads. The PHOSPHOLIPID BILAYER is the basic structure of membranes.

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9 hydrophilic Polar heads are hydrophilic water loving hydrophobic Nonpolar tails are hydrophobic water fearing Phospholipids make membranes Selectively Permeable- Selectively Permeable- able to control what crosses

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10 Polar, Hydrophilic head Polar, Hydrophilic head Nonpolar, Hydrophobic Fatty acid tails

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11 FLUID MOSAIC MODEL Cell membranes also contain proteins within the phospholipid bilayer. This model for the structure of the membrane is called the: FLUID MOSAIC MODEL FLUID- because individual phospholipids and proteins can move around freely within the layer, like its a liquid. MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.

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12 Proteins Are Critical to Membrane Function

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13 Proteins can float in the membrane or be fixed and also have hydrophobic and hydrophilic portions. Proteins may be embedded in the outer layer or in the inner layer and some span the two layers. Hydrophobic and Hydrophilic parts of the protein molecules sit next to the Hydrophobic and Hydrophilic portions of the phospholids of the membrane. This ensures the proteins stay in the membrane.

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14 Some of the proteins have carbohydrates attached to them GLYCOPROTEINS. Glycoproteins act as chemical id tags. Blood types are the result different glycoproteins.

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15 The membrane also contains molecules of CHOLESTEROL Cholesterol regulates the fluidity of the membrane gives mechanical stability helps to prevent ions from passing through the membrane.

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16 The fluid mosaic model was developed using Freeze Fracture Studies. The fracture occurs between the two phospholipid layers. You can clearly see the exposed proteins sticking out of the two layers. Individual phospholipids are too small to see.

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17 Cell Membrane Permeability Hydrophobic pass easily Hydrophilic DO NOT

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18 Semipermeable Membrane Small molecules and larger non-polar molecules move through easily. e.g. O 2, CO 2, H 2 O

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19 Ions Hydrophilic molecules larger than water Large molecules such as proteins do not move through the membrane on their own. Semipermeable Membrane

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20 Types of Transport Across Across Cell Membranes

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21 Two Forms of Transport Across the Membrane

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22 Passive Transport Simple Diffusion Doesnt require cell energy Doesnt require cell energy Molecules move from high to low concentration Molecules move from high to low concentration Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out.

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23 HIGH to LOW Molecules move from area of HIGH to LOW concentration, down their concentration gradient.

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24 PASSIVE Diffusion is a PASSIVE process which means no cell energy is used to make the molecules move, they have a natural KINETIC ENERGY

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25 Diffusion of Liquids

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26 Diffusion through a Membrane Solute moves DOWN the concentration gradient from HIGH to LOW concentration Cell membrane

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27 Diffusion of water across a membrane Moves from HIGH water potential (low solute) to LOW water potential (high solute) Osmosis

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28 Osmosis-Diffusion of H 2 O Across A Membrane Low solute concentrationHigh solute concentration

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29 CELL 10% NaCL 90% H 2 O 10% NaCL 90% H 2 O ENVIRONMENT NO NET MOVEMENT What is the direction of water movement? The cell s at _______________. equilibrium

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30 CELL 10% NaCL 90% H 2 O 20% NaCL 80% H 2 O What is the direction of water movement?

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31 CELL 15% NaCL 85% H 2 O 5% NaCL 95% H 2 O What is the direction of water movement? ENVIRONMENT

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32 Cells in Solutions

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33 Isotonic Solution NO NET MOVEMENT OF H 2 O (equal amounts entering & leaving) Hypotonic Solution CYTOLYSIS (water diffuses into the cell) Hypertonic Solution CRENATION (water diffuses out of the cell)

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34 Cytolysis & Crenation Cytolysis Crenation

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35 Osmosis in Red Blood Cells In Isotonic Solution In Hypotonic Solution In Hypertonic Solution

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36 hypotonichypertonic isotonic hypertonicisotonichypotonic hypertonic Indicate the diffusion gradient of water with an arrow. (Plasmolyzed cell) (Crenated cell) (Turgid cell)

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37 Channel proteins are embedded in the cell membrane & have a pore for materials to cross Carrier proteins can change shape to move material from one side of the membrane to the other Transport Proteins

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38 Passive Transport Facilitated Diffusion Doesnt require energy Uses transport proteins to move high to low concentration Moves materials with the concentration gradient. Examples: Glucose or amino acids moving from blood into a cell.

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39 Facilitated Diffusion Molecules will randomly move through the pores in Channel Proteins.

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40 Facilitated Diffusion

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41 Carrier Proteins Other carrier proteins change shape to move materials across the cell membrane

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42 Active Transport Requires energy or ATP Moves materials from LOW to HIGH concentration AGAINST concentration gradient

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43 Active Transport Examples: Pumping Na + (sodium ions) out and K + (potassium ions) in against strong concentration gradients. Called Na+-K+ Pump

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44 Sodium-Potassium Pump 3 Na+ pumped out for every 2 K+ pumped in; creates a membrane potential (+) (-)

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45 Moving the Big Stuff Out Molecules are moved out of the cell by vesicles that fuse with the plasma membrane. Exocytosis Exocytosis - moving things out. This is how many hormones are secreted and how nerve cells communicate with one another This is how many hormones are secreted and how nerve cells communicate with one another.

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46 Exocytosis Large molecules that are manufactured in the cell are released through the cell membrane. Inside Cell Cell environment

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47 Exocytosis Exocytic vesicle immediately after fusion with plasma membrane.

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48 Moving the Big Stuff In Large molecules move materials into the cell by one of three forms of endocytosis Large molecules move materials into the cell by one of three forms of endocytosis. 1.Pinnocytosis 2.Receptor-mediated EndoCytosis 3.Phagocytosis

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49 1. Pinocytosis Most common form of endocytosis Most common form of endocytosis. Takes in dissolved molecules as a vesicle Takes in dissolved molecules as a vesicle.

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50 Pinocytosis Cell forms an invagination Materials dissolve in water to be brought into cell Called Cell Drinking

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51 Example of Pinocytosis pinocytic vesicles forming mature transport vesicle Transport across a capillary cell (blue).

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52 2. Receptor-Mediated Endocytosis Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc.

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53 Receptor-Mediated Endocytosis Coated pit Vesicle

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54 3. Phagocytosis Used to engulf large particles such as food, bacteria, etc. into vesicles Called Cell Eating