Membranes Chapter 7 Membrane Structure The Fluid Mosaic

Membranes Chapter 7

Membrane Structure The Fluid Mosaic Model of membrane structure contends that membranes consist of: -phospholipids arranged in a bilayer -globular proteins inserted in the lipid bilayer 2

Polar heads Fluid Mosaic love water Model of the & dissolve. cell membrane Non-polar tails hide from water. Carbohydrate cell markers Proteins Membrane movement animation

Membrane Structure Cellular membranes have 4 components: 1. phospholipid bilayer 2. transmembrane proteins 3. interior protein network 4. cell surface markers 4

Membrane Structure Membrane structure is visible using an electron microscope. Transmission electron microscopes (TEM) can show the 2 layers of a membrane. Freeze-fracturing techniques separate the layers and reveal membrane proteins. 5

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Phospholipids Phospholipid structure consists of -glycerol – a 3 -carbon polyalcohol acting as a backbone for the phospholipid -2 fatty acids attached to the glycerol -phosphate group attached to the glycerol 8

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Phospholipids The fatty acids are nonpolar chains of carbon and hydrogen. -Their nonpolar nature makes them hydrophobic (“water-fearing”). The phosphate group is polar and hydrophilic (“water-loving”). 10

Phospholipids The partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a bilayer: -fatty acids are on the inside -phosphate groups are on both surfaces of the bilayer 11

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Phospholipids Phospholipid bilayers are fluid. -hydrogen bonding of water holds the 2 layers together -individual phospholipids and unanchored proteins can move through the membrane -saturated fatty acids make the membrane less fluid than unsaturated fatty acids -warm temperatures make the membrane more fluid than cold temperatures 13

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Membrane Proteins Membrane proteins have various functions: 1. transporters 2. enzymes 3. cell surface receptors 4. cell surface identity markers 5. cell-to-cell adhesion proteins 6. attachments to the cytoskeleton 15

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Membrane Proteins Peripheral membrane proteins -anchored to a phospholipid in one layer of the membrane -possess nonpolar regions that are inserted in the lipid bilayer -are free to move throughout one layer of the bilayer 17

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Membrane Proteins Integral membrane proteins - May span the lipid bilayer (transmembrane proteins) or only a portion. -nonpolar regions of the protein are embedded in the interior of the bilayer -polar regions of the protein protrude from both sides of the bilayer 19

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Membrane Proteins Integral proteins possess at least one transmembrane domain -region of the protein containing hydrophobic amino acids -spans the lipid bilayer 21

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Membrane Proteins Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane. -b sheets in the protein secondary structure form a cylinder called a b-barrel -b-barrel interior is polar and allows water and small polar molecules to pass through the membrane 23

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Passive Transport Passive transport is movement of molecules through the membrane in which -no energy is required -molecules move in response to a concentration gradient Diffusion is movement of molecules from high concentration to low concentration 25

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Passive Transport Selective permeability: integral membrane proteins allow the cell to be selective about what passes through the membrane. Channel proteins have a polar interior allowing polar molecules to pass through. Carrier proteins bind to a specific molecule to facilitate its passage. 27

Passive Transport Channel proteins include: -ion channels allow the passage of ions (charged atoms or molecules) which are associated with water -gated channels are opened or closed in response to a stimulus -the stimulus may be chemical or electrical 28

Passive Transport Carrier proteins bind to the molecule that they transport across the membrane. Facilitated diffusion is movement of a molecule from high to low concentration with the help of a carrier protein. -is specific: Transport some materials but not all. -is passive: No energy required. -saturates when all carriers are occupied 29

Passive Transport: Facilitated Diffusion A B Facilitated diffusion (Channel Protein) Diffusion (Lipid Bilayer) Facilitated diffusion: diffusion of specific particles through transport proteins found in the membrane Transports larger or charged molecules Carrier Protein

Passive Transport: 2. Facilitated Diffusion Glucose molecules Cellular Transport From a. High Concentration High • Channel Proteins animations Cell Membrane Low Concentration Through a Go to Section: Transport Protein channel Low

Passive Transport In an aqueous solution -water is the solvent -dissolved substances are the solutes Osmosis is the movement of water from an area of high to low concentration of water -movement of water toward an area of high solute concentration 32

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Passive Transport When 2 solutions have different osmotic concentrations -the hypertonic solution has a higher solute concentration -the hypotonic solution has a lower solute concentration Osmosis moves water through aquaporins toward the hypertonic solution. 34

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Passive Transport: 3. Osmosis animation • 3. Osmosis: diffusion of water through a selectively permeable membrane • Water moves from high to low concentrations • Water moves freely through pores. • Solute (green) to large to move across.

Passive Transport Organisms can maintain osmotic balance in different ways. 1. Some cells use extrusion in which water is ejected through contractile vacuoles. 2. Isosmotic regulation involves keeping cells isotonic with their environment. 3. Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid. 37

Active Transport Active transport -requires energy – ATP is used directly or indirectly to fuel active transport -moves substances from low to high concentration -requires the use of carrier proteins 38

Active Transport Carrier proteins used in active transport include: -uniporters – move one molecule at a time -symporters – move two molecules in the same direction -antiporters – move two molecules in opposite directions 39

Active Transport Sodium-potassium (Na+-K+) pump -an active transport mechanism -uses an antiporter to move 3 Na+ out of the cell and 2 K+ into the cell -ATP energy is used to change the conformation of the carrier protein -the affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane 40

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Active Transport Coupled transport -uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule -a symporter is used -glucose-Na+ symporter captures the energy from Na+ diffusion to move glucose against a concentration gradient 42

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Bulk Transport Bulk transport of substances is accomplished by 1. endocytosis – movement of substances into the cell 2. exocytosis – movement of materials out of the cell 44

Bulk Transport Endocytosis occurs when the plasma membrane envelops food particles and liquids. 1. phagocytosis – the cell takes in particulate matter 2. pinocytosis – the cell takes in only fluid 3. receptor-mediated endocytosis – specific molecules are taken in after they bind to a receptor 45

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Bulk Transport Exocytosis occurs when material is discharged from the cell. -vesicles in the cytoplasm fuse with the cell membrane and release their contents to the exterior of the cell -used in plants to export cell wall material -used in animals to secrete hormones, neurotransmitters, digestive enzymes 49

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Short distance Movement of water across a plasma membrane • Osmosis and Water Potential • Osmosis is the movement of water from High to Low Concentration. • Water Potential measures the concentration of free water molecules. It is a measure of the tendency of these molecules to diffuse to another area. The more free water molecules, the higher the Water Potential . 51

Understanding Water Potential • Water potential = Ψ : psi (sounds like “sigh”) – measure of pressure: megapascal (MPa) or “bars” (1 MPa=10 Bars) • Ψ = 0 MPa for pure water in an open container at sea level and at room temperature – Note: 1 MPa is about 10 x atmospheric pressure at sea level

How Solutes and Pressure Affect Water Potential Equation: Ψ = Ψ S + ΨP • The solute potential “osmotic potential” (ΨS) of a solution is directly proportional to its molarity (pure water is 0) – Expresses as a negative (the more solute added, the more negative it becomes!!!) • Less “free” water • An increase in solute concentration has a negative effect on water potential • Pressure potential (ΨP) is the physical pressure on a solution (from cell wall) • Positive or negative – For example: A solution Drawn in, by a syringe, is under negative pressure; it is under positive pressure when it is being expelled by the syringe – Water in cells usually under positive pressure – Turgor pressure is the push out on the cell wall • Air in a tire

Water Potential Water potential (ψ): H 2 O moves from high ψ low ψ potential Water potential equation: ψ = ψS + ψP • Water potential (ψ) = free energy of water • Solute potential (ψS) = solute concentration (osmotic potential) • Pressure potential (ψP) = physical pressure on solution; turgor pressure (plants) – Pure water: ψP = 0 MPa – Plant cells: ψP = 1 MPa

Where will WATER move? From an area of: – higher ψ lower ψ (more negative ψ) – low solute concentration high solute concentration – high pressure low pressure

Water potential (ψ) = pressure potential (ψp) + solute (osmotic) potential (ψs) Solute potential (ψs): The effect of solute concentration. Pure water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes more negative. This causes water potential to decrease also. *As solute is added, the water potential of a solution drops, and water will tend to move into the solution. 57

Water moves from a place of high water potential to a place of low water potential. Water potential (ψ) = pressure potential (ψp ) + solute potential (ψs) (osmotic) This is an open container, so the ψp = 0 This makes the ψ = ψs The ψs =-0. 23, so ψ is -0. 23 MPa, and water moves into the solution.

Water potential (ψ) = pressure potential (ψp) + solute (osmotic) potential (ψs) • Pressure potential (ψp): • In a plant cell, pressure exerted by the rigid cell wall that limits further water uptake 60

Water Movement Across Plant Cell Membranes • Flaccid (limp) cell is placed in a hypertonic environment, the cell will lose water and undergo plasmolysis • Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall © 2017 Pearson Education, Inc.

• If a flaccid cell is placed in a hypotonic environment, the cell will gain water and become turgid © 2017 Pearson Education, Inc.

Questions: If a plant cell immersed in distilled water has a ΨS of -0. 7 MPa and a Ψ of 0 MPa, what is the cell’s Ψp? If you put it in an open beaker of solution that has a Ψ of -0. 4 MPa what would be its Ψp at equilibrium? 0. 7 MPa 0. 3 MPa

Answer will be in Bars 10 bars = 1 MPa

Calculating Solute Potential (ψ S ) ψS = -i. CRT • • i = ionization constant (# particles made in water) C = molar concentration R = pressure constant (0. 0831 liter bars/mole-K) T = temperature in K (273 + 0 C) • The addition of solute to water lowers the solute potential (more negative) and therefore decreases the water potential.

So what is the solute potential of a 0. 1 M solution of sucrose at 22 C? • • Solute potential = -i. CRT i (ionization constant) = 1 R = 0. 0831 (from handbook) T = temp K (273 + C of solution) Ωs = - (1) (0. 0831) (295) - 2. 45 bars

• Calculate the water potential of a solution of 0. 15 M sucrose. The solution is at standard temperature (273 K) and pressure (. 0831 L bars/mol K). Ψ = Ψp + Ψs = 0 bars + [-i. CRT] = 0 bars + [-(1) (0. 15 mol/L) (0. 0831 L bars/mol K) (273 K) = 0 bars + -3. 40 bars Ψ = -3. 40 bars

1. Calculate the solute potential of a 0. 1 M Na. Cl solution at 25°C. 2. If the concentration of Na. Cl inside the plant cell is 0. 15 M, which way will the water diffuse if the cell is placed in the 0. 1 M Na. Cl solution?

1. 2. 3. 4. Which chamber has a lower water potential? Which chamber has a lower solute potential? In which direction will osmosis occur? If one chamber has a Ψ of -2000 k. Pa, and the other -1000 k. Pa, which is the chamber that has the higher Ψ?