AP Biology 2014 Campbell Biology in Focus Ch. 5 Erin Eggers

Part 1: Membrane Structure

The cell membrane is composed of a phospholipid bilayer Amphipathic

Fluid mosaic model O Basic model describes the membrane as fluid- like, allowing movement of proteins within it O It is actually much more structured than that, with a variety of factors affecting the location & motion of membrane-bound proteins and other macromolecules O Presence of unsaturated fatty acid tails in phospholipids – increases fluidity O Presence of cholesterol molecules in the membrane – ‘buffers’ fluidity O Membrane proteins may be anchored to the cytoskeleton Animated tutorial:

How would you ‘construct’ a transmembrane protein?

What are the jobs of these membrane-bound proteins? Review: how do these proteins get here?

How is new membrane with its integral proteins synthesized?

Glycolipids and glycoproteins O Glyco- prefix means sugar O These molecules serve as cellular identification tags O Allow cells to sort themselves into tissue types O Allow cells of the immune system to recognize self and non-self

Selective permeability (A VERY important concept) O What molecules can cross the plasma membrane? O What molecules cannot cross freely? O Small nonpolar molecules such as hydrocarbons and gases like CO 2 and O 2 O Charged, hydrophilic, or large molecules such as Na +, K +, Ca 2+, Cl -, glucose & other sugars

How would you ‘design’ this protein channel?

Part 2: Why molecules move: Diffusion, Osmosis & Energy

Diffusion O The movement of a given molecule is random – but the NET movement of a group of molecules is to spread out towards a state of equilibrium

Diffusion is the movement of molecules down their concentration gradient Equilibrium = balanced concentration

O The movement of molecules down a concentration gradient is a spontaneous process – no energy is expended – so this kind of transport across a selectively permeable membrane is called PASSIVE TRANSPORT O In fact, potential energy is released during diffusion so it is an exergonic reaction

What factors affect the rate of diffusion? O Temperature of the solution O Size of the molecules O The concentration gradient in the system (a larger gradient stores more potential energy)

Osmosis: the passive movement (aka diffusion) of water across a selectively permeable membrane Animated tutorial: om/at05.02.html

What are the consequences of osmosis? O Osmoregulation = the control of solute concentration and water balance in a hypotonic or hypertonic environment O Paramecium caudatum in fresh water: O Marine fish vs. freshwater fish:

Water balance in plant cells O Turgid O Flaccid O Plasmolysis O Onion skin or Elodea lab activity…

Discuss diffusion demonstration using dialysis tubing.

Water potential O In animal cells, if the plasma membrane is impermeable to solutes, water will move from an area of lower solute concentration to higher solute concentration. O In plant cells, the addition of a cell wall presents another factor – pressure. O The combined effect of solutes and pressure are incorporated in a calculation of water potential, symbolized by  the greek letter psi)

Water Potential (Water always moves from the system with a higher water potential to the system with a lower water potential)

Solute Potential  S (synonymous with osmotic potential) O A measure of the potential of water to move between regions of differing concentrations across a water-permeable membrane by using this formula:  = − i C R T, where  is the osmotic potential, C is the concentration of solutes, R is the universal gas constant (i.e bars K −1 mol −1 ), and T is the temperature in Kelvin (273 + o C)

More on solute/osmotic potential… O The solute potential for pure water is zero O Any solutes make the solute potential negative because the solute potential equation is  = − i C R T O More negative (aka LOWER) solute potentials mean MORE solutes and a greater potential for water to move in that direction O Water thus moves from a higher to a lower solute potential

Pressure potential (  P ) O In an animal cell or model system (ie dialysis tube or U tube with a semi- permeable membrane), if the system is open to the atmosphere then  P = zero. O If  P = 0 then the total water potential is simply equal to the solute potential  S

Turgor pressure O “Plump” plant cells are turgid. O They have turgor pressure – which is the force of the pressure of the swollen cell against the plant cell wall O

Free water moves from regions of higher water potential to regions of lower water potential

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FACILITATED DIFFUSION O Polar, hydrophilic molecules that cannot readily cross the hydrophobic interior of the plasma membrane often diffuse via FACILITATED DIFFUSION O Channel proteins – such as aquaporins O Channel proteins are often gated – or opened only in the presence of a specific stimulus O Carrier proteins – a change in shape transports the molecule across the membrane – but still down its concentration gradient

Ligand-gated channel protein

Carrier protein-mediated facilitated diffusion. Explain this graph.

Part 3: Active Transport

Active transport: Moving solutes ‘uphill’ O When a cell needs to move a solute against its concentration gradient, energy must be expended – this is called endergonic O The cell’s energy currency is ATP O This movement is referred to as active transport

Sodium-potassium ATPase pump O This protein pump maintains a concentration gradient across the plasma membrane with a high extracellular concentration of sodium and a high intracellular concentration of potassium

Cotransport (secondary active transport) O This sucrose transporter uses the energy released by the diffusion of H+ down its electrochemical gradient to drive the movement of sucrose against its concentration gradient. O The H+ gradient is maintained by a proton pump that uses ATP. Animated tutorial:

Diffusion is slightly more complicated for ions O Ions move down their concentration gradient (the chemical gradient) O AND ions will also move in a direction that would even out charge across a membrane (the electrical gradient)

An uneven charge distribution across a membrane is called the membrane potential O When there is a membrane potential (which there always is), then ions are subject to an electrochemical gradient – which is the combination of the chemical gradient and the electrical gradient O Electrogenic pump: a transport protein that generates voltage across a cell membrane O Sodium-potassium pump in animal cells and the proton (H+) pump in plants, fungi & bacteria.

Bulk transport O Exocytosis O Endocytosis O Phagocytosis O Pinocytosis O Receptor-mediated endocytosis

Exocytosis

Endocytosis Amoeba phagocytosis:

 s = − i C R T  S is the osmotic potential i is the ionization constant; i= 1 for glucose/sucrose and 2 for NaCl C is the concentration of solutes in M R is the universal gas constant ( bars K −1 mol −1 ) T is the temperature in Kelvin (273 + o C)