Understanding Membranes and Metabolism: Plasma Membrane Evolution and Functions

1. Membranes and Metabolism Chapters 7 & 8

2. Plasma Membrane: Selectively permeable Evolution of Cells: Phospholipids most likely pre-dated cells. When mixed with water, they form spheres which are somewhat selective.

3. Ch. 7 focuses on plasma membranes but the general principles apply to all eukaryotic membranes. Chemical reactions • Functions: Site of many ____________________________ • __________________________ movement in & out of cell • __________________ signals & info. between cell and its environment • Involved in ________________________________________ • Participate in _____________________________________ Regulate Transmit energy transfer & storage Cell adhesion (Junctions)

4. MAIN INGREDIENTS: Phospholipids and Proteins (also carbohydrates) “Flashback” to Chapter 6: 1935: Davson & Danielli – Wrong model, phospholipid layer between 2 protein layers. 1950’s: Electron microscope reveals membrane is less than 10 nm thick. (Too thin for globular proteins) Davson & Danielli model is modified with beta pleated sheet of proteins and is accepted until 1972. 1972: Singer & Nicolson – Suggest current Fluid Mosaic Model

5. Fluid Mosaic Model http://www.youtube.com/watch?v=ULR79TiUj80 http://www.youtube.com/watch?v=Qqsf_UJcfBc

6. Membranes must be fluid to work properly: Membranes solidify if too cold More phospholipids with unsaturated hydrocarbons: enhance fluidity Cholesterol (wedged between phospholipids): This is BAD! regulates fluidity (add more when it’s warmer because it’s less fluid) Cells can alter lipids to adjust to temp. change (winter wheat has increased unsat. lipids)

7. Membrane Proteins: Structure Membrane = collage of many different proteins embedded in the lipid bilayer. Two main types: Integral: Unilateral (embedded part way) Transmembrane (penetrate to both sides of the membrane) Peripheral: May attach to integral proteins May be held in place by cytoskeleton (on inside) penetrate lipid bilayer not embedded AT ALL

8. Membrane Proteins cont. Functions: Cell adhesion & recognition (junctions) Attachment to cytoskeleton Receptor sites for signals Enzymes Transport proteins

9. Transport Proteins: • Provide passage through membrane for specific ions and certain polar molecules (like sugars). • May provide a ____________ tunnel through membrane. • May bind to and ____________ move substance. • Are specific for any substance they transport. Three types: • Uniport = • Symport = • Antiport = hydrophilic physically single substance, one way two substances, one way two substances, opposite ways

10. Membrane Carbohydrates: Carbohydrates (when present) are on the exterior surface of the plasma membrane only. • Allow for cell-cell recognition: • Usually oligosaccharides (<15 sugar units): Some are glycolipids (“carbo attached to lipid”) To organize cells into tissues and organs and To be able to reject foreign cells (immune system) Most are glycoproteins (“carbo attached to protein”) • Vary widely (blood types...)

11. HOW DO MEMBRANES CONTROL WHAT PASSES THROUGH? A Substance’s passage through a membrane is dependent upon: Concentration gradient Size Electric gradient (ions) Polarity (hydrophobic?) • Nonpolar (hydrophobic) molecules: • Polar (hydrophilic) molecules: A) Small, uncharged (such as hydrocarbons or O2) – B) Large, uncharged (such as glucose) – C) ALL ions (such as Na+ or H+) – dissolve & pass through easily cross easily do NOT cross easily • do NOT cross easily

12. Diffusion: Net movement of particles from higher to lower concentration – • Because diffusion does NOT require energy, it is called: • Dialysis: • Osmosis: (WITH gradient) passive transport diffusion of SOLUTE through a membrane diffusion of SOLVENT through a membrane (In organisms, it’s water)

13. Osmosis: Since it is a type of diffusion, osmosis involves NET movement of water from areas of higher water concentration (lower solute conc.) to areas of lower water conc. (higher solute conc.) • Terms used to compare to areas: Hypertonic = Hypotonic = Isotonic = • Osmotic pressure: Pressure required to halt osmosis. Ex. Plant cell walls exert a force that prevents the inward flow of water past a certain level. • Is there any movement of water once equilibrium is reached? • solution with greater SOLUTE conc. solution with lower SOLUTE conc. 2 solutions have equal SOLUTE conc. YES!!! But Net movement is zero.

14. What’s the “deal” when osmosis is applied to CELLS??? Cells without cell walls: (animal cells) • Isotonic environment = • Hypertonic environment = • Hypotonic environment = equilibrium; no net change cells crenate (shrivel) cells lyse (burst)

15. Animals regulate water by: • Living in isotonic environments: • Kidneys: • Contractile vacuoles: • Salt pumps: vertebrates that live in (or drink) salt water marine invertebrates maintain isotonic condition in body fluids freshwater paramecium pump out H2O

16. cells become flaccid (wilt) Cells with cell walls: (bacteria, fungi, plants, some protists) • Isotonic environment = • Hypertonic environment = • Hypotonic environment = cells plasmolyze (shrivel – but not cell wall) cells become turgid (firm)

17. Identify the Solution type: hypotonic isotonic hypertonic

18. CELLULAR AIDS TO DIFFUSION: Or “What a cell can do when things just aren’t moving along”... Facilitated Diffusion: diffusion with the help of transport proteins • Still considered “passive transport”...WHY? Moving with gradient, no energy needed • Aids transport of polar molecules and ions specific saturated • Proteins are _________, can be _________ and/or __________ inhibited

19. Passive Transport

20. Active Transport: Energy _________ process which ______ molecules across a membrane ________  the concentration gradient. requiring pumps AGAINST • Involved in transport of ions: Na+, K+, Mg++, Ca++, and Cl- • Energy is supplied by: ATP Example: The Sodium-Potassium Pump Antiport transport protein moves 3 Na+ ions out for every 2 K+ ions pumped in. ATP energy is used to change the conformational shape of the transport protein.

21. Active Transport

22. Ion Transport The cytoplasm of a cell is negatively charged compared to the extracellular fluid, due to an unequal distribution of anions (-) and cations (+). • Membrane potential = voltage caused by the separation of opposite charges. (electric potential energy) • Ranges from: -50 to –200 millivolts (mV) into • The different charges favor diffusion of cations ______ the cell and anions ______ (WHY?) out opposites attract Diffusion of ions is affected by: concentration gradient AND membrane potential.

23. Electrogenic Pumps: Active transport pumps that create a membrane potential (Ex. Na+/K+ Pump). • This produces voltage that can be a source of • _____________ to do work. potential energy • The potential energy produced can drive the active transport of other solutes: called cotransport • Example: Plants = Proton (H+) pump transports sucrose.

24. Sodium Potassium Pump

25. MOVEMENT OF HUGE “FREAKING” MOLECULES: Endocytosis: Intake (3 Types) • Phagocytosis— pseudopodia engulf particle & package it in a vacuole. • Pinocytosis-- vesicles take in droplets (drinking) • Receptor-mediated – specific substance binds membrane receptor protein and is brought in with a coated vesicle. Exocytosis: Export • Vesicles (from ER or Golgi) – carry macromolecule to plasma membrane & fuse with it. • Secretory cells use exocytosis.  Examples: insulin (pancreas), neurotransmitters (neurons)

26. Chapter 8: “Metabolism” Metabolism: All of an organism’s chemical processes Metabolic Pathways: Step-by-step series of enzyme-catalyzed reactions.  Two main types: • Catabolic: Release energy by breaking down complex molecules.  (Ex: cellular respiration) • Anabolic: Consume energy to build complex molecules. (Ex: protein synthesis)

27. ENERGY: BASIC PRINCIPLES Energy: Ability to do work (move matter against opposing forces like gravity) • Cells cannot make or recycle energy.  What is the ultimate source of their energy? The Sun Forms of Energy: • Kinetic: energy of motion • Potential: stored energy; based on position or arrangement (chemical)

28. Energy Transformations: Energy can be converted from one form to another. Two laws of thermodynamics: • First Law: Energy can be transformed but not created or destroyed. (Also called the Law of Conservation of Energy)

29. Second Law: Every energy transformation increases the entropy (chaos, disorder) of the universe. In other words: as energy is transformed, much of it is converted to heat, a random (low quality) state of energy Organisms must use energy to maintain organization in a random universe.

30. Free Energy: Amount of energy available to do work for cells.  Determines whether a reaction will occur spontaneously. Cells do 3 basic types of work: 1. Mechanical: movement (muscles, cilia...) 2. Transport: pumping substances across membranes 3. Chemical: exergonic vs. endergonic reactions

31. ATP (Adenosine Triphosphate): Immediate source of energy to drive cell work. • Made of adenine bonded to ribose bonded to • 3 phosphate groups. Unstable triphosphate tail, broken by hydrolysis • ATP + H2O ADP + Pi + energy    (ADP = Diphosphate) Cells can renew ATP by phosphorylating the ADP again.  Catabolic (exergonic) reactions provide the energy to re-make ATP and Anabolic (endergonic) reactions break down the ATP.

32. Activation Energy: Amount of energy needed to start a chemical reaction.  Affects speed of reactions. (∆G = change in energy. ∆G is negative for exergonic reactions) Different reactions require different amounts of energy • The Energy must produce 1 or both of the following effects: Increase collision rates of reactants Agitate the internal structure of a reactant enough to permit disruption of chemical bonds • Ways to achieve the above effects: Raise temperature Increase concentration of reactants Increase pressure on reactants Add a catalyst

33. Catalysts: Chemical agents that accelerate a reaction without being permanently changed in the process.  activation energy • They function by lowering the amount of ______________ • needed.  cannot • Catalysts speed up a reaction that could not take place on its own.

34. Characteristics of Biological Catalysts: • Called: enzymes • All are: proteins • Highly specific for a given reaction.  • Due to: shape of active site • Many can catalyze reversible reactions in either direction. • Affected by: pH, temperature, conc. of reactants and products, ionic conc., and enzyme inhibitors (poisons) • Names often end in “-ase”. Examples: ligase, catalase

35. Specificity of Enzymes: substrate active site • The ___________ binds to the enzyme’s _______ • Active site = pocket or groove on enzyme, formed with only a few amino acids, the shape conforms • to the substrate • Two Theories Explain Enzyme Action:  Lock & Key (wrong) Induced Fit

36. Actions of Enzyme / Substrate Complex: • Substrate binds active site of enzyme. • Induced fit of active site: distorts substrate’s bonds • Active site provides micro-environment: conducive to the chemical reaction • Amino acid side chains: may participate in reaction

37. Induced Fit Example

38. Cofactors: Small molecules required for proper enzyme activity (NOT proteins).               • Inorganic Cofactors: metal atoms of zinc, copper, or iron • Organic Cofactors: called coenzymes, most are vitamins Proper enzyme functioning requires that you get your vitamins and minerals, one way: Or another…

39. Enzyme Inhibitors: Chemicals that slow or stop enzyme activity.  Irreversible (covalent bond) Or reversible (hydrogen bond) • Competitive Inhibitors: resemble substrate, can bond to active site • Noncompetitive Inhibitors: bond to may be metabolic poison (DDT), many antibiotics are also noncompetitive inhibitors. another part of enzyme & warp active site

40. Allosteric Regulation of Enzymes: Allosteric site: specific receptor site other than active site • Usually only present on enzymes with 2 or more polypeptide chains. • Have 2 conformations: active and inactive • Binding of activator stabilizes active shape • Enzyme activity changes in response to relative proportions of ________________ and • ________________. activators inhibitors ←Allosteric site