MEMBRANE STRUCTURE AND FUNCTION

Transcription

1 Chapter 5 The Working Cell: Membranes, Energy, and s Chapter 5: Big Ideas Cellular respiration Membrane Structure and Function Energy and the Cell How s Function MEMBRANE STRUCTURE AND FUNCTION Membranes Organize Chemical Activities of Cell The workbench of the cell Membranes are a fluid mosaic of Phospholipid bilayer Selectively permeable Kinks (double bonds) prevent tight packing Cholesterol Stabilize membrane at body temperature Proteins Move molecules across membranes Receptors & s Figure 5. Membranes form Spontaneously Fibers of extracellular matrix (ECM) Enzymatic activity Phospholipid Cholesterol CYTOPLASM Cell-cell recognition Phospholipids spontaneously self-assemble into simple membranes critical step in evolution of first cells. Signaling molecule Receptor Attachment to the cytoskeleton and extracellular matrix (ECM) Signal transduction Transport Intercellular junctions Microfilaments of cytoskeleton Glycoprotein CYTOPLASM

2 Molecules Move Across Membranes in 2 Ways Passive Transport No work/energy needed Active Transport Requires work/energy Passive Transport = Diffusion Diffusion = particles spread from high concentration to low concentration until evenly distributed Requires no work Driven by concentration gradient. Eventually, particles reach equilibrium Example: O 2 & CO 2 in lungs Particles = small & non-polar Osmosis = diffusion of water across a membrane Osmosis = diffusion of water across a membrane Definitions: Like Diffusion, but the WATER moves Solute = substance dissolved in solution (usually water) Solution = liquid which does the dissolving Isotonic = solute concentrations equal Hypotonic = low solutes (comparatively more water) Hypertonic = high solutes (comparatively less water) Water balance between cells and surroundings = Crucial Figure 5.5 How will animal cells be affected when placed into solutions of various concentrations? When an animal cell is placed into an isotonic solution concentration of solute is equal on both sides of membrane cell volume will not change a hypotonic solution solute concentration = lower outside cell water molecules move into cell, and cell will expand / burst a hypertonic solution solute concentration = higher outside cell water molecules move out of cell, and cell will shrink Animal cell Plant cell Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O Plasma H 2 O membrane Turgid (normal) H 2 O H 2 O H 2 O H 2 O Lysed Normal Shriveled Flaccid Shriveled (plasmolyzed)

3 Water balance between cells and surroundings = Crucial Facilitated Diffusion = Cell s Tool for Osmoregulation Facilitated Diffusion = passive transport of solute molecule across membrane through a Transport Protein For an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmoregulation, the control of water balance. Big or Polar molecules (e.g. sugars, AAs, even water) need the help of a Transport Protein Relies on Concentration Gradient (uses no energy) Otherwise, might explode (lyse) or shrivel Figure 5.8_s4 Active Transport In active transport, a cell must expend energy to Transport protein move a solute against its concentration gradient. = source of energy for Protein Pump Solute binding P ADP Phosphate attaching Solute 2 P Protein changes shape. 3 Transport Phosphate P detaches. 4 Protein reversion Figure 5.UN0 Large Molecules Move Across Membranes Passive transport (requires no energy) Diffusion Facilitated diffusion HIgher solute concentration Active transport (requires energy) Osmosis HIgher free water concentration HIgher solute concentration Cells use two mechanisms to move large molecules across membranes. Exocytosis ( outside cell ) = get rid of waste materials Endocytosis ( inside cell ) = import materials cell needs material packaged within a vesicle that fuses with membrane. Solute Water Lower solute concentration Lower free water concentration Lower solute concentration

4 Cells transform energy as they perform work ENERGY AND THE CELL Cells = little chemical factories, w/ constant chemical reactions for cell maintenance, manufacture of cellular parts, and cell replication. Energy = capacity to perform work Work = movement of matter in direction it would not otherwise move if left alone Kinetic vs. Potential Energy There are two kinds of energy.. Kinetic energy is the energy of motion actually doing work.. Pedaling bicycle 2. Beating wings 3. Heat & light = kinetic energy 2. Potential energy is stored energy the capacity to do work.. Bike at top of hill 2. Water behind dam 3. Molecules in a living cell Chemical Energy Chemical energy = potential energy of molecules available for release in a chemical reaction (stored in bonds) Big Molecule w/ many bonds (High PE) Gasoline (PE) + Oxygen converts combust Smaller Molecules (Low PE) CO 2 + particulates KE (available for work) Pistons move (KE) Thermodynamics Thermodynamics = study of energy transformations between system (e.g. cell, engine, firefly) & surroundings 2 Laws of Thermodynamics Two Laws of Thermodynamics First Law = Energy Conservation Total amount energy in universe is constant Can be transferred or transformed, but not created or destroyed e.g. power plant just converting E to more usable form Second Law = Inefficiency energy conversions increase the disorder of the universe (Entropy). e.g. bedroom clean, but always gets messy again! As a system becomes more ordered, surroundings become more disordered (entropy).

5 Chemical reactions either release or store energy Energy constantly being stored & used & stored & used & stored & used Two kinds of reactions: Energy used / released (exergonic reactions) or Energy stored (endergonic reactions). Exergonic Reactions Exergonic reactions ( E out ) release energy. Reactants have more PE than products Burning wood releases the energy in glucose as heat and light (+ CO 2 and water). (e.g. letting rock roll down hill) Cellular respiration involves many steps, releases energy slowly, and uses some of the released energy to produce. Figure 5.A Endergonic Reactions Potential energy of molecules Reactants Energy Products Amount of energy released An endergonic reaction ( E in ) requires an input of energy and products have more potential energy than reactants. (e.g. rolling rock up a hill) Photosynthesis is a type of endergonic process. Energy-poor reactants (carbon dioxide and water) are used. Energy is absorbed from sunlight. Energy-rich sugar molecules are produced. Figure 5.B Cellular Metabolism Potential energy of molecules Reactants Energy Products Amount of energy required Cellular Metabolism = totality of countless endergonic and exergonic chemical reactions in a cell Energy coupling uses the energy released from exergonic reactions to drive essential endergonic reactions, usually using the energy stored in molecules.

6 drives cellular work by coupling exergonic and endergonic reactions (adenosine triphosphate) = energy currency for all cellular work Acts as an energy shuttle Bond breaks (hydrolysis) in and releases one Phosphate group -- produces KE + ADP (adenosine diphosphate) Figure 5.2A_s2 : Adenosine Triphosphate Adenine Ribose Hydrolysis Phosphate group P P P H 2 O P P P Energy ADP: Adenosine Diphosphate drives cellular work by coupling exergonic and endergonic reactions Figure 5.2C is a renewable source of energy for the cell. In the cycle, energy released in an exergonic reaction, such as the breakdown of glucose,is used in an endergonic reaction to generate. Entire pool of in body regenerated in <min! Energy from exergonic reactions ADP P Energy for endergonic reactions Chemical Reactions all have Energy Barriers HOW ENZYMES FUNCTION Although biological molecules possess much potential energy, it is not released spontaneously. An energy barrier must be overcome before a chemical reaction can begin E to break bond between phosphate groups on E to push rock to get it rolling down the hill This energy is called the activation energy (E A ).

7 s lower energy barriers Figure 5.3A s function as biological catalysts by lowering the E A needed for a reaction to begin, increase the rate of a reaction without being consumed by the reaction, and Energy Reactant Activation energy barrier Energy Reactant Activation energy barrier reduced by enzyme are usually proteins Products Products -ase = enzyme Without enzyme With enzyme A specific enzyme catalyzes each cellular reaction Each enzyme has a specific shape & function (structure = function) The specific reactant that an enzyme acts on is called the enzyme s substrate. A substrate fits into a region of the enzyme called the active site (often induced fit like glove). Figure 5.4_s available with empty active site Active site (sucrase) Lock & Key Model: Lock = enzyme, Key = substrate, Active Site = place where key fits Figure 5.4_s2 available with empty active site Active site Substrate (sucrose) Figure 5.4_s3 available with empty active site Active site Substrate (sucrose) (sucrase) 2 Substrate binds to enzyme with induced fit (sucrase) 2 Substrate binds to enzyme with induced fit H 2 O 3 Substrate is converted to products

8 Figure 5.4_s4 4 available with empty active site Active site Glucose Fructose Products are released (sucrase) Substrate (sucrose) 3 2 Substrate binds to enzyme with induced fit H 2 O Substrate is converted to products Optimal Conditions of s Temperature affects molecular motion. Most human enzymes work best at deg F. Remember, enzymes = proteins, and proteins can denature (unravel) at high temps e.g. fever over 05, pasteurize milk Optimal ph (for most enzymes) = near neutral (7.0) Acid rain kills aquatic fauna Salt concentration inhibitors CONNECTION: Many drugs, pesticides, and poisons are enzyme inhibitors A chemical that interferes with an enzyme s activity is called an enzyme inhibitor. block substrates from entering the active site and reduce enzyme s productivity Many beneficial drugs act as enzyme inhibitors, including Ibuprofen, inhibiting the production of prostaglandins, some blood pressure medicines, some antidepressants, many antibiotics, and protease inhibitors used to fight HIV. inhibitors have also been developed as pesticides and deadly poisons for chemical warfare. Figure 5.UN03 Molecules cross cell membranes by by passive transport (a) may be moving down moving against requires (b) uses diffusion (d) uses (e) of of (c) polar molecules and ions