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The Working Cell

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  1. The Working Cell Chapter 5 Part II

  2. The ATP Cycle 10 million ATP molecules spent and regenerated per second per cell • Cells spend ATP continuously – renewable resource • ATP is restored by adding a P group back to ADP • Mitochondria harvest energy from carbohydrates and lipids • Energy coupling: transfer of energy from processes that yield energy, breakdown of organic fuels  metabolic processes

  3. Enzymes • Living organisms - ‘bag of chemicals’ - countless chemical reactions constantly change your molecular makeup • Metabolism is the sum total of all chemical reactions that occur in organisms • Few metabolic reactions occur without the assistance of enzymes - specialized proteins that speed up (catalyze) reactions

  4. Activation Energy • Chemical reactions – chemical bonds in reactant molecules are broken - this process requires that molecules absorb energy from their surroundings - this energy is called activation energy • Activation energy - energy that activates reactants and triggers a chemical reaction • Enzymes enable metabolism to occur at cooler temperatures - increase supply of energy by heating - enzymes reduce the amount of activation energy needed for a chemical reaction by binding to reactant molecules

  5. Activation Energy – Jumping Bean Analogy a) a chemical reaction requires activation energy to break the bonds of the reactant molecules – the jumping beans represent reactant molecules that must overcome the barrier of activation b) an enzyme speeds the process by lowering the barrier of activation energy Fig 5.8

  6. Induced Fit • An enzyme is very selective in the reaction it catalyzes - selectivity is based on the ability of the enzyme to recognize a reactant molecule (the enzyme’s substrate) • Active site: special region of the enzyme - has a shape and chemistry that fits the substrate molecule - changes shape slightly when substrate docks to catalzye the reaction • Induced fit – entry of the substrate induces the enzyme to change shape - makes a tighter fit between the substrate and active site

  7. Enzymes • Key characteristic – an enzyme can function over and over again - after the products are released from the active site the enzyme is available to accept another substrate molecule • Many enzymes are named for their substrates, but with an –ase ending

  8. Sucrase is receptive for its substrate molecule • Substrate has the correct shape to fit the active site • Enzyme catalyzes the chemical reaction - hydrolysis of sucrose • Products – glucose and fructose, exit the active site Figure 5.9

  9. Enzyme Inhibitors • Certain molecules can inhibit a metabolic reaction - some bind to the active site, as substrate imposters - others bind at a different site, but binding causes a change in the enzyme shape • Some inhibitors act as poisons that block metabolic processes essential to an organism - insecticide malathion inhibits an enzyme required for the normal functioning of the nervous system - penicillin inhibits an enzyme bacteria use to make their cell walls

  10. Figure 5.10a, b

  11. Figure 5.10c

  12. Feedback Regulation • In some cases inhibitor binding is reversible - enables some inhibitors to regulate metabolism • If metabolism is producing more of a particular product then the cell needs - the product may inhibit an enzyme required for its production • Feedback regulation keeps the cell from wasting resources

  13. Membrane Function • Working cells must control the flow of materials to and from the environment - membrane proteins help with this task • Membrane proteins embedded in the phospholipid bilayer of the PM perform a variety of functions - transport proteins are critical

  14. a) Cytoskeleton elements may bond to membrane proteins – proteins can adhere to the fibers of the ECM to coordinate EC and IC changes b) Cell signaling – protein with a binding site that fits the specific shape of a chemical messenger, such as a hormone (chemical messenger) c) Enzymatic activity – an enzyme with its active site exposed to its substrate Fig 5.11

  15. d) Transport – a protein that spans the membrane may provide a selective channel for a particular solute e) Intercellular joining – proteins of adjacent cells may be hooked together to form various kinds of junctions f) Cell-cell recognition – proteins with short chains of sugars serve as ID tags

  16. Passive Transport: Diffusion Across Membranes • Molecules are restless – contain heat energy - makes them vibrate and wander randomly • Diffusion is one result of this movement - molecules tend to spread into the available space - diffusion is passive transport; no energy is needed • Passive transport is extremely important to all cells - O2 essential for metabolism, diffuses into RBCs and CO2 , a metabolic waste,diffuses out of them - water, one of the most important substances crossed membranes by passive transport

  17. Each solute will diffuse down its own concentration gradient Membrane -permeable to dye molecules, diffuse down their concentration gradient; at equilibrium molecules are still restless, but rate of transport is equal in both directions Fig 5.12

  18. Diffusion • Concentration gradient – a substance will diffuse from a more to a less concentrated area - a membrane diffusion is a passive transport: uses no energy • Membrane plays a regulatory role by being selectively permeable - small molecules pass through more readily than large - but can also be impermeable to some very small molecules that are too hydrophilic • Facilitated diffusion – small hydrophilic molecules use transport proteins that act as selective channels

  19. Osmosis and Water Balance in Cells • Osmosis - passive transport of water across a selectively permeable membrane - a membrane separating 2 solutions with different solute concentrations is permeable to water but not to the solute • Hypertonic solution - higher concentration of solute • Hypotonic solution – lower solute concentration - higher water concentration

  20. Osmosis and Water Balance in Cells • Water will diffuse across the membrane along its concentration gradient - hypotonic solution (higher water content)  hypertonic solution - reduces the difference in solute concentrations - changes the volumes of the 2 solutions - when solute concentrations become the same water molecules will move at the same rate in both directions • Isotonic – solution of equal solute concentration

  21. Membrane - separates 2 solutions with different sugar concentrations Water can pass through the membrane, but not sugar molecules Osmosis, the passive transport of water across the membrane, reduces the difference in sugar concentrations and changes the solution volumes Fig 5.13

  22. Water Balance in Animal Cells • The survival of a cell depends on its ability to balance water uptake and loss • A RBC immersed in an isotonic solution - cell’s volume remains constant - the cell gains and loses water at the same rate - the cell is isotonic to its surroundings because the 2 solutions have the same total concentration of solutes

  23. Osmoregulation • Marine animals – isotonic to seawater - hypotonic environment: cell gains water, swells, and may burst (lyse) - hypertonic environment: cell shrivels and can die from water loss - cells must have a way to balance an excessive uptake or excessive loss of water • Osmoregulation – control of water balance in animals - freshwater fish, hypotonic to its environment: kidneys and gills work constantly to prevent excess buildup of water - Paramecium’s contractile vacuole bails out excess water continuously

  24. Figure 5.14 Animal and plant cells in different osmotic environments

  25. Water Balance in Plant Cells • Water balance in plant cells is differentbecause of their rigid cell walls - isotonic solution plant cell is flaccid (floppy) - hypotonic environment plant cell is firm (turgid), with a net inflow of water, the cell wall expands but the back pressure it exerts prevents too much water intake and bursting - hypertonic environment, plant cell loses water, shrivels, and its PM pulls away from the cell wall (plasmolysis)

  26. Plant Turgor An underwatered plant wilts due to a drop in turgor Figure 5.15

  27. Active Transport: The Pumping of Molecules Across Membranes • Active transport requires energy to move molecules across a membrane - cellular energy drives a transport protein to actively pump a solute across a membrane against the solute’s concentration gradient • Active transport enables cells to maintain internal concentrations of small solutes that differ from environmental concentrations • Sodium-Potassium pump: vital to nerve signals - a neuron has a much higher potassium ion and much lower sodium ion concentrations

  28. Active Transport Figure 5.16 • Transport proteins -specific in recognition of atoms or molecules • Transport protein - binding site accepts only a certain solute • ATP energy - pumps the solute against its concentraction gradient

  29. Exocytosis and Endocytosis: Traffic of Large Molecules • Exocytosis - secretes substances outside of the cell - too big to fit through the membrane - traffic into and out of the cell depends on the membrane to form sacs or vesciles - during protein production secretory proteins exit the cell from transport vesicles that fuse with the PM • Example: cells in tear glands use exocytosis to export salty tears

  30. Figure 5.17a

  31. Takes material into the cell within vesicles that bud inward from the PM Figure 5.17b

  32. Endocytosis • 3 types: - Phagocytosis (‘cellular eating’): a cell engulfs a particle and packages it within a food vesicle - Pinocytosis (‘cellular drinking’): a cell ‘gulps’ droplets of fluid along with its solutes by forming tiny vesicles (nonspecific) - Receptor-mediated endocytosis: is triggered by the binding of external molecules to specific PM receptor proteins that transport the specific molecules into the cell

  33. Phagocytosis An amoeba uses a pseudopodium (cellular extension) to engulf food and package it into a food vacuole. A similar process is used by WBCs of your immune system to destroy invaders Figure 5.18

  34. Cholesterol Uptake • Cholesterol circulates in the blood in low-density lipoproteins (LDLs) • Proteins embedded in LDLs attach to receptor proteins in liver cell PMs – enables liver cells to take up LDLs and process the cholesterol Fig 5.19

  35. The Role of Membranes in Cell Signaling • Cellular communication – cells talk to each other • by chemical signaling across their PM • begins with the reception of an extracellular signal, such as a hormone, by a specific PM receptor protein • Triggers a chain reaction • The signal transduction pathway • consists of proteins and other molecules that relay the signal and convert it to chemical forms that can function within the cell • chemical responses, activation of metabolic functions, and structural responses, rearrangement of the cytoskeleton

  36. Runner ‘psyched up’ for a race, adrenal gland cells secrete epinephrine into the bloodstream • Signal reaches muscle cells and is recognized by PM receptor proteins • Triggers responses (glycogen into glucose) without the hormone ever entering the cell Figure 5.20

  37. Evolution Connection: Evolving Enzymes • Organisms use many different enzymes • How could such diversity arise? • Analysis of genetic sequences suggests: • distinct enzymes came about through molecular evolution: one ancestral gene randomly duplicated, and the 2 copies of that gene diverged over time through genetic mutations that produced 2 distinct enyzmes Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

  38. Directed Evolution • A lab procedure in which genes for a particular enzyme were mutated at random, • produced new enzymes that recognized new substrates • Case Study: lactase - enzyme that splits the milk disaccharide into its 2 component monosaccharides - many copies of the gene were mutated at random - mutated enzymes were screened for those that could best perform a new function - selected enzymes were subjected to another round of duplication, mutation, and selection • Natural selection and directed evolution

  39. The Enzyme Lactase Figure 5.21