Chapter 4

1. Chapter 4 How Cells Work

2. Energy • Energy is central to life • Universal relationship between energy and work • Ultimate energy source = SUN • Plants transform light energy into chemical energy (C6H12O6) • Photosynthesis

3. What is energy? • Energy = capacity to do work • Work = movement against an opposing force

4. The Energy Currency Molecule • Adenosine triphosphate, ATP • Structure of ATP • Adenosine • Ribose + adenine • 3 phosphate groups • Negatively charged molecules that repel each other • 2 HIGH ENERGY phosphate bonds

5. How do cells use ATP? • Breakage of the last bond (release of a phosphate group) releases energy and allows the cell to do work • ATP = STORED ENERGY = Potential • ADP = CELL PERFORMED WORK = Kinetic

6. How Cells Use ATP ATP → ADP + Pi + energy

7. How do cells use ATP? • From where does the energy to make ATP from ADP come? • Covalent bonds in macromolecules! • Cells recycle the ADP and phosphates • This process requires ENERGY!! • Analogous to recharging a battery: • The components are in the battery – energy needs to be added to the battery to make it useable

8. Energy Reactions and Cycles • Endergonic Reactions • Require energy • i.e. Synthesis of glucose from CO2 and water during photosynthesis • Exergonic Reactions • Release energy • i.e. Breakdown of glucose to CO2 and water by aerobic respiration

9. Endergonic Exergonic

10. Enzymes • Reaction characteristics • Exergonic reactions in living things may not occur very quickly • Energy of Activation (high temperature, light) is needed to start the reaction • Amount of energy needed to start a reaction • Many different reactions are needed to complete a task • These reactions are linked together

11. Activation Energy

12. Ways to Lower the Energy of Activation • Enzymes • Protein catalysts that lower the amount of energy needed to get the chemical reaction going • They maintain their original chemical composition while causing a change in the substrate (reactant) • The specific shape of the enzyme allows it to catalyze only one reaction • Active site = place on the enzyme that binds substrate • Since the enzyme does NOT change its shape, it is REUSABLE

13. Figure 5.5

14. Figure 5.6

15. Figure 5.7

16. Altering the Rate of an Enzymatic Reaction • One can alter the rate by altering two key factors: 1. Temperature 2. pH 3. Coenzymes and Cofactors 4. Allosteric Regulators 5. Salt Concentration

17. Altering Temperature • Gradual ↑ in temperature will INCREASE the rate of the reaction • How? By an increase in the speed at which the molecules are moving • This results in increased collisions of the enzyme and substrate • Extremely low temperatures will SLOW DOWN or STOP the reaction • Why? The enzyme and substrate are moving too slow to collide • Extremely high temperatures will STOP the reaction • Why? Because the enzyme will be denatured!

18. Altering pH • Alterations in pH will STOP the reaction because the enzyme will be denatured! • Remember, a small pH change does NOT correlate with a small change in the pH of the environment!! • Why? pH scale is logarithmic

19. Figure 5.8

20. Enzyme Questions • The presence of an enzyme _____ the required energy of activation of a chemical reaction. • Generally, as the amount of substrate is increased, the rate of the reaction _____. • Raising the temperature to over 50C ___ the rate of an enzymatic reaction. • Lowering the pH for an enzyme that works best in a highly acidic environment ___ the rate for the reaction.

21. Ways that substances can move across the PM • Passive • Process that does NOT require energy • Includes: • Diffusion • Osmosis • Active • Process that DOES REQUIRE energy! • Includes: • Endocytosis • Phagocytosis • Pinocytosis • Receptor-Mediated Endocytosis • Exocytosis

22. How exactly do things move back and forth across a cell’s plasma membrane??

23. Selective Permeability • Protein channels located in the plasma membrane act as channels • Each channel passes only a certain kind of molecule (some are specific, some non-specific) • Types of selective permeability • Selective Diffusion • Facilitated Diffusion • Active Transport

24. Selective Diffusion • Movement of molecules from high concentration to low concentration • Channels may act as ‘open doors’ • Example includes ion channels • Ion channels allow passage of any ion that can fit in the channel • Essential roles in nervous system signaling

25. Diffusion • Oxygen, Carbon dioxide (CO2) and lipids can pass across the PM using diffusion • One way in which water and other substances can move across the PM • RANDOM movement of molecules in a solution from regions of HIGH concentration to regions of LOW concentration • HIGH low • Random movement occurs until equilibration occurs • Until there is NO NET MOVEMENT in any particular direction • NOTE: Individual molecules are still moving – but there is no overall directionality!

28. DiffusionTerms to know: • Concentration gradient • A system that is imposed on a solution by molecules present in that solution. • Ex. Sugar in water • When sugar is dropped in water, the sugar molecules break up and dissolve over time. The individual sugar molecules moving into the water move DOWN their concentration gradient – they are moving from the cube of sugar to spread out in the water where there is no sugar.

29. Osmosis • Movement of WATER ONLY across the PM from the side with more water (less solute) to the side with less water (more solute) • Water passes into and out of a cell down its concentration gradient (DIFFUSION) • DIFFERENT from diffusion in that water movement depends upon the concentration of other substances in solution

30. OsmosisTerms to know: • Osmotic concentration • Concentration of ALL molecules dissolved in a solution • Hypertonic • The solution with higher solute concentration • Hypotonic • The solution with lower solute concentration • Isotonic • Solutions are isotonic when the solute concentrations of both are equal

31. Figure 4.27

32. OsmosisOsmotic pressure • Generated by movement of water into a cell by osmosis • Ex. Red blood cell, Figure 4.28

33. Osmosis

34. What is another way that cells can take in food and liquids? • Diffusion • Osmosis • Endocytosis • Phagocytosis • Pinocytosis • Receptor-Mediated Endocytosis • Exocytosis

35. Endocytosis • Allows for BULK PASSAGE of food and liquids INTO the cell • Two types: • Phagocytosis • “Cell eating” • Pinocytosis • “Cell drinking” • The PM engulfs the particle(s) forming a vesicle thus allowing a means of entry into the cell

36. EndocytosisPhagocytosis: • “Cell eating” • Material that the cell takes in may include particulate, digested particles or other fragments of organic matter Pinocytosis: • “Cell drinking” • Material that the cell takes in is liquid

37. Phagocytosis and Pinocytosis

38. Endocytosis, continued • Rates of endocytosis vary among cells! • Ex. Muscle cells during exercise

39. Exocytosis • Process by which material is discharged from the cell • Material to be discharged is packaged into vesicles inside the cell (by what organelle?) • Vesicles then make they way (along what?) to the plasma membrane for secretion into the cell exterior

40. Exocytosis

41. Problems with endocytosis • Expensive for the cell • Cell uses a lot of its membrane to form vesicles • Non selective • Anything can enter the cell through

42. Everyday ScienceHypercholesterolemia • Human genetic disease • Receptors are normally embedded in the PM • In patients with HC, the receptors are not help in place by clathrin • This results in a failure of cholesterol uptake into the cell (failure of the mouse-trap triggering mechanism) thus leaving the cholesterol to travel though the bloodstream and bind to arteries

44. Discussion Question • Do you think muscle cells have a higher or lower rate of endocytosis during exercise?