Metabolism and Energy in the Cell ATP to ADP Cycle

Metabolic Pathways

1. Metabolism and Energy in the Cell

2. Go, Grow and Glow

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18. The sugars you take have a sufficient supply of stored energy in their chemical bonds to power your activities for the day. However, the cells in your body cannot directly use these sugars to fill up its energy demands. Introduction

19. The energy from these organic molecules must be transferred to another molecule called adenosine triphosphate (ATP) . You will mostly encounter the molecule adenosine triphosphate and enzymes in studying photosynthesis and metabolism. ATP is commonly known as the energy currency of the cell. It powers all the metabolic activities of the cell, whereas enzymes are biological catalysts that hasten chemical reactions.

20. METABOLISM

21. Metabolism refers to all the chemical reactions that transform energy inside the body. It takes place in an organized, step-by-step sequence called metabolic pathways , wherein the product of one reaction becomes the substrate of another reaction.

22. Enzymes affect metabolic reactions to occur fast enough to sustain the building and breaking down of molecules in the cell. There are two types of metabolism: anabolism and, catabolism.

23. ANABOLISM -the building of large molecules from small molecules. It is very similar to how cars are assembled, which requires a lot of energy because the small components are being assembled to form the whole.

24. Likewise, in anabolism, small molecules or subunits join to form large ones. In this way, energy is necessary. A good example of the anabolic process in plants is photosynthesis in which simple inorganic molecules such as carbon dioxide and water join to form the macromolecule glucose.

25. CATABOLISM -the breaking down of large molecules to form small molecules. It is similar to when your car is disassembled or meets an accident; it releases sound and heat.

26. In catabolism, large molecules degrade into smaller molecules or subunits, and energy is released. A good example of catabolism is cellular respiration, wherein large organic molecules of proteins are broken down into their simplest usable forms called amino acids.

27. Sample Diagram of Anabolism and Catabolism Metabolic Pathways

28. ATP: The Energy Currency of the Cell

29. Adenosine triphosphate (ATP) is made up of an organic molecule adenosine, plus a tail of 3 phosphate groups. The phosphates are connected by high-energy bonds.

30. Most of the energy in ATP is found in the bonds of the triphosphate group . It is the release of the phosphate at the tip of the triphosphate that makes energy available. This process energizes the molecule receiving the phosphate group to be used in later reactions. The process is called phosphorylation .

31. After the removal of the phosphate end, the remaining molecule is now called adenosine diphosphate (ADP) . ATP provides energy to other molecules inside the cell by breaking the bonds that hold the phosphate groups together which releases energy. Thus, the cell can perform mechanical, chemical, and transport processes.

32. ATPs in the cell are spent continuously. Fortunately, they are recyclable sources. ATP can be restored after use by adding a phosphate group back to ADP. The continuous breakdown of ATP to ADP drives all life processes; hence the trillion cells in your body must cleave 1 to 2 billion ATP molecules to ADP every minute.

33. The possible source of such ATP is the continuous recycling of ADP and AMP (Adenosine Monophosphate) to ATP. This is where food is essential. The cell has to be nourished with nutrients for the continuous supply of ATP.

34. Thus, energy coupling happens during the transfer of energy from processes that yield energy, such as the breaking down of food, to processes that consume energy, such as cell division and muscle contraction.

35. Grade 12 Biology Activity: The ATP-ADP Cycle Objective : To understand the mechanisms of ATP synthesis, breakdown, and its role in cellular energy transfer. Activity : Read the passage below and complete the questions that follow.

36. ATP and ADP – The Cellular Energy Cycle Adenosine triphosphate (ATP) is often referred to as the “energy currency” of the cell. It is the primary molecule used to store and transfer energy in cells. ATP is made up of adenosine (adenine + ribose) and three phosphate groups. When the bond between the second and third phosphate group is broken, ATP is converted to adenosine diphosphate (ADP) and a free phosphate (Pi), releasing energy in the process. The conversion of ADP back into ATP requires energy, which is typically obtained from the breakdown of glucose during cellular respiration. This cycle of ATP converting into ADP and then being regenerated back into ATP is continuous and essential for maintaining life.

37. ATP stores and releases energy by adding or removing phosphate groups in a cycle. When a cell needs energy, ATP breaks down into ADP (adenosine diphosphate) and an inorganic phosphate (Pi), releasing energy that can be used for cellular processes like muscle contraction, active transport, and chemical reactions. When energy is available (from food molecules like glucose), ADP is converted back into ATP by adding a phosphate group. ATP → ADP + Pi + Energy ADP + Pi + Energy → ATP This continuous cycle is vital for life, as cells need a constant supply of energy to function.

38. Questions : Understanding ATP Breakdown: a. What happens to ATP when it releases energy? b. What is the by-product of ATP breakdown, and how is energy involved? Energy in Cellular Processes: List three cellular activities that depend on the energy released from ATP. Briefly describe the role of ATP in one of these activities. ATP Regeneration: a. What process converts ADP back into ATP? b. Identify the source of energy that is used to regenerate ATP in cells.

39. Creative Thinking: Draw a simple diagram of the ATP-ADP cycle. Be sure to label: ATP ADP Phosphate (Pi) Energy release Energy input for ATP formation

40. The Gibbs free energy (G) of a system is a measure of the amount of usable energy (energy that can do work) in that system. The change in Gibbs free energy during a reaction provides useful information about the reaction's energetics and spontaneity (whether it can happen without added energy). Gibbs free energy, enthalpy, and entropy

41. In a practical and frequently used form of Gibbs free energy change equation, ΔG is calculated from a set values that can be measured by scientists: the enthalpy and entropy changes of a reaction, together with the temperature at which the reaction takes place. Gibbs free energy, enthalpy, and entropy

42. Gibbs free energy, enthalpy, and entropy

43. Let’s take a step back and look at each component of this equation.

44. ∆H is the enthalpy change. Enthalpy in biology refers to energy stored in bonds, and the change in enthalpy is the difference in bond energies between the products and the reactants. A negative ∆H means heat is released in going from reactants to products( exothermic reactions ), while a positive ∆H means heat is absorbed ( endothermic reactions ).

45. ∆S is the entropy change of the system during the reaction. It is the measurement of randomness or disorder If ∆S is positive, the system becomes more disordered during the reaction (for instance, when one large molecule splits into several smaller ones). If ∆S is negative, it means the system becomes more ordered.

46. Temperature (T) determines the relative impacts of the ∆S and ∆H terms on the overall free energy change of the reaction. (The higher the temperature, the greater the impact of the ∆S term relative to the ∆Hterm.) Note that temperature needs to be in Kelvin (K) here for the equation to work properly.

47. Reactions with a negative ∆G release energy, which means that they can proceed without an energy input (are spontaneous ). In contrast, reactions with a positive ∆G need an input of energy in order to take place (are non-spontaneous ).

48. Reactions that have a negative ∆G release free energy and are called exergonic reactions. (Handy mnemonic: EXergonic means energy is EXiting the system.)

49. A negative ∆G means that the reactants, or initial state, have more free energy than the products, or final state. Exergonic reactions are also called spontaneous reactions, because they can occur without the addition of energy.

50. Reactions with a positive ∆G (∆G > 0), on the other hand, require an input of energy and are called endergonic reactions. In this case, the products, or final state, have more free energy than the reactants, or initial state.

51. Endergonic reactions are non-spontaneous , meaning that energy must be added before they can proceed. You can think of endergonic reactions as storing some of the added energy in the higher-energy products they form.

52. It’s important to realize that the word spontaneous has a very specific meaning here: it means a reaction will take place without added energy, but it doesn't say anything about how quickly the reaction will happen.

53. As you can see from the equation, both the enthalpy change and the entropy change contribute to the overall sign and value of ∆G. When a reaction releases heat (negative ∆H) or increases the entropy of the system, these factors make ∆G more negative. On the other hand, when a reaction absorbs heat or decreases the entropy of the system, these factors make ∆G more positive.

54. Coupled Reactions How is the energy released by ATP hydrolysis used to power other reactions in a cell? In most cases, cells use a strategy called reaction coupling, in which an energetically favorable reaction (like ATP hydrolysis) is directly linked with an energetically unfavorable (endergonic) reaction.

55. Coupled Reactions How is the energy released by ATP hydrolysis used to power other reactions in a cell? The linking often happens through a shared intermediate, meaning that a product of one reaction is “ picked up ” and used as a reactant in the second reaction.

56. ENZYMES AND HOW THEY WORK

57. The cell is considered a metabolic machinery. Its metabolic efficiency is regulated by biological catalysts called enzymes, which are mostly globular protein molecules. Enzymes are made of proteins with occasional nonprotein parts. They are usually named by adding the suffix - ase to the name of the substrate.

58. For example, an enzyme that acts on a protein is called protease , while one that acts on a lipid is called lipase . Enzymes are proteins usually in the tertiary or quaternary structure that are coupled with cofactors or coenzymes for their catalytic activity.

59. For example, an enzyme that acts on a protein is called protease , while one that acts on a lipid is called lipase . Enzymes are proteins usually in the tertiary or quaternary structure that are coupled with cofactors or coenzymes for their catalytic activity.

60. Coenzymes are transiently associated with an inactive enzyme called apoenzyme to make a functional enzyme, referred to as holoenzyme. Coenzymes are molecules that must be present for an enzyme to be able to catalyze a chemical reaction.

61. Two particularly important coenzymes that serve as electron carriers are Nicotinamide Adenine Dinucleotide (NAD) and Flavin Adenine Dinucleotide (FAD). During dehydrogenation, they are respectively reduced to NADH and FADH.

62. Other nonprotein cofactors are more or less permanently associated with the enzyme protein. Metal ions such as copper, zinc, magnesium, and iron cofactors are sometimes referred to as metal activators.

63. Enzymes are highly specific protein catalysts. A given enzyme can only act or work on only one type of compound, which is called the substrate , and it can only perform one type of reaction. On the surface of each enzyme molecule is one small area or few areas called the active site where the substrates bind.