Energy and Cell Metabolism

• Requirements of the cell

  • A way to encode and transmit information
  • A membrane separating the inside of the cell from the outside
  • ENERGY
• The laws of thermodynamics determine whether a chemical reaction requires or releases free energy (i.e., available to do work)
• Whether this reaction will require energy or release it depends on the net change in Gibbs Free Energy (ΔG)

  • Gibbs Free Energy is the amount of energy in a system available to do work.
• The difference (or net change) in Gibbs Free Energy between the reactants and the products of a chemical reaction is written as ΔG
• If the products of a reaction have more free energy than the reactants, then ΔG is positive.

  • Reactions with a positive ΔG require an input of energy and are endergonic
• If the reactants of a reaction have more free energy than the products, then ΔG is negative.

  • Reactions with a negative ΔG release energy and are exergonic, and they will occur spontaneously
• Even exergonic (spontaneous) reactions don’t always proceed quickly on their own because of activation energy
• In order to get from reactants to products, the reacting atoms must go through a transition state that actually has higher free energy than the reactants.

  • This is called the activation energy
• Enzymes can reduce the activation energy, but they do not change the total ΔG of the reaction
• Enzymes! 1.When the activation energy is low, the reaction is faster.

  2. Enzymes are able to reduce the activation energy by stabilizing the transition state.

  3. The rate of the reaction increases because the activation energy is reduced.
• Nonspontaneous endergonic reactions (those that REQUIRE energy) are often coupled to spontaneous exergonic reactions (those that RELEASE energy): Coupled Reactions
• So the energy released from a spontaneous, exergonic reaction essentially “fuels” the nonspontaneous endergonic reaction

  • these reactions are usually coupled by enzymes, which can bind to both sets of reactants
• Cell metabolism: the sum of all chemical rxs that occur in a cell

  • Driven by the building and breaking down of carbon sources to harness or release energy.
• Sometimes the cell needs to build molecules (e.g., synthesis of RNA strand in transcription)

  • Anabolism: Building of molecules; requires free energy
• Sometimes the cell needs to break down molecules (e.g., for energy stored in bonds)

  • Catabolism: Breakdown of molecules; net release of free energy
• Adenosine Triphosphate (ATP) stores energy in a form that cells can use to perform work

  • Hydrolysis of ATP drives many reactions in cells.
  • The hydrolysis of ATP is spontaneous and produces free energy (exergonic
• Adenosine Triphosphate (ATP) stores energy in a form that cells can use to perform work

  • The phosphate groups of ATP are negatively charged and repel each other making ADP more stable than ATP.
  • This energy can be used to do work
• The energy associated with ATP hydrolysis (breakdown) and production is “intermediate,” making it a useful form of energy “currency”

  • Thus, ATP is central to all cellular functions and thus acquiring it is a major priority for all cells
• All living organisms can be divided into two groups based on how they obtain energy: Phototrophs and chemothrophs
• The goal of cellular respiration is to break down the food you eat and make ATP out of it
• Cellular respiration is a series of catabolic reactions
• Why is ATP better than glucose for storing energy?

  • Glucose has A LOT of energy, but it’s “locked up” in its very stable bonds
• Why is ATP better than glucose for storing energy?

  • ATP can also store a lot of energy in its phosphate bonds, but this energy is way more accessible
• Why is ATP better than glucose for storing energy?

  • ATP a nice “package” for storing potential energy – can readily be released to drive exergonic reactions in the cell and, in turn, easily get replenished when coupled to more exergonic hydrolysis reactions
• ATP is produced in 2 ways in cellular respiration:

  • Substrate-Level Phosphorylation and Oxidative Phosphorylation of ADP
• Substrate-level phosphorylation: A phosphate is transferred from one molecule to another

  • Phosphate transferred from an enzyme substrate to ADP to form ATP via a coupled reaction (occurs in first few steps of cellular respiration)
• Oxidative phosphorylation: Electron movement is coupled to ATP synthesis

  • Occurs through a series of coupled oxidation and reduction, or “redox” reactions that transfer electrons from one molecule to another
• Why is transferring electrons from one molecule to another (i.e., redox reactions) such a great way to make ATP?

  • Electrons can contain a lot of potential energy
  • During a redox rx, when an electron is transferred from a “donor” molecule to an “acceptor” molecule, some of that energy is released
• Why is transferring electrons from one molecule to another (i.e., redox reactions) such a great way to make ATP?

  • Thus, way more efficient process than 1:1 substrate-level phosphorylation
• Electrons come from the bonds in fuel molecules

  • The first few steps of cellular respiration are dedicated to extracting electrons from the bonds in glucose (and its subsequent derivatives) via oxidation, and then transferring them to electron carriers via reduction
• Electron Carriers: NADH and FADH2

  • The oxidized forms of these carriers are NAD+ and FAD; when you add 2 e - and H +, you convert carriers to their reduced forms, NADH and FADH 2.
• Electron carriers: NADH and FADH2

  • These electron carriers will carry electrons to the final step of cellular respiration, the only time in cellular respiration when oxidative phosphorylation occurs
• Aerobic cellular respiration has 4 stages

  1. Glycolysis --> Fermentation

  2. Pyruvate oxidation

  3. Citric acid cycle

  4. Oxidative phosphorylation
• Stage 1: Glycolysis

  • Where are the reactions taking place? Cytoplasm
  • What are the inputs? Glucose, 2 ATP
  • What are the outputs? 4 ATP, 2 NADH, 2 pyruvate
  • What is the ATP “payoff”? 2 net ATP
  • Where is the potential energy stored throughout the process?
  • Chemical bonds, ATP, Electron carriers
• Glycolysis is a series of 10 anaerobic chemical reactions that occur in the cytoplasm
• The starting molecule for glycolysis is a six-carbon molecule, glucose (C 6H 12O6); the end products are two three-carbon molecules, pyruvate (C 3H 3O3)
• Glycolysis can be divided into 3 phases1.Destabilization: Glucose is prepared for the next two phases by the addition of two phosphate groups.

  • This process requires an input of two molecules of ATP.
  • Traps glucose inside cell
  • Destabilizes molecule
• Glycolysis can be divided into 3 phases

  2.Cleavage: 6 carbon glucose split into two 3-carbon sugars (“glyco-lysis”)
• Glycolysis can be divided into 3 phases3. Pay-off phase:

  1. Oxidize each 3-C molecule, reduce NAD+ to NADH
  2. Hydrolyze each 3-C molecule (exergonic), which is coupled to phosphorylation of ADP -> ATP (endergonic) via substrate-level phosphorylation
  3. Repeat
  4. What’s left over = 2 pyruvates
• After Glycolysis is either Fermentation (Anaerobic) or Aerobic respiration