Energy and Metabolism Practice Flashcards

Bioenergetics and Cellular Metabolism

• Bioenergetics: This is the term used by scientists to describe the concept of energy flow through living systems, such as cells.

• Stepwise Chemical Reactions: Cellular processes, including the building (synthesis) and breakdown (degradation) of complex molecules, occur through a series of stepwise chemical reactions.

• Metabolism Defined: The sum total of all chemical reactions that take place inside cells is referred to as the cell's metabolism. This includes both the reactions that consume energy and those that generate it.

• Energy Consumption and Replenishment: Just as organisms must consume food to replenish energy, individual cells must obtain energy constantly to power reactions that are non-spontaneous.

Metabolic Pathways

• Definition of Metabolic Pathway: A metabolic pathway is a series of interconnected chemical reactions. It starts with a specific molecule, modifies it step-by-step through metabolic intermediates, and eventually produces a final product.

• Enzymatic Catalysis: The chemical reactions in these pathways do not occur spontaneously at sufficient rates. Each step is facilitated or catalyzed by a specific protein called an enzyme.

• The Two Primary Types of Pathways:

  • Anabolic Pathways: These pathways involve synthesizing large, complex molecules (polymers) from smaller ones (monomers). This process requires an input of energy. Example: Building a protein from amino acids.

  • Catabolic Pathways: These pathways involve the degradation or breaking down of large molecules into smaller ones. This process releases energy. Example: Breaking down sugar for fuel.

• Energy Balance: Both anabolic and catabolic pathways are essential for maintaining a cell's balance of energy.

The Metabolism of Sugar: A Case Study

• Sugar as Energy Storage: Sugar molecules, such as glucose, store a significant amount of energy within their chemical bonds.

• Photosynthesis (Anabolic):

  • Organisms such as plants use sunlight to convert carbon dioxide ($CO_2$) and water ($H_2O$) into glucose ($C_6H_{12}O_6$).

  • Reaction Equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

  • Energy Input: This process requires an input of energy initially captured from sunlight. During the light reactions, energy is provided by adenosine triphosphate (ATP).

  • ATP as Currency: Adenosine triphosphate (ATP) acts as the primary energy currency of all cells, used for immediate work, similar to how a dollar is used as currency for goods.

• Sugar Breakdown (Catabolic):

  • In cells requiring oxygen, energy is harvested by breaking down glucose. This is essentially the reverse of photosynthesis.

  • Reaction Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6H_2O + 6CO_2$

  • Waste Products: Oxygen is consumed, and carbon dioxide is released as a waste product.

Thermodynamics and Energy Systems

• Thermodynamics: The study of energy and energy transfer involving physical matter.

• System vs. Surroundings:

  • System: The specific matter under study (e.g., a pot of water on a stove).

  • Surroundings: Everything outside the defined system.

• Types of Systems:

  • Open System: Energy and matter can be exchanged with the surroundings. Biological organisms are open systems because they take in energy (sunlight or food) and release heat and waste to the environment.

  • Closed System: A system that cannot exchange energy or matter with its surroundings.

• Definition of Energy: The ability to do work or create change. It exists in various forms, including electrical, light, and heat energy.

The Laws of Thermodynamics

• First Law of Thermodynamics: Also known as the law of conservation of energy.

  • The total amount of energy in the universe is constant.

  • Energy may be transferred from one place to another or transformed into different forms, but it cannot be created or destroyed.

  • Examples: Light bulbs transform electrical energy into light and heat. Gas stoves transform chemical energy into heat. Plants transform sunlight into chemical energy.

• Second Law of Thermodynamics: Explains why energy transfers are never 100% efficient.

  • In every energy transfer, some energy is lost in an unusable form, typically heat energy.

  • Heat Energy: Defined as energy transferred from one system to another that is not performing work.

  • Entropy: A measure of randomness or disorder within a system. Higher disorder equals higher entropy ($S$).

  • As energy is lost to the surroundings, the system becomes less ordered and more random.

  • The second law states that entropy in the universe always increases during energy transfers.

  • Biological Implication: Living things are highly ordered and require constant energy input to maintain a state of low entropy.

Kinetic and Potential Energy

• Kinetic Energy: Energy associated with objects in motion.

  • Examples: A moving wrecking ball, a speeding bullet, a walking person, and the rapid movement of molecules in air (heat).

• Potential Energy: Energy that is stored due to an object's position, structure, or state.

  • Positional Example: A wrecking ball lifted above the ground has potential energy due to gravity. When it falls, that potential energy is converted back into kinetic energy.

  • Structural Example: A compressed spring or a pulled rubber band stores potential energy.

• Chemical Energy: A specific type of potential energy stored within chemical bonds.

  • In food molecules, the breaking of molecular bonds releases this energy for cellular use.

Free Energy and Activation Energy

• Free Energy ($G$): Specifically refers to the energy associated with a chemical reaction that is available to do work after accounting for losses (entropy/heat).

• Change in Free Energy ($\Delta G$): Used to quantify the transfer of energy during a reaction.

• Exergonic Reactions:

  • $\Delta G$ is a negative number (\Delta G < 0).

  • Energy is released from the system (energy exits).

  • Products have less free energy than the reactants.

  • These are considered spontaneous reactions, though "spontaneous" does not mean "fast" (e.g., the rusting of iron is a slow spontaneous reaction).

• Endergonic Reactions:

  • $\Delta G$ is a positive value (\Delta G > 0).

  • Energy is absorbed/required.

  • Products have more stored energy than the reactants.

  • These are non-spontaneous reactions; they cannot occur without an external input of free energy.

• Activation Energy ($E_a$): The small amount of energy input required for all chemical reactions to begin, even exergonic ones. This energy allows the reactants to reach a transition state.

Enzymes: Biological Catalysts

• Catalyst: A substance that helps a chemical reaction occur.

• Enzymes: Molecules (mostly proteins) that catalyze biochemical reactions by lowering the activation energy ($E_a$).

• Key Characteristics of Enzymes:

  • They speed up reactions that would otherwise occur too slowly to sustain life.

  • They do not change the $\Delta G$ of a reaction. They do not change whether a reaction is exergonic or endergonic.

  • They remain unchanged and are not consumed by the reaction; they can be reused multiple times.

• The Active Site:

  • The specific location on the enzyme where the reactant binds.

  • The reactants that bind to the enzyme are called substrates.

  • The active site has a unique environment dictated by the side chains (R groups) of its constituent amino acids (which can be acidic, basic, polar, non-polar, etc.). This environment is specific to a particular substrate.

• Induced Fit Model: Replaces the older "lock and key" model. It posits that as the enzyme and substrate interact, the enzyme undergoes a mild conformational shift to create an ideal binding arrangement.

How Enzymes Lower Activation Energy

• Optimal Orientation: Bringing two or more substrates together in the correct position for them to react.

• Optimal Environment: Providing a specific pH or polarity within the active site that favors the reaction.

• Distorting Substrates: Compromising or stressing the chemical bond structure of the substrate, making it easier for the bonds to break.

• Direct Participation: Taking part in the chemical reaction itself, provided the enzyme returns to its original state afterward.

Enzyme Regulation and Inhibition

• Environmental Factors: Enzymes have an optimal temperature, pH, and salt concentration range.

  • Denaturation: Extreme conditions can cause an irreversible change in the enzyme's three-dimensional shape, rendering it non-functional.

• Competitive Inhibition: An inhibitor molecule similar in shape to the substrate binds to the active site, physically blocking the substrate.

• Noncompetitive (Allosteric) Inhibition: An inhibitor binds to an allosteric site (a location other than the active site). This induces a conformational change that reduces the enzyme's affinity for the substrate or prevents its catalysis.

• Allosteric Activation: An activator molecule binds to an allosteric site to induce a conformational change that increases the affinity of the active site for the substrate.

• Cofactors and Coenzymes:

  • Helper molecules required for optimal enzyme function.

  • Cofactors: Inorganic ions like iron ($Fe^{2+}$) or magnesium ($Mg^{2+}$).

  • Coenzymes: Organic helper molecules, often derived from vitamins (e.g., Vitamin C is a coenzyme for collagen synthesis).

Feedback Inhibition in Metabolic Pathways

• Mechanism: Cells use the products of a metabolic reaction to inhibit further production. This prevents waste by slowing down pathways when products are abundant.

• Amino Acids and Nucleotides: Production of these molecules is frequently controlled via feedback inhibition.

• ATP Regulation:

  • ATP acts as an allosteric inhibitor for enzymes involved in its own production (sugar catabolism).

  • Conversely, ADP (adenosine diphosphate) acts as an allosteric activator for those same enzymes, signaling the cell to produce more ATP when levels are low.

Career Connection: Pharmaceutical Drug Development

• Drug Targets: Many drugs function by inhibiting specific enzymes in metabolic pathways.

• Statins: These are drugs used to reduce cholesterol. They inhibit the enzyme HMG-CoA reductase, which synthesizes cholesterol from lipids.

• Acetaminophen (Tylenol): This drug inhibits the enzyme cyclooxygenase, though its exact mechanism in reducing fever and pain is not fully understood.

• Development Process: Scientists identify a target, understand the biological pathway, design molecules to activate or block it, and conduct in vitro experiments and clinical trials before seeking U.S. Food and Drug Administration (FDA) approval.