Cells and Energy Flow: Thermodynamics

Cells and Energy Flow

• Energy is the capacity to perform work or cause alteration
• Kinetic energy is the energy of motion, like water flowing over a waterfall
• Potential energy is stored energy, such as chemical energy in food
• Energy flows, it does not cycle

Laws of Thermodynamics

• The Law of Conservation of Energy states that energy is neither created nor destroyed, but changes form, for e.g. photosynthesis
• The Law of Entropy states that energy transformations lose usable energy
• No energy conversion is ever 100% efficient
• Energy transformation makes the universe less organized, with disorder increasing

Producers and Energy

• Producers use energy to create organized structures in biological molecules

Carbohydrate Metabolism

• Potential energy stored in carbohydrates powers muscle movement
• All living organisms rely on a constant supply of solar energy

Cells and Entropy

• Cellular processes mediate energy transformations
• Every cellular process increases the total entropy in the universe
• Less energy is available for useful work as a result
• Glucose breaks down into carbon dioxide and water over time
• Glucose is more organized and less stable than its breakdown products
• Photosynthesis uses energy from the sun to create glucose from carbon dioxide and water

Metabolism and Energy Transformations

• Metabolism sums up all chemical reactions in a cell
• Free energy (delta G) measures energy available for work post-reaction
• Delta G equals the free energy of products minus the free energy of reactants
• Exergonic reactions have a negative delta G, meaning they are spontaneous and release energy, as products have less free energy than reactants
• Endergonic reactions have a positive delta G, they are non-spontaneous and require energy, as products have more free energy than reactants

ATP: Energy Currency

• ATP is unstable with high potential energy
• ATP is nucleotide formed of adenine, ribose sugar (adenosine), and three phosphate groups
• ATP supplies energy for synthesis of macromolecules (chemical work), pumping substances across membranes (transport work), and muscle contraction (mechanical work)

ATP Cycle

• ATP is not stored in cells
• ATP captures energy released by exergonic reactions
• ATP carries chemical energy for endergonic reactions
• Cells use ATP hydrolysis in two ways to facilitate energy-requiring reactions
• ATP energizes a reactant or changes the shape of a reactant

Metabolic Pathways and Enzymes

• Metabolic pathways link reactions in a sequence
• Metabolic pathways begin with a reactant, go through intermediates, and end with a final product
• In a pathway A→B→C→D→E→F→G, "A" is the initial reactant/substrate, B, C, D, E, and F are intermediates, and "G" is the end product
• Enzymes are protein catalysts
• Substrates are reactants in enzyme-catalyzed reactions
• Each enzyme accelerates a specific reaction
• Each reaction in a metabolic pathway needs unique enzyme
• All the enzymes are needed for the end product to form
• Enzymes complex with substrate at their active site
• This changes the shape of the active site
• The enzyme undergoes a slight change in it's shape to bind the substrate via the induced fit model
• Once a reaction completes, the product is released and the active site returns to its original shape
• During degradation, the substrate is broken down into smaller products
• During synthesis, substrates are combined to produce a larger product
• Energy of Activation is the energy needed to start a reaction
• EA prevents spontaneous degradation in cells
• Enzymes lower the energy of activation by bringing substrates into contact and speeding up the reaction

Factors Affecting Enzymatic Rate

• Increased substrate concentration increases enzyme activity, due to more collisions between substrate molecules and enzyme
• Increased temperature increases enzyme activity
• Warmer temperatures cause more effective collisions between enzyme and substrate
• Hot temperatures denature and destroy enzymes
• Enzymes have optimal pH for reaction rate
• pH changes alter enzyme shape, reducing enzymatic speed
• Extreme pHs denature enzymes

Enzyme Cofactors

• Cofactors are molecules that activate enzymes
• FAD, NAD+, and NADP+ are cofactors
• FAD and NAD+ work in cellular respiration
• NADP+ operates in photosynthesis
• Coenzymes are nonprotein organic molecules
• Vitamins are organic compounds needed in the diet for coenzyme synthesis

Enzyme Inhibition

• Inhibitors decrease enzyme activity
• Competitive inhibition occurs when both substrate and inhibitor compete to bind to the active site
• Noncompetitive inhibition occurs when the inhibitor binds somewhere other than the active sire, at an allosteric site

Oxidation-Reduction (Redox) Reactions

• Electrons transfer between molecules
• Oxidation is electron loss
• Reduction is electron gain
• Both occur simultaneously
• One molecule/atom accepts the electron from another
• In NaCl production, sodium is oxidized and chlorine is reduced
• OILRIG: Oxidation Is Loss, Reduction Is Gain

Photosynthesis and Cellular Respiration

• Photosynthesis equation: 6CO2 + 6H2O + sun energy → C6H12O6 + 6O2
• Carbon dioxide is reduced and water is oxidized
• Cellular respiration equation is opposite of photosynthesis: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
• Glucose is oxidized and oxygen is reduced

Photosynthesis in Chloroplasts

• Chloroplasts use solar energy to convert water and carbon dioxide into carbohydrates
• Hydrogen atoms move from water to carbon dioxide, forming glucose
• Water is oxidized and carbon dioxide is reduced
• Reduction of carbon dioxide to glucose stores 686 kilocalories of energy per mole in chemical bonds
• The coenzyme NADP+ is involved
• NADP+ + 2e- + H+ → NADPH

Cellular Respiration in Mitochondria

• Mitochondria oxidize carbohydrates to build ATP
• Oxygen is consumed, and carbon dioxide produced
• Glucose is oxidized (loses hydrogen atoms) and oxygen is reduced (gains hydrogen atoms)
• When oxygen gains hydrogen atoms, it becomes water
• Cells oxidize glucose incrementally
• Energy is stored/converted to ATP
• Coenzyme NAD+ is involved
• NAD+ + 2e- + H+ → NADH