Chapter 6: Energy, Cells and Biological Reactions
Chapter 6: Energy, Cells, and Biological Reactions
Energy Fundamentals
Definition of Energy: The ability to do work or move matter.
Types of Energy:
Potential Energy: Stored energy that is available to do work. For example, a ball at the top of a hill has high potential energy.
Kinetic Energy: The energy actively being used to perform work. For example, a ball rolling down a hill has high kinetic energy.
Energy Units:
Calories (cal): The amount of energy required to raise the temperature of 1 gram of water by 1 degree Celsius.
Kilocalories (kcal): 1 ext{ kcal} = 1000 ext{ calories}. Often used to measure energy in food.
Laws of Thermodynamics
First Law of Thermodynamics (Law of Conservation of Energy):
Energy cannot be created or destroyed. It can only be converted from one form to another. For example, chemical energy in food is converted to kinetic energy for movement or heat energy to maintain body temperature.
Second Law of Thermodynamics:
Every energy transformation or reaction results in some energy being lost to the environment, primarily in the form of heat.
All energy transformations lead to an increase in entropy.
Entropy: A measure of randomness or disorganization within a system. Natural processes tend to move towards greater disorder.
Metabolism
Definition: The sum of all chemical reactions that occur within a cell. This includes both the building up and breaking down of molecules.
Types of Metabolic Reactions:
Endergonic Reactions:
Definition: Reactions that require an input of energy to proceed.
Characteristics: The products of endergonic reactions contain more potential energy than the reactants.
Example: Photosynthesis, where 6 ext{CO}2 + 6 ext{H}2 ext{O} (reactants with low energy) are converted into ext{C}6 ext{H}{12} ext{O}6 + 6 ext{O}2 (products with high energy) using light energy.
Example (Building Up): The formation of a polypeptide chain from amino acids: ext{NH}2 ext{CHRCOOH} + ext{NH}2 ext{CHRCOOH} o ext{NH}2 ext{CHRCONHCHRCOOH} + ext{H}2 ext{O}. This reaction builds a larger molecule, thus requiring energy input.
Exergonic Reactions:
Definition: Reactions that release energy as they proceed.
Characteristics: The products of exergonic reactions contain less potential energy than the reactants.
Example: Cellular respiration, where 6 ext{O}2 + ext{C}6 ext{H}{12} ext{O}6 (reactants with high energy) are broken down into 6 ext{CO}2 + 6 ext{H}2 ext{O} (products with low energy), releasing energy for cellular use.
Example (Breaking Down): The dissociation of sulfuric acid: ext{H}2 ext{SO}4 o 2 ext{H}^+ + ext{SO}_4^{-2}. This reaction breaks a larger molecule into smaller ions, thereby releasing energy.
Electrons and Energy Transfer
Role of Electrons: Electrons are crucial carriers of energy throughout biological reactions.
Redox Reactions (Oxidation and Reduction):
Oxidation: The loss of electrons from a molecule or atom. This often involves a decrease in potential energy.
Reduction: The gain of electrons by a molecule or atom. This often involves an increase in potential energy.
Requirement: Oxidation and reduction always occur together; one molecule is oxidized while another is reduced.
Electron Transport Chain (ETC):
A series of protein complexes embedded in a membrane (e.g., mitochondrial inner membrane or chloroplast thylakoid membrane).
These proteins shuttle electrons from a high-energy donor molecule to a low-energy acceptor molecule.
As electrons move down the chain, their potential energy is gradually harvested to do work, such as pumping protons or synthesizing ATP.
ATP (Adenosine Triphosphate)
Nature: ATP is the primary energy currency of the cell, storing much of the cell's immediately usable energy in its phosphate bonds.
Hydrolysis Reaction (Energy Release):
When ATP reacts with water, its terminal phosphate group is removed, releasing a significant amount of energy.
Equation: ext{ATP} + ext{H}2 ext{O} o ext{ADP} + ext{P}{ ext{i}} + ext{Energy}
This is an exergonic reaction, making energy available for cellular processes (e.g., muscle contraction, active transport, synthesis of macromolecules).
Phosphorylation (ATP Production):
The reverse process, where a phosphate group is added to ADP (Adenosine Diphosphate), requiring an input of energy.
Equation: ext{ADP} + ext{P}{ ext{i}} + ext{Energy} o ext{ATP} + ext{H}2 ext{O}
This is an endergonic reaction, with energy typically supplied by exergonic processes like cellular respiration.
Coupled Reactions: The cycles of ATP hydrolysis and phosphorylation are vital linkages in cellular metabolism:
Exergonic reactions (e.g., glucose breakdown) release energy, which is used to power the endergonic synthesis of ATP from ADP and inorganic phosphate.
The ATP then undergoes exergonic hydrolysis, releasing energy to fuel other endergonic cellular processes (e.g., building complex molecules, active transport).
Enzymes
Definition: Biological catalysts, which are typically organic molecules (most often proteins) that accelerate the rate of specific biochemical reactions without being consumed in the process.
Mechanism of Action:
Lower Activation Energy: Enzymes drastically reduce the activation energy required for a reaction to occur. Activation energy is the minimum energy input needed to start a chemical reaction.
Bring Reactants into Contact: Enzymes provide a specific active site where reactants (called substrates) bind. This binding positions the substrates correctly and facilitates the interactions necessary for the reaction to proceed.
Key Characteristics of Enzymes:
Specificity: Each enzyme is specific to a particular type of reaction or a particular substrate due to the unique shape of its active site.
Reusable: Enzymes are not consumed in the reaction; they are released unchanged after catalyzing a reaction and can be used again.
Sensitive to the Environment: Enzyme activity is highly dependent on environmental conditions:
Temperature: Each enzyme has an optimal temperature range. Deviations (too high or too low) can alter enzyme structure and reduce activity. High temperatures can cause denaturation (irreversible loss of shape and function).
pH: Each enzyme has an optimal pH range. Extreme pH values can disrupt the enzyme's structure and active site, leading to denaturation and loss of function.
Cofactors and Coenzymes: Some enzymes require additional non-protein chemical compounds to function properly.
Cofactors: Inorganic ions, often metal ions (e.g., iron, magnesium, zinc).
Coenzymes: Organic molecules (often derived from vitamins, e.g., NAD$^+$, FAD).
Examples of Enzymes and Their Applications/Significance:
Food Industry: Enzymes in fruit juices like bromelain (pineapple), actinidin (kiwi), papain (papaya/pawpaw), and ficain (figs) can break down proteins, which is why they prevent gelatin (Jello) from setting. Papain is also used in meat tenderizers.
Lactase: An enzyme that breaks down lactose (milk sugar). Individuals missing lactase suffer from lactose intolerance.
Phenylketonuria (PKU): A genetic disorder where an individual is missing an enzyme required to break down the amino acid phenylalanine. This leads to the toxic buildup of phenylalanine in the body.
Regulation of Enzyme Activity
Cellular Regulation: Cells meticulously regulate their chemical reactions, often employing mechanisms to control enzyme activity.
Negative Feedback Loop: A common regulatory mechanism where the end product of a metabolic pathway inhibits an enzyme earlier in the pathway. This prevents the overproduction of the product.
Mechanisms of Enzyme Inhibition:
Competitive Inhibition: An inhibitor molecule structurally similar to the substrate binds reversibly to the enzyme's active site, competing with the substrate and preventing it from binding.
Non-competitive Inhibition: An inhibitor molecule binds to an allosteric site (a site other than the active site) on the enzyme. This binding causes a conformational change in the enzyme's active site, reducing or eliminating its ability to bind the substrate or catalyze the reaction, even if the substrate is present.