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Questions and Answers
What is the primary source of energy for photosynthetic organisms?
Which type of energy conversion occurs when muscle cells use chemical energy from food?
According to the First Law of Thermodynamics, what happens to energy?
What does the Second Law of Thermodynamics state about energy conversions?
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Which of the following best describes entropy?
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What type of reaction results in a release of energy and is characterized by a negative ΔG?
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What does an endergonic reaction require to proceed?
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In regards to free-energy changes, what happens if the concentrations of reactants are increased?
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Which processes are involved in metabolism?
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What happens when a cell creates a concentration gradient?
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What is the relationship between enthalpy (H), free energy (G), and entropy (S) expressed by?
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What is the role of coupling in biochemical reactions?
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Which of the following describes kinetic energy?
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What is the primary purpose of coupling reactions in metabolic pathways?
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What is the overall $ ext{ΔG}$ when the reactions $A ightarrow B$ and $C ightarrow D$ are coupled together?
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How does ATP contribute to energy transfer in cells?
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What role do electron carriers like NAD+ play in metabolic processes?
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What factor influences the specificity of enzymes?
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What is feedback inhibition in enzymatic reactions?
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Which compound is formed during the hydrolysis of ATP?
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What is the role of cofactors in enzyme activity?
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Which of the following describes an irreversible inhibitor?
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What is an example of a coenzyme that plays a role in transporting electrons?
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At what pH does the majority of human enzymes function optimally?
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The induced fit model describes which aspect of enzyme function?
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Why is a high ATP to ADP ratio beneficial for cells?
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Study Notes
Energy Conversion
- Cells obtain energy in many forms, and have mechanisms that convert energy from one form to another
- Radiant energy (from the sun) is the ultimate source of energy for life
- Photosynthetic organisms capture about 0.02% of the sun’s energy that reaches Earth, and convert it to chemical energy in bonds of organic molecules.
Biological Work
- Matter is anything that has mass and takes up space
- Energy is the capacity to do work (change in state or motion of matter)
- Energy is expressed in units of work (kJ) or units of heat energy (kcal)
- 1 kcal = 4.184 kj
Potential and Kinetic Energy
- Potential energy is the capacity to do work as a result of position or state
- Kinetic energy is the energy of motion, when work is performed
Chemical Energy
- Chemical energy is the potential energy stored in chemical bonds
- Chemical energy of food molecules is converted to mechanical energy in muscle cells
Laws of Thermodynamics
- Thermodynamics governs all activities of the universe
- Biological systems are open systems that exchange energy with their surroundings
The First Law of Thermodynamics
- Energy cannot be created or destroyed
- Energy can be transferred or converted from one form to another, including conversions between matter and energy
- The energy of any system plus its surroundings is constant
- Organisms must capture energy from the environment and transform it to a form that can be used for biological work.
The Second Law of Thermodynamics
- When energy is converted from one form to another, some usable energy (energy available to do work) is converted into heat that disperses into the surroundings
- As a result, the amount of usable energy available to do work in the universe decreases over time (Entropy increases)
- Heat is the kinetic energy of randomly moving particles.
Entropy
- The measure of the disorder or randomness of energy
- Organized, usable energy has a low entropy
- Disorganized energy, such as heat, has a high entropy
- No energy conversion is ever 100% efficient
- The total entropy of the universe always increases over time.
Energy and Metabolism
- Metabolism: all chemical reactions taking place in an organism
- Includes many intersecting chemical reactions
- Anabolism: pathways in which complex molecules are synthesized from simpler substances
- Catabolism: pathways in which larger molecules are broken down into smaller ones.
Enthalpy
- Enthalpy is the total potential energy of a system
- Every specific type of chemical bond has a certain amount of bond energy (the energy required to break that bond)
- Enthalpy is equivalent to the total bond energy.
Free Energy
- Free energy is the amount of energy available to do work under the conditions of a biochemical reaction
- Enthalpy (H), free energy (G), entropy (S); and absolute temperature (T) are related by the equation: H = G + TS
- As entropy increases, the amount of free energy decreases.
Changes in Free Energy
- Changes in free energy can be measured, even though the total free energy of a system cannot
- The equation ΔG=ΔH−TΔS can be used to predict whether a particular chemical reaction will release energy or require an input of energy.
Exergonic Reactions
- Exergonic reaction: releases energy and is a “downhill” reaction, from higher to lower free energy
- ΔG is a negative number for exergonic reactions.
Endergonic Reactions
- Endergonic reaction: a reaction in which there is a gain of free energy
- ΔG has a positive value: the free energy of the products is greater than the free energy of the reactants
- Requires an input of energy from the environment.
Diffusion
- Diffusion is an exergonic process
- Randomly moving particles diffuse down their own concentration gradient-no energy input required
- A cell must use energy to produce a concentration gradient.
Free-Energy Changes and the Concentrations of Reactants/Products
- Free-energy changes in a chemical reaction depend on the difference in bond energies between reactants and products
- Also depends on concentrations of both reactants and products
- A reaction that proceeds forward and in reverse at the same time eventually reaches dynamic equilibrium.
Changes in Free Energy (cont’d.)
- If the reactants have much greater free energy than the products, most of the reactants are converted to products and vice-versa
- If the concentration of reactants is increased, the reaction will “shift to the right” and vice-versa
- The reaction always shifts in order to reestablish equilibrium.
Coupled Reactions
- Endergonic reactions are coupled to exergonic reactions
- The thermodynamically favorable exergonic reaction provides energy required to drive a thermodynamically unfavorable endergonic reaction
- In a living cell the exergonic reaction often involves the breakdown of ATP.
Coupled Reactions (cont’d.)
- Two reactions taken together are exergonic:
- A → B ΔG = +20.9 kJ/mol (+5 kcal/mol)
- C → D ΔG = −33.5 kJ/mol (−8 kcal/mol)
- Overall ΔG = −12.6 kJ/mol (−3 kcal/mol)
- Reactions are coupled if pathways are altered for a common intermediate link:
- A + C → I ΔG = −8.4 kJ/mol (−2 kcal/mol)
- I → B + D ΔG = −4.2 kJ/mol (−1 kcal/mol)
- Overall ΔG = −12.6 kJ/mol (−3 kcal/mol).
ATP, Energy Currency of the Cell
- Adenosine triphosphate (ATP): Nucleotide consisting of adenine, ribose, and three phosphate groups
- The cell uses energy that is temporarily stored in ATP
- Hydrolysis of ATP yields ADP and inorganic phosphate.
ATP Donates Energy
- Hydrolysis of ATP can be coupled to endergonic reactions in cells, such as the formation of sucrose
- ATP + H2O → ADP + Pi
- ΔG = −32 kJ/mol (or −7.6 kcal/mol)
- glucose + fructose → sucrose + H2O
- ΔG = +27 kJ/mol (or +6.5 kcal/mol)
- glucose + fructose + ATP → sucrose + ADP + Pi
- ΔG = −5 kJ/mol (−1.2 kcal/mol).
ATP Donates Energy (cont’d.)
- The intermediate reaction in the formation of sucrose is a phosphorylation reaction: phosphate group is transferred to glucose to form glucose-P
- glucose + ATP → glucose-P + ADP
- glucose-P + fructose → sucrose + Pi.
Maintaining a High Ratio of ATP to ADP
- A typical cell contains more than 10 ATP molecules for every ADP molecule
- High levels of ATP makes its hydrolysis reaction more strongly exergonic, and more able to drive coupled endergonic reactions
- The cell cannot store large quantities of ATP
- ATP is constantly used and replaced.
Energy Transfer in Redox Reactions
- Energy is transferred through the transfer of electrons from one substance to another
- Oxidation: substance loses electrons
- Reduction: substance gains electrons
- Redox reactions often occur in a series of electron transfers
- For cellular respiration, photosynthesis, and many other chemical processes.
Electron Carriers Transfer Hydrogen Atoms
- Redox reactions in cells usually involve the transfer of a hydrogen atom
- An electron, along with its energy, is transferred to an acceptor molecule such as nicotinamide adenine dinucleotide (NAD+), which is reduced to NADH
- XH2 + NAD+ → X + NADH + H+.
Electron Carriers (cont’d.)
- An electron progressively loses free energy as it is transferred from one acceptor to another
- In cellular respiration, NADH transfers electrons to another molecule
- Energy is then transferred through a series of reactions that result in formation of ATP
- NADP+ is not involved in ATP synthesis
- Electrons of NADPH are used to provide energy for photosynthesis.
Other Important Electron Carriers
- Flavin adenine dinucleotide (FAD): nucleotide that accepts hydrogen atoms and their electrons
- Reduced form is FADH2
- Cytochromes: proteins that contain iron
- The iron component accepts electrons from hydrogen atoms, then transfers the electrons to some other compound.
Enzymes
- Cells regulate rates of chemical reactions with enzymes, which increase speed of a chemical reaction without being consumed by the reaction
- Example: Catalase has the highest known catalytic rate; it protects cells by destroying hydrogen peroxide (H2O2)
- Most enzymes are proteins, but some types of RNA molecules also have catalytic activity.
Activation Energy
- All reactions have a required energy of activation
- Even a strongly exergonic reaction may be prevented from proceeding by the activation energy required to begin the reaction
- Energy of activation (EA) or activation energy: the energy required to break existing bonds and begin a reaction.
Enzyme - Substrate Complex
- An enzyme controls the reaction by forming an unstable intermediate complex with a substrate
- When the ES complex breaks up, the product is released
- Enzyme molecule is free to form a new ES complex:
- enzyme + substrate(s) → ES complex
- ES complex → enzyme + product(s).
Active Sites
- Enzymes bind to substrates at active sites to position them close together to speed up the reaction.
- Induced fit: binding of substrate to enzyme causes a change in shape to enzyme
- Distorts the chemical bonds of the substrate
- Proximity and orientation of reactants, plus strains in their chemical bonds, facilitate the breakage/formation of products.
Enzyme Specificity
- Enzymes are specific due to shape of active site and its relationship to the shape of the substrate
- Some are specific only to a certain chemical bond (e.g. lipase splits ester linkages in many fats)
- Scientists classify enzymes into six classes that catalyze similar reactions
- Each class is divided into many subclasses.
Cofactors
- Many enzymes require cofactors
- Some enzymes have two components: an apoenzyme and a cofactor
- Neither alone has catalytic activity
- Enzyme functions only when the two are combined.
- Cofactors may be a specific metal ion (e.g. Iron, copper, zinc, and manganese)
Coenzymes
- Coenzymes are organic, nonpolypeptide compounds that bind to the apoenzyme and serve as a cofactor
- Most are carrier molecules:
- NADH, NADPH, and FADH2 transfer electrons
- ATP transfers phosphate groups
- Coenzyme A transfers groups derived from organic acids
- Most vitamins are coenzymes or components of coenzymes.
Optimal Temperature
- Each enzyme has an optimal temperature.
Heat-Tolerant Archaea
- Certain archaea have enzymes that allow them to survive in extreme habitats.
Optimal pH
- Each enzyme has an optimal pH
- Optimal pH for most human enzymes is 6 to 8.
Metabolic Pathways
- Metabolic pathway: the product of one enzyme-controlled reaction serves as substrate for the next in series of reactions
- Removal of intermediate and final products drives the sequence of reactions in a particular direction
- Enzymes can bind to one another to form a multienzyme complex that transfers intermediates in the pathway from one active site to another.
Regulating Enzymatic Activity
- The cell regulates enzymatic activity
- Gene control: a specific gene directs synthesis of each type of enzyme
- Gene may be switched on by a signal from a hormone or other signal molecule
- Amounts of enzymes influence overall cell reaction rate.
The Cell Regulates Enzymatic Activity (cont’d.)
- The product of one enzymatic reaction may control activity of another enzyme in a sequence of enzymatic reactions
- When concentration of a product is low, the sequence of reactions proceeds rapidly
- When concentration of a product is high, reactions stop.
Feedback Inhibition
- Feedback inhibition is a type of enzyme regulation in which the formation of a product inhibits an earlier reaction in the sequence.
- Removal of the allosteric inhibitor allows the enzyme to bind its substrates (enzyme is active).
Enzyme Inhibition
enzymes are inhibited by certain chemical agents:
Enzyme Inhibition
- Irreversible inhibition: inhibitor permanently inactivates or destroys an enzyme when the inhibitor combines with one of the enzyme’s functional groups, either at the active site or elsewhere
- Many poisons are irreversible enzyme inhibitors, such as mercury and lead, nerve gases, cyanide.
Enzyme Inhibitors
- Some drugs used to treat bacterial infections directly or indirectly inhibit bacterial enzyme activity
- Example: sulfa drugs compete with PABA for the active site of the bacterial enzyme
- Example: penicillin and related antibiotics irreversibly inhibit the bacterial enzyme transpeptidase
- Drug resistance is a growing problem.
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Description
Explore the principles of energy conversion, including how cells obtain and transform energy. This quiz covers various forms of energy, including potential, kinetic, and chemical energy, as well as the laws of thermodynamics governing these processes.
