2015 WASSCE BIOLOGY PAPER 1 QUESTIONS AND ANSWERS (QUIZ)
Energy, Enzymes, Biological Reactions, and 2015 WASSCE BIOLOGY PAPER 1 QUESTIONS AND ANSWERS (QUIZ).
Life, Energy, and the Laws of Thermodynamics
Energy
- Energy is the capacity to do work.
- Kinetic energy is the energy of motion.
- Potential energy is energy stored in an object because of its location or chemical structure.
- Energy may be readily converted between potential and kinetic states.
- Potential energy that can be released in a chemical reaction is chemical energy.
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Cells and the Flow of Energy
- Two energy laws are basic to understanding energy use patterns at all levels of biological organization.
- The first law of thermodynamics states that energy cannot be created or destroyed, but can only be transferred or transformed.
- The second law of thermodynamics states that one usable form of energy cannot be completely converted into another usable form.
- As a result of these laws, we know that the entropy of the universe is increasing and that only a flow of energy from the sun maintains the organization of living things.
- Thermodynamics is the study of energy flow between a system and its surroundings during chemical and physical reactions.
- A system that does not exchange energy or matter with its surroundings is known as an isolated system.
- A system that exchanges energy but not matter with its surroundings is known as a closed system.
- A system that exchanges both energy and matter with its surroundings is known as an open system.
Free Energy and Spontaneous Reactions
- A spontaneous reaction is one that will occur without the input of energy from the surroundings.
- A spontaneous reaction releases free energy that is available to do work.
- Factors that oppose the completion of spontaneous reactions, such as the relative concentrations of reactants and products, produce an equilibrium point at which reactants are converted to products and products are converted back to reactants, at equal rates.
- Organisms reach equilibrium only when they die.
- Reactions with a negative ΔG are spontaneous; they release free energy and are known as exergonic reactions.
- Reactions with a positive ΔG require free energy and are known as endergonic reactions
Metabolism
- Metabolism is the biochemical modification and use of energy in the synthesis and breakdown of organic molecules.
- A catabolic reaction releases the potential energy of a molecule by breaking it down to a simpler molecule (ΔG is negative).
- An anabolic (biosynthetic) reaction uses energy to convert a simple molecule to a more complex molecule (ΔG is positive).
- Typically, individual reactions operate in metabolic pathways.
- Individual reactions in a particular pathway can be catabolic or anabolic; it is the sum of the reactions that makes the pathway catabolic or anabolic.
- A metabolic pathway is a series of reactions that proceed in an orderly, step-by-step manner.
- Enzymes speed reactions by lowering the energy of activation when they form a complex with their substrates.
- Enzymes regulate metabolism because, in general, no reaction occurs unless its enzyme is present.
- Which enzymes are present determines which metabolic pathways will be utilized
Adenosine Triphosphate (ATP):
- The Energy Currency of the Cell
- The hydrolysis of ATP releases free energy that can be used as a source of energy for the cell.
- Using enzymes, a cell can couple the exergonic reaction of ATP hydrolysis to make an otherwise endergonic (anabolic) reaction proceed spontaneously.
- The ATP used in coupling reactions is replenished by reactions that link ATP synthesis to catabolic reactions.
- ATP thus cycles between reactions that release free energy and reactions that require free energy.
Role of Enzymes in Biological Reactions
- What prevents many exergonic reactions from proceeding rapidly is that they need to overcome an energy barrier (the activation energy, Ea) to get to the transition state.
- Enzymes are catalysts that greatly speed the rate at which spontaneous reactions occur because they lower the activation energy.
- Enzymes usually are specific: they catalyze reactions of only a single type of molecule or a group of closely related molecules.
- Catalysis occurs at the active site, which is the site where the enzyme binds to the substrate (reactant molecule). After combining briefly with the substrate, the enzyme is released unchanged when the reaction is complete.
- Many enzymes require a cofactor, a nonprotein group that binds to the enzyme, for catalytic activity. Some cofactors are ions; others are small organic molecules called coenzymes. Some coenzymes bind loosely to enzymes, whereas others, called prosthetic groups, bind tightly.
- Enzymes reduce the activation energy by inducing the transition state of the reaction, from which the reaction can move easily in the direction of either products or reactants.
Three major mechanisms contribute to enzymatic catalysis by reducing the activation energy:
- Enzymes bring reacting molecules together;
- Enzymes expose reactant molecules to altered charge environments that promote catalysis;
- Enzymes change the shape of a substrate molecule.
Conditions and Factors That Affect Enzyme Activity
- When the substrate is abundant, the rate of a reaction is proportional to the amount of enzyme. At a fixed enzyme concentration, the rate of a reaction increases with substrate concentration until the enzyme becomes saturated with reactants. At that point, further increases in substrate concentration do not increase the rate of the reaction
- Many cellular enzymes are regulated by non-substrate molecules called inhibitors. Competitive inhibitors interfere with reaction rates by combining with the active site of an enzyme; noncompetitive inhibitors combine with sites elsewhere on the enzyme
- Allosteric regulation resembles noncompetitive inhibition except that regulatory molecules may either increase or decrease enzyme activity. Allosteric regulation often carries out feedback inhibition, in which a product of an enzyme-catalyzed pathway acts as an allosteric inhibitor of the first enzyme in the pathway
- Many key enzymes are regulated by chemical modification, by substances such as ions and certain functional groups. The modifications change enzyme conformation, resulting in increased or decreased activity.
- Typically, an enzyme has optimal activity at a certain pH and a certain temperature; at pH and temperature values above and below the optimum, the reaction rate falls off
RNA-Based Biological Catalysts (Ribozymes)
- RNA-based catalysts called ribozymes speed some types of biological reactions; these include cutting and splicing reactions in which surplus segments are removed from RNA molecules and linking reactions that combine amino acids into polypeptide chains.
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