Chapter 3 Flashcards
What is the measure of disorder in a system called?
- enthalpy
- second law of thermodynamics
- free energy
- catabolism
- entropy
Entropy
(The measure of disorder in a system is called entropy. The second law of thermodynamics states that systems will change spontaneously toward arrangements with greater entropy. Therefore, cells must release heat into the environment around them, causing increased disorder outside of the cell to account for the increased internal order that the cell creates by assembling polymers and macromolecules. In this way, cells satisfy the second law of thermodynamics.)
Reactions that use energy to drive the synthesis of molecules inside the cell are most specifically considered
_______.
Anabolic
(Catabolic reactions are those that break down food molecules to release useful energy. Reactions that use energy to drive the synthesis of molecules inside the cell are most specifically considered anabolic. Anabolic reactions use the energy harnessed by the catabolic breakdown of food to drive the synthesis of the many molecules that form the cell. Together, anabolic and catabolic reactions constitute the metabolism of the cell.)
Which of the following represents energy in its most disordered form?
- chemical bond energy
- potential energy
- electromagnetic (light) energy
- heat energy
- kinetic energy of a moving object
Heat energy
(Heat is energy in its most disordered form. In contrast to the highly ordered state of a chemical bond, heat energy is the random jostling of molecules and is therefore not organized at all. As cells perform the chemical reactions that generate order within, some energy is inevitably lost in the form of heat. Because the cell is not an isolated system, the heat energy produced by the cell is quickly dispersed into the cell’s surroundings where it increases the intensity of the thermal motions of nearby molecules. This increases the entropy of the cell’s environment and keeps the cell from violating the second law of thermodynamics.)
Which of the following does not occur in cells?
- the use of heat to burn foodstuffs and transport glucose
- the conversion of sunlight into energy stored in chemical bonds during photosynthesis
- the use of chemical energy to transport organelles through the cytosol
- the metabolism of nutrients to produce useful energy stores
The use of heat to burn foodstuffs and transport glucose.
(Cells do not use heat to burn foodstuffs; they use a series of catabolic reactions that allow them to break down foods and capture some of this chemical energy to power other activities in the cell. Some of this chemical energy is lost as heat. Normal cellular activities include the use of chemical energy to transport organelles through the cytosol, the conversion of sunlight into energy stored in chemical bonds during photosynthesis, and the metabolism of nutrients to produce useful energy stores.)
Which energy conversion characterizes photosynthesis?
-electromagnetic (light) energy → kinetic energy
-electromagnetic (light) energy → heat energy
electromagnetic (light) energy → oxidation energy
-electromagnetic (light) energy → CO2
-electromagnetic (light) energy → chemical bond energy
Electromagnetic (light) energy → chemical bond energy
(During photosynthesis, light energy is converted into the chemical bond energy of sugar molecules, consuming CO2 in the process. Like all cellular reactions, some energy is released as heat during photosynthesis, in turn increasing the kinetic energy of molecules in the surrounding environment, but this is not the key form of energy conversion.)
Which of the following does not describe an oxidation reaction?
- the conversion of Fe2+ to Fe3+
- the conversion of a chlorine atom to Cl-
- the addition of oxygen atoms to a molecule
- the removal of electrons from a molecule
The conversion of a chlorine atom to Cl-
(The term oxidation derives from the addition of oxygen atoms to a molecule, but a redox reaction does not require the participation of oxygen to occur. For example, consider the reaction that converts Fe2+ to Fe3+. Fe2+ is oxidized when it loses an electron to become Fe3+. Any time atoms or molecules lose electrons, they are said to be oxidized. Therefore, the conversion of a chlorine atom to Cl– is not an oxidation. Instead, in gaining an electron to form Cl–, the chlorine atom is reduced.)
Compared to adding heat to the system, what is the advantage of using an enzyme to overcome an energy barrier?
- An enzyme generates multiple different products using multiple pathways.
- An enzyme speeds up a reaction more than heat does.
- An enzyme is specific for one desired pathway and end product.
- An enzyme can catalyze a reaction in many different ways.
An enzyme is specific for one desired pathway and end product.
(Energy barriers must be overcome for molecules to be converted to products. Energy in the form of heat can increase the energy of molecules, allowing them to overcome the energy barriers and be converted to product. This process is nonspecific and the molecule can be converted to several different products through several pathways. An enzyme is specific for one pathway and one end product. An enzyme lowers the activation energy of the barrier, allowing the molecule to be converted to a specific product. Since enzymes are regulated, product can be generated under certain desirable circumstances.)
All four possible reactions in the animation are energetically favorable; the energy of the four products is lower than the energy of the original starting molecule. Why does the starting molecule not completely and quickly convert to its possible products before the addition of heat or an enzyme?
- When heat was added to the reaction, only some of the products were produced.
- An activation energy barrier exists that must be overcome for conversion to products.
- The starting molecule can only form some of the products quickly.
- The starting substrate does quickly convert to the four products.
An activation energy barrier exists that must be overcome for conversion to products.
(An activation energy barrier must be overcome for molecules to be converted to product. Even though the energy of the products is lower than that of the starting molecule, the rate of conversion is very slow because of this activation energy barrier. Adding heat raises the energy of the starting molecule allowing it to overcome the energy barriers and be converted to one of a number of possible products. Enzymes lower the activation energy barrier for one pathway, increasing the rate of reaction of that specific pathway.)
In thermodynamics, what does the term “free energy” refer to?
- energy that can be harnessed to do work or drive chemical reactions
- energy that cannot be harnessed to do work or drive chemical reactions
- energy required to initiate a chemical reaction
- energy that cells borrow from the environment
- excess energy from a reaction that a cell does not use
Energy that can be harnessed to do work or drive chemical reactions.
(Free energy is the energy released from a chemical reaction that can be used to perform work or drive chemical reactions. Cells use free energy for a wide range of purposes, many of which order the cell. However, in addition to free energy, chemical reactions release heat energy, which dissipates into the surrounding environment of the cell and increases disorder outside of the cell so that the second law of thermodynamics is not violated. An excellent example of the use of free energy is the catabolic breakdown of glucose. The free energy released during glucose catabolism is used to form molecules like ATP, which can be used to power a whole host of processes in the cell. While glucose catabolism yields usable free energy, it also releases substantial amounts of heat, which might put the commonly heard phrase “cells burn glucose” in better context.)
Which statement about enzymes is not true?
- An enzyme can force an energetically unfavorable reaction to take place inside the cell.
- Enzymes reduce the activation energy required to initiate a spontaneous reaction.
- Enzymes can help build large polymers.
- Enzymes can speed up energetically favorable reactions.
An enzyme can force an energetically unfavorable reaction to take place inside the cell.
(Enzymes by themselves cannot make an energetically unfavorable—that is, a nonspontaneous reaction—take place in the cell. Yet, many processes that occur in cells are nonspontaneous, including anabolic pathways. How is this accomplished? It is accomplished through coupled reactions, where an unfavorable reaction is coupled to a second, highly favorable reaction, such as the hydrolysis of ATP. An example of coupled reactions is shown below, where ATP hydrolysis is coupled to drive the otherwise unfavorable reaction of glucose and fructose to form sucrose.)
For a biochemical reaction, _______ energy is the term for the extra energy boost required to initiate an energetically favorable reaction within the cell.
Activation
(Activation energy is the extra energy boost required to initiate an energetically favorable reaction. Enzymes increase the rate of chemical reactions because they lower the activation energy barrier, making the reaction more likely to proceed. It is important to remember that enzymes don’t alter the free-energy change (that is, the difference in energy between the reactants and products), nor do enzymes consume or change in the process of catalysis.)
For the simple reaction Y → X, the equilibrium constant K is equal to which of the following equations?
- [Y] × [X]
- [Y] / [X]
- [X] / [Y]
- [X] × [Y]
[X] / [Y]
(The ratio of substrate to product at the equilibrium point is called the reaction’s equilibrium constant, K, and is represented as [product] / [substrate]. Thus, for the simple reaction Y → X, the equilibrium constant (K) is equal to [X] / [Y].
In a cell, the rate at which an enzyme will encounter its substrate depends on which of the following?
- the size of the enzyme
- the way that the cytosol is structured
- the concentration of other proteins in the cytosol
- the concentration of the substrate
The concentration of the substrate
(In a cell, the rate at which an enzyme will encounter its substrate depends on the concentration of the substrate. The most abundant substrates, present in the cell at a concentration of about 0.5 mM, will collide with their enzymes approximately 500,000 times per second. Although the cytosol is a densely packed gel, substrates can diffuse through the cytosol as rapidly as they move through water. Because proteins generally diffuse through the cytosol quite slowly, the rate at which an enzyme encounters its substrate does not commonly depend on the enzyme size.)
Small molecules diffuse through the cytosol very efficiently by doing which of the following?
- moving randomly, knocked around by colliding with other molecules
- taking the shortest path from one location to another
- forming non covalent bonds with other molecules
- binding to larger molecules
- migrating along cytoskeletal microtubules
Moving randomly, knocked around by colliding with other molecules
(A molecule traverses the cytosol by taking a “random walk.” Small molecules tend to move through the cytosol via diffusion rather than any sort of directed transport, including interaction with the cytoskeletal system. Although it doesn’t sound very efficient, small molecules in solution are bounced around by colliding with other molecules. Such interactions allow them to diffuse through the cytosol randomly but rapidly. Larger molecules move more slowly than small ones, so binding to a large molecule would not help a small molecule to rapidly make its way around the cytosol.)
Which statement is true about the removal of a terminal phosphate from ATP?
- The reaction is associated with a positive change in ΔG°.
- The reaction is a condensation reaction.
- The reaction is energetically favorable.
The reaction is energetically favorable.
(The hydrolysis of the terminal phosphate group of ATP is energetically favorable and is coupled to many otherwise energetically unfavorable biosynthetic reactions. Because of its large negative ΔG°, the hydrolysis of ATP is integral to the chemical economy of the cell.)
