BIO 113 - Chapter 8: Intro to Metabolism

Which reactions release energy?

Catabolic

Which reactions consume energy? Which reactions build up larger molecules?

Anabolic

Which reactions break down molecules?

Catabolic

Which reactions are considered “uphill”? Which type of reaction is photosynthesis?

Anabolic

Which type of reaction is cellular respiration?

Catabolic

In living systems which reactions require enzymes to catalyze them?

Anabolic, catabolic

Energy is the capacity to cause change, do work, or move matter against opposing forces. It exists in various forms. Contrast kinetic energy with potential energy.

Kinetic energy is associated with the relative motion of objects, whereas potential energy refers to an object not presently moving; it is the energy that matter possesses because of its location or structure.

Which type of energy does water behind a dam have? A mole of glucose?

Water behind a dam has potential energy. A mole of glucose also has potential energy, although more specifically, glucose has chemical energy, a term used by biologists to refer to the potential energy available for release in a chemical reaction.

According to the first law of thermodynamics, what can and cannot happen to energy?

Energy can be transferred and transformed, but it cannot be created or destroyed.

The second law of thermodynamics states that every energy transfer or transformation increases the entropy (molecular disorder) of the universe. Some call this the “you always lose rule”. What always happens in each energy transfer that makes this an apt expression?

During every energy transfer or transformation, some energy is converted to thermal energy and released as heat, becoming unavailable to do work. A consequence of the loss of usable energy as heat to the surroundings is that each energy transfer or transformation makes the universe more disordered, increasing entropy of the universe.

What is meant by a spontaneous process?

A process that occurs without an overall input of energy; a process that is energetically favorable.

What is free energy? What is its symbol?

Free energy is the portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell. Free energy is symbolized by the letter G.

Once we know the value of ΔG for a reaction we can use it to predict whether it will be spontaneous. For an exergonic reaction, is ΔG negative or positive?

An exergonic reaction proceeds with a net release of free energy. Because the chemical mixture loses free energy, ∆G is negative for an exergonic reaction.

Is cellular respiration an endergonic or an exergonic reaction? What is ΔG for this reaction?

Cellular respiration is an exergonic reaction. The ∆G for this reaction is: ∆G = –686 kcal/mol.

Is photosynthesis endergonic or exergonic? What is the energy source that drives it?

Photosynthesis is an endergonic reaction. Plants get the required energy to make a mole of glucose from the environment by capturing light energy from the sun and converting its energy into chemical energy. The ∆G for this reaction is: ∆G = +686 kcal/mol.

To summarize, if energy is released, ΔG must be positive/negative?

ΔG is negative.

List the three main kinds of work that a cell does. Give an example of each.

1. Chemical work, the pushing of endergonic reactions that would not occur spontaneously, such as the synthesis of polymers from monomers.

2. Transport work, the pumping of substances across membranes against the direction of spontaneous movement, possible examples include the sodium-potassium pump and proton pump.

3. Mechanical work, such as the beating of cilia, the contraction of muscle cells, and the movement of chromosomes during cellular reproduction.

What is energy coupling?

In cellular metabolism, energy coupling is the use of energy released from an exergonic reaction to drive an endergonic reaction.

When the terminal phosphate bond is broken, a molecule of inorganic phosphate Pi is formed, and energy is _____

Released

In many cellular reactions, a phosphate group is transferred from ATP to some other molecule in order to make the second molecule less stable. What term is now used to describe the second molecule?

Phosphorylated intermediate

If you could not regenerate ATP by phosphorylating ADP, how much ATP would you need to consume each day?

If ATP could not be regenerated by the phosphorylation of ADP, humans would use up nearly their body weight in ATP each day.

What is a catalyst?

A catalyst is a chemical agent that selectively increases the rate of a reaction without being consumed by the reaction.

What is activation energy (E_A)?

Activation energy is the amount of energy that reactants must absorb before a chemical reaction will start; also called free energy of activation.

What effect does an enzyme have on E_A?

An enzyme catalyzes a reaction by lowering the EA barrier.

What is an enzyme?

A macromolecule serving as a catalyst, a chemical agent that increases the rate of a reaction without being consumed by the reaction. Most enzymes are proteins.

What is a substrate?

The reactant on which an enzyme works.

What is an active site?

Typically, a pocket or groove on the surface of the enzyme where the substrate binds and catalysis occurs.

What are products?

A material resulting from a chemical reaction.

Describe the six steps of enzyme action.

1. Substrates enter the active site; enzyme changes shape such that its active site enfolds the substates (induced fit).

2. Substrates are held in the active site by weak interactions, such as hydrogen bonds and ionic bonds.

3. The active site lowers E_A and speeds up the reaction.

4. Substrates are converted to products.

5. Products are released.

6. Active site is available for two new substrate molecules.

What is meant by induced fit?

Induced fit is the slight change in shape of the active site of an enzyme so that it binds more snugly to the substrate.

Explain how protein structure is involved in enzyme specificity.

Enzymes are proteins, and proteins are macromolecules with unique three-dimensional configurations. The specificity of an enzyme results from its shape, which is a consequence of its amino acid sequence. The specificity of an enzyme is attributed to a compatible fit between the shape of its active site and the shape of the substrate.

Enzymes use a variety of mechanisms to lower activation energy. Describe four of these mechanisms.

1. In reactions involving two or more reactants, the active site provides a template on which the substrates can come together in the proper orientation for a reaction to occur between them.

2. As the active site of an enzyme clutches the bound substrate, the enzyme may stretch the substrate molecules toward their transition-state form, stressing and bending critical chemical bonds that must be broken during the reaction.

3. The active site may also provide a microenvironment that is more conducive to a particular type of reaction than the solution itself would be without the enzyme.

4. Amino acids in the active site may directly participate in the chemical reaction.

Many factors can affect the rate of enzyme action. Explain the effects of each factor listed here.

1. Initial concentration of substrate: the more substrate molecules that are available, the more frequently they access the active sites of the enzyme molecules.

2. pH: with some exceptions, the optimal pH values for most enzymes fall in the range of pH 6–8.

3. Temperature: up to a point, the rate of an enzymatic reaction increases with increasing temperature, partly because substrates collide with active sites more frequently when molecules move rapidly. Above that temperature, however, the speed of the enzymatic reaction drops sharply. All enzymes have an optimal temperature at which its reaction rate is greatest.