Unit 5 Study Guide: Enzymes, Metabolism, and Cellular Respiration

5a. Describe the role of enzymes and how they function.

Metabolism is the chemistry of life. There are thousands of chemical reactions occurring in a single cell, and most of those chemical reactions rely on enzymes.

  • What is an enzyme?
  • What kind of macromolecule makes up an enzyme?
  • What is the function of an enzyme?
  • What is a substrate?

An enzyme is a protein that acts as a biological catalyst. A catalyst is a substance that greatly accelerates a chemical reaction without actually being a reactant in that reaction. In other words, a catalyst (and therefore an enzyme) does not get changed into another substance (a product). An enzyme interacts with reactants in such a way as to make it much more likely for those reactants to chemically react, turning them into products. We call the reactants of a catalyzed reaction "substrates". An enzyme operates by temporarily binding to substrates. The rate of the reaction (its speed) when it is catalyzed by an enzyme is typically at least one million times the rate without the help of an enzyme. This is why enzymes are absolutely vital. Without enzymes, the biochemical reactions of metabolism would occur much too slowly to support life. Importantly, since an enzyme does not get altered in a reaction in which it participates, it is reusable, and it can continue to catalyze the same sort of reaction if more substrate is present. Be sure you are familiar with the basics of enzymes by reviewing the Enzymes section of this chapter.

 

5b. Recognize and explain the difference between matter and energy.

Organisms are examples of open thermodynamic systems, because organisms must exchange matter and energy with their surroundings.

  • What is energy?
  • How is energy different from matter?

As you learned in previous units, matter is the material stuff of the universe. Matter occupies space and has mass. Matter is made up of atoms. Energy is not material. It does not have mass, and it does not occupy space. Energy is often described as the capacity to perform work or bring about some sort of change. There are countless examples. A human performs work by flexing a muscle. A tiny cell within a human performs work by transporting particles into or out of the cell or by oxidizing fuel molecules. There are many different forms of energy (light energy, mechanical energy, heat energy, etc.), and we can broadly classify energy into two categories:

  • Potential energy is energy in a stored form. It may be used, but it is not currently being used. The energy in food is an example of chemical potential energy.
  • Kinetic energy is energy that is being used at the moment. A falling object, for example, has kinetic energy.

Energy can readily be converted between forms. For example, a book that falls from a shelf converts potential energy into kinetic energy. By moving the book back to the shelf, a person can convert kinetic energy into potential energy. The metabolism of life involves countless interactions between matter and energy and countless conversions between energy forms, so it is important to understand the distinction between matter and energy. Review section 4.1 of this chapter to bolster your understanding.

 

5c. Explain the role of cellular respiration.

Any living cell is able to extract energy from fuel and temporarily store that energy in the form of ATP or some similar energy currency. This primary processing of fuel is called glycolysis. Only certain cells under the right conditions are able to continue where glycolysis leaves off, allowing much more usable energy to be extracted from the fuel. That additional processing of energy is called cellular respiration. Glycolysis and cellular respiration both extract usable energy from fuel by undergoing oxidation/reduction (or redox) reactions.

  • What is oxidation?
  • What is reduction?
  • How does cellular respiration accomplish its redox reactions?

Oxidation is the loss of electrons from a particle (like a fuel molecule), whereas reduction is the gain of electrons. Since electrons are not destroyed in chemical reactions, oxidation occurs only if reduction also occurs. When something is oxidized (loses electrons), something else gets reduced (gains electrons). Organism extract energy from fuel molecules by oxidizing those fuel molecules. In cellular respiration, there is a substance (external to the process) that ultimately accepts the electrons that have been removed from the fuel. For aerobic organisms, that substance is oxygen, and when oxygen accepts those electrons (along with protons) from the fuel molecules, the oxygen get reduced into water. Cellular respiration is important because it allows for maximal oxidation of fuels, which maximizes the amount of energy that can be extracted and stored as ATP. Be sure to review glycolysis and cellular respiration by reading sections 4.2 and 4.3 and by watching these lectures.

 

5d. Account for the matter inputs and outputs to glycolysis, pyruvate oxidation (preparatory reaction) the Krebs/Citric Acid cycle, and the electron transport chain.

Each component of the oxidation of glucose is a set of reactions that can be summarized by a reaction equation that lists the inputs (reactants) and the outputs (products) of that process. The component processes that comprise the complete oxidation of glucose are:

  • Glycolysis
  • Pyruvate Oxidation
  • The Citric Acid Cycle
  • Oxidative Phosphorylation (including Electron Transport and Chemiosmosis)
What are the material inputs and outputs for each of these processes?

Inputs

Process

Outputs

Glucose

NAD+

ADP

Glycolysis

Pyruvate

NADH

ATP

Pyruvate

NAD+

Coenzyme A

Pyruvate Oxidation

Carbon Dioxide

Acetyl Coenzyme A

NADH

Acetyl Coenzyme A

NAD+

FAD

ADP

Citric Acid Cycle

Carbon Dioxide

NADH

FADH2

ATP

ADP

NADH

FADH2

O2

Oxidative Phosphorylation

ATP

NAD+

FAD

H2O

It’s useful to look at illustrations to make sense of these inputs and outputs, so pay careful attention the figures in this section as you read the text. It will also be helpful to view these lectures on the citric acid cycle and electron transport.

 

5e. Describe the source and fate of energy in glycolysis, pyruvate oxidation (preparatory reaction) , the Krebs/Citric Acid cycle, and the electron transport chain.

The goal of the oxidation of a fuel (like glucose) is to transfer energy from that fuel into a versatile form of energy storage known as ATP (adenosine triphosphate). During the many reactions that make up glycolysis and cellular respiration, many transfers of energy take place. These transfers involve the original fuel (glucose), intermediate fuels, energy-carrying coenzymes (NAD and FAD), and ATP. Because of the third law of thermodynamics, some energy is lost as heat in each transfer, and that lost heat energy becomes unavailable to perform work in the cell.

What are the sources of energy and the fates of that energy in each of the following processes?

  • Glycolysis
  • Pyruvate Oxidation
  • The Citric Acid Cycle
  • Oxidative Phosphorylation (including Electron Transport and Chemiosmosis)

In glycolysis, energy starts out in the original fuel, glucose. By oxidizing glucose, some usable energy gets transferred into ATP and some into NADH. The intermediate fuel (pyruvate) that is left over contains usable energy, as well. In the oxidation of pyruvate, some of that usable energy gets transferred to more NADH. This leaves only acetyl coenzyme A as the remaining fuel, which still contains usable energy. The citric acid cycle completes the oxidation of the remaining fuel (acetyl coenzyme A), and the usable energy that’s extracted gets transferred to more NADH, to more ATP, and to FADH2. The carbon dioxide that remains from the fuel contains no usable energy (it is spent fuel). Oxidative phosphorylation serves to collect all of the usable energy that got transferred to NADH and FADH2 (in the earlier processes) and the usable energy is transferred to even more ATP. Refresh your understanding of this complex set of processes by viewing these lectures on glycolysis, the citric acid cycle and electron transport.

 

5f. Apply the laws of thermodynamics and conservation of matter to metabolism.

Recall that the first law of thermodynamics states that energy is conserved (it cannot be created or destroyed; it can only be transferred and transformed). In ordinary chemical reaction (like biochemical reactions), matter is also conserved. Therefore, the overall amount of energy and matter entering the processes of glycolysis and cellular respiration is the same as the overall amount of energy and matter exiting those processes. What has changed are the forms of that energy and matter. Energy enters as the potential chemical energy in the bonds of the glucose molecule. Some of that energy gets released as heat (unavailable for cellular work), and some of that energy ultimately gets stored in the bonds of ATP molecules. Matter enters as glucose and oxygen and, after many rearrangements of atoms, matter leaves as carbon dioxide and water. Review the principles of thermodynamic here.

 

Unit 5 Vocabulary

This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.

Try to think of the reason why each term is included.

  • Acetyl Coenzyme A
  • ADP
  • ATP
  • Carbon Dioxide
  • Catalyst
  • Cellular Respiration
  • Chemiosmosis
  • Citric Acid Cycle
  • Coenzyme A
  • Electron Transport
  • Energy
  • Enzyme
  • FAD
  • FADH2
  • Glucose
  • Glycolysis
  • Kinetic Energy
  • Krebs Cycle
  • Matter
  • NAD+
  • NADH
  • Oxidation
  • Oxidative Phosphorylation
  • Oxygen
  • Potential Energy
  • Pyruvate
  • Pyruvate Oxidation
  • Reaction Rate
  • Redox
  • Reduction
  • Substrate
  • Water
  • Work
Last modified: Wednesday, July 17, 2019, 5:48 PM