BIO101 Study Guide

Unit 5: Enzymes, Metabolism, and Cellular Respiration

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

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

Organisms are examples of open thermodynamic systems because organisms must exchange matter and energy with their surroundings. As we reviewed 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. We often describe energy 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. When someone moves the book back to the shelf, they 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 Energy and Metabolism, More on Energy, and Energy and Thermodynamics.

 

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

• What are the laws of thermodynamics?
• How do these laws affect biological processes?

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 reactions (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 these 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. ATP is formed when ADP and inorganic phosphate combine. Matter enters as glucose and oxygen and, after many rearrangements of atoms, matter leaves as carbon dioxide and water. Review the principles of thermodynamics in subunit 2.2. Also review Energy and Thermodynamics, Metabolism Part 1, and Metabolism Part 2.

 

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

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

Metabolism is the chemistry of life. Thousands of chemical reactions occur in a single cell – most of these chemical reactions rely on enzymes.

An enzyme is a protein that serves 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). The enzyme interacts with the reactants to make it more likely for the reactants to chemically react, turning them into products. We call the reactants of catalyzed reactions substrates. An enzyme operates by temporarily binding to substrates.

The rate of the reaction (its speed) when it is catalyzed by an enzyme is usually at least one million times faster than without the help of an enzyme. This is why enzymes are absolutely vital. Without enzymes, biochemical reactions of metabolism would occur much too slowly to support life.

Most importantly, enzymes are reusable since they do not get altered during the reaction – they can continue catalyzing the same sort of reaction until all of the substrate is depleted.

Review Energy and Metabolism and Metabolism Part 3 to make sure you are familiar with the basics of enzymes.

 

5d. Explain the role of cellular respiration

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

Any living cell can extract energy from fuel and temporarily store that energy in the form of ATP, or a similar energy currency. This primary processing of fuel is called glycolysis. Only certain cells under the right conditions can continue where glycolysis leaves off, allowing much more usable energy to be extracted from the fuel. This 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.

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). Organisms extract energy from fuel molecules by oxidizing these fuel molecules.

During 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 these electrons (along with protons) from the fuel molecules, the oxygen gets 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.

Review glycolysis and cellular respiration in Introduction to Glycolosis, Cellular Respiration, The Citric Acid Cycle and Oxidative Phosphorylation, Glycolysis, More on Glycolysis, and The Process of Glycolysis.

 

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

• What are the material inputs and outputs for each of these processes?

Each component of the oxidation of glucose contributes to a series 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, and oxidative phosphorylation (including electron transport and chemiosmosis).

 

 

It is useful to review illustrations to make sense of these inputs and outputs, so pay careful attention to the figures in The Citric Acid Cycle and Oxidative Phosphorylation. You should also review subunits 5.5 and 5.6.

 

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

• What are the sources of energy and the fates of that energy in glycolysis, pyruvate oxidation, the Citric Acid Cycle, and oxidative phosphorylation?

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). Many transfers of energy take place during the many reactions that make up glycolysis and cellular respiration. These transfers involve the original fuel (glucose), intermediate fuels, energy-carrying coenzymes (NAD and FAD), and ATP. Due to the third law of thermodynamics, some energy is lost as heat during each transfer. This lost heat energy becomes unavailable to perform work in the cell.

During 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. During 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 is 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.

The final acceptor of the electron is the molecule oxygen which subsequently changes to water as the final waste product. Refresh your understanding of this complex set of processes by reviewing subunits 5.4, 5.5, and 5.6.

 

Unit 5 Vocabulary

You should be familiar with these terms as you prepare for the final exam.

• 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