Unit 5: Enzymes, Metabolism, and Cellular Respiration

5a. Explain the difference between matter and energy

  • What is energy?
  • How is energy different from matter?
  • Are matter and energy interchangeable?
  • What forms of energy and matter exist?

Organisms are open thermodynamic systems because they must exchange matter and energy with their surroundings. Matter, which is made up of atoms, is the material stuff of the universe. As we reviewed in previous units, it occupies space and has mass.

Energy is not material. It does not have mass or 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 a 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.

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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?
  • What does it mean that energy and matter are "conserved"?
  • What is the relationship between ADP and ATP in metabolic processes?

Recall that the First Law of Thermodynamics states 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.

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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 which 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 them 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 a 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. To review, see:


5d. Explain the role of cellular respiration

  • What is oxidation and reduction?
  • What are the inputs and outputs of cellular respiration?
  • 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 specific 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 the maximal oxidation of fuels, which maximizes the amount of energy that can be extracted and stored as ATP.

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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?
  • What factors may affect the rate of these processes (faster or slower)?
  • What is the role of ADP in 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 make up the complete oxidation of glucose are glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation (including electron transport and chemiosmosis).

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 helps to review illustrations to make sense of these inputs and outputs.

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5f. Describe the differences in the inputs and outputs in the processes of glycolysis, pyruvate oxidation (preparatory reaction), the Krebs/citric acid cycle, and the electron transport chain

  • What are the sources and fates of 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 the fuel into a versatile form of energy storage like ATP (adenosine triphosphate). Many energy transfers occur during the reactions during glycolysis and cellular respiration. These transfers involve the original fuel (glucose), intermediate fuels, energy-carrying coenzymes (NAD and FAD), and ATP. Some energy is lost as heat during each transfer due to the Third Law of Thermodynamics. This lost heat energy becomes unavailable to perform work in the cell.

During glycolysis, energy is first 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 also contains usable energy. 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 collects all of the usable energy that was transferred to NADH and FADH2 (in the earlier processes), and the usable energy is transferred to more ATP.

The final acceptor of the electron is the molecule oxygen, which subsequently changes to water as the final waste product.

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Unit 5 Vocabulary

This vocabulary list includes terms you will need to know to successfully complete the final exam.

  • acetyl coenzyme A
  • aerobic organism
  • ATP (adenosine triphosphate)
  • catalyst
  • cellular respiration
  • citric acid cycle
  • FADH2
  • glucose
  • glycolysis
  • kinetic energy
  • NADH
  • oxidation
  • oxidative phosphorylation
  • potential energy
  • pyruvate
  • pyruvate oxidation
  • rate of reaction
  • redox
  • reduction
  • substrate