Cells and Molecules of Life: Chapter 4.1-4.2 PDF

## Cells and Molecules of Life ### 4.1 Enzymes and metabolism **A Metabolism** _NOT: new cells replace old cells_ Many different chemical reactions are taking place in an organism at any given moment in order to keep the organism alive. The sum of all the chemical reactions that take place in a...

## Cells and Molecules of Life ### 4.1 Enzymes and metabolism **A Metabolism** _NOT: new cells replace old cells_ Many different chemical reactions are taking place in an organism at any given moment in order to keep the organism alive. The sum of all the chemical reactions that take place in an organism is called *metabolism*. Metabolism is divided into *catabolism* and *anabolism*. * **Catabolism** * It refers to the breaking-down reactions in an organism. * In catabolic reactions, complex molecules are broken down into simpler molecules. * Energy is released during the reaction. * **Anabolism** * It refers to the building-up reactions in an organism. * In anabolic reactions, complex molecules are synthesized from simpler molecules. * Energy is required to drive the reaction. **Diagram:** * Two diagrams are present in the document. The first one shows the process of Catabolism and Anabolism. The first one consists of a complex molecule that releases energy and yields simpler molecules, while the second one shows the opposite: simpler molecules being synthesized into a complex molecule by absorbing energy. **Examples:** * **Catabolism** * Respiration: Glucose molecules are broken down into carbon dioxide and water to release energy. * Breakdown of complex molecules in food like starch, proteins, and lipids into simpler molecules during digestion. * **Anabolism** * Photosynthesis: Glucose molecules are synthesized from carbon dioxide and water using energy from sunlight. * Synthesis of complex molecules like starch, proteins, and lipids from simpler molecules for storage or building body structures. ### **B Role of enzymes in metabolism** For every reaction, a certain amount of energy must be supplied to the reactants before the reaction can occur. For example, you need to strike a match against a matchbox to supply energy for the match to burn (which is a reaction between the match and oxygen). **Diagram:** * This figure represents an analogy for the activation energy in a chemical reaction: a rock needs to be pushed to the hilltop before it can roll down. The analogy shows that the energy required to push the rock corresponds with the *activation energy* required in a chemical reaction. **Analogy:** * In our body, *enzymes* are present which function as biological catalysts. They help lower the activation energy so that chemical reactions can take place at body temperature at a faster rate. * **Enzyme:** Catalase * **Action:** catalyses (催化 speeds up) the catabolic reaction of hydrogen peroxide * **Type:** enzyme * **Reactant:** hydrogen peroxide * **Products:** into oxygen and water. **Background Chemistry:** * **Energy profiles of catabolic and anabolic reactions** **Diagram:** * Two graphs comparing the energy released/absorbed during a catabolic reaction (first diagram) and an anabolic reaction (second diagram). The first diagram shows that catabolic reactions have a negative change in energy (energy is released), whereas anabolic reactions have a positive change in energy (energy is absorbed). **Key findings:** * In catabolic reactions, the energy level of the products is lower than that of the reactants (energy released). * In anabolic reactions, the energy level of the products is higher than that of the reactants (energy absorbed). ### **Practical 4.1: Demonstration of the action of enzymes** **Procedure:** 1. Label five test tubes A to E. 2. Add 5cm³ of hydrogen peroxide solution to a test tube (A). Observe whether gas is evolved. 3. Add 5cm³ of hydrogen peroxide solution to test tubes B to E. In each tube, put a cube of tissue (potato, apple, liver or meat) with sides of 0.5cm into the solution. Loosely cap the tubes to collect any gas evolved. 4. Wait for a while to collect enough gas. Test the gas with a glowing splint. **Results and Discussion:** * **Tube A:** This is a control setup. It is used to show that no oxygen is evolved from hydrogen peroxide solution if no tissue is added. The breakdown of hydrogen peroxide is very slow at room temperature. * **Tubes B to E:** gas is released from tubes B to E after tissues are added. This gas is confirmed to be oxygen because it relights a glowing splint. **Conclusion:** These results show that potato, apple, liver and meat can catalyse the breakdown of hydrogen peroxide. This is likely due to the presence of *catalase* in these tissues. ### **Key Learning:** 1. What are Metabolism, Catabolism and Anabolism? * **Metabolism:** The sum of the chemical reactions that take place in an organism. * **Catabolism:** The breaking-down reactions in an organism. * **Anabolism:** The building-up reactions in an organism. 2. What is the role of enzymes in metabolism? * Enzymes are biological catalysts. They speed up chemical reactions in organisms by lowering the activation energy of the reactions. ### 4.2 Actions and properties of enzymes **A Actions of enzymes** * **Enzymes** are special types of *proteins* produced in organisms. On each enzyme molecule, there is an *active site* which binds to *substrate* molecules during reactions. * The active site has a *specific shape*. Only substrate molecules that fit its shape can bind to it. **Diagram:** * Four steps represent the action of an enzyme. (Figure 4.3) 1. The *substrate molecule* binds to the *active site* of an enzyme molecule to form an *enzyme-substrate complex*. The formation of the *enzyme-substrate complex* greatly lowers the *activation energy* of the reaction. 2. The *substrate molecule* is converted into *products*. 3. The *products* leave the *active site*. 4. The *enzyme molecule* is released in its original form. It can be reused. It can bind to another *substrate* molecule. * The importance/significance of forming *enzyme-substrate complex* * The reaction shown above is a *catabolic reaction*. In a catabolic reaction, the enzyme binds to substrate molecules and helps split them apart. In an *anabolic reaction*, the enzyme binds to substrate molecules and helps join them together. **B. Properties of enzymes** * **Property/Feature** | **Explain** ---|--- Enzymes are biological catalysts | Enzymes speed up metabolic reactions in organisms by lowering the activation energy of the reactions. The actions of enzymes are specific | Each enzyme has a unique active site, which has a specific shape. It only acts on substrates that can fit into its active site. Therefore, each enzyme can catalyse one type of reaction only. Hence, enzymes are said to be specific in action. **Diagram:** * Figure 4.4 shows the difference between substrates that can fit into the active site and those that cannot. * A *substrate* that can fit into the active site will result in an *enzyme-substrate complex* and product production. * A *molecule* that cannot fit into the active site will not result in the formation of an *enzyme-substrate complex* and no reaction will take place. * **The specificity of enzyme actions** can be explained by the *lock-and-key hypothesis*. A key of a specific shape fits only one lock. Similarly, an enzyme with an active site of a specific shape binds only to a particular type of substrate. **Diagram:** * Figure 4.5 shows the specificity of enzyme action using the lock-and-key hypothesis analogy. * The lock corresponds to the *active site* of the enzyme. * The key corresponds to the suitable *substrate* molecules that can bind to the *active site*. * Only one *key* (substrate) can fit into the *lock* (active site). * This analogy exemplifies the specificity of an enzyme for a specific substrate. * **Enzymes are proteins:** Their structure and hence their activity are easily affected by *temperature* and *pH*. Most enzymes are *denatured* at *high temperatures* and at *extreme pH*. **All answers:** * **Enzymes are reusable:** They can bind to other *substrate molecules* after a reaction is complete. * **Enzymes are needed in relatively small amounts:** Since enzymes can be reused, they are usually needed in small amounts only **Key learning:** 1. How does an enzyme work? * Each enzyme has an *active site*. *Substrate(s)* binds to the *active site* to form an *enzyme-substrate complex*. The *substrate(s)* is then converted into *product(s)*, which then leaves the *active site*. The *enzyme* is released in its *original form*. 2. Why are enzyme actions specific? * Each enzyme has a unique *active site*, which has a *specific shape*. It only acts on *substrates* that can fit into its *active site*. *The specificity of enzyme actions can be explained by the lock-and-key hypothesis*. 3. What are the properties of enzymes? * Enzymes are *biological catalysts*. * The actions of enzymes are *specific*. * Enzymes are *proteins*. Their structure and hence their activity are easily affected by *temperature* and *pH*. * Enzymes are *reusable*. * Enzymes are needed in *relatively small amounts*. **DSE Bio 2018 IA Q27** 'Lock and Key' is a scientific model which is a selective representation used to explain that enzymes: * **A** are *biological catalysts*. * **B** are *specific* in action. * **C** are *protein* in nature. * **D** are required in *small amounts*. **Checkpoint:** * **Which property of enzymes is illustrated in the diagram on the right?** * **A** Enzymes are *catalysts*. * **B** Enzymes are *specific* in action. * **C** Enzymes are *proteins*. * **D** Enzymes are *reusable*. * **The shape of active site is specific that only allow specific substrate fit into it.**