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Biology - Cell and Energy

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    Study Guide Biology – Cell and Energy 1. The Laws of Thermodynamics Life depends on order. Inside every cell, molecules must stay organized for the cell to survive and function properly. To keep this order, cells need a constant supply of energy . In science, energy is defined as the ability to do work. For living things, that “work” includes growing, moving, thinking, and staying alive. To understand how energy behaves, scientists rely on a set of well - tested rules called the laws of thermodynamics . There are two main laws that are especially important for biology. 1.1 The First Law of Thermodynamics: Energy Is Conserved The first law of thermodynamics states that energy cannot be created or destroyed . It can only change from one form to another. This means that in a closed system — one where no energy enters or leaves — the total amount of energy always stays the same. A good example of a closed system is the universe as a whole. Even though the total energy stays constant, the type of energy can change. For example, when gasoline burns in a car engine, its chemical energy is transformed into other forms of energy, such as heat , sound , and motion . The energy was not lost — it just changed form. 1.2 The Second Law of Thermodynamics: Energy Becomes Less Useful The second law of thermodynamics explains why energy transformations are never perfectly efficient. It states that the amount of usable energy in a closed system is constantly decreasing. This happens because of entropy , which refers to the level of disorder or randomness in a system. Over time, entropy always increases in a closed system. In simple terms, systems naturally tend to move from order to disorder.

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    Study Guide 1.3 Why This Matters for Living Things Living systems are not perfectly efficient. Every time your body moves, your brain thinks, or your cells carry out chemical reactions, energy is transferred. During each transfer, some energy becomes unusable — often released as heat. Because of this energy loss, organisms must take in more energy than they actually use just to stay alive and organized. This is why living things constantly need food or another energy source. Without new energy coming in, life cannot continue. 2. Chemical Reactions Chemical reactions are happening all the time inside living things. These reactions allow cells to grow, repair themselves, and stay alive. However, most chemical compounds do not react with each other automatically. They usually need an extra push to get started. 2.1 Energy Is Needed to Start Reactions Most chemical reactions in living organisms must be energized . This means energy has to be added to the system before the reaction can happen. Energy helps separate the atoms in molecules so they can rearrange and form new substances. Without this input of energy, many important reactions in cells would never occur. 2.2 Activation Energy: The “Spark” That Starts It All To begin a chemical reaction, a small amount of energy — called activation energy — is required. You can think of activation energy as the “spark” that starts a reaction. For example, hydrogen and oxygen can combine to form water at room temperature, but they still need activation energy to begin reacting. 2.3 Two Types of Chemical Reactions Chemical reactions can be grouped based on how energy is involved:

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    Study Guide 2.3.1 Exergonic Reactions An exergonic reaction is one that releases energy . In these reactions, the products have less energy than the reactants. The extra energy is given off to the surroundings. 2.3.2 Endergonic Reactions An endergonic reaction is one that absorbs energy from the environment. The products of these reactions have more energy than the reactants. A good example is photosynthesis in plants. During this process, plants capture energy from sunlight and store it in carbohydrates. 2.4 How Reactions Happen Faster: Catalysts The activation energy needed for both exergonic and endergonic reactions can come from heat energy or chemical energy . However, reactions can also happen with the help of special substances called catalysts . A catalyst speeds up a chemical reaction without being changed or used up. It works by lowering the amount of activation energy required. For example, hydrogen and oxygen can react in the presence of platinum. In this case, platinum acts as a catalyst. 2.5 Enzymes: Nature’s Catalysts In living systems, the most important catalysts are enzymes . Enzymes are protein molecules that make chemical reactions happen quickly and efficiently inside cells. Without enzymes, most chemical reactions in cells would occur too slowly to support life. This makes enzymes absolutely essential for all living organisms.

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    Study Guide 3. Quiz Chemical Reactions 1. Question Fill in the blank: An endergonic reaction is one in which the products: Answer Choices • end up with less energy than the reactants • have more energy than the reactants • have the same amount of energy as the reactants Correct Answer have more energy than the reactants Why This Is Correct An endergonic reaction requires an input of energy to occur. As a result, the products store more energy than the reactants . This is the opposite of an exergonic reaction , in which energy is released and the products have less energy than the reactants. 2. Question Fill in the blank: Which of the following statements is FALSE ? Answer Choices • Catalysts are substances that speed up chemical reactions. • Catalysts increase the activation energy of a reaction. • Catalysts remain unchanged when they speed up a chemical reaction. Correct Answer Catalysts increase the activation energy of a reaction.

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    Study Guide Why This Is Correct Catalysts work by lowering the activation energy , not increasing it. By reducing the energy needed to start a reaction, catalysts allow chemical reactions to occur faster and more efficiently . The other statements are true because catalysts do speed up reactions and are not used up or permanently changed during the process. 4. Enzymes Inside every living cell, thousands of chemical reactions are happening all the time. These reactions would be far too slow to support life on their own. That’s where enzymes come in. 4.1 What Are Enzymes? Enzymes are biological catalysts. This means they speed up chemical reactions without being used up or changed themselves. Every enzyme is very specific — it usually helps with only one particular reaction . Because cells carry out so many different reactions, they contain thousands of different enzymes , each doing a specific job. • The substance an enzyme acts on is called the substrate • The substances produced by the reaction are called the end products 4.2 Enzymes Are Proteins All enzymes are made of proteins , which are long chains of amino acids. Each enzyme has a special region called the active site . The active site has a shape that closely matches its substrate. This allows the enzyme and substrate to fit together precisely — much like a lock and key. 4.3 How Enzymes Work Here’s what happens step by step: 1. The substrate binds to the enzyme at the active site

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