Biology - Cell and Energy - Quick Guides | Biology

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Study GuideBiology–Cell and Energy1. The Laws of ThermodynamicsLife depends on order. Inside every cell, molecules must stay organized for the cell to survive andfunction properly. To keep this order, cells need a constant supply ofenergy. In science,energyisdefined as the ability to do work. For living things, that “work” includes growing, moving, thinking, andstaying alive.To understand how energy behaves, scientists rely on a set of well-tested rules called thelaws ofthermodynamics. There are two main laws that are especially important for biology.1.1 The First Law of Thermodynamics: Energy Is ConservedThefirst law of thermodynamicsstates thatenergy cannot be created or destroyed. It can onlychange from one form to another.This means that in aclosed system—one where no energy enters or leaves—the total amount ofenergy always stays the same. A good example of a closed system is the universe as a whole.Even though the total energy stays constant, thetypeof energy can change. For example, whengasoline burns in a car engine, itschemical energyis transformed into other forms of energy, suchasheat,sound, andmotion. The energy was not lost—it just changed form.1.2 The Second Law of Thermodynamics: Energy Becomes Less UsefulThesecond law of thermodynamicsexplains why energy transformations are never perfectlyefficient. It states that the amount ofusable energyin a closed system is constantly decreasing.This happens because ofentropy, 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 tomove from order to disorder.

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Study Guide1.3 Why This Matters for Living ThingsLiving systems are not perfectly efficient. Every time your body moves, your brain thinks, or your cellscarry out chemical reactions, energy is transferred. During each transfer, some energy becomesunusable—often released as heat.Because of this energy loss, organisms must take inmore energy than they actually usejust to stayalive and organized. This is why living things constantly need food or another energy source. Withoutnew energy coming in, life cannot continue.2. Chemical ReactionsChemical reactions are happening all the time inside living things. These reactions allow cells to grow,repair themselves, and stay alive. However, most chemical compounds donotreact with each otherautomatically. They usually need an extra push to get started.2.1 Energy Is Needed to Start ReactionsMost chemical reactions in living organisms must beenergized. This means energy has to be addedto the system before the reaction can happen.Energy helpsseparate the atomsin 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 AllTo begin a chemical reaction, a small amount of energy—calledactivation energy—is required. Youcan 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 stillneed activation energy to begin reacting.2.3 Two Types of Chemical ReactionsChemical reactions can be grouped based on how energy is involved:

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Study Guide2.3.1 Exergonic ReactionsAnexergonic reactionis one thatreleases energy. In these reactions, the products havelessenergythan the reactants. The extra energy is given off to the surroundings.2.3.2 Endergonic ReactionsAnendergonic reactionis one thatabsorbs energyfrom the environment. The products of thesereactions havemore energythan the reactants.A good example isphotosynthesisin plants. During this process, plants capture energy fromsunlight and store it in carbohydrates.2.4 How Reactions Happen Faster: CatalystsThe activation energy needed for both exergonic and endergonic reactions can come fromheatenergyorchemical energy. However, reactions can also happen with the help of special substancescalledcatalysts.Acatalystspeeds up a chemical reaction without being changed or used up. It works byloweringthe amount of activation energyrequired.For example, hydrogen and oxygen can react in the presence of platinum. In this case, platinum actsas a catalyst.2.5 Enzymes: Nature’s CatalystsIn living systems, the most important catalysts areenzymes. Enzymes are protein molecules thatmake chemical reactions happen quickly and efficiently inside cells.Without enzymes, most chemical reactions in cells would occur too slowly to support life. This makesenzymes absolutely essential for all living organisms.

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Study Guide3. Quiz Chemical Reactions1. QuestionFill in the blank:Anendergonic 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 reactantsCorrect Answerhave more energy than the reactantsWhy This Is CorrectAnendergonic reactionrequires aninput of energyto occur. As a result, theproducts store moreenergy than the reactants.This is the opposite of anexergonic reaction, in which energy is released and the products haveless energy than the reactants.2.QuestionFill in the blank:Which of the following statements isFALSE?Answer Choices•Catalysts are substances that speed up chemical reactions.•Catalysts increase the activation energy of areaction.•Catalysts remain unchanged when they speed up a chemical reaction.Correct AnswerCatalysts increase the activation energy of a reaction.

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Study GuideWhy This Is CorrectCatalysts work bylowering the activation energy, not increasing it. By reducing the energy neededto start a reaction, catalysts allow chemical reactions to occurfaster and more efficiently.The other statements are true because catalysts do speed up reactions and arenot used up orpermanently changedduring the process.4. EnzymesInside every living cell, thousands of chemical reactions are happening all the time. These reactionswould be far too slow to support life on their own. That’s whereenzymescome in.4.1 What Are Enzymes?Enzymesare biological catalysts. This means they speed up chemical reactions without being usedup or changed themselves. Every enzyme is very specific—it usually helps withonly one particularreaction.Because cells carry out so many different reactions, they containthousands of different enzymes,each doing a specific job.•The substance an enzyme acts on is called thesubstrate•The substances produced by the reaction are called theend products4.2 Enzymes Are ProteinsAll enzymes are made ofproteins, which are long chains of amino acids. Each enzyme has a specialregion called theactive site.The active site has a shape that closely matches its substrate. This allows the enzyme and substrateto fit together precisely—much like a lock and key.4.3 How Enzymes WorkHere’s what happens step by step:1.The substrate binds to the enzyme at theactive site

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