UNIT 4: Enzymes and Cell Energy WHAT are ENZYMES??? Enzymes… Are CATALYSTS: speed up chemical reactions that would otherwise happen too slowly to support life. Catalysts DO NOT make reactions happen that couldn’t happen by themselves. Are PROTEINS. Their amino acid sequences determines their shapes, which are important for their functions. Are not consumed during the reaction—the same enzyme can be used repeatedly AND, BECAUSE of its shape, an Enzyme… SUBSTRATES ENZYME PRODUCTS ACTIVE SITE ENZYME-SUBSTRATE COMPLEX Is specific to the reaction it catalyzes. The reactants (called substrates) fit perfectly into a place on the enzyme called the active site. The shape of the enzyme active site makes it substrate-specific. The enzyme may slightly change shape (gives substrates a squeeze in a process called induced fit) that allows the reaction to happen. Finally, the products are released from the active site, and the enzyme can be reused over and over again for more reactions. Only functions in narrow pH and temperature ranges, where it has the proper shape. changing the temperature or pH (acidity) may change an enzyme’s shape It may no longer stick to its substrates, and therefore not function. An enzyme that has lost its proper shape and does not function is said to be denatured. HOW do Enzymes Work? TEMPERATURE PRODUCTS MADE (ENZYME FUNCTION) 37°C ENZYME WORKS BEST AT THIS CONDITION DENATURED ENZYME Favorable chemical reactions involve reactants going through an unfavorable, awkward transition state. Unfavorable states are considered “high energy” and favorable states are considered “low energy.” The amount of energy required to overcome the transition state is called the activation energy Enzymes work when the induced fit, or squeeze provided by the enzyme, makes the transition state more favorable In other words, an enzyme lowers the activation energy of a reaction

CHEMICAL ENERGY AND LIFE Enzymes work by making this transition phase more stable. In other words, enzymes lower the activation energy of a reaction. A chemical reaction that might otherwise occur once every thousand years can happen in milliseconds with the help of the proper enzyme. CHEMICAL ENERGY AND LIFE ENERGY is the ability to do work. Cells require energy for metabolic reactions, active transport, cell division, and maintaining homeostasis. We obtain energy from food, but energy ultimately comes from the sun. Glucose is the preferred energy source, but we can obtain it from other carbs, lipids and even proteins CHEMICAL ENERGY in food is stored in chemical bonds as potential energy. When bonds are broken, energy of electrons is released. Some is lost as heat, but some portion of it can be converted into a usable form in the bonds of ATP. We use ATP energy to “pay for” unfavorable reactions ATP is a better form of energy because it contains smaller packets of energy than glucose If we released the energy from food molecules (like glucose) all at once, it would be wasteful and destructive ATP, Adenosine TriPhosphate is a nucleotide composed of: Adenine, a nitrogen base Ribose, a 5 carbon sugar Three phosphate groups Potential energy is stored in the phosphate-phosphate bonds ATP is like a rechargeable battery ATP = “charged” (3 phosphates), high PE ADP = “uncharged” (2 phosphates, adenosine diphosphate), low PE

METABOLIC PATHWAYS: CELLULAR RESPIRATION AND PHOTOSYNTHESIS METABOLISM: the sum of all chemical reactions in an organism. METABOLIC PATHWAYS: a series of sequential reactions in which the product of one reaction is the reactant for the following reaction. TWO major metabolic pathways in life: CELLULAR RESPIRATION Organic molecules (like glucose) broken down to release energy for cell use PHOTOSYNTHESIS Light energy from sun is converted to chemical energy in the form of glucose The relationship between these two pathways results in the continual flow of energy within an organism as well as within an ecosystem. Plants and other organisms that can make their own food are known as autotrophs Humans and other organisms that have to obtain food are known as heterotrophs CELLULAR RESPIRATION CELLULAR RESPIRATION - AEROBIC (requires O2) ENERGY PRODUCTION (pp. 228-232) Cellular respiration is the breakdown of glucose in the presence of oxygen to “make” ATP. The oxygen required for cellular respiration is inhaled into the lungs, diffuses into the blood, and is delivered to the mitochondria of the body cells by red blood cells. The glucose needed is obtained through digestion. The glucose is transported in the blood and enter the body cells via facilitated diffusion by protein channels. Remember that this happens even in autotrophs like plants, too! (Plants DO require oxygen, but they produce more than they need) OVERALL EQUATION: Two major parts: Glycolysis Oxidative Respiration. Oxidative respiration can be further divided into two parts: the Krebs Cycle & the Electron Transport Chain KEY PLAYERS: Glucose, a six-carbon monosaccharide ATP, the final energy molecule Pyruvic acid, a three-carbon product of glycolysis O2 gas, the final electron acceptor of the ETC Electron Carriers: NADH and FADH2, carry high-energy electrons CO2 gas and H2O

start PAYDAY RESPIRATION STEP 1: GLYCOLYSIS RESPIRATION STEP 2: OXIDATIVE RESPIRATION Glycolysis – Means “sugar-breaking”. Occurs in the cytosol of the cell. does not require oxygen. The splitting of glucose, or glycolysis, occurs very quickly in a 10-step process with the aid of enzymes, producing two 3-C molecules known as pyruvic acid. In addition, when the bonds of glucose are broken, the high energy electrons that are released are caught by NADH, a molecule that acts as an electron carrier. This electron energy will be converted to ATP later in the process. Glycolysis requires 2 ATP to occur, but results in the formation of 4 ATP, for a net gain of 2 ATP. Reaction: C6 H12 O6 + 2 ATP → 2 pyruvic acid + 4 ATP + 2 NADH Net Energy Gain = 2 ATP + 2 NADH Glycolysis releases less than ¼ of the chemical energy stored in glucose. Most of its potential energy remains bound in the pyruvic acid formed from glycolysis. In aerobic conditions, meaning O2 is available, the pyruvic acid formed from the breakdown of glucose during glycolysis enters the mitochondria of the cell where the enzymes of oxidative respiration complete the breakdown of glucose to produce CO2, H2O, and ATP. Reaction: 2 pyruvic acid + O2 + 2NADH → 6CO2 + 6H2O + 32ATP 6 + +6 +6 + 2 + +2 + +30 + 2 4 STEP 2A: KREBS CYCLE Series of reactions that occur in the mitochondria, in which the energy stored in pyruvic acid is released in the form of high-energy electrons when bonds are broken and pyruvic acid is completely broken down to CO2. There are only 2 additional ATP produced in the Krebs Cycle; most of the energy released is captured in the form of electron energy, producing additional NADH. In addition, a second type of electron carrier is utilized, producing 2 “filled” FADH2. Net Energy Gain = 2 ATP + 8 NADH + 2 FADH2 Wastes Generated = CO2 start Krebs Cycle PAYDAY 2 + 8 + 2

- 2 N Y 30+ CO2 H2O STEP 2B: ELECTRON TRANSPORT CHAIN (“PAY DAY”) the electron carriers, NADH and FADH2 “dump” their electrons. These electrons are passed along a series of molecules embedded in the inner membrane of the mitochondria of eukaryotic cells. This same process occurs in the cell membrane of prokaryotic cells. As the electrons “fall” down the ETC, the energy they release is used to power an enzyme known as ATP synthase, which attaches phosphate groups to ADP to produce ATP. This process is known as oxidative phosphorylation because oxygen must be present. It is the electronegativity of oxygen that “pulls” the electrons down the ETC. As the electrons are collected by oxygen, water is produced from oxygen. Net Energy Gain = __~ 32 ATP_______ STAGE WHERE # ATP Needs O2? Wastes GLYCOLYSIS CYTOPLASM 2 N - KREBS CYCLE OUTER MITOCHONDRIA Y CO2 ELECTRON TRANSPORT CHAIN INNER 30+ H2O ALTERNATIVE PATHWAY : ANAEROBIC (WITHOUT OXYGEN) FERMENTATION The Krebs Cycle and ETC depend on oxygen (they are aerobic) to take electrons from the electron carriers. If no oxygen is present, NADH and FADH2 all fill up with high energy electrons and have no where to put them, and cellular respiration comes to a halt. A backup mechanism exists called anaerobic fermentation. In anaerobic conditions, the cell still uses glycolysis and generates 2 ATP and 2 pyruvic acid, along with 2 full NADH. The cell can empty the electron carriers by converting the 2 pyruvic acids into other chemicals. LACTIC ACID FERMENTATION: In human muscle cells, pyruvic acid is converted to lactic acid Lactic acid buildup contributes to the muscle soreness experienced during intense exercise Some bacteria and fungi do this and are important in producing the lactic acid in cheeses and yogurt ALCOHOLIC FERMENTATION: In yeast and some bacteria, the pyruvic acid are converted to alcohol. This is important in commercial brewing and baking. Overall, much less energy (only 2 ATP compared to 32) is produced by fermentation

STEPS OF PHOTOSYNTHESIS: Photosynthesis is the metabolic pathway that provides energy for most ecosystems Occurs in the chloroplasts of plants and other photosynthetic eukaryotes like algae Occurs in cyaneobacteria—photosynthetic bacteria Overall Equation: CHLOROPLASTS: Abundant in the leaf cells of most plants Structure: Thylakoids-flattened saclike membranes arranged in stacks, where the light-dependent reactions occur Grana—stacks of thylakoids Stroma—fluid-filled space that is outside the grana, where light-independent reactions take place. PIGMENTS: light-absorbing colored molecules act like antennas for absorbing energy Different pigments absorb different wavelengths of light Chlorophyll—most abundant pigments, absorb violet-blue the strongest, reflect green Accessory pigments—absorb blue and green, reflect yellow, orange and red. The pigments we see if fall leaves when chlorophyll production stops. Ex: Beta-carotene. STEPS OF PHOTOSYNTHESIS: LIGHT-DEPENDENT REACTIONS: requires light Chlorophyll in thylakoid membrane absorbs light energy and uses it to excite, and strip away electrons from H2O Water broken apart, forming O2 gas High energy electrons are passed along an electron transport chain (ETC) used to make ATP and are re-excited again by light, and transferred to electron carrier NADPH. CALVIN CYCLE (Light-independent rxns)—does not require light, in the stroma CO2 gas is used as a source of carbon and oxygen atoms to make glucose. NADPH used as a source of high energy electrons and Hydrogen ATP is used as a source of energy FACTORS AFFECTING PHOTOSYNTHESIS: Light—more light, more photosynthetic activity Water—needed to supply electrons. Not enough water, slow or no photosynthesis Temperature—photosynthesis functions best at 0-32 C