Cells and Energy Chapter 4

Preview Vocabulary Metabolic processes take one of two directions: They synthesize (build up material and store energy). or They catabolize (break material down and release energy). Chemosynthesis and photosynthesis are synthesizing processes that capture the energy needed for life and store it in sugars. Cellular respiration and fermentation are catabolic processes that break down sugars and deliver energy to sustain life.

The chemical energy used for most cell processes is carried by ATP. ATP stands for Adenosine Triphosphate ATP is a molecule made up of adenosine and three negatively charged phosphate groups. The energy carried by ATP is released when a phosphate group is removed from the molecule. The bonds holding the 3rd group in ATP is unstable and very easily broken. When the 3rd phosphate is removed, energy is released and ATP becomes ADP, or adenosine diphosphate. ADP is a lower-energy molecule.

The ATP Cycle The breakdown of ATP to ADP and the production of ATP from ATP can be represented by a cycle. ATP  ADP releases energy. ADP  ATP requires energy.

Ways of Obtaining Energy Heterotrophs: obtain energy by breaking down carbon compounds (food) in the presence or absence of oxygen. Animals, fungi, and some protistans are heterotrophs. Carbohydrates: 4 energy calories per mg Lipids: 9 energy calories per mg Protein: 4 energy calories per mg Autotrophs: use chemical energy to build their own food molecules. Plants and most algae photosynthesize, many bacteria chemosynthesize. Photosynthesis: converts light energy into carbon compounds. Chemosynthesis: converts chemical energy into carbon compounds.

Photosynthetic organisms are producers. Photosynthesis is the process whereby light energy is converted to chemical energy and carbon is fixed into organic compounds. In the presence of light, plants transform carbon dioxide and water into carbohydrates and release oxygen. 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2 carbon dioxide + water + light → sugar + oxygen Plants then use the sugars to produce complex carbohydrates such as starches. COMMON MISCONCEPTION Plants DO NOT get energy from photosynthesis. Rather, they use light energy to build sugars. They then use the sugars to build ATP via cellular respiration (just like animals).

Photosynthesis in plants occurs in chloroplasts. Chloroplasts are membrane-bound organelles found in the leaves photosynthetic organisms. Chlorophyll is the green pigment molecule in chloroplasts that is directly involved in photosynthesis. Thylakoids: have membranes that contain chlorophyll (where light is absorbed). Grana: stacks of thylakoid. Stroma: fluid surrounding thylakoids where Calvin cycle occurs.

The reactions of photosynthesis occur in two main stages. http://www.mhhe.com/biosci/bio_animations/02_MH_Photosynthesis_Web/ Light-Dependent Reactions: Capture energy from sunlight and use this energy to produce ATP and NADPH. ATP and NADPH are the energy required to power the Calvin cycle. Occur within and across the thylakoid membranes. This is the “photo” phase – uses light. Light-Independent Reactions: Also called the dark reactions or the Calvin cycle. Use the ATP and NADPH produced by the light reactions to build simple sugars. Occurs in the stroma of the chloroplast. This is the “synthesis” phase – builds sugars.

Calvin Cycle (Dark) Reactions: Visual Overview of Photosynthesis BOTH REQUIRE LIGHT (SOMEWHAT): Even the dark reactions in most plants occurs during daylight – because that is the only time the light reactions can operate AND the dark reactions depend on the light reactions!!! Light Reactions: -carried out by molecules in thylakoid membranes -convert light E to chemical E of ATP and NADPH -split H2O and release O2 to the atmosphere Calvin Cycle (Dark) Reactions: -take place in stroma -use ATP and NADPH produced in light reactions to convert CO2 into simple sugars -return ADP, inorganic phosphate, and NADP+ to the light reactions 9 9

Overview of Light-Dependent Reactions http://www. sumanasinc http://highered.mheducation.com/olc/dl/120072/bio13.swf Energy from sunlight is captured by chlorophyll a found in chloroplasts. The energy is transferred to electrons that enter an electron transport chain found in the thylakoid membranes. Water molecules are broken down into H+ ions and electrons, and oxygen is released as a waste product. The water molecules provide the H+ ions needed to establish a concentration gradient to produce ATP, and also provide the electrons lost by chlorophyll a when the molecule is excited by light. The two electron transport chains of the light reactions create ATP and NADPH – both of which will be used to power the Calvin cycle to make sugars.

Overview of Light-Independent Reactions http://highered. mheducation Carbon dioxide enters the leaf through the stomata. ATP and NADPH from the light-dependent reactions transfer energy to the Calvin cycle and keep the cycle going. As the cycle turns, simple sugars are built and exported. The products of the Calvin cycle are simple sugars, ADP, and NADP. The ADP and NADP are recycled back to the light-dependent reactions.

Cellular respiration makes ATP by breaking down sugars. Animals eat other organisms for food, but food is not a direct source of energy for cells. Instead, all organisms break down molecules from food to produce ATP. Because it occurs in the presence of oxygen, cellular respiration is known as aerobic respiration. COMMON MISCONCEPTION Animals are NOT the only organisms that use cellular respiration. All living organisms use some type of respiration (aerobic or anaerobic) to produce ATP…this includes bacteria, protistans, fungi, plants, and animals!!!

OVERVIEW OF CELLULAR RESPIRATION The chemical formula for cellular respiration is: 6O2 + C6H12O6 → 6 CO2 + 6 H2O + Energy oxygen + glucose → carbon dioxide + water + energy The reactants of cellular respiration are: oxygen (O2) & glucose (C6H12O6) The products of cellular respiration are: carbon dioxide (CO2) and water (H2O) The 4 main stages of cellular respiration are: Glycolysis Intermediate Step Krebs Cycle (Citric Acid Cycle) Electron Transport / Oxidative Phosphorylation

Cellular respiration occurs in mitochondria. Mitochondria are membrane-bound organelles found in the cells of organisms that perform cellular respiration. Mitochondria have a double membrane that serves to compartmentalize the reactions of cellular respiration. Cristae: folds created by convoluted inner membrane, increases the surface area for the electron transport chain. Matrix: aqueous solution in the center of mitochondria where chemical reactions of Krebs cycle occur. Inner-Membrane Space: area where a hydrogen ion gradient is established using energy from electron transport chain – gradient powers the production of ATP from ADP and Pi.

The Players for Cellular Respiration Mitochondria – site of cellular respiration in cells. Glucose – energy source broken down to release ATP. NADH & FADH2 – coenzymes that shuttle electrons from Glycolysis & The Krebs Cycle to the Electron Transport Chain. Glycolysis – begins the breakdown of glucose into two molecules of pyruvate. Intermediate Step – converts pyruvate from glycolysis into Acetyl CoA for entry into Krebs cycle Krebs Cycle – completes the breakdown of glucose. CO2 – waste product of cellular respiration. O2 – required for aerobic respiration. Electron Transport Chain – establishes a concentration gradient of hydrogen across the inner membrane – gradient powers production of ATP.

Cellular respiration occurs in four main stages. http://www.sumanasinc.com/webcontent/animations/content/cellularrespiration.html http://www.mhhe.com/biosci/bio_animations/MH01_CellularRespiration_Web/index.html Glycolysis Takes place in cytoplasm of cell just outside the mitochondria. Begins the breakdown of glucose into 2 molecules of pyruvate. Produces 2 ATP, 2 NADH, and 2 pyruvate Intermediate Step Takes place just inside the mitochondria. Converts pyruvate from glycolysis into acetyl CoA, which can enter the Krebs cycle. Produces 2 NADH and 2 acetyl CoA; releases carbon-dioxide waste Krebs (Citric Acid) Cycle Takes place in the matrix of the mitochondria. Completes the breakdown of glucose. Produces 2 ATP, 6 NADH, and 2 FADH2, releases carbon dioxide waste Electron Transport Occurs across the inner membrane of the mitochondria. Uses electrons donated from NADH and FADH2 to establish a hydrogen ion gradient across the inner membrane which is used to power the production of ATP. Produces approximately 34 ATP

Glycolysis occurs in the cytoplasm. Intermediate step occurs just inside the mitochondria. Krebs cycle occurs in the matrix of the mitochondria. Electron transport occurs across the inner membrane of the mitochondria – concentration gradient occurs in inner-membrane space.

Overview of Glycolysis http://highered. mcgraw-hill Occurs in the cytoplasm & produces 4 ATP, but uses 2 ATP in the process, so there is a net of 2 ATPs produced. Products are 2 pyruvic acids, 2 NADH molecules, and 2 molecules of ATP. The first set of reactions in cellular respiration is glycolysis Glycolysis is the process in which 1 molecule of glucose is broken in half, producing 2 molecules of pyruvic acid. Occurs in the cytoplasm & produces 4 ATP, but uses 2 ATP in the process, so there is a net of 2 ATPs produced. Products are 2 pyruvic acids, 2 NADH molecules, and 2 molecules of ATP.

Intermediate Step Converts Pyruvate into Acetyl CoA The pyruvate produced during glycolysis cannot enter into the Krebs cycle. The conversion of pyruvate to acetyl CoA is the junction between glycolysis (step 1) and the Krebs cycle (step 2). If oxygen is present, the pyruvate from glycolysis enters the mitochondrion. Each pyruvate is then converted into a molecule of Acetyl CoA. CO2 waste is released and NAD+ is also used to form more NADH.

Overview of Krebs (Citric Acid) Cycle http://highered. mheducation In the presence of oxygen, the acetyl CoA produced during the intermediate step passes to the second stage of cellular respiration: the Krebs Cycle, which occurs in the matrix of the mitochondria. During the Krebs cycle, acetyl CoA is broken down into carbon dioxide in a series of energy-extracting reactions. Every time you exhale, you expel the CO2 produced by the Krebs cycle. The Krebs cycle completes the breakdown of glucose by extracting the remaining hydrogens and electrons from glucose and transferring them to the energy shuttles NAD+ and FAD2+, which will carry these to the electron transport chain. Because citric acid is the first product of the Krebs cycle, this event is also known as the Citric Acid cycle.

Overview of Electron Transport Chain http://highered. mheducation The electron transport chain uses the high-energy electrons from the Krebs cycle to convert ADP to ATP. Remember that these electrons are brought to the ETC from the Krebs by NADH and FADH2. When high energy electrons transport down the ETC, their energy is used to transport H+ ions across the inner membrane of the mitochondria…this creates a (+) charge on the inside of the membrane and a (–) charge in the matrix of the mitochondria. As a result of this charge difference, H+ ions escape through channel proteins called ATP synthase causing it to rotate. Each time it rotates, the enzyme ATP synthase grabs a low energy ADP and attaches a phosphate, forming high-energy ATP. The final electron acceptor for the electrons coming off of the ETC is OXYGEN – this oxygen combines with electrons and hydrogens to form the water that is a waste product of cellular respiration.

Cellular respiration and photosynthesis are like mirror images.

Respiration in the Absence of Oxygen (Anaerobic Respiration) When oxygen is NOT present, glycolysis is followed by a different pathway called fermentation. Fermentation releases energy from food molecules in the absence of oxygen Because fermentation does not require oxygen, it is said to be anaerobic. The 2 main types of fermentation are: alcoholic fermentation lactic acid fermentation Aside from the original 2 ATP’s made during glycolysis, the only energy produced is that which is in the bonds of: ethyl alcohol -- C2H6O lactic acid – C3H5O3

As you can see, the role of fermentation is simply to provide glycolysis with a steady supply of NAD+. By itself, fermentation does NOT produce ATP. Instead, it allows glycolysis to continue to produce ATP in the absence of oxygen.

Fermentation and its products are important in several ways. Fermentation produces the lactic acid waste product that builds up in muscle cells and causes a burning feeling. Cheese, bread and yogurt are just a few of the foods made by fermentation. The waste products of fermentation give cheeses their different flavors and textures. Fermentation produces the lactic acid waste product that builds up in muscle cells and causes a burning feeling. Cheese, bread and yogurt are just a few of the foods made by fermentation. The waste products of fermentation give cheeses their different flavors and textures.

Fermentation and Aerobic Respiration Compared Both processes use glycolysis to break down glucose and other organic fuels into pyruvate. The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration. Cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule.