Cellular Energy

Energy in the World of Life Sustaining life’s organization requires ongoing energy inputs Assembly of the molecules of life starts with energy input into living cells

Energy Conversion Only about 10% of the energy in food goes toward building body mass, most is lost in energy conversions

Energy’s One Way Flow The total amount of energy available in the universe to do work is always decreasing Each time energy is transferred, some energy escapes as heat (not useful for doing work) On Earth, energy flows from the sun, through producers, then consumers Living things need a constant input of energy

The Energy of Life Energy can come in many forms…Light, heat, electricity, etc. Cells receive their energy from certain chemical fuels. The main type of chemical fuel that provides energy for all types of cells is ADENOSINE TRIPHOSPHATE. (ATP) ATP is composed of a nitrogen- containing compound called ADENINE, a 5-carbon sugar called RIBOSE and three PHOSPHATE GROUPS.

ADP Cells can also use a compound called ADENOSINE DIPHOSPHATE, which is identical to ATP, but with one less phosphate group.

Energy from ATP & ADP Energy is released into the cell by breaking the bonds between the phosphate groups in ATP. When bonds break, energy is released and the ATP becomes ADP. Cells can also store energy by “charging” ADP with phosphate groups. Think of ADP and ATP as a rechargeable battery.

ATP as Stored as Energy The body contains only a small amount of ATP at a time. Only enough to run cellular functions that need to be taken care of immediately. Instead, cells use large organic molecules such as glucose to store mass amounts of energy that will later be turned into ATP. A single molecule of glucose has over 90 times the energy of a single ATP molecule.

Autotrophs and Heterotrophs Autotrophs harvest energy directly from the environment, and obtain carbon from inorganic molecules Plants and most other autotrophs make their own food by photosynthesis, a process which uses the energy of sunlight to assemble carbohydrates from carbon dioxide and water Animals and other heterotrophs get energy and carbon by breaking down organic molecules assembled by other organisms

Two Main Metabolic Pathways Aerobic metabolic pathways (using oxygen) are used by most eukaryotic cells Anaerobic metabolic pathways (which occur in the absence of oxygen) are used by prokaryotes and protists in anaerobic habitats

Aerobic Respiration In modern eukaryotic cells, most of the aerobic respiration pathway takes place inside mitochondria Like chloroplasts, mitochondria have an internal folded membrane system that allows them to make ATP efficiently

Overview of Carbohydrate Breakdown Pathways Photoautotrophs (make their own energy via the sun) make ATP during photosynthesis and use it to synthesize glucose and other carbohydrates Most organisms, including photoautotrophs, make ATP by breaking down glucose and other organic compounds

energy glucose CO2 H2O O2 energy Photosynthesis Aerobic Respiration Figure 7.2 The global connection between photosynthesis and aerobic respiration. Note the cycling of materials, and the oneway flow of energy (compare Figure 5.5). Aerobic Respiration energy Figure 7-2 p118

Overview of Aerobic Respiration Three stages Glycolysis The Krebs cycle Electron Transport Chain (ATP formation) C6H12O6 (glucose) + O2 (oxygen) → CO2 (carbon dioxide) + H2O (water) + energy (ATP)

Aerobic Respiration glucose In the Cytoplasm Glycolysis 2 NADH 4 ATP (2 net) 2 ATP Glycolysis 2 NADH 2 pyruvate Krebs Cycle 6 CO2 2 ATP 8 NADH, 2 FADH2 In the Mitochondrion Figure 7.3 Animated Overview of aerobic respiration. The reactions start in the cytoplasm and end in mitochondria. Electron Transfer Phosphorylation H2O oxygen 32 ATP Figure 7-3 p119

Glycolysis – Glucose Breakdown Starts The reactions of glycolysis convert one molecule of glucose to two molecules of pyruvate (pyruvic acid) for a net yield of two ATP An energy investment of ATP is required to start glycolysis

Glycolysis GLYCOLYSIS is the process in which one molecule of glucose is broken in half, producing two molecules of PYRUVIC ACID, a 3-carbon compound. What do we put into Glycolysis? 2 Molecules ATP ATP helps break apart glucose with their energy. 2 Molecules of NAD+ NAD+ is a special chemical that helps carry high-energy electrons to other parts of the cellular respiration process to make ATP. Once NAD+ has electrons, it becomes known as NADH

Glycolysis Two ATP are used to split glucose (1) Enzymes react with this newly spit glucose, and released electrons, to form 2 NADH (2) Four ATP are formed in this process from ADP (3) Glycolysis ends with the formation of two three-carbon pyruvic acid molecules (4) 1 2 3 4

Second Stage of Aerobic Respiration The second stage of aerobic respiration completes the breakdown of glucose that began in glycolysis Occurs in mitochondria Includes a set of reactions known as the KREBS CYCLE

The Krebs Cycle At the end of glycolysis, about 90% of the chemical energy that was available in glucose is still unused…still locked in the high-energy electrons trapped in the newly-formed pyruvic acid. To extract that remaining energy, the body uses oxygen to take these remaining electrons and convert them to energy for the cell to use. This is WHY we need to breathe to stay alive!

The Krebs Cycle During the KREBS CYCLE, pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions. During the Krebs Cycle, the first compound that is made is CITRIC ACID, so therefore, the Kreb Cycle is also often referred to as the CITRIC ACID CYCLE. The Krebs Cycle begins when the pyruvic acid made in glycolysis enters the mitochondria of the cell.

The Krebs Cycle One carbon from the pyruvic acid breaks off and becomes a part of a molecule of carbon dioxide, CO2 (1). Pyruvic acid then loses high-energy electrons, NAD+ comes along and grabs electrons and becomes NADH. (2) The pyruvic acid goes through a few more chemical reactions and becomes CITRIC ACID. (3) SO FAR, WHAT IS MADE?  CITRIC ACID, CO2 1 2 3

The Krebs Cycle 4 5 6 As the Kreb Cycle continues, the CITRIC ACID is broken down into a 5-carbon compound, then a 4-carbon compound. Along the way, two more molecules of CO2 are released. (4) Along the way, high-energy electron carriers NAD+ and a new carrier FAD, gain electrons and become NADH and FADH2. (5) In the end, only one molecule of ATP is generated. (6) SO… FROM ONE MOLECULE OF PYRUVIC ACID, 4 NADH, 1 FADH2 AND 1 ATP ARE PRODUCED. 4 5 6

Aerobic Respiration’s Big Energy Payoff Many ATP are formed during the third and final stage of aerobic respiration Electron Transport Chain Occurs in mitochondria Results in attachment of many phosphates to many ADPs to form many ATPs

Electron Transport Chain From the Kreb Cycle, we obtain the high-energy electron carriers NADH and FADH2. These electron carries make their way from the Kreb Cycle to the next and final portion of the cellular respiration process…the ELECTRON TRANSPORT CHAIN (E.T.C.)   The ELECTRON TRANSPORT CHAIN uses the high-energy electrons from the Kreb Cycle to convert ADP to ATP.

Electron Transport Chain High-energy electrons from NADH and FADH2 are passed into and along the E.T.C. (1) The E.T.C. is composed of a series of special proteins that are located in the inner membrane of the mitochondria. These electrons are passed from one protein to the next The energy from the electrons, force hydrogen from one side of membrane to the next. (2) In this process, electrons react with hydrogen, combine them with oxygen to form water. (3) 2 1 3

Electron Transport Chain All these hydrogen ions that have moved across the membrane build up a high positive charge and cause the opposite side to become oppositely charged. Because of this difference of charges, hydrogen ions want to escape and do so through special proteins called ATP SYNTHASE. (4) 4

Electron Transport Chain Every time a hydrogen ion passes through ATP SYNTHASE, the protein spins, attaching a phosphate group to ADP, producing ATP. (5) On average, every pair of high-energy electrons that moves down the electron transport chain provides enough energy to convert 3 ADP molecules into 3 ATP molecules. 5

Summary: The Energy Harvest Typically, the breakdown of one glucose molecule yields 36 ATP Glycolysis: 2 ATP The Krebs cycle: 2 ATP Electron Transport Chain: 32 ATP

Fermentation

Fermentation When oxygen is not present, glycolysis turns to a new pathway known as fermentation, instead of the Krebs Cycle. FERMENTATION releases energy from food molecules in the absence of oxygen. During fermentation, cells convert NADH back into NAD+ by giving high-energy electrons back to pyruvic acid. Now, with more NAD+, more ATP can finally be made. Fermentation is an ANAEROBIC process, which means oxygen is not used.

Two Fermentation Pathways ALCOHOLIC FERMENTATION Yeasts and a few other microorganisms use this process. Ethyl alcohol and carbon dioxide are given off as wastes.   Pyruvic Acid + NADH  Alcohol + CO2 + NAD+ Alcoholic fermentation is what causes bread dough to rise. When the yeast in bread runs out of oxygen, it begins to ferment, giving off carbon dioxide bubbles. Usually all alcohol given off is evaporated off during the baking process.

Fermentation

Figure 7.10 Animated Alcoholic fermentation. A Alcoholic fermentation begins with glycolysis, and the final steps regenerate NAD+. The net yield of these reactions is two ATP per molecule of glucose (from glycolysis). B One product of alcoholic fermentation in Saccharomyces cells (ethanol) makes beer alcoholic; another (CO2) makes it bubbly. Holes in bread are pockets where CO2 released by fermenting Saccharomyces cells accumulated in the dough. The micrograph shows budding Saccharomyces cells. Figure 7-10b p127

Two Fermentation Pathways LACTIC ACID FERMENTATION In many cells, pyruvic acid built up by fermentation can be converted to a new substance called LACTIC ACID. LACTIC ACID helps convert NADH back to NAD+ to make ATP. Pyruvic Acid + NADH  Lactic Acid + NAD+ LACTIC ACID is produced in your muscles during rapid exercise when oxygen is not readily available to your muscle tissues.

Sunlight as an Energy Source Energy flow through nearly all ecosystems on Earth begins when photosynthesizers intercept energy from the sun Photosynthetic organisms use pigments to capture the energy of sunlight and convert it to chemical energy – the energy stored in chemical bonds

Pigments: The Rainbow Catchers Different wavelengths of light form colors of the rainbow Pigment An organic molecule that selectively absorbs light of specific wavelengths Chlorophyll a The most common photosynthetic pigment Absorbs violet and red light (appears green)

Photosynthetic Pigments Collectively, chlorophyll and accessory pigments absorb most wavelengths of visible light Certain electrons in pigment molecules absorb photons of light energy, boosting electrons to a higher energy level Energy is captured and used for photosynthesis

Some Photosynthetic Pigments Figure 6.3 Examples of photosynthetic pigments. Left, photosynthetic pigments can collectively absorb almost all visible light wavelengths. Right, the light-catching part of a pigment (shown in color) is the region in which single bonds alternate with double bonds. These and many other pigments are derived from evolutionary remodeling of the same compound, as is heme (Section 5.6). Heme is a red pigment.

6.4 Overview of Photosynthesis In plants and other photosynthetic eukaryotes, photosynthesis occurs in chloroplasts Photosynthesis occurs in two stages

Two Stages of Photosynthesis Light-dependent reactions First stage of photosynthesis Light energy is transferred to ATP and NADPH Water molecules are split, releasing O2 Occurs in the grana of a chloroplast Light-independent reactions Second stage of photosynthesis Energy in ATP and NADPH drives the creation of glucose and other carbohydrates from CO2 and water Occurs in the stroma of a chloroplast

Chloroplast

6CO2 + 6H2O + light energy → C6H12O6 + 6O2 Photosynthesis 6CO2 + 6H2O + light energy → C6H12O6 + 6O2

3D ANIMATION: Photosynthesis Bio Experience 3D

Light-Dependent Reactions Light dependent reactions use energy from the sun to produce ATP and NADPH. Light Dependent reactions take place using two special proteins that are trapped in the membrane of chloroplasts… PHOTOSYSTEM II PHOTOSYSTEM I (Photosystem II comes first but it is named “II” because it was discovered after Photosystem I)

Light-Dependent Reactions Photons of light hit PSII (1) and excite electrons (2). These electrons, travel out of PSII where it interacts with hydrogen ions and forces hydrogen ions (3) through the membrane and into the thylakoid (4). To replace the lost electrons in PSII, water is split and oxygen is given off (5). 1 3 2 5 4

Light-Dependent Reactions 2. The excited electron, travels through the membrane, through other proteins and eventually reaches PS1(6). Light hits PS1, re-energizing that electron(7) which will help create the electron carrier NADPH (8). 8 7 6

Light-Dependent Reactions 3. Finally, hydrogen ions, which are trapped in the thylakoid (9), find their way to ATP synthase (10). The ions travel up ATP synthase, spinning it and giving it energy to turn ADP into useful ATP (11). 11 10 9

Light-Independent Reactions Light-Independent Reactions are also known as the CALVIN CYCLE. So what’s carried over from the light-dependent reactions? NADPH ATP What’s needed for the light-independent reactions? CO2 What’s going to be produced? Glucose (C6H12O6) GLUCOSE is used as an energy storage molecule for the plant.

Light-Independent Reactions 6 carbon dioxide (CO2) molecules enter the reaction from the atmosphere (1) and combine with other molecules to form special 3-carbon molecules. 1

Light-Independent Reactions These 3-carbon molecules are bombarded with energy from ATP and electrons from NADPH. (2) This excites the 3-carbon molecules and pumps them full of energy. ATP turns into ADP and the NADPH turns back into NADP+. 2

Light-Independent Reactions Two of the 3-carbon molecules go on to produce a 6-carbon sugar, called GLUCOSE (C6H12O6). (3) The remaining 3-carbon molecules get recycled and start the Calvin Cycle over again. (4) 4 3