Energy in a Cell Chapter 9

Cell Energy Energy is essential to life Plants trap light energy in sunlight and store it in the bonds of certain molecules to use later Other organisms get energy from eating those green plants What processes can you name that require energy?

Adenosine Triphosphate (ATP) An energy molecule in the cell that allows for quick and easy access to energy when needed by the cell’s organelles. A type of chemical energy Releases energy when the chemical bonds are broken A-P-P-P

Forming ATP Phosphate groups are negatively charged Negative doesn’t like being next to negative A small amount of energy is required to attach one phosphate group to adenosine (AMP) When a second phosphate group is added, this requires a lot more energy (ADP) When a third phosphate group is added, this requires an even greater amount of energy (ATP) The process of forming ATP requires much energy

Breaking down ATP Energy of ATP becomes available to a cell when the molecule is broken down When a cell requires energy, ATP goes to the cell, attaches to the binding site, and a phosphate group is broken off – this gives off energy for the cell and the ATP molecule becomes ADP (fig. 9.2 pg. 223) In order for ADP to become ATP again it goes to the mitochondria and gets recharged (another phosphate group gets attached)

Uses of cell energy Energy is VERY important on the cellular level Making new molecules Building membranes and cell organelles Cells use energy to maintain homeostasis Kidneys use energy to move molecules and ions in order to eliminate waste substances while keeping needed substances in the bloodstream.

Photosynthesis In this section… What is photosynthesis? Where photosynthesis happens Color: How it works The two phases of photosynthesis

Photosynthesis A process of taking light energy and converting it into chemical energy This energy is stored as carbohydrates in plants Happens in two phases: Light-dependent reactions- converts light energy into chemical energy-molecules of ATP produced fuel light-independent reactions Light-independent reactions- produce glucose

Where does photosynthesis occur? In chloroplasts there are thylakoid disks/grana Light-dependent reactions happen in the thylakoid membranes

Pigments To trap the energy in the sunlight, the thylakoid membranes contain pigments. Pigments are molecules that absorb specific wavelengths of sunlight Chlorophyll is the most common type of pigment in chloroplasts

Why we see color We see colors that are reflected and not absorbed. Green pigments absorb all light except green (chlorophyll) In the fall, trees stop producing chlorophyll, which results in the different colors seen.

The Big Picture Energy Light-Dependent Reactions Light-Independent Reactions (Calvin Cycle) Stored Energy (stored as glucose)

Light-Dependent Reactions First phase of photosynthesis requires sunlight. A light-dependent reaction involves sunlight striking molecules of chlorophyll and exciting an electron. Excited electrons are passed from chlorophyll to an electron transport chain Electron transport chain- a series of proteins embedded in the thylakoid membrane

Light-dependent reactions Once in the electron transport chain, each protein in the chain passes the energized electrons along to the next protein- some energy is lost during each pass- lost energy can be used to form ATP from ADP OR to pump hydrogen ions into the center of the thylakoid disc. After electrons have traveled down the electron transport chain, they are re-energized in a second photosystem and passed down a second electron transport chain- electrons are still very energized

Light-Dependent reactions Electrons are then transferred to the stroma of the chloroplast Transferred by an electron carrier molecule called NADP+ (nicotinamide adenine dinucleotide phosphate) NADP+ can combine with two excited electrons and a hydrogen ion (H+) to become NADPH. NADPH stores the energy until it can transfer it to the stroma- this is where NADPH will play an important role in the light-independent reaction.

Restoring Electrons The chlorophyll needs to replace the electrons that were lost at the beginning of photosynthesis in order to absorb additional light to keep the process going. To replace lost electrons, the molecules of water are split in the first photostem- reaction called photolysis For every water molecule that is split, 1 Oxygen, 2 electrons, and 2 Hydrogen ions are formed Oxygen produced by photolysis is released into the air-supplies oxygen for air we breathe Electrons are returned to the chlorophyll H+ ions are pumped into the thylakoid-> they accumulate in high concentrations which causes a concentration gradient- H+ ions diffuse out of thylakoid and provide energy for production of ATP (called chemiosmosis)

Light-Independent reactions 2nd phase of photosynthesis Does NOT require light Takes place in the stroma of the chloroplast Aka Calvin cycle- called a cycle bc one of the products is needed to start the cycle over Follow the cycle on pg. 229

The Calvin Cycle 1) CARBON FIXATION-The carbon atom from CO2 bonds with a five-carbon sugar called ribulose biphosphate (RuBP) to form an unstable six carbon sugar. 2) FORMATION OF 3-CARBON MOLECULES-The six-carbon sugar immediately splits to form two three-carbon molecules. 3) USE OF ATP AND NADPH-A series of reactions involving ATP and NADPH from the light-dependent reactions converts the three-carbon molecules into phosphoglyceraldehyde (PGAL), three-carbon sugars with higher energy bonds. 4) SUGAR PRODUCTION- One out of every six molecules of PGAL is transferred to the cytoplasm and used in the synthesis of sugars and other carbohydrates. After three rounds of the cycle, six molecules of PGAL are produced. 5) RuBP IS REPLENISHED- Five molecules of PGAL, each with three carbon atoms, produce three molecules of the five-carbon RuBP. This replenishes the RuBP that was used up, and the cycle can continue.

Getting Energy to Make ATP Cellular Respiration- The process by which mitochondria break down food molecules to produce ATP. There are 3 stages of cellular respiration 1) Glycolysis- anaerobic (no oxygen required) 2) Citric acid cycle- aerobic (oxygen required) 3) Electron transport chain- aerobic (oxygen required)

3 parts of cellular respiration

Glycolysis a series of reactions in the cytoplasm of a cell in which glucose (a 6 carbon molecule) is broken down into two molecules of pyruvic acid (3 carbon molecules). ATP - it takes 2 molecules of ATP to start the process of glycolysis, and only 4 ATPs are made, therefore this process is not very energy efficient.

Glycolysis *only 2 molecules of ATP are produced from the breakdown of one glucose molecule. NAD+ (nicotinamide dinucleotide) - just as photosynthesis has the energy carrier NADP+; glycolysis has an energy carrier called NAD+. *NAD+ forms NADH when carrying an electron. At the end of glycolysis the pyruvic acid molecules produced move to the mitochondria,the powerhouses or ATP producers of the cell.

Glycolysis

Glycolysis- on a molecular level

Post-Glycolysis Post-glycolysis reactions - before the pyruvic acid molecules can enter the citric acid cycle (the next stage of cellular respiration) some modifications need to be done. pyruvic acid loses a molecule of CO2 and combines with Coenzyme A to form a molecule of Acetyl-CoA. the rxn w/ Coenzyme A makes a molecule of NADH+ H+

The Citric Acid Cycle The Citric Acid Cycle: “The breakdown of Glucose”-a series of chemical reactions similar to the Calvin Cycle, but opposite in purpose. Calvin Cycle - forms glucose in photosynthesis Citric Acid Cycle - breaks down glucose in cellular respiration Materials needed : to break down glucose, two electron carriers are needed: a) NAD+ b) FAD (flavin adenine dinucleotide)

The Citric Acid cycle The Citric Acid Cycle (CAC) produces a number of molecules: a) 1 ATP is produced b) 3 NADH + H+ are produced c) 1 FADH2 molecule is produced

Steps of the Citric Acid Cycle 1) formation of citric acid - a 2 carbon acetyl CoA combines with a 4 carbon compound called oxaloacetic acid, forming a 6 carbon molecule called citric acid. 2) formation of CO2 - one molecule of CO2 is formed from the citric acid cycle which reduces the citric acid molecule to a 5 carbon molecule called ketoglutaric acid.*from this rxn, one molecule of NADH +H+ is made from one NAD+ 3) formation of second CO2 - another molecule of CO2 is formed and released from the ketoglutaric acid; this results in a 4 carbon compound called succinic acid. *from this rxn, one molecule of ATP and one molecule of NADH + H+ are formed. 4) recycling of oxaloacetic acid - succinic acid undergoes a series of rxns which form FADH and NADH + H+ and oxaloacetic acid; this is then available for the next cycle to occur. Succinic -> fumaric -> malic -> oxaloacetic

Citric Acid Cycle

Citric Acid Cycle

Electron Transport Chain Function - move energized molecules; NADH & FADH2 pass energized molecules from protein to protein releasing small amounts of energy with each pass. Location - the inner membrane of the mitochondria

Mitochondria

Electron Transport Chain The Process: a) NADH & FADH2 pass energized molecules from protein to protein; small amounts of energy are released with each pass. b) some energy is used to form ATP, while some is used to pump H+ ions into the center of the mitochondria. c) as H+ ions are pumped into the center of the mitochondria, the center becomes more (+),while the outside becomes more (-). Since the outside is more (-) it will attract more (+)’s or more H+ ions,creating an electrochemical gradient. d) The electrochemical gradient drives the inner membrane of the mitochondria to form ATP. e) The final electron acceptor in the ETC is Oxygen. The oxygen reacts with H+ ions to form water molecules.

ETC in mitochondria

ETC Importance The importance of Oxygen (O2) If oxygen is not available for the ETC, then the chain cannot pass along energized electrons; if electrons cannot be passed, then there is no room to accept more electrons and a blockage results. Therefore, cellular respiration cannot occur. Overall production The ETC results in the production of 32 ATP molecules This is the most efficient means for production of ATP Think: Aerobic (jogging) vs, Anaerobic (sprinting) - which can be done longer?

Fermentation sometimes your cells may be deprived of oxygen for a short time fermentation can occur during extremely strenuous activities Fermentation - anaerobic process that occurs when your cells are w/o O2 for a short time. It occurs after glycolysis and provides a way to continue producing ATP until oxygen is available again. 2 main types of fermentation: a) lactic acid fermentation b) alcoholic fermentation

Lactic Acid Fermentation *occurs during anaerobic conditions when oxygen is not available as the final electron acceptor in the ETC, therefore a “back-up” occurs. What happens: a) as NADH and FADH2 try to pass their energized electrons onto the next protein in the ETC, they are rejected. b) if NADH and FADH2 cannot pass on their energized electrons, then NADH and FADH2 cannot be converted back to NAD+ & FAD, which are needed to keep the CAC and glycolysis going.

Alcoholic Fermentation *often used by yeast cells to produce CO2 and ethyl alcohol. *anaerobic process - used to make bread dough “rise” and brew alcohols.

Comparing Photosynthesis and Cellular Respiration Both use an ETC to form ATP Do opposite jobs Photosynthesis - produces high energy carbohydrates and O2 from the sun’s energy Cellular respiration - uses O2 to break down carbohydrates with much lower energy level

Photosynthesis Cellular Respiration Comparisons: Food is made or accumulated Food is Broken down Energy from sun is stored as glucose Energy from glucose is released to be used by body Carbon dioxide (CO2) is taken in CO2 is given off as a waste product Oxygen (O2) is given off as waste Oxygen is needed and is taken in Produces glucose from PGAL Produces CO2 and H2O as waste Can happen only when some light is available Can occur all day and all night Requires Chlorophyll, can only happen in plants Occurs in all living cells - plants and animals