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The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Controls traffic across the membrane Enables cell function Concept 7.1: Cellular membranes are fl...

The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Controls traffic across the membrane Enables cell function Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Predominately made of lipids and proteins Carbohydrates important as well Which lipid is the most abundant lipid in the PM? Amphipathic molecules—have hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Membrane Models: Scientific Inquiry 1950’s—First membrane seen with e- microscope 1915—Membranes isolated and chemically analyzed They found membranes were made of lipids 1925—Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer Membrane Models: Scientific Inquiry 1935—Hugh Davson and James Danielli proposed the sandwich model A PLB between two layers of proteins Membrane Models: Scientific Inquiry Later studies found problems with sandwich model Placement of membrane proteins, which have hydrophilic and hydrophobic regions The Fluidity of Membranes Membrane held together hydrophobic interactions—much weaker than covalent bonds Hydrophobic interactions—When nonpolar molecules interact with each other in polar substances (water) Interactions > in polar substances compared to nonpolar substances The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane Why might this be? The Fluidity of Membranes Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Will membranes remain more fluid as temperature drops if they have saturated or unsaturated hydrocarbons? Cholesterol is an important part of membranes Cholesterol has different effects on membrane fluidity at different temperatures Warm temps (37°C)—cholesterol makes membrane less fluid by restraining movement of phospholipids Cool temps—it maintains fluidity by preventing tight packing Need even cooler temps to solidify  Cholesterol is like a “fluidity buffer”—helps to prevent extremes Hydrophobic The Fluidity of Membranes Membranes must be fluid to work properly; they are usually about as fluid as olive oil Consequences of solidified membranes Changes in permeability Proteins may become inactive Consequences of excess fluidity Protein function impaired Extreme environments challenge structure and function of a membrane Can lead to evolutionary adaptations Evolution of Differences in Membrane Lipid Composition Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions Ability to change the lipid compositions in response to temp changes has evolved in organisms that live where temperatures vary Plants in extreme cold increase unsaturated phospholipids in autumn—keep membrane fluid in winter Some bacteria and archaea can change proportion of unsaturated phospholipids in membranes Membrane Proteins and Their Functions A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer Membranes in different cell types have different proteins Proteins determine most of the membrane’s specific functions Two Major Types of Membrane Proteins Peripheral proteins are bound to the surface of the membrane (thus, hydrophilic) Integral proteins penetrate the hydrophobic core Integral proteins that span the membrane are called transmembrane proteins Integrins are a type of integral protein Integral Membrane Proteins The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices The Role of Membrane Carbs in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual Diversity of Cells within Species Carbohydrate chains on blood cells determine the blood type Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces Face different environments, carry out different functions The distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and modified by the Golgi apparatus Concept 7.2: Membrane structure results in selective permeability A cell must exchange materials (ions, small molecules) with its surroundings This process is controlled by the plasma membrane Plasma membranes are selectively permeable, regulating the cell’s molecular traffic The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules (e.g., hydrocarbons, CO2, and O2) can dissolve in the lipid bilayer and pass through the membrane rapidly Large, polar molecules, such as sugars and H2O, do not cross the membrane rapidly without aid Membrane Permeability Which of the following molecules will diffuse most quickly across a lipid bilayer membrane? H2O O2 Cl- C6H12O6 Na+ Transport Proteins Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel Channel proteins called aquaporins facilitate the passage of water (up to 3 billion water molecules per second) Transport Proteins Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves Concept 7.3: Diffusion and Passive Transport Diffusion is the movement of molecules from an area in which they are highly concentrated to an area in which they are less concentrated At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other Diffusion and Passive Transport Passive Transport—diffusion of a substance across a membrane with no investment in energy Diffusion and Passive Transport Substances diffuse down their concentration gradient The change in concentration of a solute over distance In diffusion, a substance moves down its concentration gradient No work must be done for diffusion The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen Osmosis The diffusion of a solvent (e.g., H2O) across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides Osmosis Water Balance of Cells Without Walls Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water Considers both solute concentration and membrane permeability Tonicity of a solution depends on: Concentration of nonpenetrating solutes outside vs. inside of the cell Water Balance of Cells Without Walls Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Water Balance of Cells Without Walls Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water Water Balance of Cells Without Walls Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water Water Balance of Cells Without Walls Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Paramecium Vacuole Water Balance of Cells with Walls Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) In a hypertonic environment, plant cells lose water; membrane pulls away from the wall, a usually lethal effect called plasmolysis If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), the plant may wilt Osmosis Which of the following statements about osmosis is correct? a) If a cell is placed in an isotonic solution, more water will enter the cell than leaves the cell. b) The presence of aquaporins (proteins that form water channels in the membrane) should speed up the process of osmosis. c) If a solution outside the cell is hypertonic compared to the cytoplasm, water will move into the cell by osmosis. d) Osmosis is the diffusion of water from a region of lower water concentration to a region of higher water concentration. If a marine algal cell is suddenly transferred from seawater to freshwater, the algal cell will initially lose water and decrease in volume stay the same: neither absorb nor lose water absorb water and increase in volume Facilitated Diffusion In facilitated diffusion, transport proteins passively move molecules across the plasma membrane Solute still moving down its concentration gradient Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Channel proteins include Aquaporins, for facilitated diffusion of water Ion channels that open or close in response to a stimulus (gated channels) Two types of Transport Proteins Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane Concept 7.4: Active transport uses energy to move solutes against their gradients Facilitated diffusion is still passive because the solute moves down its concentration gradient, and the transport requires no energy Some transport proteins, however, can move solutes against their concentration gradients Which is from an area of low concentration to an area of high concentration The Need for Energy in Active Transport Active transport moves substances against their concentration gradients Active transport requires energy, usually in the form of ATP Active transport is performed by specific proteins embedded in the membranes Use carrier proteins, not channel proteins The Sodium-Potassium Pump Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system Exchanges Na+ for K+ How Ion Pumps Maintain Membrane Potential Membrane potential is the voltage difference across a membrane Voltage is created by differences in the distribution of positive and negative ions across a membrane The Electrochemical Gradient Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane A chemical force (the ion’s concentration gradient) An electrical force (the effect of the membrane potential on the ion’s movement) Ions move down/up their electrochemical gradient Chemical and electrical forces can point toward the same direction or oppose each other Active transport is needed to pump ions against their electrochemical gradient The Electrochemical Gradient An electrogenic pump is a transport protein that generates voltage across a membrane The Na-K pump = major electrogenic pump of animal cells Proton pump = main electrogenic pump of plants, fungi, and bacteria Electrogenic pumps help store energy that can be used for cellular work Figure 7.20 Pumping creates potential energy When you pump water uphill, it can perform work as it flows downhill Similarly, if substances are pumped against their concentration (or electrochemical) gradient, they can do work as they diffuse back across the membrane Figure 7.21 How this saves lives Diarrhea Body loses water and salt, loss of both make you sick Expelling waste so fast, reabsorption of salt (consuming salt and water) won’t work Patients given a sugar-salt solution Solutes absorbed by sodium-glucose cotransporters Reduces infant mortality worldwide Na-K pumps help establish a voltage across nerve cells’ plasma membranes; do these Na-K pumps use ATP or produce ATP? They produce ATP by pumping Na and K against their gradient They use ATP to pump Na and K against their gradients; this produces a voltage that stores energy for work Neither, no ATP is needed They both use and produce ATP Common Misconceptions The Na-K pump is active transport but it is not cotransport Cotransport uses a pump to create a gradient, which is then used to transport solutes into the cell down their gradients Proton pump pushes H+ outside the cell That gradient is then used to passively transport sucrose and H+ back into the cell; H+ is the escort to sucrose Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis Phagocytosis In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle Pinocytosis In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles Non-specific, any and all solutes included in gulp In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation Engulf bulk quantities of specific substances Cholesterol Concept Check As the cell grows, its plasma membrane expands; does this involve exocytosis or endocytosis? Let’s Review: Cellular membranes are fluid mosaics of lipids and proteins Sandwich model replaced by fluid mosaic model Amphipathic proteins embedded in PLBL Phospholipids move laterally, flipping across is less common Unsaturated HC tails keep membranes fluid at lower temps Cholesterol = fluidity buffer Difference in lipid composition evolutionary adaptation Integral vs. peripheral proteins Let’s Review Membrane protein functions Transport Enzymatic activity Signal transduction Cell-to-cell recognition Intercellular joining Attachment to ECM and cytoskeleton Glycoproteins = short sugar chains linked to proteins Glycolipids = short sugar chains linked to lipids GPs and GLs are on the exterior and interact with surface molecules of other cells Let’s Review: membrane structure results in selective permeability Cells must exchange molecules & ions with exterior Controlled by selective permeability of PM What passes through quickly and easily? Hydrophobic substances (CO2, O2, hydrocarbons) Why do these pass through easily? What doesn’t pass through quickly or easily? Hydrophilic/polar substances, large molecules, and ions Why? What do these require to get through the membrane? What transports water across the PM? Let’s Review: Passive Transport Passive transport—diffusion of a substance across a membrane without using energy Diffusion—spontaneous movement of substance down its conc. gradient When water diffuses this is called what? Hypertonic—solution is more concentrated outside the cell Hypotonic—solution is less concentrated outside the cell Isotonic –solutions on either side of PM equal Let’s Review: Passive Transport Let’s Review: Passive Diffusion Facilitated diffusion— a type of passive diffusion that speeds up transport of water and solutes across membrane down their conc. gradients Can use channel proteins or carrier proteins Review: Active transport Specific membrane proteins use ATP to transport solutes against their conc. gradients Na-K pump Electrochemical gradient—from conc. gradients (chemical) and electrical gradient (voltage) Determine net direction of ionic diffusion Review: Active transport Electrogenic pumps—transport proteins that help create electrochemical gradients Proton Pump Cotransport—when a transport protein enables “downhill” diffusion of one solute through the “uphill” transport of another Review: Bulk Transport by Exo- and Endocytosis Exocytosis—transport vesicles move to PM, fuse with it, release their contents Endocytosis—molecules enter cell within vesicles pinched inward from PM 3 Types Phagocytosis Pinocytosis Receptor-mediated endocytosis An Introduction to MetabolismThe living cell is a miniature chemical factory where thousands of reactions occur, The cell extracts energy and applies energy to perform work Concept 8.1: Metabolism, Laws of thermodynamics Metabolism is the totality of an organism’s chemical reactions Metabolism—manages material and energy resources of a cell Metabolism is an emergent property of life that arises from interactions between molecules within the cell Organization of the Chemistry of Life into Metabolic Pathways A metabolic pathway begins with a specific molecule and ends with a product Each step is catalyzed by a specific enzyme Mechanisms that regulate enzymes balance metabolic supply and demand Organization of the Chemistry of Life into Metabolic Pathways Catabolic pathways release energy by breaking down complex molecules into simpler compounds Degradative process Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism Organization of the Chemistry of Life into Metabolic Pathways Anabolic (biosynthetic) pathways consume energy to build complex molecules from simpler ones Forms of Energy Energy is the capacity to cause change Energy exists in various forms, some of which can perform work Work—move matter against forces (gravity, friction) Life is dependent upon cells’ capacity to transform energy from one form to another Forms of Energy Kinetic energy is energy associated with motion Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules Lost to the environment Potential energy is energy that matter possesses because of its location or structure Chemical energy is potential energy available for release in a chemical reaction Glucose is high in chemical energy Concept Check Consider an apple in an apple tree Describe the forms of energy found in an apple as it: Grows on the tree Falls Digested after you eat it The Laws of Energy Transformation Thermodynamics is the study of energy transformations An isolated system, e.g., liquid in a thermos, can’t exchange matter or energy with its surroundings In an open system, energy and matter can be transferred between the system and its surroundings Organisms are open systems Anything outside the system = surroundings The First Law of Thermodynamics 1st law- the energy of the universe is constant Energy can be transferred and transformed, but it cannot be created or destroyed It can only be converted from one form to another The Second Law of Thermodynamics 2nd law—Every energy transfer or transformation increases the entropy (disorder) of the universe When energy transferred/transformed, some energy is unusable, and is often lost as heat Alternative perspective of 2nd Law—Trend toward randomization/increasing entropy Disorder increases as bear runs Release heat & small molecules The Second Law of Thermodynamics Living cells unavoidably convert organized forms of energy to heat More random matter is = greater entropy For a process to occur without energy input, it must increase the entropy of the universe Spontaneous processes—occur without energy input; they can happen quickly or slowly Energetically favorable Nonspontaneous—process which cannot occur on its own; energy required Concept 8.2: The free-energy change A living system’s free energy (G) is energy that can do work when temperature and pressure are uniform, as in a living cell Biologists want to know which reactions occur spontaneously and which require input of energy To do so, they need to determine energy changes (G) that occur in chemical reactions Concept 8.2: The free-energy change Only processes with a negative ∆G are spontaneous Spontaneous processes can be harnessed to perform work (e.g., diffusion of solutes) A process is spontaneous and can perform work only when it is moving toward equilibrium Application biologist want to understand what metabolic reactions can happen spontaneously Free Energy and Metabolism An exergonic reaction proceeds with a net release of free energy and is spontaneous (energetically favorable) Ex “out” ΔG negative Release heat Free Energy and Metabolism An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous (not energetically favorable) En “in” Absorb free energy to stores in molecules (in bonds) (ΔG positive, G increases) Absorb heat Example: Photosynthesis Equilibrium and Metabolism Reactions in a closed system eventually reach equilibrium and then do no work Equilibrium and Metabolism Cells are not in equilibrium; they are open systems experiencing a constant flow of materials Equilibrium and Metabolism A defining feature of life is that metabolism is never at equilibrium A catabolic pathway in a cell releases free energy in a series of reactions Concept 8.3: ATP powers cellular work A cell does three main kinds of work Chemical Transport Mechanical To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one Most energy coupling in cells is mediated by ATP The Structure and Hydrolysis of ATP ATP (adenosine triphosphate) is the cell’s energy shuttle The Structure and Hydrolysis of ATP The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis Energy is released from ATP when the terminal phosphate bond is broken The Structure and Hydrolysis of ATP Is the hydrolysis of ATP exergonic or endergonic? How the Hydrolysis of ATP Performs Work The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction Overall, the coupled reactions are exergonic How the Hydrolysis of ATP Performs Work Glutamine is an amino acid What is the change in free energy? So, is this reaction exergonic or endergonic? Will this reaction happen spontaneously? How the Hydrolysis of ATP Performs Work Glutamine synthesis is a 2-step process, which includes a phosphorylated intermediate: ATP phosphorylates glutamic acid less stable Ammonia (NH3) displaces the phosphate group  glutamine How the Hydrolysis of ATP Performs Work ΔG for glutamic acid conversion to glutamine (+3.4 kcal/mol) (endergonic) + ΔG of the ATP hydrolysis (-7.3 kcal/mol) (exergonic) = Net ΔG of -3.9 kcal/mol Is the overall reaction endergonic or exergonic? Does it occur spontaneously or non-spontaneously? How the Hydrolysis of ATP Performs Work ATP drives endergonic reactions by phosphorylation (transferring a phosphate group to some other molecule) such as a reactant The recipient molecule is now called a phosphorylated intermediate Phosphorylated intermediates are key to coupling exergonic and endergonic reactions Because Pis are more reactive (less stable) The Regeneration of ATP—The ATP Cycle The ATP Cycle: ATP is a renewable resource that is regenerated by addition of a phosphate group to ADP The ATP cycle couples a cell’s energy releasing (exergonic) processes to its energy consuming (endergonic) processes The Regeneration of ATP—The ATP Cycle The regeneration of ATP from ADP and Pi is endergonic (ΔG = +7.3 kcal/mol) Non-spontaneous or spontaneous? The energy to phosphorylate ADP comes from catabolic (exergonic) reactions in the cell Cellular respiration—provides energy needed to recycle ATP ATP transfers energy from exergonic to endergonic reactions by phosphorylating (adding phosphate groups to) molecules needed in the endergonic reaction ATP is then regenerated from ADP by energy released from exergonic reactions Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers 1st and 2nd Laws of TD—what reactions will happen under given conditions Rate is not based on TD Can be so slow as to be imperceptible A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction An enzyme is a catalytic protein Figure 8.UN02 The Activation Energy Barrier Every chemical reaction between molecules involves bond breaking and bond forming The initial energy needed to start a chemical reaction is called the activation energy (EA ) Activation energy is often supplied in the form of thermal energy that the reactant molecules absorb from their surroundings Figure 8.12 How Enzymes Lower the EA Barrier Enzymes catalyze reactions by lowering the EA barrier Most exergonic reactions are spontaneous, but some require a small amount of energy to overcome EA  Endergonic reactions also need enzymes How Enzymes Lower the EA Barrier Enzymes do not affect the change in free energy (∆G); instead, they hasten reactions that would occur eventually Animation: How Enzymes Work Substrate Specificity of Enzymes The reactant that an enzyme acts on is called the enzyme’s substrate The enzyme binds to its substrate, forming an enzyme-substrate complex Enzymes and substrates are specific Held by weak bonds (ionic/hydrogen) The active site is the region on the enzyme where the substrate binds Induced Fit Catalysis in the Enzyme’s Active Site In an enzymatic reaction, the substrate binds to the active site of the enzyme The active site can lower an EA barrier by Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Direct participation in the catalytic reaction Effects of Local Conditions on Enzyme Activity An enzyme’s activity can be affected by general environmental factors, such as temperature and pH Each enzyme has an optimal temperature and an optimal pH in which it can function Optimal temp for most human enzymes = 35-40°C Optimal pH for most human enzymes = 6-8 Optimal conditions favor the most active shape for the enzyme molecule Cofactors (Enzyme Helpers) Cofactors are non-protein enzyme helpers Permanently attached or reversibly attached like substrate Inorganic cofactors = metal in ionic form Zinc, Iron, copper, magnesium Organic cofactors are called coenzymes Vitamins Vitamin B complex assists in metabolizing macromolecules Folic acid assist in synthesizing AAs important for making DNA Enzyme Inhibitors Competitive inhibitors bind to the active site of an enzyme, competing with the substrate Overcome by increasing substrate conc. Enzyme Inhibitors Noncompetitive inhibitors bind to an allosteric site (site other than the active site), causing the enzyme to change shape and making the active site less effective Enzyme Inhibitors Inhibitors include toxins, poisons, pesticides, and antibiotics The Evolution of Enzymes Enzymes are proteins encoded by genes Changes (mutations) in genes lead to changes in amino acid composition of an enzyme Altered amino acids in enzymes may alter their substrate specificity Under new environmental conditions a novel form of an enzyme might be favored Concept 8.5: Regulation of enzyme activity helps control metabolism Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes Allosteric Activation and Inhibition Many natural enzyme regulators behave like reversible noncompetitive inhibitors Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site Result may either be inhibition or stimulation of active site Allosteric Activation and Inhibition Most allosterically regulated enzymes are made from polypeptide subunits, each with an active site Each enzyme has active and inactive forms The binding of an activator stabilizes the active form of the enzyme for all subunits The binding of an inhibitor stabilizes the inactive form of the enzyme for all subunits Figure 8.19a Figure 8.19b Cooperativity is allosteric because binding by a substrate to one active site affects catalysis in a different active site Feedback Inhibition In feedback inhibition, the end product of a metabolic pathway shuts down the pathway Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed Figure 8.21 Specific Localization of Enzymes Within the Cell Cells are compartmentalized Structures within the cell help bring order to metabolic pathways Eukaryotic cells can have enzymes in specific organelles E.g., enzymes for cellular respiration are located in mitochondria Figure 8.22 Review: Metabolism transforms matter and energy Metabolism—the collection of chemical reactions that occur in an organism Enzymes—catalyze rxns by pushing metabolic pathways forward Catabolic = breaking down molecules, releasing energy Anabolic = building molecules, consuming energy Energy—capacity to cause change What types of energy are there? 1st Law TD = Conservation of Energy, Energy can’t be ___________ or _____________ 2nd Law TD = Every energy transfer or transformation increases the entropy of the universe Trend toward randomization Review: Free energy change determines reaction type Free energy is energy that can do work in the cell An exergonic reaction proceeds with a net release of free energy and is spontaneous (energetically favorable) ΔG negative Products have less free energy than reactants An endergonic reaction absorbs free energy and is not spontaneous ΔG positive Products have more free energy than reactants Review: ATP powers cellular work ATP powers cellular work by coupling what two types of reactions? Hydrolysis of ATP yields: ADP + inorganic phosphate ( ) + release free energy The exergonic rxn of ATP hydrolysis drives endergonic rxns by transferring a phosphate to a specific reactant—forming a phosphorylated intermediate ΔG positive Review: Enzymes speed up reactions by lower EA In a chemical reaction, the energy needed to break bonds or reactants is the activation energy or EA Enzymes lower the EA Where do substrates bind to enzymes? When the enzyme changes shape after this it’s called an induced fit Enzyme function can vary to due to pH, temperature, chemicals Inhibitors that bind to the active site are called competitive inhibitors Inhibitors that bind to an allosteric site are called noncompetitive inhibitors Figure 8.UN03 Regulation of enzyme activity helps control metabolism Chapter 9 Cellular Respiration and Fermentation Chapter Overview Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels Several processes are central to cellular respiration and related pathways Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without O2 Aerobic respiration consumes organic molecules and O2 and yields ATP Most common and efficient source of ATP Most eukaryotes and many prokaryotes Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2 Some prokaryotes Catabolic Pathways and Production of ATP Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration General process for obtaining energy from fuel: Organic compounds + Oxygen  CO2 + H2O + Energy Fuel can be carbohydrates, fats, or proteins It is helpful to trace cellular respiration with the degradation of glucose C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat) ΔG = -686 kcal/mol Redox Reactions: Oxidation and Reduction Relocation of electrons (e-s) during chemical reactions releases energy stored in organic molecules This energy is ultimately used to synthesize ATP Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions Fermentation and Cellular Respiration involve Redox Reactions Loss or gain of electrons (e-) Oxidation = loss of e-, increase its + charge Oxidizing agent = e- acceptor Reduction = gain of e- , reduce its + charge Reducing agent = e- donor Figure 9.UN02 Figure 9.UN01 Not all redox reactions have complete transfer of e- Some redox reactions only change the electron sharing in covalent bonds An example is the reaction between methane and O2 Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced Oxidation of Organic Fuel Molecules During Cellular Respiration As e-s transferred from glucose to oxygen, the energy state of the e-s changes Stepwise Energy Harvest via NAD+ and ETC In cellular respiration, glucose and other organic molecules are broken down in a series of steps Slow burn vs. massive explosion Energy not harvested efficiently Each step catalyzed by an enzyme At key steps e-s stripped from glucose e-s travels with a proton  a H atom Electrons from organic compounds are usually first transferred to NAD+ (nicotinamide adenine dinucleotide), a coenzyme Stepwise Energy Harvest via NAD+ and the ETC As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration NAD+ acts as an electron carrier Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP Figure 9.UN04 NADH passes the electrons to the electron transport chain (ETC) Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction O2 pulls electrons down the chain in an energy-yielding tumble The energy yielded is used to regenerate ATP Figure 9.5 An ETC is a series of proteins (and other molecules) built into the inner membrane of the mitochondria  E-s removed from glucose shuttled by NADH to “top” of ETC (higher energy end) E-s cascade down to lower energy end where O2 (terminal e-s acceptor) captures them to form H2O Small amount of energy is released at each step Oxygen essentially pulls e-s down the chain E- route: Glucose  NADH ETC Oxygen The Stages of Cellular Respiration: A Preview Harvesting of energy from glucose has three stages Glycolysis (breaks down glucose into two molecules of pyruvate) In cytosol, some ATP produced The citric acid/Krebs cycle Pyruvate oxidized to acetyl CoA Breakdown of glucose (as acetyl CoA) completed; some ATP produced In cytosol in prokaryotes, in mitochondria in eukaryotes Oxidative phosphorylation (accounts for most of the ATP synthesis) Electron transport chain and chemiosmosis ETC accepts e-s from breakdown products of previous stages At end of chain e-s combine with oxygen and H+ to form H2O In plasma membrane in prokaryotes, in inner membrane of mitochondria in eukaryotes The Stages of Cellular Respiration: A Preview Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration ATP synthesis powered by redox rxns A smaller amount of ATP is formed in glycolysis and the Krebs cycle by substrate-level phosphorylation ATP synthesis powered by removal of a phosphate group from substrate via an enzyme 1 molecule of glucose degraded by respiration  38 ATP Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis (“splitting of sugar”) breaks down glucose (6C) into two molecules of pyruvate (2-3C) Glycolysis occurs in the cytoplasm and has two major phases Energy investment phase—cell uses ATP Energy payoff phase—cell produces ATP Total of 10 steps Glycolysis occurs whether or not O2 is present, no CO2 produced Figure 9.8 Concept 9.3: Krebs cycle completes the energy-yielding oxidation of organic molecules In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) via active transport Oxidation of glucose is completed here Before the citric acid cycle can begin, pyruvate must be oxidized to acetyl Coenzyme A (acetyl CoA) This step is carried out by a multi-enzyme complex that catalyzes three reactions 3 Steps to Acetyl CoA The Citric Acid Cycle The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO2 The cycle oxidizes organic fuel derived from pyruvate, generating: 2 CO2 1 ATP 3 NADH + 3H+, and 1 FADH2 per turn 2 turns per glucose molecule because glucose broken down into 2 pyruvate molecules Figure 9.11 The Citric Acid Cycle The citric acid cycle has eight steps, each catalyzed by a specific enzyme The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain Energy Yields from Glucose via Glycolysis and Citric Acid cycle Glycolysis 2 ATP’s & 2 NADH +2H+ Pyruvate to Acetyl CoA 2 NADH + 2H+ Citric Acid Cycle 2 ATP’s, 6 NADH + 6H+ & 2 FADH2 Total = 10 NADH + 2 FADH2 + 4 ATP’s Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food  These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation The Pathway of Electron Transport The electron transport chain is in the inner membrane (cristae) of the mitochondrion In plasma membrane of prokaryotes Most of the ETC’s components are multi-protein electron carrier complexes The carriers alternate reduced and oxidized states as they accept and donate electrons, respectively E-s drop in free energy as they go down the chain and are finally passed to O2, forming H2O Figure 9.13 Electrons are transferred from NADH or FADH2 to the electron transport chain Electrons are passed through a number of proteins to O2 The ETC does not generate ATP directly It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts Chemiosmosis: The Energy-Coupling Mechanism Chemiosmosis—an energy coupling mechanism that uses energy in a H+ gradient to drive cellular work ATP synthase—enzymatic protein responsible for the synthesis of ATP from ADP and inorganic phosphate ATP synthase uses the energy of existing H+ gradient to drive phosphorylation of ATP Chemiosmosis: The Energy-Coupling Mechanism BIG PICTURE: The energy stored in a H+ gradient across a membrane couples the redox reactions of the ETC to ATP synthesis This is an example of chemiosmosis Because it’s an example of using a H+ gradient to do cellular work An Accounting of ATP Production by Cellular Respiration During cellular respiration, most energy flows in this sequence: glucose  NADH electron transport chain  proton-motive force  ATP About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP Where does the rest of the energy go? The exact number of ATP produced via cellular respiration is unknown What part of this structure will be cleaved to release energy? The oxygen atom All of the negatively charged phosphates The last negatively charged phosphate The sugar Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Most cellular respiration requires O2 to produce ATP O2 pulls e-s down the ETC Without electronegative O2, oxidative phosphorylation ceases In that case, glycolysis couples with fermentation (no ETC) or anaerobic respiration (w/ ETC) to produce ATP Anaerobic respiration vs. Fermentation Anaerobic respiration—uses ETC but O2 isn’t the terminal e- acceptor Sulfate (SO42-), nitrate (NO3-), sulfur (S) When oxygen is not present and neither is an ETC, fermentation used to metabolize glucose Uses substrate-level phosphorylation instead of ETC to generate ATP Two Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis (occurs in the cytosol) Alcohol fermentation Pyruvate reduced to ethanol and CO2 NADH electron acceptor = acetaldehyde Alcohol fermentation is the primary mode of metabolism in many microorganisms Products: wine, beer, bread 2. Lactic acid fermentation Pyruvate reduced to lactate NADH electron acceptor = pyruvate Microorganisms produce yogurt, sour cream, and cheese Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce Comparing Fermentation with Anaerobic and Aerobic Respiration Similarities All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis Differences Oxidizing NADH back to NAD+; final e- acceptors: Fermentation = an organic molecule (e.g., pyruvate or acetaldehyde) Anaerobic CR = SO42-, NO3-, S Aerobic CR = O2 Aerobic Cellular respiration = ~38 ATP per glucose molecule Anaerobic Cellular respiration = 2 ATP per glucose molecule Fermentation = 2 ATP per glucose molecule Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration Figure 9.18 Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways Glycolysis and the Krebs cycle are major intersections to various catabolic and anabolic pathways Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Glycolysis accepts a wide range of carbohydrates Proteins must be digested to AAs Can feed glycolysis or the citric acid cycle Metabolism of Other Nutrients Proteins and fats can also provide energy when carbohydrates are unavailable They are broken down and their subunits feed into aerobic cellular respiration Proteins only used if fats and carb are unavailable Fatty acids are broken down by beta oxidation and yield acetyl CoA Biosynthesis (Anabolic Pathways) Organic molecules used for food, but also used to build other substances These small molecules may come directly from food, from glycolysis, or from the citric acid cycle To build carbon skeletons of cellular molecules Amino acids from proteins incorporated into cellular proteins If we consume more than we need, we store excess fuel as fat, even if a diet is fat-free Regulation of Cellular Respiration via Feedback Mechanisms Feedback inhibition is the most common mechanism for control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of catabolism achieved by regulating the activity of enzymes at strategic points in the catabolic pathway Figure 9.UN06 Figure 9.UN06 Energy Yields from Glucose via Glycolysis and Citric Acid cycle Glycolysis 2 ATP’s & 2 NADH + 2H+ Pyruvate to Acetyl CoA 2 NADH + 2H+ Citric Acid Cycle 2 ATP’s, 6 NADH + 6H+ & 2 FADH2 Total = 10 NADH + 2 FADH2 + 4 ATP’s Chapter 10 Photosynthesis Overview: The Process That Feeds the Biosphere Photosynthesis Converts solar energy into chemical energy Directly or indirectly, photosynthesis nourishes almost the entire living world Autotrophs—“producers” Produce organic molecules from CO2 and other inorganic molecules Almost all plants are photoautotrophs Plants, algae, some protists, and some prokaryotes Overview: The Process That Feeds the Biosphere Heterotrophs—“consumers” Obtain their organic material from other organisms Almost all heterotrophs depend on photoautotrophs for food and O2 Primary consumer vs secondary? Concept 10.1: Photosynthesis Structural organization of chloroplasts allows for the chemical reactions of photosynthesis Photosynthesis & Cellular Respiration Cellular respiration C6H12O6 + 6O2  6H2O + 6CO2 + Energy (ATP + heat) Photosynthesis as a Redox Process Photosynthesis reverses the direction of electron flow compared to respiration H2O split—e-s transferred with H+s from H2O to CO2, reducing it to sugar Photosynthesis is an endergonic process The energy boost is provided by light The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids) Split H2O Release O2 Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation Uses light to power regeneration of ADP Light energy  chemical energy (ATP and NADPH) No sugar produced at this stage The Two Stages of Photosynthesis: A Preview The Calvin cycle (in the stroma) Forms sugar from CO2, using ATP and NADPH The Calvin cycle begins with carbon fixation, incorporating carbon from CO2 into organic molecules Indirect requirement of light Concept 10.2: The light reaction  The visible light spectrum consists of λs of light we can see Light also behaves as though it consists of discrete particles, called photons Photons have a fixed amount of energy Inverse relationship between λ and energy The Visible Light Spectrum Colors reflect and transmit specific wavelengths (ROY G. BIV) Red = 650-750 nm Orange = 600-650 nm Yellow = 550-600 nm Green = 500-550 nm Blue/indigo = 450-500 nm Violet = 380-450 nm Why are plants green? Pigments are substances that absorb visible light Different pigments absorb different wavelengths λs that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light Animation: Light and Pigments Measuring wavelengths of light Spectrophotometer—measures a pigment’s ability to absorb various wavelengths Sends light through pigments and measures the fraction of light transmitted at each wavelength Absorption spectrum—a graph plotting a pigment’s light absorption versus wavelength Visual of how well a pigment absorbs different λs of visible light The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis Chlorophyll a is the main photosynthetic pigment Appears blue green Chlorophyll b and carotenoids are accessory pigments They increase spectrum of light available for plant use Chlorophyll b—appears olive green Carotenoids—appear yellow & orange Carotenoids also provide photoprotection Absorb extra light energy that would damage chlorophyll Act as antioxidants Found in fruits and vegetables In autumn, chlorophyll is degraded in deciduous trees’ leaves Why do leaves change color to shades of yellow, orange, or red? Excitation of Chlorophyll by Light When a molecule absorbs a photon of light: electrons go from ground state  higher e- orbital = excited state (unstable) Compounds only absorb light corresponding to specific λs Why pigments have specific absorption spectrum Excitation of Chlorophyll by Light When excited electrons fall back to the ground state, photons are given off Fluorescence—release of energy in the form of light If illuminated, an isolated solution of chlorophyll will fluoresce, giving off red-orange light and heat Photosystems A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center Chlorophyll a, b, & carotenoids Allows plant to absorb more λs Photosystems Primary electron acceptor (PEA) in the RCC Accepts excited electrons from the pair of chlorophyll a molecules inside the RCC Primary e- acceptor is reduced as a result Transfer of e- from chlorophyll a molecule to PEA = 1st step of the light reactions Two types of photosystems in thylakoid membrane Each has unique Reaction center complex w/ unique Primary electron acceptor Photosystem II (PS II) Functions first Best at absorbing λ = 680 nm The reaction-center of PS II is called P680 chlorophyll a Photosystem I (PS I) Functions second Best at absorbing λ = 700 nm The reaction-center of PS I is called P700 chlorophyll a Linear Electron Flow Sunlight drives ATP & NADPH synthesis by energizing PS I & II embedded in thylakoid membranes of chloroplasts During the light reactions, there are two possible routes for electron flow: cyclic and linear Linear electron flow The primary pathway Involves both photosystems and produces ATP and NADPH using light energy Linear Electron Flow  1) A photon hits a pigment and its energy is passed among pigment molecules until it excites P680 Linear Electron Flow P680+ is a very strong oxidizing agent (e- donor or acceptor?) Linear Electron Flow 4) Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I E- carriers: plastoquinone (PQ), cytochrome complex, and plastocyanin (PC) 5) Energy released by the fall provides energy for ATP synthesis by creating a proton gradient 6) Light energy transferred (via light harvesting complexes) to PS I Excites P700, which loses an e- to an e- acceptor P700+ (P700 that is missing an electron) accepts an e- passed down from PS II via the ETC Cyclic Electron Flow Uses only photosystem I Produces ATP, but not NADPH No oxygen is released Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle Cyclic Electron Flow Some organisms such as purple sulfur bacteria have PS I but not PS II Cyclic electron flow is thought to have evolved before linear electron flow Cyclic electron flow may be photoprotective Protects cells from light-induced damage Chemiosmosis in Chloroplasts vs. Mitochondria Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy Process that uses membranes to couple redox rxns (in an ETC) to ATP production Mitochondria transfer chemical energy from food to ATP Chloroplasts transform light energy into the chemical energy of ATP Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities Chemiosmosis in Chloroplasts vs. Mitochondria In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH Bioflix: Photosynthesis Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle Cellular respiration—oxidizing glucose to synthesize ATP = anabolic or catabolic? Photosynthesis– builds sugar from smaller molecules, using ATP & the reducing power of electrons carried by NADPH = anabolic or catabolic?  The Calvin Cycle Carbon enters the cycle as CO2 and leaves as a sugar: glyceraldehyde 3-phospate (G3P) For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2 The Calvin cycle has three phases Carbon fixation (catalyzed by rubisco) Reduction Regeneration of the CO2 acceptor (RuBP) Figure 10.19-1 Alternative mechanisms of carbon fixation: Photorespiration Dehydration in plants sometimes requiring trade-offs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis Photorespiration: An Evolutionary Relic?* Most plants are C3 plants Initial fixation of CO2, via rubisco, forms a 3-C compound (3-phosphoglycerate) In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a 2-C compound Photorespiration: Consumes O2 and organic fuel Releases CO2 without producing ATP or sugar *An organism that was characteristic of the flora or fauna of one age and that persisted into a later age, with the later age being characterized by newly evolved flora or fauna significantly different from those that came before Alternative mechanisms of C fixation: C4 Plants C4 plants minimize the cost of photorespiration CO2 produce 4-C compound as 1st product C4 plants have 2 types of photosynthetic cells Mesophyll cells & bundle sheath cells Bundle sheaths tightly packed around vein Alternative mechanisms of C fixation: C4 Plants C4 plants use the enzyme PEP carboxylase (PEP = phosphoenolpyruvate) PEP carboxylase has a higher affinity for CO2 than rubisco does It can fix CO2 even when CO2 concentrations are low 4-C compounds produced by C4 plants in mesophyll cells Exported to bundle-sheath cells, where they release CO2 for use in the Calvin cycle Alternative mechanisms of C fixation: C4 Plants In mesophyll cell Pyruvate (3C) 3C phosphoenolpyruvate (PEP) CO2 incorporated into PEP with PEP carboxylase 4C oxaloacetate created Shuttled to Bundle-sheath cell through plasmodesmata In Bundle-sheath cell CO2 released & it enters Calvin cycle Alternative mechanisms of C fixation: C4 Plants ATP required for C4 photosynthesis Adaptation that maintains CO2 concentration in bundle sheath C4 photosynthesis minimizes photorespiration Advantageous in hot regions with intense sunlight C4 plants evolved to thrive in these regions Examples: sugarcane, corn, crabgrass, sorghum Increasing levels of CO2 & temperatures may affect C3 and C4 plants differently, perhaps changing the relative abundance of these species The effects of such changes are unpredictable and a cause for concern CAM Plants Crassulacean acid metabolism (CAM) Used by some plants, including succulents, use to fix C Cacti, pineapples Adaption to arid climates CAM Plants CAM plants open their stomata at night, incorporating CO2 into organic acids Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle Reverse behavior compared to most plants Figure 10.21 The Importance of Photosynthesis: A Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the O2 in our atmosphere 3 Kinds of Carbon Fixation Calvin Cycle (C3 photosynthesis) Both steps in mesophyll cells Light reaction in thylakoids Calvin cycle in stroma C4 photosynthesis—spatial separation of carbon fixation and Calvin cycle Mesophyll cells and bundle-sheath cells The C4 is oxaloacetate CAM photosynthesis—temporal separation of steps CO2 incorporated into 4C organic acids at night CO2 released by organic acids into Calvin cycle during the day Both steps in mesophyll cells #1) Organelle where photosynthesis occurs #2) Pore opening on leaf surface #3) Where are chloroplasts found (specifically)? Type of plant tissue made up of cells by the same name #4) Dense fluid inside inner membrane of chloroplast #5) Structure inside chloroplast consisting stacks of grana Review Photosynthesis equation: Solar Energy + 6H2O +6CO2 C6H12O6 + 6O2 PS is a Redox reaction Chloroplast split H2O, e-s of H incorporated into CO2 to create sugar Light reaction occurs in the thylakoids Splits H2O and releases O2, Produces ATP, forms NADPH Calvin Cycle is in the stroma Forms sugar from CO2 using ATP for energy and NADPH for reducing power Light is a form of electromagnetic energy Colors we see—visible light; Color based on wavelength of light Pigments absorb specific wavelengths of light Chlorophyll a = main photosynthetic pigment in plants Other accessory pigments absorb different λs and pass energy to chlorophyll a Pigments get excited when photons of light reach them Pigment’s electrons raised to a higher, unstable state When they fall e-s give off heat and/or light Photosystem = reaction-center complex surrounded by light-harvesting complexes Special set of chlorophyll molecules in RCC, absorb energy from e-s and pass it to primary electron acceptor Photosystem II Contains P680 chlorophyll a molecules in the RCC Photosystem I Contains P700 chlorophyll a molecules in the RCC Linear e- flow during light reactions uses which Photosystem(s)? If it uses more than one, is there an order? Cyclic electron flow  Uses which photosystem(s)? Produces ATP but no NADPH or O2 Chemiosmosis—Process that uses membranes to couple redox rxns (in an ETC) to ATP production Mitochondria transfer chemical energy from food to ATP Chloroplasts transform light energy into the chemical energy of ATP Calvin cycle occurs in the stroma Uses electrons from NADPH and energy from ATP 1 molecule of G3P exits the cycle per 3 CO2 molecules G3P converted to glucose and other organic molecules On hot dry days, C3 plants close their stomata to prevent dehydration Leads to O2 build up which then leads to photorespiration Photorespiration—sub O2 for CO2 Consumes O2 and organic fuel Releases CO2 and doesn’t produce ATP or sugar C4 plants—incorporate CO2 into 4C molecules (oxaloacetate) Photosynthetic cells = Mesophyll and bundle sheath cells CO2 released in bundle sheath cells from organic acid—used in Calvin cycle CAM plants: Open stomata at night Incorporate CO2 into organic acids at that time Close stomata during the day CO2 released by organic acids for use in the Calvin Cycle