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How Cells Obtain Energy Cell Respiration & Photosynthesis How Cells Obtain Energy Overview Energy Metabolism Aerobic Cell Respiration Anaerobic Cell Respiration Photosynthesis How Cells Obtain Energy Energy ● Bioenergetics – – – Flow of energy through living systems Sun if our “local” source of energy Living organisms are “open” systems; energy flows in and then out of living things 12 10 8 Column 1 Column 2 Column 3 6 4 2 0 Row 1 Row 2 Row 3 Row 4 How Cells Obtain Energy Energy ● Thermodynamics – The study of energy transfer involving physical matter – First & Second Laws of Thermodynamics must be followed in all living systems How Cells Obtain Energy Energy ● Thermodynamics – First Law of Thermodynamics ● ● ● ● The amount of energy in the universe is constant Energy is neither created nor destroyed Transferred from place to place Energy may exist in different forms – Heat, light, chemical, etc How Cells Obtain Energy Energy ● Thermodynamics – Second Law of Thermodynamics ● Energy will always be lost as heat in energy transfers How Cells Obtain Energy Energy ● ● Thermodynamics – Potential vs Kinetic Energy ● ● Potential-stored energy Kinetic-energy to do work Potential Energy – Often stored as chemical bonds in organic molecules in the cell How Cells Obtain Energy Metabolism ● The sum total of all chemical reactions in a cell – Metabolic pathways ● Series of chemical changes to molecules ● Catabolic: energy releasing reactions (exergonic & spontaneous) ● Anabolic: energy requiring reactions (endogonic & nonspontaneous) How Cells Obtain Energy Metabolism ● Free Energy & Activation Energy – Free Energy: Net energy released in catabolic reactions – Activation Energy: Small amount of energy needed to allow ALL chemical reactions to occur How Cells Obtain Energy Metabolism ● Role of Enzymes – Proteins (tertiary) – Control the rate of chemical reactions by reducing the activation energy needed for a chemical reaction to occur – Active site ● Enzyme-substrate complex – Allosteric site – Control of enzymes How Cells Obtain Energy Metabolism ● Role of Enzymes – Proteins (tertiary) – Control the rate of chemical reactions by reducing the activation energy needed for a chemical reaction to occur – Active site – Allosteric site ● ● – Activation Inhibition Control of Enzymes How Cells Obtain Energy Metabolism ● Role of Enzymes – Proteins (tertiary) – Control the rate of chemical reactions by reducing the activation energy needed for a chemical reaction to occur – Active site – Allosteric site – Control of Enzymes ● Feedback Inhibition How Cells Obtain Energy Aerobic Cell Respiration ● ● Sum total of all the catabolic (exergonic) reactions that occur in a cell Free energy released is stored as chemical energy – ● ATP (adenosine triphosphate) 3 Steps – Glycolysis – Citric Acid Cycle (Kreb’s Cycle) – Oxidative Phosphorylation (Electron Transport Chain) ATP How Cells Obtain Energy Aerobic Cell Respiration ● Glycolysis – Glucose broken down to pyruvic acid – ATP formed – NADH (an electron carrier) formed How Cells Obtain Energy Aerobic Cell Respiration ● Glycolysis – Pyruvic acid is next converted to acetyl CoA – Necessary in order for pyruvic acid to enter the Citric Acid Cycle How Cells Obtain Energy Aerobic Cell Respiration ● Citric Acid Cycle – Acetyl CoA broken down to carbon dioxide (CO 2) – ATP formed – NADH formed – FADH2 (another electron carrier) formed How Cells Obtain Energy Aerobic Cell Respiration ● Oxidative Phosphorylation – Occurs in the electron transport chain of molecules in the mitochondria of eukaryotic cells – Occurs in the cell membrane of prokaryotic cells like bacteria – NADH & FADH2 transfer electrons via hydrogen atoms – Electron chain molecules keep electrons and discard protons (H+) of the hydrogen atom How Cells Obtain Energy Aerobic Cell Respiration ● Oxidative Phosphorylation – As electrons are transferred through the chain oxygen (O2) eventually received the electrons ● O2 is the final electron acceptor – ATP Synthase (enzyme) pumps H+ back across the membrane (chemiosmosis) – 3 ATP are formed by ATP Synthase for every H+ pumped back in – Oxygen combines with H+ to form water (H20) How Cells Obtain Energy Aerobic Cell Respiration Glucose + O2 + ATP + NAD+ + ADP + P Carbon dioxide (CO2) + Water (H2O) + 38 ATP How Cells Obtain Energy Aerobic Cell Respiration Can other organic molecules yield energy for the cell? Yes, and the same metabolic pathway is used How Cells Obtain Energy Anaerobic Cell Respiration ● ● ● Occurs when cells lack O2 or can’t utilize O2 O2 was the final electron acceptor in aerobic cell respiration Lack of O2 or the inability to use O 2 forces some cells to either create their own final electron acceptor (endogenous electron acceptor) or use a compound acquired from outside the cell (exogenous electron acceptor) How Cells Obtain Energy Anaerobic Cell Respiration ● Fermentation – Endogenous electron acceptors are formed ● ● Lactic acid Ethyl alcohol (ethanol) How Cells Obtain Energy Anaerobic Cell Respiration ● Lactic Acid Fermentation – Common in animals and some bacteria – Glycolysis with one additional chemical step ● – Insures that NAD+ is regenerated Yield only 2 ATP/glucose molecule metabolized How Cells Obtain Energy Anaerobic Cell Respiration ● Ethanol Fermentation – Common in animals and some bacteria – Glycolysis with two additional chemical steps ● – Insures that NAD+ is regenerated Yield only 2 ATP/glucose molecule metabolized Glycolysis How Cells Obtain Energy Anaerobic Cell Respiration ● ● ● Exogenous electron acceptors are formed Common in some species of bacteria – sulfates (SO4-)form hydrogen sulfide (H2S, “rotten egg gas”) – nitrates (NO3-) form nitrites (NO2-) Very important to life on Earth! How Cells Obtain Energy Photosynthesis ● Process of transferring light energy into stored chemical energy by converting carbon dioxide and water into complex carbohydrates like glucose ● Oxygen is a by-product ● Photoautotrophs ● – Plants – Algae – Some bacteria Essential for life on earth as the energy in carbohydrates of photoautotrophs is passed on to all organisms in the food chain How Cells Obtain Energy Photosynthesis in Plants Leaf Structure ● Upper Epidermis ● Mesophyll ● – Cells containing chloroplasts Chloroplasts-pigmented organelles ● ● Inner & Outer Membrane Thylakoids (middle layer of folded membranes) – – ● ● Contain chlorophyll pigments Granum-stack of thalakoids Stroma-space between grana Lower Epidermis – Stoma-openings for O2 & CO2 How Cells Obtain Energy Photosynthesis in Plants Light & Chlorophyll ● Light – ● Form of electromagnetic radiation (energy) Chlorophylls (pigment molecules) – Absorb light energy and emit electrons as a result – Absorption spectum – – Chlorophyll a ● ● – Present in all plants Absorbs blue and red light, not green Chlorophyll b ● – Frequency of visible light capable of being absorbed by a chlorophyll molecule Absorbs blue and red-orange light Carotenoids How Cells Obtain Energy Photosynthesis in Plants 2 Steps ● ● Light-dependent reactions Calvin Cycle How Cells Obtain Energy Photosynthesis in Plants Light-dependent reactions ● ● ● ● Begin in a group of chlorophylls and proteins called a photosystem II found in the thylakoids Light energy is transferred to electrons in chlorophyll Electrons are lost from chlorophyll and are passed along a series of cytochrome molecules called an electron transport chain H2O breaks down to donate electrons lost from chlorophyll so chlorophyll can respond to more light energy How Cells Obtain Energy Photosynthesis in Plants Light-dependent reactions ● ● Energy from the moving electrons allows membrane pumps to move H+ from the stroma to the thylakoid space The build up of H+ in the thylakoid space prompts ATP synthase to pump the H+ back into the stroma and at the same time produce ATP How Cells Obtain Energy Photosynthesis in Plants Light-dependent reactions ● ● As the electrons passes through the electron transport chain it eventually is donated to a chlorophyll molecule in another photosystem, photosystem I More light energy excites these electrons in the pigments of photosystem I causing them to move through another set of molecules and eventually be donated to NADP+ (an electron carrier) along with a H+ to form NADPH The end result: light energy is transferred and stored as chemical energy in the ATP and NADPH molecules How Cells Obtain Energy Photosynthesis in Plants Calvin Cycle Reactions ● ● Occur in the stroma of the chloroplasts CO2 which enters from the stoma of the leaf is synthesized into sugars using the energy from ATP & NADPH generated in the light-dependent reactions How Cells Obtain Energy Photosynthesis in Plants Calvin Cycle Reactions ● Stage 1- Carbon Fixation – Inorganic CO2 molecules are bonded to ribulose biphosphate (RuBP) to form a 6- carbon organic molecule which breaks down into 2 3-carbon organic compounds How Cells Obtain Energy Photosynthesis in Plants Calvin Cycle Reactions ● Stage 2- Reduction – The 2 3-carbon organic molecules are reduced (electrons added to them) using ATP & NADPH – The resulting ADP & NADP+ return to the lightdependent reaction to be reused How Cells Obtain Energy Photosynthesis in Plants Calvin Cycle Reactions ● Stage 3- Regeneration – The 2 3-carbon organic molecules are reduced using ATP & NADPH (electrons added to them) to G3P – The resulting ADP & NADP+ return to the light-dependent reaction to be reused – After 3 turns of the Calvin Cycle some of the G3P bonds together to form glucose and the remaining G3P molecules are used to regenerate RuBP for succeeding turns of the Calvin Cycle The end result of photosynthesis is that light energy is converted to stored energy (potential energy) in the form of glucose How Cells Obtain Energy Photosynthesis in Bacteria ● ● A bacterial cell has infolded regions of the plasma membrane that function like thylakoids Although these are not contained in an organelle, such as a chloroplast, all of the necessary components are present to carry out photosynthesis