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Chapter 6 Metabolism: Energy and Enzymes. Cells and the Flow of Energy. Energy is the ability to do work. Living things need to acquire energy; this is a characteristic of life. Cells use acquired energy to: Maintain their organization
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Cells and the Flow of Energy • Energy is the ability to do work. • Living things need to acquire energy; this is a characteristic of life. • Cells use acquired energy to: • Maintain their organization • Carry out reactions that allow cells to develop, grow, and reproduce
Cells and Entropy • The term entropy is used to indicate the relative state of disorganization. • Cells need a constant supply of energy to maintain their internal organization. • Complex molecules like glucose tend to break apart into their building blocks, in this case carbon dioxide and water. • This is because glucose is more organized, and thus less stable, than its breakdown products. • The result is a loss of potential energy and an increase in entropy.
Metabolic Reactions and Energy Transformations • Metabolism is the sum of all the chemical reactions that occur in a cell. • Reactants are substances that participate in a reaction; products are substances that form as a result of a reaction. • A reaction will occur spontaneously if it increases entropy. • Biologists use the term “free energy” instead of entropy for cells.
ATP: Energy for Cells • ATP (adenosine triphosphate) is the energy currency of cells. • ATP is constantly regenerated from ADP (adenosine diphosphate) after energy is expended by the cell. • Use of ATP by the cell has advantages: • 1) It can be used in many types of reactions. • 2) When ATP → ADP + P, energy released is sufficient for cellular needs and little energy is wasted.
Photosynthesis • The overall reaction for photosynthesis can be written: • 6CO2 + 6H2O + energy → C6H12O6 + 6O2 • During photosynthesis, hydrogen atoms are transferred from water to carbon dioxide, and glucose is formed. • Water has been oxidized; carbon dioxide has been reduced. • Energy to form glucose comes from the sun.
Cellular Respiration • The overall equation for cellular respiration is opposite that of photosynthesis: • C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy • In this reaction, glucose is oxidized and oxygen is reduced to become water. • The complete oxidation of a mol of glucose releases 686 kcal of energy that is used to synthesize ATP.
Organelles and the Flow of Energy • During photosynthesis, chloroplasts capture solar energy and use it to convert water and carbon dioxide into carbohydrates that provide food for other living things. • Cellular respiration, the breakdown of glucose into carbon dioxide and water, occurs in mitochondria. • It is the cycling of molecules between chloroplasts and mitochondria that allows a flow of energy from the sun through all living things.
Cellular Respiration takes place in 4 phases Lets follow ONE molecule of glucose through its complete metabolism
Cellular Respiration takes place in 4 phases 1. Glycolysis is the breakdown of glucose into pyruvate, 2 ATP molecules are made Step 1: 2
Cellular Respiration takes place in 4 phases 2. In the transition reaction, pyruvate is broken down into acetyl CoA, no ATP is made Step 2: 2
Cellular Respiration takes place in 4 phases 3. The Citric Acid Cycle also called the “Krebs” cycle, and breaksdown Acetyl-CoA into CO2….2 ATP are made Step 3: 2 2 MITOCHONDRIA
Cellular Respiration takes place in 4 phases 4. The Electron Transport System uses the ELECTRONS removed from glucose molecules to provide engergy to make TONS of ATP…..about 32-34 ATPs!!! Step 4: 2 2 32 MITOCHONDRIA
Cellular Respiration takes place in 4 phases Therefore, ONE molecule of glucose generates 2+2+32/34 ATP molecules…….a total of 36-38! Step 2: Step 3: Step 4: Step 1: 2 2 32
Where is all this occuring?!?! Cell Outside of the Mitochondria!
ATP is the currency to run machinery within the cell….. But NADH and FADH2 run the electron transport system *which then makes the ATP!*
NAD+ and FAD • Each step of cellular respiration requires a separate enzyme. • Some enzymes use the oxidation-reduction coenzyme NAD+ (nicotinamide adenine dinucleotide). • FAD (flavin adenine dinucleotide) is sometimes used instead of NAD+.
The function of NADH and FADH2 is to carry and then donate electrons to the electron transport system…..
The function of NADH and FADH2 is to carry and then donate electrons to the electron transport system….. *remember electron transport is occuring across the mitochondria’s cristae!*
When oxygen is available, pyruvate enters the mitochondria, where it undergoes further breakdown, through the citric acid and electron transport cycles. If oxygen is not available, fermentation occurs and pyruvate undergoes reduction. Fermentation is an anaeorbic process and does not require oxygen. (“an-aeorobic" means, without oxygen) In humans, pyruvate is reduced to lactic acid during fermentation. What happens when breakdown of glucose is incomplete? FERMENTATION!!!!
In humans….. In bacteria or yeast….. The Fermentation Process…… remember, it’s ANAEOROBIC this situation only occurs when oxygen levels are LOW!
In humans….. In bacteria or yeast….. Notice, that in low oxygen you only make 2 ATP, compared to 36! this is why, when you have an oxygen debt, you get a lactic acid buildup in your muscles! AND, you pant to try to Bring more oxygen into your body to complete cellular respiration
Efficiency of Fermentation • Two ATP produced during fermentation are equivalent to 14.6 kcal; complete oxidation of glucose to CO2 and H2O represents a yield of 686 kcal per molecule of glucose. • Thus, fermentation is only 2.1% efficient compared to cellular respiration. • (14.6/686) x 100 = 2.1%
Advantages and Disadvantages of Fermentation • Fermentation can provide a rapid burst of ATP in muscle cells, even when oxygen is in limited supply. • Lactate, however, is toxic to cells. • Initially, blood carries away lactate as it forms; eventually lactate builds up, lowering cell pH, and causing muscles to fatigue. • Oxygen debt occurs, and the liver must reconvert lactate to pyruvate.
But we don’t just eat carbohydrates…..so what happens with proteins and lipids? ? ? ? ? ? ? ? ? ? ? ?
Catabolismcatabolic reactions, break it down • Molecules aside from glucose can enter the catabolic reactions of cellular respiration. • When a fat is used for energy, it breaks down into glycerol and three fatty acids; glycerol is converted to PGAL, and the fatty acids are converted to acetyl-CoA, thus both types of molecules can enter the citric acid cycle.
Fat breaks down into glycerol and three fatty acids; glycerol is converted to PGAL, and the fatty acids are converted to acetyl-CoA, and both molecules can then enter the citric acid cycle.
The carbon backbones of amino acids can also enter the reactions of cellular respiration to provide energy. The amino acid first undergoes deamination, or the removal of the amino group in the liver; the amino group becomes ammonia (NH3) and is excreted as urea. Where the carbon portion of the amino acid enters the reactions of respiration depends on its number of carbons. Catabolismcatabolic reactions, break it down
AnabolismAnabolic reactions build things up • The substrates of the pathways of cellular respiration can also be used as starting materials for synthetic reactions. • This is the cell’s metabolic pool, in which one type of molecule can be converted into another. • In this way, dietary carbohydrates can be converted to stored fat, and come substrates of the citric acid cycle can be transaminated into amino acids.
Phases of Complete Glucose Breakdown • The oxidation of glucose by removal of hydrogen atoms involves four phases: • Glycolysis – the breakdown of glucose to two molecules of pyruvate in the cytoplasm with no oxygen needed; yields 2 ATP • Transition reaction – pyruvate is oxidized to a 2-carbon acetyl group carried by CoA, and CO2 is removed; occurs twice per glucose molecule
Citric acid cycle – a cyclical series of oxidation reactions that give off CO2 and produce one ATP per cycle; occurs twice per glucose molecule Electron transport system – a series of carriers that accept electrons removed from glucose and pass them from one carrier to the next until the final receptor, O2 is reached; water is produced; energy is released and used to synthesize 32 to 34 ATP If oxygen is not available, fermentation occurs in the cytoplasm instead of proceeding to cellular respiration.
