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Chapter 6 Metabolism: Energy and Enzymes

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1 Chapter 6 Metabolism: Energy and Enzymes

2 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

3 Flow of energy The plant uses solar energy to produce the organic nutrients taken in by the moose. All of the solar energy absorbed by the plant eventually dissipates as heat, and therefore energy does not cycle; rather it flows through living things and their cells.

4 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.

5 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.

6 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.

7 The ATP cycle In cells, the exergonic breakdown of glucose is coupled to the buildup of ATP, and then the exergonic breakdown of ATP is coupled to endergonic reactions in cells. When a phosphate group is removed by hydrolysis, ATP releases the appropriate amount of energy for most metabolic reactions. The high-energy content of ATP comes from the complex interaction of the atoms within the molecule.

8 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. The reduction of carbon dioxide to form a mole of glucose stores 686 kcal in the chemical bonds of glucose.

9 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. If the energy within glucose were released all at once, most of it would be dissipated as heat. Instead, cells oxidize glucose slowly, and gradually use the energy to synthesize ATP molecules.

10 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.

11 Relationship of chloroplasts to mitochondria
Chloroplasts produce energy-rich carbohydrates. These carbohydrates are broken down in mitochondria, and the energy release is used for the buildup of ATP. Usable energy is lost due to the energy conversions of photosynthesis and cellular respiration. Then, when ATP is used as an energy source, all usable energy is converted to heat.

12 Chapter 7: Cellular Respiration

13 Cellular respiration Almost all organisms, whether they reside on land or in the water, carry on cellular respiration, which is most often glucose breakdown coupled to ATP synthesis.

14 Cellular Respiration takes place in 4 phases
Lets follow ONE molecule of glucose through its complete metabolism

15 Cellular Respiration takes place in 4 phases
1. Glycolysis is the breakdown of glucose into pyruvate, 2 ATP molecules are made Step 1: 2

16 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

17 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

18 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 ATPs!!! Step 4: 2 2 32 MITOCHONDRIA

19 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

20 Where is all this occuring?!?!
Cell Outside of the Mitochondria!

21 ATP is not only produced….but also NADH and FADH!

22 ATP is the currency to run machinery within the cell…..
But NADH and FADH2 run the electron transport system *which then makes the ATP!*

23 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 electrons received by NAD+ are high-energy electrons that are usually carried to the electron transport system. NAD+ can be used over and over again. FAD accepts two electrons and two hydrogen ions (H+) to become FADH2.

24 The function of NADH and FADH2 is to carry and then donate electrons to the electron transport system…..

25 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!*

26 What happens when breakdown of glucose is incomplete?
FERMENTATION!!!! 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.

27 The Fermentation Process…… remember, it’s ANAEOROBIC
In humans….. In bacteria or yeast….. The Fermentation Process…… remember, it’s ANAEOROBIC this situation only occurs when oxygen levels are LOW!

28 Notice, that in low oxygen you only make 2 ATP, compared to 36!
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

29 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% The inputs of fermentation include glucose, 2 ATP, and 4 ADP + 2 P. Outputs are 2 lactate, or 2 alcohol and 2 CO2, and 4 ATP (net 2 ATP).

30 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.

31 But we don’t just eat carbohydrates…
But we don’t just eat carbohydrates…..so what happens with proteins and lipids? ? ? ? ? ? ? ? ? ? ? ? Carbohydrates, fats, and proteins can be used as energy sources, and they enter degradative pathways at specific points. Catabolism produces molecules that can also be used for anabolism of other compounds.

32 The metabolic pool concept
Carbohydrates, fats, and proteins can be used as energy sources, and they enter degradative pathways at specific points. Catabolism produces molecules that can also be used for anabolism of other compounds.

33 Catabolism catabolic 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. An 18-carbon fatty acid results in nine acetyl-CoA molecules. Calculation shows that respiration of these can produce a total of 108 kcal ATP molecules. For this reason, fats are an efficient form of stored energy – three are three long fatty acid chains per fat molecule.

34 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. Carbohydrates, fats, and proteins can be used as energy sources, and they enter degradative pathways at specific points. Catabolism produces molecules that can also be used for anabolism of other compounds.

35 Catabolism catabolic reactions, break it down
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.

36 Carbohydrates, fats, and proteins can be used as energy sources, and they enter degradative pathways at specific points. Catabolism produces molecules that can also be used for anabolism of other compounds.

37 Anabolism Anabolic 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. Plants are able to synthesize all the amino acids they need. Animals, however, lack some of the enzymes necessary for synthesis of all amino acids. Adult humans can synthesize 11 of the common amino acids but cannot synthesize the other 9. These so-called essential amino acids must be supplied by the diet; those amino acids that can be synthesized are called nonessential.

38

39 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

40 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. During fermentation, glucose is incompletely metabolized to lactate or to carbon dioxide and alcohol, depending on the organism. Fermentation results in a net gain of only two ATP per glucose molecule.

41 Oxidation and phosphorylation os 2 PGAL results in 2 NADH and 2 high-energy PGAP (1, 3-bisphosphoglycerate) molecules. Next, removal of high-energy phosphate from 2 PGAP by 2 ADP produces 2 ATP and 2 PGA (3-phosphoglycerate) molecules.

42 Oxidation of 2 PGA by removal of water results in 2 high-energy PEP (phosphoenolpyruvate) molecules. In the final step, removal of high-energy phosphate from PEP by 2 ADP produces 2 ATP and 2 pyruvate molecules. There are four ATP molecules produced, and 2 invested in the first step of glycolysis for a net gain of 2 ATP.

43 Citric acid cycle At the first step, the cycle begins when an acetyl group carried by CoA combines with a C4 molecule to form citrate. Twice over, substrates are oxidized, NAD+ is reduced to NADH, and CO2 is released. ADP becomes ATP as a high-energy phosphate is removed from a substrate. Next, another substrate is oxidized, but this time with FAD reduced to FADH2. Between fumarate and oxaloacetate, a substrate is oxidized and NAD+ is reduced to NADH. The net result of this cycle of reactions is the oxidation of an acetyl group to two molecules of CO2, along with a transfer of electrons to NAD+ and FAD and a gain of one ATP. The citric acid cycle turns twice per glucose molecule.

44 Overview of the electron transport system
NADH and FADH2 bring electrons to the electron transport system. As the electrons move down the system, energy is released and used to form ATP. For every pair of electrons that enters by way of NADH, three ATP result. For every pair of electrons that enters by way of FADH2, two ATP result. Oxygen, the final acceptor of the electrons, becomes a part of water.


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