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Cells and Energy • All food consumed is ultimately broken down into glucose units before it can be utilised by the body. • The chemical energy in glucoseand other organic compounds is not used directly by cells. • Cells carry out a series of reactions that release chemical energy from glucose and transfer it to ATP. The energy is then available for use by cells. • The series of energy releasing reactions that break down organic compounds of food, releasing chemical energy and transferring it to ATP, is known as cellular respiration(or sometimes, just respiration). Not to be confused with respiration as in breathing. • Cellular respiration occurs all the time in the cells of all living things.
Energy from glucose • Process of energy transfer from glucose to ATP is not 100 per cent efficient. • About 40 per cent of the chemical energy present in glucose is transferred to ATP and the remaining 60 per cent appears as heat energy. • The heat energy produced by living cells cannot be used to drive energy-requiring activities, such as muscle contraction or transport against a concentration gradient. • Instead heat energy is used to maintain the core body temperature of animals such mammals and birds within a narrow range. Insulating layers of fat, fur or feathers traps the heat energy released from cellular respiration.
Cellular Respiration • A series of biochemical pathways • Involves many chemical reactions and enzymes • Process that gradually breaks down food (organic substances) to release small packets of energy • Small packets of energy used to convert ADP into ATP • ATP is the chemical energy currency used by cells • 3 main stages are • (i) Glycolysis • (ii) Krebs Cycle (Citric acid cycle) • (iii) Electron Transport Chain (ETC) and Chemiosmosis
Three stages of respiration • Glycolysis • Occurs in cytosol • Citric acid cycle/Kreb’s Cycle • Occurs in matrix of mitochondria • Also known as Kreb’s Cycle • Electron transport • Occurs in cristae of mitochondria
Reaction for Cellular Respiration • Strictly speaking, ‘cellular respiration’refers to the aerobic breakdown of glucose to drive the production of ATP; that is, the pathways that evolved when oxygen is available to mitochondria in eukaryotic cells. • The general simplified formula for the complete aerobic breakdown of glucose is:
What happens when there is no oxygen? • If oxygen is not available, glycolysis is followed by fermentationand no more energy in the glucose molecule will be harvested—no further ATP is produced. • This process is referred to as anaerobic respiration. • Pyruvate is converted via an anaerobicpathway to either lactic acid (in most animals) or alcohol and carbon dioxide (in most plants, and in microorganisms such as yeast and bacteria). • Fermentation is necessary as it prevents the accumulation of pyruvate and thus allows glycolysis to continue.
Anaerobic respiration in mammals • In the absence of oxygen, an enzyme present in human muscle tissue converts pyruvate to lactate (lactic acid) molecules. • The totalenergy yield for anaerobic respiration is two ATP per glucose molecule. • If strenuous exercise continues, lactate builds up in the muscles, the pH falls and pain and muscle fatigue occur. • When strenuous exercise stops, the oxygen supply to the muscles is adequate for normal needs and anaerobic respiration stops. • Accumulated lactate in muscle tissue is converted back to pyruvate and enters the Krebs cycle.
Summary of pathway ATP ATP ATP Click on a section for more information Glucose (6C) O2 Krebs Cycle ETC & Chemiosmosis Glycolysis H2O CO2 Fermentation (without O2) Alcohol and Lactic acid Conclusion and summary in diagram form and in table form
Glycolysis Splits a glucose molecule into 2 - 3 Carbon molecules called PYRUVATE. products: ATP, NADH and pyruvate
Preparation for the Citric Acid Cycle The pyruvate loses a carbon leaving the 2 carbon molecule Acetyl CoA C C CO2 products: CO2, Acetyl CoA and NADH
Glycolysis Inputs Outputs 2 Pyruvate NADH (2 ADP) 4 ATP • Glucose • NAD • (2 ATP) • 4 ADP
The Citric Acid Cycle products:CO2, ATP, NADH, FADH
Krebs cycle • Before each pyruvate enters the Krebs cycle it loses one carbon dioxide molecule • The NAD carrier picks up the hydrogen • The remaining 2 carbons bond to Coenzyme A to enter the Krebs cycle • Write as formula
Krebs cycle (cont.) • During the Krebs cycle, two more carbon dioxide molecules are given off • A total of 10 hydrogen molecules are picked up by NAD and FAD carriers • Each pyruvate molecule yields one ATP (meaning 2 per glucose molecule)
Krebs cycle Inputs Outputs Coenzyme A CO2 NADH FADH2 ATP • 2 Pyruvate • Coenzyme A • NAD • FAD • ADP
Electron Transport H+ H+ NAD H+ H+ outer membrane H+ matrix inner membrane or cristae H+ H+ H+ H+ During electron transport, electrons from ‘loaded’ acceptors (NADH and FADH2) are brought to the inner membranes of the mitochondria. The electrons are passed back and forth across the membrane from one cytochrome to another. During this process their energy is gradually decreased and used to transportH+through the membrane. Oxygen is the final electron acceptor and it joins with the H+ to produce H2O. If there is no oxygen, the electron chain cannot continue because there is no way to release electrons . products:H2O, ATP
Electron transport chain • The carriers NADH and FADH2 deliver the hydrogen ions to the inner membrane • Hydrogen ions pass into the inner membrane, passing electrons along at the same time • The hydrogen ions are used to generate 32 ATP through ATP synthase • Each oxygen molecule accepts hydrogen ions to create water as a by-product
Electron Transport Chain Inputs Outputs FAD NAD Water ATP • FADH2 • NADH • Oxygen • ADP
Outcome of the three stages • In cells of your heart, liver and kidneys, two additional molecules of ATP are generated to give a total of 38 ATP. • This is because the NADH produced during glycolysis in those cells enters the respiratory chain earlier than NADH produced in other kinds of cell.
TOTAL Cellular Respiration Inputs Outputs Carbon dioxide Water 36 ATP • Glucose • Oxygen • Water Remember: NAD, FAD and ADP are all carriers, they aren’t used up by this reaction so you don’t include them in the equation
Alcoholic Fermentation • During fermentation by yeast, pyruvate is broken down to carbon dioxide and ethanol (an alcohol). • The amounts of ethanol and carbon dioxide produced vary with different yeasts and different environmental conditions. • In wine-making, grapes are crushed to release the juice which contains sugars. Yeasts are added to this fluid, fermentation occurs which produces alcohol. When the alcohol concentration reaches about 12 per cent (v/v), this kills the yeast cells and fermentation stops. • Beer is made by fermenting sprouting barley grains using brewers’ yeast. Hops are added to give colour, taste and aroma. • Spirits are produced by fermenting various products, such as fruit juice (brandy), molasses (rum), barley grains (whisky). Spirits are distilled to increase the alcohol content in the final product to about 40 per cent (v/v).
Other substrates for respiration • The products of digestion of fats (fatty acids and glycerol) and the products of digestion of proteins (amino acids) can also enter the pathways of cellular respiration at various points. • When starved of food for a long period, even the proteins in muscles and other body tissues will be broken down to provide the energy necessary to survive. • During starvation in people, up to 97 per cent of fat tissue, 31 per cent of skeletal muscle and 27 per cent of blood can be lost. The brain, heart and diaphragm are not affected • Fats provide more energy per gram (39 kJ) than either carbohydrates or proteins (about 17 kJ each).
Link between cellular respiration and photosynthesis • Carbon dioxide and water are the waste products of respiration. • These are the basic materials that a plant uses for photosynthesis. • Photosynthesis is an endergonic (energy-requiring) reaction. • Cellular respiration is an exergonic (energy-releasing) reaction. Respiration C6H12O6 + 6O2 → 6CO2 + 6H2O + 36−38 ATP Glucose + oxygen → carbon dioxide + water + energy Photosynthesis 6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2 carbon dioxide + water + light → glucose + oxygen