Cellular Respiration Harvesting Energy Cells and Energy The

Cellular Respiration Harvesting Energy

Cells and Energy The chemical energy in glucose and 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). Cellular respiration occurs all the time in the cells of all living things — plants, animals, fungi, protists and bacteria.

Rates of Respiration Living things use energy all the time, but at varying rates. Since energy for an organism’s use is supplied by cellular respiration, the rate of cellular respiration also varies depending on the state of the organism. When animals hibernate or go into a state of torpor, their rates of energy use fall and so their rates of cellular respiration drop to a fraction of normal. Example: Brine shrimp, also called sea monkeys, form cysts when they dry out. Although they can stay in this form for years, they are alive and respire with the lowest metabolic rate ever measured. The amount of energy used is estimated to be about 1/40 000 th of a kilojoule per year per milligram of shrimp.

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.

Energy from glucose C 6 H 12 O 6 + 6 O 2 6 H 2 O + 6 CO 2 + Energy

Three stages of glycolysis Glycolysis – Occurs in cytosol Citric acid cycle – Occurs in matrix of mitochondria – Also known as Kreb’s Cycle Electron transport – Occurs in cristae of mitochondria

NADH carries electrons to ETC glycolysis prep Krebs ATP Electron Transport chain

Glycolysis Splits a glucose molecule into 2 - 3 Carbon molecules called PYRUVATE products: ATP, NADH and pyruvate

Preparation for the Citric Acid Cycle C The pyruvate loses a carbon leaving the 2 carbon molecule C Acetyl Co. A CO 2 products: CO 2, Acetyl Co. A and NADH

The Citric Acid Cycle products: CO 2, ATP, NADH, FADH

Electron Transport H+ H+ NAD H+ H+ outer membrane H+ matrix H+H+ H+ H+ inner membrane or cristae During electron transport, electrons from ‘loaded’ acceptors (NADH and FADH 2) 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 transport H+ through the membrane. Oxygen is the final electron acceptor and it joins with the H+ to produce H 2 O. If there is no oxygen, the electron chain cannot continue because there is no way to release electrons. products: H 2 O, ATP

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.

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 became available and which occur in 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 fermentation and 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 anaerobic pathway 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 total energy yield for anaerobic respiration is two ATP per glucose molecule. If strenuous exercise continues, lactate builds up in the muscles, the p. H 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.

Value of anaerobic pathway in mammals Aerobic respiration produces almost 20 times the number of ATP molecules than are produced by glycolysis. . Cells of animals and plants rely on the anaerobic pathway only if there is not enough oxygen available to continue aerobic respiration, In some cases, the rapid rate of release of energy in glycolysis can be vital. In short sprints, it is energy derived from glycolysis that gets athletes across the finishing line.

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).

Comparison of anaerobic and aerobic respiration

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 k. J) than either carbohydrates or proteins (about 17 k. J each).

Applying our understanding of cellular respiration to medicine Hyberbaric chambers Hyperbaric oxygen chambers are being used to deliver a higher concentration of oxygen to tissues than normally exists to help heal injuries. A person being treated in the chamber is placed in a room in which air pressure can be increased. One hundred per cent oxygen is then delivered to the person through a hood or mask. This ensures that a higher level of oxygen is in the bloodstream of the person involved compared with normal situations in which a person breathes air containing 21 per cent oxygen. High levels of oxygen in the blood ensure that the level of oxygen is not the limiting factor for a cell. Aerobic respiration can occur at maximum rate and a maximum yield of ATP can be expected. This means that the cell has sufficient energy to boost the repair requirements that may exist after accident or surgery.

Applying our understanding of cellular respiration to medicine PET Images to asses damage to heart muscles The heart has a high oxygen demand for ATP production by aerobic respiration. Blockage of a coronary artery (e. g. a clot) will interrupt the oxygen supply to a region of the heart, interfere with heart function and damage heart tissue. If the region of heart muscle affected by the blockage is still alive, the damage may be reversed. In such cases, bypass surgery may be done to restore the blood supply to the region. If the affected region of heart muscle has died, this damage is permanent. In such cases, surgery to restore the blood supply to the affected area is of no use and exposes the patient to unnecessary risk. It is important to be able to distinguish whether the damaged area consists of living or dead heart tissue. A non-invasive technique, known as positron emission tomography (PET), can make this distinction. PET can obtain an image of glucose uptake and use by the heart. When damaged areas of the heart are still alive, they can take up and use glucose. Dead areas of heart tissue neither take up nor use glucose.

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 (energyrequiring) reaction. Cellular respiration is an exergonic (energyreleasing) reaction. Respiration C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O + 36− 38 ATP Glucose + oxygen → carbon dioxide + water + energy Photosynthesis 6 CO 2 + 12 H 2 O → C 6 H 12 O 6 + 6 H 2 O + 6 O 2 carbon dioxide + water + light → glucose + oxygen