Introduction Living is work. To perform their many tasks, cells must bring in energy from outside sources. In most ecosystems, energy enters as sunlight. Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them. Fig. 9.1
Cellular respiration is similar to the combustion of gasoline in an automobile engine. The overall process is: Organic compounds + O 2 -> CO 2 + H 2 O + Energy Carbohydrates, fats, and proteins can all be used as the fuel, but it is traditional to start learning with glucose as the fuel molecule, because it is the one most abundantly used. C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O + Energy (ATP + heat)
4. Electrons fall from organic molecules to oxygen during cellular respiration In cellular respiration, glucose and other fuel molecules are oxidized, releasing energy. Molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of energetic electrons that give off their energy as they are transferred to oxygen. The energy from these electrons will be transferred to the energy rich bonds of ATP molecules when they help form these bonds.
5. The fall of electrons during respiration occurs in small steps Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time. Rather, glucose and other fuels are broken down gradually in a series of chemical reaction steps, each catalyzed by a specific enzyme. At key steps, hydrogen atoms are stripped from glucose and passed first to a coenzyme, like NAD + (nicotinamide adenine dinucleotide).
This changes NAD +, to NADH, which carries the energy of the electrons. Fig. 9.4
1. Respiration involves glycolysis, the Krebs cycle, and electron transport: an overview Part of this involves mitochondria in eukaryotic cells Fig. 9.6
2. Glycolysis breaks down glucose to pyruvate in 10 small steps: a closer look During glycolysis, glucose, a six carbon-sugar, is split into two, three-carbon sugars, pyruvate. Let s watch the whole thing. Each of the ten steps in glycolysis is catalyzed by a specific enzyme, and occurs in the cytoplasm. These steps can be divided into two phases: an energy investment phase and an energy payoff phase.
In the energy investment phase, ATP provides activation energy by phosphorylating glucose. This requires 2 ATP per glucose. In the energy payoff phase, 4 ATP are produced and NAD + is reduced to NADH. 2 ATP (net) and 2 NADH are produced per glucose. Fig. 9.8
The net yield from glycolysis is 2 ATP and 2 NADH per glucose. No CO 2 is produced during glycolysis. Glycolysis occurs whether O 2 is present or not. If O 2 is present, pyruvate moves into the mitochondria to the Krebs cycle and the energy stored in NADH can be converted to ATP by the electron transport chain.
As pyruvate enters the mitochondrion, a multienzyme complex modifies pyruvate to acetyl CoA which enters the Krebs cycle in the matrix. A carboxyl group is removed as CO 2. A pair of electrons is transferred from the remaining two-carbon fragment to NAD + to form NADH. The 2 carbon acetic acid combines with coenzyme A to form acetyl CoA. Fig. 9.10
3. The Krebs cycle completes the energyreleasing breakdown of organic molecules: a closer look More than three quarters of the original energy in glucose is still present in two 2 molecules of pyruvate. If oxygen is present, pyruvate enters the mitochondrion where enzymes of the Krebs cycle complete it s breakdown to carbon dioxide. Let s watch it all.
The Krebs cycle is named after Hans Krebs who was largely responsible for elucidating its pathways in the 1930 s. This cycle begins when acetic acid from acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Ultimately, the oxaloacetate is recycled and the acetate is broken down to CO 2. Each cycle produces one ATP, three NADH, and one FADH 2 (another electron carrier) per acetyl CoA.
The Krebs cycle consists of eight steps. Fig. 9.11
4. The inner mitochondrial membrane couples electron transport to ATP synthesis: a closer look Only 4 of 38 ATP ultimately produced by respiration of glucose are derived from glycolysis and the Krebs Cycle The vast majority of the ATP comes from the energy in the electrons carried by NADH (and FADH 2 ). The energy in these electrons is used in the electron transport system to power ATP synthesis.
Electrons carried by NADH and FADH are transferred to the molecules in the electron transport chain. The electrons continue along the chain, which includes several cytochrome proteins. Fig. 9.13
Fig. 9.15
Electrons from NADH or FADH 2 ultimately pass to oxygen, the so-called final electron acceptor, causing this to be called aerobic respiration. The electron transport chain generates no ATP directly. The movement of electrons along the chain does contribute to a process called chemiosmosis, which leads to ATP synthesis by oxidative phosphorylation (or ox-phos as it s referred to in small talk at biologists cocktail parties). Here s how it works:http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter9/animati ons.html
A proton gradient is produced by the movement of electrons along the electron transport chain, because several chain molecules can use the exergonic flow of electrons to pump H + from the matrix to the intermembrane space. The gradient produced involves more protons in the intermembrane space than in the matrix. This gradient represents a form of potential energy, very similar to the one in a flashlight battery.
A protein complex, ATP synthase, in the cristae actually makes ATP from ADP and P i. The energy of the proton gradient is used as the source of power to do the work of ATP synthesis. How about a little animation? http://vcell.ndsu.nodak.ed u/animations/atpgradient/ movie.htm Fig. 9.14
The ATP synthase molecules are the only place that will allow H + to diffuse back to the matrix. This exergonic flow of H + through the protein complex is used by the enzyme to generate ATP. This coupling of the redox reactions of the electron transport chain to ATP synthesis is called chemiosmosis. The energy from glucose electrons was used to move protons across a membrane (uphill, so to speak), and when they passively flowed back across the membrane (downhill), their energy was used to do the work of making ATP.
5. Cellular respiration generates many ATP molecules for each sugar molecule it oxidizes: a review During respiration, most energy flows from glucose -> NADH -> electron transport chain -> ATP. Considering the fate of carbon, one six-carbon glucose molecule is oxidized to six CO 2 molecules. Some ATP is produced during glycolysis and the Krebs cycle, but most comes from the electron transport chain.
Assuming the most energy-efficient shuttle of NADH from glycolysis, a maximum yield of 34 ATP is produced by the ETC from one glucose. This plus the 4 ATP from substrate-level phosphorylation gives a bottom line of 38 ATP per glucose molecule broken down.
1. Fermentation enables some cells to produce ATP without the help of oxygen Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic).
Anaerobic catabolism of sugars can occur by fermentation. Fermentation can generate ATP from glycolysis as long as there is a supply of NAD + to accept electrons. If the NAD + pool is exhausted, glycolysis shuts down. Under aerobic conditions, NADH transfers its electrons to the electron transfer chain, recycling NAD +. Under anaerobic conditions, various fermentation pathways generate ATP by glycolysis and produce fresh NAD + by transferring electrons from NADH to pyruvate. This, like glycolysis, happens in the cytoplasm, so it can t help the Krebs Cycle.
In alcohol fermentation, pyruvate is converted to ethanol in two steps. First, pyruvate is converted to a two-carbon compound, acetaldehyde, by the removal of CO 2. Second, acetaldehyde is reduced by NADH to ethanol. Alcohol fermentation by yeast is used in baking and alcoholic beverage making. Fig. 9.17a
During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (lactic acid). Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt. Muscle cells switch from aerobic respiration to lactic acid fermentation to generate ATP when O 2 is scarce. The waste product, lactate, was thought of to be the cause muscle fatigue, but ultimately it is converted back to pyruvate in the liver. Fig. 9.17b
Carbohydrates, fats, and proteins can all be catabolized through the same pathways. We usually learn about the breakdown of glucose, but at rest, most of our ATP s actually come from the breakdown of fatty acids into 2 carbon acetyl CoA s. Good summary 6:00 Fig. 9.19
Chloroplasts are found in most plant cells but not in animal cells. Mitochondria are found in animal cells and most plant cells. Why are mitochondria found in most plant cells?
What is the primary purpose of cellular respiration? a. To store chemical energy in glucose molecules b. To store chemical energy in carbon dioxide and water molecules c. To use chemical energy from glucose molecules d. To use chemical energy from carbon dioxide and water molecules
Cellular respiration is a chemical process and can be represented by a chemical equation. What are the products in this chemical process? a. Hydrocarbons and oxygen b. Hydrocarbons and carbon dioxide c. Water, carbon dioxide, and energy d. Water, carbon dioxide, and oxygen
Which equation shows the reactants and products of cellular respiration? a. Carbon dioxide + water sugar + oxygen b. Carbon dioxide + oxygen sugar + water c. Sugar + carbon dioxide water + oxygen d. Sugar + oxygen water + carbon dioxide
All cells need energy. Where does the energy come from in plants? Briefly trace the energy from the original source to the endpoint. In animals?
Which of these is required for aerobic cellular respiration? A. oxygen B. nitrogen C. carbon dioxide D. sunlight
In terms of energy, how are cellular respiration and photosynthesis related? a. Energy captured in photosynthesis is used to power cellular respiration b. The energy transformed in cellular respiration is used to power photosynthesis c. Photosynthesis and respiration perform the same task in terms of energy transformation d. Energy is not involved in either photosynthesis or cellular respiration
Photosynthesis and cellular respiration are interrelated processes. During which biogeochemical cycle do the biological processes of photosynthesis and cellular respiration play key roles? A. carbon cycle B. hydrogen cycle C. nitrogen cycle D. oxygen cycle