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Learn about aerobic and anaerobic respiration, the role of ATP, and the mitochondria. Discover how glucose is converted to energy and the importance of coenzymes in cellular respiration.
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What is respiration? ATP Respiration is the process by which organisms extract the energy stored in complex molecules and use it to generate adenosine triphosphate (ATP). In this way they obtain energy to fuel their metabolic pathways. ATP provides the immediate source of energy for biological processes such as active transport, movement and metabolism.
What can you remember about respiration? Write the word and balanced symbol equations (if you can!) for: • Aerobic respiration • Anaerobic respiration in animals • Anaerobic respiration in yeast
Types of respiration C6H12O6 + 6O2 6CO2 + 6H2O + 36ATP C6H12O6 2C2H5OH + 2CO2 + 2ATP ethanol C6H12O6 2C3H6O3 + 2ATP lactate During aerobic respiration, a respiratory substrate, e.g. glucose, is split in the presence of oxygen to release carbon dioxide and water. A large number of ATP molecules are produced, releasing the energy from the glucose. In anaerobic respiration, glucose is converted (in the absence of oxygen) to either lactate or ethanol. The ATP yield is low.
Where does respiration occur? Respiration occurs in all living cells. In eukaryotes the early stages of respiration occur in the cytoplasm. The later stages of respiration are restricted to the mitochondria. • Mitochondria contain highly folded inner membranes that hold key respiratory proteins (including the enzyme that makes ATP) over a large surface area. • Mitochondria provide an isolated environment to maintain optimum conditions for respiration. • Mitochondria have their own DNA and ribosomes, so can manufacture their own respiratory enzymes.
Adenosine triphosphate + + + 30.5kJ inorganic phosphate ATP H2O ADP ATP contains a sugar (ribose), a base (adenine) and three phosphate groups. adenine ribose phosphates When ATP is hydrolysed to form ADP and inorganic phosphate, 30.5kJmol-1 of energy are released.
Why ATP? Biological systems transfer the energy in glucose to ATP because unlike glucose… glucose ATP • ATP releases its energy instantly in a single reaction. • The hydrolysis of ATP releases a small amount of energy, ideal for fuelling reactions in the body.
Phosphorylation of ADP The addition of an inorganic phosphate group (Pi) to a molecule like ADP is called phosphorylation. ADP is phosphorylated during respiration. Two types of phosphorylation occur during respiration: 1. Substrate-level: glycolysis & Krebs cycle A single reaction involving the direct transfer of a phosphate group from a donor molecule to ADP. 2. Oxidative: electron transport chain A series of oxidation reactions that produce sufficient energy to form ATP from ADP and phosphate.
Coenzymes Coenzymes are molecules that bind with a specific enzyme or substrate, helping to catalyze a reaction. Breaking the bonds between coenzyme and product after a reaction is crucial, otherwise coenzyme concentration will drop, limiting respiratory rate. substrate enzyme coenzyme Three major coenzymes are used in respiration: • NAD (nicotinamide adenine dinucleotide) • CoA (coenzyme A) • FAD (flavine adenine dinucleotide)
NAD, FAD and coenzyme A NAD can accept a hydrogen molecule, forming reduced NAD (NADH). nicotinamide NAD+ + 2H NADH + H+ adenine This is used to regenerate ADP in the electron transport chain (ETC). ribose NAD Coenzyme Aaids the transition between glycolysis and the Krebs cycle, by converting pyruvate to acetyl coenzyme A. FAD,like NAD, can accept hydrogen to form reduced FAD (FADH2).
Unsure? Watch this video… Got it? Work your way through the worksheet.
Keeping track of the products For each molecule of glucose, glycolysis produces: • 2× • 2× • 2× For each molecule of glucose, the link reaction produces: • 2× • 2× • 2×
Keeping track of the products For each molecule of glucose, Krebs cycle generates: • 4× produced by decarboxylation • 6 × produced by redox reactions • 2 × produced by redox reactions • 2 × produced by substrate-level phosphorylation The NADH and FADH2 contain the potential energy originally locked in glucose. This energy is now transferred to ATP by oxidative phosphorylation in the electron transport chain.
How much ATP is produced? Process ATP in ATP produced Net ATP out glycolysis 2 4 2 link reaction 0 0 0 Krebs cycle 0 2 (per glucose) 2 (per glucose) Via the electron transport chain and chemiosmosis, each NADH can yield 2.5 ATP and each FADH21.5 ATP. From one molecule of glucose, glycolysis yields 2 NADH, the link reaction yields 2 NADH and the Krebs cycle yields 6 NADH and 2 FADH2. 10 × 2.5 = 25 ATP from NADH 2 × 1.5 = 3 ATP from FADH2 total = 2 + 2 + 25 + 3 = 32 ATP overall
Efficiency of aerobic respiration The theoretical yield of 32 ATPs for each glucose molecule is rarely achieved. In fact respiration is only about 32% efficient. • Some protons leak across the mitochondrial membrane, so not all are available to generate ATP via chemiosmosis. • Some ATP is used up moving pyruvate into the mitochondria by active transport. • Some ATP is used up moving hydrogen from reduced NAD made during glycolysis into the mitochondria. • Some energy is lost as heat. This heat helps to maintain a suitable body temperature for enzyme-controlled reactions.
Evidence for chemiosmosis The theory of chemiosmosis states that the energy in a chemical gradient established by electron movement is used to generate ATP. matrix Evidence includes: • The proton gradient across the inner membrane can be measured as it corresponds to a pH gradient. • Isolated ATP synthase enzymes can produce ATP using a proton gradient even if no electron transport is occurring. • Chemicals that block the ETC inhibit the formation of a proton gradient and prevent ATP synthesis.
Respiratory rate The respiratory rate is the rate at which an organism converts glucose to CO2 and water. It can be calculated by measuring an organism’s rate of oxygen consumption. Studies on simple animals often use a respirometer. Respirometers measure the change in gas volume in a closed system. Any change is due to the respiratory activity of the study organisms. Potassium hydroxide or soda lime is used to absorb the carbon dioxide produced, meaning any changes in volume are due to oxygen consumption.
Respiratory substrates Other substances as well as glucose can be respired. Different respiratory substrates release different amounts of energy. Respiratory substrate Mean energy value (kJg-1) carbohydrate 15.8 lipid 39.4 protein 17.0 The difference in the relative energy values of these respiratory substrates is due to the amount of hydrogen atoms present in each one. If more hydrogen atoms are available to reduce coenzymes, more energy can subsequently be generated in the electron transport chain.
volume of CO2 given out RQ = volume of O2 taken in Type of respiration Substrate RQ Respiratory quotient Respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen used in the same period of time. RQ gives an indication of the respiratory substrate being respired and whether respiration is aerobic or anaerobic. anaerobic glucose > 1 carbohydrate 1.0 aerobic protein approx. 0.9 lipid approx. 0.7
