Cellular Respiration

What you should know The role of ATP in the transfer of energy and the phosphorylation of molecules by ATP. Metabolic pathways of cellular respiration. The breakdown of glucose to pyruvate in the cytoplasm in glycolysis, and the progression pathways in the presence or absence of oxygen (fermentation). The role of the enzyme phosphofructokinase in this pathway. The formation of citrate. Pyruvate is broken down to an acetyl group that combines with coenzyme A to be transferred to the citric acid cycle as acetyl coenzyme A. Acetyl coenzyme A combines with oxaloacetate to form citrate followed by the enzyme mediated steps of the cycle. This cycle results in the generation of ATP, the release of carbon dioxide and the regeneration of oxaloacetate in the matrix of the mitochondria. Dehydrogenase enzymes remove hydrogen ions and electrons which are passed to the coenzymes NAD or FAD to form NADH or FADH2 in glycolysis and citric acid pathways. NADH and FADH2 release the high-energy electrons to the electron transport chain on the mitochondrial membrane and this results in the synthesis of ATP. ATP synthesis — high-energy electrons are used to pump hydrogen ions across a membrane and flow of these ions back through the membrane synthesises ATP using the membrane protein ATP synthase. The final electron acceptor is oxygen, which combines with hydrogen ions and electrons to form water.

Introduction to Cellular Respiration A series of metabolic pathways that brings about the release of energy from a foodstuff In doing so it also regenerates the high-energy compound Adenosine Triphosphate (ATP)

ATP Adenosine Triphosphate Molecule able to provide energy immediately Consists of Adenosine & 3 inorganic phosphate molecules Energy held within ATP is released when the terminal phosphate is broken off (by enzymes) adenosine Pi

ATP Adenosine Triphosphate This bond broken to release energy Adenosine Diphosphate (ADP) and an inorganic phosphate are produced Also, energy is required to regenerate ATP from ADP & Pi adenosine Pi

ATP acts as the link between catabolic (energy releasing reactions) and anabolic (energy requiring reactions) At any given moment some ATP molecules are undergoing breakdown (releasing energy), while others are being regenerated from ADP + Pi (using energy) This means there is a relatively fixed quantity of ATP available

PHOSPHORYLATION The addition of a phosphate group to a molecule, e.g. ADP + Pi = ATP Phosphates can also be transferred from ATP to reactants in the pathway to make them more reactive e.g. Glucose ------------> Glucose- 6- Phosphate Often a step in a pathway can only proceed if a reactant becomes phosphorylated ATP ADP

Importance of ATP formation We all need energy to function and we get this energy from the foods we eat The most efficient way for cells to harvest energy stored in food is through cellular respiration a catabolic pathway for the production of adenosine triphosphate (ATP) ATP, a high energy molecule, is expended by working cells Cellular respiration occurs in both eukaryotic and prokaryotic cells

RESPIRATION Process by which energy is released from foods by oxidation. It involves the regeneration of ATP which is a high energy compound. Consists of 3 stages GLYCOLYSIS CITRIC ACID CYCLE (KREBS CYCLE) ELECTRON TRANSPORT CHAIN

GLYCOLYSIS Takes place in the cytoplasm of the cell. Is a series of enzyme controlled steps Glucose (6C) is broken down into two molecules of Pyruvic Acid (3C) (Pyruvate) Does not require oxygen Net gain of 2 ATP

Energy investment and payoff during glycolysis Phosphorylation occurs twice. The 2nd time by phosphofructokinase The first half of the chain makes up the energy investment phase- where 2 ATP are used per glucose molecule The second half of the chain makes up the energy payoff phase-where 4 ATP are produced per glucose molecule

Phosphorylation during energy investment stage The first phosphorylation of intermediates leads to a product that can continue to a number of other pathways (E.g. fermentation in the absence of oxygen) The second phosphorylation catalysed by phosphofructokinase is irreversible and leads only to the glycolytic pathway

Energy payoff stage Hydrogen ions are released by the action of a dehydrogenase enzyme Co-enzymes NAD and FAD pick up the H+ ions to form NADH or FADH in glycolysis and the citric acid pathways NADH and FADH release high energy electrons to the electron transport chain on the mitochondrial membrane Resulting in the synthesis of ATP

GLYCOLYSIS GLUCOSE PYRUVIC ACID 2 ATP 2 ADP+Pi 2 NAD 2 NADH2 4 ADP+Pi

MITOCHONDRIA Citric Acid Cycle takes place in the matrix Electron Transport Chain takes place on the cristae

CITRIC ACID CYCLE Takes place in the matrix of the mitochondria. Requires oxygen Pyruvate/Pyruvic acid converted to Acetyl which then combines with Coenzyme A (2C) Further Hydrogen ions are released and bind to NAD, forming NADH Acetyl CoA combines with oxaloacetate a 4C compound to form 6C citrate This stage involves the regeneration of oxaloacetate

CITRIC ACID CYCLE Citrate is converted back to oxaloacetate by a series of enzyme controlled reactions. During the cycle, carbon is released in the form of carbon dioxide, hydrogen is released and binds to NAD/FAD and ATP is formed.

CITRIC ACID CYCLE PYRUVIC ACID (3C) CO2 ACETYL COA (2C) 5C COMPOUND 4C COMPOUND 4C oxaloacetate 2NAD 2NADH2 CO2 2FAD 2FADH2 ADP+Pi ATP CO2 2NAD 2NADH2

ELECTRON TRANSPORT CHAIN Takes place on the cristae of the mitochondria. The reduced NAD/FAD transfer the high energy electrons to a chain of carriers called the cytochrome system Energy from the electrons is used to pump H+ from the inner matrix to the intermembrane space This maintains a higher conc of hydrogen ions in the intermembrane space, so.. The return flow of H+ ions rotates part of the membrane protein ATP synthase and ATP is generated The final electron acceptor is oxygen which combines with hydrogen ions and low energy electron to form water

ELECTRON TRANSPORT CHAIN The transfer of one H molecule releases 3 ATP molecules This is called oxidative phosphorylation

ELECTRON TRANSFER SYSTEM ADP +Pi ADP +Pi ADP +Pi NADH2 WATER SERIES OF HYDROGEN CARRIERS NAD OXYGEN ATP ATP ATP

ATP PRODUCTION SUMMARY Each NADH2 molecule produces 3 ATP 12 NADH2 = 36ATP from Kreb’s cycle 2 ATP from glycolysis 38 ATP in total

What you should know Substrates for respiration. Starch and glycogen, other sugar molecules, amino acids and fats. Regulation of the pathways of cellular respiration by feedback inhibition — regulation of ATP production, by inhibition of phosphofructokinase by ATP and citrate, synchronisation of rates of glycolysis and citric acid cycle. Energy systems in muscle cells. Creatine phosphate breaks down to release energy and phosphate that is used to convert ADP to ATP at a fast rate. This system can only support strenuous muscle activity for around 10 seconds, when the creatine phosphate supply runs out. It is restored when energy demands are low. Lactic acid metabolism. Oxygen deficiency, conversion of pyruvate to lactic acid, muscle fatigue, oxygen debt. Types of skeletal muscle fibres Slow twitch (Type 1) muscle fibres contract more slowly, but can sustain contractions for longer and so are good for endurance activities. Fast twitch (Type 2) muscle fibres contract more quickly, over short periods, so are good for bursts of activity.

Substrates for respiration Starch and glycogen (carbohydrates) are broken down to glucose Maltose and sucrose (carbohydrates) can be converted to glucose or glycolysis intermediates Proteins can be broken down to amino acids and converted to intermediates of glycolysis and the citric acid cycle Fats can be broken down into fatty acids and glycerol. Glycerol is converted to a glycolytic intermediate and fatty acids converted for use in the citric acid cycle

Regulation of Cellular Respiration The cell conserves its resources by only producing ATP when required Feedback inhibition regulates and synchronises the rates of the glycolytic and citric acid cycle pathways

If more ATP than the cell needs is produced the ATP inhibits phosphofructokinase slowing glycolysis High concentrations of citrate also inhibit phosphofructokinase When citrate concentration drops the enzyme is no longer inhibited

Energy systems in muscle cells During strenuous muscle activity the cell breaks down its reserves of ATP and releases energy Muscle cells can only store enough ATP for a few muscle contractions Muscle cells have an additional source of energy

Energy systems in muscle cells Creatine phosphate acts as a high energy reserve available to muscle cells during strenuous exercise During strenuous exercise creatine phosphate breaks down releasing energy and phosphate which are used to convert ADP to ATP by phosphorylation

This system can only support strenuous muscle activity for around 10 seconds before the supply of creatine phosphate runs out When ATP demand is low, ATP from cellular respiration restores the levels of creatine phosphate

Lactic acid metabolism If strenuous exercise continues the cells respire anaerobically as they do not get enough oxygen Neither the citric acid cycle nor electron transport system can generate the ATP required Only glycolysis is able to provide more ATP This results in pyruvate being converted to lactic acid It involves the transfer of hydrogen from NADH produced during glycolysis to pyruvic acid to produce lactic acid NAD is regenerated to maintain ATP production during glycolysis Only 2 molecules of ATP are produced from each molecule of glucose Lactic acid metabolism

As lactic acid builds in the muscles it causes fatigue An oxygen debt builds up When the oxygen debt is repaid, the lactic acid is converted back to pyruvic acid which then enters the aerobic pathway

ANAEROBIC RESPIRATION Oxygen debt builds up Glucose (6C) Pyruvic Acid (2 X 3C) Lactic Acid (2 X 3C) Oxygen debt repaid

AEROBIC V ANAEROBIC 38 2 Aerobic Respiration Anaerobic Respiration Number of ATP molecules per glucose molecule 38 2 Products of reaction (other than ATP) Carbon dioxide and water Lactic acid Location in cell mitochondrion cytoplasm

Types of skeletal muscle Skeletal muscles bring about movement of the body Two types of skeletal muscle fibres Type 1- Slow Twitch Muscle Fibres These contract slowly, but sustain contractions for longer Good for endurance activities Rely on aerobic respiration to generate ATP Have many mitochondria Have a large blood supply Have a high concentration of myoglobin which is good at storing oxygen (myoglobin also extracts oxygen from the blood) Major storage fuel is fats

Type 2- Fast Twitch Muscle Fibres Muscle fibres contract quickly Over short periods of time Good for bursts of activity Generates ATP through glycolysis Have only a few mitochondria Lower blood supply Major storage fuels are glycogen and creatine phosphate

Most human muscle tissue contains both slow and fast twitch fibres Athletes show distinct patterns of muscle fibres that reflect their sporting activities Fast twitch fibres are responsible for strength. Sports requiring sudden bursts of maximum activity, such as in sprinting, throwing, jumping and lifting rely on fast twitch fibres. Slow twitch fibres are responsible for stamina and suppleness. The action of slow twitch fibres is dependent on aerobic respiration. So, the supply of oxygen is important. The presence of large quantity of myoglobin is necessary. Therefore, slow twitch fibres give the characteristic red colour.