Chapter 5 Cell Respiration and Metabolism 5-1

Metabolism Is all reactions in body that involve energy transformations Divided into 2 categories: Catabolism breaks down molecules and releases energy Is primary source of energy for making ATP Anabolism makes larger molecules and requires energy Source of body’s large energy-storage compounds 5-3

Metabolism continued 5-4

Glycolysis 5-5

Glycolysis Is metabolic pathway by which glucose is converted into 2 pyruvates Does not require oxygen Overall net equation is: glucose + 2NAD + 2ADP + 2Pi  2 pyruvates + 2NADH + 2 ATP 5-6

Glycolysis continued Glycolysis is exergonic - produces net of 2ATPs and 2NADHs However, glucose must be activated with 2ATPs (phosphorylation) before energy can be obtained Phosphorylation traps glucose inside cell 5-7

Lactic Acid Pathway To avoid end-product inhibition NADHs produced in glycolysis need to give Hs away In absence of O2 NADH gives its Hs to pyruvate creating lactic acid (anaerobic respiration or fermentation) Makes muscles feel fatigued 5-10

Lactic Acid Pathway continued Only yields a net gain of 2 ATPs per glucose RBCs don’t have mitochondria; use only lactic acid pathway Occurs in skeletal and heart muscle when oxygen supply falls below critical level During heavy exercise or vascular blockage Only in skeletal and heart muscle 5-11

Glycogenesis and Glycogenolysis For osmotic reasons cells can’t store many free glucoses Instead store glucose as glycogen (glycogenesis) Skeletal muscle and liver store lots of glycogen Glycogenolysis clips glucose out of glycogen as glucose 6-phosphate. Breaking down of glycogen Phosphate groups trap molecules in cells 5-12

Glycogenesis and Glycogenolysis continued 5-13

Glycogenesis and Glycogenolysis continued Skeletal muscles use trapped glucose-6-phosphate for own energy needs Only liver has glucose-6-phosphatase that removes phosphate groups So it can secrete glucose 5-14

Aerobic Respiration 5-16

Aerobic Respiration Begins when pyruvate formed by glycolysis enters mitochondria CO2 is clipped off pyruvate forming acetyl CoA (coenzyme A is a carrier for acetic acid) CO2 goes to lungs Energy in acetyl CoA is extracted during aerobic respiration in mitochondria 5-17

Krebs Cycle Begins with acetyl CoA combining with oxaloacetic acid to form citric acid In a series of reactions citric acid converted back to oxaloacetic acid to complete the pathway 5-18

Krebs Cycle continued 5-20

Fat and Protein Metabolism 5-31

Fats and Proteins as Energy Sources Fats can be hydrolyzed to glycerol and fatty acids These can be modified to run through Kreb’s Proteins can be broken down to amino acids Which can be deaminated and run through Kreb’s These pathways can be used to interconvert carbohydrates, fats, and proteins 5-32

Energy Storage When more energy is taken in than consumed, ATP synthesis is inhibited Glucose converted into glycogen and fat 5-33

Acetyl CoA Is a common substrate for energy and synthetic pathways 5-34

Fat Synthesis (Lipogenesis) Acetyl CoAs can be linked together to form fatty acids Fatty acids + glycerol = Fat (triglycerides) Occurs mainly in adipose and liver tissues Fat is major form of energy storage in body Yields 9 kilocalories/g Carbos and proteins yield only 4/g 5-35

Lipolysis Is breakdown of fat into fatty acids and glycerol Via hydrolysis by lipase Acetyl CoAs from free fatty acids serve as major energy source for many tissues Found in the liver 5-36

Brown Fat Amount of brown fat greatest at time of birth Is major site for thermogenesis in the newborn Brown fat produces an uncoupling protein, causing H+ to leak out of inner mitochondrial membrane Less ATP is produced, causing electron transport system to be more active Heat produced instead of ATP 5-38

Ketone Bodies Triglycerides are continually broken down and resynthesized Ensures blood will contain fatty acids for aerobic respiration During fasting and diabetes lots of fat is broken down Causes high levels of ketone bodies Which are fat metabolites Gives breath an acetone smell Cells cannot have big storages of glycogen 5-39

Amino Acid Metabolism Nitrogen (N) is ingested primarily as protein Which is used in body as amino acids Excess is excreted mainly as urea 5-40

Nitrogen (N) Balance Nitrogen balance = N ingested minus N excreted Positive N balance: more Nitrogen ingested than excreted Negative N balance: less Nitrogen ingested than excreted In healthy adults amount of N excreted = amount ingested -if this is not happening, diet is not balanced Excess amino acids can be converted into carbos and fat 5-41

Essential and Non-essential Amino Acids 20 amino acids are used to build proteins 12 can be produced by body 8 must come from diet (= essential amino acids) 5-42

Gluconeogenesis Occurs when amino acids or other non-carbos are converted to Keto acids, then pyruvate, then glucose 5-46

Uses of Different Energy Sources Different cells have different preferred energy substrates Brain uses glucose as its major source of energy 5-48

Summary of Metabolic Conversions of Nutrients Figure 22-2: Summary of metabolism