第七章 細胞如何釋放儲存的能量 前言

細胞如何製造ATP 能量釋放途徑主要型式之比較 有氧呼吸的總覽

醣解作用(Glycolysis): 能量釋放途徑的第 一階段

有氧呼吸的第二階段

pyruvate + coenzyme A + NAD+ 有氧呼吸的第二階段 前置步驟和Kreb循環 pyruvate + coenzyme A + NAD+ acetyl-CoA + NADH + CO2 One of the carbons from pyruvate is released in CO2 Two carbons are attached to coenzyme A and continue on to the Krebs cycle

What is Acetyl-CoA? A two-carbon acetyl group linked to coenzyme A CH3 C=O Coenzyme A Acetyl group

Coenzyme Reductions During First Two Stages Glycolysis 2 NADH Preparatory reactions 2 NADH Krebs cycle 2 FADH2 + 6 NADH Total 2 FADH2 + 10 NADH

有氧呼吸的第三階段

電子傳遞磷酸化作用 (Electron Transport Phosphorylation) Occurs in the mitochondria Coenzymes deliver electrons to electron transport systems Electron transport sets up H+ ion gradients Flow of H+ down gradients powers ATP formation

Electron Transport Electron transport systems are embedded in inner mitochondrial compartment NADH and FADH2 give up electrons that they picked up in earlier stages to electron transport system Electrons are transported through the system The final electron acceptor is oxygen

Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT

Making ATP: Chemiosmotic Model INNER COMPARTMENT ADP + Pi

(per molecule of glucose) 能量報酬之摘要 (per molecule of glucose) Glycolysis 2 ATP formed by substrate-level phosphorylation Krebs cycle and preparatory reactions Electron transport phosphorylation 32 ATP formed

Energy Harvest from Coenzyme Reductions What are the sources of electrons used to generate the 32 ATP in the final stage? 4 ATP - generated using electrons released during glycolysis and carried by NADH 28 ATP - generated using electrons formed during second-stage reactions and carried by NADH and FADH2

Energy Harvest Varies NADH formed in cytoplasm cannot enter mitochondrion It delivers electrons to mitochondrial membrane Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion Electrons given to FAD yield less ATP than those given to NAD+

Energy Harvest Varies Skeletal muscle and brain cells Liver, kidney, heart cells Electrons from first-stage reactions are delivered to NAD+ in mitochondria Total energy harvest is 38 ATP Skeletal muscle and brain cells Electrons from first-stage reactions are delivered to FAD in mitochondria Total energy harvest is 36 ATP

Efficiency of Aerobic Respiration 686 kcal of energy are released 7.5 kcal are conserved in each ATP When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP Efficiency is 270 / 686 X 100 = 39 percent Most energy is lost as heat

ATP形成途徑 發酵作用途徑 乳酸發酵 酒精發酵 無氧電子傳遞

electrons, hydrogen from NADH GLYCOLYSIS C6H12O6 2 ATP energy input 2 ADP 2 NAD+ 2 NADH 4 ATP energy output 2 pyruvate 2 ATP net ETHANOL FORMATION 2 H2O 2 CO2 2 acetaldehyde electrons, hydrogen from NADH Fig. 7.10, p. 119 2 ethanol

Carbohydrate Breakdown and Storage Glucose is absorbed into blood Pancreas releases insulin Insulin stimulates glucose uptake by cells Cells convert glucose to glucose-6-phosphate This traps glucose in cytoplasm where it can be used for glycolysis

Energy Reserves Glycogen makes up only about 1 percent of the body’s energy reserves Proteins make up 21 percent of energy reserves Fat makes up the bulk of reserves (78 percent)

Making Glycogen If glucose intake is high, ATP-making machinery goes into high gear When ATP levels rise high enough, glucose-6-phosphate is diverted into glycogen synthesis (mainly in liver and muscle) Glycogen is the main storage polysaccharide in animals

Using Glycogen When blood levels of glucose decline, pancreas releases glucagon Glucagon stimulates liver cells to convert glycogen back to glucose and to release it to the blood (Muscle cells do not release their stored glycogen)

Energy from Fats Most stored fats are triglycerides Triglycerides are broken down to glycerol and fatty acids Glycerol is converted to PGAL, an intermediate of glycolysis Fatty acids are broken down and converted to acetyl-CoA, which enters Krebs cycle

Energy from Proteins Proteins are broken down to amino acids Amino acids are broken apart Amino group is removed, ammonia forms, is converted to urea and excreted Carbon backbones can enter the Krebs cycle or its preparatory reactions

Processes Are Linked Aerobic Respiration Photosynthesis Reactants Sugar Oxygen Products Carbon dioxide Water Photosynthesis Reactants Carbon dioxide Water Products Sugar Oxygen