Cell Respiration and Metabolism

Human physiology

Metabolism•All reactions that involve energy

transformations.•Divided into 2 Categories:

▫Catabolic: Release energy. Breakdown larger molecules into smaller

molecules.▫Anabolic:

Require input of energy. Synthesis of large energy-storage

molecules.

Aerobic Cell Respiration•Oxidation-reduction reactions:

▫Break down of molecules for energy.

▫Electrons are transferred to intermediate carriers, then to the final electron acceptor: oxygen. Oxygen is obtained from the blood.

Glycolysis•Breakdown of glucose for energy in the

cytoplasm.•Glucose is converted to 2 molecules of

pyruvic acid (pyruvate).•Each pyruvic acid contains:

▫3 carbons▫3 oxygens▫4 hydrogens

•4 hydrogens are removed from intermediates.

Glycolysis

•Each pair of H+ reduces a molecule of NAD.

•Produces:▫2 molecules of NADH and 2 unbound H+

▫2 ATP•Glycolysis Pathway:•Glucose + 2 NAD + 2 ADP + 2 Pi

2 pyruvic acid + 2 NADH and 2 ATP

Glycolysis•Glycolysis is exergonic.

▫Energy released used to drive endergonic reaction: ADP + Pi ATP

•Glucose must be activated first before energy can be obtained.▫ATP consumed at the beginning of

glycolysis.

Glycolysis•ATP ADP + Pi

•Pi is not released but added to intermediate molecules (phosphorylation).

•Phosphorylation of glucose, traps the glucose inside the cell.

•Net gain of 2 ATP and 2 NADH.

Glycolysis

Lactic Acid Pathway•Anaerobic respiration:

▫Oxygen is not used in the process.

•NADH + H+ + pyruvic acid lactic acid and NAD.

•Produce 2 ATP/ glucose molecule.

Lactic Acid Pathway

•Some tissues adapted to anaerobic metabolism:▫Skeletal muscle: normal daily occurrence.

▫RBCs do not contain mitochondria and only use lactic acid pathway.

•Cardiac muscle: ischemia

Glycogenesis and Glycogenolysis

•Increase glucose intracellularly, would increase osmotic pressure.

•Must store carbohydrates in form of glycogen.

• Glycogenesis: formation of glycogen from glucose.• Glycogenolysis: conversion of glycogen to glucose-

6-phosphate.▫ Glucose-6-phosphate can be utilized through glycolysis.

Glycogenesis and Glycogenolysis

•Glucose-6-phosphate cannot leak out of the cell.

•Skeletal muscles generate glucose-6-phosphate for own glycolytic needs.

•Liver contains the enzyme glucose-6-phosphatase that can remove the phosphate group and produce free glucose.

Cori Cycle•Lactic acid produced by anaerobic

respiration delivered to the liver.•LDH converts lactic acid to pyruvic

acid.•Pyruvic acid converted to glucose-6-

phosphate:▫Intermediate for glycogen.▫Converted to free glucose.

•Gluconeogenesis: conversion to non-carbohydrate molecules through pyruvic acid to glucose.

Aerobic Respiration•Aerobic respiration of glucose, pyruvic acid is formed by glycolysis, then converted into acetyl coenzyme A (acetyl CoA).

•Energy is released in oxidative reactions, and is captured as ATP.

Aerobic Respiration•Pyruvic acid enters interior of mitochondria.

•Converted to acetyl CoA and 2 C02.

•Acetyl CoA serves as substrate for mitochondrial enzymes.

Acetyl CoA enters the Krebs Cycle.

overview

Krebs Cycle•Acetyl CoA combines with oxaloacetic

acid to form citric acid.•Citric acid enters the Krebs Cycle.•Produces oxaloacetic acid to continue

the pathway.•1 GTP, 3 NADH, and 1 FADH2•NADH and FADH2 transport electrons

to Electron Transport Cycle.

CAC

Electron Transport•Cristae of inner mitochondrial membrane contain molecules that serve as electron transport system.

•Electron transport chain consists of FMN, coenzyme Q, and cytochromes.

ETC Chain•Each cytochrome transfers electron

pairs from NADH and FADH2 to next cytochrome.

•Oxidized NAD and FAD are regenerated and shuttle electrons from the Krebs Cycle to the ETC.

•Cytochrome receives a pair of electrons.

•Iron reduced, then oxidized as electrons are transferred.

ETC Chain•Cytochrome a3 transfers electrons to O2 (final electron acceptor).

•Oxidative phosphorylation occurs:▫Energy derived is used to phosphorylate ADP to ATP.

Coupling ETC to ATP•Chemiosmotic theory:•ETC powered by transport of electrons, pumps H+ from mitochondria matrix into space between inner and outer mitochondrial membranes.

Coupling ETC to ATP•Proton pumps:•NADH-coenzyme Q reductase complex:

▫Transports 4 H+ for every pair of electrons.

•Cytochrome C reductase complex:▫Transports 4 H+.

•Cytochrome C oxidase complex:▫Transports 2 H+.

Coupling ETC to ATP•Higher [H+] in inter-membrane space.•Respiratory assemblies:

▫Permit the passage of H+.•Phosphorylation is coupled to oxidation,

when H+ diffuse through the respiratory assemblies:▫ ADP and Pi ATP

Coupling ETC to ATP•Oxygen functions as the last electron acceptor.▫Oxidizes cytochrome a3.

•Oxygen accepts 2 electrons.•O2 + 4 e- + 4 H+ 2 H20

ATP Produced•Direct phosphorylation:•Glycolysis:

▫2 ATP •Oxidative phosphorylation:

▫2.5 ATP produced for pair of electrons each NADH donates.

▫1.5 ATP produced for each pair of electrons FADH2 donates ((activates 2nd and 3rd proton pumps).

▫26 ATP produced.

Metabolism of Lipids

• When more energy is taken in than consumed, glycolysis inhibited.

• Glucose converted into glycogen and fat.

Lipogenesis

• Formation of fat.

• Occurs mainly in adipose tissue and liver.

• Acetic acid subunits from acetyl CoA converted into various lipids.

Metabolism of Lipids•Lipolysis:

▫Breakdown of fat.•Triglycerides glycerol + fa

•Free fatty acids (fa) serve as blood-borne energy carriers.

lipase

Beta-oxidation

• Enzymes remove 2-carbon acetic acid molecules from acid end of fa.

• Forms acetyl CoA.

• Acetyl CoA enters Krebs Cycle.

Metabolism of Proteins•Nitrogen is ingested primarily as

protein.•Excess nitrogen must be excreted.•Nitrogen balance:

▫Amount of nitrogen ingested minus amount excreted.

• + N balance:▫ Amount of nitrogen ingested more than amount

excreted.• - N balance:

▫ Amount of nitrogen excreted greater than ingested.

• Adequate amino acids are required for growth and repair. A new amino acid can be obtained by:

• Transamination: ▫ Amino group (NH2) transferred from one amino acid to form

another.

• Process by which excess amino acids are eliminated.

• Amine group from glutamic acid removed, forming ammonia and excreted as urea.

Deamination

• Energy conversion: amino acid is deaminated.

• Ketoacid can enter the Krebs Cycle.

Use of different energy sources.