Chapter 5 - Cell Respiration and Metabolism Metabolism - the sum of all the chemical reactions that...

Chapter 5 - Cell Respiration and Metabolism

Metabolism - the sum of all the chemical reactions that occur in the body. It is comprised of: anabolism – synthesis of molecules,

requires input of energy catabolism – break down of

molecules, releases energy

We have been designed to liberate energy from food molecules by aerobic cellular respiration. This process is described as aerobic because oxygen is required. Why is oxygen required?

Oxygen is the final e- acceptor.Aerobic cellular respiration occurs in four

stages:glycolysis

transition reactionKrebs cycle (citric acid cycle)electron transport pathway

C6H12O6 + 6O2 6CO2 + 6H2O + ATP

WARNING: What follows is a simplified and

condensed version of cellular respiration and is only appropriate for this

class.

• Glycolysis – glucose must be “activated” by the addition of two phosphate groups P . The addition of the P also traps glucose within the cell.

2ADP + Pi 2ATP

C6H12O6 2 C3H4O3

glucose pyruvic acid

2NAD + 4H 2NADH2

AMP + P ADP + Pi ATP

This reaction is reversible. High energy energy bonds hold the P .

When NAD is reduced, a pair of hydrogen atoms donates a pair of e-, one of which then binds one proton and the other proton follows along = NADH + H+. We simplify this with NADH2.

• If oxygen is not present to take the e- from NADH2, then the e- will be donated to pyruvic acid =

Lactic acid pathway (anaerobic respiration)

The final product is lactic acid. This meta-bolic pathway only yields 2 ATP/ molecule.

• Cells cannot store very much glucose because the glucose affects the cell’s osmolarity (see ch. 6). The glucose is converted to glycogen (or fatty acids) for storage = glycogenesis.glucose glu–6–P glu– 1—P

glycogen [glu + glu + glu + glu + …

glycogen]

• The stored glycogen can be converted to glucose for use by the cell or secreted into the bloodstream (this only in the liver) = glycogenolysis.

[glycogen glu + glu + glu + glu + …]

glycogen glu–1–P glu– 6—P

liver enzymes

glucose glucose

• Only liver cells contain glucose – 6 – phosphatase, which removes P from glu – 6—P and liberates free glucose. The free glucose is then transported out of the cell into the blood stream.

• Gluconeogenesis = liver cells contain another enzyme, lactic acid dehydrogenase, which converts lactic acid to pyruvic acid. The pyruvic acid is then converted to glu – 6—P (this is the reverse of glycolysis).

• The glu – 6—P can then be used to make glycogen or released as free glucose.

• The production of glucose from noncarbohydrate molecules, e.g. lactic acid, amino acids, fatty acids, is gluconeogenesis. (The Cori cycle is the production of glu from lactic acid.)

Continuation of Aerobic Respiration

• Transition reaction = pyruvic acid moves into the matrix of the mitochondrion. CO2

is cleaved off and at the same time Coenzyme A is added.Coenzyme A is derived from the vitamin pantotenic acid.

NAD + 2H NADH2

2C3H4O3 + 2CoA 2C2H3O-CoA + 2CO2

pyruvic acid coenzyme A acetyl-CoA carbon

dioxide

Krebs Cycle• Acetic acid (2C) is added to

oxaloacetic acid (4C) to form citric acid (6C). CO2 is enzymatically removed.

3NAD+6H 3NADH2

2C2H3O-CoA 4CO2

FAD+2H FADH2 2ADP+Pi 2ATP

Electron Transport System• e- are passed along a chain of

molecules to O2, which acts as the final e- acceptor.

max 30 ADP+Pi 30 ADP

2 H+ + 2e- + ½ O2 H2O

• If the last cytochrome remained in a reduced state, it would be unable to accept more e-. E- transport would then progress only to the next-to-last cytochrome. This process would continue until all of the elements of the chain remained in the reduced state. At this point, the system would stop and no ATP could be produced in the mitochondrion. With the system incapacitated, NADH2 and FADH2 could not become oxidized by donating their electrons to the chain and, through inhibition of Krebs cycle enzymes, no more NADH2 and FADH2 could be produced in the mitochondrion. The Krebs cycle would stop and respiration would become anaerobic.

• Lipids and proteins can also be used in aerobic respiration. Some organs preferentially use molecules other than glucose. Why were we designed this way?To prevent depletion of glucose and

conserve it for the brain.• We can store only small amounts of

ATP. If food molecules are still available and ATP concentration continues to rise, glycolysis is inhibited and glucose is converted into glycogen and fats instead = glycogenesis and lipogenesis

Lipogenesis

• Excess glucose does not complete respiration but instead is converted into glycerol and fatty acids. The acetyl-CoA subunits from the transition reaction are added together to produce fatty acids. This occurs primarily in adipose tissue and the liver.

Lipolysis

• Triglycerides are hydrolyzed into glycerol and free fatty acids (FFA) by lipolysis.

• In some tissues glycerol can be converted into phosphoglyceraldehyde.

• FFAs are a major energy source and are metabolized by -oxidation.

Amino Acids• Excess amino acids (a.a.) in the diet are not

simply stored as additional protein – instead they are deaminated and the carbon skeleton is either respired or converted to carbohydrates or fats.

• Adequate amounts of amino acids are required for growth and repair. Some a.a. can be make by rearranging parts of carbohydrates and essential a.a. A new amino acid can be obtained by transamination. – Amine group (NH2) transferred from one amino acid to

form another amino acid and a keto acid.– Catalyzed by a specific enzyme (transaminase).

• Excess amino acids are processed for excretion by oxidative deamination. The amine group is removed and converted to urea, which is then excreted by the kidneys.

• Not all cells can use glucose as the energy source.

• Blood contains a variety of energy sources:– Glucose and ketone bodies, fatty acids,

lactic acid, and amino acids.

• Different tissues preferentially use different energy molecules. – Blood [glucose] maintained as many

organs spare glucose.

• Why?