Metabolism

 

Autotrophs are organisms that utilize inorganic sources of carbon, such as carbon dioxide and calcium carbonate (limestone).

 

Heterotrophs are organisms that utilize organic sources of carbon, such as glucose.

 

Phototrophs are organisms that get their energy from sunlight (or artificial light).

 

Chemotrophs are organisms that get their energy from chemical reactions.

 

Photoautotrophs are organisms that get their carbon from CO2 and their energy from sunlight, therefore, plants, algae, and cyanobacteria are photoautotrophs.

 

Chemoheterotrophs get their carbon and their energy both from breaking down organic molecules.All animals, including humans, and all parasitic organisms, including those that cause infectious diseases, are chemoheterotrophs.

 

Chemoautotrophs and photoheterotrophs also exist, but are beyond the scope of this course.

 

Metabolism is the sum total of all of the chemical reactions occurring within a cell.

 

Anabolism is the process of building up more complex substances from simpler substances.Anabolic reactions in cells frequently employ dehydration synthesis as a

mechanism for building complex macromolecules.Dehydration synthesis removes a molecule of water from two simpler molecules, or monomers, as they are joined to form a dimer.Additional monomers may then be added by dehydration synthesis to form polymers, or complex molecules composed of many repeating units, or monomers.Anabolic reactions are endergonic reactions, meaning that they require an input of energy.

 

Catabolism is the process of breaking down complex molecules into simpler molecules.Catabolic reactions often employ hydrolysis in order to break down complex molecules by adding a molecule of water to the simple molecules that are the products.However, other catabolic reactions include decarboxylation, which involves the removal of a molecule of carbon dioxide, and dehydrogenation, the removal of two hydrogen ions and two electrons.Catabolic reactions are exergonic, that is they release energy that can be captured and used to do cellular work or to carry out anabolic reactions.

 

The unit of energy exchange that is used to capture energy released by catabolic reactions and to transfer that energy to perform cellular work or to carry out anabolic reactions is adenosine triphosphate (ATP), a nucleotide, composed of the nitrogenous base adenine, the five-carbon sugar ribose, and three phosphate units coupled together.ATP captures small quantities of energy by binding one phosphate unit to adenosine diphosphate (ADP) to synthesize, by dehydration synthesis, a high-energy molecule of ATP.The ATP can then give up some of that energy by being hydrolyzed to ADP and a phosphate unit.The phosphate can either be captured in the form of inorganic phosphate (a phosphate anion) or as an organic phosphate, that is, it can be attached to a metabolite or organic intermediate in a metabolic reaction.

 

ATP can be produced in catabolic reactions in several ways.One of the more common ways that occurs in many of the catabolic reactions is called substrate phosphorylation, whereby the phosphate molecule is taken from one of the organic phosphate intermediates, or metabolites, and combined with ADP by dehydration synthesis.Substrate phosphorylation takes place in glycolysis, the decarboxylation of metabolites such as pyruvic acid, and in the Krebs cycle.

 

Another very efficient means of producing ATP is by oxidative phosphorylation, where the phosphate is in inorganic anionic form.This inorganic phosphate is coupled to ADP using energy released by the movement of hydrogen ions (protons) down a concentration gradient, across a membrane, by way of a channel protein that also serves as an enzyme that catalyzes the dehydration synthesis of ATP.These hydrogen ions are then combined with electrons that have passed from molecule to molecule through a sequence of proteins in the membrane.This sequence of membrane proteins is called the electron transport system, and the movement of hydrogen ions through the channel protein-enzyme is called chemiosmosis.The final electron acceptor of the hydrogen ions and electrons is oxygen, thus the products of oxidative phosphorylation are H2O and ATP.Oxidative phosphorylation is employed in aerobic respiration, which is carried out on the plasma membrane of prokaryotic cells, but on the inner membrane of the mitochondrion in eukaryotic cells.

 

A third process for producing ATP, which occurs in photosynthetic autotrophs, is called photophosphorylation.Photophosphorylation takes place in plants, algae, cyanobacteria, and some eubacteria.However, since these organisms are autotrophs, they are not causative agents of infectious diseases.Therefore, photophosphorylation is discussed in courses such as cellular and molecular biology, botany, phycology, general microbiology, and bacteriology.

 

Chemoheterotrophic organisms may be divided into three groups, those that must always utilize aerobic respiration, and therefore, get most of their ATP by oxidative phosphorylation; those that must always utilize fermentation and get all of their ATP by substrate phosphorylation; and those that can do both.Animals, including humans, belong to the first group.Causative agents of infectious diseases come from all three groups.Since the final electron acceptor in oxidative phosphorylation is oxygen, while the final electron acceptor in substrate phosphorylation is an organic compound, we can distinguish among these three groups by their need for oxygen.

 

Strict, or obligate, aerobes must have oxygen to grow, and cannot grow without it.They will grow only on the surface of a liquid growth medium (broth).Strict, or obligate, anaerobes can only grow deep in the broth culture where oxygen cannot reach them because they are inhibited or even poisoned by oxygen.Facultative anaerobes can grow in the presence of oxygen using aerobic respiration, or in the absence of oxygen using fermentation.Most causative agents of infectious disease are facultative anaerobes, but many are obligate anaerobes, and others are strict aerobes.

 

The nutrient most frequently used by most organisms as a source of energy through catabolism of that nutrient is glucose, a six-carbon-atom sugar or hexose.The catabolism of glucose is called glycolysis (glyco=glucose, lysis=break down), and glycolysis occurs in all catabolic schemes, whether the organisms are strict aerobes, facultative anaerobes, or obligate anaerobes.

 

The starting substance, or substrate, for glycolysis is usually glucose, but can be any six-carbon simple sugar or hexose that can be readily isomerized into glucose.So, we say that the substrate for glycolysis is glucose.Two additional reactants that are required for glycolysis to proceed are ATP and a carrier molecule, called NAD, which carries hydrogen ions and electrons.Two ATP molecules are required to make the hexose (6-carbon-atom sugar) molecule into a hexose biphosphate molecule, which is unstable, and immediately splits into two triose (3-carbon-atom sugar) phosphate molecules.Then, two NAD molecules are used to accept the hydrogen ions (H+) and electrons (e-) from the

two triose phosphate molecules, causing the triose phosphate molecules to take on additional phosphate to become triose biphosphate molecules.These two triose biphosphate molecules give up one phosphate immediately to two ADP molecules to synthesize two ATP molecules.After a dehydration step in which the two triose phosphate molecules lose one molecule of water each, the phosphate is rearranged to make two high-energy, or unstable, triose phosphate molecules which give up their phosphates to two more ADP molecules to synthesize two ATP molecules.The final organic product is called pyruvic acid. (Some textbooks call it pyruvate, which is the name of itís ionic form when it is in solution in the cytoplasm.)

 

Summarizing glycolysis:

a)      The starting organic compound or substrate is glucose, a hexose, which is a 6-carbon-atom monosaccharide.

b)      Additional reactants in the sequence of reactions are 2 ATP, 2 NAD, and 4 ADP molecules.

c)      The products of glycolysis include 2 ADP, 2 NADH + H+ (reduced NAD), 2 H2O, 4 ATP and 2 Pyruvate molecules.

d)      The net output of ATP by glycolysis is 2 ATP molecules per molecule of glucose broken down by the process.

 

Thus, glycolysis yields 2 molecules ofATP from a molecule of glucose, but it also creates some problems for the cell:

1)      The pyruvate molecules are toxic and must be converted to other molecules to be removed from the cell.

2)      The NADH + H+ must be re-oxidized back into NAD

3)      Something must accept the electrons and hydrogen ions (H+)

 

Cells solve these problems in several ways:

1)      Most eucaryotes and many prokaryotes carry out aerobic respiration, whereby

a)      the pyruvate is converted to carbon dioxide (CO2) and water (H2O), through the Krebs cycle,

b)      the NADH + H+ is oxidized to NAD through the electron transport chain,

c)      the electrons and hydrogen ions are accepted by oxygen through chemiosmosis.

d)      34 to 36 additional ATP molecules are produced.

 

2)      Some eucaryotes and many procaryotes carry out fermentation, whereby

a)      the pyruvate is converted to another organic acid or an alcohol during which

b)      the NADH + H+ is oxidized to NAD, and

c)      an organic compound accepts the electrons and hydrogen ions,

d)      no additional ATP is produced.

 

Organisms that carry out strictly aerobic respiration require oxygen and are called obligate aerobes.

Organisms that carry out strictly fermentation are obligate anaerobes.

Organisms that can carry out both are called facultative anaerobes.

 

Anabolic Reactions essentially are the reverse ofthese catabolic sequences, utilizing the energy from ATP to synthesize polysaccharides from monosaccharides, polypeptides from amino acids, nucleic acids from nucleotides, and fats from two-carbon units called acetyl groups derived from excess pyruvate.

 

Anerobic

Glycolysis

Reactants

Products

Comments

Net

1 Glucose

2 pyruvic acid

2 water

 

 

2 ATP

4 ADP

4 ATP

2 ADP

Substrate-level Phosphorlation

2 ATP

2 NAD+

2 NADH + H+

Sent to ETC if respiration

Sent to fermentation if not

2 NADH+

†††††††††††††††††††††††††††††††††††††††††††††††††††

Anerobic or Aerobic

Fermentation Completion

Reactants

Products

Comments

Net

2 Pyruvic acid

2 Lactic acid

or

2 Ethanol + 2CO2

Anerobic

 

Aerobic

0 ATP

2 NADH

2 NAD

 

 

 

Aerobic

Krebs cycle

Reactants

Products

Comments

Net

2 Pyruvic acid

6 CoA

6 CO2

 

 

2 ADP

2 ATP

Substrate-level Phosphorlation

2 ATP

8 NAD+

8 NADH + H+

Sent to ETC

8 NADH+

2 FAD

2 FADH2

Sent to ETC

2 FADH2

 

Electron Transport Chain (ATP synthesis is called chemiosmosis)

Reactants

Products

Comments

Net

10 NADH

10 NAD

30 ATP

Oxidative Phosphorlation

30 ATP

2 FADH2

2 FAD

4 ATP

Oxidative Phosphorlation

4 ATP

2H + O

Water

 

 

 

Scientist donít completely understand how enzymes lower the activation energy of chemical reactions, the general sequence is as follows:

 

Coenzymes come from vitamins.B vitamins are mentioned.

 

Fermentation = Glycolysis + Fermentation Completion = 2 ATP

Respiration = Glycolysis + Krebs + ETC= 36 or 38 ATP

 

In eucaryote cells, 38 ATP Ė 2 ATP to get Acetyle in the cell = 36 ATP

In prokaryote cells = 38 ATP

 

All organisms, including microbes, can be classified metabolically according to their nutritional pattern Ė their source of energy and their source of carbon