BIO 208 - Microbiology - Review of Important Concepts on Metabolism

Review - Important Concepts for Lectures over Metabolism

I assume that you have had an introduction to the basics of metabolism in an introductory biology course. The metabolism you learned was probably entirely focused on the types of metabolism that animal (maybe plant) cells carry out --aerobic respiration; perhaps you were exposed to lactic acid fermentation (When muscles are working very hard, they may be temporarily depleted of oxygen, muscle cells can perform lactic acid fermentation for a short period of time. The lactic acid end products are secreted by the muscle cells into your tissues, and you feel the lactic acid as muscle soreness). The microorganisms are tremendously more diverse and complex in metabolic patterns than are Eucarya and I want to spend our time emphasizing what microbes can do, not just covering what you have already had in other courses.

So, if you do not remember the basics of metabolism you will need to review. The following pages should serve as a reminder. If it doesn’t all come back to you then read Chapter 5 in the text. If you have not had chemistry you will also need to read Chapter 2.

Review of oxygen tolerance:

· Obligate anaerobe – does not require O₂.

· Aerotolerant anaerobe – does not require O₂ for .

· Microaerophile – needs a little O₂ for metabolism, but less than amount present in the atmosphere.

· Facultative anaerobe – can switch its metabolism based on whether or not O₂ is present.

· Aerobe (obligate aerobe) – requires O₂ for metabolism.

Review of nutritional patterns:

Source of energy Source of carbon

Chemicals CO₂ (used by autotrophs)

organic Organic molecules (-C-C-C-) (used by heterotrophs)

inorganic

Light

Most common combinations of Energy gaining strategy plus Carbon gaining strategy

Chemoorgano heterotrophs

Chemolitho autotrophs

Photo autotrophs

Photo heterotrophs

You should also know

Definitions of metabolism, anabolism, and catabolism

That ATP (Adenosine Tri Phosphate) is made to store energy and used to release energy – it is the energy “currency” for the cell.

Pyruvate is a key intermediate molecule in many catabolic pathways.

Should understand basics of oxidations - reductions

Remember - A loss of an electron is called an oxidation; a gain of an electron is called a reduction (remember as: LEO the lion says GER).

In biological molecules it is usually the entire H atom (electron and proton) that is lost or gained, but not always. Sometimes the electrons are separated from the proton and only the electrons are lost or gained; and sometimes it may be one H atom + 1 electron (from a second H atom) that are lost or gained.

In any pair of molecules you can distinguish which is the oxidized and which is the reduced:

Oxidized state Reduced state:

Contains more oxygen atoms OR Contains fewer oxygen atoms OR

fewer hydrogen atoms AND more hydrogen atoms AND

therefore has fewer electrons and is therefore has more electrons and is

less negative or more positive more negative or less positive

Example pairs:

Glucose Pyruvate

C₆H₁₂O₆ C₃H₄O₃

NAD⁺ NADH

Sulfate Hydrogen sulfide

SO₄ H₂S

All cells need:

1. A source of carbon for making cellular molecules.

There are two strategies for obtaining carbon:

a. Recycle the C already present in some organic (-C-C-) molecule

b. Use CO₂ from the atmosphere

2. A source of energy for performing all cellular work (building molecules, transport across the plasma membrane, locomotion, etc.)

Energy is created by harvesting the electrons present in:

a. Organic molecules.

(specifically the electrons in the H atoms in the molecules)

Hydrogen – showing the proton and electron

like a sugar or an amino acid

b. Inorganic molecules.

electrons in molecules like

ammonia

hydrogen sulfide

The more electrons a molecule has, the more energy the molecule is capable of yielding – so look at glucose compared to hydrogen sulfide – which molecule should yield the most energy? (glucose – 12 H vs. 2 in H₂S)

The electrons that are released when bonds are broken have to go somewhere, so they get passed from the donor (the molecule that you started with that had all the electrons) to intermediate electron carriers.

NAD⁺ is a soluble carrier present in the cytoplasm. It is lacking 1 electron (1 H) and so it can accept 1 electron (1 H). As it accepts the electron, it is reduced to NADH.

Text Box:

Oxidized state

fewer H, fewer e-

more positive (NAD⁺)

Reduced state

more H, more e-

NAD⁺ is in limiting quantities in the cell and it must be regenerated if energy production is to continue.

There are 2 ways to regenerate NAD⁺from NADH :

1. NADH passes the electron to an organic molecule like pyruvate – this process is called fermentation - as NADH loses the electron it becomes oxidized to NAD⁺ again. As pyruvate accepts the electron it becomes reduced to acetic acid or to ethanol, etc., which are excreted from the cell, carrying waste electrons with them. Acetic acid, ethanol, etc. still have electrons, so potential energy is lost in the fermentation strategy.

2. NADH travels to the cytoplasmic membrane and passes the electron off to the electron transport chain. This process is called respiration.

(NADH then becomes NAD⁺ )

The electrons are passed along the chain, generating two types of usable energy along the way – electrochemical gradient and ATP - until they reach a final electron acceptor, an inorganic molecule which can be:
a. oxygen (aerobic respiration) OR		As oxygen accepts electrons it will become reduced to H₂O
b. some other inorganic molecule (anaerobic respiration)	like nitrate	or sulfate
	becomes reduced to nitrite (NO₂)	becomes reduced to hydrogen sulfide (H₂S)

Note – fermentation is NOT anaerobic respiration. By definition respiration requires both an electron transport chain and an inorganic terminal electron acceptor. Fermentation does not employ an electron transport chain and the terminal electron acceptor is an organic molecule. Fermentation takes place in the absence of oxygen, it can occur in anoxic and anaerobic environments, but it is not respiration!

Comparison of Respiration vs Fermentation in Chemoorganotrophs

	Respiration		Fermentation
Initial electron donor:	organic molecule		organic molecule
examples:	carbohydrates, amino acids, lipids		carbohydrates, amino acids, lipids
Intermediary electron carrier(s):	NADH, FADH₂, carriers in the electron transport chain		NADH
Final electron acceptor	inorganic molecule		organic molecule
examples:	O₂	CO₂, NO₃, SO₄	pyruvate
final electron acceptor reduced to:	H₂O	CH₄, NO₂, H₂S	lactic acid, acetic acid, ethanol, etc.
example organisms	Mitochondria, E. coli, Pseudomonas, S. aureus	Methanogens, E. coli, Pseudomonas, Sulfate-reducing bacteria	Bifidobacterium, Lactobacillus, E. coli, Clostridium, Bacteroides
Potential net ATP yield:	as many as 38 if starting with 1 glucose by aerobic respiration with an electron transport chain containing all the cytochromes – but often far fewer than 38 - but still more than 2.		2

Comparison of Respiration in Chemoorganotrophs vs Chemolithotrophs

	Chemoorganotroph		Chemolithotroph
Initial electron donor:	organic (-C-C-) molecule		inorganic molecule
examples:	carbohydrates, amino acids, lipids		hydrogen gas, ammonia, nitrate, hydrogen sulfide
Electron donor oxidized to:	CO₂		water, nitrate, nitrite, sulfuric acid
Final electron acceptor	inorganic molecule		inorganic molecule
examples:	O₂ (aerobic respiration)	CO₂, NO₃, SO₄ (anaerobic respiration)	O₂ (aerobic respiration)
electron acceptor reduced to:	H₂O	CH₄, NO₂, H₂S	H₂O
example organisms	Mitochondria, E. coli, Pseudomonas, S. aureus	Methanogens, E. coli, Pseudomonas, Sulfate-reducing bacteria	Alcaligenes, Nitrosomonas, Nitrobacter, Thiomargarita