From the molecules of life, to the simpler organisms

Paula B. Matheus Carnevali

Part II

Nutrient requirements

Over 95 % of cell dry weight is made up of:

• Carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorous, potassium, calcium, magnesium, and iron.

Electron movement (electron transport chains, oxidation-reduction reactions) provide energy for use in work, and allow molecules’ reduction during biosynthesis

Metabolisms

AutotrophsCO2 sole or Principal

biosyntheticCarbon source

HeterotrophsReduced, preformed,

Organic moleculesFrom other organisms

Carbon source Energy source

ChemotrophsOxidation of organic

Or inorganiccompounds

OrganotrophsOrganic molecules

LithotrophsReduced inorganic

molecules

PhototrophsLight

Electron source

Major nutritional types

Major nutritional types of microorganisms

From Prescott et al,, 2005

What is the energy needed for?

• Chemical work: involves the synthesis of complex biological molecules from much simpler precursors,

• Transport work: requires energy input in order to take up nutrients, eliminate wastes, and maintain ion balance,

• Mechanical work: energy is required to change physical location of organisms, cells and structures within the cells.

Light energy

Phototrophs (photosynthesis), Chemolitotrophs

Chemical energy

Photolithoautotrophs and Chemolithoautotrophs transform CO2 into

biological molecules

Carbon source for Chemoheterotrophs

Free-energy and Equilibrium

ΔG º’ = - 2.303 RT.logKeq

R is the gas constantT is the absolute temperature

•When ΔG º’ is negative, K is greater than 1 and the reaction goes to completion as written = exergonic reaction

•When ΔG º’ is positive, K is less than 1 and the reaction is not favorable (little product will be formed at the equilibrium) = endergonic reaction

Cells energy currency: ATP

Oxidation-reduction reactions

• Electron donor/Electron acceptor

• Equilibrium constant is the Standard reduction potential (Eo) = measure of the tendency of a donor to lose electrons

• Redox couples with more negative reduction potentials will donate electrons to couples with more positive potentials and greater affinity for electrons.

Electron movement and reduction potentials.

From Prescott et al., 2005

When electrons move from a donor to an

acceptor with a more positive

redox potential, free energy is

released

NAD: nicotinamide adenine dinucleotide

Electron movement requires the participation of carriers to transport electrons between different

locations

Photosynthesis

Photosynthetic organisms capture light energy and use

it to move electrons from water (and other electron

donors) to electron acceptors, such as NADP+ , that have more negative

reduction potentials. These electrons can flow back to

more positive acceptors and provide energy for ATP

production.

Enzymes

Enzymes accelerate reactions by lowering the activation energy

An overview of metabolism

Metabolism refers to the sum of the biochemical reactions required

for energy generation and the use of energy to synthesize cell material

from small molecules in the environment

Patterns of energy release

Fermentation the energy substrate is oxidized and degraded without the participation of an exogenous or externally derived electron acceptor

Energy-yielding metabolism can make use of exogenous or externally derived electron acceptors

• Metabolic pathways consist of enzyme-catalyzed reactions arranged so that the product of one reaction serves as a substrate for the next.

• The uniqueness of microbial metabolism lies in the diversity of the sources from which it generates ATP and NADPH.

• Carbohydrates and other nutrients serve two functions in the metabolism of heterotrophic organisms: they are oxidized to release energy, and supply carbon for the synthesis of new cell constituents.

Glycolysis

Glucose + 2ADP + 2Pi + 2NAD+ →

2 Pyruvate + 2ATP + 2 NADH + 2H+

Fermentation

NADH produced in the glycolytic pathway must be oxidized back to NAD+

Pyruvate or one of its derivatives can be used as an electron and hydrogen acceptor for the reoxidation of NADH

This may lead to the production of more ATP

A lot of energy is released

when pyruvate is degraded

aerobically to CO2. The substrate

of the Krebs cycle is acetyl-

CoA

Electron transport chain

The electron transport chain is

composed of a series of electron

carriers that operate together to transfer

electrons form donors, like NADH

and FADH2, to acceptors, such as

O2. Electron transport at these

points may generate proton and electrical gradients.

Oxidative phosphorylationthe process by which the energy from electron transport is used to make ATP

As many as three ATP molecules may

be synthesized from ADP and Pi when a pair of electrons pass

from NADH to an atom of O2

Anaerobic respirationNot as efficient as aerobic respiration

•NO3- + 2e- + 2H+ → NO2- + H2O(Dissimilatory nitrate reduction)

•2NO3- + 10e- + 12H+→ N2 + 6H2O(Denitrification)

•SO42- + 8e- + 8H+ → S2- + 4H2O

Bacterial Growth

Microbial growth curve in a closed system. The growth of organisms reproducing by binary fission can be plotted as the logarithm of the number of viable cells versus the incubation time

Log n

um

ber

of

via

ble

cells

Time

Microbial responses to environmental factors

From Prescott et al,, 2005

The influence of environmental factors on

growth