L18: Catabolism

Question 1

Photo autotrophic

Answer 1

Photosynthetic

Use sunlight to produce chemical energy (CO2 as C source)

Example: cyanobacteria, photosynthetic bacteria, microalgae

Question 2

Chemolithoautotrophic organisms

Answer 2

Oxidise inorganic compounds -> produce chemical energy (CO2 as C source)

Some specialised bacteria

Question 3

Heterotrophic organisms

Answer 3

Oxidise organic compounds (originally produced by autotrophic organisms) to harvest chemical energy (and C)

Fungi, protozoans and many bacteria (and all animals)

Question 4

Complementary processes in heterotrophic organisms

Answer 4

Catabolism

Anabolism

Question 5

Catabolism

Answer 5

Larger more complex molecules are broken down into smaller, simpler molecules -> release energy

Fuelling reactions supply ATP, reducing power and precursor metabolites

Question 6

Anabolism

Answer 6

Synthesis of more complex molecules from simpler ones with input of energy

Question 7

How is energy generated during catabolism?

Answer 7

Degradation of fuel molecules during catabolism -> releases energy ‘stored’ in chemical bonds

Cells harvest energy in form of e

Energy is harvested in redox reactions -> make ATP

Question 8

Redox reactions

Answer 8

Involve pair of e carrier compounds (redox)

Donor: becomes oxidised

Acceptor: becomes reduced

Reduced acceptor can then act as donor and original donor can act as acceptor

Question 9

Electron transport chains (ETCs)

Answer 9

E carrier compounds can be linked in cascade through which e is passed(ETC)

Important e carrier pairs involved in catabolism: NAD+ (nicotinamide adenine dinucleotide)/NADH, FAD (flavin adenine dinucleotide)/FADH2, also ubiquinone (coenzyme Q), flavoproteins, cytochromes

Question 10

ATP

Answer 10

The universal energy currency

High energy molecule because of energy required to maintain bonds between bulky, -vely charged phosphate groups

Energy harvested from catabolic reactions: stored in ATP and released to drive energy-requiring cellular activities

Link between catabolism and anabolism

Question 11

Exergonic

Answer 11

Energy yielding reactions

Question 12

Endergonic

Answer 12

Energy requiring reactions

Question 13

Catabolism involving aerobic respiration

Answer 13

Macromolecules are degraded to smaller, simpler molecules (glycolysis, TCA cycle)

Some ATP is produced during glycolysis and TCA cycle (substrate level phosphorylation)

More ATP is produced in ETC from reducing generated in glycolysis & TCA cycle (oxidative phosphorylation: harvest of e in redox reactions, ETC & proton motive force)

Question 14

Glycolysis

Answer 14

Conversion of glucose to pyruvate

1. Embden-Meyerhof Payhway: most common pathway. Generates pyruvate from glucose in multi-step process. Occurs in all major groups of microbes. Functions in presence or absence of O2. Generates energy (ATP, reducing power) & compounds for biosynthesis
2. Pentose Phosphate Pathway: most microbes have this alternative/complementary pathway -> pyruvate
3. Entner-Doudoroff Pathway: some G-ve bacteria use this pathway -> pyruvate. Soil bacteria e.g. Pseudomonas, Rhizobium. Not used in prokaryotes or most G+ves

Question 15

TCA cycle: conversion of pyruvate to CO2

Answer 15

1. Pyruvate is oxidised to acetyl-CoA (2-C)
2. Acetyl-CoA is condensed with oxaloacetate to form citric acid (6-C)
3. Citric acid is oxidised and CO2 is released

Generate energy (ATP and reducing power). Compounds for biosynthesis formed. Oxaloacetate is regenerated

Most microbes have TCA cycle: aerobic bacteria, most fungi and algae, free living protozoa

Some microbes do not: TCA cycle enzymes may be present and involved in making biosynthesis intermediates

Question 16

ATP from substrate level phosphorylation

Answer 16

Phosphate group from phosphate-containing intermediate of glycolysis pathway or TCA cycle -> transferred directly to ADP to yield ATP (occurs only during some reaction steps in glycolysis and TCA cycle.e.g. In final step in glycolysis)

Question 17

Harvesting electrons as reducing power

Answer 17

E are harvested by electron carriers (EC) during degradation of fuel molecules

E are packets of energy, and ECs use them to reduce other compounds in redox reactions: to harvest e NAD(and FAD) form redox pairs with fuel molecules during fuel molecule degradation, e transferred to NAD -> NADH, e transferred to FAD -> FADH2, ECs acquired reducing power from fuel molecules

Question 18

Harvesting e as reducing power example: NAD acquires reducing power when it reacts with malate in TCA cycle

Answer 18

e is transferred from malate to NAD -> malate is oxidised to oxaloacetate

NADH is reduced to NADH (reducing power transferred from malate to NADH)

NADH has reducing power able to be transferred to an ETC

Question 19

ATP from oxidative phosphorylation

Answer 19

Process by which ATP is synthesised as result of e transport driven by oxidation of chemical energy source

1. e harvested by ECs during fuel molecule degradation enter an ETC
2. Passage of e through ETC by series of interlinked EC pairs
3. Generation of proton motive force from ETC
4. Formation of ATP using proton motive force

Involves redox

Question 20

Step 1 of ATP from oxidative phosphorylation

Answer 20

Harvest of e by ECs during fuel molecule degradation:

NAD & FAD -> redox pairs with fuel molecules during degradation

e are transferred to NAD and FAD (producing NADH and FADH)

NADH and FADH transfer e to ETC

Question 21

Step 2 of ATP from oxidative phosphorylation

Answer 21

(Passage of e through ETC)

ECs transfer e to ETC

e flow to redox pairs (ECs) having progressively more +ve reduction potentials

e are eventually transferred to terminal e acceptor

Difference in reduction potential between EC redox pair and terminal e acceptor redox pair determines free energy released

Terminal e acceptor redox pair in aerobic respiration O2 is final acceptor and is reduced to H2O

Question 22

Step 3 in ATP from Oxidative Phosphorylation

Answer 22

(Generation of proton motive force from ETC)

Energy released in ETC is used to pump protons (H+) to outside of membrane

Charge difference set up across membrane (i.e potential energy)

Potential energy from chemical and electrical potential differences: proton motive force

Question 23

Step 4 in ATO oxidative phosphorylation

Answer 23

(Formation of ATP using proton motive force)

Electrochemical potential energy released during flow of protons back across membrane down charge and proton conc gradients

Energy used to form ATP, catalysed by ATP synthase embedded in membrane: a rotary motor

Question 24

How much energy does aerobic respiration yield?

Answer 24

Glycolysis (substrate level phosphorylation): ATP yield = 2

TCA cycle (substrate level phosphorylation): ATP yield = 2

ETC (Oxidative phosphorylation): ATP yield=28

-> total ATP yield (in eukaryotes) during aerobic respiration: ATP yield=32

Question 25

Catabolism by fermentation

Answer 25

Key requirements for maintenance of glycolysis pathway is presence of EC NAD

NAD accepts e during fuelling reactions of glycolysis -> formation of a phosphate transferring intermediate essential to formation of ATP by substrate level phosphorylation (SLP)

-> glycolysis will cease in absence of supply of NAD to accept e

Question 26

Some heterotrophic microbes do not respire

Answer 26

Some lack on ETC

Sometimes those with ETC find themselves in environment where there are no suitable terminal e acceptors

In both cases has to be way to regenerate NAD

Overcome problem by fermentation (energy yielding process in which organic molecule is oxidised without exogenous e acceptor)

Pyruvate (end product of glycolysis) acts as e acceptor which: regenerates NAD, ensures ATP formation by SLP is maintained, produces variety of fermentation by-products

Question 27

Common microbial fermentatikns

Answer 27

Depending on pathway used, range of alcohols and organic acids can be produced by particular microbes

Named on basis of major alcohol or acid formed

Question 28

How much energy does fermentation yield?

Answer 28

Fermentation only yields ATP by substrate level phosphorylation

Fermentation produces 2 ATP molecules per molecule of glucose compared to approx. 32 ATP yielded by aerobic respiration

Difference implies that microbial growth bu fermentation will be much slower than by aerobic respiration (less ATP per fuel molecule degraded)