Chapter 6 Outline
Metabolism of Microorganisms
Introduction
6.1 Enzymes and Energy in Metabolism
• Enzymes Catalyze All Cellular Reactions
• Enzymes increase the probability that chemical reactions will occur
• Enzymes are not changed by the reactions and can be reused
• Enzyme activity is highly specific
• Enzymes act on substrates
• Enzymes Act through Enzyme-Substrate Complexes
• Enzymes align substrate molecules in such a way that a reaction is
energetically favorable
• Substrates bind to the enzyme at the active site, which is specific to the
substrate
• Enzymes lower the activation energy so a reaction is more likely to occur
• Enzymes weaken chemical bonds in the substrate
• Enzymes can
• be made entirely of protein or contain a complementing substance,
such as:
– a metal ion (cofactor)
– an organic molecule (coenzyme)
• Enzymes Often Team Up in Metabolic Pathways
• A metabolic pathway is a sequence of chemical reactions
• each reaction is catalyzed by a different enzyme
• the product of one reaction serves as the substrate for the next
• Enzyme Activity Is Regulated and Can Be Inhibited
• Feedback inhibition hinders metabolic pathways
• It inhibits an enzyme in the pathway so no product is available to
feed the next reaction
• Other types of inhibition include
• changing the shape of an active site (noncompetitive inhibition)
• blocking an active site (competitive inhibition)
• Energy in the Form of ATP is Required for Metabolism
• ATP (adenosine triphosphate) is the cellular “energy currency,” providing
energy for:
• movement
• cell division
• protein synthesis, etc.
• Energy is released from ATP when the bond holding the last phosphate
group on the molecule is broken, producing:
• adenosine diphosphate (ADP)
• a free phosphate group
• Adding a phosphate group to a molecule is called phosphorylation
• ATP cannot be stored because it is relatively unstable
•
energy must be stored in more stable forms like glycogen or lipids
(in prokaryotes)
6.2 The Catabolism of Glucose
• Glucose Contains Stored Energy That Can Be Extracted
• Energy in glucose is released slowly by converting to ATP through
metabolic pathways
• Cellular Respiration Is a Series of Catabolic Pathways for the Production of ATP
• Cells make ATP by harvesting energy through cellular respiration
• If oxygen is consumed while making ATP, it is aerobic respiration
• if not, it is anaerobic respiration
• Glycolysis Is the First Stage of Energy Extraction
• Glycolysis is the splitting of 1 glucose molecule into 2 pyruvate molecules
• This releases 6 ATP and 2 NADH molecules
• The Krebs Cycle Extracts More Energy from Pyruvate
• The Krebs cycle is also called the citric acid cycle
• Before entering the Krebs cycle, enzymes
• remove a carbon from each pyruvate molecule
• combine the carbon with coenzyme A (CoA) to form acetyl-CoA
– This releases 2 NADH and 2 CO2
• The Krebs cycle is like a constantly turning wheel
• picking up pyruvate molecules from glycolysis
• spitting out carbon dioxide, ATP, NADH, and FADH2
• For each two pyruvate molecules that enter the cycle, the following
molecules are formed
• 4 CO2
• 2 ATP
• 6 NADH
• 2 FADH2
• Oxidative Phosphorylation Is the Process by Which Most ATP Molecules Form
• Pairs of electrons are passed from one chemical substance to another
(electron transport), releasing energy
• The energy released is used to combine phosphate with ADP to form ATP
• The electron transport chain is composed of electron carriers called
cytochromes
• NADH and FADH2 provide the source electrons for oxidative
phosphorylation
• Oxygen accepts the electron pair at the end of the chain, acquires 2
protons, and becomes water
• As the electrons move down the chain they use energy, which is harnessed
to pump protons out of the cell (chemiosmosis)
• The protons outside the membrane build up a concentration gradient
• A channel opens and the protons flow in through a channel in ATP
synthase
• ATP synthase harnesses the energy from the flowing protons to
phosphorylate ADP into ATP
6.3 Other Aspects of Catabolism
•
Other Nutrients Represent Potential Energy Sources
• Many mono-, di-, and polysaccharides can be energy sources for
prokaryotes
• They must all be prepared before being processed by
• glycolysis
• the Krebs cycle
• oxidative phosphorylation
• Chemical bonds in fats store large amounts of energy, making fats good
energy sources
• Cells use proteins for energy when fats and carbohydrates are lacking
• Deamination is the replacement of the amino group in a protein with a
carbonyl group in protein breakdown
• Fatty acids are broken down through beta oxidation
• Anaerobic Respiration Produces ATP Using Other Final Electron Acceptors
• In anaerobic respiration, anaerobes use molecules other than oxygen as the
final electron receptor in the electron transport chain
• Anaerobic respiration produces less ATP than aerobic respiration
• Fermentation Produces ATP Using an Organic Final Electron Receptor
• Fermentation is used when oxygen and other alternative electron acceptors
are unavailable
• Pyruvate can be converted to lactic acid to reform NAD+ coenzymes so
glycolysis can produce ATP from glucose
• Eukaryotes also perform fermentation, such as the yeast used in alcoholic
fermentation to create alcoholic beverages
6.4 The Anabolism of Carbohydrates
• Photosynthesis Is a Process to Acquire Chemical Energy
• In photosynthesis, light energy is converted to chemical energy, which is
stored as an organic compound
• In prokaryotes, it is carried out in the cell membrane, in eukaryotes in
organelles called chloroplasts
• The green pigment chlorophyll a absorbs light energy
• Some bacteria use other pigments, such as bacteriochlorophylls
• Some archaea use bacteriorhodopsin Photosynthesis is divided into two
sets of reactions:
• energy-fixing reactions
• carbon-fixing reactions
6.5 Patterns of Metabolism
• Autotrophs and Heterotrophs Get Their Energy and Carbon in Different Ways
• Autotrophs synthesize their own foods from simple carbon sources like
carbon dioxide
• Photoautotrophs use light as their energy source
• Chemoautotrophs use inorganic compounds as their energy source
• Heterotrophs gain energy and carbon from outside sources
• Photoheterotrophs use light as their energy source and organic
compounds as their source of carbon
•
•
•
Chemoheterotrophs use organic compounds both for energy and
carbon sources
Saprobes feed exclusively on dead organic matter
Parasites feed on living organic matter