Harvesting Energy

CELLULAR RESPIRATION & FERMENTATION

Photosynthesis and respiration provide the energy needed for life

This energy ultimately comes from the sun

RESPIRATION

Harvesting of energy from food molecules

Performed at the cellular level

This energy can then be stored for later use

RESPIRATIONRespiration is a catabolic process: large molecules are broken down and the energy released from bonds is used for:

maintenancegrowth (anabolic process)reproduction

The energy released is transformed into ATP

Summary Equation for Aerobic Respiration

C6H12O6 + 6O2 6CO2 + 6H2O glucose oxygen carbon water

dioxide

What’s happening?

C6H12O6 + 6O2 6CO2 + 6H2O glucose oxygen carbon water

dioxide

Glucose is losing electrons - oxidation

Oxygen is gaining electrons - reduction

Energy released

This doesn’t happen at once

Much energy lost as heat

Energy conserved if smaller reactions take place

STAGES OF RESPIRATION• Aerobic cellular respiration can be divided into three (or

four) main stages:#1 Glycolysis - cytoplasm

#1.5 Transition step

cytoplasm mitochondria

#2 Krebs Cycle - inner

compartment (matrix)

#3 Electron Transport

Pathway - Inner

membrane

GLYCOLYSIS• Occurs within eukaryotic cytoplasm• Multi-step metabolic pathway• Partial oxidation of glucose (6

carbon)• No oxygen required• Products:

– 2 ATP (net)– 2 NADH– 2 pyruvate

(3 carbon)

NADH• The reduced coenzyme NADH is also

produced during cellular respiration– Nicotinamide adenine dinucleotide– High energy molecule– Can be “spent” to make more ATP later

TRANSITION STEP• The pyruvate produced in glycolysis (etc.)

– Enters the mitochondria– Is converted into acetyl CoA (2 carbon)– Enters the Krebs Cycle

• Products:– 2 NADH

– 2 CO2 formed

– 2 acetly CoA

KREBS CYCLE• a.k.a., Citric Acid Cycle • Occurs within mitochondrial matrix• Multi-step metabolic pathway• Remnants of glucose completely

oxidized• Products:

– 2 ATP– 6 NADH

– 2 FADH2

– 4 CO2

GLYCOLYSIS and KREBS

• Several high-energy molecules are produced during glycolysis and the Krebs cycle– 4 ATP– 10 NADH– 2 FADH2

• Most of the energy harvested from glucose is in the form of reduced coenzymes

• However, only ATP is readily usable to perform cellular work

• The Electron Transport Pathway oxidizes NADH and FADH2 to produce more ATP

ELECTRON TRANSPORT PATHWAY

• Occurs within the inner mitochondrial membrane• Electrons are removed from NADH and shuttled through a series of

electron acceptors– Energy is removed from the electrons

with each transfer• This energy is used to make ATP

– NADH 3 ATP

– FADH2 2 ATP

– O2 is the terminal electron acceptor

• ½O2 + 2H+ + 2e- H2O

Generation of ATPChemiosmosis

Electrons attract H+ and pull them through transport proteins to outer-compartment of mitochondria

H+ then diffuse back through ATP synthase channels making ATP andwater

ENERGY YIELD• 4 ATP (glucose, krebs)• 10 NADH 30 ATP

• 2 FADH2 4 ATP (electron transport)

• 38 ATP total

• This total yield depends on different things

THEORETICAL YIELD

• Theoretical yield of 38 ATP not generally reached because:– Intermediates in central pathways

siphoned off as precursor metabolites for biosynthesis

– Electrons of NADH generated in cytosol often shuttled into mitochondria as FADH2

– Each NADH typically yields slightly less than 3 ATP

BURNING OTHER STUFF

• Glucose can be oxidized to yield ATP

• Other biomolecules can also be oxidized to yield ATP– These molecules are

converted to either glucose or to an intermediate in the catabolism of glucose

O2 REQUIREMENT

• ~38 ATP produced per glucose molecule– 34 ATP from ETP

• Requires adequate supply of oxygen

• Under conditions of insufficient oxygen, ATP yields can be severely reduced

What happens when O2 is unavailable?

• Some cells cannot obtain energy when deprived of O2

– e.g., human heart cells

– “Obligate aerobes”

• Some cells normally perform aerobic respiration, but can still obtain energy when O2 is lacking

– e.g., skeletal muscle cells, S. cerevisiae (yeast), E. coli

– “Facultative anaerobes”

• Others do not use O2 to obtain energy

– e.g., Clostridium botulinum, an “obligate anaerobe”

– e.g., Streptococcus pyogenes, an “aerotolerant anaerobe”

FACULTATIVE ANAEROBES

• In the absence of O2, aerobic respiration is impossible– Glycolysis still occurs

• Net ATP production: 2 ATP– 2 is significantly less than thirty-something

• NAD+ is converted to NADH– NADH is not useful to the cell if energy is not extracted– The absence of NAD+ is detrimental to the cell– NADH must be converted back to NAD+

» “Fermentation”

FERMENTATION• NADH is produced during glycolysis

– Energy in NADH cannot be used– NADH must be oxidized to replenish NAD+

• No payoff– NADH is oxidized to NAD+

– Pyruvate is reduced to _______• (Different substances in different

organisms)• Human muscle: pyruvate lactic acid

• Yeast: pyruvate ethanol & CO2

• Other cells many other molecules– Total energy yield of fermentation

is the 2 ATP generated in glycolysis

FERMENTATION• Skeletal muscles normally undergo aerobic respiration

• During strenuous exercise, O2 may be rapidly depleted

– Fermentation can continue to provide energy

– Pyruvate lactic acid• Lactic acid builds up• Buildup causes muscle

fatigue & pain• Lactic acid ultimately

removed

FERMENTATION

• Saccharomyces cerevisiae (yeast) normally undergoes aerobic respiration

• O2 is not always available

– Fermentation can continue to provide energy

– Pyruvate ethanol & CO2

• Ethanol ultimately toxic

FERMENTATION• Many other organisms also undergo fermentation

– Some are facultative anaerobes– Some are obligate fermenters

• Pyruvate is converted into a host of different molecules by a host of different organisms– Many of these molecules

are commercially important