Cellular Cellular RespirationRespiration

The Big Question:The Big Question: Why do we Why do we breathe?breathe?

• Living systems require Living systems require energyenergy

• This energy comes This energy comes from the breakdown of from the breakdown of food moleculesfood molecules

• The breakdown The breakdown requires Orequires O22 and and

releases COreleases CO22 as a as a

waste productwaste product

• SO, we breathe in OSO, we breathe in O22

and exhale COand exhale CO22..

The overall reaction for Cellular The overall reaction for Cellular Respiration:Respiration:

*As we go through this whole process, note where *As we go through this whole process, note where reactantsreactants are used and are used and productsproducts are made. are made.

WarningWarning: the steps of cell respiration are long and tedious…but with good reason!: the steps of cell respiration are long and tedious…but with good reason!

CC66HH1212OO6 6 + 6 O + 6 O2 2 6 H 6 H22O + 6 COO + 6 CO22 + + 36 ATP36 ATP glucose oxygen gasglucose oxygen gas water carbon dioxide water carbon dioxide energy energy

Transferring energy means Transferring energy means moving electronsmoving electrons

• When bonds are broken and remade, electrons are transferred. – This is represented below in the loss of and gain of hydrogen atoms, which are

made up of an electron and an H+ (hydrogen proton)

• A molecule that gains electrons is reduced and a molecule that loses electrons is oxidized.– Just remember OiL RiG:

Oxidation is a Loss of electrons, Reduction is a Gain of electrons.

Cell respiration is about transferring energy from glucose to ATP and…

Redox Redox (reduction-oxidation)(reduction-oxidation) reactions are always reactions are always

paired!paired!Ele

ctr

on

Donor

Electr

o

n Acc

epto

r

Oxidizing a sugar molecule and Oxidizing a sugar molecule and reducing oxygen releases a great reducing oxygen releases a great

deal of energydeal of energy• You could literally burn sugar – reducing oxygen to H2O and oxidizing

C6H12O6 to CO2 – BUT this releases energy very rapidly in the form of heat. This means that none of the energy from the sugar gets stored…

• So instead we release the energy a little bit at a time and stockpile it to make a source of energy the cell can readily use: ATP!

Let’s review…Let’s review…

• Objective of Cellular RespirationObjective of Cellular Respiration: To harvest the energy stored in food and transfer to a molecule that we can use – ATP

• Where is the energy stored? In bonds, in the form of high energy electrons. So we need to get these high energy electrons from food to ATP

• When electrons are transferred, it’s called a redox reaction. The molecule that loses the electrons is oxidized and the molecule that gains the electrons is reduced.

How is energy picked up & How is energy picked up & transferred in cell transferred in cell

respiration?respiration?Energy Carrying MoleculesEnergy Carrying Molecules (“Electron Taxi’s”) (“Electron Taxi’s”)

• NAD+ (nicotinamide adenine dinucleotide)

A coenzyme derived from niacin, carries electrons in cell respiration

NAD+ + H+ + 2e- NADH

• FAD++ (flavin adenine dinucleotide)A coenzyme derived from riboflavin, also carries electrons in cell respiration

FAD++ + 2H+ + 4e- FADH2

And the final electron taxi is…And the final electron taxi is…ATP!ATP!

ATP (adenosine triphosphate) is found in all cells

• made of adenine, a ribose sugar, and 3 phosphate groups (PO4-)

• a great deal of energy is stored in unstable bonds between the phosphates…this energy can be transferred by attaching a phosphate to other molecules (a process called phosphorylation)

Overview of Cellular Overview of Cellular RespirationRespiration

It occurs in THREE main stages… Note the LOCATION of each.It occurs in THREE main stages… Note the LOCATION of each.

Glycolysis OverviewGlycolysis Overview

•Glycolysis has an Glycolysis has an energy investment energy investment phase and an energy phase and an energy payoff phase – it payoff phase – it takes some ATP to takes some ATP to get it running but get it running but you end up with more you end up with more than you put in.than you put in.

•Here you see •Here you see glucose being broken glucose being broken down into two G3P down into two G3P molecules, USING molecules, USING ___________.___________.

Glycolysis details (part one)

two ATP

•Here’s the Here’s the payoff. The first payoff. The first part “costs” 2 ATP part “costs” 2 ATP and here we MAKE and here we MAKE ___________, ___________, giving us a net of giving us a net of ___________.___________.

We also transfer We also transfer our first our first electrons to NADelectrons to NAD++, , forming NADH forming NADH (which can then (which can then dump these dump these electrons into the electrons into the electron transport electron transport chain)!chain)!

Glycolysis details (part two)

four ATP

TWO ATP

Glycolysis, in a nut shell…Glycolysis, in a nut shell…

• Converts 6-carbon glucose to two 3-carbon pyruvic acids

• Produces 2 ATP (net) and 2 NADH• Occurs in the cytoplasm• Does not require oxygen as a reactant!• Prokaryotic cells (bacteria) and eukaryotic

cells all perform glycolysis• Q: Where is most of the energy (from

glucose) after glycolysis? ______________the pyruvic acid!!

Glycolysis, the movieGlycolysis, the movie

Before we enter the next step Before we enter the next step (Krebs (Krebs

cycle),cycle), Pyruvic Acid must be Pyruvic Acid must be “groomed”“groomed”

(this happens only when O2 is present in the cell)

Step 2: The Krebs cycleStep 2: The Krebs cycle• Occurs inOccurs in the matrix of the

mitochondria (see next slide)

• Involves a huge enzyme complex

• The pointThe point – to finish completely oxidizing what’s left of glucose by transferring the electrons and H+ to NAD+ and FAD

• ProductsProducts per one Acetyl-CoA: 3 NADH, 1 ATP, and 1 FADH2 and also CO2 as waste.

• RememberRemember – there are 2 Acetyl-CoA’s per glucose!

Here it is – the “powerhouse” Here it is – the “powerhouse” of the eukaryotic cellof the eukaryotic cell

• MatrixMatrix: where the Krebs cycle occurs

• Intermembrane Intermembrane space:space: where the H+ will be pumped during the next stage of cellular respiration

• Outer membraneOuter membrane is pretty porous – inner membrane is VERY selective

Krebs cycle in excruciating Krebs cycle in excruciating detaildetail

(don’t sweat the details here…just look at what goes in & what (don’t sweat the details here…just look at what goes in & what comes out)comes out)

Krebs cycle, the movieKrebs cycle, the movie

Checkpoint QuestionsCheckpoint Questions::

•How many ATP have we made so far?•How many ATP have we made so far? (from glycolysis & Krebs Cycle)(from glycolysis & Krebs Cycle) __________

•How many more ATP do we have to •How many more ATP do we have to make? ____make? ____•Where is most of the energy (from •Where is most of the energy (from glucose) after the Krebs cycle?glucose) after the Krebs cycle? _________________ _________________

C6H12O6 + 6 O2 6 H2O + 6 CO2 + 36 ATP

Remember our overall reaction? Which reactants have we used already? Which products have we made already? What is left?

four

32

NADH & FADH2

So what do we do with all of the So what do we do with all of the electrons now on NADH & FADHelectrons now on NADH & FADH22??

( (Time for step 3!!)Time for step 3!!)

Electron Transport Chain & Electron Transport Chain & ChemiosmosisChemiosmosis

• • The The ETC ETC is a series of electron transport proteins embedded in the is a series of electron transport proteins embedded in the innerinner mitochondrial membrane mitochondrial membrane

• • This protein chain passes down energy in the form of electrons This protein chain passes down energy in the form of electrons which originate from which originate from NADHNADH & & FADHFADH22

• In an electron transport system, electrons pass from carrier molecule In an electron transport system, electrons pass from carrier molecule to carrier molecule through a series of oxidation-reduction reactions. to carrier molecule through a series of oxidation-reduction reactions. During each transfer, some energy is released.During each transfer, some energy is released.

• As the electrons progress, some of their energy is used by the • As the electrons progress, some of their energy is used by the carriers (you will see how shortly). The progressively lower-energy carriers (you will see how shortly). The progressively lower-energy electrons are “pulled” down the chain because each successive carrier electrons are “pulled” down the chain because each successive carrier is is more electronegativemore electronegative than the carrier before it! than the carrier before it!

• During each of the electron transfers, the energy released is used to pump H+ ions against their concentration gradient, into the mitochondrial intermembrane space.

• The end result is a high concentration of H+ in the intermembrane space. Now which way do they want to diffuse?

You know this stuff…You know this stuff…• Diffusion of something will occur down its concentration

gradient and towards equilibrium.• Diffusion is favorable – and thus it releases energy• When H+ diffuses back into the matrix through the

protein ATP synthase, it releases energy and this energy drives the production of ATP from ADP and Pi (Pi = inorganic Phosphate, PO4

-)

• So here we’re coupling an energy-releasing process (H+ diffusing into the matrix) with an energy-consuming process (making ATP from ADP). These coupled processes are called Chemiosmosis.

• Wait! What about those traveling electrons?? At the end of the electron chain, the final electron acceptor is the very electronegative O2, which combines with a pair of H+ in the matrix to form _____. H2O

Electron transport, the Electron transport, the moviemovie

Argh, poisons attack Argh, poisons attack mitochondria!!mitochondria!!

They kill by stopping ETC and They kill by stopping ETC and Chemiosmosis.Chemiosmosis.

Glycolysis gives 2 ATP. Add Krebs and Glycolysis gives 2 ATP. Add Krebs and Chemiosmosis and you’ve got Chemiosmosis and you’ve got 36 - 38 ATP36 - 38 ATP

total.total.

So, So, whywhy do you need do you need oxygen??oxygen??

It’s all about the It’s all about the electron transport chain!electron transport chain!

What happens when the oxygen What happens when the oxygen runs out? Depends on the runs out? Depends on the

organism!organism!

Sometimes, there Sometimes, there areare other ways of other ways of getting energy…getting energy…• When OXYGEN is NOT readily available to allow Cellular

Respiration (an aerobic process) to occur, Pyruvic Acid can follow a different chemical pathway after glycolysis: Fermentation.

• There are two types of fermentation, alcoholic and lactic acid fermentation.

Alcoholic fermentationAlcoholic fermentation• Occurs in some bacteria, fungi, and yeast.

• Allows organism to still produce some ATP (but not as much as with electron transport chain). How? Remember 2 net ATP made during Glycolysis…but Glycolysis soon stops due to lack of NAD+!

• Fermentation step simply replenishes supply of NAD+ by accepting 2 electrons and an H+ from each NADH made in glycolysis. Glycolysis continues with NAD+ produced in fermentation.

• Several commercial uses to this process: Making bread, beer & wine!

Lactic acid fermentationLactic acid fermentation• Occurs in some bacteria, plants and animals.

• Again, allows organism to still produce some ATP by simply replenishing supply of NAD+ needed for Glycolysis to continue.

• Lactic acid is produced in muscles during rapid exercise when there is not enough oxygen to carry out aerobic respiration. Lactic acid is toxic to muscle tissue however -- causes “burn” or soreness.

• Eventually lactic acid flushed away from muscle tissue, taken to liver where converted back to pyruvic acid.

We eat – and it all comes We eat – and it all comes down to making ATPdown to making ATP

What do we do What do we do with ATP and with ATP and some of the some of the byproducts of byproducts of catabolism catabolism (breakdown)?(breakdown)?

Make more Make more biomolecules!biomolecules!!!