Cellular Respiration

 How organisms convert food into usable energy

The Overall Process

 Organic molecule + Oxygen → Carbon dioxide + Water + Energy + O 2  C 6 H 12 O 6 + 6O 2 (ATP and heat) → 6CO 2 + 6H 2 0 + Energy  Glucose is a very high energy molecule, carbon dioxide and water are not.

Cellular Respiration

• Cellular respiration occurs in ALL eukaryotes (Plants, animals, fungi, protists) and some bacteria • Glycolysis (the first step) occurs in ALL organisms

ATP

 Energy from food is not used directly  Energy is transferred in the

form of ATP

 Food is broken down and the energy released forms ATP, storing energy in the bonds between phosphate groups

ATP is Recycled

ATP is formed by combining ADP and Pi. Energy is released when ATP is split.



Glucose is Oxidized During Cellular Respiration

Glucose loses electrons to Oxygen, which can then bind with H ions and form water  Molecules with lots of C-H bonds are good sources of fuel, as they have lots of energy and electrons  i.e. fat

Remember Activation Energy, Ea?

 Glucose releases a lot of energy when it is combusted  However, this doesn’t happen spontaneously due to Ea  Enzymes lower this energy of activation and allow the reaction to occur in many steps

Electrons Do Not Pass Directly to O 2

 Electrons are stripped from glucose one by one  They are passed to NAD+ to form NADH

NADH delivers electrons to the ETC

 Electrons travel down the Electron Transport Chain  Each step of the ETC is more electronegative (wants the electrons more!)  Small amounts of energy are released in each step  Electrons eventually reach Oxygen (remember Oxygen is one of the most electronegative elements!)

Why So Many Steps?

•Glucose contains a lot of energy!

•If all of the energy is released at once, you would get an explosion •Lots of energy is lost as light and heat

Cellular Respiration

3 Parts of Cellular Respiration

 Glycolysis   Glucose is oxidized into Pyruvate Some ATP is made, along with some NADH  Krebs Cycle  Decomposes Pyruvate to Carbon Dioxide, making some ATP   Produces NADH and FADH 2 ETC/Oxidative Phosphorylation  Produces FADH 2

lots

of ATP from NADH and

Glycolysis

 Literally means “sugar splitting”  Takes place in

cytosol



Does not require mitochondria

Net Effects of Glycolysis

 Glucose → 2 Pyruvate + 2 ATP + 2NADH



ATP Synthesized via Substrate-level Phosphorylation

Phosphate group is transferred from the sugar to ADP by an enzyme, forming ATP

Pyruvate → Acetyl CoA

 Before entering the Krebs cycle, Pyruvate releases a Carbon Dioxide molecule  The remaining 2 carbons are oxidized to form acetate, producing NADH  They then combine with a molecule called Coenzyme A to form Acetyl CoA

The Krebs Cycle

 Acetyl CoA enters the Krebs Cycle (aka Citric Acid cycle)

For every glucose...

 2 Acetyl CoAs enter the Krebs cycle   4 CO 2 molecules are released 6 NAD+ molecules are reduced to NADH  2 FADH 2 molecules are formed from FADH+  2 ATP molecules are formed from ADP and Pi via Substrate-Level phosphorylation

Krebs Cycle

In even more detail...

Glycolysis + Krebs

From 1 Molecule of Glucose we have created:

2ATP + 2NADH + 2NADH

(glycolysis) (Pyruvate → Acetyl CoA) +

6NADH + 2 FADH2 + 2ATP

(Krebs cycle) =

10NADH + 4ATP + 2FADH 2



Most ATP is Produced via the Electron Transport Chain

A series of proteins, each is more electronegative than the protein before it  So electrons give off a little energy each time as they are passed down the chain  Electrons eventually end up at O 2

The Mitochondrial ETC

 Found in the inner membrane (cristae)

NADH and FADH 2 Hand off Electrons to the ETC

 NADH and FADH2 drop off their electrons at different points in the ETC  NADH drops off electrons to FMN at the top of the chain  FADH2 drops off electrons a little ways into the chain  FADH 2 will produce less ATP

Chemiosmosis

 Each time an electron moves, an H+ ion is pumped across the membrane  This creates an electrochemical gradient as there is a much greater concentration of H+ ions in the intermembrane space  The only way H+ ions can diffuse into the matrix is through ATP synthase

Pumping Protons

ATP Synthase

Mechanism of ATP Synthase

 Diffusing protons turn a cylinder (like water pushing a mill)  This causes the cylinder to essentially spin – changing the shape of the enzymatic regions of the protein  This activates the enzyme, synthesizing ATP

ATP Synthase produces ATP as H+ ions diffuse through it

Oxygen reduced to Water

 Electrons at the end of the chain are donated to oxygen.  Oxygen, with these new electrons, can combine with H+ ions to form water.  4H + + 4e + O 2 → 2H 2 O

For each molecule of Glucose...

 34 ATPs are synthesized via oxidative phosphorylation  Each NADH produces 3 ATP  10NADH*3ATP = 30 ATP  Each FADH2 produces 2 ATP  2FADH2*2ATP = 4 ATP 

Total = 34 ATP (roughly)



(plus 6 molecules of Water)

Review of Respiration

Chemical Summary

C 6 H 12 O 6 + 6O 2 + 38ADP + 38P i → 6CO 2 + 6H 2 0 + 38 ATP

Efficiency of Cellular Respiration

 40% of the energy is converted to ATP  Although it may sound low, this is a remarkably high amount  Cars at the most convert about 25% of their fuel to usable energy  The rest of the energy is lost as heat per the 2 nd Law of Thermodynamics (entropy!)

Cellular Respiration

• Occurs in all eukaryotes and some bacteria • Glycolysis occurs in

cytosol

ALL cells of • The rest of respiration occurs in

mitochondria

• Krebs cycle in the

mitochondrial matrix

• ETC is in the

inner membrane

Review

•

Glyolysis

– Sugar split, forming ATP and NADH • •

Krebs cycle

– sugar fully broken down to CO 2 FADH 2 forming NADH and

Oxidative phosphorylation –

lots of ATP made via chemiosmosis

Aerobic Respiration

 The process we have learned is aerobic respiration  Oxygen is necessary  Glycolysis → Krebs Cycle → Oxidative Phosphorylation  Lots of ATP produced

Anaerobic Respiration

 ATP can be made without oxygen (just not as much)  Glycolysis makes 2 ATP  NAD+ can oxidize glucose to pyruvate, generating ATP 

But

we must regenerate NAD+ as it is converted to NADH in glycolysis  This is solved by fermentation

Alcohol Fermentation

 In alcohol fermentation pyruvate is converted to ethanol, regenerating NAD+  Yeast are used to make alcoholic beverages  Generates CO 2

Lactic Acid Formation

 Muscle cells can generate some ATP without oxygen  Glycolysis occurs  Fermentation converts pyruvate to lactate, regenerating NAD+

 

Lactic Acid Build-Up

When muscle cells cannot get oxygen fast enough (i.e. when you're sprinting) Lactic Acid Fermentation occurs Build up of lactic acid is what makes muscles feel heavy and tired   Lactic acid is carried to liver where pyruvate can be regenerated Lactic acid is also important in the making of cheese and yogurt

Glycolysis Evolved Early

 Glycolysis evolved long before oxygen was present in the atmosphere  All cells undergo glycolysis  Aerobic respiration provides much more ATP, but some organisms can survive only by fermentation  Some organisms (and cells) can switch switch between fermentation and aerobic respiration  i.e. muscle cells  If oxygen is present, pyruvate enters Krebs cycle, if not it heads to fermentation

Other Sources of Fuel



Starch

and

glycogen

can be broken down to glucose 

Proteins

can be converted to intermediates of respiration 

Fatty Acids

can be broken down to 2 carbon fragments that enter the Krebs cycle via Beta Oxidation  Provides lots of ATP

Feedback Inhibition

 When excess ATP is present, it shuts off glycolysis, preventing accumulation of ATP and preserving energy stores