Chapter 7 - Cellular Respiration and Fermentation

Cellular Respiration

catabolic process which harvests chemical energy from organic compounds

• energy releasing process (exergonic)

Aerobic Respiration

uses oxygen in the breakdown of organic molecules to produce energy

Organic compound + O2 → CO2 + H2O + energy

• oxygen draws in electrons due to its high electronegativity

• uses a variety of biomolecules like carbohydrates, lipids, proteins

• most of the time, glucose is broken down

Anaerobic respiration

uses molecules other than oxygen in the breakdown of organic molecules to produce energy

Principle of redox (what is reducing agent, oxidizing agent, etc.)

• during a chemical reaction, electrons are transferred and organic molecules release stored energy

  • this energy released is used to make ATP

• reducing agent becomes oxidized as it loses electrons (becomes more positive)

• oxidizing agent becomes reduced as it accepts electrons (becomes more negative)

Oxidation in cellular respiration

• glucose (C6H12O6) is oxidized and oxygen is reduced

  • Reducing agent always starts with hydrogen and loses it

  • Oxidizing agent doesn’t start with hydrogen but gains it

• electrons lose potential energy during the transfer

• organic molecules are not consumed instantly due to their high activation energy

stepwise energy harvest in cellular respiration

glucose is harvested in a stepwise process

• electrons are stripped from glucose step by step

• electrons are then transferred along with a proton as a hydrogen atom but are not passed directly onto oxygen

• during the intermediate steps, electrons are passed to electron carriers (coenzymes)

NAD+

Nicotinamide Adenine Dinucleotide

• main carrier in cellular respiration

• oxidizing agent (accepts electrons)

• cycles between NAD+ (oxidized) and NADH (reduced)

• extremely versatile and is involved in many of the redox reaction steps of cellular respiration

Dehydrogenases

specialized enzymes which catalyze the transfer of 2 electrons and 1 proton to NAD+

• a second proton (H+) is also removed which is freefloating in the solution

  • important in generating ATP due to its accumulation

3 steps of electron transfer in cellular respiration

1. Electrons are passed from glucose to NAD+ making NADH

2. NADH transfers the electrons to a series of carriers (in multiple steps)

3. final electron transfer is to oxygen at the end of the electron transport chain

• steps within the ETC release small amounts of energy

• oxygen pulls electrons down the chain due to its high electronegativity

3 stages of cellular respiration

1. Glycolysis

2. Pyruvate Oxidation and the Citric Acid Cycle

3. Oxidative Phosphorylation

What is the goal of cellular respiration

to generate ATP released during electron transfers

• energy released is used to add a phosphate group to ADP

two methods of phosphorylation

1. Oxidative phosphorylation

2. Substrate-level phosphorylation

oxidative phosphorylation

inorganic phosphate is added to ADP to produce ATP at the end of cellular respiration

Substrate-level phosphorylation

phosphate group is transferred from a substrate to ADP

• occurs during glycolysis and the citric acid cycle

How much potential energy in glucose and ATP?

glucose - 686kcal/mol of potential energy (stored in chemical bonds)

ATP - 7.3kcal/mol of potential energy

how many moles of ATP does cellular respiration create and how much energy is that?

32 molecules of ATP x 7.3 = 233.6kcal/mol

Glycolysis

“sugar splitting”

• core process used by all cells

• occurs in the cytosol unlike cellular respiration (in eukaryotes)

• may occur with or without oxygen

• splits a single 6C glucose into two 3C pyruvate molecules

Glucose structure (draw it)

pyruvate structure (draw it)

Energy investment phase of Glycolysis

1. use ATP to create phosphorylated intermediate

2. Alteration of carbon bonds to turn phosphorylated intermediate into fructose

3. fructose gets phosphorylated using energy (transform ATP into ADP)

4. 6C sugar is split into two 3C sugars (G3P and DHAP)

5. Isomerase turns DHAP into G3P, so now there’s two G3P molecules

structures of G3P and DHAP (draw them)

Energy Payoff Phase of Glycolysis

6. two steps

a) G3P is oxidized by transferring electron from NADH to NAD+ where energy is harvested

b) energy harvested is used to add a phosphate group to the oxidized G3P to create a high energy molecule

7. phosphate group is transferred to 2 ADP molecules through exergonic reactions to create 2 ATP molecules

8. enzyme moves phosphate groups to create high energy molecules

9. double bond is created through the removal of water which increases energy of substrate molecules

10. phosphate group is transferred to 2 ADP which creates 2 molecules of pyruvate and ATP

net reactions of glycolysis

• Glycolysis → 2 pyruvate + 2 H2O

• 4 ATP formed - 2 ATP used → 2 ATP

• 2 NAD+ + 4 e- + 4 H+ → 2 NADH + 2H+

how efficient is cellular respiration in terms of extracting energy from glucose

• less efficient since only 1/3 of energy is left

What component is the oxidizing agent in the basic cellular respiration equation?

Oxygen (gets reduced as it accepts H+)

How many electron(s) and hydrogen atom(s) are transfered to NAD+ from a redox reaction

2 electrons go to every molecule and hydrogen becomes free floating in the cytoplasm

during steps 1 and 3 of glycolysis, what is happening

phosphorylating glucose intermediate

(phosphate comes from ATP)

what is produced during steps 7 and 10 of glycolysis

ATP is produced by phosphorylating ADP

fermentation

allows continuous production of ATP by substrate-level phosphorylation of glucose

• requires the transformation of NADH to NAD+ for the cycle to continue

• happens through the reduction of pyruvate or its derivatives

• occurs in a cell when no oxygen is available

cons of fermentation

Since it occurs in a cell when no oxygen is available:

• far less efficient as less ATP is returned

• but is the only efficient way of producing energy for some cells in extreme conditions

anoxic

lack of oxygen

what determines whether a cell will use aerobic respiration or fermentation

presence or absence of oxygen

how many molecules of ATP produced in aerobic respiration vs. fermentation

32 ATP molecules in aerobic respiration

2 ATP molecules in fermentation

2 types of fermentation

• Alcohol fermentation

• lactic acid fermentation

What do bacteria/fungi and muscle cells use fermentation for

bacteria/fungi - dairy production

muscle cells - carried to liver and converted back into pyruvate

where does pyruvate oxidation take place

mitochondria (for eukaryotes)

cytosol (for prokaryotes)

what happens after glycolysis

• pyruvate (in the presence of oxygen) enters either mitochondria (for eukaryotes) or stays in cytosol (in prokaryotes)

• oxygen is used to harvest the ¾ energy (1/4 harvested from glycolysis) that is left in pyruvate

pyruvate oxidation

• involves multiple enzymatic steps to oxidize pyruvate into acetyl-CoA

• occurs in the matrix of the mitochondria

• NAD+ is reduced to NADH

• acetyl-CoA then continues into the citric acid cycle

steps of pyruvate oxidation

1. 1 carbon is removed from pyruvate to create CO2

2. leftover molecules gives up energy for NAD+ to be reduced into NADH

• this stores energy which is used later in ATP production

3. CoA is added to replace the removed carbons spot

• creates Acetyl CoA

Products of Citric Acid Cycle

• 3 NADH

• 2 CO2

• 3 H+

• 1 ATP

• 1 FADH2

for every turn of the cycle

how many turns of the citric acid cycle produces 1 glucose molecule

2 turns

steps of the citric acid cycle

1. Acetyl CoA enters and transfers its two-carbon group onto oxaloacetate and then CoA leaves

2. intermediate molecule rearranges

3. intermediate is oxidized as it releases one carbon as CO2, NAD+ is reduced to capture energy

4. second carbon is released as CO2 and another NAD+ is reduced into NADH with a freefloat H+

5. substrate-level phoshorylation creates a small amount of ATP

6. electrons are transferred from the substrate to FAD to form FADH2 for later ATP production

7. Last NADH is produced with a freefloat H+ and original 4 carbon molecule (oxaloacetate) is regenerated to restart the cycle

Flavin Adenine Dinucleaotide (FAD)

an electron carrier similar to NAD+ that gets reduced to FADH2

FAD binds to 2 H+ and 2 electrons to form FADH2

How much ATP is produced during fermentation and where does it come from?

only 2 ATP molecules from every cycle

• comes from glycolysis

where in the cell does pyruvate oxidation/citric acid cycle occur? How does this differ between eukaryotes and prokaryotes?

occurs in mitochondrial matrix in eukaryotes

cytosol in prokaryotes

what is the initial molecule of the citric acid cycle?

oxaloacetate

what are the products of pyruvate oxidation

NADH and Acetyl CoA and CO2 per molecule of pyruvate

• (2 of each per molecule of glucose)

how much energy is stored, and in what form(s), during each turn of the citric acid cycle

3 molecules of NADH per turn

2 moles of FADH2 per turn

Oxidative Phosphorylation - where its energy is stored, what the process requires, and two processes involved

• most of the energy from glucose remains stored in NADH and FADH2

• this process requires enzymes embedded in the inner membrane of mitochondrion in eukaryotes or cell membrane of prokaryotes

• involves two processes

  • Electron Transport Chain

  • Chemiosmosis

Electron Transport Chain (ETC)

Series of redox reactions through membrane-bound electron carriers

• has several multi-protein complexes

what are the complex proteins of ETC

prosthetic groups which are required for catalysis

• many contain iron to bind electrons

ETC Proteins

FMN (flavin mononucleotide)

• first protein of the chain

FeS (iron sulfur protein)

• found in complexes 1, 2, and 3

Cyt (cytochrome)

• contains heme (iron)

Steps of ETC

1. Complex 1

• NADH becomes oxidized into NAD+ as it releases 2 electrons that are passed onto FMN in complex 1

• FMN passes electrons onto FeS

• FeS passes electrons to Q (shuttle molecule)

• pumps out hydrogen into inter membrane space, and returns NAD+ to the matrix

2. Complex 2

• FADH2 interacts with complex 2 and becomes oxidized, passing its electrons onto FeS

• FeS passes electrons to Q (shuttle molecule)

• NOT a proton pump

3. Ubiquinone (Q)

• moves electrons received from Complexes 1 and 2 and moves them to Cyt b in complex 3

4. Complex 3

• Cyt b passes electrons to FeS to Cyt c1

• pumps out hydrogen into intermembrane space

5. Cyt C (shuttle protein)

• moves electrons received from complex 3 and moves them to Cyt a in complex 4

6. Complex 4

• cyt a passes electrons to cyt a3

• cyt a 3 moves electrons out of complex 4

• electrons combine with oxygen (last electron acceptor) and H+ to form H2O and more H+ are pumped

7. continuous pumping of H+ creates the proton-motive force (electrochemical gradient between matrix and inter membrane space)

Shuttle molecules

small hydrophobic molecules that move electrons from complex to complex

• not proetins

• not firmly in place on the membrane which allows them to move around

how does proton pumping work

transfer of electrons releases energy which is used to pump H+ into the intermembrane space membrane space

• complexes 1, 3, and 4 are proton pumps, but not complex 2

• NADH moves 3 H+ for every 2 H+ from FADH2

Proton-motive force

electrochemical gradient produced by the ETC

• used to drive ATP production through membrane-bound ATP synthase

• ATP synthase moves hydrogen down gradient which phosphorylates ADP into ATP

ATP synthase

• synthesizes ATP from ADP and inorganic phosphate using the H+ gradient

• protons bind to rotor causing it to spin and phosphorylation happens

• spinning of the rotor powers the production of ATP through catalytic site on the knob

Chemiosmosis

• Process where a proton gradient across a membrane is used to drive cellular work to produce ATP

• works as an energy-coupling system

• energy is created through the gradient from redox reactions

• overlaps and connects to the ETC, but they aren’t the same, they just happen at the same time

  • needs the ETC chain in order to work (since ETC creates the H+ gradient)

• ATP synthesis occurs here

what steps of cellular respiration require oxygen and what steps do not

Glycolysis - doesnt need oxygen

pyruvate oxidation - needs oxygen

citric acid cycle - requires oxygen

Oxidative phosphorylation (ETC and chemiosmosis) - requires oxygen

which step of cellular respiration is a true cycle

Citric Acid Cycle (AKA Krebb’s Cycle)

• since the initial molecule, oxaloacatate, is regenerated at the end of the cycle

ETC in anaerobic respiration

final electron acceptor may be sulfate, nitrate, sulfur, or fumarate

• final electron chain is less electronegative than oxygen so less energy is produced overall

• generates more ATP than fermentation but less than aerobic respiration

facultative anaerobes

can survive using fermentation/anaerobic respiration OR aerobic respiration

ex. listeria monocytogenes

obligate anaerobes

can ONLY use fermentation or anaerobic respiration and cannot survive in oxygen

• oxygen is toxic to them since they’ve become adapted to survive without it

• Ex. clostridium botulinum

How much ATP is produced in cellular respiration

1 NADH = 2.5 ATP

1 FADH2 = 1.5 ATP

• 32 molecules of ATP

what other energy resources besides glucose are used in cellular respiration and how

carbohydrates - large polysaccharides can be broken down into monomer sugars

proteins - digested into amino acids and then deaminated before entering glycolysis or converts directly into Acetyl CoA

fats - digested to glycerol and fatty acids

• Glycerol becomes G3P

• Fatty acids go through beta oxidation to produce Acetyl CoA and NADH/FADH2