Ch 9 Cellular Respiration

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Catabolic Pathways

Are metabolic pathways that release stored energy by breaking down complex molecules.
- Electron transfer plays a major role in these pathways.
- compounds that can participate in exergonic reactions can act as fuels.

Fermentation

A partial degradation of sugars that occurs without the use of oxygen.

Aerobic respiration

Where oxygen is consumed as a reactant along with the organic fuel
- is a catabolic pathway

Anaerobic Respiration

Some prokaryotes use substances other than oxygen as reactants in a similar process that harvests chemical energy without using any oxygen at all.

Cellular respiration

Includes both aerobic and anaerobic processes.
- but usually is referring to the aerobic process.

Glucose

C6 H12 O6
- Breakdown of glucose is exergonic

Exergonic

there is a negative G in this reaction where the products have less energy than the reactants. and the reaction can happen spontaneously without an input of energy.

Redox Reactions

Oxidation - Reduction reactions.
The two are always paired.
Not all redox reactions involve a complete transfer of electrons - some change the degree of electron sharing in covalent bonds.

Oxidation

The loss of electrons from one substance is an oxidation.

Reduction

The addition of electrons to another substance.
Negatively charged electrons added to an atom reduce the amount of positive charge of that atom.

Reducing agent

Is the electron DONOR.

Oxidizing agent

is the electron ACCEPTOR - it oxidizes another agent by removing its electron.

In cellular respiration

Glucose is oxidized and oxygen is reduced meaning that electrons are added to oxygen.
The electrons lose potential energy along the way and energy is released.
In respiration, the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for ATP synthesis.

Electron fate during respiration

Electrons are stripped from glucose - each electron travels with a proton - as a hydrogen atom.
- the H atoms are NOT directly transferred to oxygen - but are passed first to an electron carrier (co-enzyme NAD + (nicotinamide adenine dinucleotide) derivative of vitamin niacin.

NAD +

Is an electron acceptor and functions as an oxidizing agent during respiration.

How does NAD+ trap electrons?

From glucose enzymes called dehyrogenases remove a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate which is glucose which oxidizes and releases them.
2 electrons and 1 proton is sent to electron carrier NAD + and the other proton is released as a hydrogen ion into the surrounding solution.
NAD then becomes NADH because there is one extra negative charge and it is now officially reduced.
NAD has its charge neutralized when its reduced to NADH

NADH

Each molecule can be 'tapped' for energy to make ATP when the electrons complete their 'fall' down an energy gradient from NADH to oxygen.

Electron Transport Chain

cellular respiration uses ETC to break the fall of electrons to oxygen into several energy releasing steps.
Is made up of mostly proteins (inside the inner membrane space of mitochondria of eukaryotic cells and the plasm membrane of prokaryotes.
At the end of this chain O2 captures these electrons where there is lower energy and forms water with hydrogen.
- electron transfer is exergonic and negative.
- with each redox reaction a small amount of energy is lost until they reach oxygen.
- each 'downhill' carrier is MORE electronegative than its 'uphill' neighbor.
- oxygen is the most electronegative but stable - the electrons moving down the chain come from glucose.

Stages of cellular respiration

Three metabolic stages:
1. Glycolysis
2. The citric acid cycle
3. Oxidative phosphorylation: electron transport and chemiosmosis

Glycolysis

Occurs in the cytosol
begins by breaking glucose into two molecules of a compound pyruvate.

Citric Acid Cycle

takes place in the mitochondrial matrix of eukaryotic cells or the cytosol of prokaryotes - completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide.

Oxidative Phosphorylation

Is powered by the redox reactions of the electron transport chain.
- eukaryotic cells - the inner membrane of the mitochondrion is the site of ETC and chemiosmosis
- in prokaryotes the site is in plasma membrane.
- oxidative phosphorylation accounts for 90% of the ATP produced.

Substrate level phosphorylation

This mode of ATP synthesis occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP rather than adding an inorganic phosphate to ADP (like in oxidative phosphorylation.
Some ATP is made by this direct transfer of the phosphate group from an organic substance to ADP by an enzyme.
-A smaller amount of ATP is produced through glycolysis and the citric acid cycle.

Glycolysis

Means sugar splitting.
Glucose - 6 carbon sugar is split into 3 carbon sugars.
- these sugars are then oxidized and their atoms rearranged to form 2 molecules of pyruvate.
Glycolysis has 2 phases:
1. Energy investment - the cell spends 2 ATP.
2. Energy Payoff- ATP is produced by substrate level phosphorylation and NAD+ is reduced to NADH by electrons released from the oxidation of glucose.
NET YIELD
2 ATP
2 NADH
No CO2 is released in glycolysis it's all in pyruvate produced.
Glycolysis occurs whether or not Oxygen is present.

Process of Glycolysis - Energy investment phase

1. Glucose enters the cell and is phosphorylated by the enzyme hexokinase which transfers a phosphate group from ATP to the sugar.
2. Glucose 6 phosphate is converted to its isomer fructose 6 posphate.
3.This enzyme transfers a phosphate group from ATP to the sugar using another ATP molecule. - the sugar is now split in half.
4. The enzyme divides glucose into two 3 carbon molecules.

Glycolysis - Energy Payoff Phase

6.glyceraldehyde 3 phosphate is oxidized by the transfer of electrons and H to NAD+ froming NADH.
- this reaction is exergonic. - the enzyme uses the released energy to attach a phophate group to the oxidized substrate.
- inorganic phosphates come from the ions that are always in the cytosol.
7. Glycolysis then produces ATP by substrate level phosphorylation - the phosphate group is added to ADP in an exergonic reaction.
9. PEP is formed from glyceraldehyde 3 phosphate by creating a double bond to form in the substrate by extracting a water molecule.
10. The last reaction of glycolysis produces more ATP by transferring the phosphate group from PEP to ADP.
- this step occurs twice for each glucose molecule so 2 ATP are produced.
- oxygen needs to be present for the chemical energy in pyruvate to be extracted by the citric acid cycle.

Citric Acid Cycle

Pyruvate enters mitochondrion via active transport because its a charged molecule and needs a transport protein.
- pyruvate is converted to a compound Acetyl coenzyme A, acetyl CoA. (this is the junction between glycolysis and the citric acid cycle.
Three things happen
1. pyruvates COO- is removed and given off as a molecule of CO2.
2. the remaining carbon is oxidized and transferred from NAD to NADH.
3.Then CoA is attached to the acetate which makes acetly group reactive.
The molecule is now ready to feed the actyl group into the citric acid cycle to be oxidized further.

Citric Acid Cycle or Krebs Cycle.

NADH FADH2 link glycolysis and the citric acid cycle to continue on to oxidative phosphorylation.
Occurs in mitochondrial matrix in eukaryotic cells.
1. Pyruvate from glycolysis enters matrix with 2 molecules. It's broken down into three CO2 molecules.
The cycle generates 1 ATP per turn by substrate level phosphorylation.
For each turn of the citric acid cycle - two carbons enter and two carbons leave in an oxidized form of CO2.
CoA joins the cycle by combining with the compound oxaloacetate forming citrate.

Energy Yield from Citric Acid Cycle

For each acetyl group entering the cycle 3 NAD+ are reduced to NADH. Electrons are transferred to FAD which accepts 2 electrons and 2 protons to become FADH2.
- GTP is sometimes produced in animal cells which is similar to ATP.
Only 2 ATP molecules are released from citric acid cycle.

Electron Transport Chain

Is located in the inner membrane of the mitochondrion in eukaryotic cells.
- prokaryotes on the inside of the plasma membrane.
(the folding of the inner membrane INCREASES the surface area of the cell.
- electrons are travelling DOWN the chain, losing energy as the electrons are transferred through redox reactions.
- electrons removed from glucose and donated by NADH are given to the first protein complex in the ETC.
- Each oxygen atom also picks up a pair of hydrogen ions from the aqueous solution, forming water.
- FADH2 also donates electrons to the chain.
- FADH2 donates its' electrons at complex 2 in the ETC therefore resulting in one third less energy for ATP b/c its farther down the chain. FADH2 and NADH both donate 2 electrons - but FADH 2 pumps fewer protons into intermembrane space.

Prosthetic groups

Nonprotein components essential for the catalytic functions of certain enzymes.

Cytochromes

Electron carriers between ubiquinone and oxygen.
They have a prosthetic group called a heme group that accepts and donates electrons.

ATP synthase

Protein complex in the inner membrane of mitochondrion or in plasma membrane of prokaryotic organisms.
- this enzyme makes ATP from ADP and inorganic phosphate.
- ATP synthase uses the energy of an existing ion gradient to power ATP synthesis.
- the power source for ATP synthase is a difference in the concentration of H on opposite sides of the inner mitochondrial membrane.

Chemiosmosis

The process in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP.
osmos = push in greek
- process refers to hydrogen across a membrane.
chemiosmosis couples the exergonic flow of electrons from NADH and FADH2 to pump H across the membrane from mitochondrial matrix into the intermembrane space.
- the hydrogen usually diffuses down its gradient back into the matrix.
ATP synthase is the only route for H to move out of the matrix.

ATP Synthase parts

Its a multisubunit complex with four parts- each made of polypeptides.
- protons move onto the binding sites on the rotor - causing it to spin in a way that catalyzes ATP from ADP and inorganic phosphate.
- Each H ion makes one complete turn before leaving the rotor and passing into the matrix.

Proton motive force

The hydrogen gradient that results from H+ being pumped from matrix to intermembrane space.
- emphasizes the capacity of the gradient to perform work.
- this force drives H back across the membrane through the H channels via ATP synthase.

Chemiosmosis

Is an energy coupling mechanism that uses energy stored in the form of an H gradient across a membrane to drive cellular work.

Energy flow during respiration

Glucose = NADH = electron transport chain = proton - motive force = ATP.

NADH

each one that transfers a pair of electrons from glucose to the electron transport chain contributes enough to the proton motive force to generate a maximum of about 3 ATP.

FADH2

is responsible for transport of only enough hydrogen for the synthsis of 1.5 to 2 ATP>

Full energy yield of cellular respiratin

one glucose molecule could generate a maximum of 34 ATP produced by oxidative phophorylation plus 4 ATP (net) from substrate level phosphorylation to give a total yield of 38 ATP or 36 ATP if there was less efficient shuttling.
About 40% of the potential chemical energy in glucose has been transferred to ATP.
The rest of the stored energy is lost as heat.

Anaerobic Respiration and Fermentation

Oxidative phosphorylation ceases because there's no oxygen to pull electrons down the chain.
- these ways can oxidize organic fuel and generate ATP without the use of oxygen.
COMPARISON:
1.both use glycolysis to oxidize glucose to pyruvate with a net yield of 2 ATP.
2.NAD + is the oxidizing agent that accepts electrons from food during glycolysis.
3. in fermentation the final electron acceptor is pyruvate in lactic acid fermentation or acetaldehyde in alcohol fermentation.
both anaerobic respiration and fermentation yields only 2 ATP>

Anaerobic respiration

takes place in prokaryotic organisms that live in environments without oxygen.
- they have an electron transport chain but do not use oxygen as a final electron acceptor.
- less electronegative substances can serve as final acceptor.
- uses glycolysis

Fermentation

A way of harvesting chemical energy without using either oxygen or electron transport chain.
- fermentation is an expansion of glycolysis that allows continuous generation of ATP by the substrate level phosphorylation of glycolysis.
- the transfer of electrons from NADH is made to pyruvate which is the end product of glycolysis.
- NETs 2 ATPS.
2 Types
Alcohol fermentation
lactic acid fermentation
- uses glycolysis

Alcohol Fermentation

Pyruvate is converted to ethanol.
step 1 - CO2 is released from pyruvate which is converted into acetaldehyde.
step 2 - acetaldehyde - is reduced by NADH to ethanol.- this regenerates NAD+ needed for the continuation of glycolysis.
- yeast carries out alcohol fermentation.

Lactic acid fermentation

pyruvate is reduced directly by NADH to form lactate as an end product -
with NO release of CO2
- Lactate is the ionized form of lactic acid.
- used in making cheese and yogurt.
- human muscle cells make ATP by lactic acid fermentation when oxygen is scarce.

Obligate anaerobes

Carry out only fermentation or anaerobic respiration and in fact cannot survive in the presence of oxygen.

Facultative anaerobes

yeasts and many bacteria can make enough ATP to survive using either fermentation or repiration.

Ancient theories of glycolysis

old prokaryotes probably used glycolysis to make ATP before O2 was present in our atmosphere.

Fuel for glucose

through catabolism the break down of carbohydrates, fats and proteins are all used as fuel for cellular respiration.
- Food also provides the carbon skeleton that cells require to make their own molecules.

Beta oxidation

breaks fatty acids down to two carbon fragments whcih enter the citric acid cycle as acetyl CoA. NADH and FADH2 are also generated during beta oxidation.

Regulation of cellular respiration via feedback mechanisms

most common mechanism - feedback mechanism.- the end product of the anabolic pathway inhibits the enzyme that catalyzes an early step of the pathway.
- through catabolism - is ATP concentration drops respiration speeds up.

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