CHAPTER 11: Cellular Respiration: Every Breath You Take summary

Cellular respiration

• Cellular respiration is a metabolic process that rearranges atoms in molecules of food through multiple steps to ensure that stored food is available to cells.
• The pathway involves redox reactions that move electrons between molecules and energy transfer. This occurs in the following steps:
  • During respiration in cells, electrons are transferred fro food molecules to NADH electron carrier then to oxygen.
  • By the end of respiration,  food energy is transferred to ATP, an energy carrier.
  • The cells then rearrange food molecules into carbon dioxide and water as waste products.
• Glucose is the simple sugar used to study cell respiration pathway

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy

Transferring energy to ATP

• Respiration is an exergonic process.
• As cells rearrange glucose and oxygen atoms during respiration, the molecules go downhill in energy.
• The potential energy in molecules is reduced slowly as ATP forms.

 Taking things one step at a time

• The cellular respiration process occurs in three small pathways.
  • Glycolysis
  • Krebs cycle
  • Oxidative phosphorylation.

Glycolysis

• The main glycolysis events are as follows
  • The bonds between glucose atoms and form pyruvate atoms.
  • Various enzymes facilitate energy transfer from intermediates to ATP.
  • The enzymes reduce NAD⁺ and oxidize glucose.

Glycolysis steps

• An enzyme activates phosphate group transfer from ATP to glucose to form glucose-6-phosphate and ADP
• Glucose 6 phosphate undergoes isomerization reaction to form fructose 6 phosphates.
• Another enzyme facilitates phosphate transfer from ATP  to fructose-6-phosphate to fructose-bis-1,6- phosphate.
• An enzyme breaks to fructose-bis-1,6- phosphate into dihydroxyacetone and glyceraldehyde-3-phosphate.
• An enzyme converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate.
• Electrons are transferred to NAD⁺ from glyceraldehyde -3- phosphate to form 1,3-bisphosphoglycerate.
• An enzyme promotes phosphate transfer to ADP  from form 1,3-bisphosphoglycerate.
•  To form ATP and 3-phosphoglycerate.
• An enzyme allows bond rearrangement to form 2-phosphoglycerate.
• Water is removed from 2-phosphoglycerate to form phosphoenolpyruvate.
• Phosphoenolpyruvate is then broken down into pyruvate molecules and ATP is formed.

Fermentation

• Fermentation occurs as either alcohol fermentation or lactic acid fermentation
  • Lactic acid fermentation: cells utilizing this process produce 2 pyruvate molecules, 2NADH + H and 2 ATP.
    • The enzymes facilitate electron transfer from NADH to pyruvate making it become lactic acid.
    • Lactic acid is a cell waste product.
  • Alcohol fermentation:  it is complex compared to lactic acid fermentation.
    • Involves enzymes that transfer electrons to acetaldehyde from NADH  converting it to ethanol.

Krebs cycle

• The process occurs in the mitochondrion.
• The pathway promotes the oxidation of food intermediates.
• Enzymes reduce electron carriers and oxidize food intermediates.
• Enzymes promote the transfer of energy to ATP.

Steps of Krebs cycle

• Enzymes combine acetyl-CoA with oxaloacetate to form citrate.
• An enzyme rearranges citrate bonds to form isocitrate.
• The isocitrate molecule is then converted to α-ketoglutarate.
• α-ketoglutarate is then converted to succinyl-CoA.
• Succinyl-CoA is then converted to succinate.
• Succinate is oxidized to fumarate.
• Fumarate is then converted to malate.
• Malate is converted oxaloacetate.

 Transferring electrons along the electron transport chain

• Electron transport chains accept electrons from electron donors and pass them through protein complexes to a final electron acceptor.
• The electrons move from one complex to another complex within the chain.
• Electron donors include NADH and FADH₂.

Transferring energy from food to ATP

• Glycolysis and Krebs cycle enzymes transfer electrons from intermediates to electron carriers.
• Electron carriers that are reduced transfer electrons and energy to the electron transport chain.
• The complexes in the electron transport chain release energy while they pass through the chain.
• The ATP synthase protein allows hydrogen ions to move through the membrane.

Steps of the chemiosmotic theory of oxidative phosphorylation

• NADH donates energy and electrons to the electron transport chain.
• Electrons move in a series of redox reactions through the electron transport chain until they are picked by oxygen, the final electron acceptor.
• The energy from redox reactions is used by protein complexes I, II and III to pump hydrogen ions across the membrane.
• ATP synthase enables the recrossing of ions to the inner mitochondrial membrane. As the protons pass, energy is used to join ADP and Phosphate to form ATP.

How many ATP molecules are made from a single glucose molecule?

• Two ATP molecules are produced in glycolysis.
• Two ATP for a glucose molecule is produced in the Krebs cycle.
• Two NADH molecules are produced in glycolysis. One NADH resembles 2 ATP hence a total of 4 ATP for 2 NADH molecules.
• Two FADH₂ in the Krebs cycle are produced per glucose. This total up to 4 ATP molecules.
• 8 NADH molecules are produced per glucose during pyruvate oxidation and the Krebs cycle. This is equal to 24 ATP.
• Total ATP molecules are 2+2+4+4+24 =36

Breaking down complex carbohydrates, proteins, and fats

• Food molecules besides glucose need to be broken down by enzymes into intermediates in glycolysis or Krebs cycle.
• The food molecule intermediates are then oxidized they transfer their energy to ATP.

Feeding complex carbohydrates into the system

• Simple sugars need very few reactions to convert them into intermediates of the glycolysis pathway.
• The cell respiration process digests the complex carbohydrate like starch common in human food t simple intermediates. The amylase enzyme breaks the starch complex into glucose molecules that fit right in the glycolysis process.

Burning fat

• Lipases break oils and fats into fatty acids and glycerol.
• The glycerol and fatty acids enter the cell respiration process in two regions:
  • Glycerol is converted by an enzyme into glyceraldehyde- 3 phosphate which is a glycolysis intermediate.
  • Fatty acids can be broken down by enzymes into 2 carbon fragments. The 2 carbon fragments can be converted into acetyl-CoA, a Krebs cycle intermediate.

Breaking down proteins

• Proteases break down proteins into individual amino acids.
• Enzymes induce two amino acid alterations to make them intermediates of glycolysis pathway or Krebs cycle:
  • Amino acids undergo the deamination process to remove amino groups to enter the Krebs cycle or glycolysis pathway.
  • After deamination, enzymes convert the remaining amino acid into an intermediate for glycolysis or Krebs cycle.

Reversing the flow of matter and energy

• Because metabolic pathways rely on enzymes, regulation of enzymes controls the direction of metabolic reactions.
• ATP can also act as an inhibitor of metabolic pathways.
• ATP inhibits many cellular respiration enzymes:
  • Phosphofructokinase
  • Pyruvate dehydrogenase
  • Citrate synthase

Packing on the fat

• Intermediates from the Krebs cycle and glycolysis pathway can be redirected into fat synthesis metabolic pathways.
• Enzymes utilize glyceraldehyde-3-phosphate from the glycolysis pathway to generate glycerol.
• Enzymes use the excess molecules of acetyl-CoA to produce fatty acids.

Building muscle

• To build proteins for muscles or other tissues, amino acids have to be joined together in long chains.
• Adult humans can make a range of 11 to 20 amino acids important for the synthesis of proteins.
• The other amino acids can be retrieved from diet and hence they are referred to as essential amino acids.
• Enzymes generate many amino acids from the Krebs cycle intermediates.

Revision

In general, describe what happens during photosynthesis
Process by which light energy is converted to chemical energy

What are the reactants of photosynthesis
Carbon dioxide, water

What are the products of photosynthesis
Glucose, oxygen

Where in the cell does photosynthesis occur
Chloroplast

In general, describe what happens during cellular respiration
extracts the chemical energy stored in food (glucose) and uses it to generate ATP energy for cellular work.

What are the reactants in cellular respiration
Glucose, oxygen

What are the products of cellular respiration
Carbon dioxide, water

Define the following terms and tell which reaction (photosynthesis or cellular respiration) is? Anabolic
Metabolic process by which larger molecules are synthesized from smaller ones…photosynthesis

Define the following terms and tell which reaction (photosynthesis or cellular respiration) is? Catabolic
Metabolic process that breaks down large molecules into smaller ones…Cellular respiration

Define the following terms and tell which reaction (photosynthesis or cellular respiration) is? Exergonic
Chemical reaction that releases energy…Cellular respiration

Define the following terms and tell which reaction (photosynthesis or cellular respiration) is? endergonic
Chemical reaction that requires an input of energy…Photosynthesis

Write the chemical equation for cellular respiration
C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy

define oxidation
lose of electrons

define reduction
gain of electrons

Cellular respiration is described as a stepwise redox reaction…What substance is oxidized?
Glucose -> Carbon dioxide

Cellular respiration is described as a stepwise redox reaction…What substance is reduced?
Oxygen -> water

What are the roles of NAD+ and FAD+ in cellular respiration
As electrons are stripped from the glucose, each electron travels with a hydrogen ion (H+). The hydrogen atoms are not directly transferred to oxygen, but instead are first passed to an electron carrier, either the coenzyme NAD+ or FAD++

What happens to the amount of potential energy of electrons as they shift from carbon and hydrogen toward oxygen in cellular respiration
The electrons lose potential energy as they “fall” down the electron transport chain, just as a ball loses potential energy as it rolls downhill. The energy released does not make ATP directly

What happens to the amount of potential energy of electrons as they shift from carbon and hydrogen toward oxygen in cellular respiration….How is the energy used
The electron transport chain is an energy converter that uses the exergonic flow of electrons from NADH and FADH2 to pump H+ across the membrane.

List the four phases of cellular respiration
a) Glycolysis – oxidation of glucose to pyruvate
b) Transition reaction – oxidation of pyruvate to acetyl CoA
c) Krebs cycle
d) Oxidadtive phosphorylation – electron transport and chemiosmosis

Phase 1: glycolysis
Label the diagram by filling in the boxes with the correct molecule
a) Where does this process occur?
b) What molecule does it start with?
c) Which step(s) show the transfer of a phosphate form ATP to an intermediate?
d) Which step shows a reduction reaction?
e) Which step shows the splitting of a 6-c compound into two 3-c PGALs?
f) Which steps are included in the energy-investment phase?
g) Which steps are included in the energy-yielding phase?
h) How many ATP molecules, per glucose, are used in this series of reactions?
i) How many NADH, per glucose, are produced?
j) How many ATP, per glucose, are produced
a) cytosal
b) glucose
c) 1,3
d) 5
e) 4
f) 1,3
g) 6,9
h) 2
i) 2
j) 4

In summary glycolysis:
a) Occurs where?
b) Starts with?
c) Ends with?
d) Yields how much ATP (net)?
e) Produces ATP through what process?
a) cytosal
b) glucose
c) pyruvate
d) 2
e) substrate phosphorylation

What is meant by substrate- level phosphorylation
process in which ATP is formed by transferring a phosphate from a metabolic substrate to ADP/ the mode of ATP synthesis when an enzyme transfers a phosphate group froma substrate molecule to ADP

Why is glycolysis thought to be one of the earliest of all biochemical processes to have evolved
Ancient prokaryotes are thought to have used glycolysis to make ATP long before oxygen was present in Earth’s atmosphere

What reactant is not utilized during the glycolysis stage but must be available for cellular respiration to follow?
oxygen

Phase 2 : the transition reaction – the oxidation of pyruvate
a) Occurs where?
b) Starts with?
c) Produces?
d) Yields how much ATP?
a) cytoplasm
b) pyruvate
c) acetyl- CoA
d) 0

Phase 3: the krebs cycle (aka citric acid cycle)
a) Occurs where?
b) Starts with?
c) Produces?
d) Yields how much ATP?
e) Produces ATP through what process?
a) mitochondrial marix
b) acetyl-CoA
c) CO2, NADH, FADH2, ATP
d) 2
e) Substrate level phosphorylation

What is the major function of the krebs cycle?
The major function of the Krebs cycle is to transfer most of the chemical energy from the breakdown of pyruvate (through acetyl CoA) to NAD+ and FAD during redox reactions.

By the end of the krebs cycle, what has become of the six carbon atoms in the glucose molecule?
The 6 carbon atoms in glucose become 6 carbon dioxide molecules by the end of the Krebs cycle.

Considering both the transition reaction and the krebs cycle together how many of each molecule lusted below are produced per glucose molecule
a) NADH
b) FADH2
c) CO2
d) ATP
a) 2+6= 8
b) 2+4=2
c) 6
d) 2

Phase 4: the electron transport chain
a) Occurs where? (be specific)
b) Starts with?
c) Produces?
d) Yields how much ATP?
e) Produces ATP through which process?
a) inner mitochondrial membrane-cristae of the mitochondria
b) NADH and FADH2
c) NAD+, FAD+, ATP
d) 32
e) Oxidative phosphorylation

What is meant by oxidative phosphorylation
Process by which ATP production is tied to an electron transport system that uses oxygen as the final acceptor/ energy released by the electron transport chain to make ATP from ADP

What is the final electron acceptor in the electron transport chain
oxygen

As electrons are transported through the electron transport chain they lose potential energy. This energy is used to do what work
Make ATP from ADP and P

Describe in detail what happens in the electrons transport chain. What happens to the electrons and H+?
The electron transport chain is a collection of proteins in the inner mitochondrial membrane. The folding of the inner membrane to form cristae increases its surface area, providing space for thousands of copies of the chain in each mitochondrion. During electron transport along the chain, electron carriers alternate between the reduced and oxidized states as they accept and donate electrons. Each component of the chain becomes reduced when it accepts electrons from its “uphill” neighbor, which has a lower affinity for electrons (is less electronegative). It then returns to its oxidized form as it passes electrons to its “downhill”, more electronegative neighbor.

What is chemiosmosis and how is it generated?
Chemiosmosis is the process in which energy stored in the form of a hydrogen ion (H+) gradient across a membrane is used to drive cellular work such as the synthesis of ATP. It is generated by the electron transport chain.

What is the role of dehydrogenases in cellular respiration?
Dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2 protons) from glucose, thereby oxidizing it.

What is the role of ATP synthase complexes in cellular respiration?
ATP synthase complexes are the protein enzymes that actually make ATP from ADP and inorganic phosphate.

Indicate if each of the following is true of:
Substrate- level phosphorylation or oxidative phosporylation. Produce ATP by adding a phosphate to ADP
O

Indicate if each of the following is true of:
Substrate- level phosphorylation or oxidative phosporylation. Involves the direct transfer of a phosphate form an intermediate molecule to ADP
S

Indicate if each of the following is true of:
Substrate- level phosphorylation or oxidative phosporylation. Couples the addition of a phosphate to ADP with the exergonic slide of electrons down the electron transport chain
O

Indicate if each of the following is true of:
Substrate- level phosphorylation or oxidative phosporylation. Accounts for 90% of ATP production in aerobic respiration
O

Indicate if each of the following is true of:
Substrate- level phosphorylation or oxidative phosporylation. oxygen used as the terminal electron acceptor
O

Use the diagram below to answer the questions that follow:
Outer membrane
Inner membrane
Cytoplasm
Matrix
Intermembrane space
Crista
Site of glycolysis
Site of krebs cycle
High H+
Location of ATP synthase molecules
Location of electrons transport chain
A Outer membrane
C Inner membrane
F Cytoplasm
D Matrix
B Intermembrane space
E Crista
F Site of glycolysis
D Site of krebs cycle
B High H+
E Location of ATP synthase molecules
C Location of electrons transport chain

Explain why respiration is considered exergonic.
Respiration is exergonic because energy is released when large high-energy molecules (glucose) are broken down into smaller molecules.

What is the energy released in cellular respiration used for?
This released energy is used to drive the reaction that synthesizes ATP from ADP.

What is the main reason that energy is harvested into ATP molecules gradually in phases in cellular respiration? What happens to most of the energy released?
If energy is released from fuel all at once, it cannot be harnessed efficiently for constructive work. If a gasoline tank explodes, it cannot drive a car very far. Cellular respiration does not oxidize glucose in a single explosive step, but by a series of smaller steps that extracts energy slowly and efficiently.

Account for the 36 ATP molecules produced from the complete oxidation of the glucose molecule.
2 ATP from glycolysis
2 ATP from Krebs
~ 32 ATP from ETC

Some desert animals such as the kangaroo rat never have to drink water. Explain how kangaroo rats can obtain the water they need to survive from the dry seeds they eat.
Kangaroo rats obtain the water they need from the digestion of seeds through the process of cell respiration. Water is produced at the end of the electron transport chain when oxygen, as the final electron acceptor, is reduced.

How is aerobic respiration different from anaerobic respiration (fermentation
Aerobic means “with oxygen”, the process of using oxygen to reduce glucose and make ATP. Anaerobic means “without oxygen”, and so there is no oxidative phosphorylation process to make ATP

What phase of cellular respiration is included in fermentation?
Glycolysis generates 2 ATP whether oxygen is available or not.

What happens during fermentation when oxygen is not available to accept the electrons from the electron transport chain?
Fermentation is an extension of glycolysis that allows continuous generation of ATP by the substrate level phosphorylation of glycolysis. For this to occur there must be a sufficient supply of NAD+ to accept electrons during the oxidation step of glycolysis. The two fermentation pathways recycle NAD+ from NADH, thus restoring the cell’s pool of NAD+, an oxidizing agent.

a) Alcoholic fermentation converts glucose to
ethanol (alcohol)

b) Alcoholic fermentation is utilized by what organisms?
Bacteria and yeast (for brewing, winemaking, breadmaking)

c) Lactic acid fermentation converts glucose to
lactic acid

d) Lactic acid fermentation is utilized by what organisms?
Human muscle cells, bacteria and fungi (for making cheese and yogurt)

What are the advantages and disadvantages of fermentation?
Advantages
Can make ATP energy in the absence of oxygen

Disadvantages
Can only make 2 ATP by substrate-level phosphorylation, wheras oxidative phosphorylation can male up to 16 times more (32 ATP) from a single molecule of glucose