Topic 8.1 Cell Respiration

Question 1

8.1.1 State that oxidation involves the ____ of electrons from an element, whereas reduction involves a ____ of electrons; and that oxidation frequently involves gaining _______ or losing ________, whereas reduction frequently involves losing _______ or gaining ________.

Answer 1

Oxidation involves the loss of electrons from an element, whereas reduction involves a gain of electrons; oxidation frequently involves gaining oxygen or losing hydrogen, whereas reduction frequently involves losing oxygen or gaining hydrogen.

Oxidation results in many C-O bonds & a compound with lower potential energy

Reduction results in many C-H bonds and a compound with higher potential energy

Oxidation Is Loss; Reduction Is Gain

Question 2

8.1.2 Outline the process of glycolysis, including phosphorylation, lysis, oxidation and ATP formation.

Answer 2

Glycolysis uses no oxygen, occurs in cytosol of cell; no required organelles; sugar-splitting occurs efficiently in aerobic & anaerobic environments; happens in both prokaryotes & eukaryotes.

1. Glucose is phosphorylated: phosphates from 2 ATPs phosphorylate glucose to form hexose diphosphate (2 phosphate groups provided by 2 molecules of ATP)
2. Lysis of hexose diphosphate: splits into 2 molecules of triose phosphate
3. Each triose phosphate molecule undergoes oxidation: loses 2 H⁺, which are accepted by NAD⁺ (hydrogen carrier) to becomes NADH + H⁺
   
   • As NADH is being formed, released energy is used to add inorganic phosphate to remaining 3 carbon compound → results in two 3-carbon compounds, each carrying two phosphate groups
4. Enzymes remove phosphate groups: 2 pyruvate molecules are formed by removing 2 phosphate groups from each molecule.
   
   • these phosphates groups are given to ADP to form ATP

Summary

• 2 ATPs used to start process
• total 4 ATPs produced - net yield of 2 ATPs
• 2 molecules NADH + H⁺ produced
• involves phosphorylation, lysis, oxidation & ATP formation
• occurs in cytoplasm
• is a metabolic pathway controlled by enzymes; whenever ATP levels in cell are ↑, negative feedback inhibition will block 1^st enzyme of pathway (stops or slows process)
• 2 pyruvate molecules produced at end of pathway

Question 3

8.1.3 Draw and label a diagram showing the structure of a mitochondrion as seen in electron micrographs.

Answer 3

• Inner membrane (cristae)
• Outer membrane
• Intermembrane space
• Matrix

Question 4

8.1.4 Explain aerobic respiration, including the link reaction.

Answer 4

Aerobic respiration

• Aerobic cell respiration is continued from glycolysis with oxidation of pyruvate
• occurs in mitochondria of cells; begins with link reaction

Link reaction

1. once glycolysis has occurred & O₂ is present, pyruvate enters matrix of mitochondrion via active transport
2. pyruvate is decarboxylated (loses CO₂ - now a waste gas) and oxidized (loses 2H⁺ to NAD⁺) → called oxidative decarboxylation (the link reaction!)
   
   • forms 2-carbon acetyl group
3. acetyl group combines with coenzyme A (CoA) to form acetyl CoA

Link reaction controlled by system of enzymes. Formation of acetyl CoA = most significant b/c can enter Krebs cycle to continue aerobic respiration to produce ATP

Question 5

8.1.4 Explain the Krebs cycle and the role of NADH + H⁺.

Answer 5

Krebs Cycle

1. occurs in matrix of mitochondrion; acetyl CoA combines with 4-carbon compound to form 6-carbon compound
2. 6-carbon compound undergoes decarboxylation & oxidation to form 5-carbon compound
   
   • reduces NAD⁺ to NADH + H⁺ – stored energy
   • creates CO2 - waste product
3. 5-carbon compound undergoes decarboxylation & oxidation to form 4-carbon compound
   
   • again, reduces NAD+ to NADH + H⁺ → stored energy
   • creates CO₂ → waste product
4. 4-carbon compound undergoes substrate-level phosphorylation which results in several products
   
   • NADH + H⁺
   • coenzyme FAD reduced to FADH₂
   • reduction of ADP to form ATP
5. During these steps, 4-carbon compound is changed to re-form starting compound of the cycle → is ready to accept new acetyl group and repeat cycle

​Kreb cycle runs 2X per glucose molecule entering cell respiration (each glucose → 2 pyruvate → 2 acetyl CoA. NADH + H⁺ important when releasing 2e^- into electron transport chain, producing more ATP

Summary of products resulting from breakdown of 1 glucose molecule:

• 2 ATP molecules
• 6 molecules of NADH + H⁺
• 2 molecules FADH₂
• 5? molecules CO₂ (released)

Question 6

8.1.4 Explain the electron transport chain and the role of oxygen.

Answer 6

Electron Transport Chain

Chain of electron carriers (most are cytochromes - with heme group) located inside inner membrane. They are set closely & pass e^- down due to energy gradient (more electronegative down the chain).

1. electrons from oxidative reactions in earlier stages of cell respiration (provided by NADH + H⁺ & FADH₂) pass along the chain
2. NADH + H⁺ donates 2e^- to first carrier in chain
   
   • These 2e^-s pass along the chain and release energy from one carrier to the next
   • At 3 locations along the chain, enough energy is released to produce ATP via ATP synthase (enzyme located in inner mitochondrial membrane)
3. FADH₂ also donates e^-s but at a later, lower free energy level than NADH + H⁺
   
   • produces 2 ATPs in comparison to NADH + H⁺​’s 3 ATPS

Role of Oxygen

• Important at end of electron transport chain
• 2 de-energized electrons combine with oxygen, which also accepts 2H⁺ to form H₂O
  
  • occurs in matrix at surface of inner membrane (cristae)
• if no oxygen available, then NADH + H⁺ can’t be reconverted into NAD⁺ → runs out → link reaction & Krebs cycle put on hold → e^- can’t pass through e^- transport chain
  
  • Glycolysis still continues, but only produces enough ATP (2) for its own process; aerobic respiration produces 30 ATP per glucose

Question 7

8.1.5 Explain oxidative phosphorylation in terms of chemiosmosis.

Answer 7

1. when e^-s pass through e^- transport chain, they release energy
2. energy is used to pump protons (H⁺) from matrix across inner membrane to intermembrane space
3. as protons move across, they create a concentration gradient
4. protons move down conc. gradient (back into matrix) through ATP synthase, enzyme embedded in inner membrane
5. as protons move through channels of ATP synthase, they release energy
6. energy is used to synthesize ATP from ADP
   
   • this process is called oxidative phosphorylation, and chemiosmosis (pumping of protons & their movement down concentration gradient) is necessary for oxidative phosphorylation to work

Question 8

8.1.6 Explain the relationship between the structure of the mitochondrion and its function.

Answer 8

• Matrix: watery substance that contains ribosomes, naked loop of DNA, and many enzymes required for link reaction and Krebs cycle
• Inner membrane: electron transport chain & ATP synthase found here → vital for oxidative phosphorylation
• Cristae: tubular projections of inner membrane increase surface area available for oxidative phosphorylation
• Intermembrane space: small volume of space into which protons are pumped. Small volume allows high conc. gradient to be reached quickly → vital for chemiosmosis
• Outer membrane: separates contents of mitochondrion from rest of cell → provides good environment for cell respiration