IB HL BIOLOGY C1.2 cell respiration

What is the purpose of NAD?

• A coenzyme (molecule needed for an enzyme to work)

• Key in CR because it is a hydrogen carrier (Can be reduced and oxidised)

What is NAD’s role in CR?

• Is an oxidiser (oxidises/dehydrogenates other molecules)

• it becomes reduced itself (NADH)

• Important in CR because there are some stages where a substrate needs to be oxidized

• Also important for oxidative phosphorylation at end of CR (carries e and H+)

What is the first stage of cellular respiration? Does this occur regardless of aerobic or anaerobic type CR?

Glycolysis

yes

Where does glycolysis occur? What is the general summary?

• Cytoplasm

• Splits 1 molecule glucose (6C) into 2 x pyruvate (3C)

• 10 steps

Describe the 4 main phases of glycolysis

1. phosphorylation

• 2 molecules of ATP is used to phosphorylate glucose

  • The 2 Pi is added to glucose

• Glucose is now unstable (in form of fructose-1,6-bisphosphate)

2. Lysis

• the fructose-1,6-bisphosphate splits into 2 TP/G3P

3. Oxidation of each G3P molecule

• reduces 2 molecules of NAD —> NADH

• Released energy adds another Pi to G3P

4. ATP formation/Substrate level phosphorylation (From each G3P)

• Enzymes remove Pi to be added to ADP to make ATP

• Generates 2 x ATP for each G3P

• Forms the final product of pyruvate

What are the reactants and products of glycolysis?

Reactants: 1 glucose, 2 ATP

Products: 2 NET ATP, 2 pyruvate, 2 NADH

What is substrate level phosphorylation?

• The process of generating ATP from ADP and a Pi taken from the substrate

Does aerobic or anaerobic produce more ATP?

Aerobic

Does glycolysis need O2 to occur? What happens if it continues in anaerobic conditions?

No

in a short time, the NAD of the cell would all be turned into NADH —> glycolysis would stop

What is the purpose of anaerobic cell respiration?

• Allows the regeneration of NAD so that glycolysis can continue (and some ATP is still produced)

• Also removes pyruvate buildup so glycolysis continues

How is NAD regenerated in humans and yeast?

In humans: lactic acid fermentation

• Pyruvate is converted into lactate by reducing pyruvate using NADH

  • This oxidizes NADH back into NAD (goes back into glycolysis)

In yeast: alcohol fermentation

• 1. Conversion of pyruvate into ethanal

• Through decarboxylating the pyruvate (Releases CO2)

• 2. Conversion of ethanal to ethanol

• The ethanal was reduced by NADH into ethanol

• This regenerates the NAD for glycolysis

• This makes 2 products = CO2 + ethanol

When would anaerobic respiration be useful?

• Much faster than aerobic, so it can generate a relatively large amount of ATP for short exercises

List the stages in cellular respiration

1. Glycolysis

2. Link reaction

3. Krebs cycle

4. ETC/chemiosmosis

What happens in the link reaction? 3 steps

• (Before: After glycolysis, the 2 pyruvates move into the mitochondria matrix via active transport)

1. decarboxylation of pyruvate

• converts it into acetyl (2C)

• CO2 released as waste

2. Oxidation of acetyl

• Reduces NAD to NADH

3. Formation of Acetyl CoA

• Acetyl groups combine with CoA

What are the 4 steps in Krebs cycle

1. Formation of citrate

• The acetate from acetyl CoA binds with oxaloacetate (4C)

• This releases coA to continue link reaction

2. Oxidative decarboxylation

• Removes CO2

• Reduces NAD

• Oxidizes citrate (6C —> 5C)

3. Oxidative decarboxylation

• Removes CO2

• Reduces NAD

• Makes 1x ATP (substrate-level phosphorylation)

• 5C —> 4C

*Now, all 6C from OG glucose molecule has been released as 6CO2

*For cycle to continue, oxaloacetate must be regenerated

4. Oxidation of 4C (Regeneration of oxaloacetate)

• Reduces NAD

• Reduces FAD

• Now oxaloacetate is available

Summarize the Krebs cycle

• Completed the breakdown of OG glucose molecule

• Generates majority of NADH and FADH2 that will deliver e to ETC

How many times does the Krebs cycle turn

2

(1 for each glucose/pyruvate/acetyl CoA)

What is the budget (for H carriers) at each step in CR?

*Per glucose molecule

1. Glycolysis (2 NADH)

2. Link (2 NADH)

3. Krebs (6 NADH, 2 FADH)

4. ETC (1 NADH gives 3 ATP, 1 FADH2 gives 2 ATP)

Products of link reaction

• The products are 2 acetates (2C), which combine with coA to make 2acetyl COA

Where does ETC occur?

• Inner mitochondria membrane/mitochondria matrix

What structures are involved in ETC?

• 4 membrane bound protein complexes w/2 electron carriers

Describe the 3 mains steps in ETC

1. NADH delivers 2 e (carried from glycolysis/link/krebs) to Complex I

• These electrons power the pumping of H+ across the membrane (from matrix to IM space)

2. FADH2 also delivers 2 e to the ETC, but at complex II (pumps less H+)

3. e are transported along ETC (pumps more H+ along the way)

What is created from the ETC

• Proton gradient

  • The e from the 10 NADH and 2 FADH2 pump many H+ into the IM space

  • Since this space is narrow, and H+ can’t diffuse across membranes, this establishes a high [ ] gradient of H+ in IM space compared to matrix side

• Important to generate ATP, as H+ want to move down gradient but can’t

How is ATP finally generated (last step in CR)?

• H+ can only flow down [ ] in one pathway, down ATP synthase (protein channel)

• This flow of H+ (PMF) generates energy needed to phosphorylate ADP (using Pi) to make ATP

  • Through oxidative phosphorylation

• ATP synthase has a turbine, which generates ATP when H+ flow

• This process is chemiosmosis (flow of H+ down electrochemical gradient, driving ATP formation)

How many H+ is needed to phosphorylate one ADP into ATP

• Average is 3, but ranges 2-4

How much ATP is made at the end of CR?

• 30-34 ATP

• 4-8 ATP were used for transporting molecules in/out of mito

What is the role of O2 in the ETC? What does it do?

• Its the terminal electron acceptor

• after e have passed along ETC, they need to go somewhere —> they are accepted by oxygen

  • To do this, each molecule of O2 splits, and accepts 4e, 4H+, forming 2 molecules of H2O

What happens if O2 is not present at end of aerobic respiration

• More electrons cannot join ETC (if O2 doesn’t accept it)

• Thus, NAD and FAD cannot be regenerated (by oxidation) —> No longer a supply of NAD and FAD to continue the link/krebs

• Creates a “molecular traffic jam”

What property determines the energy content of a respiratory substrate? Why?

• The amount of H available when the molecule is broken down

  • The more H, the more NAD can be reduced

  • The more reduced NAD produced, the more protons transported across IMM (Generates more PMF, more ATP)

• *However, more O2 is also required

Do lipids or carbohydrates reduce more NAD?

• Lipids, since they are composed of long chains of carbon with hydrogens

In increasing order, list the energy content of lipids, proteins and carbohydrates as respiratory substrates

• Carbohydrates, proteins, lipids

How do lipids enter cell respiration vs. carbohydrates?

• Carbohydrates can enter glycolysis (after being modified by the liver)

• lipids can’t be broken down through glycolysis

Can lipids and carbs both undergo anaerobic respiration? Why?

• No, only carbs. Since lipids can’t be broken down in glycolysis (the previous step)

How does a lipid molecule serve as a respiratory substrate (how does it enter the CR cycle)

• It’s first broken down into glycerol and fatty acids

  • glycerol is used in glycolysis

  • fatty acids are broken down into acetyl groups —> directly participate in the link reaction to form acetyl CoA, then enter Krebs

What reaction is the formation of ATP? Hydrolysis?

• Endergonic (energy from oxidation of nutrients is stored in high energy bond. There’s an input of energy, and ATP has higher potential energy than ADP)

• Exergonic (energy is released)

Why are there high energy bonds in ATP? which bonds are broken during hydrolysis of ATP

• Due to the negatively charged phosphate groups repelling eachother (unstable covalent bond = high energy)

• These unstable bonds are easily hydrolyzed (low Ea)

• Energy is released when the bond holding the 2nd to 3rd phosphate is broken

Where do the electrons in ETC come from

• The splitting of H atoms from NADH/FADH2

Summarize the ETC and chemiosmosis together

• While electrons move down ETC, energy is released in small amounts to pump H+ from matrix into IM space

• This creates a proton gradient, accumulating H+ in IM space

• Protons move down gradient by difussion through channels called ATP synthase

• As H+ move from IM to matrix, ATP synthase harnesses this energy and phosphorylates ADP to make ATP

Define chemiosmosis

Formation of ATP driven by proton gradient

Fatty acids yield how much more ATP than carbohydrate

20%