Biology: Topic 5: Catabolism

What is the first law of thermodynamics?

• Energy can be transferred or transformed, but it cant be created or destroyed

• metabolism:

  • Anabolism- make things and use energy - reduce disorder

  • Catabolism - break down things and release energy - increase disorder

What is the second law of thermodynamics?

• in the universe the degree of disorder (entropy) can only increase

How to photosynthesis and respiration work together?

Complementary processes

What does it mean to become oxidised?

• giving away an electron

• The substance that becomes oxidised and gives away an electron is the reducing agent

What does it mean to become reduced?

• the substance that accepts an electron becomes reduced

• It is also the oxidising agent

• This is more electronegative than the other substance, meaning its happier to accept the electron

What is the equation for cellular respiration, and what becomes oxidised/reduced?

• Glucose (C6H12O6) + 6 O2 → 6 CO2 + 6H2O + energy

• Glucose gives away electrons and becomes oxidised - its more unstable, contains more energy - becomes CO2, more stable, less energy

• O2 is accepting the electrons and becomes reduced

• Energy can be stored temporarily in certain molecules such as a NADH or ATP

How is glucose stored in cells?

• as starch (plants)

• As glycogen (animals)

What are the main steps of aerobic respiration?

1. Glycolysis

2. Pyruvate oxidation

3. Citric acid cycle

4. Oxidative phosphorylation

Explain the steps of glycolysis

• occurs in cytosol

Energy investment phase - glucose + 2 ATP invested - output 2ADP + 2P

1. Hexokinase enzyme converts glucose to glucose 6-phosphate using ATP to phosphorylate glucose

2. Glucose 6-phosphate continues to be broken down in multiple steps - the main regulatory enzyme is phosphofructokinase - 1 more ATP molecule needed

3. Ending up with 2 × 3carbon molecules

Energy payoff phase - 2 NADH, 4 ATP, 2H+, 2 pyruvate, 2H2O made- 2NAD+ invested

4. An enzyme is used to take 2H+ (and 2 electrons) from each 2× 3 carbon molecules (partially oxidising glucose) and gives it to NAD+ carriers reducing them to NADH (2 total- accepts 2 electrons, 1 H, and 2H+ is released) - P is also added to 3c molecules

5. The P is cut from the 3c molecules and uses substrate level phosphorylation to make 2 ATP using an enzyme

6. 3c molecules are converted again and again- releasing water

7. The 3c molecules undergo substrate level phosphorylation again to make 2 ATP

8. End w 2 × 3c molecules called pyruvate

Net = 2 pyruvate, 2 ATP, 2 NADH, 2H+, 2H2O

• doesn’t require O2- likely evolved before photosynthesis

• Limitations

  • NAD+ stores

  • If O2 not present, NADH accumulates, cells runs out of NAD+

What is substrate level phosphorylation?

• simplest way to make ATP

• Enzyme is used to add a single P to ADP

• Not efficient but its simple

• Done in all cells

Explain the steps of pyruvate oxidation

1. Pyruvate from glycolysis travels from the cytosol to the mitochondrial matrix

2. Oxidised further into 2× 2C molecules called Acetyl CoA

3. For each pyruvate molecule- One C molecule is lost as CO2, NAD+ is reduced to NADH, H+ is released

How did mitochondria originate?

Thought to have originated from an endosymbiosis event where an aerobic heterotrophic prokaryote was engulfed by a host cell

How did the chloroplast develop?

Endosymbiosis event where a photosynthetic prokaryote was engulfed

What happens during the citric acid cycle (krebs cycle)

1. Acetyl CoA enters the cycle, the CoA leaves, and acetyl is joined by 4C oxaloacetate (final product of the cycle)

2. Forms 6C citric acid

3. Citric acid loses 2x carbons as 2 CO2 molecules

4. 3 NAD+ are reduced to 3 NADH - 3H+ released

5. FAD is reduced to FADH2

6. Substrate level phosphorylation makes 1 ATP

7. Oxaloacetate - final product that enters the cycle again

8. This is for 1 pyruvate molecule- output is double for 1 glucose molecule

• series of oxidation reaction steps to completely oxidise glucose

Explain oxidative phosphorylation

• Electron transport chain + chemiosmosis

• high energy electron transporters travel form the matrix to the inner membrane with a series of proteins embedded

1. 6NADH donate their electrons to protein complex I - becomes re-oxidised to 6NAD+ (replenishing stores to recycle)

2. 2FADH2 donates electrons to protein complex II - becomes re-oxidised to 2FAD

3. Electrons travel along the electron transport chain from complexes I to IV - from least to most electronegative - The final electron acceptor at the end of the ETC is O2 - which is reduced to water

4. ubiquinone (Q) and cytochrome c (Cyt c) move within the inner membrane to pass electrons between complexes

5. At each step electrons are losing energy that the cell can use to do work - Complexes I, III, IV use this energy to pump H+ against its concentration gradient from the matrix into the intermembrane space

6. Chemiosmosis: ATP synthase uses this H+ gradient to diffuse H+ back into the matrix down its conc gradient, powering the rotor, and creates ATP from ADP by oxidative phosphorylation

7. Approx 28 ATP produced from 1 glucose at the end of chemiosmosis