BIOCHEM - Glucose Metabolism

Question 1

What are the four stages of glucose metabolism?

• what occurs at each stage?
• where does each occur
• what are the net products of each stage?

Answer 1

(can occur with or without oxygen)
1. Glycolysis
- glucose to pyruvate
- occurs in cytosol
- 2 x ATP, 2 x NADH

(aerobic)
2. Pyruvate oxidation
- pyruvate to Acetyl CoA
- occurs in inner mitochondrial matrix
- 1 x CO2, 1 x NADH

3. Citric acid cycle
   - Acetyl CoA to CO2, NADH2+, FADH2
   - occurs in the inner mitochondrial matrix
   - 2 x CO2, 3 x NADH, 1 x FADH, 1 x ATP/GTP
4. Electron transport chain
   - oxidative phosphorylation (ETC & chemiosmosis)
   - occurs in inner mitochondrial membrane and space
   NADH2+, FADH2 to ATP —> NAD, FAD, H2O, ATP

Question 1

Describe the stages of glycolysis.

• where does it occur?
• how much ATP is generated?
• what are the phases of glycolysis?

Answer 1

Involves energy investment phase and energy yielding phase.

Occurs in cytosol in cytoplasm of cells.

phase 1 - priming:
- glucose to fructose-1,6-diphosphate
phase 2 - cleaving:
- fructose-1,6-diphosphate - DHAP + glyceraldehyde-3-phosphate
phase 3 - oxidation:
- glyceraldehyde-3-phosphate - 2 x pyruvate

ATP is generated in glycolysis through ‘substrate level phosphorylation’
- yields:
2 x ATP molecules
2 x NAHD2

Question 2

Describe the stages of pyruvic oxidation:

• where does it occur?
• what are the products?

Answer 2

(in presence of oxygen)

CO2 is removed from pyruvate to form an acetyl group.
NAD+ removes 2 e- from pyruvate to form NADH & H+
Coenzyme A is added to acetyl to form acetyl-CoA

Occurs in the mitochondrial matrix.

Produces 0 x ATP, 2 x NAD, 2 x CO2

Question 3

Describe the stages of the citric acid cycle:

• where does it occur?
• what are the products of citric acid cycle?

Answer 3

Occurs in the inner mitochondrial matrix.

The citric acid cycle is a bond breaking pathway that produces little ATP (2 x ATP).

Acetyl CoA enters and combines with oxoacetate - undergoes a series of redox reactions.

Produces reduced electron carriers NADH, FADH+ which go into ETC.

What are the products of TCA cycle?
Per Acetyl CoA molecule:
2 x CO2, 1 x ATP, 3 x NADH, 1 x FADH

Question 4

What are the two main functions of the ETC?

Answer 4

Two functions:

• regenerates electron carriers
  (NADH and FADH pass their electrons to the ETC, turning back into NAD and FAD)
• makes a proton gradient
  (builds a proton gradient across the inner mitochondrial membrane, with a higher concentration of H+ in the intermembrane space and a lower concentration in the matrix)

Question 5

Describe the stages of the electron transport chain.

What occurs in each stage of oxidative phosphorylation?

Answer 5

The ETC & chemiosmosis are the final stages of cellular respiration.

Oxidative phosphorylation:
Electrons are passed between molecules. Energy released in these electron transfers is used to form an electrochemical gradient.

Stages of oxidative phosphorylation:

1. Delivery of electrons by NADH and FADH
2. Electron transfer and proton pumping:
   Electrons are passed down the chain moving from high to low energy level. The released energy is used to pump H+ ions, moving them out of the matrix and into the inter membrane space establishing an electrochemical gradient.
3. The final electron acceptor is oxygen - electrons combind with protons and form water.
4. Gradient-driven synthesis of ATP
   H+ ions flow down their gradient and back into the matrix, passing through ATP synthase (enzyme).
   ATP synthase harnesses the flow of protons to synthesize ATP from ADP + P.

Question 6

What are carbohydrates?

Answer 6

One of the three macronutrients in human diet.

Contain carbon, hydrogen & oxygen (CHO).

Metabolised into glucose for energy – skeletal muscle and brain preferred fuel source.