Metabolic Pathways in the Cell

Question 1

What is a metabolic pathway?

Answer 1

It is a sequence of enzyme-catalyzed reactions that lead to the conversion of a substance into a final product.

Question 2

Metabolic pathways can be broadly divided into 2 categories based on their effects. Name them.

Answer 2

Anabolic pathways (building up)
Catabolic pathways (breaking down)

Question 3

List the important metabolic pathways in humans. (7, but they are more than 7…very many)

Answer 3

1. Glycolysis
2. Citric acid cycle (Krebs’ cycle)
3. Oxidative phosphorylation
4. Pentose phosphate pathway - synthesis of pentoses and release of the reducing power needed for anabolic reactions
5. Urea cycle
6. Fatty acid β-oxidation - fatty acids breakdown into acetyl-CoA to be used in Krebs’ cycle
7. Gluconeogenesis- glucose synthesis from smaller precursors to be used by the brain

Question 4

Which enzyme chemically modifies glucose by phosphorylation when glucose is at a higher concentration in the cell than in the bloodstream?

Answer 4

Enzyme hexokinase.

Question 5

What does enzyme hexokinase convert glucose into, and what is the product now used in?

Answer 5

Glucose-6-phosphate, and it’ll be used in glycogen synthesis, production of other carbon compounds by the pentose-phosphate pathway, or degraded in order to produce energy i.e. glycolysis.

Question 6

In glycolysis, glucose is broken down to give pyruvate. In a flowchart, briefly show that metabolic pathway.

Answer 6

Glucose —–>glucose-6-phosphate—->fructose-6-phosphate—–>fructose-1,6-biphosphate—->glyceraldehyde 3-phosphate—–>1,3 biphosphoglycerate (the 2 NADH are given out at this point)—-> 3 phosphoglycerate—-> 2 phosphoglycerate —-> phosphoenol pyruvate —-> pyruvate (2 ATP are given out)

Question 7

List the enzymes involved in glycolysis.

Answer 7

Hexokinase
Isomerase
Phosphofructokinase
Aldolase
Mutase
Phosphoglycerate kinase
Enolase
Pyruvate kinase

Question 8

What is the committed step of glycolysis?

Answer 8

The step where fructose-1,6-biphosphate is formed.

NB: In enzymology, the committed step is an effectively irreversible enzymatic reaction that occurs at a branch point during the biosynthesis of some molecules.

Question 9

What are the end products in glycolysis?

Answer 9

2 pyruvate molecules, 2 ATP molecules, 2 NADH molecules

Question 10

__________ are the most abundant proteins in the mitochondrial outer membrane.

Answer 10

Porins/voltage-dependent anion channels

(SIDE NOTE: They have significant roles in diverse cellular processes including regulation of mitochondrial ATP and calcium flux.)

Question 11

Where does Krebs’ cycle/citric acid cycle/tricarboxylic acid cycle take place?

Answer 11

In the mitochondrial matrix.

Here acetyl-CoA is oxidized to form carbon dioxide and coenzymes are reduced, which generate ATP in the electron transport chain.

Question 12

Krebs’ cycle serves as the final steps in carbon skeleton oxidative catabolism for ______________, ____________, and ______________.

Answer 12

carbohydrates, amino acids and fatty acids.

(NB: Each oxidative step in Krebs’ cycle reduces a coenzyme such as NADH [nicotinamide adenine dinucleotide] or FADH2 [flavin adenine dinucleotide].)

Question 13

Show the metabolic pathway in the Krebs’ cycle. (Within, show where CO2, NADH and FADH2 will be given out)

Answer 13

Acetyl-CoA —> Citrate —> Isocitrate –(CO2)–> alpha-Ketoglutarate –(CO2)–> Succinyl-CoA –(ATP)–> Succinate –(FADH2)–> Fumarate –(NADH)–> Malate –(NADH)–> Oxaloacetate

Question 14

State the significance of Krebs’ cycle.

Answer 14

a) It is the final pathway of oxidation of glucose, fats and amino acids.
[AA are deaminated and get converted to pyruvate and other intermediates of the cycle. Fatty acids undergo β-oxidation to form acetyl CoA, which enters the Krebs cycle.]
b) It is the major source of ATP production in the cells.
c) It plays an important role in gluconeogenesis and lipogenesis and interconversion of amino acids.
d) The genetic defects of the Krebs cycle enzymes are associated with neural damage. [Reduced ATP generation results in the withdrawal of 𝝰-ketoglutarate and formation of glutamate, which forms glutamine, an important neurotransmitter in the brain.]

Question 15

List the enzymes involved in Krebs cycle.

Answer 15

Aconite
Isocitrate dehydrogenase
𝝰-ketoglutarate dehydrogenase
Succinyl-CoA synthetase
Succinic dehydrogenase
Fumarase
Malate dehydrogenase
Citrate synthase

Question 16

What is oxidative phosphorylation?

Answer 16

It is a cellular process to harness the reduction of oxygen to generate high-energy phosphate bonds in the form of ATP. It is the disposal of the electrons released by glycolysis and citric acid cycle.

Question 17

Where does oxidative phosphorylation take place in eukaryotes and in prokaryotes?

Answer 17

Eukaryotes - mitochondrial inner membrane
Prokaryotes - cytoplasm

Question 18

Why is oxidative phosphorylation important?

Answer 18

• Being the final process of cellular respiration in eukaryotes, it provides bulk ATP for living organisms, and ATP is the main energy source for maintaining life activity.
• It involves the formation of reactive oxygen species and the regulation of apoptosis.

Question 19

NADPH (Nicotinamide Adenine Dinucelotide Phosphate Hydrogen) is one of the products of the Pentose Phosphate Pathway. What are some of its functions?

Answer 19

1. Cholesterol synthesis
2. Fatty acid synthesis
3. Reduction of glutathione (protects the cell from reactive oxygen species)

Question 20

What is oxidative stress?

Answer 20

Oxidative stress is an imbalance of free radicals and antioxidants in the body, which can lead to cell and tissue damage.

Question 21

State where each of the following metabolic pathways occur in the cell:
a) Glycolysis
b) Citric acid cycle/Kreb’s Cycle/Tricarboxylic acid cycle
c) Oxidative phosphorylation
d) Pentose phosphate pathway
e) Urea cycle/Ornithine cycle
f) Fatty acid β-oxidation

Answer 21

a) Cytoplasm (cytosol of cytoplasm)
b) Mitochondrial matrix
c) Mitochondrial inner membrane of eukaryotic cells and cytoplasm of prokaryotes
d) Cytosol of liver cells, adrenal cortex, mammary glands
e) Occurs in hepatocytes; first two steps take place in mitochondrial matrix, the rest of the steps take place in the cytosol
f) Mitochondrial matrix, peroxisome (for long chain fatty acids)

Question 22

What is the relationship between Fatty acid β-oxidation and Kreb’s cycle?

Answer 22

In Fatty acid β-oxidation, fatty acids are broken down into acetyl-CoA, to be used by the Kreb’s cycle.

Question 23

a) What is gluconeogenesis?
b) Where does it occur in vertebrates?
c) When does it occur?
d) What are the three main precursors used in gluconeogenesis?

Answer 23

a) A metabolic pathway that results in the biosynthesis of glucose from certain non-carbohydrate carbon substrates.
b) It occurs in the liver, and to a lesser extent, the cortex of the kidneys.
c) It occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.
d) Lactate (from anaerobic glycolysis in exercising muscle and red blood cells via the Cory cycle), glycerol (released from the breakdown of triglycerides in adipose tissue), amino acids (mainly alanine)