Metabolic pathways

Question 1

Metabolism

Answer 1

Set of life sustaining chemical transformations within cells or organisms.

Question 2

Catabolism

Answer 2

Breaking down of organic matter. Release energy.

Question 3

Anabolism

Answer 3

Building up of cell components. Consume energy.

Question 4

Metabolic pathway

Answer 4

Chemical transformed through a series of steps into another chemical, by a sequence of enzymes.

Question 5

Cellular respiration

Answer 5

The complete breakdown of glucose can be summarized with the
following reaction:
C6H12O6 + 6 O2 + 32 ADP + 32 P → 6 CO2 + 6 H2O + 32 ATP + heat
- Glucose is completely oxidized to CO2.
- Oxygen is completely reduced to water.
- Electron carriers behave as intermediary, many NAD+ are reduced
throughout cellular respiration and it drops the electrons to the
electron transport chain, in which oxygen is finally reduces.
- Series of redox reaction.

Question 6

Steps of cellular respiration

Answer 6

1. Glycolysis
2. Pyruvate oxidation
3. Citric acid cycle
4. Oxidative phosphorilation

Question 7

1. Glycolysis

Answer 7

Glucose → G-6-P→2xPyruvate 
Gain: 2x Pyruvate, 2 ATP, 2 NADH
Happens in the cytosol.
No O2 needed. 
Energy requiring phase: 2 ATP, 2NAD+ 
Energy releasing phase: 4 ATP,
2 NADH,

Question 8

2. Pyruvate oxidation

Answer 8

Happens in mitochondria, matrix.
Gain: 2x Acetyl CoA, 2 NADH, 2 CO2 (per molecule of glucose)
Needs oxygen.
Pyruvate goes into mitochondria and completely oxidizes.
Irreversible reaction!

Question 9

3. Citric acid cycle

Answer 9

Series of redox reactions.
Fueled by Acetyl-CoA.
Gain: 2 ATP, 4 CO2, 6 NADH, 2 FADH2 (per molecule of glucose)(per cycle, cut in half).
Happens in mitochondria.
Combines with oxaloacetate to form Citrate.

Question 10

4. Oxidative Phosphorylation

Answer 10

Electron transport chain:
Happens in inner membrane of mitochondria.
Gain: 10 NAD+, 2 FAD, 6 H2O
1) Regeneration of NAD+ and FAD: required for glycolysis and Krebs cycle
2) Electron carrier are oxidized, oxygen is reduced to water
3) Creation of a proton gradient across the inner mitochondrial membrane
Followed by chemiosmosis.

Question 11

4. Chemiosmosis (oxidative phosphorylation)

Answer 11

The proton gradient created by the electron transport chain is used to
power the ATP synthase.
Gain: 28 ATP.
Catalyzes the addition of phosphate to ADP creating ATP.
Four H+ ions must flow back into
the matrix through ATP synthase
to allow the synthesis of one ATP
molecule.
Each NADH yields about 2.5 ATP.

Question 12

Who is the final acceptor of electrons?

Answer 12

CO2 (form H2O with protons).

Question 13

Lactic fermentation

Answer 13

IF OXYGEN IS NOT AVAILABLE.
Electrons are dropped onto pyruvate, which is reduced to lactate.
Do not produce energy.
Regeneration of Cytosolic NAP+ to allow glycolysis to keep happening.
Location: skeletal muscle cells & red blood cells (all the time: no mitochondria)
Happens in cytosol.
Conditions: lack of oxygen in muscles cells.

Question 14

Glycogen Metabolism

Answer 14

G-6-P↔️Glycogen.
As the synthesis and degradation pathways use different enzymes, one can be activated while the other is inhibited.
G-1-P→Glycogen : Glycogen synthesis.
Enzyme: glycogen synthase
Energy storage.
Glycogen→G-1-P : Glycogenolysis (Glycogen breakdown).
Enzyme: Glycogen phosphorylase

Question 15

Glycogen usage in muscle cells

Answer 15

Energy production for muscle contraction.

Question 16

Glycogen usage in liver

Answer 16

Glucose production for maintaining
glycemia.
Activates Gluconeogenesis at the same time.

Question 17

Gluconeogenesis

Answer 17

Pyruvate→Oxaloacetate→G-6-P→Glucose

Sources of carbon: AA, Glycerol, Lactate (Exam).

Question 18

What pathways in the liver can be used to maintain glycemia? (Carbohydrate metabolism)

Answer 18

Gluconeogenesis: Secrete glucose
Glycogenolysis: Secrete glucose
Glycogen synthesis: Store glucose as glycogen.

Question 19

Amino acid metabolism

Answer 19

Amino acids can be used in times of energetic need to produce glucose or ketone bodies.
1. Gluconeogenic amino
acids can be transformed
into glucose.
2. Ketogenic amino acids are
broken down to acetyl-CoA
and thus can be converted to
fatty acids or ketone bodies

Question 20

Lipid metabolism

Answer 20

Triglycerides ↔️ Glycerol, Fatty acids
Lipogenesis
Lipolysis

Question 21

Lipogenesis

Answer 21

Glycerol, Fatty acid→TAG
Lipogenesis occurs in the liver and in the adipose tissue, when energy and glucose are high.
Triglycerides synthetized in the liver are sent to the adipose tissue through VLDL (very low density lipoproteins).
Used to store energy.

Question 22

Lipolysis

Answer 22

TAG→Glycerol, Fatty acids
Used to obtain energy from fat.
Lipolysis takes place in the cytoplasm of adipocytes in response to hormones.
INSULIN suppresses lipolysis.
EPINEPHRINE activates lipolysis.

Question 23

β-Oxidation

Answer 23

Fatty Acids→Acetyl CoA
Fatty acids can produce ATP:
β-oxidation produce acetyl-CoA and acetyl-CoA can be oxidized through the citric acid cycle.

Question 24

Why can’t Fatty acids contribute to the maintenance of glycemia?

Answer 24

Acetyl-coA can’t be transformed in pyruvate: thus fatty acids can’t be
transformed into glucose.

Question 25

Ketogenesis

Answer 25

Acetyl-CoA→Ketone bodies
Happens in the liver under prolonged
FASTING conditions.
Purpose: production of a source of energy usable to all tissue (including the brain).
=> Limit glucose consumption from the brain.
Acetyl-CoA uses ketogenic amino acids to form ketone bodies.

Question 26

Ketone body oxidation

Answer 26

Most tissue can use
ketone bodies to
produce ATP (including
the brain).
Exception: liver (it
produces them) and red
blood cells (lack
mitochondria).

Question 27

Protein metabolism

Answer 27

Amino acid oxidation
AA→NH2 group, carbon skeleton
The amino group must be removed.
The urea cycle transform the resulting ammonia in a less toxic product: urea.
Amino acids can be used to
produce ATP with ketogenic and gluconeogenic AA.
Storage of amino acids excess happens in the liver in the FED STATE!