Senin, 23 Februari 2009

Respirasi sel

Overview Respirasi sel
Respirasi sel : pelepasan energi dari karbohidrat dan sintesis ATP
Perlu oksigen (O2)
Melepaskan karbondioksida (CO2)
glucose split to carbon dioxide and water.

Oksidasi glukosa (exergonic) akan mendorong sintesis ATP (endergonic), coupled reaction

Satu molekul glukosa menghasilkan 36 to 38 molekul ATP (efficiency of approx. 40%)
NAD+ and FAD
Nicotinamide adenine dinucleotide), flavin adenine dinucleotide
NAD+ used more often
accept two electrons plus a hydrogen ion (H+) Reduced or oxidized?
The NAD+ cycle
Fase pemecahan molekul glukosa
Oksidasi glukosa dengan menghilangkan atom hidrogen melibatkan 4 fase
Glycolysis – pemecahan molekul glukosa menjadi 2 molekul asam piruvat dalam sitoplasma; no oxygen needed; yields 2 ATP
Transition reaction – piruvat dioksidasi menjadi gugus asetil dengan 2-carbon, dikatalisis oleh CoA (acetyl CoA); CO2 is removed; twice per glucose molecule

Citric acid cycle –reaksi oksidasi siklis yang melepaskan CO2 , menghasilkan satu molekul, NADH, FADH; berlangsung 2 kali per molekul glukosa
Electron transport system – a series of carriers that accept electrons removed from glucose and pass them from one carrier to the next until the final receptor, O2 is reached; water is produced; energy is released and used to synthesize 32 to 34 ATP
If oxygen is not available, fermentation occurs in the cytoplasm instead of proceeding to cellular respiration.

The four phases of complete glucose breakdown
Diluar mitokondria: Glycolysis
Glycolysis berlangsung di sitoplasma
dan merupakan reaksi pemecahan glukosa menjadi 2 molekul asam piruvat

Glycolysis is found in all organisms

Glycolysis does not require oxygen.

(Insert Fig. 7.4a)
Energy-Investment Steps
two ATP are used to activate glucose,
Glucose splits into two C3 molecules (PGAL).
PGAL carries a phosphate group from ATP.
From this point on, each C3 molecule undergoes the same series of reactions.
Energy-Harvesting Steps
Oksidasi PGAL berlangsung dengan menghilangkan elektron dan diikuti dengan ion hidrogen, keduanya ditangkap oleh koenzim NAD+:
2 NAD+ + 4H → 2 NADH + 2 H+
The oxidation of PGAL and subsequent substrates results in four high-energy phosphate groups used to synthesize ATP in substrate-level phosphorylation.

Glycolysis summary
2 NAD+
4 ADP + 2 P

Jika tersedia oksigen- pyruvate enters the mitochondria.
Jika tidak tersedia oxygen, fermentation occurs
Fermentation - anaeorbic (does not require oxygen), in humans lactic acid is produced.

mitochondrion –
double membrane
intermembrane space between the two layers.
Cristae -folds of inner membrane that jut out into the matrix, the innermost compartment
transition reaction and citric acid cycle occur in the matrix
the electron transport system is located in the cristae.

Transition Reaction
1) connects glycolysis to the citric acid cycle
2) Pyruvate is converted to a C2 acetyl group attached to coenzyme A (together called acetyl CoA)
3) CO2 is released.
4) NAD+ is converted to NADH + H+
Citric Acid Cycle
1) Jalur metabolisme siklis yang berlangsung pada matriks mitokondria
2) acetylCoA joins a C4 molecule, and C6 citrate results.
AcetylCoA will be oxidized to CoA and 2 CO2 molecules.

4) oxidation occurs when NAD+ accepts electrons (happens 3 times) and FAD accepts electrons once.
Gain 1 ATP per acetyl CoA (substrate-level phosphorylation)

Citric acid cycle inputs and outputs per glucose molecule
2 acetyl groups
6 NAD+
2 ADP + 2 P
Electron Transport System
located in the cristae of mitochondria
series of protein carriers (some are cytochromes), pass electrons from one to the other.
3) electrons removed from NADH and FADH2 and enter the electron transport system.
pair of electrons is passed from carrier to carrier
energy is released and used to form ATP molecules by (oxidative phosphorylation)

6) Oxygen receives energy-spent electrons at the end of the electron transport system.
7) oxygen combines with hydrogen, and water forms:
Where did the hydrogen come from?

Organization of Cristae
electron transport system is located in the cristae
consists of protein complexes and mobile carriers.
The carriers use the energy released by electrons as they move down the carriers to pump H+ from the matrix into the intermembrane space of the mitochondrion.

pH gradient is established with few H+ in the matrix and many in the intermembrane space.
Is the pH of the matrix high or low?
Is the pH of the intermembrane space high or low?
cristae contain an ATP synthase complex through which hydrogen ions
Which way do they flow?
Energy used to synthesis ATP (chemiosmosis)

Accounting of energy yield per glucose molecule breakdown
Advantages and Disadvantages of Fermentation
Fermentation can provide a rapid burst of ATP in muscle cells, even when oxygen is in limited supply.
Lactate, however, is toxic to cells.
Initially, blood carries away lactate as it forms; eventually lactate builds up, lowering cell pH, and causing muscles to fatigue.
Oxygen debt occurs, and the liver must reconvert lactate to pyruvate.
Efficiency of Fermentation
Two ATP produced during fermentation are equivalent to 14.6 kcal; complete oxidation of glucose to CO2 and H2O represents a yield of 686 kcal per molecule of glucose.
Thus, fermentation is only 2.1% efficient compared to cellular respiration.
(14.6/686) x 100 = 2.1%
Metabolic Pool and Biosynthesis
Degradative reactions, which occur in catabolism, break down molecules and are exergonic.
Synthetic reactions, which occur during anabolism, tend to be endergonic.
Catabolism drives anabolism because catabolism results in ATP buildup used by anabolism.
Molecules aside from glucose can enter the catabolic reactions of cellular respiration.
When a fat is used for energy, it breaks down into glycerol and three fatty acids; glycerol is converted to PGAL, and the fatty acids are converted to acetyl-CoA, thus both types of molecules can enter the citric acid cycle.

The carbon backbones of amino acids can also enter the reactions of cellular respiration to provide energy.
The amino acid first undergoes deamination, or the removal of the amino group; the amino group becomes ammonia (NH3) and is excreted.
Where the carbon portion of the amino acid enters the reactions of respiration depends on its number of carbons.
The substrates of the pathways of cellular respiration can also be used as starting materials for synthetic reactions.
This is the cell’s metabolic pool, in which one type of molecule can be converted into another.
In this way, dietary carbohydrates can be converted to stored fat, and come substrates of the citric acid cycle can be transaminated into amino acids.
The metabolic pool concept
Chapter Summary
During cellular respiration, glucose is oxidized to CO2 and H2O; this exergonic reaction drives ATP buildup.
Four phases of cellular respiration occur:
1) Glycolysis, in the cytosol, is the breakdown of glucose to two pyruvates, with the formation of 2 NADH and net gain of 2 ATP.
2) A transition reaction takes place to convert pyruvate into acetyl-CoA, with CO2 given off; two NADH result in total.

3) The acetyl group enters the citric acid cycle, located in the matrix of the mitochondria; complete oxidation follows, and two CO2, three NADH, one FADH2, and two ATP are formed – the entire cycle runs twice per glucose molecule.
4) The final stage of glucose breakdown is the electron transport system located in the cristae of the mitochondria; electrons from NADH and FADH2 are passed down a chain of carriers until O2 is reached and H2O is formed. ATP is formed during oxidative phosphorylation via chemiosmosis.

Fermentation involves glycolysis, followed by the reduction of pyruvate to lactate or alcohol and CO2; in humans, it provides a quick burst of energy but triggers oxygen debt.
Carbohydrate, protein, and fat can be used for energy, and their components can be used for synthesis of needed compounds; both anabolism and catabolism use the same metabolic pool of reactants.

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