Energy and respiration
Some Basics
What Respiration actually is…
"Respiration is the chemical process of releasing energy from organic compounds"
It is important that you understand the correct terms for some of the processes;
Gas exchange
The movement of oxygen into an organism and carbon dioxide out of an organism.
Breathing
The ventilation movements that are needed in some larger animals so that efficient gas exchange can take place. It involves ribs, intercostal muscles, the diaphragm and lungs.
Respiration
The process by which complex organic molecules are broken down to release energy.
Aerobic respiration
Requires oxygen to fully oxidise the organic molecule. This releases lots of energy.
Anaerobic respiration
The breakdown of the molecule without oxygen. This releases much less energy.
At GCSE aerobic respiration was presented in one equation:
C6H12O6 + 6 02 6 H2O + 6 CO2 + ENERGY
glucose + oxygen water + carbon dioxide + energy
However, this is a gross
over-simplification. In reality it is a long series of enzyme-controlled reactions.
ATP
ATP (adenosine triphosphate) is the universal currency of energy. It is a small
molecule with 3 phosphate groups (P) attached to an adenosine molecule i.e.
Adenosine-P-P-P
During respiration, high energy C-C, C-H and C-OH bonds are broken. Lower energy bonds are formed and the difference is released and used to attach a P to Adenosine-P-P (ADP adenosine diphosphate), making ATP.
When energy is required at a later time by a cell, it can use the ATP and break a P off the end. This releases the energy needed (30.6kJ for every ATP, ADP + P). The more ATPs used, the more energy is released.
For aerobic respiration to occur, the cell needs to possess mitochondria.
An outline of the stages of respiration
Stages of Respiration
Glycolysis (splitting of sugar)
What: formation of pyruvic acid from glucose
Where: cytoplasm of a cell
Krebs Cycle
What: removal of hydrogen from pyruvic acid
Where: matrix of the mitochondria
Electron Transport Chain / Oxidative Phosphorylation
What: using hydrogen to produce ATP (see chemiosmotic theory)
Where: inner mitochondrial membrane.
Electron/hydrogen carriers
Many of the reactions in the stages of respiration involve oxidation by the
removal of electrons or hydrogen atoms (H). These are transferred to electron/hydrogen
carriers. Ultimately they are passed to oxygen to form water right at the very
last stage of respiration.
Two important electron/hydrogen carriers are NAD (nicotinamide adenine dinucleotide), and FAD (flavin adenine dinucleotide).
Oxidation/Reduction
Oxidation is the addition of oxygen, the removal of hydrogen or the loss of electrons.
Reduction is the addition of hydrogen, the removal of oxygen or the gain of electrons.
A quick way to remember this is 'OILRIG':
Oxidation Is Loss, Reduction
Is Gain
Glycolysis
1.
Glucose is phosphorylated twice to make a 6C sugar phosphate. 2 ATPs are used to supply the P groups. This makes the glucose more reactive and so…
2.
The 6C sugar phosphate breaks down to form 2, 3-carbon sugar phosphates, called triose phosphates (TP).
3.
Hydrogen is removed from each of the 2 TP molecules. The hydrogens are passed to 2 NADs (the NADs are reduced). 2 ATPs are made directly from the conversion of each TP to pyruvic acid (written shorthand as PA or called pyruvate) as the phosphate groups are removed.
Into glycolysis
Out of glycolysis
1 Glucose (6C)
2 Pyruvic acids (3C)
2 NAD
2 reduced NAD (2 NADH + H+)
2 ATP
4 ATP
Net Gain = 2 ATP and 2 NADH + H+
This stage occurres in the
cytoplasm. The next stage occurs in the mitochondria. From here, the reaction
will only proceed if oxygen is available.
The Link Reaction
1.
The PA molecules enter the mitochondrion.
2.
CO2 and hydrogen are removed from each PA to create 2 2-C molecules. The hydrogen is transferred to NAD
3.
The 2-C molecule is then combined with coenzyme A (CoA) to form the 2C compound, acetylCoA. (CoA is a vitamin derivative which acts as a transporter of the 'acetate' psrt left from the PA molecule)
Into Link
Out of link
2 PA and 2 CoA
2 Acetyl CoA
2 CO2
2 NAD
2 reduced NAD (2 NADH + H+)
Krebs Cycle
This is also known as the citric acid cycle or the tricarboxylic acid cycle.
For each glucose molecule, there were 2 pyruvic acid molecules formed, (and therefore 2 acetylCoA molecules formed) so the whole cycle takes place twice for every glucose molecule respired.
1.
Each acetylCoA (2C) combines with an oxaloacetic acid (4C) to make a 6C compound (citric acid).
2.
In a series of steps, for
each 6C compound,
2 CO2 molecules are released,
3 NAD molecules are reduced,
2 FAD molecules are reduced,
1 ATP molecule is made directly.
3.
The 4C compound is regenerated (by the removal of the 2 Cs in 2 CO2 molecules) so that the cycle can begin again with more molecules of acetylCoA.
Into Krebs
Out of Krebs
2 acetylCoA
2 oxaloacetate
4 CO2
6 NAD
6 reduced NAD (6 NADH + H+)
2 FAD
2 reduced FAD (2 FADH + H+)
2 ADP + P
2 ATP