RESPIRATION
Respiration is the conversion of chemical energy in organic compounds into
energy able to be used in the cells.
In respiration, glucose is rapidly broken down in a series of steps controlled by
enzymes. ADP is present in all of the cells. The energy released causes the
conversion of ADPATP. ATP, Adenosine triphosphate, the high energy
molecule, stores the chemical energy for later use. When ATP is converted to
ADP (adenosine diphosphate) energy is released and used for cell processes.
The energy cycle in each cell.
The overall equation for respiration
6O2 + C6H12O6+ 38 ADP + 38 P  6 CO2 + 6 H2O + 38 ATP
Respiration occurs in 3 stages
1. Glycolysis
2. The krebs cycle (citric acid cycle)
3. The respiratory chain (electron transport system)
Glycolysis
Glycolysis is the firs stage of respiration and occurs in the cytoplasm. Glucose
is broken down into pyruvate(pyruvic acid) molecules and ATP
Glucose  pyruvate + ATP
Glycolysis yields very little energy (only 2 ATP molecules form) but does not
require oxygen i.e. it is an anaerobic step.
The Krebs cycle
Pyruvate from the cytoplasm is first changed to acetyl coenzyme A (acetyl
coA) and carbon dioxide inside mitochondria.(called the transition reaction)
The acetyl component of the acetyl co A is then broken down in the Krebs
cycle, a cycle of acids, into carbon dioxide, H atoms and ATP.( The coenzyme A is not broken down and returns to pick up another acetyl group.)
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Occurs in the matrix of the mitochondria
Uses pyruvic acid as its raw material
Cycle involves many steps and many reactions
It is an aerobic stage, O2 needs to be present.
The steps of the cycle result in pyruvate being broken down to bring
about the release of carbon dioxide and hydrogen
Carbon dioxide is released form the cell as a waste product of
respiration
A hydrogen carrier, NAD or FAD takes the released hydrogen, to the
respiratory chain. The hydrogen produced contains lots of energy in its
electrons. The energy associated with the electrons is involved in the
production of more ATP, most of the energy of respiration, in the
respiratory chain.
The Respiratory chain/ the electron transfer system.
Most of the energy of the respiration process is released in this stage and is
stored as ATP. It occurs on the inner membrane of each mitochondrion, the
cristae, whose foldings greatly increase the surface area. 34 molecules of
ADP are converted into 34 molecules of ATP in this stage.
The hydrogen molecules break up into hydrogen ions and electrons. The
electrons pass along a series of cytochromes,(electron carriers) each one at
a lower energy level. The energy released at each stage changes the
ADPATP. The last cytochrome is cytochrome oxidase. Oxygen enters at this
stage and combines with the electrons to form O2-. The O2- combines with the
H+ to form water.
2H+ +O2- =H2O
This water is another waste product.
Aerobic and Anaerobic respiration(fermentation)
Respiration can either occur in the presence of oxygen (aerobic respiration)
or in the absence of oxygen (fermentation)
If oxygen is present during respiration then the Krebs cycle and the respiratory
chain occur and much energy is produced.
If oxygen is absent, the pyruvic acid produced during glycolysis is
1. Converted into lactic acid and energy by animals or
2. Converted into ethyl alcohol, carbon dioxide and energy by plants
If fermentation occurs, only 2 ATP molecules are produced (compared with
38 during aerobic respiration).
The energy remains locked up in the end products.
Fermentation cannot continue indefinitely, as there is a build up of toxic
products, Lactic acid and ethanol, that inhibit further metabolic activity.
Many plants and animals are facultative fermenters- they respire without O2
for a certain time and then respire with O2.
Factors affecting the rate of respiration
The rate of respiration depends on
1. The amount of oxygen present in the cells
2. the amount of glucose or fat present in the cell
3. the cell or body’s demand for energy
4. temperature: the higher the temperature, the greater the rate of
respiration