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 ADPàATP. 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 à 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 à 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 co-enzyme A is not broken down and returns to pick up another acetyl group.)
· 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 ADPàATP. 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