respiration Flashcards
what is respiration?
A series of exothermic chemical reactions where one form of chemical energy (glucose) is converted to a more useful form (ATP)
- ATP molecules transfer less energy when hydrolysed than glucose does when oxidised. ATP more flexible than glucose, and more manageable quantities of energy
- energy more rapidly transferred from ATP as only one bond has to be hydrolysed to turn ATP into ADP
AEROBIC RESPIRATION- requires oxygen and produces CO2, water and ATP
anaerobic respiration- absence of oxygen and produces lactate (animals) or ethanol and CO2 (in plants and fungi), but only a little ATP
glycolysis
First stage of both aerobic and anaerobic respiration
- occurs in the cytoplasm
- when glucose is split into two of the 3-carbon molecules, pyruvate
- glucose —> phosphorylated glucose requires hydrolysis of 2 ATP molecules to provide Pi and energy. Glucose is phosphorylated
- phosphorylated glucose is split into two triose phosphate molecules
- triose phosphate then forms pyruvate, an acid that is also three carbon. This requires 2 NAD, which is reduced to 2 NADH, as a hydrogen is removed from each of the two triose phosphate molecules.
- ATP is formed from ADP + Pi during the formation of pyruvate from triose phosphate. 2 ATPs are produced per triose phosphate, so 4 ATPs overall
The overall yield from one glucose molecule undergoing glycolysis is therefore: two molecules of ATP, 4 are produced but two are used up in initial phosphorylation of glucose
two molecules of NADH
two molecules of pyruvate
- The name of the formation of ATP during respiration is substrate-level phosphorylation
link reaction and Krebs cycle
If oxygen is available pyruvate from glycolysis enters the mitochondrial matrix by ACTIVE TRANSPORT
- the link reaction links glycolysis to the Krebs cycle and this also occurs in the mitochondria
- in krebs cycle and link reaction NADH is produced, but in the calvin cycle and light-dependant reaction it is NADPH
oxidative phosphorylation
- occurs in inner membranes of the mitochondria, cristae
- chemical energy in high energy electrons, in reduced co-enzymes comes from glucose. The chemical energy is transferred through a series of chemical reactions
1. NADH is oxidised into NAD, releasing H+ and electrons into the matrix. FADH is also oxidised to FAD. The electrons of the hydrogen atom are donated to the first molecule of the electron transfer chain
2. the electrons are moved down the electron transport chain through carrier proteins. The carrier it moves to is reduced and the one it moves from is oxidised. As the electrons flow along the chain, the energy they release causes the active transport of H+ across the inner mitochondrial membrane and into inter-membranal space
3. The H+ move through the inner mitochondrial membrane to the intermembrane space by active transport
4. The H+ then accumulate in the inter-membranal space and then move down a concentration gradient, through the ATP synthase molecule. The energy transferred from this forms ATP from ADP and Pi
5. The electrons in the final carrier molecule are then accepted by an oxygen molecule and react with H+ to form H20
6. 1/2 O2 + 2e- + 2H+ —-> H20. The oxygen molecule is the terminal electron acceptor
This process is the chemiosmotic theory of oxidative phosphorylation
The importance of oxygen in respiration is to act as the final acceptor of the hydrogen atoms produced in glycolysis and the Krebs cycle. Without its role in removing hydrogen atoms at the end of the chain, H+ and e- would back up along the chain and the process of respiration would come t a halt
Called oxidative phosphorylation as oxidation is required to move electrons along the electron transport chain, and phosphorylation occurs to ADP molecules when forming ATP
releasing energy in stages
In general, the greater the energy that is released in a single step, the more of it is released as heat and the less there is available for more useful purposes. When energy is released a little at a time, more of it can be harvested for the benefit of the organisms. For this reason, the electrons carried by NAD and FAD aren’t transferred in one explosive step.
Instead they are passed along a series of electron transfer carrier molecules, each of which is at a slightly lower energy level. The electrons therefore move down an energy gradient. The transfer of electrons down this gradient allows their energy to be released more gradually and therefore more usefully
alternative respiratory substrates
Sugars aren’t the only substrates that can be oxidised by cells to release energy. Both lipids and protein may, in certain circumstances, be used as respiratory substrates, without first being converted to carbohydrate
- respiration of lipids. Before being respired, lipids are first hydrolysed to glycerol and fatty acids. The glycerol is the phosphorylated and converted to triose phosphate which enters the glycolysis pathway and subsequently the Krebs cycle. The fatty acid component is broken down into 2-carbon fragments which are converted to acetyl coenzyme A. This enters the Krebs cycle
The oxidation of lipids produces 2-carbon fragments of carbohydrate and many hydrogen atoms. The hydrogen atoms are used to produce ATP during oxidative phosphorylation. For this reason lipids release more than double the energy of the same mass of carbohydrate - Respiration of a protein
Protein is another potential source of energy. It is first hydrolysed to its constituent amino acids. These have their amino group removed (deamination) before entering the respiratory pathway at different points depending on the number of carbon atoms they contain.
3-carbon compounds are converted to pyruvate, while 4 and 5 carbon compounds are converted to intermediates in the Krebs cycle
anaerobic respiration
In the absence of oxygen, neither the Krebs cycle nor oxidative phosphorylation can occur. This leaves only anaerobic process of glycolysis as potential source of ATP. For glycolysis to continue, its products of pyruvate and hydrogen most be constantly removed. In particular, the hydrogen must be released from NADH in order to regenerate NAD. Without this, the already tiny supply of NAD in cells will be entirely converted to NADH, leaving no NAD to take up the hydrogen newly produced from glycolysis
Glycolysis would then grind to a halt
- occurs in the cytoplasm of the cell ONLY
anaerobic respiration in animals
Occurs as a means of overcoming a temporary shortage of oxygen
- lactate production occurs most commonly in muscles as a result of strenuous exercise. In these conditions oxygen may be used up more rapidly than it can be supplied and therefore an oxygen debt occurs. Often essential, that the muscles continue to work despite the shortage of oxygen, eg if an organism is fleeing from a predator.
-each pyruvate molecule produced takes up two hydrogen atoms from NADH to form lactate
pyruvate + NADH —> lactate + NAD
catalysed by lactate dehydrogenase
- anaerobic respiration doesn’t produce CO2, as lactate and pyruvate are both 3 carbon molecules
- lactate is broken down by oxygen in the liver. It is converted back into pyruvate. Lactate is oxidised. This can then be further oxidised to release energy, or converted into glycogen. This happens when oxygen is once again available
The reaction to form lactate reforms NAD. This can be used in the reaction to form pyruvate again, allowing anaerobic respiration to continue, and therefore allowing more ATP to be produced
- lactate will cause cramp and muscle fatigue if it is allowed to accumulate in the muscle tissue. As lactate is an acid, it also causes pH changes which effect enzymes
Although the muscle has a certain tolerance to lactate, it is nevertheless important that it is removed y the blood and taken to the liver to be converted to glycogen
anaerobic respiration in plants
Same process but produces ethanol and CO2, as well as the oxidised NAD. Occurs in organisms such as bacteria and fungi, eg yeast, as well as in some cells of higher plants, eg root cells under waterlogged conditions
- the pyruvate molecule at the end of glycolysis loses a molecule of CO2 and accepts a hydrogen from reduced NAD to produce ethanol
- this form of anaerobic respiration can be exploited by humans in the brewing industry to produce ethanol
energy yields from anerobic respiration
- substrate-level phosphorylation in glycolysis and the Krebs cycle provides energy. This is the direct transfer of a phosphate from a respiratory intermediate to ADP to produce atp
- oxidative phosphorylation in the electron transfer chain. This is the indirect linking of energy from phosphate to ADP to produce ATP involving energy from the hydrogen atoms that are carried on NAD and FAD. Cells produce most of their ATP this way
In anaerobic respiration, pyruvate is converted to either ethanol or lactate. Consequently, it is not available for the Krebs cycle. Therefore in anaerobic respiration neither the Krebs cycle nor the electron transfer chain can take place. The only ATP produced is therefore that formed by glycolysis. This leads to anaerobic respiration being very inefficient. Doesnt produce many ATP’s per glucose- only 2. BUT has a very high rate as lots of ATPs are produced per second
