WHAT EFFECT DOES SUBSTRATE HAVE ON THE RATE OF RESPIRATION IN SACCHAROMYCES CEREVISIAE
Raja Awais Ali Biology Coursework
WHAT EFFECT DOES SUBSTRATE HAVE ON THE RATE OF RESPIRATION IN SACCHAROMYCES CEREVISIAE?
AIM
The aim of this investigation is to examine what effects different substrates have on the respiration of yeast. I will investigate this by measuring the amount of carbon dioxide evolved during anaerobic respiration. The volume of CO2 gas will be collected using a gas syringe.
BACKGROUND INFORMATION
YEAST
, also known as yeast, is a micro organism that uses saprophytic digestion to break down substrates. This is achieved through releasing specific enzymes to break down specific substrates, but if yeast does not contain a certain types of enzyme then it cannot break down its substrate. The more the enzyme of a particular substrate, the faster the rate of breakdown and therefore the more CO2 is produced. This will help me to test how much CO2 each substrate produces. Yeast can also respire aerobically and anerobically depending on the availability of O2. If there is plentiful of O2 then yeast would respire aerobically with sugars, producing H2O and CO2 as waste products. However, if no oxygen is available then the fermentation would occur which converts sugars into CO2 and ethanol.
RESPIRATION
Respiration is the process by which energy is released energy from glucose in the presence of Oxygen, forming carbon dioxide and water as waste products. Glucose releases energy in a series of reactions that take place inside components of the cell. The stages are briefly explained below:
GLYCOLYSIS
To get the sugar in a more reactive form it is produced to fructose-1,6-bisphosphate by the addition 2 phosphate molecules. This process is a phosphorylation reaction. The fructose-1,6-bisphosphate is then broken down into 2 molecules of glyceraldehydes-3-phosphate, which comprises of 3C each. The glyceraldehydes-3-phosphate converted into pyruvate via the oxidation process where each GAL3P molecule releases 2 hydrogen ions and 2 electrons. The electrons are then transferred to NAD to produce NADH (reduced NAD) and the energy is used to produce 4ATP from 4ADP and 4Pi. Finally there is a net yield of 2 molecules of ATP, and 2 molecules of pyruvate which is used in the link reaction and 2 molecules of reduced NAD which carries on to the link reaction.
LINK REACTION
In the link reaction the 2 molecules of pyruvate leave the cytoplasm of the cell and enter the mitochondrial matrix. This is an oxidation reaction where 2 NAD molecules oxidise 2 pyruvate molecules into 2 acid molecules. These 2 molecules of acetic acid then go on to combine with 2 coenzyme-A molecules to form Acetyl Co enzyme A. in the end of this stage 2 molecules of reduced NAD form, 2 molecules of CO2 is lost and most importantly, Acetyl Co enzyme A is formed through the conversion of pyruvate. This is then used in the next stage of respiration.
KREBS CYCLE
At the start Acetyl Coenzyme A , combines with Citrate Synthase an enzyme as well and a 4 carbon molecule called oxaloacetate, forming Citrate. Then, Citrate goes through the process of oxidative decarboxylation which forms a 5 carbon molecule called oxoglutarate.at this point NADH is produced and CO2 is removed. In the latter stages of the krebs cycle, the oxoglutarate is changed into a 4 carbon oxaloacetate molecule. NADH is made and 1 molecule ATP is also made. The volume of CO2 that is produced in the krebs cycle is important as this is the dependant variable.
ELECTRON TRANSPORT CHAIN
In this stage all of the NADH and FADH that has been produced in the previous stages is converted into ATP. This takes place in the cristae of the mitochondria. The NADH and FADH electrons move. When the electrons pass from one carrier to another, a series of reduction and oxidation reactions take place which releases energy in the process. This energy is used to pump H+ ions from the matrix into the intermembrane space, thus creating a gradient where the concentration of the H+ ions in the intermembranal space is higher than it s in the matrix. The inner membrane contains enzymes called ATP Synthase and The H+ ions diffuse through these enzymes causing energy to be released which is used to synthesise ATP through phosphorylation. The process is called because the final terminal electron acceptor is oxygen which picks up the electrons from the chain and the H+ ion from the matrix to form H20 as a waste product. This reaction is catalysed by the enzyme Cytochrome Oxidase
For every NADH which enters the chain and is oxidised by NADH dehydrogenase, 3 ATP are produced. For each FADH that enters the chain, 2 molecules of ATP are made.
ENZYMES
Enzymes are proteins that can effectively increase the rate of a reaction by lowering the required energy (activation energy) needed in order for the reaction to occur. Enzymes have a tertiary structure which decides the shape of the active site. The substrate must be specific to the active site because if they were not complementary to each other, then the substrate can no longer bind to the active site, thus the enzyme substrate complex does not form. The performance of enzymes can be affected in several ways some of which I have explained below.
TEMPERATURE
An increase in temperature will cause an increase in the rate of reaction because both the enzyme particles and substrate particles have gained kinetic energy. This will result in the particles to move faster, thus increasing collision frequency and the numbers of successful collisions as the particles have the required activation energy. If the temperature rises above the optimum temperature then the enzymes can become denatured. This happens because the enzyme molecule vibrates more causing the weak hydrogen bonds (holding the 3D structure of the enzyme together) to break. This eventually leads to the shape of the active site being altered. Consequently, the substrate will not be able to bind with the substrate as the shape of the active site is no longer complementary so the substrate enzyme complex can not form. This is important in my experiment because if the yeast (enzyme) was to become denatured then it would not be able to bind with the substrate (e.g. glucose) and the reaction would not be catalysed, preventing any CO2 from being formed. I must ensure that temperature is kept constant throughout.
