What is the effect on the rate of respiration of yeast cells with glucose when the temperature is varied?

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Biology Coursework

Planning

Problem

What is the effect on the rate of respiration of yeast cells with glucose when the temperature is varied?

Aim

The aim of the experiment is to investigate the effect of temperature on the rate of respiration of yeast cells with glucose. As yeast cells use up glucose in respiration, carbon dioxide gas is given off. Measurements of the volume of carbon dioxide gas given off within a set amount of time can be used to measure the rate of reaction. A fast rate of reaction would be indicated by a large volume of carbon dioxide gas being collected within this set amount of time. A small volume of carbon dioxide gas collected within the same amount of time would indicate a slower rate of reaction. The rate of reaction will be fastest at the optimum temperature for working enzymes in yeast cells.

Procedure for preliminary experiment

List of apparatus

A packet of fast acting yeast

A tub of D-glucose

Distilled water

A spatula

A rapid weighing balance

A 100ml measuring cylinder

A conical flask

A count down timer

A gas syringe

A delivery tube with rubber bungs tightly fitted at the ends joined to the gas syringe

2 clamp stands

An electrical water bath

Method 

The equipment above was collected and set up as shown below and the electrical water bath was set to 20ºC:

  • 25g of yeast and 12.5g of glucose were measured out into a 100ml beaker using a weighing balance.
  • Using a measuring cylinder, 50ml of distilled water at room temperature was measured.
  • Pour this into the beaker of yeast and glucose.
  • Dissolve the yeast, glucose and water together by stirring with a glass rod.
  • Pour this solution into the conical flask in the water bath.
  • Place the rubber bung on the top of the conical flask, making sure that it is tightly fitted.
  • A count down timer that was set to 2 minutes was started.
  • When this time elapsed the volume of gas collected was recorded and another count down timer set to 2 minutes was started.
  • After each 2 minutes the timer was reset and started immediately.

Results

Table to show the amount of carbon dioxide produced by the yeast solution at 20°C within 10 minutes.

Table to show the amount of carbon dioxide produced by the yeast solution at 30°C within 10 minutes.

Table to show the amount of carbon dioxide produced by the yeast solution at 40°C within 10 minutes.

Prediction based on the results of the preliminary experiment

I predict that the temperature of the yeast will affect its rate of respiration because enzymes are involved. I predict that the optimum temperature for the respiration of yeast cells is between 35ºC and 40ºC. The most volume of carbon dioxide will be collected within the set amount of time between these temperatures, indicating that the yeast cells will respire fastest between these temperatures. I predict that the rate of respiration in the yeast would start of at a low rate and as the temperature increased until 40ºC, the rate of respiration would also increase. Beyond 40ºC the amount of carbon dioxide collected within the set amount of time will start to decrease. At around 60ºC the rate of respiration will rapidly decrease, having collected only a very small volume of carbon dioxide gas. This is due to the enzymes involved being denatured.

Justification of prediction  

These predictions were based on my research on respiration and enzymes from textbooks and from the Internet. As the yeast cells begin to take up the glucose in solution during the first few stages of the experiment, they respire aerobically:

Glucose + oxygen                    carbon dioxide + water + energy

Glycolysis takes place in the cytoplasm, whereby glucose is converted to pyruvic acid. Oxygen is present at this stage and the pyruvic acid enters the mitochondrion. Here it is converted to acetyl coenzyme A. A molecule of carbon dioxide is given off. At the beginning of the Krebs’ Cycle acetyl coenzyme is present. It releases a further two molecules of carbon dioxide gas. Hydrogen atoms released from the Krebs’ Cycle are passed along the hydrogen and electron carrier system. Final products of this system include 3 molecules of ATP. Finally, the hydrogen atoms combine with oxygen to form water, which allows the reactions to continue. Anaerobic respiration of the yeast cells begins to take place in the conical flask when there is no longer any oxygen available.      

Glucose                      carbon dioxide + ethyl alcohol + less energy

Enzymes have active sites.  This is the binding site on the surface of the enzyme molecule for the substrate molecules.  An enzyme’s active site has regions that form chemical bonds, holding the substrate(s) in place.  Other groups of atoms in the active site speed up the chemical reaction of substrate(s) (reactants) to products.  Enzymes are very specific as they can only catalyse a reaction in which the substrate molecule has a complementary shape to that of its active site.  The substrate molecules that react together on the active site are likened to a ‘key’ and the active site to a ‘lock’.  If the substrate molecule does not have the correct configuration (shape) to fit into the active site (i.e. the key does not fit into the lock), then no reaction can take place.  

In general, the rate of an enzyme-controlled reaction is doubled with a rise of 10°C over a range from 0°C – 40°C.  This is the temperature coefficient, which has a value of two.  Above 40°C the rate of reaction falls off and declines rapidly and at 60°C or above the reaction stops altogether.  This is because enzymes are proteins.  Proteins have specific tertiary structures.  The polypeptide chains making up proteins bend and folds to form precise compact globular shapes.  Ionic bonds, hydrogen bonds, disulphide (covalent) bonds and hydrophobic interactions between polypeptide chains maintain this shape.  I am basing this prediction also partly on the collision theory. The collision theory says that increasing the temperature will speed up the respiration, because the glucose particles will have more energy, and they will collide with the active site of the yeast particles with greater frequency and force. At high temperatures the bonds holding the polypeptide chains together are broken as atoms gain enough heat energy and subsequently kinetic energy to overcome the bonds.  The polypeptide chains then open up and become randomly arranged.  The specific shape of the active site becomes altered so substrate molecules can no longer fit in so the reaction stops. (Biology Foundation 5.1.3 Enzymes).

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Table to show reasons for choice of apparatus

Modifications

In light of the results from my preliminary experiment, it was foreseeable that improvements were needed to be made so as to increase the reliability of the results obtained, making them more accurate. One of these was to replicate the experiment three times for each temperature. A ...

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