Investigation into the effects of different

Authors Avatar

Alota boka

Investigation into the effects of different

Carbohydrate substrates on the growth of Yeast.

Introduction:

Yeasts are Eukaryotic – unicellular, heterotrophic, non-photosynthetic organisms, with cell walls made of Chitin and belong to the kingdom of Fungi. They are usually about 5-10 microns in size. They are usually found in waste water and on the skin surface of fruits .Ecologically, yeasts are decomposers that secrete enzymes which dismantle the complex carbon compounds of plant cell walls and animal tissue which they convert into sugar, which is used for its living processes i.e. (growth) .the type of yeast used in this investigation, is Saccharomyces cerevisiae (S.cerevisiae) .Also known as baker’s yeast and is widely used in the baking and in alcoholic beverages industries.  This is the taxonomy of S.cerevisiae:

Kingdom; Fungi        Family; Saccharomycetaceae

Phylum; Ascomycota        Genus; Saccharomyces

Class; Saccharomycestes        Species; Saccharomyces cerevisiae

Order; Saccharomycetates

Its genome is comprised of about 13,000,000 base pairs and 6,275 genes, which is all compactly organised on to 16 chromosomes. It is said that it shares about 23% of its genome with humans. This is a diagram to show the cellular structure of S.cerevisiae;

The cell wall being made out of Chitin (Polysaccharide) gives the yeast tolerance to acidity, where they can grow in a pH value range of 4-4.5.moreover this results in the enzymes involved in fermentation and respiration to operate in an optimum pH environment of (5.5), and so within the yeast cell it is largely unaffected by the external extreme changes in pH. The cell wall also acts like a filter, which allows only molecules of a moderate size to diffuse through. And, of course the plasma membrane acts as a partially permeable membrane, with carrier proteins (for active transport of glucose), enzymes known as permeases which aid with the transport of substances, and channel proteins(for the facilitated diffusion of glucose).They have an active transport system, which metabolizes glucose through the glycolytic pathway, This system is effective at low glucose concentrations, but if concentrations are high the system will be inhibited  , if this occurs then glucose is transported via facilitated diffusion, which is less efficient for the yeast.

S.cerevisiae contain enzymes such as yeast invertase, sucrase and maltase, which are involved in the hydrolyses of the large polymers into small easily transported monomers, (this process is known as extra-cellular digestion).However, they do not contain enzymes to hydrolyse Starch.

Reproduction; Yeast reproduce asexually by “Budding”. it is the most common mode of vegetative growth in yeasts, and is a typical reproductive characteristic of S.cerevisiae. They have a diploid number of 16 chromosomes which remains intact , since the budding involves mitosis, where cell division occurs resulting in the  in the production of genetically identical cells, the only source of variation is via a mutation which may occur during the replication of the DNA.during Mitosis  (Interphase) there is an increase in the number of DNA , as DNA replication occurs and the subsequent mitotic stages develop .This results in the size of the mother cell to reach a critical size. When this occurs yeast buds are initiated, as when reached this critical size the cell wall weakens, due to the tension. This allows the “forcing out” of the cytoplasm into an area bounded by new cell wall material. This is a diagram representing the cell division cycle of S.cerevisiae

However the growth and reproduction of the S.cerevisiae depends on the ATP energy supply, as it is needed for the active transport of glucose and for cell division .Carbohydrates e.g. Sugar supplies this energy, via the process of respiration. This occurs within the mitochondria in the yeast’s cell .It is said that the main respiratory substrate for yeast is glucose. Yeasts are facultive anaerobes, which mean that they can switch from aerobic to anaerobic (and vice versa) depending on the amount of oxygen present in the medium, in which they are growing in. In Aerobic respiration, where oxygen supply is plentiful along with a supply of sugar (e.g. glucose)

Glycolysis occurs in the cytoplasm of the yeast cell , where glucose is oxidised into pyruvate, kreb’s cycle and the link reaction occur in the matrix of the mitochondria, in kreb’s cycle; reduced co- enzymes and carbon dioxide are produced. Finally in the ETC (Electron transfer chain) as the electrons from the reduced co-enzymes are passed along the electron carriers, energy is released, this is then used to phosphorylate ADP to result in the production of ATP energy, the last electron acceptor is oxygen, and so combines with the electron and the H+ protons to produce the waste product of water. This is an equation for the aerobic respiration of yeast;

C6H12O +   6O2                        6CO2 + 6 H2O + 36-38 ATP

However, in low oxygen conditions, anaerobic respiration occurs, in yeast this is called fermentation. This is when since there is not oxygen as a final electron acceptor in the ETC and so the co-enzyme will remain reduced (NADH), and so the only way to oxidise it is by oxidising pyruvate, which is converted into Ethanol. This is an equation for anaerobic respiration/fermentation;

Join now!

C6H12O6                        2 CO2   + 2 C2H5OH + 2 ATP 

As can be seen from the equations, anaerobic respiration/fermentation produces only 2 ATP molecules whilst aerobic respiration produces 36/38 ATP molecules and so aerobic respiration produces much more ATP per glucose molecule than fermentation/anaerobic respiration. So aerobic respiration produces more energy for the yeast growth.

In this investigation I will be investigating the effects of different carbohydrate substrates on yeast growth. I have chosen to be using a monosaccharide (glucose), a disaccharide (maltose) and a polysaccharide (starch).

Glucose;         Maltose;             ...

This is a preview of the whole essay