Yeast
Table of Contents
Structure of Yeast
Yeast cells are far more complex than bacteria and are larger in size
In the image to the side, the yeast are the yellow balls. The green balls and red rods are bacteria. Here you can see the size difference
Yeast are Eukaryotic cells, like you.
So their DNA is kept safe in a Nucleus
They have an endoplasmic reticulum for Protein synthesis
They have Golgi for packaging
They have a Vacuole to control water content
Yeast have a cell membrane to control the movement of solutes
Yeast have a cell wall for structural support
And, they have Mitochondria for Cellular Respiration
Yeast Life Processes
Movement
Yeast don't have any way of moving by themselves
Their movement for the life processes, refers to movement of the cytoplasm inside of the cells. Looking closeup to the cells as in the clips below you can see the movement
Reproduction
Yeast grow and create little 'buds' that break off and grow into new cells
Sensitivity
Yeast can sense each other. This is important for Yeast cell sexual reproduction
Growth
Yeast absorb nutrients such as sugar. Sugar diffuses into the cell and the cell grows bigger. Once large enough the cell will start to bud
The warmer it is the faster the sugar can diffuse into the cell and so the faster the cell will grow
Respiration
Beer makers use Yeast in an oxygen free environment (anoxic) to force the Yeast to do anaerobic respiration.
The Yeast then take the sugars and turn them into Alcohol and Carbon Dioxide
When making bread, any alcohol that is made will evaporate away when the bread is cooked
The Carbon dioxide gets trapped in the dough, causing the dough to rise
Yeast can do both Aerobic and Anerobic Respiration
Aerobic Respiration: Glucose + Oxygen = Carbon Dioxide + Water
Anaerobic Respiration: Glucose = Alcohol + Carbon Dioxide
As you can see in this clip, the warmer the environment that the yeast are in, the faster the rate of respiration
This is because the reactant, sugar, is able to diffuse into the cell and the product carbon dioxide is able to diffuse away
Note, that the yeast in the boiled water did not do anything, this is because the enzymes of the cell have all denatured and so they wont work
Excretion
Yeast excrete the products of cellular respiration as waste products
Aerobic = Carbon Dioxide and Water
Anaerobic = Carbon Dioxide and Alcohol
In the clip to the side you can see the excreted Carbon dioxide forming bubbles
This excretion of Carbon Dioxide is important in Bread making and in making frothy beer
Nutrition
Yeast release enzymes into the environment that digest their food source
This is Extracellular digestion
Amylase is an extracellular digestive enzyme that is secreted by yeast cells
Amylase cuts up starch into sugar (glucose)
This sugar diffuses into the cell and the cell grows bigger
The warmer it is the faster the sugar will diffuse into the cell so the faster it can grow bigger
These enzymes are proteins
Because they are proteins, they are sensitive to pH and to Temperature
Enzymes
What are enzymes
Enzymes are proteins that do things
They either:
break down things - catabolic
build things up - anabolic
Enzymes hold chemicals momentarily allowing them to react with other things this causes the building or the breaking
The chemical being held is the substrate. The shape of the substrate matches the active site on the enzyme
The active site is where the chemical is held
Each type of enzyme has a specific active site shape, this will fit only the targeted substrate
Once the substrate has bonded to the active site, the enzyme usually changes shape slightly, this forces a change on the substrate increasing the chance of the chemical reaction occurring
Once the reaction has occurred the enzyme changes shape again, releasing the products
In other words:
The enzymes active site binds to its target substrate, a chemical reaction occurs, turning the substrate into a product, the product is then released
Enzymes and Activation Energy
For a chemical reaction to occur the reactants must bang into each other fast enough (with enough energy). This is the activation energy - the energy required for the reaction to be activated
Also, the reactants need to hit in the correct orientation - like to react they need to hit the correct parts - like spider man going through a window or slamming into a wall
Enzymes lower the energy required for the chemical reaction to occur by grabbing hold of the molecule and then holding it and even bending it in the correct way for the other reactant to collide with it
So, by holding the reactant the right way the collisions with the other reactant are more likely to result in a reaction
The hotter it is the more energy both reactants have, so the more likely it is that the reactants will collide whilst one is bound in the enzyme, and the more likely it is to collide with enough force for the reaction to occur
At the optimum temperature for an enzyme it will be conducting its reaction at its fastest rate. This means that the molecules are colliding with sufficient energy for the reaction to occur, whilst the enzyme holds the reactant in place. In other words, the molecules are colliding with sufficient kinetic energy for the activation energy to be overcome.
Factors effecting Enzyme activity
Enzymes work by having chemical substrates land in their active sites
Temperature
Chemical substrates must diffuse through water to land on the enzymes active site.
The warmer the water is, the faster the rate of diffusion, because the water molecules themselves are moving faster
Colder = slower water = slower diffusion = slower collisions = slower enzyme action
Hotter = faster movement within the water = faster collisions = faster enzyme activity
The enzyme works the fastest at its optimum temperature
Past the optimum temperature, the atoms of the enzyme itself start to move to much and the active site changes shape. If the active site changes shape, then the substrate cannot fit. So, that enzyme can no longer participate in the reaction - this is why the graph drops so quickly after the optimum temperature.
This change in shape is called denaturation, we say the protein has denatured. After this it will not work.
Heat Collisions and Activation Energy
Hotter = more successful collisions
Enzyme holds onto the substrate so that the site for the reaction is exposed - increasing the likelihood of a reaction should the other reactant collide
This way the enzyme reduces the ACTIVATION ENERGY required for a reaction
The HOTTER it is, the more collisions will occur, and the more energy the collisions will have, so more collisions will have sufficient energy for the reaction to occur
Denatured
Proteins are held in their shape with a range of chemical bonds
Some of these bonds are very weak like H to OH attraction (Hydrogen bond)
The proteins shape is its nature
Heat causes atoms to move
When it is too hot, the atoms in the proteins move too much and the weaker chemical bonds (like the hydrogen bonds) will break
This causes the protein to change its shape
This new shape is not its natural shape, so it is called denatured
Proteins, including enzymes denature because of:
Heat - they shake themselves apart
pH - the H+ ions or OH- ions get inbetween the weak hydrogen bonds and break them, causing the protein to unfold, change its shape, change its nature, denature, and stop working
Yeast Environment
The most important factors in the Yeasts Environment
Water - allows things float - food, enzymes
Food - for cellular respiration and growth
Temperature - how fast things float and diffuse!
pH - can cause proteins to denature
Its all about how the environment effects the enzymes and the diffusion of reactants and products.
"They all float, higher temperature means they float faster"
Yeast releases digestive enzymes such as Amylase
Amylase diffuses away from the cell
Amylase cuts starches into glucose
Glucose diffuses into the Yeast cell
Glucose used for Anaerobic Respiration and growth
Higher temperature speeds up the rate of diffusion and thus the rate of respiration, growth and reproduction (budding)
After Anaerobic Respiration the Ethanol and Carbon Dioxide diffuses away from the cell.
The warmer it is the faster the diffusion
How to make Bread
Ingredients:
2 Tablespoons Sugar
Warm water
1/2 Tablespoon Yeast
1/2 tsp Salt
1 Tablespoon Oil
2 cups Flour
Steps
Dissolve the sugar in warm water
Add yeast, then wait 5 - 10 minutes
The yeast are rehydrated by the water and have sugar to eat, so they are 'activated' from their dormant state
The yeast mixture should be foamy if it has worked
Mix in flour, oil and salt
Kneed the dough
Leave to rise for 1 hour
CO2 is released and is captured in the dough
Kneed again and place onto the dish
Bake - the CO2 bubbles expand in the heat of the oven. Also, all microbes are killed during this step, preserving the bread
How Yeast has impacted Society
The Internal Assessment reminder
Section 2: Use of FUNGI (yeast) in food production
1. Name and describe the microorganism used in bread making.
2. Describe the steps to produce bread and explain the role of fungi (yeast) in this process.
3. Link the life processes of the fungi (yeast) to the production of bread.
a) Describe the two most important yeast life processes in the production of bread.
b) Explain how these life processes help to produce bread.
c) Link the physical characteristics of bread to these life processes.
4. Discuss the effect of different environmental factors (temperature and time) on the production of the bread.
a) Explain how the environmental factors affect fungi (yeast) life processes.
b) Explain how the affected life processes impact the production of bread.
5. Explain how the produced bread is preserved through elimination of other unwanted microorganisms.
6. Discuss the impacts of this knowledge of microorganisms and food production on your everyday life.