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Unlock This CourseYeast is a fascinating single-celled fungus that plays a crucial role in food production around the world. From bread making to brewing, yeast's ability to respire and produce useful byproducts makes it incredibly valuable. In this session, we'll explore how to investigate yeast respiration scientifically, understanding both the biology behind it and its practical applications.
Key Definitions:
Yeast is perfect for respiration investigations because it's safe to handle, responds quickly to changes and produces measurable results. Unlike human cells, yeast can survive without oxygen, making it ideal for comparing different types of respiration.
Yeast cells are remarkable because they can switch between two different types of respiration depending on whether oxygen is available. This flexibility makes them incredibly useful in food production and perfect subjects for scientific investigation.
When investigating yeast respiration, we need to understand the two main pathways yeast can use to break down glucose for energy.
Equation: Glucose + Oxygen → Carbon dioxide + Water + Energy
Produces lots of energy (38 ATP molecules per glucose). Yeast grows rapidly but doesn't produce alcohol.
Equation: Glucose → Carbon dioxide + Ethanol + Energy
Produces less energy (2 ATP molecules per glucose). Creates alcohol and carbon dioxide - perfect for bread and brewing!
Both types produce carbon dioxide, which we can measure. Only anaerobic respiration produces ethanol (alcohol), which is why bread dough rises and beer contains alcohol.
Bread makers rely on anaerobic respiration! When yeast is mixed into dough, there's limited oxygen available. The yeast respires anaerobically, producing carbon dioxide bubbles that make the bread rise and ethanol that evaporates during baking, leaving behind the characteristic bread flavour.
A good scientific investigation requires careful planning. When studying yeast respiration, we need to consider what we're measuring, what variables we're changing and what we're keeping the same.
The most common and reliable method is measuring carbon dioxide production. Since both types of respiration produce CO₂, we can use this as evidence that respiration is occurring.
Understanding and controlling variables is essential for reliable results. Let's explore the key factors that affect yeast respiration rates.
These are the factors you deliberately alter to see their effect on respiration rate:
Higher temperatures increase enzyme activity up to about 40°C. Above this, enzymes denature and yeast dies. Try 20°C, 30°C, 40°C and 50°C.
More glucose means more substrate for respiration. Test different concentrations: 0%, 5%, 10%, 15% glucose solutions.
Yeast prefers slightly acidic conditions (pH 4-6). Very acidic or alkaline conditions slow respiration or kill yeast.
This is what changes as a result of your independent variable - usually the rate of carbon dioxide production measured in cm³/minute or bubbles/minute.
These must stay constant to ensure fair testing:
Commercial breweries carefully control temperature (usually 18-22°C for ales), sugar concentration from malted grains and pH levels to optimise yeast respiration. They monitor CO₂ production to track fermentation progress, with the entire process taking 1-3 weeks depending on the beer type.
Here's a step-by-step method for investigating how temperature affects yeast respiration rate:
Always wear safety goggles when handling chemicals. Be careful with hot water baths - use tongs to handle equipment. Wash hands after handling yeast. Ensure good ventilation as CO₂ can accumulate.
Once you've collected your data, it's time to analyse what it tells us about yeast respiration.
You should observe that respiration rate increases with temperature up to about 40°C, then drops dramatically at 50°C as the yeast enzymes become denatured.
Your results should show that yeast respiration is controlled by enzymes, which work faster at higher temperatures until they become denatured. This explains why bakers use warm (not hot) water when making bread dough.
Understanding yeast respiration isn't just academic - it's the foundation of massive food industries worth billions of pounds globally.
Bakers control temperature, sugar content and timing to optimise yeast respiration. The CO₂ creates air pockets, while alcohol evaporates during baking.
Brewers use controlled anaerobic respiration to produce alcohol and CO₂. Different yeast strains and conditions create different flavours and alcohol strengths.
Wine makers rely on natural or added yeasts to ferment grape sugars. Temperature control is crucial - too hot kills yeast, too cold slows fermentation.
Modern biotechnology companies are developing new yeast strains that can produce biofuels, medicines and even synthetic materials through controlled respiration processes. Some genetically modified yeasts can now produce insulin for diabetics or create sustainable alternatives to petroleum-based products.