Plant Gas Exchange » Gas Exchange Practical

What you'll learn this session

Study time: 30 minutes

How to design and carry out gas exchange experiments with plants
Methods for measuring the rate of photosynthesis
Techniques for investigating factors affecting gas exchange in plants
How to collect, analyse and interpret experimental data
Common errors and limitations in plant gas exchange experiments

Introduction to Plant Gas Exchange Practicals

Plants exchange gases with their environment primarily through tiny pores called stomata. During photosynthesis, plants take in carbon dioxide and release oxygen, while during respiration, they take in oxygen and release carbon dioxide. Being able to measure and investigate these processes is a crucial skill in biology.

Key Definitions:

Gas exchange: The process by which oxygen and carbon dioxide move between an organism and its environment.
Stomata: Tiny pores on the surface of leaves that control gas exchange and water loss.
Photosynthesis: The process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen.
Respiration: The process by which organisms release energy from glucose, consuming oxygen and producing carbon dioxide.

Investigating Photosynthesis Rate

There are several practical methods we can use to measure the rate of photosynthesis in plants. Each method has its advantages and limitations.

🌱 Counting Oxygen Bubbles

One of the simplest methods to measure photosynthesis involves counting oxygen bubbles released by an aquatic plant like Elodea (pondweed).

Equipment needed:

Beaker or test tube
Elodea (pondweed)
Sodium hydrogen carbonate solution (source of CO₂)
Light source (lamp)
Stopwatch
Ruler

Procedure:

Cut a fresh piece of Elodea and place it in a test tube with sodium hydrogen carbonate solution.
Position the test tube a set distance from a lamp.
Allow the plant to adjust for 5 minutes.
Count the number of bubbles produced in 1 minute.
Repeat for reliability.

🔬 Using a Gas Pressure Sensor

A more accurate method involves measuring changes in gas pressure as plants photosynthesise.

Equipment needed:

Gas pressure sensor
Plant material (leaves or aquatic plants)
Sealed container
Light source
Data logger

Procedure:

Place plant material in a sealed container with the gas pressure sensor.
Record initial pressure.
Expose to light for a set period.
Record the change in pressure over time.
Calculate the rate of gas exchange.

Investigating Factors Affecting Photosynthesis

Several environmental factors affect the rate of photosynthesis. By designing controlled experiments, we can investigate how each factor influences gas exchange in plants.

Light Intensity Investigation

Light is essential for photosynthesis. We can investigate how changing light intensity affects the rate of photosynthesis.

💡 Method

Use the bubble counting method with Elodea, but vary the distance between the lamp and the plant. Remember that light intensity is inversely proportional to the square of the distance.

Distances to try: 5cm, 10cm, 15cm, 20cm, 25cm

📊 Results Analysis

Plot a graph of bubble count (y-axis) against light intensity or distance from lamp (x-axis). As light intensity increases, the rate of photosynthesis should increase until another factor becomes limiting.

⚠ Control Variables

Keep these factors constant:

Temperature
CO₂ concentration
Same plant species/size
Same volume of water
Time period for counting

Carbon Dioxide Concentration Investigation

Carbon dioxide is a key reactant in photosynthesis. We can investigate how its concentration affects the rate of photosynthesis.

🧬 Experimental Setup

Use different concentrations of sodium hydrogen carbonate solution to provide different levels of CO₂:

0% (distilled water)
0.1%
0.2%
0.5%
1.0%

Count bubbles produced in 1 minute for each concentration.

📈 Expected Results

As CO₂ concentration increases, the rate of photosynthesis should increase until another factor (like light intensity) becomes limiting. The graph should show a positive correlation that eventually plateaus.

Investigating Stomatal Density and Distribution

Stomata are the primary sites of gas exchange in leaves. We can investigate their density and distribution to understand how plants are adapted for efficient gas exchange.

🔎 Stomatal Impression Method

Equipment needed:

Clear nail varnish
Microscope slides and coverslips
Forceps
Microscope
Leaves from different plants or different parts of the same plant

Procedure:

Apply a thin layer of clear nail varnish to the leaf surface.
Allow to dry completely (about 10-15 minutes).
Carefully peel off the nail varnish using forceps.
Place the peel on a microscope slide and add a coverslip.
Observe under a microscope and count the number of stomata in the field of view.
Calculate stomatal density (number of stomata per mm²).

📝 Comparing Upper and Lower Leaf Surfaces

Most plants have different stomatal densities on the upper and lower surfaces of their leaves. This adaptation helps reduce water loss while maintaining gas exchange.

Investigation:

Take impressions from both the upper and lower surfaces of the same leaf.
Count stomata in multiple fields of view for each surface.
Calculate average stomatal density for each surface.
Compare results and explain any differences observed.

Expected results: Most terrestrial plants have more stomata on the lower surface to reduce water loss while maintaining gas exchange.

Case Study Focus: Xerophytes and Stomatal Adaptations

Xerophytes are plants adapted to dry conditions. They often have specialised stomatal adaptations to reduce water loss while maintaining sufficient gas exchange for photosynthesis.

Examples of adaptations:

Sunken stomata (found in pine needles and marram grass)
Stomata in grooves or pits (found in oleander)
Hairy leaves that trap moist air (found in many desert plants)
CAM photosynthesis where stomata open at night (found in cacti and succulents)

You could compare stomatal impressions from a xerophyte (like a cactus or succulent) with a mesophyte (like a typical garden plant) to observe these adaptations.

Common Errors and Limitations

Understanding potential sources of error is crucial for evaluating the reliability of your experimental results.

⛔ Potential Errors in Bubble Counting

Bubbles may vary in size, affecting oxygen volume measurements.
Some bubbles may stick to the plant or equipment.
Human error in counting rapidly produced bubbles.
Difficulty maintaining constant light intensity.
Temperature fluctuations affecting enzyme activity.

Improvements: Use a gas syringe or pressure sensor for more accurate measurements. Control temperature using a water bath.

🔧 Improving Experimental Design

Always include control experiments.
Take multiple readings and calculate averages.
Control all variables except the one being investigated.
Use fresh plant material for each experiment.
Allow time for the plant to adjust to new conditions.
Consider using digital sensors for more precise measurements.

Analysing and Presenting Your Results

After collecting data from your gas exchange experiments, you need to analyse and present your findings effectively.

📊 Creating Effective Graphs

For investigating factors affecting photosynthesis:

Plot the independent variable (e.g., light intensity, CO₂ concentration) on the x-axis.
Plot the dependent variable (rate of photosynthesis) on the y-axis.
Include error bars if you have calculated standard deviation.
Label axes clearly with units.
Add a title that describes what the graph shows.
Draw a line of best fit (usually a curve for photosynthesis experiments).

💭 Interpreting Your Results

When discussing your findings, consider:

How does your data support or contradict your hypothesis?
Can you identify the limiting factors at different points on your graph?
How do your results compare with published data or theory?
What biological explanations can you provide for the patterns observed?
What improvements could be made to your experimental design?

Real-World Application: Climate Change and Plant Gas Exchange

Understanding plant gas exchange has important implications for climate change research. As atmospheric CO₂ levels rise, scientists study how this affects photosynthesis rates in different plant species.

Research suggests that while increased CO₂ can initially boost photosynthesis (the "CO₂ fertilisation effect"), many plants eventually acclimatise. Additionally, other factors like temperature stress and water availability can limit any potential benefits.

Scientists use sophisticated gas exchange equipment to measure how different plant species respond to elevated CO₂ levels, helping to predict future changes in ecosystems and crop productivity.

🧠 Test Your Knowledge!