Nutrition - Plants » Photosynthesis Practical - Oxygen Evolution

Introduction to Photosynthesis Practical - Oxygen Evolution

One of the best ways to understand photosynthesis is to see it happening in real life! When plants photosynthesise, they produce oxygen as a waste product. We can actually see and measure this oxygen being released, especially when using water plants like pond weed (Elodea). This practical investigation is brilliant because it gives us visible proof that photosynthesis is really happening.

Key Definitions:

• Oxygen Evolution: The release of oxygen gas during photosynthesis as a by-product.
• Elodea: A common water plant (pond weed) used in photosynthesis experiments.
• Light Intensity: How bright the light is, which affects the rate of photosynthesis.
• Variable: A factor that can change in an experiment (like light intensity, temperature).

The Practical Method

This experiment is quite straightforward, but getting good results needs careful planning. The basic idea is to put a piece of pond weed in water, shine light on it and count the oxygen bubbles it produces.

Equipment You'll Need

Before starting any experiment, make sure you have all the right equipment. Here's what you'll need for investigating oxygen evolution:

🧪 Basic Equipment

Large beaker or tank, fresh Elodea shoots, lamp with known wattage, ruler for measuring distances

⌛ Measuring Tools

Stopwatch, thermometer, measuring cylinder (if collecting gas), calculator

💧 Water Setup

Tap water (left to stand for 24 hours), or pond water, test tube for gas collection (optional)

Step-by-Step Method

Follow these steps carefully to get the best results from your oxygen evolution experiment:

Setting Up the Experiment

Start by cutting a fresh piece of Elodea about 10cm long. Make sure the cut is clean and underwater to prevent air bubbles getting trapped. Place the plant in your beaker of water with the cut end pointing upwards.

Top Tip

Cut the Elodea underwater with sharp scissors. This prevents air getting into the plant's tubes and gives you more accurate results. The fresher the plant, the better your results will be!

Controlling the Variables

To make this a fair test, you need to control all variables except the one you're investigating (usually light intensity). Keep the temperature constant, use the same piece of plant and maintain the same water conditions throughout.

💡 Light Distance Method

Place your lamp at different distances from the plant (10cm, 20cm, 30cm, etc.). Closer = more intense light. Count bubbles for exactly 5 minutes at each distance. Remember: light intensity follows the inverse square law!

Collecting and Recording Data

Good data collection is crucial for reliable results. You have two main options for measuring oxygen production:

Bubble Counting Method

This is the simpler method. Count the number of bubbles produced in a set time (usually 5 minutes). Make sure you start timing as soon as you switch the light on and count every single bubble that rises from the cut end of the plant.

📜 Recording Tips

Use a table with columns for light distance, time, number of bubbles and bubbles per minute

🔁 Repeat Tests

Do each test at least 3 times and calculate an average to make your results more reliable

⏳ Timing

Keep timing consistent - exactly 5 minutes for each test. Use a stopwatch for accuracy

Gas Collection Method

For more accurate results, you can collect the oxygen gas in an upturned test tube filled with water. As oxygen is produced, it displaces the water. You can then measure the volume of gas collected.

Understanding Your Results

Once you've collected your data, it's time to make sense of what it tells you about photosynthesis!

Expected Results Pattern

You should find that as light intensity increases (lamp gets closer), more oxygen bubbles are produced. This proves that light intensity affects the rate of photosynthesis. However, there might be a limit - if you get the lamp too close, the heat might actually slow things down!

Case Study Focus

In a typical experiment, students found that moving a 60W lamp from 50cm to 10cm from the plant increased bubble production from 8 bubbles per minute to 45 bubbles per minute. However, at 5cm, the rate dropped to 30 bubbles per minute due to overheating.

Common Problems and Solutions

Even experienced scientists face challenges with this experiment. Here are the most common issues and how to fix them:

Troubleshooting Guide

Don't worry if your first attempt doesn't go perfectly - that's normal! Science is about learning from problems and improving your method.

❌ No Bubbles Appearing

Check your plant is fresh and healthy. Make sure the cut end is clean and pointing upwards. Try using pond water instead of tap water, as it contains more dissolved minerals that plants need.

🔥 Inconsistent Results

This usually happens when temperature varies too much. Keep your setup away from draughts and direct sunlight. Let the plant settle for 10 minutes before starting each test.

Real-World Applications

Understanding oxygen evolution isn't just useful for exams - it helps us understand how life on Earth works!

Why This Matters

Every breath you take contains oxygen that was produced by photosynthesis. Plants and algae in oceans, forests and even your garden are constantly producing the oxygen we need to survive. By understanding how light affects this process, we can better protect our environment.

Amazing Fact

Ocean algae produce about 70% of the world's oxygen - much more than forests! This is why protecting our oceans is so important for maintaining oxygen levels in our atmosphere.

Exam Tips and Key Points

Here's what you absolutely must remember for your iGCSE Biology exam: