Welcome to the Core Practicals: Respiration, Photosynthesis, and Plant Growth

Biology is more than just reading textbooks; it is a practical science! In this chapter, we explore the hands-on experiments that prove how organisms breathe, how plants make food, and how they grow. These "Core Practicals" are vital because they appear in your exams, especially Paper 3: General and Practical Principles in Biology. We will break down five key experiments, focusing on the "how" and the "why."

Note: For a deeper dive into the theory behind these processes, you can cross-reference Topic 5: Energy for Biological Processes and Topic 9: Control Systems.


Core Practical 9: Factors Affecting the Rate of Respiration

How do we measure something we can’t see, like a gas being used up? We use a piece of equipment called a respirometer. This practical allows us to calculate the rate of aerobic or anaerobic respiration in small organisms (like woodlice or germinating seeds).

How it Works

1. Absorbing Carbon Dioxide: As the organisms breathe, they take in \(O_{2}\) and release \(CO_{2}\). To measure only the oxygen used, we place potassium hydroxide (KOH) or soda lime in the chamber. This chemical absorbs all the \(CO_{2}\) produced.
2. Pressure Change: Because the \(CO_{2}\) is absorbed and \(O_{2}\) is being used up, the air pressure inside the tube drops.
3. The Result: This drop in pressure pulls a drop of colored liquid (manometer fluid) toward the organisms. The distance the liquid moves tells us the volume of oxygen consumed.

Key Mathematical Skills

To find the volume of oxygen used, you treat the capillary tube like a cylinder. You may be asked to use the formula:
\(V = \pi r^{2} h\)
Where \(r\) is the radius of the tube and \(h\) is the distance the fluid moved.

Important Considerations

  • Temperature Control: Respiration is controlled by enzymes. You must use a water bath to keep the temperature constant, or it becomes a confounding variable.
  • Safety and Ethics: When using living organisms like woodlice, you must handle them carefully and ensure they do not come into direct contact with the corrosive KOH.

Quick Tip: If you are measuring anaerobic respiration (e.g., in yeast), you must ensure no oxygen is present. Scientists often use a layer of liquid paraffin (oil) over the yeast suspension to block out air.


Core Practical 10: Effects of Different Wavelengths of Light on Photosynthesis

Plants don’t just need "light"—they are picky about which colors (wavelengths) they use! This practical investigates how different colors of the spectrum affect the speed of photosynthesis.

The Method

1. The Setup: We usually use an aquatic plant, like Elodea (pondweed), placed in a solution of sodium hydrogen carbonate (which provides plenty of \(CO_{2}\)).
2. The Variable: We place different colored filters (red, blue, green) between the light source and the plant.
3. Measuring the Rate: We count the number of oxygen bubbles released over a set time, or more accurately, use a gas syringe to measure the volume of \(O_{2}\) produced.

What to Expect

Plants usually have a high rate of photosynthesis in blue and red light but a very low rate in green light. This is because chlorophyll reflects green light (which is why leaves look green!) rather than absorbing it.

Key Takeaway: The independent variable is the wavelength (color) of light, and the dependent variable is the volume of oxygen produced per minute (the rate).


Core Practical 11: Presence of Different Chloroplast Pigments using Chromatography

Have you ever wondered why leaves turn different colors in the autumn? It's because they contain a mixture of pigments, including chlorophyll a, chlorophyll b, carotene, and xanthophyll. We use chromatography to separate them.

Step-by-Step Separation

1. Extraction: Grind up leaves (like spinach) with a solvent (like propanone) to extract the pigments.
2. Spotting: Place a concentrated drop of this extract on a piece of chromatography paper or a Thin Layer Chromatography (TLC) plate.
3. Running: Place the paper in a solvent. As the solvent moves up the paper, the pigments move with it. Pigments that are more soluble in the solvent move faster and further.

Calculating the \(R_{f}\) Value

To identify the pigments, we calculate the Retention Front (\(R_{f}\)) value:
\(R_{f} = \frac{\text{distance moved by pigment}}{\text{distance moved by solvent}}\)

Common Mistake: Always measure from the middle of the pigment spot! Also, remember that \(R_{f}\) values are always less than \(1.0\). If you get a number higher than \(1\), you’ve got your fraction upside down!


Core Practical 8: Factors Affecting Water Uptake using a Potometer

Plants lose water through their leaves in a process called transpiration. We measure the rate of water uptake using a potometer.

Using the Potometer

  • The "Air-Tight" Rule: The most important part of this practical is ensuring the seal between the plant shoot and the potometer is completely airtight. We usually use petroleum jelly (Vaseline) to seal the joints.
  • Underwater Cutting: You must cut the plant shoot underwater. This prevents air bubbles from entering the xylem, which would break the continuous column of water (the cohesion-tension model).
  • Variables to Test: You can change the wind speed (using a fan), humidity (using a plastic bag), or light intensity to see how they affect the rate.

Did you know? A potometer actually measures water uptake, not transpiration directly. Some water is used for photosynthesis or keeping cells turgid, but since about \(99\%\) of water taken up is transpired, it is a very good estimate!


Core Practical 14: Effect of Gibberellin on Amylase Production

This practical looks at how plant hormones (growth regulators) control development. Gibberellins are hormones that signal seeds to break dormancy and start growing.

The Starch Agar Assay

1. The Science: When a cereal seed (like barley) germinates, gibberellin triggers the production of the enzyme amylase. Amylase breaks down starch into sugar.
2. The Setup: We soak seeds in different concentrations of gibberellin and then place them on starch agar plates.
3. The Test: After a few days, we flood the plate with iodine. Iodine turns starch blue-black.
4. The Result: Areas where amylase has been produced will have no starch left. These look like clear "halos" or zones around the seed. A larger clear zone means more amylase was produced.

Quick Review: This experiment demonstrates the antagonistic or synergistic roles of plant hormones and how they regulate gene expression to produce enzymes during growth.


Summary Table for Revision

Respiration (CP9): Uses a respirometer; measures \(O_{2}\) uptake; uses KOH to absorb \(CO_{2}\).
Photosynthesis (CP10): Uses pondweed; measures \(O_{2}\) bubbles; tests wavelength/color.
Pigments (CP11): Uses chromatography; identifies pigments via \(R_{f}\) values.
Water Uptake (CP8): Uses a potometer; measures transpiration rate; must be airtight.
Plant Growth (CP14): Uses starch agar; tests gibberellin; measures clear zones (amylase activity).

Don't worry if these setups seem complicated! The key is to focus on the variables (what you change and what you measure) and the limitations (what might make the results inaccurate, like air leaks in a potometer or temperature fluctuations in a respirometer).