Required practicals 4-6: root tip squashes, heart rate and the potometer

Introduction to Required Practicals 4–6

Welcome to your guide for the Unit 2 practicals! These three activities are a vital part of your OxfordAQA International AS Biology course. They allow you to move from the microscopic world of mitosis to the physiological study of heart rate, and finally to the mass transport of water in plants. These practicals aren't just for lab days; you will be tested on your ability to plan, analyze, and evaluate these specific procedures in your exams.

Don’t worry if these seem complicated at first. We will break them down step-by-step so you can master both the "how" and the "why."

Required Practical 4: Root Tip Squashes and Mitosis

In this practical, we look at where a plant grows most rapidly: the root tip. This area contains a region called the meristem, where cells are constantly dividing by mitosis.

The Procedure

The goal is to produce a single layer of cells so that light can pass through them under an optical microscope, allowing us to see the chromosomes.

Step-by-Step Guide:

1. Cutting: Cut a small sample (about \(5\text{mm}\)) from the very tip of a growing root (e.g., onion or garlic).
2. Softening (Acid Hydrolysis): Place the root tip in dilute hydrochloric acid. This breaks down the murein or cellulose cell walls and the "glue" (middle lamella) between cells so they can be squashed easily later.
3. Staining: Rinse the tip and add a stain. Stains are essential because DNA/chromosomes are naturally colorless; the stain binds to them so they become visible.
4. Squashing: Place the tip on a slide, add a cover slip, and push down firmly with your thumb. Important: Do not smear the cover slip sideways, or you will break the chromosomes. Pushing straight down creates a thin, single layer of cells.

Analyzing the Results

Under the microscope, you will see cells at different stages of the cell cycle: interphase (G1, S, G2) and the four stages of mitosis: prophase, metaphase, anaphase, and telophase.

The Mitotic Index:
This is a formula used to calculate how fast a tissue is growing. It is the ratio of cells undergoing mitosis to the total number of cells observed.
\( \text{Mitotic Index} = \frac{\text{Number of cells with visible chromosomes}}{\text{Total number of cells observed}} \)

Quick Review of Magnification:
Remember the standard formula from Unit 1:
\( \text{magnification} = \frac{\text{size of image}}{\text{size of object}} \)

Key Takeaway: The root tip squash allows us to visualize the cell cycle. Success depends on effective staining and creating a very thin "squash" to see individual nuclei.

Required Practical 5: Investigating Human Heart Rate

This practical moves into human physiology. You will investigate how a specific variable affects heart rate or pulse rate.

Variables to Investigate

Common variables you might test include:
- Exercise: Comparing resting heart rate to heart rate after physical activity.
- Caffeine: Measuring the effect of a caffeinated drink over time.
- Body Position: Comparing heart rate while lying down, sitting, and standing.

Key Calculations

You need to know the relationship between heart rate, stroke volume, and cardiac output:
\( \text{cardiac output} = \text{heart rate} \times \text{stroke volume} \)

Note: Heart rate is usually measured in beats per minute (bpm).

Ensuring Valid Data

To get reliable results, you should:
- Control other variables: If testing exercise, ensure the participant hasn't had caffeine recently. Keep the environmental temperature the same.
- Recovery time: If doing repeated trials, allow the heart rate to return to a stable "resting" state before the next test.
- Sample size: Test multiple people or repeat the test on the same person to calculate a mean and identify anomalies.

Did you know? Heart rate is myogenic, meaning the signal for a beat starts within the heart muscle itself (at the sinoatrial node), but it is modified by the nervous system in response to your variables!

Key Takeaway: When studying heart rate, consistency is key. Always use a resting "control" measurement for comparison.

Required Practical 6: Using a Potometer

A potometer is a piece of equipment used to measure the rate of water uptake in a leafy shoot. While we use it to estimate transpiration, it actually measures water uptake (some water is used for photosynthesis or kept for cell turgidity).

Setting Up the Potometer

This experiment is famously tricky because any air bubbles in the wrong place can break the continuous water column (the cohesion-tension mechanism).
1. Cut the shoot underwater: This prevents air from entering the xylem.
2. Airtight seals: Use petroleum jelly (Vaseline) around the rubber bungs and joints to ensure no air leaks in and no water leaks out.
3. Introduce a bubble: Lift the capillary tube out of the water briefly to let one air bubble in, then put it back.
4. Measure: Use a timer to see how far the bubble moves along the scale in a set time (e.g., \(10\) minutes).

Factors Affecting the Rate

You can change the environment around the plant to see how the rate of water uptake changes:
- Light intensity: Use a lamp at different distances (affects stomatal opening).
- Temperature: Use a heater or a cool room (affects kinetic energy of water molecules).
- Humidity: Place a plastic bag over the shoot (reduces the water potential gradient).
- Air movement: Use a fan to simulate wind (removes the "boundary layer" of water vapor).

The Rate Calculation

The rate of uptake is often calculated as the distance moved by the bubble divided by time:
\( \text{Rate} = \frac{\text{Distance}}{\text{Time}} \)
If you know the radius (\(r\)) of the capillary tube, you can calculate the volume of water taken up using the formula for a cylinder:
\( V = \pi r^2 l \) (where \(l\) is the distance moved).

Key Takeaway: The potometer measures water uptake. The most important part of the setup is ensuring it is airtight and that the shoot is cut underwater.

Quick Summary Checklist

Before your exam, make sure you can:
- Explain why acid and stain are used in root tip squashes.
- Identify the stages of mitosis in a photograph or diagram.
- Calculate a Mitotic Index from a set of data.
- Use the cardiac output formula.
- Describe how to make a potometer airtight.
- Explain how environmental factors like humidity or wind change the rate of transpiration.

Remember: In practical questions, always look for ways to improve precision (using better equipment) and reliability (repeating the experiment and calculating a mean).