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

Introduction to Required Practicals 4-6

Welcome to your study guide for the second set of required practicals! These activities are a vital part of your Unit 5 assessment. While you won't sit a practical exam, the written papers will ask you to describe these procedures, evaluate techniques, and analyze data. These three practicals bridge the gap between cell biology, human physiology, and plant transport. Don't worry if the details seem a bit overwhelming at first—we will break them down step-by-step!

Practical 4: Root Tip Squashes and Mitosis

This practical involves preparing a microscope slide to observe cells dividing by mitosis. We use root tips because they contain meristematic tissue, which is a region of rapid cell division and growth.

The Procedure: Preparing the Squash

1. Growth and Harvesting: Grow a plant (like an onion or garlic) so that fresh roots emerge. Cut a small section (about 5mm) from the very tip of the root.
2. Acid Hydrolysis: Place the root tips in warm hydrochloric acid. This is a crucial step because it breaks down the pectins in the cell walls, allowing the cells to be separated easily later.
3. Staining: Rinse the roots and add a stain (such as orcein or toluidine blue). Chromosomes are naturally invisible under a light microscope; the stain binds to the DNA, making the chromosomes appear dark and visible.
4. The Squash: Place the stained tip on a slide, add a coverslip, and press down firmly and vertically with your thumb. Important: Do not smear the coverslip sideways! You want a layer that is only one cell thick so light can pass through.

Observations and Calculations

Under the microscope, you will see cells at different stages of the cell cycle: Interphase (the longest stage), Prophase, Metaphase, Anaphase, and Telophase. You may be asked to identify these stages from photographs or drawings in your exam.

The most common calculation here is the Mitotic Index. This tells us the proportion of cells actively undergoing division:

\(\text{Mitotic Index} = \frac{\text{Number of cells with visible chromosomes (in mitosis)}}{\text{Total number of cells observed}}\)

Quick Tip: If the Mitotic Index is very high, it suggests the tissue is growing rapidly or perhaps indicates an abnormal growth like a tumor.

Key Takeaway: The goal is to produce a single-cell thick layer where stained chromosomes can be clearly identified to calculate the rate of division.

Practical 5: Investigation of Variables Affecting Heart Rate

This practical explores how the human body responds to different stimuli. You might investigate variables such as exercise intensity, caffeine intake (if ethical/safe), or temperature.

Methodology and Variables

1. Measuring the Pulse: The heart rate is usually measured in beats per minute (BPM). This can be done manually by feeling the radial pulse at the wrist or using a digital heart rate monitor.
2. Establishing a Baseline: Before changing any variable, the subject must sit quietly to determine their resting heart rate.
3. Controlling Variables: To make the results valid, you must control factors like the subject's age, fitness level, sex, and previous caffeine intake.

Biological Principles and Data

You need to link this practical to the theory in Unit 4. Heart rate is controlled by the autonomic nervous system. The sinoatrial node (SAN) acts as a pacemaker. When you exercise, chemoreceptors detect a drop in pH (due to more \(CO_2\)), and pressure receptors detect changes in blood pressure. These send signals to the medulla oblongata in the brain, which then sends impulses via the sympathetic nervous system to increase the heart rate.

A key formula you might need to use when analyzing heart data is:
\(\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}\)

Key Takeaway: Always ensure you have a control (resting rate) and repeat the measurements to identify any anomalies and calculate a reliable mean.

Practical 6: Using a Potometer

A potometer is a piece of apparatus used to estimate the rate of transpiration by measuring the rate of water uptake by a leafy shoot.

Setting Up the Potometer

Setting this up is famously tricky! Here are the steps to ensure accuracy:
1. Cut the shoot underwater: This prevents air bubbles from entering the xylem, which would break the continuous column of water (the cohesion-tension mechanism).
2. Assemble underwater: Insert the shoot into the potometer while submerged to ensure no air enters the system.
3. Airtight seals: Use Vaseline (petroleum jelly) to seal all joints. Any leak will prevent the water from being drawn up effectively.
4. The Air Bubble: Introduce one small air bubble into the capillary tube. As the plant transpires, it pulls water from the tube, and the bubble moves.

Measuring the Rate

To calculate the rate of water uptake, you measure the distance the bubble travels in a set amount of time. You can calculate the volume of water taken up using the formula for a cylinder:
\(\text{Volume} = \pi r^2 d\)
(where \(r\) is the radius of the capillary tube and \(d\) is the distance the bubble moved).

Variables Affecting the Rate

You can investigate several environmental factors (related to Unit 2.2.8):
- Light intensity: Use a lamp at different distances. More light opens stomata, increasing transpiration.
- Temperature: Higher temperatures increase the kinetic energy of water molecules, increasing evaporation.
- Humidity: High humidity decreases the water potential gradient between the leaf and the air, slowing transpiration.
- Air movement: Using a fan blows away the humid air near the leaf, maintaining a steep water potential gradient and increasing transpiration.

Evaluation: Is Water Uptake the same as Transpiration?

Critical Point: The potometer measures water uptake, but not all water taken up is transpired. Some water is used for:
1. Photosynthesis
2. Maintaining cell turgidity
Therefore, the potometer provides an estimate of transpiration, not an exact measurement.

Key Takeaway: The potometer relies on an airtight system to measure how environmental changes affect the speed of water movement through a plant.

Quick Review of Skills

In your exam, remember these general practical tips for Units 4-6:
- Precision: Use equipment with smaller graduations (e.g., a capillary tube with a small diameter).
- Reliability: Repeat the experiment at least three times at each level of the independent variable to calculate a mean and identify anomalies.
- Validity: Ensure all controlled variables (like temperature in the root tip squash or light intensity in the heart rate practical) are kept constant so that only the independent variable affects the results.