Practical skills Study Guide

Plants perform photosynthesis to create glucose, their food, using light energy, carbon dioxide, and water. The rate at which this process occurs depends on several factors, known as limiting factors. These include light intensity, carbon dioxide concentration, and temperature. When one of these factors is in short supply, it will limit the rate of photosynthesis, even if other factors are abundant. For example, in dim light, increasing the carbon dioxide concentration will not increase the rate of photosynthesis because light intensity is the limiting factor.

To investigate these factors, experiments are typically set up using an aquatic plant like pondweed (Elodea). The rate of photosynthesis can be measured by counting the number of oxygen bubbles produced by the plant over a set period, or by collecting the oxygen gas and measuring its volume. By changing one factor at a time (the independent variable) while keeping all other factors constant (control variables), we can observe its effect on the rate of oxygen production (the dependent variable). For instance, to investigate light intensity, a lamp can be moved closer or further away from the pondweed, while ensuring carbon dioxide and temperature remain constant.

Careful measurement and control are crucial. For safety, ensure electrical equipment is kept away from water. When collecting data, repeat readings and calculate averages to improve reliability. Analysing the results involves plotting graphs to visualise the relationship between the independent variable and the rate of photosynthesis. This helps identify the point at which a factor becomes limiting. Evaluating the investigation involves identifying potential sources of error and suggesting improvements, such as maintaining a constant temperature with a water bath or using a CO2 indicator solution to monitor carbon dioxide levels more precisely.

Ecological sampling involves using specific techniques to estimate the distribution and abundance of organisms in a particular habitat without counting every single individual. This is crucial for studying ecosystems effectively, as directly counting all organisms is often impossible. Distribution refers to where organisms are found, while abundance describes how many organisms are present in a given area. Common sampling techniques include using quadrats and transects.

Quadrats are square frames of a known area, typically 0.25 m² or 1 m², used to sample plants or slow-moving animals. To ensure representative sampling, quadrats should be placed randomly. Random sampling minimises bias, meaning the selected areas accurately reflect the variation across the entire habitat. This can be achieved by generating random numbers for coordinates within the study area. For example, if a field is 10 meters by 10 meters, random numbers between 0 and 10 can be used to select x and y coordinates for placing the quadrat.

Transects are lines along which measurements are taken. They are useful for studying changes in distribution and abundance across an environmental gradient, such as changes in light intensity, moisture, or altitude. A line transect involves simply recording the species touching the line at regular intervals. A belt transect involves placing quadrats at regular intervals along the line, allowing for more detailed abundance data collection.

Once samples are collected, the data can be analysed to estimate population size or density. For example, if 10 dandelions are counted in a 0.25 m² quadrat, the estimated density is 40 dandelions per square meter (10 / 0.25). Environmental factors, both biotic (living, e.g., presence of other species, competition) and abiotic (non-living, e.g., light, temperature, pH, moisture, soil type), can be measured alongside organism distribution and abundance. This allows for investigation of correlations between environmental factors and organism presence or quantity. Comparing data from different areas or over time helps to understand ecological patterns and the impacts of various factors on populations.