Photosynthesis

Calculate the rate of photosynthesis if a piece of pondweed produces $0.45\text{ cm}^3$ of oxygen gas in $5\text{ minutes}$.

Step 1: Identify the formula and values.

• Formula: $\text{Rate} = \frac{\text{Volume of } O_2}{\text{Time}}$
• $\text{Volume} = 0.45\text{ cm}^3$, $\text{Time} = 5\text{ min}$

Step 2: Substitute into the equation.

• $\text{Rate} = \frac{0.45}{5}$

Step 3: Calculate the final answer.

• $\text{Rate} = 0.09\text{ cm}^3\text{/min}$

Calculate the rate of reaction if a hydrogencarbonate indicator takes $250\text{ seconds}$ to change colour.

Step 1: Identify the formula and values.

• Formula: $\text{Rate} = \frac{1}{\text{Time}}$
• $\text{Time} = 250\text{ s}$

Step 2: Substitute into the equation.

• $\text{Rate} = \frac{1}{250}$

Step 3: Calculate the final answer.

• $\text{Rate} = 0.004\text{ s}^{-1}$ (or $4.0 \times 10^{-3}\\text{ s}^{-1}$)

Light Intensity and the Inverse Square Law

Why does a torch seem so much dimmer when you take just a few steps back? This rapid drop in brightness follows the Inverse Square Law, which states that light intensity is inversely proportional to the square of the distance ($d$) from the source.

• If you double the distance, the light intensity divides by four ($2^2$).
• If you triple the distance, the intensity divides by nine ($3^2$).

You can calculate relative light intensity in arbitrary units (a.u.) using this formula:

$\text{Light Intensity} = \frac{1}{d^2}$

Explaining Limiting Factors

A car can only go as fast as its most restricted part allows, whether that is the engine power or the fuel supply. In biology, environmental conditions can act as a limiting factor, restricting the overall speed of the biological process.

• Light Intensity: Photosynthesis is an endothermic reaction, meaning it requires an external energy input. Increasing light intensity supplies more energy to the chlorophyll for splitting water molecules. This directly increases the reaction rate until the chloroplasts are fully saturated with energy.
• Carbon Dioxide: $CO_2$ is a crucial reactant. Higher concentrations of $CO_2$ increase the frequency of successful collisions between $CO_2$ molecules and the active sites of enzymes, specifically Rubisco. This drives the reaction faster until all enzyme active sites are continuously occupied.
• Temperature: Because photosynthesis relies on enzymes, warming the plant initially increases kinetic energy, causing more enzyme-substrate collisions. However, heating past the optimum point will denature the enzymes, causing the rate of reaction to drop sharply to zero.

Safe and Ethical Organism Use

Biologists must commit to the safe and ethical use of living organisms during investigations. Before starting, you must conduct a risk assessment to manage hazards, such as keeping electrical lamps away from water.

• To ensure biosafety, students should wash their hands after handling pondweed and ensure all equipment is sanitised.
• Aquatic plants can suffer physiological stress and tissue damage from excessive heat. You must avoid overheating the specimen by using an LED lamp or placing a glass heat shield (a beaker of water) between a traditional bulb and the plant.
• Some commonly used species, such as green Cabomba, are classed as invasive in the UK.
• To prevent ecological damage, you must never dispose of these plants in local waterways; they must be destroyed or placed in domestic waste.