Introduction to Decomposition and Decay

Have you ever wondered why a fallen tree in a forest eventually disappears, or why a forgotten sandwich in your bag starts to smell and change color? This is all due to decomposition. In this chapter, we will explore how materials are recycled in nature, what factors speed up or slow down this process, and how we can use this knowledge to keep our food fresh for longer. This topic is part of Topic 9: Ecosystems and material cycles and is specific to the GCSE Biology (1BI0) course (Biology only).

What is Decomposition?

Decomposition is the process where dead organisms and waste products are broken down. This is vital for life on Earth because it recycles nutrients back into the soil, allowing plants to grow and the cycle of life to continue. The stars of this show are decomposers.

Decomposers are microorganisms, specifically bacteria and fungi. They break down organic matter by releasing enzymes into their environment to digest dead material and then absorb the dissolved nutrients.

Factors Affecting the Rate of Decay

The "rate of decay" is simply how fast something decomposes. Decomposers are living organisms, so they work best when conditions are just right for their enzymes and metabolism. There are three main factors you need to know:

1. Temperature

Temperature has a huge impact on the rate of decay because it affects enzyme activity.
• In the cold: Decomposers work very slowly because enzymes have low kinetic energy.
• Warm conditions: The rate increases as enzymes and substrates move faster and collide more often.
• Too hot: If it gets too hot, the enzymes in the decomposers become denatured, and the rate of decay drops to zero as the microorganisms die.

2. Water (Moisture)

Decomposers need water to survive and function. Moisture makes it easier for them to digest food and prevents them from drying out. Many chemical reactions involved in decay require water as a solvent.

3. Oxygen Availability

Most decomposers need to respire aerobically to get the energy they need to grow and reproduce. This requires oxygen. If there is more oxygen available, the decomposers can respire faster, leading to a quicker rate of decay.
Note: Some decay can happen without oxygen (anaerobic), but it is usually much slower.

Calculating the Rate of Decay

In your exam, you might be asked to calculate how fast something is decaying. Don't worry, the formula is straightforward! It is usually the change in a variable (like mass) divided by the time taken.

The Formula:
\( \text{Rate of decay} = \frac{\text{change in mass}}{\text{time}} \)

Example Calculation:
If a compost heap loses \( 10\text{kg} \) of mass over \( 5 \) days, what is the rate of decay?
\( \text{Rate} = \frac{10\text{kg}}{5\text{days}} = 2\text{kg/day} \)

Food Preservation Methods

We can use our knowledge of decay to stop food from spoiling. To preserve food, we simply remove one or more of the conditions that decomposers need to thrive.

Refrigeration and Freezing: This lowers the temperature. In a fridge, decomposers work very slowly; in a freezer, the rate of decay is almost zero because it is too cold for enzymes to function and the water is frozen.

Drying (Dehydration): This removes water. Without water, microorganisms like bacteria and fungi cannot perform the chemical reactions needed to grow or digest the food. Examples include dried fruit or beef jerky.

Canning and Vacuum Packing: This removes oxygen. By sealing food in a can or a vacuum bag, we prevent aerobic decomposers from respiring. Did you know? Cans are also heated to high temperatures to kill any bacteria already inside before sealing!

Adding Salt or Sugar: This is a clever trick! High concentrations of salt or sugar draw water out of the decomposer cells by osmosis, which kills them or stops them from growing.

Composting

While we want to stop decay in our food, we want to encourage it in a compost heap! Gardeners use compost heaps to recycle garden waste into natural fertilizer. To make the "perfect" compost, they optimize the conditions we discussed earlier:
• Warmth: The process of decay itself produces heat, and compost bins are often insulated to keep that heat in.
• Moisture: Gardeners might water their compost heap if it gets too dry.
• Oxygen: Many people use a garden fork to "turn" the compost. This adds oxygen into the center of the pile so the bacteria can respire aerobically and decay the waste faster.

Quick Review & Common Mistakes

Key Takeaway: Decay is carried out by bacteria and fungi. It is fastest when it is warm, moist, and oxygen-rich.

Common Mistake to Avoid: Don't say that decomposers "eat" the food like we do. Instead, use the scientific explanation: they release enzymes onto the dead matter to break it down and then absorb the nutrients.

Common Mistake to Avoid: When talking about temperature, students often say the bacteria "die" in the fridge. They usually don't! They just work much, much slower. They only die (and their enzymes denature) if the temperature gets too high.

Summary Table for Revision

Factor: High Temperature (but not boiling)
Effect on Decay: Increases (Faster)

Factor: Low Temperature
Effect on Decay: Decreases (Slower)

Factor: Lack of Oxygen
Effect on Decay: Decreases (Slower)

Factor: Lack of Water
Effect on Decay: Decreases (Slower)