Physical Cycles + Decay

- Precipitation: Rain, hail, snow, and sleet.
- Evapouration: When water becomes water vapour.
- Transpiration: Loss of water vapour from leaves.
- Condensation: Water vapour turning into a liquid. 

The water cycle:

Water plays four important roles in biology:
1) Maintain habitats.
2) It maintains internal fluids and transport systems.
3) Is needed for chemical reactions.
4) Is a reactant in photosynthesis.

The Carbon Cycle

Carbon in the Sea:

- Many sea creatures create shells from calcium carbonate (CaCO₃), which contains a significant amount of carbon.
- When they die, they fall to the seabed, where their crushed shells eventually form limestone.
- Acid rain weathers limestone, releasing CO₂ back into the atmosphere.

Carbon Sink: Any place where carbon is stored.

Carbon Processes

Feeding: When herbivores eat plants, they absorb the carbon in the plant and use it to make their own molecules. When animals eat each other, they obtain carbon. 

Combustion: Burning fossil fuels releases CO₂ into the atmosphere. (This leads to climate change.) Humans also carry out combustion.

Fossilisation: When some living things die, a lack of oxygen in the soil may prevent them from fully decomposing, instead forming fossil fuels.

Decomposition: When plants and animals die, bacteria and fungi (decomposers) can break down their bodies and use the carbon to make their own molecules.

Example Q:

- Researchers found there was more CO₂ in the air during winter than summer. Why?
- Similar to summer, the trees have more leaves, which leads to increased photosynthesis, which in turn results in a higher proportion of oxygen compared to carbon dioxide.

The Nitrogen Cycle

- There's 78% nitrogen in the air.
- The nitrogen in the air is too inert for plants to use.
- They get the nitrogen they need through active transport or nitrates in the soil.
- Animals get the nitrogen they need through eating and breathing.
- Plants and animals need nitrogen to make protein.

Bacteria: (involved in the nitrogen cycle)

1) Decomposers: Bacteria and fungi convert proteins (urea) into ammonia (NH₃), which then converts to ammonium ions (NH₄).

2) Nitrifying bacteria: They convert ammonium ions into nitrates.

3) Denitrifying bacteria: This unhelpful process converts nitrates back into nitrogen gas.

4) Nitrogen-fixing bacteria: They convert N₂ gas into ammonium ions (NH₄⁺) or nitrates (NO₃⁻).

- Nitrogen-fixing bacteria either reside in the soil or inside root nodules, which are found in legumes of plants belonging to the pea and bean family (inuminous plants), such as clovers.
→ This is an example of mutualism/symbiosis.

- Lightning (which has lots of energy) can also fix nitrogen from the air by combining it with O₂ to form nitrates (NO₃).
- Urea is also a nitrogen-containing waste product.

- Nitrification = Nitrites (NO₂⁻) → Nitrates (NO₃⁻)
- Denitrification = Nitrates → Nitrogen gas in atmosphere

Decay

- Decay: The slow breakdown of plants' and animals' bodies into simple molecules, such as water, allows for recycling.
- Plastic and metal don't decay.

- Decomposers: Microorganisms like bacteria and fungi cause decay. They can break down any biodegradable material and turn it into compost.

Experiment:

Which mass bag will be lost due to weathering?
1. The soil will lose mass as it contains many respiring decomposers, releasing CO₂; limewater will go from clear to cloudy.
2. It retains its bulk, unlike heated soil that kills the decomposers.

- Detritus: Remains of dead and decaying plants and animals.

- Detrivores: Animals like woodlice, earthworms, and maggots that depend on detritus for food.

- Detritivores don't eat detritus and break it down into small pieces. They enhance the surface area (SA) of the detritus, significantly accelerate the decay process, and play a crucial role in the food chain.

Decomposers

Factors Speeding Up Decay:

- There are many decomposers present, which leads to a faster decomposition rate.
- Warmth; therefore, enzymes will work faster.
- There is plenty of oxygen, which leads to increased rates of oxygen.
- All metabolic reactions occur in solution when there is some moisture present.

Optimum Temperature:

- 25°C = Bacteria
- 37°C = Fungi

- In a compost heap, decomposers respire, releasing heat → speeding up decay.

Saprotroph (saprophyte):

- An organism that uses extracellular digestion to digest what's around them.
e.g., fungi.
→ The organism releases enzymes and absorbs the products.

Fungal Mycelium:

- Contains mass branching, thread-like hyphae. Through mycelium, a fungus absorbs nutrients from its environment in a two-stage process.
1. Hyphae secrete enzymes onto dead wood sources, breaking down biological molecules into smaller molecules.
2. The mycelium then absorbs these small molecules.

Biogas

- Most bacterial decomposers respire aerobically.
- Some decomposers can respire anaerobically.
→ These anaerobic bacteria produce biogas containing methane and CO₂.
- Marshes, septic tanks, and sewers produce biogas.
→ It is a cheap fuel.
- It's now possible to produce biogas in huge generators.