Welcome to GattPrep!
Hey there, science sleuth! 🧠🌿
You may have heard that nitrogen is a gas in the air, but did you know it’s also in the food you eat and even in your DNA? Nitrogen is everywhere—but most of it is in a form we can’t use. So how do plants and animals get it? That’s where the nitrogen cycle comes in!
In this lesson, we’ll explore how nitrogen moves through the air, soil, plants, and animals—and why this cycle is essential for life on Earth.
Nitrogen (N) is a colourless, odourless gas that makes up about 78% of the air we breathe. But in this form (N₂), it’s too stable for plants and animals to use directly.
The nitrogen cycle is the natural process by which nitrogen moves between the atmosphere, soil, plants, animals, and bacteria.
This cycle makes nitrogen available in forms that living things can use, especially nitrates (NO₃⁻) and ammonium (NH₄⁺).
| Process | Description | Example |
|---|---|---|
| Nitrogen Fixation | Conversion of nitrogen gas (N₂) into usable forms (ammonia or nitrates) | Done by nitrogen-fixing bacteria in legume roots or lightning |
| Nitrification | Conversion of ammonia (NH₃) into nitrites (NO₂⁻) and then nitrates (NO₃⁻) | Carried out by soil bacteria |
| Assimilation | Plants absorb nitrates from the soil | Used to make proteins |
| Ammonification | Decomposers turn dead plants/animals into ammonia | Bacteria/fungi break down waste |
| Denitrification | Conversion of nitrates back into nitrogen gas (N₂) | Done by denitrifying bacteria in low-oxygen soil |
You can draw or label a simple diagram showing:
N₂ in the air → lightning or root nodules
Ammonia and nitrates in soil
Plants absorbing nitrates
Animals feeding on plants
Decomposition and return to the atmosphere
Nitrogen is needed for:
🌱 Plant growth (used to make proteins and chlorophyll)
💪🏽 Muscle development in animals (proteins are nitrogen-based)
🧬 DNA (genetic material contains nitrogen)
Without the nitrogen cycle:
Soil becomes poor
Crops fail
Ecosystems collapse
Growing Beans in a School Garden
Legumes like beans and peanuts have special nodules on their roots that house nitrogen-fixing bacteria. These bacteria help “trap” nitrogen from the air and convert it into a form the plant can use—boosting soil fertility naturally!
🧠 Problem:
Your school farm is planting maize, and your teacher asks how nitrogen in the air eventually helps the maize plant grow. Explain the process.
✅ Step-by-Step Solution:
Nitrogen gas (N₂) in the air is fixed into nitrates by bacteria or lightning
Nitrates mix into the soil
The maize plant absorbs the nitrates through its roots
The plant uses them to build proteins for growth
✔️ Answer: Through nitrogen fixation and nitrification, nitrogen gas becomes nitrates in the soil, which the maize plant uses to grow.
1. Fill in the blanks
(a) About ______% of the air is nitrogen.
(b) The process where bacteria change nitrates back into nitrogen gas is called ______.
Answers:
(a) 78%
(b) denitrification
2. Match the Process to the Action
| Action | Process |
|---|---|
| (i) Plants absorb nitrates | A. Assimilation |
| (ii) Bacteria turn ammonia into nitrates | B. Nitrification |
| (iii) Dead organisms turn into ammonia | C. Ammonification |
Answers:
(i) → A, (ii) → B, (iii) → C
3. Short Answer
Why can’t plants use nitrogen gas (N₂) directly from the air?
Sample Answer: Because nitrogen gas is too stable and must first be changed into a usable form like nitrates or ammonium by bacteria or lightning.
Nitrogen makes up most of the air but is unusable in that form.
The nitrogen cycle turns nitrogen gas into forms plants and animals can use.
Key steps: fixation, nitrification, assimilation, ammonification, denitrification.
This cycle supports life by helping build proteins and DNA.
Think about your last meal. Which foods in it came from plants or animals that needed nitrogen to grow? Write down three and explain how nitrogen helped them reach your plate!
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