The Invisible Living World: Beyond Our Naked Eye — NCERT Solutions

Concept: Different microorganisms have distinct characteristics and roles.

(i) I live in every kind of environment, and inside your gut.

Answer: Bacteria

Bacteria are found everywhere — in water, soil, air, extreme environments (hot springs, cold zones), and inside the human body, especially in the gut (intestine), where they help in digestion.

(ii) I make bread and cakes soft and fluffy.

Answer: Yeast (a type of fungus, e.g., Saccharomyces cerevisiae)

Yeast ferments sugar and releases carbon dioxide (CO₂) gas. The CO₂ bubbles get trapped in the dough, making it rise and become soft and fluffy. This is why yeast is used in making bread, cakes, and pastries.

(iii) I live in the roots of pulse crops and provide nutrients for their growth.

**Answer: Rhizobium (nitrogen-fixing bacteria)**

Rhizobium bacteria live in the root nodules of leguminous (pulse) crops such as peas, beans, and lentils. They fix atmospheric nitrogen into nitrogen compounds, which enrich the soil and provide nutrients to the plant, promoting healthy growth.

Aim: To test that microorganisms need optimal temperature, air, and moisture for their growth.

Materials required: 6 slices of bread (same type), 6 zip-lock/polythene bags, water, a refrigerator, a warm place (like near a window or in sunlight).

Experimental Design:

Set up 6 groups as follows:

Procedure:

1. Take 6 slices of bread.
2. For groups A, C, D: Lightly sprinkle water on the bread to make it moist.
3. For groups B, E, F: Keep the bread dry.
4. Place each slice in a separate bag. Seal the bags for groups C, E, F. Leave bags open for A, B, D.
5. Keep groups A, B, C at room temperature (warm place).
6. Keep groups D, E, F in the refrigerator.
7. Observe all groups after 3–5 days.

Expected Observations:

• Group A (moist + air + warm): Maximum fungal/mould growth — confirms all three conditions support microbial growth.
• Group B (dry + air + warm): Little or no growth — lack of moisture inhibits growth.
• Group C (moist + no air + warm): Reduced growth — lack of air slows growth.
• Group D (moist + air + cold): Little or no growth — low temperature inhibits growth.
• Groups E and F: No growth — absence of multiple conditions prevents microbial growth.

Conclusion: Microorganisms grow best when optimal temperature, air (oxygen), and moisture are all present. Absence of any one of these conditions slows down or prevents their growth, proving that microorganisms need all three conditions for optimal growth.

Given: Slice 1 — near the sink (warm, moist, open to air); Slice 2 — in the refrigerator (cold, less moisture, closed).

Observations after three days:

Reasons:

• Slice near the sink: The area near the sink is warm and humid (moist). Microorganisms (mainly fungi/mould spores) present in the air settle on the bread. The warm temperature, moisture, and availability of nutrients in the bread provide ideal conditions for the microorganisms to grow and multiply rapidly. Hence, mould grows on this slice within 3 days.

• Slice in the refrigerator: The refrigerator maintains a low temperature (around 4°C). At such low temperatures, the growth and reproduction of microorganisms is greatly slowed down or inhibited. Therefore, the bread in the refrigerator shows little or no mould growth even after 3 days.

Conclusion: Microorganisms need warmth and moisture to grow. Cold temperatures (as in a refrigerator) inhibit microbial growth, which is why refrigeration is used to preserve food.

Given: Curd left out at room temperature for a day becomes more sour.

Concept: Curd contains Lactobacillus bacteria, which are responsible for its formation and taste.

Two possible explanations:

Explanation 1 — Continued bacterial activity:
Curd contains live Lactobacillus bacteria. When left at room temperature, these bacteria continue to ferment the lactose (milk sugar) present in the curd. During fermentation, they produce more lactic acid. As the concentration of lactic acid increases over time, the curd becomes more and more sour.

$$\text{Lactose} \xrightarrow{\text{Lactobacillus}} \text{Lactic acid} + \text{other products}$$

Explanation 2 — Growth of additional microorganisms:
When curd is left open at room temperature, other microorganisms from the surrounding air (such as additional bacteria) may also settle on it and grow. These microorganisms may also produce acidic compounds during their metabolic activities, further increasing the sourness of the curd.

Conclusion: The increased sourness is primarily due to the continued production of lactic acid by Lactobacillus bacteria (and possibly other microbes) at warm room temperature, which is more favourable for their growth than cold temperatures.

Note: Based on the chapter context (Activity 2.8, Aanandi's experiment, and the description of yeast fermentation), Flask A contains sugar solution with yeast kept in a warm place, and it is connected via a delivery tube to test tube B containing lime water.

(i) What happens to the sugar solution in flask A?

Answer: Yeast (a microorganism/fungus) is present in the sugar solution in flask A. In the warm conditions, yeast ferments the sugar (glucose/sucrose). During fermentation, yeast breaks down the sugar and produces:

• Carbon dioxide (CO₂) gas — which bubbles out of the solution.
• Alcohol (ethanol) — which remains in the solution.
• A small amount of energy is also released.

As a result, the sugar solution in flask A undergoes fermentation, producing CO₂ gas and alcohol. The solution may also develop a slightly different (alcoholic) smell.

(ii) What do you observe in test tube B after four hours? Why do you think this happened?

Answer — Observation: The lime water in test tube B turns milky (white/cloudy).

Reason: The CO₂ gas produced during fermentation in flask A travels through the delivery tube into test tube B containing lime water. Carbon dioxide reacts with calcium hydroxide (lime water) to form calcium carbonate (CaCO₃), which is insoluble and appears as a white precipitate, turning the lime water milky:

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 \downarrow + \text{H}_2\text{O}$$

This confirms that the gas produced by yeast during fermentation is carbon dioxide.

(iii) What would happen if yeast was not added in flask A?

Answer: If yeast was not added to flask A, no fermentation would take place. Without yeast, the sugar solution would remain unchanged — no CO₂ gas would be produced. As a result:

• No gas would pass through the delivery tube into test tube B.
• The lime water in test tube B would remain clear (would not turn milky).
• There would be no change in the smell or composition of the sugar solution in flask A.

This confirms that yeast is essential for fermentation and for the production of CO₂ gas.