Chapter 2 Important Questions: The Invisible Living World: Beyond Our Naked Eye

✔ Correct Answer: (c) Antonie van Leeuwenhoek

Leeuwenhoek was the first person to clearly see and describe tiny living things like bacteria and blood cells using improved microscopes he built himself.

✔ Correct Answer: (b) Cell → Tissue → Organ → Organ system → Organism

This is the correct hierarchical sequence:

$\text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{Organ system} \rightarrow \text{Organism}$

✔ Correct Answer: (c) Safranin

Safranin is a red-coloured stain used in Activity 2.2 that gives a pinkish colour to onion cells, making them clearly visible under the microscope.

✔ Correct Answer: (c) Carbon dioxide

During fermentation, yeast breaks down sugar and releases $\textbf{CO}_2$, which forms bubbles in the dough, making it soft and fluffy.

✔ Correct Answer: (d) Chloroplast

Chloroplasts contain chlorophyll and are responsible for photosynthesis. They are present only in plant cells, not in animal cells.

✔ Correct Answer: (b) Cork

In 1665, Robert Hooke observed a thin slice of cork under his microscope and noticed small empty compartments resembling a honeycomb, which he called cells.

✔ Correct Answer: (c) Legumes (beans, peas, lentils)

Rhizobium lives in root nodules of legumes and traps nitrogen from the air, making it available for plant growth and increasing soil fertility.

✔ Correct Answer: (c) Lactobacillus

Lactobacillus feeds on lactose in milk, multiplies, and produces lactic acid, which ferments the milk and converts it into curd.

✔ Correct Answer: (d) Nucleus

The nucleus is the control centre of the cell. It regulates all cellular activities including growth, reproduction, and life processes.

✔ Correct Answer: (b) Lack a well-defined nucleus and nuclear membrane

Bacteria have a nucleoid region instead of a membrane-bound nucleus. This is the key feature that distinguishes bacterial cells from plant, animal, fungal, and protozoan cells.

Nucleoid. Since bacteria lack a well-defined nucleus and nuclear membrane, their genetic material is found in an irregularly shaped region called the nucleoid.

Tissue. In the hierarchy of life organisation, a tissue is formed when a group of similar cells come together to perform a specific function, e.g., muscle tissue.

Decomposition. During decomposition, microorganisms like bacteria and fungi break down fallen leaves and dead animal bodies into simpler nutrients that return to the soil and help plant growth.

Vacuole. The vacuole in plant cells is large and stores important substances, helps get rid of waste, and provides strength and support to the cell. In animal cells, vacuoles are usually absent or very small.

Spirulina. Spirulina is a microalga that contains more than 60% protein by body weight. It is considered a superfood and is used as a health supplement. It can also be farmed for livelihood opportunities.

False. Viruses are acellular (not made of cells). They are microscopic entities that can only multiply by entering a living cell, which makes them different from all other microorganisms.

True. The cell wall is an extra outer layer present in plant cells (and fungal and bacterial cells) that provides rigidity and structural support, making plant cells firm and compact.

False. Yeast belongs to the group called fungi, not bacteria. It is a unicellular fungus that grows well in warm conditions and is used in making bread, cakes, and fermented foods.

True. Microalgae are microscopic plant-like organisms that make their own food using sunlight through photosynthesis and in the process release oxygen, contributing to more than half of Earth's total oxygen supply.

False. In animal cells, vacuoles are usually absent; if present, they are very small. It is plant cells that have a large central vacuole that stores substances and maintains cell shape.

The labelled part 'X' is the Vacuole.

The vacuole is a large, fluid-filled space found in plant cells. It stores important substances, removes waste, and helps maintain the shape and firmness of the plant cell by providing support.

The region labelled 'Y' is the Nucleoid.

Unlike the nucleus of a plant cell, the nucleoid has no nuclear membrane surrounding it. The nucleus in plant and animal cells is membrane-bound, whereas the nucleoid in bacteria is simply an irregularly shaped region within the cytoplasm.

A cell is the basic structural and functional unit of all living organisms. The term was first used by Robert Hooke in 1665 after he observed small, empty, honeycomb-like compartments in a thin slice of cork under his microscope, and named them 'cells.'

Unicellular organisms are made up of a single cell that performs all life functions (e.g., Amoeba or bacteria). Multicellular organisms are made up of many cells that are specialised for different functions (e.g., humans or plants).

The cell membrane is a porous outer layer that encloses the cytoplasm and nucleus of the cell. It allows the entry of essential materials needed for life processes and the exit of waste materials, thereby separating the cell's interior from its environment.

Decomposition is the process by which microorganisms break down the complex substances of fallen leaves and dead animals into simpler, nutrient-rich substances that return to the soil. The two main types of microorganisms responsible are bacteria and fungi.

Biogas is a mixture of gases released when microorganisms (bacteria) decompose plant and animal waste in an oxygen-free environment. It is primarily composed of methane ($\text{CH}_4$) and carbon dioxide ($\text{CO}_2$), and is used as a fuel for cooking, heating, and generating electricity.

Glycerin is added to the onion peel on the slide to prevent the cells from drying out. It also improves the clarity and visibility of the cells under the microscope, making the cellular structures easier to observe.

Pickles and murabbas contain high concentrations of salt or sugar, which act as preservatives. This high concentration does not allow microorganisms to grow on the food, thus preventing spoilage.

The two cell types are the muscle cell (spindle-shaped) and the nerve cell (elongated with branches). The spindle shape allows muscle cells to contract and relax, while the long, branched shape of nerve cells (neurons) helps them carry messages quickly to different parts of the body.

A tissue is a group of similar cells that work together to perform a specific function (e.g., muscle tissue). An organ is formed when different types of tissues are organised together to carry out a more complex function (e.g., the stomach), making an organ structurally and functionally more complex than a tissue.

Manure is a dark-coloured, nutrient-rich material formed from the decomposition of fruit and vegetable peels, dried leaves, and plant waste by microorganisms such as bacteria and fungi. It increases soil fertility and helps plants grow better.

Plant Cell vs Animal Cell:

These differences reflect the different lifestyles of plants and animals — plants make their own food (autotrophs) and need structural support, while animals obtain food from outside (heterotrophs).

Rhizobium are bacteria that live in swollen regions called root nodules on the roots of leguminous plants such as beans, peas, and lentils. These bacteria trap nitrogen from the air and convert it into a form that plants can use for growth.

Because of this biological nitrogen fixation, the soil around legume crops naturally becomes enriched with nitrogen. This is why legume farmers do not need to add chemical nitrogen fertilisers — the Rhizobium bacteria supply the required nitrogen, keeping the soil healthy and fertile naturally.

Yeast is a unicellular fungus that grows well in warm conditions. When added to warm sugar solution, yeast breaks down the sugar through a process called fermentation to release energy. During this process, carbon dioxide ($\text{CO}_2$) gas is produced as a byproduct.

This $\text{CO}_2$ gas accumulates inside the test tube and has nowhere to go except into the balloon, causing it to inflate. This experiment demonstrates that yeast produces $\text{CO}_2$ during fermentation, which is also the reason why dough rises when yeast is added.

Bread left near a sink is exposed to moisture, warmth, and air — all of which are ideal conditions for the growth of microorganisms like moulds (fungi). These microorganisms land on the bread from the environment and multiply rapidly, causing it to decay.

Bread kept in a refrigerator remains mould-free because the low temperature slows down or stops the growth of microorganisms. This experiment shows that microorganisms need optimal temperature, moisture, and air for their growth:

$\text{Microbial growth} \propto \text{Warmth} + \text{Moisture} + \text{Air}$

Microalgae are microscopic, plant-like organisms that live in water, soil, air, and on trees. They make their own food using sunlight (photosynthesis) and release oxygen. Examples include Spirulina and Chlorella.

Two important roles of microalgae:

1. Oxygen production: Microalgae produce more than half of Earth's total oxygen supply through photosynthesis.

2. Cleaning water and biofuel: Microalgae help in purifying water and are used to produce biofuel, making them valuable for both environmental and energy purposes.

Structure of a Plant Cell

[DIAGRAM: Draw a large rectangular cell with rounded corners and label the following parts:]

• Cell wall — outermost thick layer

• Cell membrane — thin layer just inside cell wall

• Nucleus — large oval structure near centre, with nuclear membrane

• Cytoplasm — jelly-like fluid filling the cell

• Vacuole — large central fluid-filled space

• Chloroplasts — oval green structures scattered in cytoplasm

• Mitochondria — small bean-shaped structures in cytoplasm

Functions of each part:

A plant cell is different from an animal cell because it has a cell wall, chloroplasts, and a large central vacuole, which are absent in animal cells. These structures allow plants to make their own food, maintain firm structure, and store nutrients effectively.

Experiment: Conditions Required for Microbial Growth

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

Materials Required: 6 slices of bread, 3 airtight containers, 1 open plate, refrigerator, warm place, water.

Procedure:

• Label all setups clearly.

• Observe all slices after 3–5 days and record changes.

Expected Observations:

Conclusion:

Microorganisms grow best when all three conditions — optimal temperature (warmth), moisture, and air — are present simultaneously:

$\text{Best microbial growth} = \text{Warmth} + \text{Moisture} + \text{Air}$

Removing any one of these conditions slows or stops microbial growth, confirming that all three are essential.

Roles of Microorganisms in Daily Life

Microorganisms, though invisible to the naked eye, play extremely important roles in our lives.

1. Role in Food Preparation (Fermentation)

Yeast (a unicellular fungus) is used in making bread, cakes, idli, dosa, and bhatura. It ferments sugar and releases $\text{CO}_2$, making the dough rise and become soft and fluffy.

Lactobacillus (a bacterium) ferments lactose in milk to produce lactic acid, converting milk into curd. It also helps in fermenting batter for idli and dosa.

$\text{Sugar} \xrightarrow{\text{Yeast}} \text{CO}_2 + \text{Alcohol} + \text{Energy}$

2. Role in Soil Fertility

Rhizobium bacteria live in the root nodules of legumes like beans, peas, and lentils. They trap atmospheric nitrogen and convert it into a usable form for plants — a process called biological nitrogen fixation. This naturally enriches the soil with nitrogen, reducing the need for chemical fertilisers.

3. Role in Environmental Cleaning (Decomposition)

Microorganisms like bacteria and fungi act as decomposers. They break down dead plant material, fallen leaves, and animal waste into simpler, nutrient-rich substances through decomposition. This:

• Returns nutrients to the soil

• Helps form manure (compost) that increases soil fertility

• Cleans the environment of dead organic matter

Some bacteria also decompose plant and animal waste in oxygen-free conditions to produce biogas ($\text{CH}_4 + \text{CO}_2$), used as a clean fuel.

Summary Table:

Thus, microorganisms are essential partners in maintaining life, food, and ecological balance on Earth.

Levels of Organisation in Living Organisms

All living organisms are built from the same basic unit — the cell. These units are organised in increasingly complex levels:

$\text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{Organ system} \rightarrow \text{Organism}$

Level 1: Cell

The cell is the basic unit of life, just as a brick is the basic unit of a wall. Cells are of different shapes and sizes depending on their function. For example, nerve cells (neurons) are long and branched to carry messages quickly, while muscle cells are spindle-shaped to contract and relax.

Level 2: Tissue

A tissue is a group of similar cells working together to perform a specific function. For example, muscle tissue in the stomach wall helps churn food. A group of similar cheek cells forms the epithelial tissue lining the inside of the mouth.

Level 3: Organ

An organ is formed when different tissues are organised together to perform a more complex function. For example, the stomach is an organ made of muscle tissue, epithelial tissue, and secretory tissue working together to digest food.

Level 4: Organ System

An organ system is formed when several organs work together to perform a major body function. For example, the digestive system consists of the mouth, food pipe (oesophagus), stomach, intestines, and other organs working together for digestion.

Level 5: Organism

An organism is a complete living being made up of all organ systems functioning in coordination. For example, a human being or a plant is a complete organism.

Summary Table:

This organisation allows simple building blocks (cells) to come together to form complex living beings capable of carrying out all life processes efficiently.

Fermentation by Yeast and Lactobacillus

Role of Yeast (Activity 2.8)

Yeast is a unicellular fungus that grows in warm conditions. When added to dough (flour + sugar + warm water), yeast breaks down the sugar and produces:

• Carbon dioxide ($\text{CO}_2$) — causes the dough to rise and become soft and fluffy

• Alcohol — gives the dough a characteristic smell

This process is used in making bread, cakes, idli, dosa, and bhatura.

$\text{Sugar} \xrightarrow{\text{Yeast (warm conditions)}} \text{CO}_2 + \text{Alcohol} + \text{Energy}$

Role of Lactobacillus (Activity 2.9)

Lactobacillus is a bacterium present in curd. When a spoonful of curd is added to lukewarm milk and kept in a warm place, Lactobacillus:

• Feeds on lactose (milk sugar)

• Multiplies rapidly in warm conditions

• Produces lactic acid, which curdles the milk and forms curd

• Makes the curd sour due to lactic acid

In cold conditions (refrigerator), curd does not form because low temperature inhibits bacterial growth.

Fermentation vs Decomposition

Conclusion: Both fermentation and decomposition are carried out by microorganisms and involve breaking down complex substances, but they differ in their substrates, products, and applications. Fermentation benefits food preparation, while decomposition benefits the environment by returning nutrients to the soil.