NCERT MCQ Solutions for Class 8 Science Curiosity Chapter 2 The Invisible Living World – Beyond Our Naked Eye

Early lenses evolved into microscopes, allowing people to see what the naked eye could not. This invention revealed an entire hidden world of microorganisms and enabled the study of their size, shape, and role in nature for the first time in scientific history.

In 1665, Robert Hooke used a microscope to observe cork. He noticed small compartments that reminded him of a honeycomb and called them “cells.” This was the first time the term “cell” was used in science to describe the basic structural unit of life.

Around the 1660s, Antonie van Leeuwenhoek made improved lenses and microscopes. He became the first to clearly observe living microorganisms like bacteria and blood cells. His detailed observations laid the foundation of microbiology, earning him the title of the Father of Microbiology.

Yeast is a fungus that grows well in warmth. It ferments the sugar in the dough, producing carbon dioxide. This gas forms bubbles, causing the dough to rise and become soft and fluffy. The smell and volume of the dough change.

During fermentation, yeast breaks down sugars and releases carbon dioxide (CO₂). This gas forms bubbles in the dough, making it soft and airy. This process to demonstrate how microorganisms carry out life processes and change their environment.

The bacteria like *Lactobacillus* ferment milk to form curd. These bacteria grow well in warm conditions. Bowl A, kept in a warm place, allowed faster fermentation. Bowl B, kept in a cooler spot, did not form curd quickly due to slower bacterial activity.

The *Lactobacillus*, a type of bacteria, ferments lactose (milk sugar) into lactic acid, leading to curd formation. This bacterium works best in warm environments and is commonly used at home and in food industries for preparing fermented milk products.

Viruses are acellular microorganisms that do not reproduce independently. They need a host’s living cell to multiply. They infect plants, animals, and even bacteria. This behavior distinguishes them from other microorganisms like bacteria or fungi.

Bacterial cells lack a well-defined nucleus. Instead, they contain a nucleoid, where genetic material is concentrated. This is in contrast to plant or animal cells that possess a nuclear membrane enclosing the nucleus.

The Legumes like peas and lentils have root nodules containing *Rhizobium* bacteria. These bacteria convert atmospheric nitrogen into forms usable by the plant, thus enhancing soil fertility without chemical fertilizers. This natural process supports sustainable agriculture practices.

During fermentation, yeast or helpful bacteria break down sugars in batters. The carbon dioxide produced creates bubbles that lighten the texture. This gas formation is responsible for making foods like idlis and dosas soft and spongy, as explained in the text.

Salt and sugar act as preservatives in pickles and murabbas. Their high concentration prevents the growth of microbes, keeping the food safe for longer periods. This demonstrates how some microorganisms are inhibited by specific chemical environments.

Microalgae, plant-like microscopic organisms, release oxygen while making food through photosynthesis. They produce over half of Earth’s oxygen. This critical role highlights their importance in maintaining ecological balance and supporting aquatic life.

“Spirulina”, a microalga, is labeled a superfood because it is rich in protein (over 60% of its weight) and contains vitamin B₁₂, essential for human health. Its low fat and sugar content, making it beneficial for dietary use.

The foldscope is a low-cost, paper-based microscope mentioned in the chapter. It helps users, especially students, to view microorganisms that are invisible to the naked eye. Though less powerful than laboratory microscopes, it democratizes access to scientific observation.

The three basic parts of a cell are the cell membrane, cytoplasm and nucleus. These components are found in both plant and animal cells and are essential for maintaining life processes. Other structures like cell wall or chloroplast are additional and found only in specific cells.

Plant cells have an extra outer covering called a cell wall. It provides rigidity and support, which allows plant cells to remain firm and compact. This wall is absent in animal cells and is unique to plant structure and function.

The vacuole in plant cells is a large, empty-looking structure. It stores substances, removes waste and helps maintain cell shape. While animal cells may have small vacuoles, they are larger and more prominent in plant cells for maintaining internal balance.

A cell wall is present only in plant cells. It is not found in animal cells. This structure gives shape, support and strength to plant cells, allowing them to remain rigid and maintain structure under various environmental conditions.

These cells appear as thin, flat and polygonal in shape. They form the protective inner lining of the mouth and are an example of animal epithelial cells.

Unicellular organisms as those made of only one cell. Examples include *Amoeba* and *bacteria*. These organisms perform all life functions within a single cell, unlike multicellular organisms that have multiple cells organized into tissues and organs.

The nucleoid is the region in bacterial cells where genetic material is found. It is not surrounded by a membrane like a true nucleus. This distinguishes bacterial cells from plant and animal cells that have a well-defined nucleus.

Fungal cells have a cell wall but no chloroplasts. Hence, they cannot make their own food through photosynthesis, unlike plant cells. This key structural and functional difference separates fungi from plants in their method of energy production.

Plastids are special structures found in plant cells. It identifies chloroplasts as one type of plastid. They contain chlorophyll and enable photosynthesis. Other plastids store food or pigments. These are absent in animal cells, showing another structural difference between plants and animals.

A cell’s shape and structure relate to its function. For example, muscle cells are spindle-shaped for contraction and nerve cells are long with branches to transmit messages. This correlation between structure and function is a key idea in biology.

The correct sequence is provided in the chapter as Cell → Tissue → Organ → Organ system → Organism. Each level represents increasing complexity in biological organisation. Cells form tissues, which group into organs, which combine into systems to form the entire living organism.

The yolk of an ostrich egg is the largest known cell, measuring about 130 mm to 170 mm in diameter. It also contains other non-cellular materials like the shell and nourishing fluid, making it an exceptional biological structure.

Some fungi are multicellular. They consist of many cells arranged in structures like hyphae and sporangia. In contrast, bacteria and protozoa are unicellular. Fungi may live in soil or food and can be beneficial or harmful based on their function.

Pond or stagnant water contains many tiny living organisms such as protozoa, algae and bacteria. These are visible under a microscope and exhibit movement, helping students discover the hidden microbial world around them.

Bacteria and fungi decompose plant and animal waste in compost pits. This results in nutrient-rich manure. Microorganisms play a vital role in recycling matter in ecosystems by turning complex substances into simpler, usable forms.