Class 9 Science Exploration Chapter 2 MCQ – Cell: The Building Block of Life

Every living organism, whether a tiny bacterium or a large tree, is made up of cells. The cell is the smallest unit that can carry out all basic life processes, making it the fundamental unit of both structure and function.

Unicellular organisms consist of only a single cell that performs all life functions on its own. Yeast and bacteria are classic examples, while fish, humans, and plants are multicellular organisms made up of millions of cells working together.

In 1665, Robert Hooke used a self-designed microscope with about 200–300X magnification to observe a thin slice of cork. He saw small box-like compartments and named them ‘cells’, making him the first person to observe and name a cell.

When two points are closer than 0.1 mm, the human eye cannot distinguish them as separate objects — they appear as one. This minimum distance at which two points can be seen as distinct is called the limit of resolution, which is 0.1 mm for the human eye.

Unlike light microscopes that use visible light, electron microscopes use a beam of electrons to produce images. This allows them to reveal cell structures at the nanometre scale with remarkable clarity, far beyond what a light microscope can show.

The cell membrane, also called the plasma membrane, is a thin boundary that surrounds the cell, protects its contents, and defines the individuality of a cell. Every living cell has this membrane as a universal feature.

The cell membrane does not let everything pass through it. It carefully controls what enters and exits the cell — allowing some substances through while blocking others. This selective control is essential for maintaining the cell’s internal environment.

Osmosis is specifically the movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) through a selectively permeable membrane. Diffusion, on the other hand, is the movement of any particles from higher to lower concentration and does not require a membrane.

The concentration of solutes inside potato cells is higher than in plain water. So water moves from plain water (dilute) into the potato cells (concentrated) through osmosis. This gain of water causes the potato piece to swell.

In a hypertonic solution, the solute concentration outside the cell is greater than inside. As a result, water moves out of the cell through osmosis, causing the cell to shrink. This is why a potato piece placed in concentrated salt solution becomes soft and shrinks.

The fluid-mosaic model describes how the cell membrane is structured. It consists of a lipid bilayer (two layers of fat molecules) with proteins embedded in it. The molecules can move sideways making it “fluid,” and the proteins are arranged like tiles in a “mosaic” pattern.

While all living cells have a cell membrane, plants, fungi, and bacteria have an additional rigid layer outside it called the cell wall. This extra layer provides structural support and protection. Animal cells do not have a cell wall, which is why they can change shape easily.

The plant cell wall is primarily made of cellulose, a carbohydrate formed by many glucose units linked together. Cellulose gives the cell wall its rigidity and strength, helping plants withstand environmental stresses like wind and rain.

Eukaryotic cells (eu = true, karyon = nucleus) have a well-defined, membrane-bound nucleus and several membrane-bound organelles. Plant and animal cells are eukaryotic. In contrast, prokaryotic cells like bacteria do not have a membrane-bound nucleus.

Prokaryotic cells are much smaller than eukaryotic cells. Their typical diameter is 1 to 10 μm. Eukaryotic cells, being more complex with many organelles, are larger — ranging from 10 to 100 μm in diameter.

Inside mitochondria, glucose and other molecules are broken down through cellular respiration to release energy. This energy is stored as ATP (Adenosine Triphosphate), which is used to power almost all activities of the cell — hence the name “powerhouse.”

ATP (Adenosine Triphosphate) acts as the energy currency of the cell. When glucose is broken down in mitochondria during cellular respiration, the released energy is captured and stored in ATP molecules, which are then used for various cellular activities.

The Golgi apparatus receives proteins and lipids from the endoplasmic reticulum, modifies them, sorts them, and packages them into vesicles for transport to other parts of the cell, secretion outside the cell, or lysosome formation — much like a post office processes and dispatches packages.

Ribosomes are tiny structures found either freely in the cytoplasm or attached to the endoplasmic reticulum. Their primary role is to synthesise proteins, which are essential for building and maintaining the cell and carrying out its functions.

SER does not have ribosomes on its surface (hence it looks smooth) and is specialised for the synthesis and storage of fats (lipids) and hormones. RER, on the other hand, has ribosomes attached to it and is mainly involved in protein synthesis and secretion.

Lysosomes are single membrane-bound sacs filled with powerful digestive enzymes. They act as the cell’s clean-up crew — breaking down unwanted proteins, carbohydrates, fats, and damaged organelles, keeping the cell healthy and free of waste.

Chlorophyll is the green pigment found inside chloroplasts. It captures light energy from sunlight, which is then used to convert carbon dioxide and water into glucose during photosynthesis. This is why plants appear green. 

Leucoplasts (leukos = white) are colourless plastids found mainly in non-green parts of plants like roots and seeds. They store food materials such as starch, oils, or proteins. For example, leucoplasts in potato cells store starch.

The large central vacuole in a mature plant cell is surrounded by a selectively permeable membrane and filled with cell sap — a watery fluid containing water, minerals, sugars, and waste materials. It helps maintain pressure inside the cell, keeping the plant firm and upright.

In a plant cell, the large vacuole stores water and maintains internal pressure (turgor pressure), which keeps the plant firm. When water is scarce, the vacuole loses water, the cells become less firm, and the plant wilts.

During mitosis, one parent cell divides to produce two daughter cells. Each daughter cell receives the same DNA and the same number of chromosomes as the parent cell, making them genetically identical. Mitosis is used for growth, repair, and replacement of cells.

Meiosis involves two successive divisions. The parent cell first divides into two cells, then each of those divides again, producing four daughter cells (gametes). Each gamete has half the number of chromosomes of the parent cell. This is important so that when two gametes fuse during fertilisation, the original chromosome number is restored.

Normal animal cells stop dividing when they touch neighbouring cells — a control mechanism called contact inhibition. Cancer cells lose this control and keep dividing without stopping, leading to the formation of tumours that can invade nearby tissues and spread to other parts of the body.

Cell Theory has three key principles: all living organisms are made of cells, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells. Rudolf Virchow contributed this third principle, explaining why cells cannot form spontaneously from non-living matter.

By not having a nucleus, mature RBCs gain extra space inside the cell, which is used to pack more haemoglobin — the protein responsible for carrying oxygen. This makes them more efficient at transporting oxygen throughout the body. However, since they lack a nucleus, they cannot repair or reproduce themselves, and they survive for only about 120 days.