Chapter 2 Important Questions: Cell: The Building Block of Life

Osmosis is the movement of water through a selectively permeable membrane from an area with more water and less solute (dilute solution) to an area with less water and more solute (concentrated solution) until the concentrations in the two areas become equal.

Cristae are finger-like projections formed by the folded inner membrane of mitochondria. They increase the surface area available for chemical reactions and facilitate energy production during cellular respiration.

Totipotency is the special ability of plant cells by which any living plant cell, even a fully mature cell from a permanent tissue, can develop into a complete plant if provided with suitable nutrients and favourable conditions. This concept was proposed by Gottlieb Haberlandt in 1902.

Prokaryotic cells lack a well-defined nucleus; their genetic material is present in a region called the nucleoid as a single circular DNA molecule. They also lack membrane-bound organelles and are generally smaller and simpler. Eukaryotic cells, on the other hand, have a well-defined nucleus enclosed by a nuclear membrane and contain several membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Bacteria are examples of prokaryotic cells, while plant and animal cells are eukaryotic.

Mitochondria are called the powerhouses of the cell because they supply the energy needed for most cellular activities. During cellular respiration, glucose and other molecules are broken down inside the mitochondria to release energy, which is stored in the form of ATP (Adenosine Triphosphate). ATP acts as the energy currency of the cell and is used to fuel various cellular processes. The inner membrane is folded into cristae, increasing the surface area for these energy-producing reactions.

The fluid-mosaic model explains the structure of the cell membrane. According to this model, the membrane consists of a lipid bilayer — two layers of fat molecules with water-attracting heads facing outward and water-repelling tails facing inward. Proteins are embedded within this lipid bilayer and act as gatekeepers, helping substances pass through. The molecules can move sideways, flip, and rotate within the membrane, making it fluid. Since proteins are arranged like tiles in a mosaic pattern, it is called the mosaic model.

Rough Endoplasmic Reticulum (RER) has ribosomes attached to its surface, giving it a rough appearance under an electron microscope. It is mainly involved in protein synthesis and secretion. Smooth Endoplasmic Reticulum (SER), on the other hand, lacks ribosomes on its surface and therefore appears smooth. SER is involved in the synthesis and storage of fats (lipids) and hormones. Both types form a network within the cytoplasm and are continuous with the outer nuclear membrane.

Contact inhibition is the process in many animal cells by which cell division usually stops when cells come in contact with neighbouring cells. It ensures that cells grow in a controlled and orderly manner, maintaining the proper structure and functioning of tissues. When contact inhibition fails, cells lose control over their division and keep dividing uncontrollably. This leads to the formation of tumours, which may become cancerous (malignant) and can invade nearby tissues or spread to other parts of the body.

The Golgi apparatus consists of stacks of flattened, sac-like structures located in the cytoplasm. It acts like the cell's post office — it receives proteins and lipids from the endoplasmic reticulum, modifies them, sorts them, and packages them into vesicles for transport to their final destinations, including secretion outside the cell or delivery to lysosomes. Lysosomes are single membrane-bound sacs filled with powerful digestive enzymes. They break down unwanted proteins, carbohydrates, fats, and damaged or worn-out organelles, thereby cleaning and maintaining the health of the cell. The products of this breakdown are released into the cytoplasm and may be reused. Together, the Golgi apparatus and lysosomes form an important intracellular processing and waste management system.

Plastids are special organelles found only in plant cells and are classified into three types based on pigment content and function. Chloroplasts contain the green pigment chlorophyll and are responsible for photosynthesis — they absorb sunlight and produce food for the plant. Inside the chloroplast, a semi-fluid stroma stores the sugars and starch produced during photosynthesis. Chromoplasts are found in flower petals and fruits and contain pigments other than chlorophyll — such as yellow, orange, or red pigments. These bright colours attract pollinators and fruit-eating animals, aiding in pollination and seed dispersal. Leucoplasts are colourless plastids that lack pigments and are responsible for storing food materials such as starch, oils, or proteins. They are commonly found in roots and other non-photosynthetic parts of plants, for example in potato and taro cells where they store starch.

Programmed Cell Death (PCD) is a genetically regulated and organised process of selective cell destruction. It is not a random event but a carefully controlled mechanism essential for maintaining the health and development of an organism. PCD plays a vital role in normal development, cellular quality control, and immune function. For example, during embryonic development, the formation of fingers involves PCD — the cells between the developing digits are selectively destroyed, separating the fingers. Without PCD, humans would be born with webbed hands. PCD also helps eliminate damaged, infected, or unnecessary cells, preventing them from causing harm to the body. When PCD fails to occur properly, it can contribute to diseases such as cancer (where cells that should die continue to survive and multiply) or degenerative disorders.

Mature Red Blood Cells (RBCs) in humans are enucleate, meaning they do not have a nucleus. During their maturation, the nucleus is expelled to make more space inside the cell. The main advantage of this feature is that it provides more room for haemoglobin, the protein responsible for transporting oxygen. This allows RBCs to carry a larger amount of oxygen to all cells of the body, making them more efficient at their primary function. However, the absence of a nucleus means that RBCs cannot repair themselves or divide to form new cells. As a result, they have a limited lifespan of approximately 120 days, after which they must be replaced by new RBCs produced in the bone marrow. This is a fine example of how structural modification in a cell is directly related to its specialised function.

Gametes (sperm and egg cells) are normally formed by meiosis, which reduces the chromosome number to half. If gametes were formed by mitosis instead, each gamete would contain the full number of chromosomes (diploid) instead of half (haploid). During fertilisation, when two such gametes fuse, the resulting offspring would have double the normal chromosome number. Over successive generations, the chromosome number would keep doubling, leading to abnormal chromosome numbers in cells. This would cause serious genetic disorders, developmental abnormalities, and could potentially make reproduction unsustainable. Additionally, mitosis produces genetically identical cells, so gametes formed by mitosis would lack genetic variation. This would eliminate the genetic diversity that is essential for evolution and adaptation of species. Meiosis, therefore, is crucial not only for maintaining the correct chromosome number but also for generating genetic diversity through variation in gametes.

The nucleus is the most prominent organelle in a eukaryotic cell and is often called the control centre of the cell because it regulates all cellular activities. Structure of the Nucleus: The nucleus is surrounded by a double-layered covering called the nuclear membrane (nuclear envelope). This membrane has small openings called nuclear pores, which allow the transfer of materials such as RNA and ribosomal subunits between the nucleus and the cytoplasm. Inside the nucleus is a dense, round body called the nucleolus, which is the site of synthesis of ribosomal subunits. These subunits pass through the nuclear pores into the cytoplasm, where they assemble into complete ribosomes. The nucleus also contains chromatin material, which is an entangled mass of thread-like structures composed of DNA and specific proteins. Relationship between Chromatin, Chromosomes, and DNA: In a non-dividing cell, the genetic material exists as chromatin — loosely arranged thread-like structures. When the cell is about to divide, this chromatin condenses and gets organised into rod-shaped structures called chromosomes. Each chromosome is made up of DNA tightly wound around proteins. The functional segments of DNA are called genes, which carry genetic information for inheritance of characters from parents to offspring. Functions of the Nucleus: (i) It controls all the metabolic activities of the cell. (ii) It stores genetic information in the form of DNA. (iii) It is essential for cell division as chromosomes are replicated here. (iv) It controls protein synthesis by directing the production of mRNA. (v) The nucleolus within it is responsible for the production of ribosomal subunits. A labelled diagram of the nucleus should show: double-layered nuclear membrane, nuclear pores, nucleolus, chromatin material, and the overall spherical shape of the nucleus within the cytoplasm.

Aim: To demonstrate the process of osmosis using potato pieces placed in plain water and salt solution. Materials Required: Two beakers (A and B), two potato pieces of roughly equal size, plain water, 20 per cent salt solution, a weighing balance, and a kitchen knife. Procedure: (1) Using a kitchen knife, carefully cut a potato into two pieces of roughly equal size. (2) Measure and record the initial weight of both pieces using a weighing balance. (3) Fill Beaker A with plain water and place one potato piece in it. (4) Fill Beaker B with 20 per cent salt solution and place the other potato piece in it. (5) Leave both beakers undisturbed for about one hour or until a visible change is noticed. (6) Remove both pieces and measure their final weights. (7) Calculate the difference between the initial and final weights of each piece. Observations: The potato piece in Beaker A (plain water) increases in weight and becomes firmer. The potato piece in Beaker B (salt solution) decreases in weight and becomes softer or limp. Explanation and Conclusion: The cell membrane of potato cells is selectively permeable — it allows water molecules to pass through but not sugar or salt molecules. In Beaker A, the concentration of water outside the cell (plain water) is higher than inside the cell, so water moves into the potato cells by osmosis, increasing their weight and firmness. In Beaker B, the concentration of the salt solution outside the cell is higher than inside, so water moves out of the potato cells into the surrounding solution by osmosis, decreasing their weight and making them soft. This experiment clearly demonstrates that osmosis is the movement of water through a selectively permeable membrane from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration). Precaution: It is important to cut both potato pieces to roughly equal size and measure their initial weights to ensure a fair comparison between the two conditions.