GSEB Class 11 Biology Solutions Chapter 8 Cell The Unit of Life

Question 1. Which of the following is not correct?
1. Robert Brown discovered the cell.
2. Schleiden and Schwann formulated the cell theory.
3. Virchow explained that cells are formed from pre-existing cells.
4. A unicellular organism carries out its life activities within a single cell.
Answer: 1. Robert Brown discovered the cell.
In simple words: The first statement, "Robert Brown discovered the cell," is not correct among the given choices.

Exam Tip: Be careful when identifying incorrect statements; sometimes options that seem correct might have subtle inaccuracies. Always double-check historical facts in biology.

Question 2. New cells generate from
1. bacterial fermentation
2. regeneration of old cells
3. pre-existing cells
4. abiotic materials
Answer: 3. pre-existing cells
In simple words: New cells always come from existing cells that divide, not from old cells regenerating or non-living substances.

Exam Tip: Remember Virchow's famous dictum, "Omnis cellula e cellula," which states that all cells arise from pre-existing cells.

Question 3. Match the following
(1) Cristae
(2) Cisternae
(3) Thylakoids
(i) Flat membranous sacs in stroma
(ii) Infoldings in mitochondria
(iii) Disc-shaped sacs in Golgi apparatus.
Answer:
1. (ii)
2. (iii)
3. (i)
In simple words: Cristae are the folds within mitochondria. Cisternae are the flattened sacs in the Golgi apparatus. Thylakoids are the flat sacs found in the stroma of chloroplasts.

Exam Tip: To score well on matching questions, first identify the structures you are familiar with, and then logically deduce the remaining matches.

Question 4. Which of the following is correct:
1. Cells of all living organisms have a nucleus.
2. Both animal and plant cells have a well-defined cell wall.
3. In prokaryotes, there are no membrane-bound organelles.
4. Cells are formed de novo from abiotic materials.
Answer: 1. Cells of all living organisms have a nucleus.
In simple words: The statement that cells of all living organisms contain a nucleus is considered correct among these options.

Exam Tip: Carefully read each option to identify potential inaccuracies, and choose the most accurate statement based on general biological principles.

Question 5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
Answer: A mesosome in a prokaryotic cell is a unique membranous structure. It forms from the plasma membrane extending inward into the cell. These are essentially folds of the cell membrane. The mesosome performs several key functions:

• They assist in cell wall formation, DNA replication, and distributing genetic material to daughter cells.
• They play a role in respiration and secretion processes, helping to increase the surface area of the plasma membrane and boost its enzymatic content.

Exam Tip: When describing cell structures, always specify the type of cell (e.g., prokaryotic) and clearly list its main functions to get full marks.

Question 6. How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
Answer: Neutral solutes move across the plasma membrane by simple diffusion. This process occurs along the concentration gradient, meaning they go from a region of higher concentration to lower concentration. Water movement by diffusion is known as osmosis.
Polar molecules cannot move across the membrane through simple diffusion. They need a carrier protein in the membrane to help their transport. Some ions or molecules are moved across the membrane against their concentration gradient, from lower to higher concentration. This process uses energy (ATP) and is called active transport.
In simple words: Neutral particles cross the cell membrane easily by diffusion, moving from a crowded area to a less crowded one. Polar particles need special proteins to cross; some even need energy to move against their natural flow.

Exam Tip: Distinguish clearly between simple diffusion, facilitated diffusion (for polar molecules), and active transport (which requires energy and moves against a gradient).

Question 7. Name two cell-organelles that are double membrane-bound. What are the characteristics their functions and draw labelled diagrams of both.
1. Endoplasmic reticulum (ER)
2. Golgi Complex
Answer:
1. **Endoplasmic Reticulum (ER) Characteristics:** The endoplasmic reticulum separates the intracellular space into two distinct areas: the luminal compartment (inside the ER) and the extraluminal compartment (cytoplasm). The ER often has ribosomes attached to its outer surface; this is called the rough endoplasmic reticulum (RER). When ribosomes are absent, the ER appears smooth and is known as the smooth endoplasmic reticulum (SER).
**Function:**
1. They are extensive and connect continuously with the nucleus's outer membrane.
2. The smooth endoplasmic reticulum is the primary location for lipid synthesis.
3. In animal cells, steroid hormones and similar lipids are made in the SER.
2. **Golgi Complex Characteristics:** Golgi bodies consist of many flat, disc-shaped sacs called cisternae, typically 0.5 µm to 1.0 µm in diameter. These cisternae are arranged in stacks. A varied number of cisternae are present in a Golgi complex. The Golgi cisternae are arranged concentrically near the nucleus, with a distinct convex 'cis' or forming face and a concave 'trans' or maturing face. The cis and trans faces of the organelle are different but remain interconnected.
**Functions:** The Golgi apparatus primarily performs the task of packaging materials. These materials are then either sent to intracellular targets or secreted outside the cell. Substances to be packaged in the form of vesicles from the ER merge with the cis face of the Golgi apparatus and proceed towards the maturing face. This explains why the Golgi apparatus stays closely associated with the endoplasmic reticulum. Many proteins made by ribosomes on the endoplasmic reticulum are changed within the cisternae of the Golgi apparatus before they are released from its trans face. The Golgi apparatus is an important place for making glycoproteins and glycolipids.
In simple words: Two double membrane-bound organelles are the Endoplasmic Reticulum and the Golgi Complex. The ER has two types, rough and smooth, which help make proteins and lipids. The Golgi Complex looks like stacked sacs and is vital for packaging and modifying cellular materials.

Exam Tip: For organelles, remember their structure, whether they are single or double-membraned, and their specific roles in cell function. Also, note any connections they have with other organelles.

Question 8. What are the characteristics of prokaryotic cells?
Answer: Characteristics of Prokaryotic cells:

• Prokaryotic cells are typically smaller and reproduce more quickly than eukaryotic cells.
• They possess a cell wall around the cell membrane and a fluid matrix, the cytoplasm, that fills the cell. They do not have a well-defined nucleus.
• Some prokaryotes, such as bacteria, have extra small circular DNA known as plasmids, separate from the genomic DNA. These plasmids give unique characteristics to these bacteria.
• Prokaryotic cells do not contain membrane-bound organelles, which are found in eukaryotes, except for ribosomes.
• A specialized, distinct form of the cell membrane, called the mesosome, is present in prokaryotes. These are membranous structures created by extensions of the plasma membrane into the cell.
• Most prokaryotic cells, like bacterial cells, have chemically complex cell envelopes. These envelopes comprise three tightly bound layers: the outermost glycocalyx, the cell wall, and the plasma membrane.
• Ribosomes consist of two subunits, 50s and 30s units. When these are present together, they form 70s prokaryotic ribosomes.
• Reserve materials in prokaryotic cells are kept in the cytoplasm as inclusion bodies, which float freely.

Exam Tip: Focus on the key distinguishing features of prokaryotes: lack of true nucleus and membrane-bound organelles, presence of cell wall, and unique structures like plasmids and mesosomes.

Question 9. Multicellular organisms have a division of labour. Explain.
Answer: The presence of various organ systems within the body of multicellular organisms is a distinct example of division of labor. Most cells in multicellular organisms develop from a single cell, the zygote, which divides to form many cells. Groups of these newly formed cells become differentiated or specialized to carry out specific functions. This creates a 'division of labor' among cells that coexist in the multicellular organism. Cells with a common origin that perform a similar but specific function make up a tissue (for instance, muscle tissue). Various types of tissues then join together to form an organ, which performs one or more specific functions (such as the digestive, excretory, or reproductive systems).
In simple words: Multicellular organisms divide tasks among different cells and organs. Cells specialize to do particular jobs, and these specialized cells work together in groups to form tissues and organs, each with its own role, like digestion or reproduction.

Exam Tip: When explaining division of labor, start with cell specialization, progress to tissue and organ formation, and then describe how organ systems collectively perform complex functions.

Question 10. The cell is the basic unit of life. Discuss in brief.
Answer: Every cell in an organism performs all its biological activities independently. It releases energy by breaking down food molecules. Using this energy, the cell produces complex molecules. Cells also play a significant part in heredity. In plants, cells perform photosynthesis. Each cell can reproduce itself and has its own life cycle. Furthermore, the cell manages and maintains its own processes. Therefore, the cell is considered the basic unit of life.
In simple words: Cells are the fundamental building blocks of all living things. They can do everything needed for life, like getting energy, making new things, passing on traits, and even reproducing themselves.

Exam Tip: When discussing the cell as the basic unit of life, highlight its independence in carrying out essential biological functions like metabolism, reproduction, and heredity.

Question 11. What are nuclear pores? State their function.
Answer: The nuclear envelope contains many small openings known as nuclear pores. In some instances, these pores can occupy up to 10% of the nuclear envelope's total area. The two membranes of the nuclear envelope become continuous at the edge of each pore. A nuclear pore has a complex structure, sometimes featuring a diaphragm septum or a plug of electron-dense material called nucleoplasm. Some pores also have membranous annuli or small, pocket-shaped outgrowths called blends.
A pore, along with these additional structures, is known as a pore complex. The primary role of nuclear pores is to control the movement of substances into and out of the nucleus. They serve as passages through which RNA and protein molecules travel in both directions between the nucleus and the cytoplasm.
In simple words: Nuclear pores are tiny holes in the nucleus's outer layer. They act like gates, controlling what goes in and out of the nucleus, like RNA and proteins, so the cell can work correctly.

Exam Tip: Emphasize that nuclear pores are not just simple openings but complex structures that regulate molecular traffic between the nucleus and cytoplasm, crucial for gene expression.

Question 12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
Answer: Lysosomes are membrane-bound, sac-like structures created through the packaging process in the Golgi apparatus. Isolated lysosomal vesicles have been found to contain a rich variety of hydrolytic enzymes, such as lipases, proteases, and carbohydrates. These enzymes are capable of breaking down carbohydrates, proteins, lipids, and nucleic acids.
Vacuoles are membrane-bound spaces found in the cytoplasm. They store water, sap, excretory products, and other non-useful cellular substances. A membrane called the tonoplast encloses the vacuole, which helps transport many ions and other materials into the vacuole against a concentration gradient.
In simple words: Lysosomes are like a cell's recycling center, full of enzymes to break down waste. Vacuoles are storage bags, holding water, waste, and other things, often helping to maintain cell shape.

Exam Tip: When comparing organelles, clearly state their primary functions and any specialized structures (like tonoplast for vacuoles or hydrolytic enzymes for lysosomes) to highlight their differences.

Question 13. Describe the structure of the following with the help of labelled diagrams.
1. Nucleus
2. Centrosome
Answer:
1. **Nucleus:** Robert Brown first described the nucleus as a cell organelle in 1831. Later, Flemming named the nuclear material that stained with basic dyes "chromatin."
The interphase nucleus contains highly extended and complex nucleoprotein fibers called chromatin, a nuclear matrix, and one or more spherical bodies known as nucleoli. Electron microscopy has shown that the nuclear envelope, which consists of two membranes separated by a space (10 to 50 nm) called the perinuclear space, acts as a barrier between the materials inside the nucleus and those in the cytoplasm. The outer membrane usually connects continuously with the endoplasmic reticulum and carries ribosomes. In several places, the nuclear envelope is broken by tiny pores, formed by the merging of its two membranes. These nuclear pores allow RNA and protein molecules to move in both directions between the nucleus and the cytoplasm. The nuclear matrix or nucleoplasm holds the nucleolus and chromatin.
Nucleoli are spherical structures found in the nucleoplasm. The content of the nucleolus mixes continuously with the rest of the nucleoplasm as it does not have a membrane. It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells that are actively producing proteins.
2. **Centrosome:** A centrosome is an organelle that typically contains two cylindrical structures called centrioles. These are surrounded by amorphous pericentriolar materials. Both centrioles in a centrosome lie perpendicular to each other, each organized like a cartwheel. They are composed of nine evenly spaced peripheral fibrils of tubulin. Each peripheral fibril is a triplet, and adjacent triplets are connected.
The central part of the centriole is also proteinaceous and is called a hub. This hub connects with the tubules of the peripheral triplets through radial spokes made of protein. Centrioles form the basal body of cilia or flagella and the spindle fibers that give rise to the spindle apparatus during cell division in animal cells.
In simple words: The nucleus is the cell's control center, holding genetic material and the nucleolus, which makes ribosomes. It has a double membrane with pores. The centrosome, found in animal cells, has two centrioles that help organize cell division and form structures like cilia.

Exam Tip: For complex organelles like the nucleus and centrosome, ensure you describe their structural components (membranes, pores, chromatin, centrioles) and their key functions clearly and concisely.

Question 14. What is centromere? How does the position of the centromere form the basis of the classification of chromosomes? Support your answer with a diagram showing the position of the centromere on different types of chromosomes.
Answer: Every chromosome has a main constricted region called the centromere. Disc-shaped structures called kinetochores are found on the sides of the centromere. Chromosomes can be categorized into four types based on the centromere's position:

• **Metacentric:** This chromosome type has a centromere located in the middle, creating two arms of equal length.
• **Sub-Metacentric:** In this type, the centromere is positioned slightly away from the center, resulting in one shorter arm and one longer arm.
• **Acrocentric:** The centromere in this chromosome is situated near its end, forming one extremely short arm and one very long arm.
• **Telocentric:** This chromosome features a centromere at its very end, making it terminal.

Exam Tip: Clearly define the centromere and kinetochore. For chromosome classification, remember the four types and associate each with the precise position of the centromere and the resulting arm lengths.