CBSE Class 11 Biology Chapter 10: Cell - The Unit of Life NCERT Solutions

Solution: The statement that is not correct is (a). Robert Brown discovered the nucleus of the cell, but it was Robert Hook who is credited with the discovery of the cell itself in 1665. Schleiden and Schwann proposed the cell theory, and Virchow later added that cells arise from pre-existing cells. Unicellular organisms, by definition, perform all life functions within a single cell.

Solution: New cells generate from (c) pre-existing cells. This principle is a cornerstone of cell theory, often referred to as the law of biogenesis. It states that all cells arise from pre-existing cells through cell division. Options (a), (b), and (d) are incorrect as they do not represent the fundamental mechanism of cell reproduction.

Solution: The correct matching of the terms from Column I to Column II is as follows:

Column I | Column II

(a) Cristae | (ii) Infoldings in mitochondria

(b) Cisternae | (iii) Disc-shaped sacs in Golgi apparatus

(c) Thylakoids | (i) Flat membranous sacs in stroma

Explanation: Cristae are the inner folds of the mitochondrial membrane. Cisternae are the flattened sacs that make up the Golgi apparatus. Thylakoids are the membrane-bound compartments inside chloroplasts where the light-dependent reactions of photosynthesis take place.

Solution: The correct statement is (3) In prokaryotes, there are no membrane bound organelles.

Explanation:

1. This is incorrect because prokaryotic cells, such as bacteria, lack a well-defined nucleus; their genetic material is located in a region called the nucleoid.
2. This is incorrect. Plant cells have a well-defined cell wall, but animal cells do not.
3. This is correct. Prokaryotic cells are characterized by the absence of membrane-bound organelles like mitochondria, chloroplasts, and the nucleus.
4. This is incorrect. According to cell theory, cells are formed only from pre-existing cells, not de novo from abiotic materials.

Solution: A mesosome is a convoluted, invaginated membranous structure found in the cytoplasm of prokaryotic cells. It is formed by the inward folding of the plasma membrane.

Functions of mesosomes include:

• Cell wall synthesis: They play a role in the formation of the cell wall during cell division.
• DNA replication: Mesosomes are associated with the replication of the bacterial chromosome.
• Distribution of genetic material: They help in the equal distribution of replicated DNA into daughter cells during cell division.
• Increased surface area: They increase the surface area of the plasma membrane, which enhances the efficiency of various enzymatic activities and processes like respiration and secretion.
• Secretion: They assist in the process of secretion of substances out of the cell.
• Bacterial respiration: They are involved in carrying out respiratory functions, similar to mitochondria in eukaryotic cells.

Solution: The plasma membrane is selectively permeable, meaning it controls the passage of substances into and out of the cell.

Movement of neutral solutes: Neutral solutes, particularly small nonpolar molecules, can move across the plasma membrane through simple passive diffusion. This process involves the movement of molecules from a region where their concentration is high to a region where their concentration is low, without the requirement of cellular energy.

Movement of polar molecules: Polar molecules, such as glucose and amino acids, and ions cannot easily diffuse across the lipid bilayer of the plasma membrane because the membrane's interior is hydrophobic. Therefore, they require assistance for transport.

Transport of polar molecules: Polar molecules and ions are transported across the plasma membrane by facilitated diffusion or active transport.

Facilitated diffusion is a passive process that requires the help of specific membrane proteins (carrier proteins or channel proteins) to move substances across the membrane down their concentration gradient. It does not require energy.

Active transport is an energy-requiring process (usually using ATP) that moves substances across the membrane against their concentration gradient, with the help of specific carrier proteins.