Cell: The Unit of Life

Correct Answer: B

Eukaryotes are organisms whose cells contain a true nucleus and membrane-bound organelles. They include a wide variety of life forms such as protists, plants, animals, and fungi. These organisms are generally more complex than prokaryotes, which lack a true nucleus and membrane-bound organelles.

Correct Answer: B

Eukaryotic cells show extensive compartmentalisation of the cytoplasm because of the presence of membrane-bound organelles. These organelles, such as mitochondria, endoplasmic reticulum, and Golgi apparatus, divide the cytoplasm into distinct regions. This organization allows different cellular processes to occur efficiently and simultaneously, making eukaryotic cells more complex and specialized compared to prokaryotic cells.

Correct Answer: B

Eukaryotic cells have a well-defined and organised nucleus that contains the genetic material. This nucleus is surrounded by a nuclear envelope, which is a double membrane structure that separates the nuclear contents from the cytoplasm. The nuclear envelope also regulates the movement of substances in and out of the nucleus, ensuring proper cellular function.

Correct Answer: B

Eukaryotic cells possess complex locomotory and cytoskeletal structures. Locomotory structures such as cilia and flagella help in movement, while the cytoskeleton provides shape, support, and internal organization to the cell. These structures are highly developed and contribute to the complexity and functionality of eukaryotic cells.

Correct Answer: B

In eukaryotic cells, genetic material (DNA) is organized into well-defined structures called chromosomes. These chromosomes are located inside the nucleus and are associated with proteins. This organization helps in proper storage, replication, and transmission of genetic information during cell division.

Correct Answer: B

All eukaryotic cells are not identical because they vary in structure and function depending on the organism and cell type. For example, plant cells, animal cells, fungal cells, and protist cells all have differences in organelles, shape, and functions. This diversity allows eukaryotic organisms to perform specialized functions efficiently.

Correct Answer: B

Plant cells have certain structures that are absent in animal cells, such as a cell wall, plastids (including chloroplasts), and a large central vacuole. The cell wall provides rigidity and protection, plastids are involved in photosynthesis and storage, and the large vacuole helps in maintaining cell turgor and storing substances. Animal cells lack these structures, making plant and animal cells structurally different.

Correct Answer: C

Animal cells contain centrioles, which are cylindrical structures involved in cell division and the formation of spindle fibers. These structures are generally absent in most plant cells. This is one of the key differences between plant and animal cells.

Correct Answer: B

The detailed structure of the cell membrane became clear only after the development of the electron microscope in the 1950s. This advanced microscope provided very high resolution, allowing scientists to observe fine structural details of the membrane that were not visible with earlier microscopes.

Correct Answer: B

Chemical studies on the cell membrane, particularly using human red blood cells (RBCs), helped scientists understand the structure of the plasma membrane. RBCs were ideal for study because they lack internal organelles, making it easier to isolate and analyze their membranes. These studies contributed significantly to the development of models explaining membrane structure.

Correct Answer: B

Scientific studies on cell membranes, especially using human red blood cells, revealed that the plasma membrane is mainly composed of lipids and proteins. Lipids form a bilayer that provides structure and flexibility, while proteins are embedded within or attached to the membrane and perform functions such as transport, signaling, and enzymatic activity.

Correct Answer: B

The cell membrane is mainly composed of lipids and proteins. The major lipids are phospholipids, which are arranged in a bilayer. In this arrangement, the hydrophilic (water-loving) heads face outward towards the aqueous environment, while the hydrophobic (water-repelling) tails face inward, forming a stable barrier. This structure is essential for maintaining the integrity and selective permeability of the membrane.

Correct Answer: B

In the phospholipid bilayer of the cell membrane, the polar (hydrophilic) heads face the outer aqueous environment on both sides, while the hydrophobic tails face inward, away from water. This arrangement creates a stable structure that acts as a selective barrier, allowing certain substances to pass while restricting others.

Correct Answer: C

In the phospholipid bilayer, the hydrophilic (polar) heads face the aqueous environment, while the hydrophobic (nonpolar) tails face inward. This arrangement protects the nonpolar tails of saturated hydrocarbons from water, ensuring membrane stability and proper functioning. This structure is essential for maintaining the integrity and selective permeability of the cell membrane.

Correct Answer: B

Apart from phospholipids, the cell membrane also contains cholesterol. Cholesterol helps in maintaining membrane fluidity and stability. It prevents the membrane from becoming too rigid in cold conditions and too fluid in high temperatures, thereby ensuring proper functioning of the membrane.

Correct Answer: B

Further biochemical studies showed that, in addition to lipids, cell membranes also contain proteins and carbohydrates. Proteins are involved in transport, signaling, and enzymatic functions, while carbohydrates are present on the outer surface as glycoproteins or glycolipids and play roles in cell recognition and communication.

Correct Answer: B

The proportion of proteins and lipids in the cell membrane is not fixed and varies among different cell types. Some membranes may have more proteins for active transport and signaling, while others may have more lipids for structural stability. This variation allows membranes to perform specialized functions depending on the needs of the cell.

Correct Answer: B

In human erythrocytes (red blood cells), the plasma membrane contains approximately 52% proteins and 40% lipids, with the remaining small percentage made up of carbohydrates. This composition reflects the need for flexibility, stability, and efficient transport functions in red blood cells.

Correct Answer: B

Membrane proteins are classified into integral and peripheral types based on their ease of extraction from the membrane. Integral proteins are embedded within the lipid bilayer and are difficult to remove, while peripheral proteins are loosely attached to the membrane surface and can be easily extracted.

Correct Answer: B

Peripheral proteins are located on the surface of the cell membrane and are loosely attached. In contrast, integral proteins are embedded within the lipid bilayer and may be partially or completely buried in the membrane. This difference is based on their position and interaction with the lipid bilayer.

Correct Answer: B

The fluid mosaic model was proposed by S. J. Singer and Garth Nicolson in 1972. It describes the cell membrane as a dynamic and fluid structure with proteins embedded in a phospholipid bilayer.

Correct Answer: B

The lipid bilayer has a quasi-fluid nature, which allows proteins embedded in it to move laterally. This fluidity is a key feature of the fluid mosaic model of the cell membrane.

Correct Answer: B

The movement of lipids and proteins within the membrane is referred to as fluidity. It reflects the quasi-fluid nature of the lipid bilayer in the fluid mosaic model.

Correct Answer: B

Membrane fluidity is essential for various cellular processes such as cell growth, formation of intercellular junctions, secretion, endocytosis, and cell division, as it allows flexibility and movement of membrane components.

Correct Answer: B

The plasma membrane plays a crucial role in the transport of molecules across it, regulating the entry and exit of substances to maintain cellular homeostasis.

Correct Answer: C

The plasma membrane is selectively permeable, meaning it allows some molecules to pass through while restricting others, helping maintain the internal environment of the cell.

Correct Answer: C

Passive transport refers to the movement of molecules across the membrane without the use of energy, typically along the concentration gradient.

Correct Answer: C

Neutral solutes move across the membrane by simple diffusion, which occurs along the concentration gradient from higher concentration to lower concentration without the use of energy.

Correct Answer: C

Water can move across the membrane from a region of higher concentration to lower concentration, typically through the process of osmosis.

Correct Answer: C

Osmosis is the movement of water molecules across a selectively permeable membrane from a region of higher concentration to lower concentration.

Correct Answer: B

Polar molecules cannot pass through the nonpolar lipid bilayer directly, so they require carrier proteins present in the membrane to facilitate their transport.

Correct Answer: C

Active transport is the movement of ions or molecules from lower to higher concentration against the concentration gradient. This process requires energy in the form of ATP, as seen in the Na+/K+ pump.

Correct Answer: B

The cell wall is a non-living rigid structure that forms the outer covering of the plasma membrane in fungi and plants, providing support and protection.

Correct Answer: B

The cell wall provides shape to the cell, protects it from mechanical damage and infection, helps in cell-to-cell interaction, and acts as a barrier to undesirable macromolecules.

Correct Answer: B

In algae, the cell wall is primarily made up of cellulose, galactans, mannans, and minerals such as calcium carbonate, which provide strength and protection.

Correct Answer: B

In higher plants, the cell wall consists mainly of cellulose, hemicellulose, pectins, and proteins, which provide structural support and strength.

Correct Answer: B

The primary cell wall of young plant cells is capable of growth. As the cell matures, growth decreases and a secondary cell wall is formed on the inner side of the primary wall.

Correct Answer: B

The middle lamella is mainly composed of calcium pectate and acts as a cementing layer that holds or glues adjacent plant cells together.

Correct Answer: B

Plasmodesmata are cytoplasmic connections that pass through the cell wall and middle lamella, linking the cytoplasm of adjacent plant cells.