Chapter – 2B : Cell | Chapter Solution Class 9

(c) Robert Hooke

Explanation: Robert Hooke was the first scientist to observe cells in a piece of cork using a microscope in 1665. He described the structures as “cells” because they resembled the small rooms (cellula) in a monastery.

(a) True nucleus

Explanation: Eukaryotic cells possess a well-defined nucleus enclosed by a nuclear membrane. This separates the genetic material from the rest of the cytoplasm, distinguishing them from prokaryotic cells, which lack a nuclear membrane.

(a) Cell of chinagrass

Explanation: The longest plant cells are found in chinagrass (Boehmeria nivea), which is known for its long fibers. These fibers are used in making high-quality paper and textiles.

(c) Egg of ostrich

Explanation: The ostrich egg is the largest animal cell known. It is a single cell that can weigh up to 1.4 kg, making it the biggest cell found in nature.

(b) Bacterial Cell & (c) Plant Cell

Explanation:

• Plant cells have a cell wall made of cellulose for structure and protection.
• Bacterial cells have a cell wall made of peptidoglycan for support and shape.
• Animal cells lack a cell wall, instead, they have only a plasma membrane.

(b) Bacteria

Explanation: A nucleoid is a region in prokaryotic cells (like bacteria) where genetic material (DNA) is located. Unlike eukaryotic cells, bacteria lack a true nucleus, and their DNA is not enclosed within a nuclear membrane.

(c) Plant Cell

Explanation: Plastids (e.g., chloroplasts, chromoplasts, and leucoplasts) are exclusive to plant cells. They play a role in photosynthesis, storage, and pigment synthesis. Fungal and animal cells do not have plastids.

(b) Lysosome

Explanation: Lysosomes contain digestive enzymes that help in breaking down cellular waste, damaged organelles, and pathogens. When a cell is damaged beyond repair, lysosomes release their enzymes, leading to cell self-destruction (autolysis), hence the name “suicide bag of the cell.”

(c) Mitochondria

Explanation: Mitochondria generate ATP (Adenosine Triphosphate) through cellular respiration, providing energy for various cellular activities. Due to this function, mitochondria are known as the “powerhouse of the cell.”

1. Chlorophyll
2. Leucoplast
3. Nucleus
4. Ribosomes
5. Protein
6. Prokaryotic
7. Robertson

True

False

Explanation: Singer and Nicholson proposed the fluid mosaic model of the plasma membrane, not the unit membrane.

True

False

Explanation: Leucoplasts are colorless plastids that store starch, fats, and proteins, not found in green leaves.

True

False

Explanation: Mitotic apparatus is present in eukaryotic cells, aiding in cell division.

Thylakoid

Explanation: Nuclear membrane, Nucleoplasm, and Nucleolus are all parts of the nucleus, whereas Thylakoid is a part of the chloroplast.

Tonoplast

Explanation: Chloroplast, Chromoplast, and Leucoplast are types of plastids, whereas Tonoplast is the membrane of a vacuole, not a plastid.

Ribosomes

Explanation: Mitochondria, Plastids, and Nucleus are membrane-bound organelles, whereas Ribosomes are non-membranous organelles.

(a) Mitochondria

(b) Protein Synthesis

(c) Nucleus

Plasma lemma is another term for the plasma membrane, which surrounds the cell and regulates the exchange of materials.

Singer and Nicholson

Thomas Huxley

Cytoplasm

Endoplasmic Reticulum (ER)

Ribosomes

Polysome or Polyribosome

The fluid portion of the cytoplasm that surrounds organelles.

Mitochondria

Plasmodesmata are cytoplasmic bridges that connect adjacent plant cells, allowing for the exchange of molecules and communication. They are present in plant cell walls.

The zone of exclusion refers to the area in a cell where certain organelles or cellular structures are not present, often observed around the nucleus or vacuoles in the cytoplasm.

Lamina densa is a dense layer of the basal lamina, composed of collagen and glycoproteins, providing structural support to epithelial tissues.

Plasmolysis occurs when water moves out of a plant cell due to a hypertonic solution, causing the plasma membrane to shrink away from the cell wall. It does not occur in animal cells because they lack a rigid cell wall.

The G.E.R.L system stands for Golgi body, Endoplasmic Reticulum, and Lysosomes. It functions in intracellular digestion, lysosome formation, and protein transport within the cell.

The nuclear membrane (nuclear envelope) is a double-layered membrane that surrounds the nucleus, separating it from the cytoplasm. It controls the exchange of materials between the nucleus and cytoplasm through nuclear pores.

A kinetosome (or basal body) is a microtubule-organizing structure found at the base of cilia and flagella. It helps in the formation and anchoring of these motile structures.

The fluid mosaic model, proposed by Singer and Nicholson, describes the plasma membrane as a dynamic structure made of a phospholipid bilayer with embedded proteins. The membrane is fluid, allowing the movement of proteins and lipids.

Plastids are classified into three main types:

Chloroplasts

• Color: Green (due to chlorophyll)
• Function: Photosynthesis (conversion of light energy into chemical energy)
• Location: Found in green leaves, stems, and algae

Chromoplasts

• Color: Red, yellow, or orange (due to carotenoids)
• Function: Provides color to flowers and fruits, attracting pollinators
• Location: Found in petals, fruits, and roots (e.g., carrot, tomato)

Leucoplasts

• Color: Colorless
• Function: Stores starch (amyloplasts), proteins (proteinoplasts), and fats (elaioplasts)
• Location: Found in seeds, tubers, and roots

Structure of Chloroplast:

• Envelope: Double-membrane structure (outer and inner membrane).
• Stroma: A fluid-filled matrix containing enzymes, ribosomes, and DNA.
• Grana: Stacks of disc-shaped thylakoids that contain chlorophyll for photosynthesis.
• Thylakoid Membrane: Site of the light-dependent reaction of photosynthesis.
• Stromal Lamellae: Connects grana stacks.

Function:

• Performs photosynthesis by capturing sunlight and converting it into ATP.
• Produces glucose and oxygen as end-products.

Structure of Centrosome:

• Found in animal cells, made up of two centrioles placed perpendicular to each other.
• Each centriole is composed of nine triplets of microtubules arranged in a cylindrical manner.
• Surrounded by pericentriolar material, which contains proteins required for spindle formation.

Function:

• Helps in cell division by forming spindle fibers.
• Forms basal bodies for cilia and flagella development.

Characteristics of Prokaryotic Cells:

• Lack a true nucleus (DNA is free-floating in the cytoplasm).
• No membrane-bound organelles (e.g., mitochondria, Golgi body).
• Small in size (1-10 µm).
• Reproduce by binary fission (asexual reproduction).
• Example: Bacteria, Archaea.

Characteristics of Eukaryotic Cells:

• Contain a true nucleus enclosed in a nuclear membrane.
• Have membrane-bound organelles (e.g., mitochondria, ER, Golgi apparatus).
• Larger in size (10-100 µm).
• Reproduce by mitosis or meiosis.
• Example: Plants, Animals, Fungi, Protists.

Plastid Structure:

• Types: Chloroplasts, Chromoplasts, Leucoplasts.
• Chloroplasts: Double membrane, stroma, grana, and thylakoids.
• Function: Photosynthesis and storage of food.

Mitochondrion Structure:

• Double membrane: Outer membrane is smooth, inner membrane has folds called cristae.
• Matrix: Contains enzymes, ribosomes, and mitochondrial DNA.
• Function: ATP production through aerobic respiration (“powerhouse of the cell”).