Question 1

Which of the following is not correct?
(a) Robert Brown discovered the cell.
(b) Schleiden and Schwann formulated the cell theory.
(c) Virchow explained that cells are formed from pre-existing cells.
(d) A unicellular organism carries out its life activities within a single cell.

Accepted Answer

Option (a) is not correct. Robert Brown did not discover the cell; he discovered the nucleus. The cell was first observed by Robert Hooke in 1665. Schleiden and Schwann formulated the cell theory, and Virchow explained that cells arise from pre-existing cells. A unicellular organism indeed carries out all life activities within a single cell. — Robert Hooke discovered cells by observing cork under a microscope. Robert Brown discovered the nucleus inside cells. Schleiden and Schwann proposed the cell theory stating all living organisms are made of cells. Virchow added that cells arise from pre-existing cells. Hence, statement (a) is incorrect.

Question 2

New cells generate from
(a) bacterial fermentation
(b) regeneration of old cells
(c) pre-existing cells
(d) abiotic materials

Accepted Answer

Option (c) pre-existing cells is correct. According to cell theory, all cells arise from pre-existing cells through cell division. — The cell theory states that new cells are formed only by the division of existing cells, not from abiotic materials or fermentation. Regeneration is a process of repair but new cells originate from existing cells.

Question 3

Match the following

## Column I

(a) Cristae
(b) Cisternae
(c) Thylakoids

## Column II

(i) Flat membranous sacs in stroma
(ii) Infoldings in mitochondria
(iii) Disc-shaped sacs in Golgi apparatus

Accepted Answer

The correct matching is:
(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 membrane folds of mitochondria that increase surface area for respiration.
- Cisternae are flattened membrane-bound sacs that make up the Golgi apparatus.
- Thylakoids are flat membranous sacs found in the stroma of chloroplasts where photosynthesis occurs. — Cristae increase surface area inside mitochondria for ATP production. Cisternae are the structural units of Golgi apparatus involved in packaging. Thylakoids contain chlorophyll and are sites of light reactions in photosynthesis.

Question 4

Which of the following is correct:
(a) Cells of all living organisms have a nucleus.
(b) Both animal and plant cells have a well defined cell wall.
(c) In prokaryotes, there are no membrane bound organelles.
(d) Cells are formed de novo from abiotic materials.

Accepted Answer

Option (c) is correct. Prokaryotic cells lack membrane-bound organelles. Options (a) and (b) are incorrect because not all living organisms have nuclei (prokaryotes lack nuclei) and only plant cells have a well-defined cell wall, not animal cells. Option (d) is incorrect as cells arise from pre-existing cells. — Prokaryotes do not have membrane-bound organelles like mitochondria or nucleus. Animal cells lack cell walls but plant cells have them. Cell theory states cells come from existing cells, not abiotic materials.

Question 5

What is a mesosome in a prokaryotic cell? Mention the functions that it performs.

Accepted Answer

A mesosome is an infolded structure of the plasma membrane found in prokaryotic cells. It appears as vesicles, tubules, or lamellae.

Functions:
1. It increases the surface area of the plasma membrane to facilitate cellular processes like respiration.
2. It helps in the distribution of daughter DNA molecules during cell division.
3. It is involved in the secretion of extracellular enzymes.
4. It may play a role in cell wall formation during cell division. — Mesosomes are thought to be sites for cellular respiration in prokaryotes, similar to mitochondria in eukaryotes. They also help in DNA replication and cell division by anchoring DNA and assisting cytokinesis.

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?

Accepted Answer

Neutral solutes move across the plasma membrane by simple diffusion. Since they are non-polar or uncharged, they can pass through the lipid bilayer without assistance.

Polar molecules, being charged or hydrophilic, cannot easily diffuse through the hydrophobic lipid bilayer. They require facilitated transport mechanisms such as:
1. Facilitated diffusion via channel proteins or carrier proteins.
2. Active transport using energy (ATP) to move molecules against the concentration gradient.
3. Endocytosis or exocytosis for bulk transport.

Thus, polar molecules are transported by specific transport proteins or vesicular transport mechanisms. — The plasma membrane is selectively permeable. Neutral molecules like oxygen and carbon dioxide diffuse freely. Polar molecules like glucose or ions need protein channels or pumps to cross the membrane due to their polarity and charge.

Question 7

Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.

Accepted Answer

Two double membrane-bound organelles are mitochondria and chloroplasts.

Characteristics:
- Both have an outer and inner membrane.
- Mitochondria have inner membrane folds called cristae.
- Chloroplasts have internal membrane structures called thylakoids arranged in stacks called grana.

Functions:
- Mitochondria are the sites of cellular respiration and ATP production.
- Chloroplasts are the sites of photosynthesis in plant cells.

[Labelled diagrams should show outer membrane, inner membrane, cristae in mitochondria; outer membrane, inner membrane, thylakoids, grana, and stroma in chloroplasts.] — Mitochondria generate energy by breaking down glucose molecules. Chloroplasts capture light energy to synthesize food. Their double membranes help compartmentalize functions and increase efficiency.

Question 8

What are the characteristics of prokaryotic cells?

Accepted Answer

Characteristics of prokaryotic cells:
1. Lack a well-defined nucleus; DNA is naked and located in nucleoid region.
2. Lack membrane-bound organelles.
3. Cell wall is present (made of peptidoglycan in bacteria).
4. Usually unicellular.
5. Smaller in size (1-10 µm).
6. Reproduce by binary fission.
7. Have 70S ribosomes.
8. May have flagella for movement. — Prokaryotes are simpler cells without compartmentalization. Their genetic material is not enclosed in a membrane. They have a rigid cell wall and reproduce asexually.

Question 9

Multicellular organisms have division of labour. Explain.

Accepted Answer

Division of labour in multicellular organisms means that different cells, tissues, and organs perform specific functions efficiently. This specialization allows organisms to carry out complex life processes.

For example, in humans, nerve cells transmit signals, muscle cells contract to produce movement, and red blood cells transport oxygen. This division enhances survival and functioning. — Specialization allows cells to become efficient at particular tasks, reducing redundancy and increasing overall organism efficiency. It is a key feature distinguishing multicellular from unicellular organisms.

Question 10

Cell is the basic unit of life. Discuss in brief.

Accepted Answer

The cell is the basic unit of life because:
1. All living organisms are made up of cells.
2. Cells carry out all life processes such as metabolism, growth, and reproduction.
3. Cells contain genetic material that controls their activities.
4. New cells arise from pre-existing cells.

Thus, the cell is the fundamental structural and functional unit of all living beings. — Cells form the building blocks of organisms. The cell theory establishes the importance of cells in biology. Without cells, life as we know it would not exist.

Question 11

What are nuclear pores? State their function.

Accepted Answer

Nuclear pores are openings in the nuclear envelope that allow the exchange of materials between the nucleus and cytoplasm.

Functions:
1. They permit the passage of RNA and ribosomal subunits from the nucleus to the cytoplasm.
2. They allow the entry of proteins and molecules needed inside the nucleus.
3. They regulate the transport of molecules to maintain nuclear environment. — Nuclear pores are essential for communication between nucleus and cytoplasm, enabling gene expression and protein synthesis.

Question 12

Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.

Accepted Answer

Lysosomes are membrane-bound organelles containing hydrolytic enzymes that digest cellular waste, foreign particles, and worn-out organelles. They act as the cell's digestive system.

Vacuoles are large membrane-bound sacs primarily involved in storage of substances like water, nutrients, and waste. In plant cells, the central vacuole maintains turgor pressure.

Thus, lysosomes are mainly involved in digestion and recycling, while vacuoles function in storage and maintaining cell rigidity. — Though both are part of the endomembrane system, lysosomes have enzymes for breakdown, whereas vacuoles serve as storage compartments.

Question 13

Describe the structure of the following with the help of labelled diagrams.
(i) Nucleus
(ii) Centrosome

Accepted Answer

(i) Nucleus:
- The nucleus is a spherical or oval structure surrounded by a double membrane called the nuclear envelope.
- The envelope has nuclear pores for material exchange.
- Inside is the nucleoplasm containing chromatin (DNA and proteins) and a nucleolus.
- The nucleolus is involved in ribosome synthesis.

(ii) Centrosome:
- The centrosome is a microtubule organizing center near the nucleus.
- It consists of two centrioles arranged perpendicular to each other.
- Centrioles are cylindrical structures made of microtubules.
- Centrosomes play a key role in cell division by forming spindle fibers.

[Labelled diagrams should show nuclear envelope, pores, nucleolus, chromatin for nucleus; centrioles and their arrangement for centrosome.] — The nucleus controls cell activities and stores genetic material. The centrosome organizes microtubules and is crucial during mitosis.

Question 14

What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.

Accepted Answer

A centromere is the constricted region of a chromosome that holds two sister chromatids together.

Classification based on centromere position:
1. Metacentric: Centromere is in the middle, arms are equal.
2. Submetacentric: Centromere is slightly off-center, arms are unequal.
3. Acrocentric: Centromere is close to one end, one very short arm.
4. Telocentric: Centromere is at the end, only one arm visible.

This classification helps in identifying and studying chromosomes during cell division.

[Diagram should illustrate chromosomes with centromere positions labeled accordingly.] — The centromere's position affects chromosome shape and behavior during mitosis and meiosis. It is a key feature in cytogenetics.

Question 15

Who first described the nucleus and in which year?

Accepted Answer

Robert Brown in 1831 — The nucleus was first described by Robert Brown in 1831, marking an important discovery in cell biology.

CELL: STRUCTURE AND FUNCTIONS

CELL: STRUCTURE AND FUNCTIONS — Study Notes

8.1 What is a Cell?

8.2 Cell Theory

8.3 An Overview of Cell

Practice Questions — CELL: STRUCTURE AND FUNCTIONS

All 19 Chapters in Biology