Cell: The Unit of Life

What to remember

• The cell is the structural and functional unit of life. All living organisms are made of cells, and every new cell arises from a pre-existing cell (omnis cellula e cellula, Virchow).
• Prokaryotic cells have no membrane-bound nucleus or organelles, and have 70S ribosomes. Eukaryotic cells have a true nucleus, membrane-bound organelles and 80S ribosomes (70S in mitochondria and plastids).
• The cell membrane is described by the fluid mosaic model (Singer and Nicolson, 1972): a quasi-fluid lipid bilayer with proteins floating in it. The endomembrane system includes the ER, Golgi apparatus, lysosomes and vacuoles only.
• Mitochondria make ATP, plastids carry out photosynthesis or storage, the nucleus holds the chromosomes, and the 9+2 arrangement of microtubules forms the axoneme of cilia and flagella.

What is a cell: cell theory, sizes and shapes

Anton van Leeuwenhoek first saw and described a living cell. Robert Brown later discovered the nucleus. Matthias Schleiden, a botanist, reported in 1838 that all plants are made of cells. Theodor Schwann, a zoologist, reported in 1839 that animal cells have a thin outer layer, the plasma membrane, and that plant cells have a cell wall. Together they proposed the cell theory: all living organisms are composed of cells and their products. Rudolf Virchow in 1855 explained that new cells arise from pre-existing cells (omnis cellula e cellula). This modified the original hypothesis.

Cell theory in short:

• 1. All living organisms are composed of cells and products of cells.
• 2. All cells arise from pre-existing cells.

Cells differ greatly in size and shape.

Shapes vary too. Red blood cells are disc-shaped, white blood cells are amoeboid, tracheid cells are elongated, nerve cells are long and branched, and columnar cells are tall and narrow. Cells may be round, oval, spherical or flattened.

Prokaryotic cells

Prokaryotes include bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO. They are generally smaller and multiply faster than eukaryotes. They may be bacillus (rod), coccus (spherical), vibrio (comma) or spirillum (spiral) in shape.

All prokaryotic cells have a cell wall surrounding the plasma membrane, except mycoplasma, which has no cell wall. The cytoplasm has no membrane-bound organelles. The genetic material, a single circular DNA, lies in a region called the nucleoid, which has no nuclear membrane.

Cell envelope. It is a tightly bound three-layered structure: the outermost glycocalyx, then the cell wall, then the plasma membrane. Each layer has its own function, but together they act as one protective unit.

• Glycocalyx may be a loose sheath called the slime layer, or a thick and tough layer called the capsule.
• The cell wall gives shape and strong structural support, and prevents the cell from bursting or collapsing.
• Bacteria that take up the Gram stain and appear purple are called Gram positive. Those that do not retain the stain are called Gram negative. The difference lies in the nature of the cell envelope.
• The plasma membrane is semipermeable and has the same basic structure as the eukaryotic membrane.

Mesosome. It is an infolding of the plasma membrane in the form of vesicles, tubules or lamellae. Mesosomes help in cell wall formation, DNA replication and distribution to daughter cells. They also help in respiration, secretion and increase the surface area and enzyme content of the membrane.

Chromatophores. In cyanobacteria and in purple and green bacteria, membranous extensions of the cytoplasm contain pigments and are called chromatophores.

Ribosomes. These are 70S, made of a 50S and a 30S subunit. They are the site of protein synthesis. Several ribosomes can attach to one mRNA to form a polysome.

Inclusion bodies. Reserve material is stored in the cytoplasm as inclusion bodies, which are not bounded by any membrane. Examples are phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles are found in some blue-green and purple and green photosynthetic bacteria.

Plasmids. Many bacteria have small circular DNA outside the genomic DNA, called plasmids. They give special features such as resistance to antibiotics. Plasmid DNA is used in genetic engineering.

Surface structures. The bacterial flagellum has three parts: filament, hook and basal body. It helps in motility. Pili are elongated tubular structures made of a special protein. Fimbriae are small bristle-like fibres that help bacteria attach to rocks in streams and to host tissues. Pili and fimbriae are not for movement.

Eukaryotic cells: cell membrane and cell wall

Eukaryotic cells have a membrane-bound nucleus and membrane-bound organelles. Plant and animal cells differ in several ways, listed in the table at the end of these notes.

Cell membrane. Chemical studies on the human red blood cell membrane show about 52 per cent protein and about 40 per cent lipid. Lipids are arranged in a bilayer, with the polar hydrophilic heads towards the outside and the non-polar hydrophobic tails towards the inside. Cholesterol is also present. Carbohydrates are present in small amounts, as glycoproteins and glycolipids.

Singer and Nicolson in 1972 proposed the fluid mosaic model. The lipid bilayer is quasi-fluid, so proteins can move sideways within it. This fluidity helps in cell growth, formation of intercellular junctions, secretion, endocytosis and cell division. Proteins are of two kinds: peripheral proteins lie on the surface, and integral proteins are partially or totally buried in the bilayer.

Water moves by osmosis. Non-polar molecules cross the bilayer easily, but polar molecules need carrier proteins.

Cell wall. The cell wall is a non-living, rigid structure outside the plasma membrane of plant cells. It gives shape, protects from mechanical damage and infection, helps cell-to-cell interaction and acts as a barrier to undesirable macromolecules. In algae it is made of cellulose, galactans, mannans and minerals like calcium carbonate. In plants it is made of cellulose, hemicellulose, pectins and proteins. A young plant cell has a primary wall that is capable of growth. The secondary wall forms on the inner side when the cell matures. The middle lamella is made of calcium pectate and holds neighbouring cells together. Plasmodesmata are cytoplasmic bridges that connect neighbouring cells through the wall.

Endomembrane system

The endomembrane system includes the endoplasmic reticulum, Golgi complex, lysosomes and vacuoles. Their functions are coordinated. Mitochondria, chloroplasts and peroxisomes are not part of it.

Materials made in the ER move to the Golgi cis face and leave from the trans face. In Amoeba the contractile vacuole removes excess water, and food vacuoles are formed by engulfing food. Plant vacuoles concentrate ions and amino acids.

Mitochondria, plastids and ribosomes

Mitochondria. They are sausage-shaped or cylindrical. Typical diameter is about 0.2 to 1.0 µm, average about 0.5 µm. Each is bound by two membranes. The outer membrane is smooth. The inner membrane is folded into cristae, which increase the surface area. The inner space is the matrix. Mitochondria are the site of aerobic respiration and make ATP, so they are called the powerhouses of the cell. They have a single circular DNA, a few RNA molecules, 70S ribosomes and the components needed for their own protein synthesis. They divide by fission.

Plastids. They are found in plant cells and in euglenoids. They are classified by their pigments.

Leucoplasts are of three kinds: amyloplasts store starch, elaioplasts store oils and fats, and aleuroplasts store proteins.

The chloroplast has two membranes. The space inside is the stroma. It contains flattened membranous sacs called thylakoids, stacked into grana, and connected by stroma lamellae. Chlorophyll is in the thylakoids. The stroma has enzymes for carbohydrate and protein synthesis, small circular DNA and 70S ribosomes.

Ribosomes. They are made of RNA and protein and are not membrane-bound. Eukaryotic ribosomes are 80S, made of 60S and 40S subunits. Prokaryotic ribosomes are 70S, made of 50S and 30S subunits. The S stands for the Svedberg unit, which measures the sedimentation coefficient, that is, the rate of sedimentation in a centrifuge. S values depend on size and shape, so they do not add up exactly (60S + 40S = 80S).

Cytoskeleton, cilia and flagella, centrosome

The cytoskeleton is a network of filamentous proteinaceous structures in the cytoplasm. It helps in mechanical support, motility and maintenance of cell shape.

Cilia and flagella. They are hair-like outgrowths of the cell membrane. Cilia are small and work like oars, moving the cell or the surrounding fluid. Flagella are longer and cause cell movement. Both are covered by the plasma membrane. The core, called the axoneme, has nine pairs of peripheral doublet microtubules and two central microtubules, called the 9+2 arrangement. The central pair is connected by bridges and enclosed by a central sheath. Each peripheral doublet is linked to the sheath by a radial spoke. Cilia and flagella arise from a basal body, which has the same structure as a centriole (nine triplets, no central tubule).

Centrosome and centrioles. The centrosome has two cylindrical centrioles surrounded by amorphous pericentriolar material. The two centrioles lie at right angles to each other. Each centriole has nine evenly spaced peripheral fibrils, each a triplet of microtubules. The centre is proteinaceous, called the hub, and each triplet is joined to the hub by a radial spoke. This gives a cartwheel arrangement (9+0). The centrosome forms the spindle apparatus during animal cell division.

Nucleus and chromosomes

Robert Brown discovered the nucleus. Flemming named the stainable material of the nucleus chromatin. The interphase nucleus has nuclear envelope, nucleoplasm, nucleolus and chromatin.

• Nuclear envelope: two parallel membranes separated by a perinuclear space of about 10 to 50 nm. The outer membrane is continuous with the ER and often has ribosomes. The envelope has nuclear pores, through which RNA and protein move in and out.
• Nucleolus: a spherical structure in the nucleoplasm, not bound by a membrane. It is the site of rRNA synthesis. Cells that make many proteins have larger and more numerous nucleoli.
• Chromatin: a network of nucleoprotein fibres made of DNA, histones (basic proteins), non-histone proteins and RNA. Euchromatin is loosely packed, lightly stained and transcriptionally active. Heterochromatin is densely packed, darkly stained and inactive.

Each chromosome has a centromere. On each side of it is a disc-shaped kinetochore, to which spindle fibres attach. The position of the centromere decides the shape of the chromosome.

Some chromosomes have a secondary constriction at a constant location. The small segment beyond it is called the satellite, and such chromosomes are called satellite chromosomes.

Microbodies. Many minute membrane-bound vesicles containing various enzymes are present in both plant and animal cells. They are called microbodies.

Plant cell and animal cell compared

NEET traps

• 1. Schleiden studied plants and Schwann studied animals. Virchow stated that cells arise from pre-existing cells.
• 2. 70S ribosomes (50S + 30S) are prokaryotic. 80S ribosomes (60S + 40S) are eukaryotic. Mitochondria and chloroplasts have 70S ribosomes.
• 3. Mycoplasma has no cell wall. It is the smallest cell, so do not confuse it with a virus.
• 4. Pili are tubular structures made of a special protein and fimbriae are bristle-like fibres. Neither is for movement. Flagella are for movement.
• 5. Mesosomes are infoldings of the plasma membrane. Chromatophores are pigment-bearing membranes in photosynthetic bacteria.
• 6. Peripheral proteins lie on the surface. Integral proteins are embedded in the bilayer.
• 7. Mitochondria and chloroplasts are not part of the endomembrane system, since their functions are not coordinated with it.
• 8. Rough ER makes proteins. Smooth ER makes lipids and steroids.
• 9. Centriole and basal body have 9+0 triplets. Axoneme of cilium has 9+2 doublets.
• 10. Metacentric means middle. Telocentric means terminal. Acrocentric means near the end.

One-liners

• 1. Leeuwenhoek first saw a living cell.
• 2. Robert Brown discovered the nucleus.
• 3. Fluid mosaic model was proposed in 1972.
• 4. The cell envelope of bacteria has three layers: glycocalyx, cell wall, plasma membrane.
• 5. Plasmids are extra-chromosomal circular DNA in bacteria.
• 6. Prokaryotic ribosomes are 70S.
• 7. Nucleolus is the site of rRNA synthesis.
• 8. Cristae are folds of the inner mitochondrial membrane.
• 9. Grana are stacks of thylakoids.
• 10. Amyloplasts store starch.
• 11. Lysosomes contain hydrolytic enzymes.
• 12. The Golgi complex has a cis face and a trans face.
• 13. The vacuolar membrane of plant cells is the tonoplast.
• 14. Middle lamella is made of calcium pectate.
• 15. Kinetochore is the attachment site of spindle fibres on the chromosome.

Practice questions

1. Who proposed the fluid mosaic model of the plasma membrane?

   1. Singer and Nicolson
   2. Robert Brown and Virchow
   3. Schleiden and Schwann
   4. Leeuwenhoek and Flemming
   Answer

   A. Singer and Nicolson

   The fluid mosaic model was proposed by Singer and Nicolson in 1972.

2. Which of the following is the smallest known cell?

   1. Cyanobacterium
   2. Mycoplasma
   3. Human red blood cell
   4. Bacillus
   Answer

   B. Mycoplasma

   Mycoplasma (PPLO) is the smallest cell, about 0.3 µm.

3. Ribosomes of prokaryotic cells have a sedimentation coefficient of

   1. 80S
   2. 60S
   3. 70S
   4. 40S
   Answer

   C. 70S

   Prokaryotic ribosomes are 70S, made of 50S and 30S subunits.

4. Which of the following is NOT a part of the endomembrane system?

   1. Golgi complex
   2. Endoplasmic reticulum
   3. Lysosomes
   4. Mitochondria
   Answer

   D. Mitochondria

   Mitochondria, chloroplasts and peroxisomes are not coordinated with the endomembrane system.

5. The site of ribosomal RNA synthesis in the nucleus is the

   1. nuclear pore
   2. heterochromatin
   3. nucleolus
   4. perinuclear space
   Answer

   C. nucleolus

   The nucleolus is the site of active rRNA synthesis.

6. The arrangement of microtubules in a centriole is

   1. nine peripheral doublets with two central tubules
   2. nine peripheral singlets with two central tubules
   3. nine peripheral doublets with no central tubule
   4. nine peripheral triplets with no central tubule
   Answer

   D. nine peripheral triplets with no central tubule

   A centriole has nine triplets joined to a central hub by radial spokes (cartwheel, 9+0).

7. A chromosome whose centromere is close to one end, giving one very short and one very long arm, is

   1. acrocentric
   2. metacentric
   3. telocentric
   4. sub-metacentric
   Answer

   A. acrocentric

   Acrocentric chromosomes have the centromere close to one end. In telocentric ones it is terminal.

8. Small bristle-like fibres that help bacteria attach to rocks in streams and to host tissues are

   1. gas vacuoles
   2. fimbriae
   3. plasmids
   4. mesosomes
   Answer

   B. fimbriae

   Fimbriae are bristle-like fibres that help attachment.

9. The membrane that surrounds the vacuole of a plant cell is called the

   1. cristae
   2. middle lamella
   3. plasmalemma
   4. tonoplast
   Answer

   D. tonoplast

   The vacuolar membrane is the tonoplast.

10. The middle lamella between two plant cells is mainly made of

    1. hemicellulose
    2. calcium pectate
    3. cellulose
    4. chitin
    Answer

    B. calcium pectate

    Middle lamella is a layer of calcium pectate that holds neighbouring cells together.

11. Leucoplasts that store oils and fats are called

    1. elaioplasts
    2. amyloplasts
    3. aleuroplasts
    4. chromoplasts
    Answer

    A. elaioplasts

    Elaioplasts store oils and fats. Amyloplasts store starch and aleuroplasts store proteins.

12. Which organelle of a eukaryotic cell contains a single circular DNA and 70S ribosomes?

    1. Lysosome
    2. Golgi apparatus
    3. Mitochondrion
    4. Rough endoplasmic reticulum
    Answer

    C. Mitochondrion

    Mitochondria have circular DNA, a few RNA molecules and 70S ribosomes.

13. Proteins that are partially or totally buried in the lipid bilayer are called

    1. histone proteins
    2. glycolipids
    3. integral proteins
    4. peripheral proteins
    Answer

    C. integral proteins

    Integral proteins are embedded in the bilayer. Peripheral proteins lie on the surface.

14. Which of the following prokaryotes has no cell wall?

    1. Mycoplasma
    2. Vibrio
    3. Bacillus
    4. Cyanobacteria
    Answer

    A. Mycoplasma

    All prokaryotes have a cell wall except mycoplasma.

15. How many microtubules are present in the axoneme of a cilium?

    1. 27
    2. 20
    3. 11
    4. 29
    Answer

    B. 20

    Nine doublets (18) plus two central singlets give 20. Twenty-seven would be nine triplets, as in a centriole.

16. The cell organelle that holds hydrolytic enzymes active at acidic pH is the

    1. mitochondrion
    2. ribosome
    3. nucleolus
    4. lysosome
    Answer

    D. lysosome

    Lysosomes contain lipases, proteases and carbohydrases that work best at acidic pH.

17. Given below are two statements: Statement I: Schleiden was a zoologist who studied animal cells. Statement II: Virchow explained that new cells arise from pre-existing cells. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Schleiden was a botanist who studied plants. Virchow stated omnis cellula e cellula.

18. Given below are two statements: Statement I: Mesosomes are infoldings of the plasma membrane in prokaryotes. Statement II: Mesosomes help in DNA replication and in its distribution to daughter cells. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct. Mesosomes also help in cell wall formation and respiration.

19. Given below are two statements: Statement I: Plasmids are small circular DNA molecules present outside the genomic DNA of some bacteria. Statement II: Plasmids are the main genetic material of a bacterial cell and lie inside a nuclear membrane. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Plasmids are extra-chromosomal. The main DNA lies in the nucleoid, which has no membrane.

20. Given below are two statements: Statement I: The surface of rough ER has no ribosomes attached to it. Statement II: Smooth ER is the site of lipid synthesis. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Rough ER has ribosomes on its outer surface. SER synthesises lipids.

21. Given below are two statements: Statement I: Prokaryotic ribosomes are 80S. Statement II: Eukaryotic ribosomes are made of 50S and 30S subunits. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    B. Both Statement I and Statement II are incorrect

    Prokaryotic ribosomes are 70S. Eukaryotic ribosomes are 80S, made of 60S and 40S subunits.

22. Which of the following statements are correct? 1. The nucleolus is bound by a membrane. 2. The nucleolus is the site of rRNA synthesis. 3. Cells making large amounts of protein have larger and more numerous nucleoli.

    1. 2 and 3 only
    2. 1 and 2 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    A. 2 and 3 only

    The nucleolus has no membrane. Statements 2 and 3 are correct.

23. Which of the following statements are correct? 1. Aleuroplasts store proteins. 2. Elaioplasts store starch. 3. Amyloplasts store starch.

    1. 2 only
    2. 1 and 3 only
    3. 1 and 2 only
    4. 2 and 3 only
    Answer

    B. 1 and 3 only

    Elaioplasts store oils and fats, not starch.

24. Given below are two statements: Statement I: Centrioles have a cartwheel arrangement of nine peripheral triplets. Statement II: Centrioles form the basal bodies of cilia and flagella. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct. The basal body has the same structure as a centriole.

25. Given below are two statements: Statement I: The outer membrane of the nuclear envelope has no ribosomes on it. Statement II: The perinuclear space between the two nuclear membranes is about 10 to 50 nm wide. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    The outer nuclear membrane often bears ribosomes and is continuous with the ER.

26. Given below are two statements: Statement I: Cellulose is a major component of the plant cell wall. Statement II: The middle lamella is made mainly of cellulose. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    The middle lamella is made of calcium pectate.

27. Assertion (A): Mitochondria are called the powerhouses of the cell. Reason (R): Mitochondria make ATP during aerobic respiration. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    ATP production is the reason for the name.

28. Assertion (A): Proteins can move sideways within the plasma membrane. Reason (R): The lipid bilayer of the membrane is quasi-fluid. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    The fluid nature of the bilayer allows lateral movement.

29. Assertion (A): Prokaryotic cells lack a membrane-bound nucleus. Reason (R): Their DNA is linear and held within a nuclear envelope with histones. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    Prokaryotic DNA is circular and lies in the nucleoid with no envelope.

30. Assertion (A): Smooth ER synthesises steroid hormones in animal cells. Reason (R): Smooth ER has no ribosomes on its surface. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    B. Both A and R are true but R is not the correct explanation of A

    SER makes lipids and steroids, but the lack of ribosomes does not explain this.

31. Assertion (A): Chromoplasts carry out photosynthesis. Reason (R): Chromoplasts contain carotenoid pigments. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    Chromoplasts give colour to flowers and fruits. Chloroplasts perform photosynthesis.

32. Assertion (A): Chloroplasts can make some of their own proteins. Reason (R): Chloroplast thylakoids contain chlorophyll. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    B. Both A and R are true but R is not the correct explanation of A

    The stroma has DNA and 70S ribosomes, which explain protein synthesis. Chlorophyll does not.

33. Match List I with List II. List I: a. Mitochondria b. Rough ER c. Lysosomes d. Nucleolus List II: i. Intracellular digestion by hydrolytic enzymes ii. rRNA synthesis iii. ATP synthesis iv. Protein synthesis and secretion Choose the correct answer from the options given below.

    1. a-iv, b-iii, c-i, d-ii
    2. a-ii, b-i, c-iv, d-iii
    3. a-iii, b-iv, c-i, d-ii
    4. a-iv, b-i, c-ii, d-iii
    Answer

    C. a-iii, b-iv, c-i, d-ii

    Each organelle is matched to its function.

34. Match List I with List II. List I: a. Amyloplast b. Elaioplast c. Aleuroplast d. Chromoplast List II: i. Stores proteins ii. Stores starch iii. Contains carotenoids iv. Stores oils and fats Choose the correct answer from the options given below.

    1. a-ii, b-iii, c-iv, d-i
    2. a-iv, b-i, c-iii, d-ii
    3. a-ii, b-i, c-iii, d-iv
    4. a-ii, b-iv, c-i, d-iii
    Answer

    D. a-ii, b-iv, c-i, d-iii

    Leucoplast types store starch, oil or protein. Chromoplasts hold carotenoids.

35. Match List I with List II. List I: a. Centriole b. Axoneme c. Nucleolus d. Kinetochore List II: i. Attachment site of spindle fibres ii. Cartwheel arrangement iii. 9+2 arrangement of microtubules iv. No limiting membrane Choose the correct answer from the options given below.

    1. a-iv, b-i, c-ii, d-iii
    2. a-iv, b-i, c-iii, d-ii
    3. a-ii, b-iii, c-iv, d-i
    4. a-i, b-iii, c-iv, d-ii
    Answer

    C. a-ii, b-iii, c-iv, d-i

    Each structure is matched to its feature.

36. Match List I with List II. List I: a. Metacentric b. Sub-metacentric c. Acrocentric d. Telocentric List II: i. Centromere slightly away from the middle ii. Centromere in the middle iii. Centromere at the terminal end iv. Centromere close to one end Choose the correct answer from the options given below.

    1. a-i, b-ii, c-iii, d-iv
    2. a-ii, b-i, c-iv, d-iii
    3. a-ii, b-i, c-iii, d-iv
    4. a-iii, b-iv, c-ii, d-i
    Answer

    B. a-ii, b-i, c-iv, d-iii

    The names describe the centromere position.

37. Match List I with List II. List I: a. Schleiden b. Schwann c. Virchow d. Singer and Nicolson List II: i. Fluid mosaic model ii. Omnis cellula e cellula iii. All plants are made of cells iv. Animal cells have a thin outer layer, the plasma membrane Choose the correct answer from the options given below.

    1. a-ii, b-iii, c-i, d-iv
    2. a-iii, b-iv, c-ii, d-i
    3. a-iv, b-ii, c-iii, d-i
    4. a-iii, b-ii, c-i, d-iv
    Answer

    B. a-iii, b-iv, c-ii, d-i

    Each scientist is matched to the contribution.

38. Match List I with List II. List I: a. Mesosome b. Plasmid c. Fimbriae d. Chromatophore List II: i. Pigment-bearing membranous extension ii. Infolding of the plasma membrane iii. Extra-chromosomal circular DNA iv. Bristle-like fibres for attachment Choose the correct answer from the options given below.

    1. a-ii, b-i, c-iii, d-iv
    2. a-iv, b-iii, c-ii, d-i
    3. a-ii, b-iii, c-iv, d-i
    4. a-iv, b-i, c-ii, d-iii
    Answer

    C. a-ii, b-iii, c-iv, d-i

    Each prokaryotic structure is matched to its description.

39. Treating a bacterial culture to remove a small circular DNA molecule makes it lose antibiotic resistance. The removed DNA was a

    1. plasmid
    2. mesosome
    3. nucleoid
    4. glycocalyx
    Answer

    A. plasmid

    Plasmids are extra-chromosomal circular DNA that can carry antibiotic resistance.

40. A pancreatic cell that secretes large amounts of digestive enzyme proteins would be expected to have abundant

    1. smooth ER and chloroplasts
    2. centrioles and cell wall
    3. chromoplasts and tonoplast
    4. rough ER and Golgi apparatus
    Answer

    D. rough ER and Golgi apparatus

    Secreted proteins are made on rough ER and packaged by the Golgi apparatus.

41. A cell has a nucleoid with circular DNA, 70S ribosomes and no cell wall. It is most likely

    1. an Amoeba
    2. Mycoplasma
    3. a cyanobacterium
    4. a yeast cell
    Answer

    B. Mycoplasma

    Only mycoplasma among these is a prokaryote with no cell wall.

42. A freshwater Amoeba takes in extra water by osmosis. The structure that removes the excess water is the

    1. food vacuole
    2. Golgi apparatus
    3. contractile vacuole
    4. lysosome
    Answer

    C. contractile vacuole

    In Amoeba, the contractile vacuole expels excess water.

43. A human red blood cell membrane has about 52 per cent protein and about 40 per cent lipid. About how much protein is in 200 mg of such membrane?

    1. about 104 mg
    2. about 16 mg
    3. about 52 mg
    4. about 80 mg
    Answer

    A. about 104 mg

    52 per cent of 200 mg is 104 mg.

44. A cell is found to have unusually large and numerous nucleoli. This cell most probably

    1. is dividing by fission
    2. is a prokaryote
    3. has no ribosomes
    4. is making large amounts of protein
    Answer

    D. is making large amounts of protein

    Active protein-making cells need more rRNA, so they have larger and more numerous nucleoli.