Chapter 12 — Cells, tissues and classification
The cell is a working unit
Living organisms have many levels of organisation. A cell is the basic structural and functional unit in the school-level account of life. A tissue is a group of cells with related roles; organs combine tissues; organ systems combine organs to carry out broader functions. A heart is an organ, not one giant muscle cell. Blood is a connective tissue even though it is liquid, because it contains cells suspended in a matrix and has transport and defence functions. In an MCQ, identify the scale of the named object before matching its function.
The cell membrane separates the cell's interior from its surroundings and controls much exchange. It is selectively permeable, meaning different substances cross by different routes and rates. It is not a rigid wall that blocks all movement. A plant cell also has a cell wall outside the membrane; in plants it is mainly cellulose in the standard school comparison. A plant cell can have a large central vacuole, and photosynthetic plant cells have chloroplasts. Animal cells do not have a cellulose cell wall or chloroplasts. Yet not every plant cell contains a chloroplast: underground root cells normally do not photosynthesise in the same way as green leaf cells.
The nucleus in a eukaryotic cell holds much of the cell's genetic material. Prokaryotic cells such as typical bacteria lack a membrane-bound nucleus, though they have genetic material. “No nucleus” must not be read as “no DNA.” Mitochondria are important sites of energy-releasing cellular respiration in eukaryotes; chloroplasts are sites of photosynthesis in suitable plant and algal cells. Ribosomes make proteins. These short pairings are useful, but real cellular roles overlap and interact. A mitochondrion does not create energy from nothing: it participates in transforming energy stored in molecules into forms a cell can use.
Membrane movement and osmosis
Diffusion is a net movement from a region of higher concentration toward lower concentration under the specified conditions. It results from random particle motion; no cell's intention is required. Oxygen can diffuse across suitable biological surfaces when the gradient allows it. Osmosis concerns water movement through a selectively permeable membrane, driven by differences in water potential. A school-level shortcut says water moves from a more dilute solution toward a more concentrated one across such a membrane. This is helpful for simple two-solution diagrams, but the full description depends on water potential and pressure as well as solute concentration.
A plant cell placed in a very concentrated external solution may lose water and its membrane can pull away from the cell wall, a phenomenon called plasmolysis. In a dilute surrounding solution, water can enter and the cell may become turgid because the wall resists further expansion. An animal cell lacks a rigid cell wall, so its response to strongly hypotonic conditions differs. These comparisons require that the membrane is intact and the relevant solutes do not freely equilibrate across it. If a question omits those conditions, do not overstate a precise outcome.
Active transport differs from simple diffusion because it uses cellular energy to move substances in ways that can oppose a concentration gradient. A cell membrane is therefore not a passive sieve. A typical exam stem might describe mineral uptake by root cells where the ion is already more concentrated inside; active transport is the expected concept. But not every movement through a membrane is active: water movement in osmosis and many gas exchanges are passive. Identify the direction and energy requirement stated in the stem.
Plant and animal tissues
Plant meristematic tissue contains cells that divide and contribute to growth. Apical meristems near tips support growth in length; lateral meristems can contribute to thickening in relevant plants. Permanent tissues include simple tissues such as parenchyma, collenchyma and sclerenchyma, and complex transport tissues such as xylem and phloem. Parenchyma often performs storage or photosynthesis depending on location. Collenchyma provides flexible support, while mature sclerenchyma provides stronger mechanical support and has thickened walls. These are functional patterns, not a claim that every cell in a whole plant has one tissue type.
Xylem conducts water and dissolved mineral nutrients mainly from roots toward aerial parts in the familiar simplified model. Phloem transports products of photosynthesis from source regions to sink regions; movement can be in different directions depending on source and sink. The two tissues differ in component cells and function. A quick memory trap is “food goes down, water goes up.” Water transport is generally upward in the school account, but phloem movement should not be restricted to down only. A growing shoot can receive sugar through phloem from a mature leaf.
Animal epithelial tissue covers or lines surfaces and has protective, absorptive or secretory roles depending on type and location. Connective tissue supports and joins structures; examples include bone, cartilage, blood and tendons. Muscle tissue contracts to produce movement. Nervous tissue receives and transmits signals through specialised cells. A question may ask about the tissue lining an organ, connecting muscle to bone, or producing contraction. “Tendon” connects muscle to bone; “ligament” connects bone to bone in the elementary distinction. These are different connective-tissue examples, not synonyms.
Muscle types also differ. Skeletal muscle is usually under voluntary control and attached to bones. Smooth muscle occurs in many internal organ walls and usually works involuntarily. Cardiac muscle is the specialised muscle of the heart. A candidate should not conclude that all striped muscle is voluntarily controlled, because cardiac muscle has striations yet contracts without voluntary command. Likewise, “involuntary” does not mean unregulated; nervous and hormonal systems influence these tissues.
Classification as a testable hypothesis
Biological classification groups organisms by shared characteristics and evolutionary relationships. It is revised as evidence improves. Older school lists may divide life into five kingdoms; modern systems also use higher-level domains and molecular evidence. A police-exam question must identify which system it is using. If it explicitly asks for the five-kingdom scheme, answer within that scheme; if it asks what distinguishes prokaryotes from eukaryotes, use cell structure. Do not silently combine ranks from two different classification frameworks.
In a broad three-domain view, Bacteria and Archaea are distinct prokaryotic lineages, while Eukarya includes plants, animals, fungi and diverse protists. A fungus is not a plant simply because it stays fixed in one place. Fungi obtain organic nutrients rather than performing the chlorophyll-based photosynthesis typical of green plants; their cell walls also differ in composition. A virus is acellular and depends on a host cell to replicate, so it does not fit neatly into a list of cellular kingdoms. This is a conceptual distinction, not a statement about whether a virus is important or harmful.
Within animals, vertebrates have a vertebral column at the relevant life stage, while invertebrates lack one. Fish, amphibians, reptiles, birds and mammals are familiar vertebrate groups, but the exact boundaries of these everyday labels can be refined by modern taxonomy. A whale is a mammal despite living in water; a bat is a mammal despite flying. Habitat and movement alone are poor classifiers. Look for defining traits such as mammalian milk production, feathers in birds, and the broader anatomical and genetic evidence used by biologists.
Worked identification route
A diagram shows a rectangular cell with a cell wall, a large vacuole, a nucleus and green plastids. The wall and plastids support a plant-cell identification; the nucleus shows it is eukaryotic. Yet shape alone would have been insufficient because many cells can look rectangular in drawings. A second diagram lacks a membrane-bound nucleus and has a cell membrane and genetic material in a nucleoid region: a prokaryotic cell is the basic classification. Do not infer that the second cell is “empty” or cannot carry out life processes.
Suppose a question gives three tissue statements: A transports water in plants, B transports sugars from source to sink, and C contracts in the heart. Match A to xylem, B to phloem and C to cardiac muscle. To avoid a memory slip, identify the material moved or action performed before reading the answer choices. If an option labels phloem as “downward-only food pipe,” reject that extra absolute claim even if it mentions food. Precise wording matters more than partial familiarity.
Inside a cell: follow a product rather than memorise labels
A cell that secretes a protein offers a useful route through several organelles. The instructions for the protein are encoded in DNA; an RNA copy helps carry the information to ribosomes. Ribosomes associated with rough endoplasmic reticulum can make proteins that enter the secretion pathway. The endoplasmic reticulum and Golgi apparatus help process and direct these products; transport vesicles carry them between compartments and towards the cell surface. The Golgi apparatus therefore packages and sorts many products but is not the site that translates the genetic message into a protein. If a stem asks specifically for protein synthesis, choose ribosome. If it asks for modification and packaging in this pathway, consider Golgi apparatus.
Smooth endoplasmic reticulum lacks the attached ribosomes that give rough endoplasmic reticulum its appearance. It participates in lipid synthesis and other functions that vary with cell type. Lysosomes contain enzymes that break down materials in an acidic internal environment. A vacuole may store substances and help regulate water balance; the large central vacuole of many mature plant cells contributes to turgor. The phrase “storage sac” is a useful introduction but does not exhaust vacuole function. Learn a principal role for each organelle, then identify the exact operation requested in the question.
Mitochondria and chloroplasts contain their own DNA and ribosomes, yet they depend heavily on products encoded by nuclear genes. Calling them completely independent cells inside a cell would therefore be wrong. Prokaryotes can carry out respiration and make ATP without mitochondria, using cellular structures that include their plasma membrane. Absence of an organelle does not prove absence of the chemical process associated with that organelle in another kind of cell. The reliable contrast is structural: typical bacteria lack membrane-bound organelles such as mitochondria and a membrane-bound nucleus, but have a membrane, DNA and ribosomes.
Microscopy: size, magnification and resolution
Magnification describes how much larger an image appears than the object. Resolution describes the ability to distinguish nearby details as separate. Enlarging a blurred image increases its size without necessarily revealing new structures. A multiple-choice item can test this distinction by offering “higher magnification always gives more detail.” The word always makes the claim unsound because useful detail also depends on resolution, illumination, contrast and specimen preparation. A diagram is an interpretation of structure; its bright colours and thick boundaries need not represent the natural appearance of a living cell.
In a conventional compound microscope, total magnification is found by multiplying eyepiece and objective magnifications. A 10× eyepiece with a 40× objective gives 400× total magnification. This is not 50×, because the lenses provide successive magnification rather than two numbers to add. Suppose a printed image of a cell is 20 mm long at a stated magnification of 500×. Actual length = image length ÷ magnification = 20/500 mm = 0.04 mm = 40 micrometres. Convert both lengths into the same unit before comparing them. One millimetre is 1,000 micrometres.
A scale bar often remains more useful than a printed magnification after a diagram is resized. If a bar labelled 10 micrometres occupies 8 mm on the page and the cell spans 32 mm, the cell is four bar-lengths long, so its represented length is 40 micrometres. This ratio survives proportional enlargement of the entire image. It fails if the image is stretched unevenly or if the bar is changed separately. For exam diagrams, use the scale provided rather than judging a cell's size from how much of the page it occupies.
Osmosis cases: predict the direction before naming the result
Use the terms hypotonic, hypertonic and isotonic relative to the cell or comparison solution. For an elementary membrane problem, assume the specified solute cannot pass freely but water can. A hypotonic external solution has a lower effective solute concentration than the cell contents; water tends to enter. A hypertonic external solution tends to draw water out. Isotonic conditions produce no sustained net water movement due to this concentration difference. Water molecules can still move in both directions at equilibrium. “No net movement” does not mean every molecule has stopped.
Consider identical plant cells placed in dilute and concentrated solutions. In the dilute solution, water enters and the vacuole expands until wall pressure opposes further net entry. The cell becomes turgid. In the concentrated solution, water loss reduces volume inside the wall and can produce plasmolysis. The cellulose wall does not shrink in precisely the same way as the living contents. In an animal cell, strong water entry can cause swelling and rupture because a comparable rigid wall is absent. These are controlled comparison cases, not instructions to use any particular solution in a medical setting.
A classroom investigation may compare equal-sized potato strips in different solutions. Initial mass of a strip is 5.0 g and final mass after careful surface blotting is 5.5 g. Percentage mass change = (5.5 − 5.0)/5.0 × 100 = +10%. A second strip falls from 5.0 g to 4.5 g, giving −10%. The signs distinguish gain and loss. Standardise strip dimensions, temperature, immersion time and blotting method; otherwise the comparison may reflect unequal surface water or sample size. A zero mass change can help estimate an isotonic condition under that procedure, but it does not establish that all cell processes have stopped.
Cell division: separate chromosome number from DNA quantity
Cell division supports growth, replacement and reproduction, but not every division has the same outcome. In the standard diploid-cell model, mitosis followed by division of the cytoplasm produces two daughter cells with the same chromosome number as the original cell. Meiosis involves two nuclear divisions after one round of DNA replication and typically produces haploid products. In animals, meiosis is central to gamete formation. In plants, the relationship includes spore production and a life cycle with different generations; the shortcut “meiosis directly makes all plant gametes” is not a universal description.
Chromosomes are counted by centromeres in elementary chromosome-number problems. Before DNA replication, a chromosome contains one chromatid. After replication, it consists of two sister chromatids joined at the centromere until they separate. Replication doubles DNA quantity but does not immediately double the chromosome count by this convention. For a hypothetical diploid organism with 2n = 8, the replicated cell still has eight chromosomes, now represented by sixteen chromatids. Ordinary mitosis produces daughter nuclei with eight chromosomes each. Meiosis produces haploid products with four chromosomes each when the process proceeds normally.
Do not mix this eight-chromosome teaching example with human chromosome numbers. In the usual human somatic-cell account, 46 chromosomes form 23 pairs; a typical gamete has 23 chromosomes. Fertilisation combines two haploid sets. Mature mammalian red blood cells are a familiar exception to claims that every human cell contains a nucleus with 46 chromosomes. Questions about chromosome number must specify the organism, cell type and stage. A correct numerical fact becomes a distractor when applied to the wrong cell.
Classification: extract evidence from a short description
Taxonomic ranks in the conventional school sequence run kingdom, phylum or division, class, order, family, genus and species. Moving towards species generally narrows the group. A family may contain several genera; a genus may contain several species. Binomial names contain a genus name followed by a specific epithet. The genus begins with a capital letter, the epithet with a lower-case letter, and both are conventionally italicised in print. The second word alone is not the complete species name. Scientific naming avoids many ambiguities of local common names, but names and classifications may be revised when evidence warrants.
Build classification answers from several traits. A multicellular organism with a chitin-containing cell wall and absorptive nutrition fits a fungal comparison more closely than a plant comparison. An organism with feathers belongs to the bird group even if it does not fly. Flight alone cannot distinguish a bat from a bird or an insect. An animal with gills at one stage is not automatically a fish: many amphibian larvae have gills. Ask whether the trait is diagnostic for the group being tested and whether the life stage matters.
Within a five-kingdom question, Monera includes the prokaryotes of that historical scheme, while Protista groups diverse mainly unicellular eukaryotes in the simplified account. Do not substitute a domain name as though it were a kingdom in that same list. The three-domain framework separates Bacteria and Archaea, whereas the historical five-kingdom framework groups prokaryotes differently. A question that names the framework supplies the rule for interpreting the options. Evolutionary relationships are the scientific reason classifications are reviewed; the existence of more than one teaching framework does not mean any arbitrary grouping is equally valid.
Retrieval practice with reasons
Case 1: A cell lacks a membrane-bound nucleus but contains DNA and ribosomes. Is it necessarily unable to synthesise protein? No. Ribosomes provide the machinery for protein synthesis; the absence of a nuclear membrane supports a prokaryotic identification rather than absence of genetic information. Case 2: A secreted protein has been made but needs processing and sorting. Which organelle is most closely associated with that later task? The Golgi apparatus, with the understanding that transport and processing involve a pathway rather than one isolated organelle.
Case 3: A 12 mm cell image has a magnification of 300×. Its represented actual length is 0.04 mm, or 40 micrometres. Case 4: A plant strip changes from 8.0 g to 7.2 g. Its mass change is −0.8 g and percentage change is −10%, consistent with net water loss under a suitably controlled osmosis experiment. Case 5: A diploid cell with 12 chromosomes replicates its DNA before mitosis. It then has 12 replicated chromosomes, not 24 chromosomes by the stated counting convention. Explain each result aloud before checking the numerical answer.
Recall and review
Explain the cell→tissue→organ→system sequence using one human and one plant example. Identify cell membrane, wall, nucleus, chloroplast, mitochondrion and ribosome by role. Distinguish diffusion, osmosis and active transport by gradient and energy. Compare xylem with phloem and tendon with ligament. Finally, explain why a whale's aquatic habitat does not make it a fish and why a virus does not simply belong in a cellular kingdom. Attempt the chapter practice set, then use the explanations to revisit any uncertain concepts.
Figure 12. Original chapter diagram.
Figure 12.1. Count chromosomes by centromeres
Practice 32 questions
Choose one answer for each item. The answer key and explanations follow this chapter set.
1. The rigid cell wall of a plant cell is made mainly of which carbohydrate?
A. Cellulose B. Chitin C. Starch D. Glycogen
2. Match the vertebrate group with a typical school-level feature. List I: A. Fishes (traditional Pisces); B. Amphibia; C. Aves; D. Mammalia. List II: 1. Moist, permeable skin without scales; 2. Mammary glands; 3. Feathers; 4. Gills in typical adult aquatic members.
A. A-1, B-4, C-3, D-2 B. A-4, B-3, C-1, D-2 C. A-4, B-1, C-2, D-3 D. A-4, B-1, C-3, D-2
3. Match the animal group with a distinguishing feature. List I: (a) Cnidaria (traditional Coelenterata); (b) Porifera; (c) Platyhelminthes; (d) Arthropoda. List II: (i) flatworms with no true body cavity; (ii) pore-bearing body without true tissues; (iii) stinging cnidocytes; (iv) jointed appendages and an exoskeleton.
A. a-iii, b-ii, c-i, d-iv B. a-ii, b-iii, c-i, d-iv C. a-iii, b-i, c-ii, d-iv D. a-iv, b-ii, c-i, d-iii
4. Starfish and sea urchin belong to which phylum?
A. Coelenterata B. Arthropoda C. Echinodermata D. Mollusca
5. The roundworm Ascaris, a round and unsegmented worm, belongs to which phylum?
A. Annelida B. Nematoda (Aschelminthes) C. Mollusca D. Platyhelminthes
6. The silverfish, commonly found in old books and damp cupboards, belongs to which phylum?
A. Arthropoda B. Annelida C. Mollusca D. Chordata
7. Snails, octopuses and oysters belong to which animal phylum?
A. Mollusca B. Porifera C. Annelida D. Echinodermata
8. Members of which vertebrate class breathe through gills in the larval stage and through lungs and skin as adults?
A. Amphibia B. Aves C. Pisces D. Reptilia
9. Which is the only mammal capable of true, sustained flight?
A. Colugo (flying lemur) B. Flying squirrel C. Sugar glider D. Bat
10. Which of the following animals is a true fish?
A. Silverfish B. Rohu C. Starfish D. Jellyfish
11. Which of the following mammals lays eggs?
A. Platypus B. Dolphin C. Kangaroo D. Bat
12. Consider the following statements:
Statement I: All members of phylum Chordata possess a notochord at some stage of their life.
Statement II: All members of phylum Chordata have a vertebral column (backbone).
Which of the above is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
13. Consider the following statements:
Statement I: Sponges (Porifera) generally show bilateral symmetry.
Statement II: Adult echinoderms show radial symmetry.
Which of the above is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
14. Which of the following animals is NOT cold-blooded (ectothermic)?
A. Lizard B. Frog C. Penguin D. Rohu
15. Which of the following is NOT a mammal?
A. Whale B. Dolphin C. Shark D. Bat
16. Which of the following is NOT an arthropod?
A. Spider B. Octopus C. Scorpion D. Crab
17. Which of the following statements about cold-blooded and warm-blooded animals is NOT correct?
A. Endothermic birds and mammals maintain body temperature largely by internal heat production B. A snake basking in sunlight may feel hot yet remains an ectotherm C. Ectothermic animals always feel cold to the touch D. Ectotherms rely mainly on environmental heat rather than maintaining temperature chiefly by metabolic heat
18. Stinging cells called cnidocytes, used for capturing prey and defence, are a characteristic feature of which phylum?
A. Echinodermata B. Porifera C. Platyhelminthes D. Coelenterata (Cnidaria)
19. Arrange the following milestones in cell biology in chronological order:
(A) Schleiden states that all plants are made of cells
(B) Hooke observes and names cells in cork
(C) Virchow states that all cells arise from pre-existing cells
(D) Robert Brown discovers the nucleus
A. B, A, D, C B. D, B, A, C C. B, D, C, A D. B, D, A, C
20. Why are viruses generally described as acellular rather than as prokaryotic cells?
A. They lack a cellular organisation and depend on host cells to replicate B. They contain a large membrane-bound nucleus C. They are made of many specialised tissues D. They have chloroplasts but no mitochondria
21. Apart from the nucleus, which of the following cell organelles contains its own circular DNA and can make some of its own proteins?
A. Mitochondria B. Golgi apparatus C. Lysosome D. Endoplasmic reticulum
22. Match List-I (Cell organelle) with List-II (Function):
List-I: (a) Smooth endoplasmic reticulum (b) Rough endoplasmic reticulum (c) Vacuole (d) Chloroplast
List-II: (i) Storage of water, food and waste (ii) Synthesis of lipids and detoxification of chemicals (iii) Synthesis and transport of proteins, with ribosomes attached (iv) Photosynthesis
A. a-iii, b-ii, c-i, d-iv B. a-ii, b-iii, c-i, d-iv C. a-ii, b-iii, c-iv, d-i D. a-i, b-iii, c-ii, d-iv
23. Match List-I (Organelle) with List-II (Function):
List-I: (a) Golgi apparatus (b) Lysosome (c) Ribosome (d) Mitochondrion
List-II: (i) Protein synthesis (ii) ATP production (iii) Packaging and dispatch of proteins (iv) Intracellular digestion
A. a-iv, b-iii, c-i, d-ii B. a-iii, b-iv, c-ii, d-i C. a-iii, b-iv, c-i, d-ii D. a-i, b-iv, c-iii, d-ii
24. Ribosomes found in the cytoplasm of eukaryotic cells are of which type?
A. 70S B. 60S C. 80S D. 90S
25. Which observer is especially associated with early descriptions of living microorganisms seen through simple microscopes?
A. Robert Hooke B. Camillo Golgi C. Anton van Leeuwenhoek D. Robert Brown
26. Which cell organelle contains hydrolytic enzymes that break down material taken into a cell and worn-out cell parts?
A. Golgi apparatus B. Lysosome C. Vacuole D. Centrosome
27. Which structure controls much of the movement of substances into and out of an animal cell?
A. Nucleolus B. Plasma membrane C. Chloroplast D. Ribosome
28. The nucleolus, present inside the nucleus, is mainly concerned with the synthesis of:
A. ATP B. Lipids C. Cellulose D. Ribosomal RNA
29. Consider the following statements:
Statement I: Centrioles are usually present in animal cells but absent in most plant cells.
Statement II: A mature plant cell usually has one large central vacuole.
Which of the above is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
30. Consider the statements. I: Chloroplasts in many plant cells contain their own DNA. II: Mature human red blood cells retain a well-defined nucleus. Which statement(s) is/are correct?
A. Only I B. Only II C. Both I and II D. Neither I nor II
31. Which of the following is NOT a prokaryotic organism?
A. Amoeba B. Blue-green alga (cyanobacterium) C. Escherichia coli D. Lactobacillus
32. Which of the following is NOT a type of connective tissue?
A. Cartilage B. Epithelial tissue C. Ligament D. Adipose tissue
Answers and explanations
1. A. The plant cell wall is made mainly of cellulose, which gives the cell a fixed, rigid shape. Chitin forms the cell wall of fungi and the exoskeleton of arthropods; starch and glycogen are storage carbohydrates of plants and animals.
2. D. Typical fishes use gills, so A-4; amphibians commonly have moist permeable skin, B-1; birds have feathers, C-3; and mammals possess mammary glands, D-2. The match is A-4, B-1, C-3, D-2. Biological groups have exceptions, so a claim that every fish breathes only through gills throughout life would be too absolute.
3. A. Cnidarians have cnidocytes (iii); sponges have a pore-bearing body lacking true tissues (ii); flatworms lack a true coelom (i); arthropods have jointed appendages and an exoskeleton (iv). The code is a-iii, b-ii, c-i, d-iv. “Coelenterata” is a traditional school name for the cnidarian group.
4. C. Sea stars and sea urchins are echinoderms: marine invertebrates with an internal calcareous skeleton and adult radial organisation. A sea star is not a fish despite its common name; fish are vertebrates. Saying sea stars have no gill-like structures would be imprecise, so that is not used as the distinction.
5. B. Ascaris and the filarial worm are nematodes — round, unsegmented worms with a tube-within-a-tube body. Annelids (earthworm, leech) are segmented, while platyhelminths (tapeworm, liver fluke) are flat.
6. A. Silverfish is a wingless insect (phylum Arthropoda), named only for its silvery scales and fish-like wriggling movement. It has jointed legs and an exoskeleton, not a backbone, so it is not a fish.
7. A. These animals belong to Mollusca. Molluscs generally have a soft body, a mantle and a muscular foot or a modified form of it; shells occur in many but not all members, as an octopus illustrates. The other options are sponges, segmented worms and echinoderms.
8. A. Amphibians such as frogs start life as gill-breathing tadpoles in water and, after metamorphosis, breathe with lungs and their moist, permeable skin. Fish keep gills throughout life; reptiles and birds use lungs from hatching.
9. D. Bats fly by flapping wings made of skin stretched over elongated fingers. Flying squirrels, colugos and sugar gliders only glide using skin flaps between their limbs; they cannot power sustained flight.
10. B. Rohu is a freshwater bony fish (class Pisces) with gills, fins and a backbone. Starfish is an echinoderm, jellyfish a coelenterate and silverfish an insect — none has a backbone, despite 'fish' in their names.
11. A. The duck-billed platypus and the echidna are monotremes — egg-laying mammals that still feed their young on milk. The kangaroo is a marsupial that gives birth to tiny live young carried in a pouch; bats and dolphins are also viviparous.
12. A. Statement I is correct: a notochord at some life stage defines chordates. Statement II is wrong: sea squirts (tunicates) and lancelets are chordates that never develop a backbone; only the subphylum Vertebrata replaces the notochord with a vertebral column.
13. B. Statement I is wrong: most sponges are asymmetrical, with no fixed body plan. Statement II is correct: adult echinoderms such as starfish are radially symmetrical (their larvae are bilateral). Almost all other animals, including vertebrates, are bilateral.
14. C. A penguin is a bird and maintains body temperature largely by internal metabolic heat, making it endothermic. The lizard, frog and rohu are generally ectothermic and rely more on external conditions for temperature regulation. “Warm-blooded” and “cold-blooded” are convenient school terms rather than claims that body temperature is perfectly constant or that ectotherms generate no heat.
15. C. The shark is a true fish that breathes through gills throughout life. Whales and dolphins breathe air with lungs, are warm-blooded and nurse their young, while bats have fur and mammary glands — all three are mammals.
16. B. The octopus is a mollusc with a soft, unsegmented body and no jointed legs. Crabs, spiders and scorpions all have jointed appendages and a chitinous exoskeleton, placing them in Arthropoda.
17. C. The false statement is that an ectotherm always feels cold. A basking snake may become hot to the touch. Ectotherms do generate some metabolic heat, but in general rely more on environmental heat for body-temperature regulation; endotherms such as birds and mammals rely much more on metabolic heat.
18. D. Coelenterates such as Hydra, jellyfish and sea anemone carry cnidocytes (cnidoblasts) on their tentacles, which give the phylum its name Cnidaria. Sponges (Porifera) have pores and choanocytes instead.
19. D. Hooke named cells in 1665, Robert Brown discovered the nucleus in 1831, Schleiden described plant cells in 1838, and Virchow gave "Omnis cellula e cellula" in 1855. So the order is B, D, A, C.
20. A. A virus is not organised as a cell; it contains genetic material within a protein coat and, in some viruses, an envelope. It replicates using host-cell machinery. Prokaryotes, by contrast, are cells with membranes, cytoplasm and cellular metabolism even though they lack a membrane-bound nucleus.
21. A. Mitochondria have their own small circular DNA and can make some of their own proteins, a leftover from the time they were probably free-living bacteria. Golgi bodies, lysosomes and the ER have no DNA of their own.
22. B. Smooth ER synthesises lipids and detoxifies chemicals (ii), rough ER, studded with ribosomes, makes and transports proteins (iii), the vacuole stores water, food and waste (i) and the chloroplast carries out photosynthesis (iv): a-ii, b-iii, c-i, d-iv.
23. C. Golgi apparatus packages and dispatches proteins (a-iii); lysosomes carry out intracellular digestion (b-iv); ribosomes synthesise proteins (c-i); mitochondria produce ATP through cellular respiration (d-ii).
24. C. Eukaryotic cytoplasmic ribosomes are 80S, whereas prokaryotic ribosomes are smaller 70S. The "S" is the Svedberg unit of sedimentation; 60S is only the larger subunit of an 80S ribosome.
25. C. Antonie van Leeuwenhoek observed living microorganisms with microscopes he made and used in the seventeenth century. Hooke named cells after viewing cork; Robert Brown described the nucleus; Camillo Golgi described the Golgi apparatus. This asks a different milestone from the Brown chronology item.
26. B. Lysosomes contain acid hydrolases that help digest engulfed material and recycle cellular components. The old nickname “suicide bag” is an oversimplification: controlled lysosomal function does not mean a lysosome normally bursts and destroys the entire cell.
27. B. The selectively permeable plasma membrane forms the cell boundary and regulates much of the exchange with the surroundings through diffusion, transport proteins and vesicle processes. The nucleolus helps assemble ribosomes; chloroplasts perform photosynthesis; ribosomes synthesize proteins.
28. D. The nucleolus lies inside the nucleus and is the site where ribosomal RNA is made and ribosome subunits are assembled. ATP comes from mitochondria, lipids from smooth ER, and cellulose forms the plant cell wall.
29. C. Both statements are correct. Animal cells have centrioles that help in cell division, while higher plant cells usually lack them. A mature plant cell has one large central vacuole that maintains turgor; animal cells have small or no vacuoles.
30. A. Only I is correct. Chloroplasts contain their own genetic material and ribosomes, consistent with their endosymbiotic origin. Mature human red blood cells lose the nucleus during development. The item uses chloroplasts to avoid repeating the earlier mitochondria-DNA question.
31. A. Amoeba is a unicellular protist with a true membrane-bound nucleus, so it is eukaryotic. Cyanobacteria (blue-green algae) and bacteria such as E. coli and Lactobacillus lack a true nucleus and are prokaryotes.
32. B. Epithelial tissue is a separate primary tissue that covers and lines surfaces with tightly packed cells. Cartilage, ligaments and adipose (fat) tissue are connective tissues, as are bone, tendons and blood.