Cell, Tissues and Cell Division

What to remember

• The cell is the basic structural and functional unit of life. Cell theory: all living things are made of cells, and new cells come only from existing cells.
• Plant cells have a cell wall, large vacuole and plastids; animal cells have centrioles and small vacuoles. Both have a nucleus, mitochondria, ribosomes and a plasma membrane.
• Mitosis gives two identical cells for growth; meiosis gives four cells with half the chromosomes for gametes. Plant tissues are meristematic or permanent; animal tissues are epithelial, connective, muscular and nervous.

Discovery and cell theory

Robert Hooke saw dead cork cells under a compound microscope of his own design and named them "cells". Anton van Leeuwenhoek was the first to see living cells, such as bacteria and protozoa. Robert Brown discovered the nucleus. Schleiden (botanist) and Schwann (zoologist) gave the cell theory; Virchow added that new cells arise from pre-existing cells. The electron microscope later showed the fine structure of organelles.

Cells differ in size and shape. The smallest cells are those of bacteria (Mycoplasma). The longest cells are nerve cells. An ostrich egg is the largest single cell. Red blood cells are round and biconcave; nerve cells are branched; muscle cells are spindle-shaped or long; plant cells are usually rectangular.

Organisms may be unicellular (Amoeba, Paramecium, Chlamydomonas, yeast) or multicellular (humans, trees). In a unicellular organism one cell does all life functions.

Prokaryotic and eukaryotic cells

A prokaryotic cell has no true nucleus and no membrane-bound organelles. Its genetic material lies free in the cytoplasm in a region called the nucleoid. Bacteria and blue-green algae (cyanobacteria) are prokaryotes. A eukaryotic cell has a true nucleus with a nuclear membrane and many membrane-bound organelles.

Cell organelles and their work

Plasma membrane is the thin, living, selectively permeable outer layer. It controls what enters and leaves the cell. Substances move by diffusion, osmosis and active transport. Osmosis is the movement of water through a selectively permeable membrane from lower to higher solute concentration.

Cell wall is non-living, rigid and found in plants, fungi and bacteria. In plants it is made of cellulose. It gives shape and protection and is fully permeable.

Nucleus is the control centre. It has a double membrane with pores, nucleoplasm, a nucleolus and chromatin. Chromatin condenses into chromosomes during division. Chromosomes carry genes made of DNA and carry hereditary information.

Cytoplasm is the jelly-like fluid between the nucleus and the membrane. Many chemical reactions happen here.

Endoplasmic reticulum (ER) is a network of membranes. Rough ER has ribosomes and makes proteins. Smooth ER has no ribosomes; it makes lipids and helps remove poisons (detoxification).

Golgi apparatus (discovered by Camillo Golgi) packs, modifies and sends out substances. In plants, the Golgi units are called dictyosomes.

Lysosomes are small sacs with digestive enzymes. They digest worn-out organelles and foreign matter. They are called "suicide bags" because they can burst and digest the whole cell when it is damaged.

Mitochondria are the "power houses" of the cell. They release energy from food as ATP in cellular respiration. They have two membranes and their own DNA and ribosomes.

Plastids are found only in plants. Chloroplasts (green, with chlorophyll) carry out photosynthesis. Chromoplasts hold colours of flowers and fruits. Leucoplasts are colourless and store starch, oil and protein.

Vacuoles store water, salts and wastes. Plant cells have one large central vacuole that keeps the cell firm (turgid). Animal cells have small or no vacuoles.

Ribosomes make proteins. They are found free in the cytoplasm or on the rough ER. They are sometimes called "protein factories".

Centrioles are found in animal cells near the nucleus. They help to form the spindle during cell division.

Tissues

A tissue is a group of similar cells doing a particular work. Several tissues make an organ, and organs make an organ system.

Plant tissues. Meristematic tissue has cells that keep dividing. Apical meristem is at the tips of roots and shoots and causes growth in length. Lateral meristem (cambium) increases thickness. Intercalary meristem is at the base of leaves or internodes (as in grasses). Permanent tissues have lost the power to divide. Simple permanent tissues are parenchyma (thin-walled, stores food; with chlorophyll it is chlorenchyma; with air spaces in aquatic plants it is aerenchyma), collenchyma (corners thickened, gives flexibility) and sclerenchyma (dead cells with lignin, gives strength; coconut husk and seed coats). Complex permanent tissues are xylem and phloem. Xylem carries water and minerals up; its parts are tracheids, vessels, xylem fibres and xylem parenchyma. Phloem carries prepared food; its parts are sieve tubes, companion cells, phloem fibres and phloem parenchyma. Epidermis is the outer protective layer; stomata in it control gas exchange and loss of water.

Animal tissues.

Tendons join muscle to bone; ligaments join bone to bone. Cartilage is found in the nose tip and ear. Blood is a fluid connective tissue. Striated (skeletal) muscles are voluntary; smooth muscles in the gut and blood vessels are involuntary; cardiac muscle is striated but involuntary and does not tire. A neuron has a cell body, dendrites and an axon.

Cell division

Cells divide for growth, repair and reproduction.

Cell cycle. The cycle has interphase (the cell grows and DNA is copied) and the division phase. Interphase is the longest phase.

Mitosis happens in body (somatic) cells. One parent cell gives two daughter cells with the same chromosome number. The stages are:

• 1. Prophase: chromatin condenses into chromosomes; nuclear membrane and nucleolus disappear; spindle forms.
• 2. Metaphase: chromosomes line up at the equator of the cell.
• 3. Anaphase: chromatids separate and move to opposite poles.
• 4. Telophase: nuclear membrane reforms; the cytoplasm divides (cytokinesis). In plants a cell plate forms; in animals the membrane pinches in.

Meiosis happens in reproductive cells to make gametes. One diploid cell gives four haploid cells in two divisions (meiosis I and II). Crossing over in prophase I swaps parts of homologous chromosomes and brings variation. Meiosis keeps the chromosome number constant over generations. Human body cells have 46 chromosomes (23 pairs); gametes have 23.

Teaching angle

In Classes VI to X these topics are taught through activities: onion peel and cheek-cell slides, comparing prepared slides, and drawing labelled diagrams. A teacher should lead learners from observation to inference, use models and charts for organelles, and avoid pure rote learning. Common learner mistakes are to call the cell wall living or to think that all cells have a nucleus.

Exam traps

• Hooke saw dead cork cells; Leeuwenhoek saw living cells.
• Mitochondria release energy; they do not "make" energy. ATP is the energy carrier.
• Lysosomes digest; ribosomes make protein; Golgi packs and sends.
• Rough ER makes proteins; smooth ER makes lipids.
• Collenchyma is living and flexible; sclerenchyma is dead and hard.
• Xylem moves water upward; phloem moves food in both directions.
• Tendon joins muscle to bone; ligament joins bone to bone.
• Mitosis gives two diploid cells; meiosis gives four haploid cells.

One-liners

• 1. Robert Hooke named the cell using cork.
• 2. Robert Brown discovered the nucleus.
• 3. Virchow said new cells come from old cells.
• 4. Bacteria have no true nucleus.
• 5. The cell wall of plants is made of cellulose.
• 6. Lysosomes are called suicide bags.
• 7. Mitochondria are the power houses of the cell.
• 8. Chloroplasts carry out photosynthesis.
• 9. Centrioles help in spindle formation in animal cells.
• 10. Apical meristem increases length of roots and shoots.
• 11. Cardiac muscle is striated but involuntary.
• 12. Crossing over occurs in prophase I of meiosis.

Practice questions

1. Who first observed and named 'cells' in a slice of cork?

   1. Robert Brown
   2. Rudolf Virchow
   3. Robert Hooke
   4. Anton van Leeuwenhoek
   Answer

   C. Robert Hooke

   Hooke saw dead cork cells and named them cells.

2. Who discovered the nucleus in the cell?

   1. Camillo Golgi
   2. Schwann
   3. Robert Hooke
   4. Robert Brown
   Answer

   D. Robert Brown

   Robert Brown discovered the nucleus.

3. Which organelle is called the power house of the cell?

   1. Lysosome
   2. Mitochondria
   3. Ribosome
   4. Golgi apparatus
   Answer

   B. Mitochondria

   Mitochondria release energy as ATP during respiration.

4. Which organelle is known as the suicide bag of the cell?

   1. Lysosome
   2. Vacuole
   3. Plastid
   4. Mitochondria
   Answer

   A. Lysosome

   Lysosomes contain digestive enzymes that can digest the cell itself.

5. Which of the following organisms has a prokaryotic cell?

   1. Yeast
   2. Bacteria
   3. Amoeba
   4. Chlamydomonas
   Answer

   B. Bacteria

   Bacteria have no true nucleus; the others are eukaryotes.

6. Cell wall of plant cells is mainly made of

   1. cellulose
   2. starch
   3. chitin
   4. protein
   Answer

   A. cellulose

   Plant cell wall is made of cellulose.

7. Protein synthesis in a cell takes place on

   1. lysosomes
   2. centrioles
   3. vacuoles
   4. ribosomes
   Answer

   D. ribosomes

   Ribosomes are the sites of protein synthesis.

8. Which plastids carry out photosynthesis?

   1. Chromoplasts
   2. Leucoplasts
   3. Chloroplasts
   4. Amyloplasts
   Answer

   C. Chloroplasts

   Chloroplasts contain chlorophyll and do photosynthesis.

9. Which tissue joins one bone to another bone?

   1. Areolar tissue
   2. Tendon
   3. Ligament
   4. Cartilage
   Answer

   C. Ligament

   Ligaments join bone to bone; tendons join muscle to bone.

10. The longest cell in the human body is the

    1. skin cell
    2. nerve cell
    3. muscle cell
    4. red blood cell
    Answer

    B. nerve cell

    Neurons can be very long.

11. The number of chromosomes in a normal human body cell is

    1. 48
    2. 44
    3. 23
    4. 46
    Answer

    D. 46

    Human somatic cells have 23 pairs, that is 46.

12. Which structure forms the spindle in animal cell division?

    1. Centriole
    2. Nucleolus
    3. Plastid
    4. Golgi body
    Answer

    A. Centriole

    Centrioles help in spindle formation.

13. Growth in the length of a root is due to

    1. phloem
    2. lateral meristem
    3. apical meristem
    4. permanent tissue
    Answer

    C. apical meristem

    Apical meristem at the tip adds length.

14. Which tissue transports water and minerals upward in plants?

    1. Xylem
    2. Parenchyma
    3. Phloem
    4. Collenchyma
    Answer

    A. Xylem

    Xylem carries water and minerals from roots to leaves.

15. Blood is an example of which type of tissue?

    1. Muscular tissue
    2. Nervous tissue
    3. Epithelial tissue
    4. Connective tissue
    Answer

    D. Connective tissue

    Blood is a fluid connective tissue.

16. Interphase of the cell cycle is the stage in which

    1. gametes are formed
    2. DNA is copied and the cell grows
    3. the cell plate forms
    4. chromosomes separate
    Answer

    B. DNA is copied and the cell grows

    DNA replication occurs in interphase.

17. Crossing over takes place during

    1. metaphase of mitosis
    2. telophase of mitosis
    3. anaphase II of meiosis
    4. prophase I of meiosis
    Answer

    D. prophase I of meiosis

    Homologous chromosomes exchange segments in prophase I.

18. A cell that is placed in a strongly salty solution will mostly

    1. make more cell wall
    2. stay unchanged
    3. lose water by osmosis
    4. gain water and burst
    Answer

    C. lose water by osmosis

    Water moves out to the higher solute concentration outside.

19. Which tissue gives flexibility to leaf stalks and young stems?

    1. Collenchyma
    2. Aerenchyma
    3. Sclerenchyma
    4. Xylem
    Answer

    A. Collenchyma

    Collenchyma is living with corners thickened and is flexible.

20. Which tissue has dead cells with lignin and forms coconut husk?

    1. Parenchyma
    2. Sclerenchyma
    3. Chlorenchyma
    4. Collenchyma
    Answer

    B. Sclerenchyma

    Sclerenchyma gives strength and is dead.

21. Cardiac muscle is

    1. unstriated and voluntary
    2. striated and voluntary
    3. unstriated and involuntary
    4. striated and involuntary
    Answer

    D. striated and involuntary

    Heart muscle has stripes but works without will.

22. Number of daughter cells formed from one cell after meiosis is

    1. four
    2. two
    3. one
    4. eight
    Answer

    A. four

    Two divisions give four haploid cells.

23. Which organelle is mainly responsible for detoxification of poisons and lipid making?

    1. Nucleolus
    2. Smooth endoplasmic reticulum
    3. Golgi apparatus
    4. Rough endoplasmic reticulum
    Answer

    B. Smooth endoplasmic reticulum

    Smooth ER makes lipids and detoxifies.

24. Which tissue of a plant is made of sieve tubes and companion cells?

    1. Xylem
    2. Cambium
    3. Phloem
    4. Epidermis
    Answer

    C. Phloem

    Phloem carries prepared food.

25. In which phase of mitosis do chromosomes line up at the equator?

    1. Telophase
    2. Prophase
    3. Anaphase
    4. Metaphase
    Answer

    D. Metaphase

    Chromosomes align at the cell equator in metaphase.

26. A student sees a cell without a nucleus, with a circular DNA lying free, and 70S ribosomes. The cell is most likely

    1. a bacterial cell
    2. an onion cell
    3. a muscle cell
    4. a cheek cell
    Answer

    A. a bacterial cell

    Naked DNA and 70S ribosomes show a prokaryote.

27. Mitosis in a cell with 20 chromosomes gives two cells each with

    1. 5 chromosomes
    2. 20 chromosomes
    3. 10 chromosomes
    4. 40 chromosomes
    Answer

    B. 20 chromosomes

    Mitosis keeps the chromosome number the same.

28. Meiosis in a cell with 20 chromosomes gives cells each with

    1. 5 chromosomes
    2. 20 chromosomes
    3. 10 chromosomes
    4. 40 chromosomes
    Answer

    C. 10 chromosomes

    Meiosis halves the number.

29. The Golgi apparatus of plant cells is also called

    1. grana
    2. dictyosome
    3. nucleoid
    4. cristae
    Answer

    B. dictyosome

    Golgi units in plants are dictyosomes.

30. Tissue that stores food and, with air spaces, helps aquatic plants float is

    1. collenchyma
    2. sclerenchyma
    3. chlorenchyma
    4. aerenchyma
    Answer

    D. aerenchyma

    Aerenchyma is parenchyma with large air spaces.

31. Consider the statements. 1. Lysosomes contain digestive enzymes. 2. Ribosomes are present only in plant cells. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Ribosomes are present in all cells.

32. Consider the statements. 1. Tendons join bone to bone. 2. Ligaments join muscle to bone. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    It is the opposite: tendon joins muscle to bone, ligament joins bone to bone.

33. Consider the statements. 1. Mitosis takes place in body cells. 2. Meiosis takes place in cells that form gametes. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

34. Consider the statements. 1. Xylem carries prepared food. 2. Phloem carries water and minerals from roots. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    The functions are reversed.

35. Consider the statements. 1. Chloroplasts are found in animal cells. 2. Leucoplasts store starch, oil and protein. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Plastids occur only in plants.

36. Consider the statements. 1. Rough ER has ribosomes on it. 2. Smooth ER takes part in making proteins. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Protein making is by rough ER.

37. Consider the statements. 1. Interphase is the longest phase of the cell cycle. 2. Cytokinesis in plants occurs by cell-plate formation. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

38. Consider the statements. 1. Intercalary meristem is found at the base of leaves in grasses. 2. Lateral meristem increases the length of the stem. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Lateral meristem (cambium) increases thickness.

39. Match the organelle with its work. 1. Golgi apparatus : packing and dispatch 2. Mitochondria : photosynthesis Which pair(s) is/are correctly matched?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Mitochondria do respiration, not photosynthesis.

40. Match the following. (a) Nucleus (b) Centriole (c) Vacuole (d) Ribosome (p) Spindle formation (q) Control centre (r) Protein making (s) Storage of water and wastes

    1. a-q, b-s, c-p, d-r
    2. a-q, b-p, c-s, d-r
    3. a-r, b-p, c-s, d-q
    4. a-p, b-q, c-s, d-r
    Answer

    B. a-q, b-p, c-s, d-r

    Nucleus controls; centriole forms spindle; vacuole stores; ribosome makes protein.

41. Match the tissue with its example. (a) Epithelial (b) Connective (c) Muscular (d) Nervous (p) Neurons (q) Skin lining (r) Cartilage (s) Heart wall

    1. a-r, b-q, c-s, d-p
    2. a-q, b-s, c-r, d-p
    3. a-q, b-r, c-s, d-p
    4. a-p, b-r, c-s, d-q
    Answer

    C. a-q, b-r, c-s, d-p

    Skin lining is epithelial; cartilage connective; heart wall muscular; neurons nervous.

42. Arrange the mitosis stages in the correct order.

    1. Prophase, metaphase, telophase, anaphase
    2. Prophase, anaphase, metaphase, telophase
    3. Metaphase, prophase, anaphase, telophase
    4. Prophase, metaphase, anaphase, telophase
    Answer

    D. Prophase, metaphase, anaphase, telophase

    The order is PMAT.

43. In a classroom, to show that onion peel cells have a cell wall but cheek cells do not, a teacher should mainly

    1. ask them to memorise a table
    2. draw the diagram only on the board
    3. let learners observe both stained slides and compare
    4. tell them the answer first
    Answer

    C. let learners observe both stained slides and compare

    Activity-based observation then inference is the suitable method.

44. Which of these is the smallest cell?

    1. Nerve cell
    2. Mycoplasma
    3. Ostrich egg
    4. Red blood cell
    Answer

    B. Mycoplasma

    Mycoplasma, a bacterium, is among the smallest cells.

45. Which part of the nucleus is the site where ribosomal material forms?

    1. Nucleolus
    2. Nucleoplasm
    3. Nuclear pore
    4. Chromatin
    Answer

    A. Nucleolus

    Ribosomal RNA is made in the nucleolus.