Cell: The Building Block of Life — Detailed Notes
All living organisms are made of cells — the basic unit at which life exists. Some organisms are unicellular (bacteria, yeast); others are multicellular (plants, animals). A group of similar cells → tissue → organ → organ system. Yet the cell always remains the fundamental unit of structure and function.
2.1 How to study cells?
The limit of resolution of the human eye is about 0.1 mm (two points closer than this, seen from ~25 cm, merge into one). Most cells are smaller than this, so we use a microscope. Robert Hooke (1665) first saw box-like compartments in a cork slice and named them ‘cells’. A microscope’s total magnification = eyepiece magnification × objective magnification (e.g. 10X × 10X = 100X). Scientists improved three features — resolution (clarity), contrast, and magnification. Electron microscopes use a beam of electrons (not light) to reveal detail at the nanometre scale (1 nm = one-billionth of a metre).
2.2 Cell membrane (plasma membrane)
A thin boundary (≈7–10 nm) that surrounds the cell, defines its individuality and is selectively permeable (lets some substances pass, blocks others). Diffusion = net movement of particles from higher to lower concentration (needs no membrane). Osmosis = diffusion of water across a selectively permeable membrane, from a dilute (more water) to a concentrated (less water) solution. Roots absorb soil water by osmosis.
Tonicity: Isotonic — outside solute = inside (no net change); Hypotonic — outside solute < inside (cell gains water, swells); Hypertonic — outside solute > inside (cell loses water, shrinks). Fluid-mosaic model: a lipid bilayer (water-attracting heads out, water-repelling tails in) with embedded proteins that act as gatekeepers; molecules move sideways (fluid), arranged like mosaic tiles.
2.2.2 Cell wall
An extra rigid covering outside the membrane in cells of plants, fungi and bacteria. It gives shape, support and lets plants stay upright. It is permeable (unlike the selectively permeable membrane). The plant cell wall is mainly cellulose (glucose units; acts as dietary roughage). In a hypertonic (concentrated) solution a plant cell undergoes plasmolysis — the membrane pulls away from the wall — but the wall keeps the cell’s outer shape; an animal cell (no wall) simply shrinks.
2.3 The cell interior — organelles
Three basic parts: plasma membrane, jelly-like cytoplasm, and a prominent nucleus. Prokaryotic cells (bacteria) — no well-defined nucleus (DNA in a region called the nucleoid), no membrane-bound organelles, 1–10 µm. Eukaryotic cells (plants, animals) — true nucleus + membrane-bound organelles, 10–100 µm.
Nucleus — double nuclear membrane with pores; contains the nucleolus (makes ribosomal subunits) and chromosomes (DNA + proteins) carrying genes; DNA is seen as chromatin in a non-dividing cell. Ribosomes — sites of protein synthesis. Endoplasmic Reticulum (ER) — network for transport/synthesis; RER (with ribosomes) makes/secretes proteins, SER (no ribosomes) makes fats (lipids) and hormones. Golgi apparatus — the cell’s ‘post office’: modifies, sorts and packs materials into vesicles. Lysosomes — ‘clean-up’ sacs of enzymes that digest waste/worn-out parts. Mitochondria — ‘powerhouse’; double membrane, inner folded into cristae; release energy as ATP by cellular respiration. Plastids (plant cells) — chloroplasts (green, chlorophyll, photosynthesis; stroma inside), chromoplasts (yellow/orange/red — flower & fruit colour), leucoplasts (colourless — store starch/oil/protein). Vacuole — large central sac of cell sap in plant cells; stores water/minerals/waste and keeps the cell firm (turgid). Mitochondria & plastids have their own DNA and ribosomes.
2.4 Cell division
New cells form from pre-existing cells — this allows growth, repair and reproduction. Two types: Mitosis — produces two genetically identical daughter cells (same chromosome number); used for growth, repair, maintenance and asexual reproduction. Meiosis — occurs only in reproductive organs; parent divides twice to give four daughter cells each with half the chromosome number (gametes), creating variation; the number is restored at fertilisation. Errors in mitosis → tumours; errors in meiosis → genetic disorders/reduced fertility.
2.5 Cell Theory
Built by Schleiden (1838, plants), Schwann (1839, animals), and Virchow (1855 — new cells arise from pre-existing cells). Classical Cell Theory: (i) all organisms are made of one or more cells; (ii) the cell is the basic unit of structure and function; (iii) all cells arise from pre-existing cells. Contact inhibition stops normal animal-cell division when cells touch neighbours; cancer cells lose this control and divide uncontrollably, forming tumours. Every cell has a definite lifespan; Programmed Cell Death (PCD) removes unneeded cells (e.g. separating fingers in an embryo).
Key Terms
Resolution, osmosis, diffusion, selectively permeable, fluid-mosaic model, cell wall (cellulose), plasmolysis, prokaryotic/eukaryotic, nucleoid, chromosome/chromatin/gene/DNA, ribosome, RER/SER, Golgi, lysosome, mitochondria/cristae/ATP, chloroplast/chromoplast/leucoplast, vacuole, mitosis/meiosis, cell theory, contact inhibition, totipotency, cell culture.
Revise, Reflect, Refine — All NCERT Exercise Answers
1Differentiate between the following pairs based on the clue in brackets: (i) Cell membrane & cell wall (permeability) (ii) RER & SER (structure) (iii) Chloroplasts & chromoplasts (pigments).
(i) The cell membrane is selectively permeable (allows only certain substances through); the cell wall is fully permeable (water and dissolved minerals pass freely). (ii) RER has ribosomes attached to its surface (looks rough); SER has no ribosomes (looks smooth). (iii) Chloroplasts contain the green pigment chlorophyll (photosynthesis); chromoplasts contain yellow/orange/red pigments (colour of flowers and fruits).
2Cell X (pure water) swells; Cell Y (concentrated salt solution) shrinks. Which statement is the correct explanation?
(iii) “Water moved into Cell X and moved out of Cell Y through the cell membrane.” This is osmosis — water enters the cell in a hypotonic medium (pure water) making it swell, and leaves the cell in a hypertonic medium (concentrated salt) making it shrink. The membrane lets water pass but not the salt.
3Identify parts (a)–(g) of the cell in Fig. 2.20 and match them with the functions (controlling activities, respiration, storage+rigidity, separates contents, structural rigidity, packs/stores from ER, manufacturing food).
The matching by function is: Nucleus — controls all cell activities; Mitochondrion — site of cellular respiration; Vacuole — storage organelle that also gives rigidity (turgidity); Cell membrane — separates cell contents from surroundings; Cell wall — provides structural rigidity; Golgi apparatus — packs and stores materials received from the ER; Chloroplast — helps manufacture food (photosynthesis). (Match each lettered label on your diagram to these organelles accordingly.)
4Which option correctly places pairs under ‘Present in plant cells’ and ‘Absent in animal cells’?
(i) — Leucoplast (present in plant cells) and Cell wall (absent in animal cells). Leucoplasts are plant plastids, and the cell wall is the structure animal cells lack. (The other options pair organelles common to both, so they are wrong.)
5Renu says all plant parts, even roots, contain plastids; Rohit says roots have no plastids as they don’t photosynthesise. Who is correct?
Renu is correct. Plastids occur in all parts of a plant, including roots — but of different types. Roots contain leucoplasts (colourless plastids that store starch, oils or proteins), not chloroplasts. Since roots are underground they lack chloroplasts (no photosynthesis), but plastids as a group are still present. So the statement that plastids are entirely absent in roots is wrong.
6How are mitochondria and chloroplasts structurally and functionally similar, and how do they differ?
Similarities: both are double-membrane-bound organelles, both contain their own DNA and ribosomes (can make some of their own proteins), and both are involved in the cell’s energy relations. Differences: the mitochondrion releases energy by breaking down glucose in cellular respiration (makes ATP) and is found in all eukaryotic cells; the chloroplast traps light energy to make food by photosynthesis (contains chlorophyll) and is found only in plant cells and some protists.
7Which pair of cell organelles contains DNA?
(ii) Mitochondria and Nucleus. Both contain DNA (and chloroplasts do too). Ribosomes, Golgi bodies and lysosomes do not contain DNA.
8A carrot in plain water stays stiff; a carrot in concentrated salt solution becomes limp. (i) hypothesis being tested (ii) improvement (iii) reason.
(i) Hypothesis: water moves into or out of cells by osmosis depending on the surrounding solution’s concentration. (ii) Improvement: use carrots of equal size/mass, record initial and final weights, keep temperature and time the same, and repeat for reliable results (a control in plain water). (iii) Reason: in plain water (hypotonic) the carrot cells gain water and become turgid → stiff and crunchy; in concentrated salt (hypertonic) the cells lose water → flaccid/plasmolysed → rubbery and limp.
9Indicate presence/absence of Chromosome, Nucleus, Mitochondria, Golgi complex, Chromoplasts in bacterial and animal cells.
Bacterial cell: Chromosome — present (as a single circular DNA in the nucleoid); Nucleus — absent; Mitochondria — absent; Golgi complex — absent; Chromoplasts — absent. Animal cell: Chromosome — present; Nucleus — present; Mitochondria — present; Golgi complex — present; Chromoplasts — absent (plastids occur only in plant cells).
10Four potato cups in water: A empty, B sugar, C salt, D sugar in a BOILED potato. (i) why water gathers in B & C (ii) why A is needed (iii) why water does NOT gather in A & D.
(i) In cups B and C the sugar/salt makes the inside hypertonic, so water moves from the beaker into the cup by osmosis through the living potato cells and collects there. (ii) Cup A (empty, living potato) is the control — it shows that without added solute no net water collects, proving the effect is due to the solute, not the potato itself. (iii) In A there is no concentration gradient; in D the potato is boiled, so its cell membranes are dead and no longer selectively permeable — osmosis cannot occur, so no water gathers.
11Identify the pair that INCORRECTLY matches organelle with function.
(ii) SER — ‘Lipid and cellulose synthesis’ is incorrect. The SER makes lipids and hormones, not cellulose (cellulose of the cell wall is made at the cell surface, not by the SER). Ribosome–protein synthesis and lysosome–digestion are correctly matched.
12What outcome do you expect if all the mitochondria are removed from a eukaryotic cell?
The cell would lose its main site of cellular respiration, so it could no longer release energy as ATP efficiently. Without this energy currency, energy-requiring processes (synthesis, transport, division, movement) would stop and the cell would soon die.
13Which phenomenon inhibits tumour formation in humans? Can plants develop tumours? Explain.
Contact inhibition normally stops animal cells from dividing once they touch neighbouring cells, preventing tumours; cancer arises when cells lose this control. Plants can also develop tumour-like growths (galls), but because plant cells have rigid cell walls and no contact inhibition and cannot migrate, such growths stay localised and do not spread like malignant animal tumours.
14The cell membrane is made of proteins and lipids. Which organelles synthesise it? Give the path from site of synthesis to the membrane.
Proteins are made on ribosomes of the RER; lipids are made by the SER. Path: RER/SER → Golgi apparatus (modified, sorted, packed into vesicles) → vesicles fuse with the plasma membrane, adding new protein and lipid to it. (Draw: RER with ribosomes → SER → Golgi → vesicles → cell membrane, with arrows.)
15What would happen if gametes were formed by mitotic division instead of meiosis?
Mitosis keeps the full (diploid) chromosome number. If gametes formed by mitosis, each gamete would carry the full set, and at fertilisation the offspring would have double the parental chromosome number — doubling every generation. This would disturb the species’ chromosome number and cause abnormalities; meiosis halves the number so fertilisation restores the correct count.
16Farmer Deepa preserves amla and lemons as pickles/murabbas/sharbat using salt and sugar. (i) scientific concept (ii) how salt/sugar prevents microbial growth (iii) a healthy recipe (iv) scientific values.
(i) The concept of osmosis (and food preservation by creating a hypertonic environment). (ii) High salt/sugar makes the surroundings strongly hypertonic, so water is drawn out of spoilage bacteria and fungi by osmosis; they become dehydrated (plasmolysed) and cannot grow — this preserves the food. (iii) e.g. a low-sugar amla murabba or a lemon-honey sharbat using jaggery instead of refined sugar. (iv) Scientific values: applying scientific knowledge to real life, reducing waste (sustainability), self-reliance and economic value, and food security.