How This Chapter Is Asked in NEET

Cell - The Unit of Life is the highest-yield chapter in this unit, and it is also the most predictable. Almost every question is one of four shapes: name the organelle from its description, give the number or the size, state the one function, or spot the odd one out. There is nothing to work out. Every mark here is decided before you enter the hall, by how exactly you learnt the wording.

The traps repeat, and in this chapter they are always the same eleven:

  • Leeuwenhoek against Robert Brown. Leeuwenhoek first saw and described a live cell. Robert Brown discovered the nucleus, in 1831. The option that reads "Robert Brown discovered the cell" is built out of this swap.
  • Schleiden against Schwann. Schleiden (1838) was the botanist. Schwann (1839) was the zoologist. Schwann is the one who named the plasma membrane and who said the cell wall is unique to plant cells.
  • "No organelles in prokaryotes", stated without the exception. The sentence is "no organelles, like the ones in eukaryotes, are found in prokaryotic cells except for ribosomes". Learn it with the except for ribosomes attached.
  • 70S against 80S, and their subunits. 70S = 50S + 30S, prokaryotic. 80S = 60S + 40S, eukaryotic. The numbers do not add up arithmetically, and they are not meant to - S is a sedimentation coefficient, Svedberg's Unit, indirectly a measure of density and size, not a mass.
  • Cristae against thylakoids against cisternae. Cristae are infoldings of the inner mitochondrial membrane. Thylakoids are flat membranous sacs in the stroma of the chloroplast. Cisternae are the disc-shaped sacs of the golgi apparatus. One of the chapter-end exercises is a straight matching item on exactly these three.
  • Cis against trans face. Cis = forming face = convex. Trans = maturing face = concave. Traffic runs ER, then cis, then trans, then out.
  • 9+2 against 9+0, doublets against triplets. The axoneme of a cilium or flagellum is 9+2, with nine doublets and a central pair. The centriole is 9+0, with nine triplets and no central tubule.
  • Mitochondria, chloroplast and peroxisomes are NOT part of the endomembrane system. They are membrane bound; that is not the test. The test is whether their functions are coordinated with the ER, golgi complex, lysosomes and vacuoles - and they are not.
  • The tonoplast moves materials AGAINST the gradient, into the vacuole. An option saying along the gradient is the built-in trap.
  • Simple diffusion against facilitated against active transport. Neutral solutes - simple diffusion. Water - osmosis. Polar molecules - a carrier protein, still down the gradient and still free. Ions against the gradient - active transport, ATP utilised, the Na+/K+ pump.
  • Membrane bound counts. Single membrane - lysosome, vacuole (the tonoplast), microbody. Double membrane - mitochondrion, chloroplast, nuclear envelope. No membrane at all - ribosome, nucleolus, centriole, inclusion bodies of prokaryotes.

One habit pays more than any other here. In a "which is NOT" question, the wrong option is almost never invented - it is a real structure or a real function borrowed from a neighbouring organelle. Protein synthesis offered as a mesosome function, hydrolytic enzymes offered as a golgi product, cisternae offered for the mitochondrion. Ask "which organelle does this actually belong to" rather than "have I heard of this".

Before You Start

Give yourself 45 minutes for 45 questions and look nothing up. Mark every question where you were choosing between two options rather than certain - those marked questions, not only the wrong ones, are your revision list.

The facts most likely to decide your score, by topic:

Cell theory. Leeuwenhoek - first saw and described a live cell. Robert Brown, 1831 - the nucleus. Flemming - named chromatin. Schleiden, 1838, botanist. Schwann, 1839, zoologist. Virchow, 1855 - new cells arise from pre-existing cells, Omnis cellula-e cellula. Cell theory today: all living organisms are composed of cells and products of cells; all cells arise from pre-existing cells.

Sizes and shapes. Mycoplasma 0.3 micrometre, the smallest cell. Bacteria 3 to 5 micrometre. Human RBC about 7.0 micrometre. Largest isolated single cell - the egg of an ostrich. Longest cells - nerve cells. Shape may vary with the function performed.

Prokaryotes. Bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). Bacillus rod like, coccus spherical, vibrio comma shaped, spirillum spiral. Cell wall in all except mycoplasma. Genetic material naked, no nuclear membrane. Genomic DNA - single circular chromosome. Plasmids - small circular DNA that confer antibiotic resistance and are used to monitor bacterial transformation. Envelope from outside in - glycocalyx, cell wall, plasma membrane; the loose glycocalyx is the slime layer, the thick tough one the capsule. Gram positive take up the stain, Gram negative do not. Mesosome - extensions of the plasma membrane as vesicles, tubules and lamellae; cell wall formation, DNA replication and distribution, respiration, secretion, more surface area and enzymatic content. Chromatophores with pigments in cyanobacteria. Flagellum - filament, hook, basal body; the filament is the longest part. Pili and fimbriae do not take part in motility. Prokaryotic ribosome 70S, about 15 nanometre by 20 nanometre, 50S plus 30S, attached to the plasma membrane; polysome = many ribosomes on one mRNA. Inclusion bodies - phosphate, cyanophycean and glycogen granules - lie free, no membrane. Gas vacuoles in blue green and purple and green photosynthetic bacteria.

Membrane and transport. Phospholipid bilayer, polar heads out, hydrophobic tails in; cholesterol also present. Human erythrocyte membrane - about 52 per cent protein and 40 per cent lipids. Integral or peripheral, by the ease of extraction. Singer and Nicolson, 1972, fluid mosaic model; fluidity is the lateral movement of proteins. Passive - no energy, along the gradient. Active - against the gradient, ATP utilised, Na+/K+ pump.

Cell wall. Non-living and rigid, in fungi and plants. Algae - cellulose, galactans, mannans, calcium carbonate. Other plants - cellulose, hemicellulose, pectins, proteins. Primary wall grows, secondary wall forms on the inner side. Middle lamella is mainly calcium pectate. Plasmodesmata connect the cytoplasm of neighbouring cells.

Endomembrane system. ER, golgi complex, lysosomes, vacuoles - functions coordinated. Mitochondria, chloroplast and peroxisomes excluded. Luminal is inside the ER, extra luminal is the cytoplasm. RER bears ribosomes, is continuous with the outer nuclear membrane, and does protein synthesis and secretion. SER is the major site of lipid synthesis and of steroidal hormones in animal cells. Camillo Golgi, 1898. Cisternae 0.5 to 1.0 micrometre, stacked parallel. Cis convex forming face, trans concave maturing face. Golgi is the important site of formation of glycoproteins and glycolipids.

Lysosome and vacuole. Lysosome - formed by packaging in the golgi, rich in hydrolases - lipases, proteases, carbohydrases - optimally active at acidic pH, digests carbohydrates, proteins, lipids and nucleic acids. Vacuole - single membrane, the tonoplast; water, sap, excretory product; up to 90 per cent of a plant cell's volume; the tonoplast pumps materials in against the gradient. Contractile vacuole in Amoeba for osmoregulation and excretion; food vacuoles in protists.

Mitochondria. Not easily visible unless specifically stained. Number depends on the physiological activity of the cell. Sausage-shaped or cylindrical, diameter 0.2 to 1.0 micrometre with 0.5 micrometre average, length 1.0 to 4.1 micrometre. Double membrane; outer is the continuous limiting boundary, inner throws cristae into the matrix and the cristae increase the surface area. Sites of aerobic respiration, produce ATP, the power houses. Matrix has a single circular DNA, a few RNA molecules, 70S ribosomes. Divide by fission.

Plastids. In all plant cells and in euglenoides; large and easily seen. Classified by pigment - chloroplast, chromoplast, leucoplast. Amyloplast starch (potato), elaioplast oils and fats, aleuroplast proteins. Chloroplasts in the mesophyll; 5 to 10 micrometre long, 2 to 4 micrometre wide; 1 per cell in Chlamydomonas to 20 to 40 in the mesophyll; double membrane with the inner one relatively less permeable. Stroma, thylakoids, grana, intergranal thylakoids, stroma lamellae, lumen. Chlorophyll is in the thylakoids. Chloroplast ribosome 70S, cytoplasmic 80S. Grana - light reactions. Stroma - dark reactions, plus enzymes for carbohydrate and protein synthesis and small double-stranded circular DNA.

Ribosomes, cytoskeleton, microbodies. George Palade, 1953. RNA plus protein, no membrane. 80S = 60S + 40S; 70S = 50S + 30S; S is Svedberg's Unit. Cytoskeleton - microtubules, microfilaments, intermediate filaments; mechanical support, motility, maintenance of shape. Microbodies - membrane bound minute vesicles with enzymes, in both plant and animal cells.

Cilia, flagella, centriole. Hair-like outgrowths of the cell membrane, covered with plasma membrane. Cilia small, work like oars, move the cell or the surrounding fluid; flagella longer, responsible for cell movement; bacterial flagella are structurally different. Axoneme 9+2 - nine peripheral doublets, one central pair, bridges, central sheath, nine radial spokes, linkers, basal body. Centrosome has two centrioles perpendicular to each other, in pericentriolar material; nine triplet fibrils of tubulin, adjacent triplets linked, proteinaceous hub, radial spokes; 9+0. Centrioles form the basal body and the spindle fibres.

Nucleus and chromosomes. Envelope of two parallel membranes, perinuclear space 10 to 50 nanometre; outer membrane continuous with the ER and bearing ribosomes; pores formed by the fusion of the two membranes carry RNA and protein in both directions. Nucleoplasm holds the nucleolus and chromatin. Nucleolus - spherical, not membrane bound, site of active ribosomal RNA synthesis, larger and more numerous in cells actively making protein. Some mature cells lack a nucleus - erythrocytes of many mammals, sieve tube cells of vascular plants. Chromatin - DNA, basic proteins called histones, non-histone proteins, RNA; about two metre of DNA in forty six (twenty three pairs) chromosomes. Centromere is the primary constriction and holds the two chromatids; kinetochores are disc shaped structures on its sides; a secondary constriction gives a satellite. Metacentric middle, sub-metacentric slightly away, acrocentric close to the end, telocentric terminal.