Q5. Assertion: the SER is abundant in cells that make steroid hormones. Reason: the SER is the major site of lipid synthesis. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: NCERT says the SER is the major site of lipid synthesis and that lipid-like steroidal hormones are made in the SER of animal cells (page 95).
Q6. What is the composition of the membrane of the human erythrocyte?
Approximately 52 per cent protein and 40 per cent lipids (NCERT page 93).
Q7. What holds neighbouring plant cells together, and what connects their cytoplasm?
The middle lamella, mainly calcium pectate, holds them together; plasmodesmata connect their cytoplasm (NCERT page 94).
Q8. Name the three parts of the cytoskeleton and two of its functions.
Microtubules, microfilaments and intermediate filaments; mechanical support, motility and maintaining cell shape (NCERT page 98).
Q9. Describe the structure of (i) the nucleus and (ii) the centrosome, with labelled diagrams. (NCERT Exercise 13)
(i) Nucleus: an envelope of two parallel membranes with the perinuclear space (10 to 50 nm) between them; the outer membrane is continuous with the ER and bears ribosomes; nuclear pores, where the two membranes fuse, let RNA and protein molecules pass both ways. Inside, the nucleoplasm holds chromatin and one or more nucleoli, which have no membrane (NCERT page 100). (ii) Centrosome: two cylindrical centrioles lying perpendicular to each other in amorphous pericentriolar material; each centriole has nine evenly spaced peripheral triplets of tubulin, adjacent triplets linked, and a proteinaceous hub joined to them by radial spokes, like a cartwheel (NCERT pages 99–100). Draw them as in the nucleus and centriole figures on this page.
Q10. Why do cells actively making proteins have larger and more numerous nucleoli?
Because the nucleolus is a site of active ribosomal RNA synthesis, and ribosomes are needed for protein synthesis (NCERT pages 91 and 100).