Cell: The Unit of LifeNEET Botany · Class 11 · NCERT Chapter 6

You can expect 2 to 3 questions from this chapter in NEET 2027. It is a high-frequency chapter. The most tested topics are: differences between prokaryotic and eukaryotic cells, ribosome types (70S vs 80S and their subunits), the fluid mosaic model of the plasma membrane, the endomembrane system (ER, Golgi, lysosomes), and organelle structure (mitochondria, chloroplast, nucleus).

The fluid mosaic model was proposed by Singer and Nicolson in 1972. It describes the plasma membrane as a fluid phospholipid bilayer in which proteins are embedded. "Fluid" means the phospholipid molecules can move laterally within each layer. "Mosaic" means proteins are scattered throughout the lipid bilayer like tiles in a mosaic. There are two types of membrane proteins: integral (intrinsic) proteins that pass through the bilayer, and peripheral (extrinsic) proteins that sit on the surface. Cholesterol is present in animal cell membranes and helps maintain membrane fluidity.

70S ribosomes are found in prokaryotes (bacteria) and also inside mitochondria and chloroplasts (reflecting their bacterial origin). A 70S ribosome is made of two subunits: a large 50S subunit and a small 30S subunit. 80S ribosomes are found in the cytoplasm of all eukaryotic cells. An 80S ribosome is made of a large 60S subunit and a small 40S subunit. The S in "70S" and "80S" stands for Svedberg units, which measure the rate of sedimentation in a centrifuge (not size). Important: 50 + 30 does not equal 70 because Svedberg units are not additive.

The endomembrane system is a network of membrane-bound compartments inside eukaryotic cells that work together to produce, modify, and ship proteins and lipids. It includes: Rough Endoplasmic Reticulum (rough ER, has ribosomes on its surface, makes proteins for export), Smooth Endoplasmic Reticulum (smooth ER, no ribosomes, makes lipids), Golgi apparatus (cis face receives vesicles from ER, trans face sends out modified proteins to lysosomes or cell surface), Lysosomes (contain hydrolytic enzymes, digest worn-out organelles and foreign material), and Vacuoles (storage). Mitochondria and chloroplasts are NOT part of the endomembrane system.

Plant cells have: cell wall (made of cellulose), large central vacuole, chloroplasts, and plasmodesmata (channels between cells). Plant cells do NOT have centrioles or lysosomes (generally). Animal cells have: centrioles (9+0 arrangement of triplet microtubules), lysosomes, and small or no vacuoles. Animal cells do NOT have cell walls or chloroplasts. Both cell types have: plasma membrane, nucleus, mitochondria, ER, Golgi apparatus, and 80S ribosomes in the cytoplasm.

Mitochondria and chloroplasts are called semi-autonomous because they have their own DNA (circular, like bacterial DNA) and their own 70S ribosomes, which means they can synthesize some of their own proteins. However, they are not fully independent: most of the proteins they need are encoded by the nuclear DNA and imported from the cytoplasm. This is consistent with the endosymbiotic theory, which proposes that mitochondria and chloroplasts evolved from ancient bacteria that were engulfed by ancestral eukaryotic cells.

Cilia and flagella are hair-like extensions of the cell membrane used for movement. Inside, they contain an axoneme: 9 pairs of peripheral microtubule doublets surrounding a central pair of single microtubules. This is called the 9+2 arrangement. Movement is powered by dynein, a motor protein, using ATP. Centrioles (found in animal cells, absent in higher plants) have a 9+0 arrangement: 9 sets of triplet microtubules arranged in a ring with NO central microtubule. Centrioles form the centrosome and help organize the spindle during cell division.