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Every living organism is built from cells, and NEET Biology treats this chapter as the foundation for everything that follows in cell division, genetics and physiology. The questions here reward precision: knowing not just what an organelle is called, but what it actually does and how it differs from similar-looking structures.
NEET weightage: High · NCERT Biology Class 11, Part 1, Chapter 8 (Cell: The Unit of Life)
All living organisms are composed of cells, the cell is the basic structural and functional unit of life, and new cells arise only from pre-existing cells. Viruses are usually treated as an exception since they are acellular and only show life processes inside a host cell.
Prokaryotic cells (bacteria, cyanobacteria) lack a true nucleus and membrane-bound organelles — their genetic material lies in a nucleoid region. Eukaryotic cells have a well-defined nucleus enclosed by a nuclear envelope and membrane-bound organelles such as mitochondria and the endoplasmic reticulum.
Proposed by Singer and Nicolson, the plasma membrane is a fluid bilayer of phospholipids with embedded proteins that can move laterally. This 'fluidity' lets the membrane fuse, bud and repair itself, and it underlies selective permeability.
The endoplasmic reticulum (rough ER studded with ribosomes for protein synthesis; smooth ER for lipid synthesis), Golgi apparatus (packaging, modification and dispatch of materials) and lysosomes (intracellular digestion via hydrolytic enzymes) work together to process and route the cell's molecules.
Both are double membrane-bound, contain their own circular DNA and 70S ribosomes, and can divide independently of the cell. Mitochondria have inner-membrane folds called cristae that increase surface area for ATP synthesis; chloroplasts (a type of plastid) contain thylakoids stacked into grana for photosynthesis.
Prokaryotic ribosomes and the ribosomes inside mitochondria/chloroplasts are 70S (made of 50S + 30S subunits); eukaryotic cytoplasmic ribosomes are 80S (60S + 40S). This size difference is a frequently tested NEET fact.
Prokaryotic / organellar ribosome
70S = 50S + 30S
Eukaryotic cytoplasmic ribosome
80S = 60S + 40S
Flagella/cilia axoneme arrangement
9 + 2 microtubule pattern
Centriole arrangement
9 + 0 (nine triplets, no central pair)
Plant cell wall — main component
Cellulose
Bacterial cell wall — main component
Peptidoglycan (murein)
Why is the mitochondrion called the 'powerhouse of the cell'?
Mitochondria carry out oxidative phosphorylation on their inner membrane, converting the chemical energy in food molecules into ATP. The cristae (folds of the inner membrane) increase the surface area available for this process, which is why cells with high energy demand (e.g. muscle cells) tend to have more mitochondria.
A cell shows a large central vacuole, a rigid outer wall, and plastids, but no centrioles. Is it a plant or animal cell?
Plant cell. A large central vacuole (for turgor pressure and storage), a cellulose cell wall, and plastids are characteristic plant-cell features. Animal cells generally have centrioles and lack a cell wall and plastids, which rules out an animal cell here.
Which organelle is primarily responsible for post-translational modification and packaging of proteins for secretion?
The Golgi apparatus. It receives vesicles from the rough ER, modifies proteins (e.g. by glycosylation) and packages them into vesicles for secretion or delivery to other organelles.
A student observes a cell under a microscope and sees ribosomes attached to the outer surface of a membrane network. What is this structure and what does it synthesise?
This is the rough endoplasmic reticulum (RER). The attached ribosomes synthesise proteins that are typically destined for secretion, the plasma membrane, or the ER/Golgi system itself.
Both mitochondria and chloroplasts contain their own DNA. What does this suggest about their evolutionary origin, and what is this idea called?
It supports the endosymbiotic theory, which proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by an ancestral eukaryotic cell and became permanent, semi-autonomous residents.
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