Cell: The Unit of Life — Part A (NEET Biology Class 11): Cell Theory, Prokaryotic Cell & Membrane

Cell Theory & the Size/Shape of Cells

🎯 NEET priority: High-yield. 5–7 questions every year — split across both parts of this chapter. Part A covers cell theory, the prokaryotic cell and the eukaryotic membrane/wall; Part B covers the organelles.

Cell theory is the idea that unifies all of biology: every living thing is made of cells, and every cell comes from a pre-existing cell. Antonie von Leeuwenhoek was the first to see and describe a live cell; Robert Brown later discovered the nucleus.

Cell theory — three names, three dates

• Matthias Schleiden (1838), a German botanist, found that all plants are made of cells.

• Theodor Schwann (1839), a German zoologist, found the same for animal cells and reported the thin outer layer we now call the plasma membrane; he also proposed that the cell wall is unique to plant cells. Schleiden and Schwann together proposed the cell theory — but it did not explain how new cells form.

• Rudolf Virchow (1855) supplied that missing piece: cells divide and new cells arise from pre-existing cells — Omnis cellula-e cellula. This gave cell theory its final, modern form: (i) all organisms are composed of cells and cell products, and (ii) all cells arise from pre-existing cells.

Unicellular vs multicellular, and cell size/shape

• A unicellular organism is a single cell capable of independent existence and every essential function of life — nothing less than a complete cell can live independently. A multicellular organism, like a human, is built of many cells.

• Mycoplasma is the smallest known cell, about 0.3 micrometre long; most bacteria are 3 to 5 micrometre. The largest single isolated cell is the egg of an ostrich. Among multicellular organisms, a human red blood cell is about 7 micrometre in diameter, and nerve cells are among the longest cells in the body.

• Cell shape varies with function: disc-like, polygonal, columnar, cuboidal, thread-like or irregular — e.g. round biconcave RBCs, branched long nerve cells, and long narrow columnar epithelial cells.

The Prokaryotic Cell: Envelope, Mesosome & Ribosomes

Prokaryotic cells — who, and the cell envelope

• Prokaryotes = bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). They are generally smaller and multiply faster than eukaryotic cells. The four basic shapes of bacteria are bacillus (rod), coccus (spherical), vibrio (comma-shaped) and spirillum (spiral).

• Most prokaryotic cells have a three-layered cell envelope: the outermost glycocalyx, then the cell wall, then the plasma membrane — every prokaryote has a cell wall except mycoplasma. Glycocalyx may be a loose slime layer or a thick, tough capsule. Based on how they take up the Gram stain, bacteria are Gram positive (stain retained) or Gram negative (stain not retained).

• The genetic material is circular DNA, naked — not enclosed by a nuclear membrane (no true nucleus). Many bacteria also carry small extra circular DNA called plasmids, which confer traits like antibiotic resistance and are used in the lab to carry foreign DNA into bacteria.

Mesosome, surface appendages, ribosomes & inclusion bodies

• A mesosome is an infolding of the plasma membrane found only in prokaryotes; it aids cell wall formation, DNA replication and its distribution to daughter cells, plus respiration and secretion.

• Motile bacteria bear flagella (filament + hook + basal body). Pili are elongated tubular protein structures; fimbriae are small bristle-like fibres that help bacteria attach to surfaces or host tissue — neither pili nor fimbriae aid motility.

• Ribosomes are the only organelle prokaryotes have, and they are non-membrane bound — found in ALL cells, prokaryotic and eukaryotic alike. Prokaryotic ribosomes are 70S, made of a 50S and a 30S subunit; several strung on one mRNA form a polyribosome (polysome), which translates that mRNA into protein.

• Inclusion bodies (phosphate granules, cyanophycean granules, glycogen granules) are free in the cytoplasm, not membrane-bound. Gas vacuoles occur in blue-green, purple and green photosynthetic bacteria.

Eukaryotic Cell: Plasma Membrane & Cell Wall

The eukaryotic cell — plant vs animal

• Eukaryotes = protists, plants, animals, fungi. A eukaryotic cell has an organised nucleus with a nuclear envelope, extensive membrane-bound organelle compartments, and genetic material organised into chromosomes.

• Plant cells uniquely have a cell wall, plastids and a large central vacuole; animal cells uniquely have centrioles — a swap NEET loves to test both ways.

Prokaryotic vs eukaryotic — the full comparison

Plasma membrane — the fluid mosaic model

• Singer and Nicolson (1972) proposed the fluid mosaic model: a phospholipid bilayer with polar heads facing outward and hydrophobic tails facing inward, studded with proteins that can move laterally (fluidity). The membrane also contains cholesterol.

• Membrane proteins are integral (partly/fully buried in the bilayer) or peripheral (sitting on the surface). In the human erythrocyte membrane, the composition is about 52 per cent protein and 40 per cent lipid — this classic ratio was worked out from RBC membrane studies.

• Movement across the membrane: passive transport needs no energy — diffusion (solutes, high → low concentration) and osmosis (water, high → low concentration); polar molecules need a carrier protein. Active transport moves molecules against their gradient (low → high) and needs ATP — e.g. the Na⁺/K⁺ pump.

Cell wall

• The cell wall is a non-living, rigid outer covering of the plasma membrane in fungi and plants — it gives shape, protects from mechanical damage and infection, and aids cell-to-cell interaction. Algal cell walls are made of cellulose, galactans, mannans and minerals like calcium carbonate; other plants' walls are cellulose, hemicellulose, pectins and proteins.

• A young cell first lays down a growable primary wall; as the cell matures, a secondary wall forms on its inner (membrane) side. The middle lamella, made mainly of calcium pectate, glues neighbouring cells together. Plasmodesmata cross the wall and middle lamella to connect the cytoplasm of adjacent cells.

Why This Matters for NEET

Why this matters for NEET

• High-value one-liners: cell theory's three names in order (Schleiden 1838 → Schwann 1839 → Virchow 1855); prokaryotic ribosome = 70S (50S+30S), the ONLY organelle prokaryotes possess; the envelope order glycocalyx → cell wall → plasma membrane; fluid mosaic model = Singer & Nicolson, 1972.

• Trap: mesosomes and cell walls belong to prokaryotes and plants respectively — never both; mycoplasma is the one prokaryote with NO cell wall. Pili/fimbriae are not for motility (that's flagella). Ribosomes are non-membrane bound in every cell, so don't list them under "membrane-bound organelles." Plant cells have plastids + large vacuole + cell wall, animal cells have centrioles — NEET frequently swaps these to test if you actually know the difference, not just that a difference exists.

Test Yourself: MCQs, PYQs & Active Recall

Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one.

Practice Questions

Q1. The scientist who first explained that new cells arise only from pre-existing cells is:

• (a) Robert Hooke

• (b) Rudolf Virchow

• (c) Theodor Schwann

• (d) Matthias Schleiden

Q2. Which of these is the smallest known cell?

• (a) Ostrich egg

• (b) Bacterium

• (c) Human RBC

• (d) Mycoplasma

Q3. Which prokaryote characteristically lacks a cell wall?

• (a) Bacillus

• (b) Spirillum

• (c) Mycoplasma

• (d) Vibrio

Q4. The three layers of a bacterial cell envelope, from outside in, are:

• (a) Cell wall, glycocalyx, plasma membrane

• (b) Plasma membrane, cell wall, glycocalyx

• (c) Glycocalyx, plasma membrane, cell wall

• (d) Glycocalyx, cell wall, plasma membrane

Q5. The prokaryotic ribosome is:

• (a) 80S, made of 60S and 40S subunits

• (b) 70S, made of 50S and 30S subunits

• (c) 70S, made of 60S and 40S subunits

• (d) 50S only

Q6. Plasmid DNA in bacteria is best known for conferring:

• (a) Antibiotic resistance

• (b) Cell wall formation

• (c) Motility

• (d) Photosynthesis

Q7. Which structure is an infolding of the plasma membrane, found only in prokaryotes?

• (a) Golgi apparatus

• (b) Plasmodesmata

• (c) Centriole

• (d) Mesosome

Q8. The fluid mosaic model of the plasma membrane was proposed by:

• (a) Robert Brown

• (b) Watson and Crick

• (c) Schleiden and Schwann

• (d) Singer and Nicolson

Q9. In the human erythrocyte membrane, the approximate composition is:

• (a) 20% protein, 70% lipid

• (b) Only lipid, no protein

• (c) 52% protein, 40% lipid

• (d) 80% protein, 10% lipid

Q10. Movement of a molecule against its concentration gradient, using ATP, is called:

• (a) Facilitated diffusion

• (b) Diffusion

• (c) Osmosis

• (d) Active transport

Q11. Which structural feature is found in plant cells but NOT in animal cells?

• (a) Centriole

• (b) Ribosome

• (c) Cell wall, plastids and a large central vacuole

• (d) Plasma membrane

Q12. The middle lamella that glues neighbouring plant cells together is made mainly of:

• (a) Calcium pectate

• (b) Cellulose

• (c) Chitin

• (d) Lignin

NEET Previous Year Questions (PYQs)

Real NEET previous-year questions on this chapter, with explanations in our own words.

Q13. Which is the correct sequence of scientists in the development of cell theory? (NEET PYQ)

• (a) Schleiden, Virchow, Schwann

• (b) Schleiden, Schwann, Virchow

• (c) Schwann, Virchow, Schleiden

• (d) Virchow, Schleiden, Schwann

Q14. Ribosomes are: (NEET PYQ)

• (a) Membrane-bound organelles found only in eukaryotes

• (b) Found only in prokaryotes

• (c) Present only in the nucleus

• (d) Non-membrane bound organelles found in all cells

Q15. The structures that help bacteria attach to host tissues or to rocks in streams, without aiding motility, are: (NEET PYQ)

• (a) Flagella

• (b) Mesosomes

• (c) Fimbriae

• (d) Plasmids

Q16. Gram staining is used to classify bacteria based on differences in their: (NEET PYQ)

• (a) Cell envelope

• (b) Nuclear material

• (c) Ribosome type

• (d) Flagellar number

Q17. The polar heads of phospholipids in the plasma membrane face: (NEET PYQ)

• (a) Outward, towards the aqueous environment

• (b) Only towards the cytoplasm

• (c) Neither side specifically

• (d) Inward, away from water

Q18. A structure made of calcium pectate that cements adjacent plant cell walls together is the: (NEET PYQ)

• (a) Middle lamella

• (b) Primary wall

• (c) Plasmodesmata

• (d) Secondary wall

Active Recall Prompt

Write everything you can recall about Cell: The Unit of Life (Part A), naming each part first: cell theory (Schleiden, Schwann, Virchow — years and contributions); cell size and shape examples; the prokaryotic cell envelope (glycocalyx, cell wall, plasma membrane, Gram stain); the mesosome and surface structures (flagella, pili, fimbriae); prokaryotic ribosomes and inclusion bodies; plasmids; the plant vs animal cell difference; the fluid mosaic model of the plasma membrane (who, when, structure, transport types); and the cell wall (composition, primary/secondary wall, middle lamella, plasmodesmata). Begin each fact with its topic and end it with a full stop.