# Cell: The Unit of Life β€” Part A (NEET Biology Class 11): Cell Theory, Prokaryotic Cell & Membrane Source: https://tracknation.in/story/cell-the-unit-of-life-part-a-neet-biology-class-11-cell-theory-prokaryotic-cell-membrane (TrackNation β€” free UPSC notes with PYQs) > NEET Class 11 Cell: The Unit of Life, Part A, mined from NCERT: cell theory (Schleiden 1838, Schwann 1839, Virchow 1855), cell size/shape diversity, the prokaryotic cell envelope (glycocalyx, cell wall, plasma membrane, Gram stain), mesosomes, flagella/pili/fimbriae, 70S ribosomes, plasmids, plant vs animal cell differences, the fluid mosaic model of the plasma membrane, passive/active transport, and the plant cell wall. With 12 MCQs, 6 NEET PYQs and an Active Recall prompt. ## Cell Theory & the Size/Shape of Cells 🎯 **NEET priority: High-yield.** *5–7 questions every year β€” 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 β€” 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** β€” 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** β€” *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 β€” 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 β€” e.g. round biconcave RBCs, branched long nerve cells, and long narrow columnar epithelial cells. ## The Prokaryotic Cell: Envelope, Mesosome & Ribosomes ### Prokaryotic cells β€” 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** β€” 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** β€” 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. ### Bacterial shape and Gram reaction β€” the match-the-pair table | **Term** | **What it means** | **Remember** | | Bacillus | Rod-shaped bacterium | *Bacillus, Lactobacillus, E. coli* | | Coccus | Spherical bacterium | *Streptococcus, Staphylococcus* | | Vibrio | Comma-shaped bacterium | *Vibrio cholerae* β€” cholera | | Spirillum | Spiral / helical bacterium | *Spirillum, Treponema* | | Gram positive | Retains the crystal-violet stain | Thick peptidoglycan wall, no outer membrane | | Gram negative | Does NOT retain the stain | Thin peptidoglycan + outer lipopolysaccharide membrane | | Glycocalyx | Outermost envelope layer | Loose = slime layer; tough = capsule | | Mycoplasma | The one prokaryote with NO cell wall | Smallest living cell; also called PPLO | ### 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 β€” neither pili nor fimbriae aid motility. - **Ribosomes** are the only organelle prokaryotes have, and they are **non-membrane bound** β€” 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 β€” 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** β€” a swap NEET loves to test both ways. ### Plant cell vs animal cell β€” the exclusive structures | **Structure** | **Plant cell** | **Animal cell** | | Cell wall | Present β€” cellulose | Absent | | Plastids | Present β€” chloroplast, chromoplast, leucoplast | Absent | | Vacuole | One large central vacuole, up to 90% of volume | Many small vacuoles, or none | | Centriole | Absent (except in lower plants like Chlamydomonas) | Present β€” forms the spindle and basal bodies | | Lysosome | Rare | Common | | Glyoxysome | Present in fat-storing seeds | Absent | | Reserve food | Starch | Glycogen | | Cytokinesis | Cell plate formation | Cleavage furrow | **Exam trap.** The single most-repeated error is putting **centrioles in plant cells** and **plastids in animal cells**. Remember the exception too: lower plants such as *Chlamydomonas* DO have centrioles, so "centrioles are absent in all plant cells" is a false statement. ### Prokaryotic vs eukaryotic β€” the full comparison | **Feature** | **Prokaryotic** | **Eukaryotic** | | Nucleus | Absent β€” naked circular DNA | True nucleus with nuclear envelope | | Membrane-bound organelles | Absent (only ribosomes present) | Present (ER, Golgi, mitochondria, etc.) | | Ribosomes | 70S (50S + 30S) | 80S (60S + 40S) in cytoplasm; 70S inside mitochondria/chloroplast | | Cell wall | Present, except in mycoplasma | Present in plants/fungi only, absent in animals | | Cell division | No mitotic spindle (binary fission) | Mitosis/meiosis with spindle apparatus | ### Plasma membrane β€” 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** β€” this classic ratio was worked out from RBC membrane studies. - Movement across the membrane: **passive transport** needs no energy β€” **diffusion** (solutes, high β†’ low concentration) and **osmosis** (water, high β†’ low concentration); polar molecules need a carrier protein. **Active transport** moves molecules against their gradient (low β†’ high) and **needs ATP** β€” 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** β€” 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 β†’ Schwann 1839 β†’ Virchow 1855); prokaryotic ribosome = **70S (50S+30S)**, the ONLY organelle prokaryotes possess; the envelope order **glycocalyx β†’ cell wall β†’ plasma membrane**; fluid mosaic model = **Singer & Nicolson, 1972**. - **Trap:** mesosomes and cell walls belong to **prokaryotes and plants respectively β€” 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** β€” 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 **Answer: (b)** β€” Rudolf Virchow (1855) proposed Omnis cellula-e cellula, giving cell theory its modern, final form. **Q2.** Which of these is the smallest known cell? - (a) Ostrich egg - (b) Bacterium - (c) Human RBC - (d) Mycoplasma **Answer: (d)** β€” Mycoplasma, at about 0.3 micrometre, is the smallest known cell; most bacteria are 3-5 micrometre. **Q3.** Which prokaryote characteristically lacks a cell wall? - (a) Bacillus - (b) Spirillum - (c) Mycoplasma - (d) Vibrio **Answer: (c)** β€” All prokaryotes have a cell wall except mycoplasma. **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 **Answer: (d)** β€” Order from outside to inside: glycocalyx, then cell wall, then 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 **Answer: (b)** β€” Prokaryotic ribosomes are 70S, formed of a 50S and a 30S subunit. **Q6.** Plasmid DNA in bacteria is best known for conferring: - (a) Antibiotic resistance - (b) Cell wall formation - (c) Motility - (d) Photosynthesis **Answer: (a)** β€” Plasmids are small extra circular DNA molecules that confer traits such as antibiotic resistance. **Q7.** Which structure is an infolding of the plasma membrane, found only in prokaryotes? - (a) Golgi apparatus - (b) Plasmodesmata - (c) Centriole - (d) Mesosome **Answer: (d)** β€” The mesosome is a membranous infolding aiding cell wall formation, DNA replication, respiration and secretion. **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 **Answer: (d)** β€” Singer and Nicolson proposed the fluid mosaic model in 1972. **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 **Answer: (c)** β€” Classic RBC membrane studies found roughly 52 per cent protein and 40 per cent lipid. **Q10.** Movement of a molecule against its concentration gradient, using ATP, is called: - (a) Facilitated diffusion - (b) Diffusion - (c) Osmosis - (d) Active transport **Answer: (d)** β€” Active transport moves molecules from lower to higher concentration and requires energy (ATP), e.g. the Na+/K+ pump. **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 **Answer: (c)** β€” Plant cells uniquely have a cell wall, plastids and a large central vacuole; animal cells have centrioles instead. **Q12.** The middle lamella that glues neighbouring plant cells together is made mainly of: - (a) Calcium pectate - (b) Cellulose - (c) Chitin - (d) Lignin **Answer: (a)** β€” The middle lamella is a layer mainly of calcium pectate holding adjoining cells together. **Q13.** A bacterium shaped like a comma is called: - (a) Bacillus - (b) Coccus - (c) Vibrio - (d) Spirillum **Answer: (c)** β€” Vibrio is the comma-shaped form β€” Vibrio cholerae is the classic example. Bacillus is rod-shaped, coccus spherical and spirillum spiral. **Q14.** Gram-negative bacteria differ from Gram-positive bacteria in having: - (a) No cell wall at all - (b) No plasma membrane - (c) An outer lipopolysaccharide membrane over a thin peptidoglycan layer - (d) A thicker peptidoglycan layer **Answer: (c)** β€” Gram-negative bacteria have a thin peptidoglycan layer covered by an outer lipopolysaccharide membrane, so they do not retain the crystal-violet stain. Gram-positive bacteria have the thick peptidoglycan wall that holds the stain. **Q15.** The mesosome of a prokaryotic cell is: - (a) A storage granule of glycogen - (b) An infolding of the plasma membrane - (c) A type of ribosome - (d) A membrane-bound nucleus **Answer: (b)** β€” A mesosome is an infolding of the plasma membrane, unique to prokaryotes. It helps in cell wall formation, DNA replication and distribution, respiration and secretion β€” effectively doing several jobs that organelles do in eukaryotes. **Q16.** Which of the following is NOT a function of the bacterial flagellum? - (a) Motility - (b) Attachment to surfaces during conjugation - (c) Propelling the cell forward - (d) Locomotion in liquid media **Answer: (b)** β€” Attachment during conjugation is the job of the PILI (sex pili), not the flagellum. The flagellum is purely for motility; fimbriae are the short bristles used for general attachment. **Q17.** The cell organelle absent in a mature plant cell but present in an animal cell is: - (a) Golgi body - (b) Centriole - (c) Mitochondrion - (d) Ribosome **Answer: (b)** β€” Centrioles are present in animal cells and absent from most plant cells β€” though lower plants such as Chlamydomonas do have them, so the absence is not absolute. **Q18.** In a prokaryotic cell, the genetic material is: - (a) RNA only - (b) Linear DNA with histones inside a nuclear envelope - (c) Linear DNA without histones inside a nucleolus - (d) Naked circular DNA without a nuclear envelope **Answer: (d)** β€” Prokaryotes have naked circular DNA in a region called the nucleoid β€” no nuclear envelope and no histone proteins. Plasmids are additional small circular DNA molecules carrying traits such as antibiotic resistance. **Q19.** Which statement about the plant cell wall is INCORRECT? - (a) The secondary wall lies on the outer side of the primary wall - (b) It is made mainly of cellulose - (c) The primary wall is capable of growth - (d) The middle lamella is made mainly of calcium pectate **Answer: (a)** β€” The secondary wall forms on the INNER side of the primary wall, towards the plasma membrane β€” not the outer side. The primary wall is the growing layer and gradually thins as the cell matures. **Q20.** Plasmodesmata in plant cells are: - (a) Cytoplasmic bridges connecting adjacent cells through the wall - (b) Infoldings of the inner mitochondrial membrane - (c) Channels within the Golgi apparatus - (d) Pores in the nuclear envelope **Answer: (a)** β€” Plasmodesmata are cytoplasmic channels passing through the cell walls of adjacent plant cells, linking their protoplasts. They are the plant equivalent of animal gap junctions. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q21.** 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 **Answer: (b)** β€” Schleiden (1838) and Schwann (1839) proposed cell theory; Virchow (1855) completed it with 'cells arise from pre-existing cells.' **Q22.** 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 **Answer: (d)** β€” Ribosomes are non-membrane bound and are present in both prokaryotic and eukaryotic cells. **Q23.** 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 **Answer: (c)** β€” Fimbriae are small bristle-like fibres that help attachment; they do not aid motility (flagella do that). **Q24.** 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 **Answer: (a)** β€” Gram positive/negative classification is based on differences in the cell envelope and its staining response. **Q25.** 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 **Answer: (a)** β€” Polar phospholipid heads face outward towards water on both sides; hydrophobic tails face inward, away from water. **Q26.** 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 **Answer: (a)** β€” The middle lamella, mainly calcium pectate, glues neighbouring cell walls together. ### 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 β€” 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.