Cell: The Unit of Life: NEET notes

1. What is a cell, and the cell theory

NCERT §8.1–§8.2

• The cell is the basic structural and functional unit of all living things. A unicellular organism lives on its own and performs every essential life function, but nothing short of a complete cell can support independent life.
• Leeuwenhoek was the first person to observe and describe a live cell; the nucleus was discovered later by Robert Brown.
• Matthias Schleiden, a German botanist, studied many plants and concluded in 1838 that all plants are made of different kinds of cells that together form tissues.
• Theodore Schwann, a zoologist, reported in 1839 that animal cells are bounded by a thin outer layer (what we now call the plasma membrane), and from his work on plant tissues he concluded that a cell wall is peculiar to plant cells.
• Schwann proposed that plant and animal bodies are made of cells and cell products; Schleiden and Schwann together are credited with the cell theory.
• Their theory could not explain where new cells come from. Rudolf Virchow in 1855 filled this gap by stating that cells divide and that new cells arise only from pre-existing cells, summed up as 'Omnis cellula-e cellula'.
• The modern cell theory therefore has two parts: all living organisms are made of cells and their products, and all cells come from pre-existing cells.

2. An overview of the cell

NCERT §8.3

• Cells whose genetic material is enclosed by a nuclear envelope are eukaryotic; cells without a membrane-bound nucleus are prokaryotic.
• Both kinds are filled with cytoplasm, a semi-fluid matrix in which most of the cell's chemical reactions take place.
• Eukaryotic cells also carry distinct membrane-bound organelles such as the endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, microbodies and vacuoles; prokaryotes lack all of these.
• Ribosomes are membrane-less and are found in every cell, prokaryotic or eukaryotic. Inside a eukaryotic cell they occur in the cytoplasm, in chloroplasts (in plants), in mitochondria and on the rough ER.
• The centrosome is a non-membrane-bound organelle present in animal cells and helps in cell division.
• Cells vary widely in size: Mycoplasmas are the smallest cells, bacteria may be 3–5 μm, a human red blood cell is about 7.0 μm across, and the largest isolated single cell is the ostrich egg.
• Nerve cells are among the longest cells, and cell shape usually reflects function: disc-like, polygonal, columnar, cuboidal, thread-like or irregular.

3. Prokaryotic cells: general features

NCERT §8.4

• Mycoplasma, PPLO (pleuro-pneumonia like organisms), bacteria and blue-green algae (cyanobacteria) all have prokaryotic cells.
• They are generally smaller than eukaryotic cells and multiply faster. Bacteria come in four basic shapes: bacillus (rod), coccus (spherical), vibrio (comma) and spirillum (spiral).
• Every prokaryote has a cell wall outside its cell membrane, with one exception: mycoplasma, which has no wall.
• The genetic material is naked: not surrounded by a nuclear membrane. Besides the single genomic DNA (a chromosome, circular), many bacteria carry small circular extra-chromosomal DNA called plasmids.
• Plasmids give bacteria special traits such as resistance to antibiotics; later in the course, plasmid DNA is used to monitor the transformation of bacteria with foreign DNA.
• Apart from ribosomes, prokaryotes have none of the organelles found in eukaryotes. They do have inclusions, and a structure special to them is the mesosome, a specialised infolding of the cell membrane.

4. Cell envelope, its modifications, ribosomes and inclusion bodies

NCERT §8.4.1–§8.4.2

• Most prokaryotes, especially bacteria, have a three-layered cell envelope: outer glycocalyx, a cell wall and an inner plasma membrane, which together act as one protective unit.
• The glycocalyx differs between bacteria: a loose sheath is called the slime layer, while a thick, tough covering is called the capsule.
• Based on how the envelope reacts to Gram's stain, bacteria are divided into Gram-positive (take up the stain) and Gram-negative (do not).
• The bacterial cell wall decides the cell's shape and is strong enough to stop the cell from either bursting or collapsing.
• Mesosomes are formed when the plasma membrane folds inwards into vesicles, tubules and lamellae. Their roles include forming the cell wall, replicating DNA and distributing it to daughter cells, respiration and secretion; they also increase the surface area of the membrane and its enzyme content.
• Cyanobacteria also have chromatophores, membranous extensions into the cytoplasm that carry the photosynthetic pigments.
• A bacterial flagellum has three parts: filament (the longest part, extending outward), hook and basal body. Pili (long tubes of a special protein) and fimbriae (short bristle-like fibres) are surface structures that play no part in motility; in some bacteria fimbriae help the cell cling to rocks in streams or to host tissue.
• Prokaryotic ribosomes are 70S, built from a 50S and a 30S subunit, and are attached to the plasma membrane. Several ribosomes reading one mRNA form a chain called a polyribosome or polysome.
• Inclusion bodies store reserve material free in the cytoplasm, with no membrane around them; examples are glycogen, phosphate and cyanophycean granules. Blue-green, purple and green photosynthetic bacteria also have gas vacuoles.

5. Eukaryotic cells and the cell membrane

NCERT §8.5, §8.5.1

• Eukaryotes include all protists, plants, animals and fungi. Their cytoplasm is divided into compartments by membrane-bound organelles, and they have a well-organised nucleus with a nuclear envelope.
• Plant cells have a cell wall, plastids and a large central vacuole, all absent from animal cells; animal cells have centrioles, which are absent from almost all plant cells.
• The membrane's fine structure could only be studied once electron microscopes were available in the 1950s; before that, chemical analysis, chiefly of human red blood cells, showed it is built mainly of lipids and proteins, with carbohydrate also present.
• The main lipids are phospholipids arranged in a bilayer with polar heads facing outward and hydrophobic tails facing inward, shielding the non-polar tails from the watery surroundings. Cholesterol is also present.
• In the human RBC membrane, protein makes up roughly 52 per cent and lipid roughly 40 per cent; the protein to lipid ratio varies between cell types.
• Membrane proteins are classed as peripheral (on the surface) or integral (partly or fully buried in the bilayer).
• The fluid mosaic model of Singer and Nicolson (1972) says the lipid layer is quasi-fluid, so proteins can move sideways within the bilayer. This fluidity is needed for cell growth, forming intercellular junctions, secretion, endocytosis and cell division.
• The membrane is selectively permeable. Movement without energy along a concentration gradient is passive transport; water moving by diffusion is osmosis. Polar molecules need carrier proteins to cross.
• Movement against a concentration gradient uses ATP and is active transport; the Na⁺/K⁺ pump is the standard example.

6. Cell wall

NCERT §8.5.2

• In plants and fungi a rigid, non-living cell wall covers the plasma membrane. Its jobs: giving the cell its shape, guarding against mechanical injury and infection, helping cells interact, and keeping out unwanted macromolecules.
• Algal walls are made of cellulose, galactans, mannans and minerals such as calcium carbonate.
• Walls of other plants contain cellulose, hemicellulose, pectins and proteins.
• A young plant cell has a primary wall that can grow; this growth gradually slows as the cell matures.
• The secondary wall is laid down on the inner side of the primary wall, that is, towards the plasma membrane.
• The middle lamella, made mainly of calcium pectate, is the layer that glues neighbouring cells together.
• Going outward from the cell: plasma membrane, secondary wall, primary wall, middle lamella. Plasmodesmata pass through the wall and middle lamella to join the cytoplasm of adjacent cells.

7. Endomembrane system: ER and Golgi apparatus

NCERT §8.5.3–§8.5.3.2

• The endomembrane system groups organelles whose functions are coordinated: endoplasmic reticulum, Golgi complex, lysosomes and vacuoles. Mitochondria, chloroplasts and peroxisomes are NOT part of it because their functions are not coordinated with these.
• The ER is a network of tiny tubular structures in the cytoplasm that splits it into a luminal (inside ER) and an extra-luminal (cytoplasm) compartment.
• ER carrying ribosomes on its surface is rough ER (RER); ER without ribosomes is smooth ER (SER). RER is abundant in cells busy making and secreting proteins and is continuous with the outer nuclear membrane.
• Lipids are synthesised mainly in the SER, which in animal cells also makes the lipid-like steroidal hormones.
• Camillo Golgi first saw the Golgi apparatus in 1898 as densely stained reticular structures near the nucleus.
• It is a stack of flat, disc-shaped sacs called cisternae, about 0.5–1.0 μm in diameter, arranged parallel to one another in stacks, with cisternae varying in number.
• The stack has two faces: the convex cis (forming) face and the concave trans (maturing) face. They are entirely different but interconnected.
• Materials packed in ER vesicles fuse with the cis face and travel towards the trans face, which explains why the Golgi lies close to the ER.
• The Golgi packages materials for intracellular targets or for secretion. Proteins made on ribosomes of the ER are modified in its cisternae before release from the trans face, and it is an important site for forming glycoproteins and glycolipids.

8. Endomembrane system: lysosomes and vacuoles

NCERT §8.5.3.3–§8.5.3.4

• Lysosomes are membrane-bound vesicles formed through packaging in the Golgi apparatus.
• Lysosomal vesicles, when isolated, carry nearly every type of hydrolytic enzyme (lipases, proteases, carbohydrases and so on), all most active at acidic pH.
• These enzymes can digest carbohydrates, proteins, lipids and nucleic acids.
• A vacuole is a membrane-enclosed space in the cytoplasm; it stores water and sap along with excretory products and other substances the cell has no use for.
• Its single membrane is called the tonoplast. In plant cells the vacuole can take up as much as 90 per cent of the cell volume.
• The tonoplast moves several ions and other materials into the vacuole against their concentration gradient, so their concentration inside is much higher than in the cytoplasm.
• In Amoeba the contractile vacuole handles osmoregulation and excretion. In many cells, protists for example, food vacuoles form when food particles are engulfed.

9. Mitochondria

NCERT §8.5.4

• Without special staining, mitochondria are hard to make out under the microscope. How many a cell holds depends on how physiologically active that cell is.
• They are typically sausage-shaped or cylindrical, about 0.2–1.0 μm in diameter (average 0.5 μm) and 1.0–4.1 μm long, though shape and size vary a lot.
• A mitochondrion has two membranes, which split its interior into two aqueous compartments, an outer one and an inner one; the inner compartment holds the matrix, a dense and uniform substance.
• The outer membrane forms a continuous boundary. The inner membrane folds into the matrix as cristae, which greatly increase its surface area.
• Each membrane carries its own specific enzymes linked to mitochondrial function.
• Mitochondria are the sites of aerobic respiration and make energy in the form of ATP, hence the name 'power houses' of the cell.
• Inside the matrix are 70S ribosomes, one circular DNA molecule, a few RNA molecules and the components required for protein synthesis.
• Mitochondria divide by fission.

10. Plastids

NCERT §8.5.5

• Plastids occur in all plant cells and in euglenoids. They are easy to see under the microscope because they are large and carry specific pigments that give plants their colours.
• Based on pigment type, plastids are chloroplasts, chromoplasts or leucoplasts.
• Chloroplasts contain chlorophyll and carotenoid pigments and trap light energy for photosynthesis.
• Chromoplasts contain fat-soluble carotenoids such as carotene and xanthophylls, which give yellow, orange or red colour.
• Leucoplasts are colourless storage plastids: amyloplasts store starch (as in potato), elaioplasts store oils and fats, and aleuroplasts store proteins.
• In green plants, the mesophyll cells of the leaves hold most of the chloroplasts. Their shape varies: discoid, oval, spherical, lens-shaped or even ribbon-like; they measure about 5–10 μm in length and 2–4 μm in width. A cell may have just one, as in Chlamydomonas, or 20–40, as in mesophyll.
• Chloroplasts are double-membraned, and the inner membrane is relatively less permeable. The space inside is the stroma, holding flattened sacs called thylakoids stacked into grana; flat membranous tubules called stroma lamellae link thylakoids of different grana.
• The stroma holds the enzymes for synthesising carbohydrates and proteins, and small, double-stranded circular DNA and ribosomes. Chlorophyll pigments are located in the thylakoids.
• Chloroplast ribosomes are 70S, smaller than the 80S cytoplasmic ribosomes.

11. Ribosomes, cytoskeleton, cilia, flagella and centrioles

NCERT §8.5.6–§8.5.9

• George Palade first observed ribosomes under the electron microscope in 1953 as dense particles. They are made of RNA and proteins and have no surrounding membrane.
• Eukaryotic ribosomes are 80S (60S + 40S subunits); prokaryotic ones are 70S (50S + 30S). The 'S' stands for Svedberg unit, the sedimentation coefficient, which indirectly reflects density and size, so subunit values do not simply add up.
• The cytoskeleton is an elaborate network of protein filaments (microtubules, microfilaments and intermediate filaments) giving mechanical support, motility and maintenance of cell shape.
• Cilia are small, oar-like structures that move either the cell or the fluid around it; flagella are comparatively longer and move the cell. Prokaryotic flagella are structurally different from eukaryotic ones.
• The eukaryotic cilium or flagellum is covered by plasma membrane. Its core, the axoneme, has nine doublets of microtubules around the edge and a pair of single microtubules in the centre: the 9+2 array.
• The central pair is linked by bridges and enclosed by a central sheath, which connects to one tubule of each peripheral doublet through a radial spoke, giving nine radial spokes. Linkers join neighbouring peripheral doublets. Cilia and flagella arise from centriole-like basal bodies.
• The centrosome usually has two cylindrical centrioles surrounded by amorphous pericentriolar material, lying perpendicular to each other.
• Each centriole is built like a cartwheel: nine evenly spaced peripheral fibrils of tubulin, each a triplet, with neighbouring triplets linked. In the proximal region the centre is a proteinaceous hub joined to the triplets by protein radial spokes; there is no central pair of microtubules.
• Centrioles form the basal bodies of cilia and flagella and give rise to the spindle fibres of the spindle apparatus during division in animal cells.

12. Nucleus and microbodies

NCERT §8.5.10–§8.5.11

• Robert Brown described the nucleus as a cell organelle in 1831; Flemming later named the material stained by basic dyes chromatin.
• A non-dividing (interphase) nucleus contains chromatin, an extended network of nucleoprotein fibres, along with the nuclear matrix and one or more spherical nucleoli.
• The nuclear envelope has two parallel membranes separated by a 10–50 nm perinuclear space. The outer membrane is usually continuous with the ER and carries ribosomes.
• Nuclear pores, formed where the two membranes fuse, allow RNA and protein to move in both directions between nucleus and cytoplasm.
• A cell usually has a single nucleus, though the number can vary. Some cells lack one altogether, such as sieve tube cells in vascular plants and mature erythrocytes in many mammals.
• Nucleoli are not membrane-bound, so their contents are continuous with the nucleoplasm. They are sites of active ribosomal RNA synthesis, and cells that make a lot of protein have larger and more numerous nucleoli.
• Along with DNA, chromatin has histones (basic proteins), RNA and some non-histone proteins. About two metres of DNA, distributed among 46 chromosomes, is packed into a single human cell.
• Every chromosome has a primary constriction, the centromere, with disc-shaped kinetochores on its sides. By centromere position chromosomes are metacentric (middle; two equal arms), sub-metacentric (slightly off-centre; one shorter and one longer arm), acrocentric (close to one end; one very short and one very long arm) and telocentric (at the tip).
• Some chromosomes have a secondary constriction at a fixed location that sets off a small fragment called the satellite.
• Microbodies are tiny membrane-bound vesicles packed with a variety of enzymes; both plant and animal cells have many of them.

Must-know facts

1. Virchow (1855) added 'Omnis cellula-e cellula' to the Schleiden (1838) and Schwann (1839) cell theory.
2. Leeuwenhoek was the first to see and describe a live cell; the nucleus was discovered by Robert Brown (1831).
3. Smallest cells: Mycoplasmas. Largest isolated single cell: ostrich egg. Human RBC: about 7.0 μm in diameter; bacteria 3–5 μm.
4. Mycoplasma is the one prokaryote without a cell wall.
5. Bacterial envelope: glycocalyx (slime layer if loose, capsule if thick and tough), cell wall, plasma membrane.
6. Mesosome functions: cell wall formation, DNA replication and distribution, respiration, secretion, more surface area and enzymes.
7. Prokaryotic ribosomes are 70S (50S + 30S), and mitochondria and chloroplasts also carry 70S ribosomes; eukaryotic cytoplasmic ribosomes are 80S (60S + 40S).
8. Human RBC membrane: about 52% protein, 40% lipid. Fluid mosaic model: Singer and Nicolson, 1972.
9. Na⁺/K⁺ pump is active transport and uses ATP.
10. Cell wall layers outward: secondary wall, primary wall, middle lamella (calcium pectate).
11. Endomembrane system = ER + Golgi + lysosomes + vacuoles; mitochondria, chloroplasts and peroxisomes are excluded.
12. SER makes lipids and, in animal cells, steroidal hormones; RER is continuous with the outer nuclear membrane.
13. Golgi: discovered by Camillo Golgi (1898); cisternae 0.5–1.0 μm; cis = forming (convex) face, trans = maturing (concave) face; site of glycoprotein and glycolipid formation.
14. Lysosomes are packaged by the Golgi and hold hydrolases (lipases, proteases, carbohydrases) that work best at acidic pH.
15. Vacuole membrane is the tonoplast; the vacuole can fill up to 90% of a plant cell.
16. Mitochondria: 0.2–1.0 μm wide (average 0.5 μm), 1.0–4.1 μm long; matrix has circular DNA, some RNA and 70S ribosomes; divide by fission.
17. Leucoplasts: amyloplast = starch, elaioplast = oils and fats, aleuroplast = proteins.
18. Chloroplasts: 5–10 μm long, 2–4 μm wide; one per cell in Chlamydomonas, 20–40 per mesophyll cell.
19. Axoneme is 9+2 (doublets); centriole is nine peripheral triplets with a central hub and no central microtubules.
20. Nucleolus is not membrane-bound and is the site of active rRNA synthesis; perinuclear space is 10–50 nm.

Common traps

Key terms

Prokaryotic cell

A cell whose genetic material lies free in the cytoplasm with no nuclear envelope and which has no membrane-bound organelles.

Plasmid

A small circular DNA outside the bacterial chromosome that carries extra traits such as antibiotic resistance.

Mesosome

An inward fold of the bacterial plasma membrane that helps with wall formation, DNA handling, respiration and secretion.

Glycocalyx

The outermost layer of the bacterial envelope, called a slime layer when loose and a capsule when thick.

Polysome

A string of ribosomes translating the same mRNA at the same time.

Inclusion bodies

Membrane-free stores of reserve material in prokaryotic cytoplasm, such as glycogen or phosphate granules.

Fluid mosaic model

The Singer–Nicolson picture of the membrane as a quasi-fluid lipid bilayer with proteins that can drift sideways.

Middle lamella

The calcium pectate layer that cements adjacent plant cells together.

Plasmodesmata

Cytoplasmic channels passing through plant cell walls that connect neighbouring cells.

Endomembrane system

The functionally linked set of ER, Golgi apparatus, lysosomes and vacuoles.

Cisternae

The flat, stacked, disc-like sacs that make up the Golgi apparatus.

Tonoplast

The single membrane surrounding the plant vacuole.

Cristae

Folds of the inner mitochondrial membrane that increase its surface area.

Grana

Stacks of thylakoids inside the chloroplast, linked by stroma lamellae.

Svedberg unit (S)

The unit of sedimentation coefficient, an indirect measure of a particle's size and density.

Axoneme

The microtubule core of a cilium or flagellum, arranged in a 9+2 pattern.

Basal body

A centriole-like structure at the base of a cilium or flagellum from which it grows.

Kinetochore

A disc-shaped structure on either side of the centromere.

Satellite

A small chromosome segment set apart by a secondary constriction.

Microbodies

Small membrane-bound vesicles containing various enzymes, present in plant and animal cells.