Prokaryotic Cells

NCERT grounding

The canonical text is NCERT Class 11 Biology, Chapter 8 (Cell — The Unit of Life), section 8.4 with its two sub-sections 8.4.1 (Cell Envelope and its Modifications) and 8.4.2 (Ribosomes and Inclusion Bodies). The chapter opens its prokaryote treatment by naming the four groups — bacteria, blue-green algae, mycoplasma and PPLO — that are generally smaller and multiply more rapidly than eukaryotic cells. Figure 8.2 of NCERT places typical bacteria at 1–2 µm, PPLO at about 0.1 µm and viruses at 0.02–0.2 µm beside a 10–20 µm typical eukaryotic cell.

NIOS Senior Secondary Biology, Lesson 4 (Cell — Structure and Function), supplements this with Table 4.1 — the standard prokaryote-vs-eukaryote comparison — and adds the explicit Svedberg-unit note that prokaryotic ribosomes sediment at 70S whereas eukaryotic ribosomes sediment at 80S in the ultracentrifuge. Together these two anchors define everything NEET has ever asked from this subtopic.

Architecture of a prokaryotic cell

A prokaryotic cell is best read as four nested envelopes around a naked chromosome. Outside-in: a glycocalyx (slime layer or capsule), a cell wall that determines shape and resists bursting, a plasma membrane that is selectively permeable and is structurally similar to the eukaryotic membrane, and finally the cytoplasm with its nucleoid, ribosomes, mesosomes and inclusion bodies. The four groups NCERT lists — bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO — share this floor-plan; archaea, recognised separately in modern taxonomy, share it too.

Two facts dominate everything else. First, the genetic material is naked — a single circular chromosome (the nucleoid) that is not enclosed in a nuclear envelope and, in true bacteria, is not packaged onto histone proteins. Many bacteria additionally carry small circular plasmids outside the genomic DNA; plasmids confer accessory phenotypes such as antibiotic resistance and are the workhorse vectors of recombinant DNA technology. Second, no membrane-bound organelle exists. There is no mitochondrion, no plastid, no endoplasmic reticulum (smooth or rough), no Golgi, no lysosome, no peroxisome and no true nucleus. The only organelle prokaryotes possess is the ribosome, which is itself non-membranous.

What replaces eukaryotic compartmentalisation is the mesosome — a specialised infolding of the plasma membrane into the cytoplasm in the form of vesicles, tubules and lamellae. The mesosome is the prokaryotic answer to "how do you do respiration without a mitochondrion, replicate DNA without a nuclear membrane and increase membrane surface area without a Golgi?" — it does all of these at once.

Four basic shapes

NCERT lists four bacterial shapes that NEET treats as a single direct-recall block. Each name maps to a Latin descriptor; the descriptor is the testable detail.

Cell envelope and its modifications

NCERT 8.4.1 is the densest NEET-asked region of the subtopic. The envelope is "a tightly bound three-layered structure" — outermost glycocalyx, then cell wall, then plasma membrane — and the three layers "act together as a single protective unit". Each layer has a distinct job.

Gram-positive vs Gram-negative

Bacteria are sorted by how they respond to the staining procedure developed by Gram. Those that retain the crystal-violet stain are Gram-positive; those that do not are Gram-negative. The chemistry of the cell wall is what creates the staining outcome — a fact NEET likes to invert into a one-line distractor.

Ribosomes, mesosome and inclusion bodies

NCERT 8.4.2 covers ribosomes and inclusion bodies; the mesosome is introduced one section earlier but belongs conceptually with this cluster because all three are cytoplasmic components of the prokaryotic cell. Together they replace what eukaryotes do with bound organelles.

The 70S ribosome

In prokaryotes ribosomes are associated with the plasma membrane of the cell. They measure about 15 nm × 20 nm and are built of two subunits — 50S (large) and 30S (small) — which together form the 70S prokaryotic ribosome. They are the site of protein synthesis. Several ribosomes may attach to a single mRNA to form a chain called a polyribosome (or polysome); the ribosomes of a polysome translate the mRNA into proteins. Eukaryotic counterparts sediment at 80S (60S + 40S) — the difference is the basis of selective bacterial antibiotics such as streptomycin, tetracycline and chloramphenicol.

The mesosome

The mesosome is the only "organelle-like" structure unique to prokaryotes. It is not an organelle in the strict sense — it has no separate membrane — it is an extension of the plasma membrane into the cell in the form of vesicles, tubules and lamellae. Its functional load is multi-purpose because the prokaryote has nowhere else to do this work.

In some prokaryotes — notably cyanobacteria — additional membranous extensions called chromatophores push into the cytoplasm carrying photosynthetic pigments. Functionally these are the cyanobacterial analogue of the thylakoid system, but they remain part of the plasma-membrane continuum and not a separate, double-membraned plastid.

Inclusion bodies — reserve, non-membranous

NCERT defines inclusion bodies in a single sentence that NEET has tested twice: "These are not bound by any membrane system and lie free in the cytoplasm." They store reserve material. The list to memorise:

Flagella, pili and fimbriae

Bacteria may be motile or non-motile. Motility, when present, comes from a flagellum — a thin filamentous extension of the cell wall composed of three parts: filament (longest, extends from the cell surface outward), hook and basal body. Bacteria show a range in the number and arrangement of flagella — monotrichous (one), lophotrichous (a tuft at one pole), amphitrichous (one at each pole) and peritrichous (all over) — but NCERT does not require the Latin names; what it does require is the three-part anatomy.

Pili and fimbriae are also surface structures but do not play a role in motility. Pili are elongated tubular structures made of a special protein and are associated with conjugational DNA transfer. Fimbriae are small bristle-like fibres sprouting from the cell that help the bacterium attach — to rocks in streams, or to host tissues during infection. NEET likes to put all three in one MCQ and ask which is the locomotor.

Worked examples

Common confusion & NEET traps