Cell Structure and Function

1Cell theory

Cell theory is three claims, built in stages. Schleiden (1838) and Schwann (1839) established that all plants and animals are made of cells and that the cell is the basic unit of life. Virchow (1855) added the third claim: omnis cellula e cellula — every cell arises from a pre-existing cell. Together they rule out spontaneous generation of cells from non-living matter.

The three claims are not optional decorations. A stem that says 'cells can arise from abiotic soup in the modern biosphere' is denying Virchow; one that treats a virus as a cell is denying the basic-unit claim, because a virus has no cellular organisation of its own.

Figure. Three equal claims. The right-hand box is Virchow's and is the one spontaneous-generation stems attack.

2Prokaryotes and eukaryotes

The deepest cut among cells is prokaryote against eukaryote. A prokaryote — a bacterium — has no nuclear membrane and no membrane-bound organelles. Its DNA is a circular molecule in a nucleoid, its ribosomes are 70S, and if it has a cell wall the wall contains peptidoglycan. A eukaryote has a true nucleus, membrane-bound organelles (mitochondria, ER, Golgi, and in plants chloroplasts), 80S cytosolic ribosomes, and — in plants — a cellulose wall.

The defining prokaryotic trait is the absence of a nuclear membrane. Listing 'no mitochondria' is true but secondary: it follows from having no membrane-bound organelles at all.

Figure. Two cell types as labelled boxes, not outlines. Anatomy of a bacterium or a plant cell needs closed curves; the characters that decide the cut do not.

3Key organelles

Four organelles carry most of the marks. Mitochondria run aerobic respiration and make ATP; they have their own circular DNA and 70S ribosomes, so they are semi-autonomous. Chloroplasts run photosynthesis in green plant cells and are semi-autonomous on the same grounds — both facts point to an endosymbiotic origin. Ribosomes are the protein factories: 70S in prokaryotes and in those two organelles, 80S in the eukaryotic cytosol. Lysosomes hold hydrolytic enzymes and are nicknamed suicide bags because a rupture digests the cell's own contents.

ER and Golgi complete the endomembrane story: rough ER is studded with ribosomes and makes proteins for export; Golgi modifies and packages them. Eukaryotic cilia and flagella share a 9+2 microtubule arrangement — nine doublets around a central pair — which is a stored structural fact, not a drawing you can approximate with boxes.

Figure. Four high-yield organelles mapped to the job NEET asks for. Semi-autonomous organelles sit on the positive side; trafficking organelles on attention. No cristae or thylakoid drawings.

4Membrane structure and transport

Singer and Nicolson's fluid mosaic model (1972) pictures the plasma membrane as a phospholipid bilayer with proteins floating in it like icebergs in a sea — some spanning the bilayer (integral), some sitting on one face (peripheral). The bilayer is fluid: lipids and many proteins diffuse laterally. Cholesterol in animal membranes modulates that fluidity.

Crossing the membrane is either passive or active. Passive transport — simple diffusion, facilitated diffusion through a protein channel or carrier, and osmosis of water — needs no ATP and always runs down a concentration gradient. Active transport pumps a solute against its gradient and spends ATP. The fluid mosaic is what makes both kinds of protein-mediated route possible: the proteins are the gates; the bilayer is the barrier.

Figure. Two lipid leaflets with one protein spanning both and one sitting on a face. Not to scale — the point is mosaic arrangement, not molecular geometry.

Choosing a transport mode

1. Ask the gradientIf the solute moves from high to low concentration, the route can be passive.
2. Ask for a proteinSmall non-polar solutes cross the bilayer alone (simple diffusion); ions and polar solutes need a channel or carrier (facilitated).
3. Ask for ATPIf the solute moves from low to high concentration, a pump must spend ATP — active transport.

5Ribosome sizes

Ribosome size is reported in Svedberg units (S), which measure sedimentation rate, not mass, and do not add arithmetically in any simple physical sense — yet the textbook identities are written as sums: the prokaryotic 70S ribosome is assembled from 50S and 30S subunits, and the eukaryotic cytosolic 80S ribosome from 60S and 40S. Mitochondrial and chloroplast ribosomes are 70S, which is part of the semi-autonomous evidence.

Remember which cell type has which pair. Mixing them — writing 70S = 60S + 40S — is a common slip. The S values are labels you look up, not numbers you derive from first principles.

Figure. Two ribosome classes. Keep the subunit pairs tied to location: 70S is not 'the small one' in a plant cytosol.

6The cell cycle

A proliferating eukaryotic cell alternates interphase with M phase. Interphase is G1 (growth, ordinary metabolism), S (DNA replication — every chromosome becomes two sister chromatids), and G2 (preparation for division). M phase is mitosis plus cytokinesis. Cells that leave the cycle temporarily or permanently sit in G0.

DNA content is the quantity that moves. If G1 DNA is called 2C, S phase doubles it to 4C; mitosis then partitions 4C into two daughters at 2C each. Chromosome number stays 2n through mitosis — only chromatid number and DNA content change in S.

Figure. Interphase as G1–S–G2, then M. Only S changes the C-value; M partitions it.

Order of phases

1. G1Cell grows; DNA is still 2C; chromosomes are single chromatids.
2. SDNA replicates to 4C; each chromosome now has two sister chromatids.
3. G2Final preparations; DNA remains 4C.
4. MMitosis plus cytokinesis returns each daughter to 2C and 2n.

7Mitosis against meiosis

Mitosis is equational: one diploid cell yields two diploid cells (2n → 2n), genetically identical to the parent, used for growth and repair. Meiosis is reductional: one diploid cell yields four haploid cells (2n → n), used to make gametes. Chromosome number halves in meiosis I, when homologous chromosomes separate; meiosis II is equational for chromatids, like a mitosis of haploid cells.

Variation is a meiosis speciality. Crossing over in prophase I and independent assortment of homologues scramble alleles; mitosis has neither, so daughter cells match the parent.

Figure. Mitosis: one 2n cell to two 2n cells. Meiosis: one 2n cell to four n cells. Counts, not chromosome shapes.

8Where meiosis creates variation

Two meiotic events shuffle genes. Crossing over — exchange between non-sister chromatids of a homologous pair — happens in pachytene of prophase I and produces recombinant chromatids. Independent assortment — random orientation of homologous pairs at metaphase I — sends maternal and paternal chromosomes into gametes in new combinations.

Mitosis has neither event, which is why it yields genetic copies. A stem that asks 'at which stage does crossing over occur' wants pachytene of prophase I, not metaphase or anaphase, and not prophase of mitosis.

Figure. Where meiosis creates variation: synapsis enables crossing over in pachytene; independent assortment at metaphase I is the second shuffle. No chiasma drawing — the named events carry the content.

Two sources of variation

1. PairingHomologous chromosomes pair as bivalents in prophase I.
2. Crossing overIn pachytene, non-sister chromatids exchange segments; recombinants appear.
3. Independent assortmentAt metaphase I each bivalent orients randomly, mixing maternal and paternal chromosomes into gametes.
4. Contrast mitosisNo pairing, no crossing over, no assortment — daughters match the parent.