Cell Structure and Function

The cell theory and the size scale

Schleiden (1838) and Schwann (1839) proposed that all living things are made of cells, and Virchow (1855) added that every cell arises from a pre-existing cell — the three pillars of cell theory in Punjab Textbook Chapter 4 and Campbell Chapter 6. Typical sizes you must recognise on sight: bacteria 1-10 μm, mitochondria 0.5-1.0 μm wide and 2-5 μm long, chloroplasts 5-10 μm, animal cells 10-30 μm, plant cells 30-100 μm, ostrich egg yolk > 7 cm. The light microscope resolves down to about 200 nm; the transmission electron microscope reaches about 0.2 nm, which is why ribosomes (20 nm) were not seen until Palade's work in the 1950s.

Prokaryotic vs eukaryotic architecture

Prokaryotes (bacteria, archaea) lack a nuclear membrane and membrane-bound organelles; their circular DNA sits in a nucleoid, ribosomes are 70S (50S + 30S), cell wall is peptidoglycan, and reproduction is binary fission. Eukaryotes have a true nucleus enclosed by a double membrane perforated by nuclear pores (~ 100 nm), 80S ribosomes (60S + 40S), and a full set of organelles. The 70S ribosomes inside mitochondria and chloroplasts are evidence for the endosymbiotic theory of Lynn Margulis, which the MDCAT tests at least once every two years.

The endomembrane system

The rough endoplasmic reticulum, studded with ribosomes, synthesises and folds membrane and secretory proteins; the smooth ER handles lipid synthesis, steroid production (abundant in testis and ovary), and detoxification (abundant in liver). Vesicles bud off and fuse with the cis-face of the Golgi apparatus, where glycosylation and sorting occur, before mature vesicles leave the trans-face for the plasma membrane or lysosomes. Lysosomes (pH ~ 4.5) carry hydrolytic enzymes for intracellular digestion; their failure causes Tay-Sachs and other storage diseases.

Energy organelles and the cytoskeleton

Mitochondria have a smooth outer membrane and a folded inner membrane (cristae) carrying the electron transport chain; the matrix holds the Krebs cycle enzymes, mitochondrial DNA, and 70S ribosomes. Chloroplasts add an inner thylakoid membrane stacked into grana, with chlorophyll-bearing photosystems doing the light reactions and stroma enzymes doing the Calvin cycle. The cytoskeleton is built from microfilaments (actin, 7 nm), intermediate filaments (8-12 nm), and microtubules (α/β tubulin dimers, 25 nm) — the last forming the spindle, cilia and flagella in the 9+2 arrangement.

Plant cell extras and recurring traps

Plant cells additionally show a cellulose cell wall, a large central vacuole (up to 90% of cell volume) maintaining turgor at ~ 0.5 MPa, plasmodesmata for cytoplasmic continuity, and chloroplasts. Common traps on past papers: lysosomes are normally absent from plant cells (the vacuole does the job); centrioles are absent from higher plant cells but present in animals; the 9+0 arrangement is found in the basal body, not the cilium itself; cristae increase surface area, they do not store DNA. Always pair the Punjab Textbook diagram with the Campbell electron micrographs when revising.

Membrane structure and transport

The Singer-Nicolson fluid mosaic model (1972) describes the plasma membrane as a phospholipid bilayer (~ 7-8 nm thick) studded with integral and peripheral proteins, with cholesterol modulating fluidity in animal cells. Transport across it falls into five categories the MDCAT keeps recycling: simple diffusion (lipid-soluble small molecules like O₂, CO₂, steroids); facilitated diffusion (glucose via GLUT transporters, ions through channels — passive but protein-mediated); osmosis (water, often through aquaporins); active transport (Na⁺/K⁺-ATPase pumping 3 Na⁺ out and 2 K⁺ in per ATP, against gradient); and bulk transport (endocytosis — phagocytosis, pinocytosis, receptor-mediated — and exocytosis). A red blood cell placed in distilled water haemolyses because water enters down the osmotic gradient; in concentrated saline it crenates. Plant cells protected by their wall undergo plasmolysis instead of lysis.