B2.3 Cell specializationIB Biology HL: Revision notes

After fertilization, unspecialized cells develop into specialized cells by differentiation. In an early embryo, gradients of signalling molecules cause different genes to be expressed in different positions.

Stem cells can divide endlessly and differentiate along different pathways. Early embryo cells are totipotent, soon becoming pluripotent; adult stem cells, e.g. in bone marrow, are multipotent. A stem cell niche (e.g. bone marrow, hair follicles) maintains stem cells or promotes their proliferation and differentiation.

Human cells range from sperm and red blood cells (a few μm) to eggs (about 110 μm), neurons with axons up to 1 m and muscle fibres several cm long.

Exchange depends on surface area; the need for exchange depends on volume. As size increases, volume grows faster than area, so SA:V falls. Cubes of different sizes model this: cells are not cubes, but scale factors act the same way.

• Flattening: erythrocytes are thin and biconcave, giving more area for their volume and a short diffusion distance for oxygen.
• Microvilli: finger-like projections on the filtrate side of proximal convoluted tubule cells greatly increase area for carrier proteins.
• Invagination: infoldings of the opposite membrane of PCT cells add area for pump proteins, with mitochondria alongside to supply ATP.

Type I pneumocytes are extremely thin (about 0.2 μm) and cover most of the alveolar surface, reducing the diffusion distance for gases.

Type II pneumocytes are cuboidal with many lamellar bodies (secretory vesicles) that discharge surfactant into the alveolar lumen, reducing surface tension so alveoli do not collapse.

The alveolar epithelium is a tissue with more than one cell type because the overall function needs different adaptations.

Both contain contractile myofibrils and appear striated.

• Striated (skeletal) muscle fibres: unbranched, very long (cm), with many nuclei; formed by fusion of cells.
• Cardiac muscle cells: branched, short (about 100 μm), usually one nucleus, joined by intercalated discs.

Is a striated fibre a cell? For: one continuous plasma membrane, contracts as a unit. Against: many nuclei, formed by fusion, far larger than typical cells. It is usually treated as an atypical cell.

Sperm: small and streamlined with little cytoplasm; a flagellum for swimming; mitochondria in the midpiece supplying ATP; an acrosome containing enzymes to digest the egg's outer layers; a haploid nucleus.

Egg (secondary oocyte): very large, with cytoplasm rich in nutrients and organelles for the early embryo; a zona pellucida (glycoprotein layer) that sperm must penetrate; cortical granules whose contents harden the zona pellucida after fertilization, preventing entry of more sperm (polyspermy); a haploid nucleus.