Stem Cells in Animals & Plants

Erythrocytes (Red Blood Cells)

• Structural Changes: To maximize oxygen transport, these cells lose their nucleus and most organelles, providing more internal space for haemoglobin.
• Morphology: The cell adopts a biconcave disc shape to increase surface area for gas exchange and develops a highly flexible membrane to squeeze through narrow capillaries.

Neutrophils (White Blood Cells)

• Structural Changes: These cells develop a characteristic lobed nucleus, which enhances their flexibility to migrate through small gaps in capillary walls to reach infected tissues.
• Organelle Adaptation: They accumulate a high density of lysosomes containing hydrolytic enzymes, which are essential for the digestion of engulfed pathogens during phagocytosis.

• Meristems: These are specific regions of active cell division in plants, such as root tips and the cambium layer in vascular bundles.

• Xylem Differentiation: Cambium cells destined for the xylem undergo lignification, where cell walls are thickened with lignin for support. The cells eventually lose their cytoplasm and end walls, becoming dead, hollow tubes for water transport.

• Phloem Differentiation: Cells becoming phloem sieve tubes lose most of their organelles and cytoplasm to reduce resistance to flow. Unlike xylem, they remain living and develop sieve plates at their end walls to allow the passage of nutrients.

• Animal vs. Plant Growth: In animals, growth is generally distributed, whereas in plants, growth is localized to meristems where stem cells are concentrated.

• Adaptation Links: When describing specialized cells, always link the structural change to the function. For example, 'loss of nucleus' (structure) leads to 'increased haemoglobin capacity' (function).

• Potency Precision: Be careful not to use 'pluripotent' and 'totipotent' interchangeably; the key difference is the ability to form extra-embryonic tissues.

• Mathematical Patterns: The number of possible cell combinations in independent assortment (related to meiosis) follows the formula $2^n$, where $n$ is the haploid number. While not a stem cell formula, it is often tested in the same unit.

• Common Error: Students often forget that phloem cells are living while xylem cells are dead. Always check the status of the cytoplasm when identifying these tissues.