Multiple Choice
1. B – Permanent interdependence is the defining feature. Colonial cells retain independence; multicellular cells are permanently committed and cannot survive alone.
2. D – No nucleus + no mitochondria + biconcave + iron-containing protein (haemoglobin) = red blood cell. Every structural feature maps directly.
3. A – Epithelial tissue is correctly described as a continuous sheet of tightly packed cells on a basement membrane. Options B, C, and D swap the structural features between tissue types.
4. C – The organ criterion is multiple tissue types. Size and cell count are irrelevant. The stomach has four tissue types; that is what makes it an organ.
5. B – Organelle → Cell → Tissue → Organ → Organ system → Organism is the correct sequence.
Activity 03 – Practice Question Answers
Question A: The cell most likely came from a colonial organism (most likely a colonial protist or alga such as Volvox). Evidence 1: The cell contains chloroplasts, indicating it is photosynthetic and eukaryotic – consistent with colonial algae. Evidence 2: The cell can survive independently after removal – the defining feature of colonial (not multicellular) organisation, where each cell retains independent viability. In a multicellular organism, most specialised cells cannot survive if isolated.
Question B: The myelin sheath is essential because it dramatically increases the speed of electrical signal transmission along the axon. Structurally, the myelin sheath consists of layers of lipid-rich membrane wrapped around the axon by Schwann cells, creating an insulating layer with exposed gaps called nodes of Ranvier. This insulation forces the electrical signal to jump from node to node (saltatory conduction) rather than propagating continuously along the entire axon membrane. This increases conduction velocity up to 100 times compared to an unmyelinated fibre of the same diameter. Without myelin, signals in the peripheral nervous system would travel too slowly to coordinate rapid muscle responses.
Question C: Similarity: Both xylem and phloem are vascular tissues that form continuous bundles running from roots through stems to leaves, and both function in transporting materials throughout the plant. Difference 1: Whereas xylem cells are dead at maturity – cell contents removed, leaving hollow lignified tubes – phloem sieve tube elements must remain living because they require ATP to actively load and unload sucrose at source and sink tissues. Difference 2: Xylem transports water and dissolved inorganic minerals unidirectionally upward from roots to leaves, driven by transpiration; whereas phloem transports dissolved organic compounds (primarily sucrose) bidirectionally – from photosynthetic source leaves to any sink tissue (growing roots, fruit, storage organs) depending on metabolic demand.
Question D: At the organelle level, specialised structures compartmentalise specific biochemical reactions within a cell – mitochondria perform aerobic respiration, ribosomes synthesise proteins – allowing incompatible processes to occur simultaneously without interference. At the cell level, organelles are integrated into a self-contained living unit capable of all life processes; a cardiac muscle cell integrates mitochondria, myofilaments, and nucleus into a unit that can contract, respond to signals, and maintain its own metabolism – no organelle alone can do this. At the tissue level, millions of cardiac muscle cells connected by intercalated discs contract simultaneously, generating sufficient pressure to move blood – an emergent property impossible for a single cell. At the organ level, the heart integrates cardiac muscle, epithelial, connective, and nervous tissue to create a self-regulating pump with one-way valves and its own electrical rhythm – no single tissue type could pump and direct blood flow. At the organ system level, the cardiovascular system connects the heart to the vessel network and blood, enabling whole-body circulation and delivery of O2 and nutrients to every cell – the heart alone cannot distribute materials to all tissues. At the organism level, all organ systems operate simultaneously under nervous and endocrine coordination to maintain homeostasis – stable body temperature, blood glucose, and pH – a level of integrated regulation that is impossible at any lower level of organisation.
