Structural Organisation in Animals and Plants

The One Thing

Plants and animals are organised hierarchically: cells form tissues, tissues form organs, and organs work together in organ systems. At every level, structure is closely related to function.

Reactions, Processes and Experiments

This topic contains no named chemical reactions or experiments. The principal biological process is organisation through increasing levels of structural complexity.

Key Terms

• Morphology: The study of the external form, shape, size, and visible parts of an organism.
• Anatomy: The study of the internal structure and arrangement of tissues and organs.
• Plant organ: A specialised plant structure, such as a root, stem, leaf, flower, fruit, or seed, made of different tissues performing specific functions.
• Root system: The underground part of a plant that anchors the plant, absorbs water and minerals, and may store food.
• Tap root: A root system with one main primary root and smaller lateral roots, commonly found in dicot plants.
• Fibrous root: A cluster of similarly sized roots arising from the base of the stem, commonly found in monocot plants.
• Stem: The aerial axis that supports leaves, flowers, and fruits and conducts water, minerals, and food.
• Leaf: A generally flattened plant organ specialised for photosynthesis, gaseous exchange, and transpiration.
• Inflorescence: The arrangement of flowers on the floral axis.
• Flower: The reproductive structure of angiosperms, usually consisting of sepals, petals, stamens, and carpels.
• Fruit: A mature ovary, sometimes with other floral parts, that usually encloses seeds.
• Meristematic tissue: Plant tissue containing actively dividing cells responsible for growth.
• Apical meristem: Meristem present at root and shoot tips that increases the length of the plant.
• Intercalary meristem: Meristem located near nodes or at the bases of leaves, helping elongation of plant parts.
• Lateral meristem: Meristem such as vascular cambium and cork cambium that increases the girth of stems and roots.
• Permanent tissue: Plant tissue formed from meristematic cells that have stopped dividing and become specialised.
• Parenchyma: A living simple permanent tissue involved in storage, photosynthesis, and repair; chloroplast-containing parenchyma is called chlorenchyma.
• Collenchyma: A living tissue with unevenly thickened cell walls that provides flexible support to growing plant parts.
• Sclerenchyma: A usually dead supporting tissue with thick, lignified walls that provides strength and rigidity.
• Xylem: Complex tissue that mainly conducts water and minerals from roots to aerial parts and also provides support.
• Phloem: Complex tissue that transports organic food from sources to areas where it is used or stored.
• Epidermis: The outer protective layer of plant organs, often covered by a cuticle and containing stomata or root hairs.
• Stomata: Small pores surrounded by guard cells that regulate gaseous exchange and transpiration.
• Epithelial tissue: Animal tissue forming coverings and linings; its cells are closely packed and rest on a basement membrane.
• Connective tissue: Animal tissue that connects, supports, binds, stores, or transports; its cells are embedded in an extracellular matrix.
• Muscular tissue: Contractile tissue responsible for movement of body parts and internal organs.
• Nervous tissue: Tissue specialised for receiving stimuli and transmitting electrical signals; it consists mainly of neurons and supporting cells.
• Simple epithelium: Epithelium consisting of a single layer of cells, suited for absorption, secretion, filtration, or diffusion.
• Compound epithelium: Epithelium with multiple cell layers, mainly providing protection.
• Areolar tissue: Loose connective tissue that fills spaces, supports organs, and helps in tissue repair.
• Adipose tissue: Connective tissue that stores fat, provides insulation, and cushions organs.
• Cartilage: Flexible connective tissue that supports structures such as the nose, ear, and joints.
• Bone: Hard connective tissue forming the skeleton, protecting organs, supporting the body, and aiding movement.
• Blood: A fluid connective tissue containing plasma, red blood cells, white blood cells, and platelets.
• Smooth muscle: Involuntary, non-striated muscle found in walls of organs such as the intestine and blood vessels.
• Skeletal muscle: Usually voluntary, striated muscle attached to bones and responsible for body movement.
• Cardiac muscle: Involuntary, striated muscle found only in the heart and adapted for rhythmic contraction.
• Neuron: The functional cell of nervous tissue, consisting of a cell body, dendrites, and an axon.
• Dorsiventral leaf: A leaf with distinct upper and lower surfaces, generally characteristic of dicot plants.
• Isobilateral leaf: A leaf with similar upper and lower surfaces, generally characteristic of monocot plants.
• Dicot stem: A stem generally having vascular bundles arranged in a ring, with cambium commonly present.
• Monocot stem: A stem generally having scattered vascular bundles, usually closed and surrounded by a sclerenchymatous sheath.
• Root anatomy: The internal arrangement of tissues in a root, including epidermis, cortex, endodermis, pericycle, vascular tissue, and pith.

Easily Confused

• Morphology and anatomy: Morphology concerns external form and visible parts, whereas anatomy concerns internal structure and tissue arrangement.
• Tap roots and fibrous roots: Tap roots have one main primary root and lateral roots and are common in dicots; fibrous roots form a cluster of similarly sized roots and are common in monocots.
• Meristematic tissue and permanent tissue: Meristematic cells divide actively; permanent tissue cells have stopped dividing and become specialised.
• Apical and lateral meristems: Apical meristems increase length, whereas lateral meristems increase girth.
• Xylem and phloem: Xylem mainly transports water and minerals; phloem transports organic food.
• Simple and compound epithelium: Simple epithelium has one cell layer and supports absorption, secretion, filtration, or diffusion; compound epithelium has multiple layers and mainly provides protection.
• Collateral and bicollateral vascular bundles: In collateral bundles, xylem and phloem lie on the same radius; in bicollateral bundles, phloem lies on both sides of the xylem.
• Exarch and endarch xylem: In roots, xylem is usually exarch, with protoxylem toward the outside; in stems, it is usually endarch, with protoxylem toward the centre.
• Dicot and monocot roots: Dicot roots commonly have a small or inconspicuous pith, whereas monocot roots commonly have a large, well-developed pith.
• Dicot and monocot stems: Dicot stems generally have vascular bundles in a ring and commonly contain cambium; monocot stems have scattered, usually closed bundles lacking cambium.
• Dorsiventral and isobilateral leaves: Dorsiventral leaves have distinct upper and lower surfaces and are generally dicot; isobilateral leaves have similar surfaces and are generally monocot.
• Smooth, skeletal, and cardiac muscle: Smooth muscle is involuntary and non-striated; skeletal muscle is usually voluntary and striated; cardiac muscle is involuntary, striated, and restricted to the heart.
• Connective and epithelial tissue: Epithelial cells are closely packed and form coverings or linings; connective tissue contains cells embedded in a comparatively abundant extracellular matrix.
• Plant and animal growth: Plant growth occurs through cell division in meristems, whereas animal growth generally involves cell division and enlargement throughout the body during development.

What Gets Asked

• Questions may require the organisation sequence cell → tissue → organ → organ system → organism. Marks are lost by omitting a level or reversing the order.
• Questions may ask for differences between dicot and monocot roots, stems, or leaves. The relevant structural features must be stated specifically: pith size, vascular-bundle arrangement, cambium, venation, mesophyll, or bulliform cells.
• Questions may ask for the functions or identifying features of meristems. The common error is confusing length increase by apical meristem with girth increase by lateral meristem.
• Questions may require classification of plant tissues as simple permanent or complex permanent tissue. Parenchyma, collenchyma, and sclerenchyma are simple permanent tissues, whereas xylem and phloem are complex permanent tissues.
• Questions may ask for the components and functions of xylem, phloem, blood, or nervous tissue. Marks may be lost by assigning water and mineral transport to phloem or organic-food transport to xylem.
• Questions may require identification of animal muscle or epithelial tissue from structural and functional features. Distinctions involving striation, voluntary or involuntary control, number of epithelial layers, and protective or exchange functions must be stated accurately.