Question 1 of 8: Plant Cell Wall, Pigments, Turgor Pressure, and Cereal Grain Structure

Question 1: Plant Cell Wall, Pigments, Turgor Pressure, and Cereal Grain Structure (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Composition of plant cell walls. The primary structural component is cellulose, a linear polymer of β-1,4-linked glucose units bundled into microfibrils that provide tensile strength (analogous to reinforcing fibres in a composite). These microfibrils are embedded in a hydrated matrix of hemicellulose (branched polysaccharides that cross-link cellulose microfibrils) and pectin (highly hydrophilic polysaccharide, concentrated in the middle lamella that cements adjacent cells together). Mature or woody tissue additionally deposits a secondary wall inside the primary wall, often impregnated with lignin, a rigid phenolic polymer that cements the fibrils and greatly increases compressive strength and resistance to enzymatic/microbial attack. Small amounts of structural glycoproteins (e.g. extensin) are also present. Because this composite wall is essentially inextensible once fully formed, it is what converts the cell's osmotic water uptake into hydrostatic (turgor) pressure instead of unlimited swelling and lysis.

(b) Definition of pigment; why leaves are green. A pigment is a molecule that selectively absorbs light in the visible spectrum, so that only the non-absorbed (reflected/transmitted) wavelengths reach the eye, giving the material its characteristic colour. Plant leaves owe their colour mainly to chlorophyll a and b, housed in the chloroplast's thylakoid membranes. Chlorophyll's conjugated porphyrin ring strongly absorbs light in the blue (≈430–450 nm) and red (≈640–660 nm) regions of the spectrum to drive photosynthesis, but absorbs only weakly in the middle (green, ≈500–560 nm) region — that unabsorbed green light is reflected/transmitted back to the observer, which is why healthy leaves appear green. (Accessory pigments — carotenoids, xanthophylls — absorb elsewhere and pass their captured energy to chlorophyll; they become visually dominant only when chlorophyll degrades, e.g. in autumn.)

(c) Turgor pressure and plant tissue rigidity. Turgor pressure is the internal hydrostatic pressure that builds up inside a plant cell as water enters by osmosis (driven by the cell's higher solute concentration) and pushes the protoplast outward against the rigid, inextensible cell wall. Because the wall resists this outward push, the wall pushes back on the protoplast with an equal and opposite pressure — the cell becomes a small pressurised vessel. At the tissue level, thousands of such pressurised cells packed together behave like a stack of inflated balloons in a box: each cell's turgor pressure keeps its neighbours from collapsing, so the tissue as a whole is firm and rigid only while turgor is maintained. When water is lost (wilting, dehydration, excessive transpiration) turgor pressure falls toward zero, the protoplast pulls away from the wall (plasmolysis in the extreme case), and the tissue becomes limp/flaccid even though the cell wall itself is undamaged — i.e. rigidity is a turgor-pressure effect layered on top of the wall's structural stiffness, not the wall's property alone.

(d) Structure of a wheat grain (caryopsis). A wheat/corn kernel is a dry, single-seeded fruit with three principal anatomical regions:

1. Pericarp (bran). The outermost layers, derived from the fused fruit wall and seed coat; fibrous, rich in cellulose, hemicellulose, and lignin, and provides the main mechanical protection for the kernel.
2. Endosperm. The bulk (≈80–85% by mass) of the kernel, providing stored energy for germination. Its outermost single cell layer, the aleurone layer, is metabolically active and rich in protein and enzymes (it synthesises α-amylase during germination); the inner starchy endosperm is mostly starch granules embedded in a protein matrix and is the fraction milled into white flour.
3. Germ (embryo). A small, oil- and protein-rich structure at the base of the kernel containing the embryonic root (radicle) and shoot (plumule); it is the living, genetically complete part of the seed.