Sec 4 Biology Plant Nutrition & Transport notes

Slicing a dicotyledonous leaf crosswise reveals a stacked design. A waxy cuticle tops and tails the leaf to limit water loss. Beneath it the transparent upper epidermis lets light straight through to the palisade mesophyll — elongated cells packed with the most chloroplasts, sitting at the light’s front door. Below, the spongy mesophyll cells are loosely packed around air spaces so gases can diffuse freely to every cell. The lower epidermis carries the stomata — pores each flanked by a pair of guard cells that open and close them — and vascular bundles vein through the mesophyll, delivering water and carting away sugar.

Xylem vessels are hollow, dead tubes: the cells lose their contents, and the walls are stiffened with lignin so the pipeline resists collapse. Water and dissolved mineral ions flow upward only — from roots to leaves — pulled by transpiration. Phloem stays alive: sieve tube elements form tubes with perforated end walls, and each is supported by a nucleated companion cell that runs its metabolism. Phloem transports sucrose and other dissolved foods in either direction — from leaves to roots in summer, from storage organs to growing buds in spring.

Water enters at the root hair cells — epidermal cells stretched into a long, thin projection. The projection gives an enormous surface area for absorption and a very short distance across, so water osmoses in swiftly: soil water has a higher water potential than the cell sap, and water always moves from higher to lower water potential. Mineral ions are taken up by active transport, the energy-requiring process from chapter 2.

From the root cortex the water passes into the xylem and climbs in the transpiration stream, drawn up mainly by transpiration pull: as water evaporates from the mesophyll walls into the air spaces and out through open stomata, the escaping molecules tug on the water columns below like a rope — the whole stream is a chain pulled from the top. If the leaf loses water faster than the roots supply it, cells lose turgor and the plant wilts — the soft droop of a thirsty lettuce.

Photosynthesis builds glucose from carbon dioxide and water using light energy trapped by chlorophyll in the chloroplasts:

carbon dioxide + water → (light + chlorophyll) → glucose + oxygen, or in symbols: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The energy in light becomes chemical energy stored in the glucose bonds — the energy that feeds the plant and, through food chains, nearly every living thing. Photosynthesis also maintains the atmosphere’s oxygen and removes carbon dioxide, which is why forests and oceans are called carbon sinks.

The raw materials must reach the mesophyll cells: carbon dioxide enters through a stoma, diffuses through the spongy-mesophyll air spaces, dissolves in the film of moisture on the cell walls, and crosses into the cells to reach the chloroplasts; water arrives continuously from the xylem.

The classic investigation uses a submerged aquatic plant (such as pondweed). Place it in water under a lamp, and count the oxygen bubbles per minute released from the cut stem — or collect the gas and test it with a relighting splint. Move the lamp to change light intensity, keep everything else constant (same plant, same water, same temperature), and let the plant adjust for a minute before each count.

Plot rate against the variable and three patterns appear. Against light intensity the curve rises steeply then plateaus: while rising, light is the limiting factor; along the plateau something else is short — usually carbon dioxide concentration or temperature. Against carbon dioxide the same shape appears, with light or temperature limiting at the top. Against temperature, the rate climbs with the enzyme-driven chemistry to an optimum and then falls as enzymes denature — the chapter 3 signature.

Translocation is the transport of dissolved food, mainly sucrose, in the phloem. Sucrose made in photosynthesising leaves flows to every consumer: growing shoots and roots, developing fruits and seeds, and storage organs like potato tubers — and in spring, stored sugar flows back up to fuel new growth. Translocation studies track these flows (using labelled carbon dioxide fed to leaves, for example) and confirm both the sucrose cargo and the two-way traffic that xylem’s one-way stream can never do.