Sec 4 Combined Science (Biology) Nutrition & Transport in Flowering Plants notes

Cut a dicot leaf across and the layers stack like a purpose-built factory. The upper epidermis is a thin, transparent skin — protection without shadow. Beneath it, the palisade mesophyll holds the leaf's densest population of chloroplasts, placed at the lightiest address in the whole plant. The spongy mesophyll below has fewer chloroplasts but plenty of air spaces connected to the outside by stomata — pores mostly on the lower surface, each guarded by two guard cells that open and close the pore. Vascular bundles — the veins — thread through the mesophyll, delivering water by xylem and collecting food by phloem.

Xylem vessels carry water and dissolved mineral salts upwards — from the roots, through the stem, into the leaf veins. Phloem transports food, mainly sucrose, in whichever direction it is needed: downward to the roots, upward to growing buds, laterally too. In a herbaceous dicot stem the two sit side by side in vascular bundles arranged in a ring, with xylem on the inside and phloem on the outside of each bundle; in a leaf vein, the bundle branches through the mesophyll with the same xylem-toward-the-top orientation. Telling them apart in a section: xylem vessels have thick, strong walls and large open lumens; phloem cells are smaller and thin-walled.

Memory hook: xylem is the water main — one-way up; phloem is the food delivery service — any direction, wherever the demand is.

Behind the root tip, epidermal cells sprout root hairs — long, narrow extensions that thread between soil particles. The adaptation logic from Chapter 1 applies: the extension multiplies the surface area in contact with the water film around soil particles, so water and dissolved mineral salts enter faster. The cell wall is thin, shortening the entry path, and the vacuole-rich cytoplasm holds a cell-sap solution that water flows towards by osmosis. No chloroplasts here — underground cells never see light.

Every root hair feeds the xylem: water entering across root hair membranes passes cell to cell into the root's central xylem, joining the upward stream that leaf transpiration will pull.

Chlorophyll, the green pigment inside chloroplasts, absorbs light energy and converts it into chemical energy, which the cell uses to build carbohydrates from carbon dioxide and water. The word equation: carbon dioxide + water → glucose + oxygen, with light energy and chlorophyll written over the arrow. The glucose is used at once in respiration, converted to starch for storage, or built into cellulose and other structures.

Why most life depends on it: photosynthesis is the entry point of energy into nearly every food chain — animals eat plants (or plant-eaters), tracing back to chlorophyll. It is also the source of the atmosphere)s oxygen, the reactant all aerobic respiration needs. Remove photosynthesis and both the food supply and the oxygen supply of the living world collapse.

The raw material route: carbon dioxide diffuses through the stomata into the leaf's air spaces, then dissolves on the moist mesophyll cell walls and diffuses into the cells — reaching the chloroplasts where the water, drawn up the xylem, is waiting.

The classic investigation uses an aquatic plant such as pondweed: count the oxygen bubbles per minute it releases under each condition. Light intensity: moving a lamp closer raises the rate — halve the distance and the bubbles roughly quadruple (light intensity varies inversely with distance squared). Carbon dioxide concentration: more dissolved carbon dioxide raises the rate until something else runs short. Temperature: warmth speeds the reactions, but excessive heat damages the system and the rate falls.

Whichever factor is nearest its minimum is the limiting factor — the one holding the rate back. In dim light, light is limiting; in bright light with stale air, carbon dioxide is limiting.

Transpiration is the loss of water vapour from the stomata. Water evaporates from the moist mesophyll cell walls into the leaf air spaces and diffuses out through open stomata. That evaporation draws a continuous column of water up the xylem — the transpiration pull — which is what lifts water from the roots to the highest leaves without any pump.

Four conditions change the rate. Air movement: wind sweeps vapour away from the stomata, keeping the gradient steep — faster. Temperature: warmer air holds and moves more vapour — faster. Humidity: damp air is already crowded with water vapour, so loss slows — slower. Light intensity: light opens the stomata, so more vapour escapes — faster.

Wilting is the failure mode: when loss by transpiration exceeds uptake by the roots, cells lose water and their firmness, leaves droop. The stomata then tend to close, which actually slows further loss — a built-in brake, at the cost of less carbon dioxide entering and slower photosynthesis.

Translocation is the transport of food — mainly sucrose — in the phloem. Photosynthesis builds sugars in the leaves; translocation carries them to every consumer inside the plant: growing buds, developing fruits, and roots that store them for next season. Unlike the xylem)s one-way water flow, translocation runs both directions along the stem — delivery to wherever the demand is.

Exam precision: water travels in the xylem, food in the phloem. Writing "translocation of water" or "transpiration of sucrose" hands the examiner an easy mark to withhold.