8. Transport in Plants (Part 1) (1/2)(Cambridge IGCSE Biology 0610 for exams in 2026, 2027 and 2028)

Plants have a sophisticated transport system to move essential nutrients. Water from the soil is vital for stems and leaves, while nutrients produced during photosynthesis in leaves are transported to other areas like roots and stems. The xylem and phloem are the primary transport vessels.

The xylem transports water and mineral ions from the roots to the stem and leaves, also providing structural support. Its structure is adapted for this function: xylem tissues lack cell contents and cross walls, forming a continuous tube for efficient water passage. Their walls are thickened with lignin, preventing collapse and providing strength.

The phloem transports food (sucrose and amino acids) from photosynthesizing leaves to non-photosynthesizing regions like roots and stems. A mnemonic to remember is 'phloem' for 'food'. Xylem and phloem vessels are collectively known as vascular bundles, found throughout the plant's root, stem, and leaves.

Understanding the position of xylem and phloem in root, stem, and leaf cross-sections of non-woody dicotyledonous plants is crucial. In roots, xylem forms an X-shape in the center, encircled by phloem. In stems, xylem is typically on the inside of vascular bundles, with phloem on the outside. In leaves, xylem is usually found above the phloem.

Plants absorb water through their roots, specifically via specialized root hair cells. These cells possess long extensions that greatly increase the surface area for efficient uptake of water by osmosis and mineral ions by active transport, enhancing the rate of absorption.

Water enters the root hair cells from the soil via osmosis, due to a higher water potential in the soil. It then moves into the root cortex cells, travels into the xylem, and ascends the stem to the leaves, eventually reaching the mesophyll cells.

An experiment using celery stalks in dyed water demonstrates the water pathway. Celery placed in plain water serves as a control. Stalks in red and blue food dyes show specific vessels (the xylem) stained, proving that water moves through these vessels and is taken up by the leaves, changing their color.