Scaling up Study Guide

Plants require water and mineral ions for survival and growth. These are absorbed from the soil by root hair cells. Root hair cells are specialised plant cells, found on the surface of roots, that have a distinctive structure perfectly adapted for this role. Each root hair cell has a long, thin extension that dramatically increases its surface area. This large surface area maximises the contact with the soil, allowing for efficient absorption.

Water is absorbed by osmosis. The cytoplasm of root hair cells has a higher concentration of solutes (dissolved substances) compared to the soil water, creating a water potential gradient. Water molecules move from an area of higher water potential (the soil) to an area of lower water potential (inside the root hair cell) across the partially permeable cell membrane. This passive process does not require energy from the plant.

Mineral ions, such as nitrates and phosphates, are also essential for plant growth. The concentration of these ions in the soil is often lower than inside the root hair cells. Therefore, absorbing mineral ions against their concentration gradient requires active transport. This process uses energy, released from respiration, to move specific mineral ions from the soil into the root hair cells. The root hair extensions provide ample surface area for the protein carriers embedded in the cell membrane that are responsible for this active transport, ensuring the plant can acquire sufficient nutrients even when scarce in the soil.

Plants require efficient transport systems to move water, minerals, and sugars throughout their body. This is achieved by two specialised vascular tissues: xylem and phloem. Both tissues are adapted in their structure to perform their specific functions effectively. Xylem, primarily responsible for transporting water and dissolved mineral ions from the roots to the rest of the plant, consists of dead cells forming continuous, hollow tubes. The cell walls are thickened with lignin, a strong, waterproof substance, which provides structural support to the plant and prevents the collapse of the vessels under the tension created by transpiration. The absence of end walls between xylem cells forms a continuous pipeline, facilitating efficient water flow. Phloem, on the other hand, transports sugars (produced during photosynthesis) from the leaves to other parts of the plant, including growing regions and storage organs. Phloem is composed of living cells, primarily sieve tube elements and companion cells. Sieve tube elements are elongated cells with perforated end walls called sieve plates, forming a continuous tube. These cells have very little cytoplasm and no nucleus, allowing for maximum space for sap flow. Companion cells, adjacent to sieve tube elements, are metabolically active and contain a nucleus and dense cytoplasm, providing energy and controlling the activity of the sieve tube elements. This structural co-adaptation ensures that both water/minerals and sugars are moved efficiently where needed for growth and survival.