Scaling up Study Guide

Plants have specialised transport tissues called xylem and phloem, which are crucial for their survival and growth. The xylem tissue is responsible for transporting water and dissolved mineral ions from the roots to the leaves and other parts of the plant. Its structure is adapted for this function: it consists of dead cells forming continuous, hollow tubes. These tubes are strengthened by rings of lignin, a waterproof substance that prevents the vessels from collapsing under pressure and makes them impermeable to water, ensuring efficient upward transport. The absence of end walls between xylem cells allows for an uninterrupted flow of water. The movement of water in the xylem is primarily driven by transpiration pull, a suction force created by the evaporation of water from the leaves.

In contrast, the phloem tissue transports sugars (produced during photosynthesis in the leaves) to all other parts of the plant where they are needed for energy or storage, such as roots, flowers, and fruits. This process is known as translocation. Phloem is composed of living cells, primarily sieve tubes and companion cells. Sieve tubes are arranged end-to-end, forming long columns. They have perforated end walls called sieve plates, which allow the flow of sap, though they have lost most of their organelles to allow for maximum space for sap flow. Companion cells are located next to sieve tubes and have a full complement of organelles, including a nucleus and many mitochondria. They provide metabolic support to the sieve tube elements, including supplying energy for the active transport of sugars into and out of the sieve tubes. This active transport is vital for maintaining the pressure gradient needed for translocation. The distinct structures of both xylem and phloem allow them to perform their specific roles efficiently, enabling the plant to thrive.

The human circulatory system is a vital organ system responsible for transporting substances throughout the body. It is a closed, double circulatory system, meaning blood is confined within vessels and passes through the heart twice for each complete circuit of the body. This efficiency is crucial for mammals, including humans, to maintain a high metabolic rate and body temperature. The system consists of three main components: the heart, the blood vessels (arteries, veins, and capillaries), and the blood itself. The heart acts as a powerful pump, propelling blood through two distinct loops or circuits. The pulmonary circuit carries deoxygenated blood from the heart to the lungs, where it picks up oxygen and releases carbon dioxide, returning oxygenated blood to the heart. The systemic circuit then pumps this oxygenated blood from the heart to all other parts of the body, delivering oxygen and nutrients, and collecting carbon dioxide and waste products, before returning deoxygenated blood to the heart. This double circulation ensures efficient delivery of oxygen and nutrients to tissues and removal of waste products, supporting the complex needs of the human body.