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

Transpiration is the loss of water vapor from leaves. Water evaporates from mesophyll cells into air spaces and diffuses out through the stomata. This process creates a transpiration pull, drawing water from the soil through roots and xylem to replace lost water. Water movement in xylem is not by osmosis as xylem cells lack living contents and membranes.

Water vapor loss is influenced by the large internal surface area provided by air spaces between mesophyll cells and the size and number of stomata. Water molecules are held together by cohesion, allowing them to be pulled up in a continuous column through the xylem.

A potometer is used to measure the rate of transpiration by observing the movement of an air bubble in a capillary tube. The apparatus must be set up underwater to prevent air bubbles in the plant's xylem and sealed to be airtight. The experiment involves introducing an air bubble, allowing it to move for a set time, and recording the distance traveled. The faster the bubble moves, the greater the rate of water uptake and transpiration. Environmental conditions like temperature and wind speed can be varied to study their effects.

Temperature, wind speed, and humidity significantly affect transpiration. Increased temperature and wind speed increase transpiration due to higher kinetic energy of water molecules and faster removal of water vapor, respectively. Conversely, increased humidity decreases transpiration because it reduces the concentration gradient for diffusion of water vapor from the leaf.

Wilting occurs when a plant loses more water through transpiration than it can absorb from the soil. This causes the plant cells to lose turgor, making the plant soft and droopy as cell walls become flaccid and unable to support the plant structure.

Translocation is the movement of sucrose and amino acids, products of photosynthesis, through phloem tubes to various parts of the plant for respiration and nutrition. Unlike xylem, phloem tubes are made of living cells. Sucrose and amino acids can be transported in different directions depending on the plant's growth stage or time of year.

Sources are parts of plants that release sucrose or amino acids (e.g., leaves during summer), while sinks are parts that use or store them (e.g., roots). During winter, storage organs like roots become sources, supplying nutrients to other parts for respiration. In spring, growing areas are sinks, and storage organs are sources. In summer, leaves are sources, and roots become sinks, storing sucrose as starch.