Scaling up Study Guide

Plants absorb water through their roots, which then moves up through the stem and into the leaves. A significant amount of this water is released into the atmosphere as water vapour through tiny pores called stomata, primarily located on the underside of leaves. This process is known as transpiration. The movement of water through the plant, driven by transpiration, is crucial for transporting essential minerals and maintaining turgor pressure, which keeps the plant rigid. While a potometer directly measures water uptake by a plant shoot, this uptake is closely linked to the rate of water loss by transpiration. Therefore, a potometer serves as a useful tool to investigate how environmental factors affect the rate of transpiration indirectly.

A simple potometer works by measuring the distance an air bubble moves in a capillary tube over a set period. A freshly cut plant shoot is sealed into the potometer, ensuring there are no air leaks, especially around the stem where it connects to the apparatus. The end of the capillary tube is placed in a beaker of water. As the plant transpires, it draws water from the apparatus, causing the air bubble to move along the capillary tube. The speed at which this bubble moves is proportional to the rate of water uptake, and thus, the rate of transpiration. Various environmental factors, such as light intensity, temperature, humidity, and air movement (wind), can affect the rate of transpiration. For example, increased light intensity generally increases transpiration rates because it encourages stomata to open wider for photosynthesis, allowing more water vapour to escape. Higher temperatures increase the kinetic energy of water molecules, leading to faster evaporation. Higher humidity reduces the water potential gradient between the leaf and the atmosphere, slowing down transpiration, while increased air movement carries away humid air, maintaining a steep water potential gradient and thus increasing transpiration. By changing one of these factors at a time while keeping others constant, we can investigate its specific effect on water uptake.

Multicellular organisms are composed of many cells, some of which are located deep within the organism's body. These cells have the same basic needs as single-celled organisms: they require a constant supply of nutrients and oxygen, and they need to remove waste products like carbon dioxide and urea. For very small organisms, the surface area to volume ratio is large enough that essential substances can diffuse directly across the outer surface to all cells, and waste products can diffuse out efficiently. However, as organisms increase in size, their volume increases much faster than their surface area. This leads to a significantly smaller surface area to volume ratio.

Imagine a cube: if you double its side length, its surface area increases by a factor of four, but its volume increases by a factor of eight. This principle applies to living organisms. In larger multicellular organisms, the vast majority of cells are too far from the external environment for simple diffusion to provide an adequate supply of resources or to remove waste effectively. The outer surface alone is not large enough to meet the metabolic demands of all the internal cells. Therefore, these organisms have evolved specialised exchange surfaces, such as lungs, gills, small intestines, and kidneys. These surfaces are adapted to maximise the rate of diffusion: they typically have a very large total surface area, are thin (short diffusion path), have a good blood supply (maintaining steep concentration gradients), and are often moist. Alongside these exchange surfaces, multicellular organisms also possess internal transport systems, such as the circulatory system in animals and the xylem and phloem in plants. These transport systems are crucial for efficiently moving absorbed nutrients and oxygen from the exchange surfaces to all cells in the body, and for carrying waste products from cells to the appropriate exchange organs for excretion. Without these adaptations, large multicellular life as we know it would not be possible.