B8: Nutrition and Transport in Flowering Plants

Use a light microscope or a supplied micrograph to identify tissues in transverse sections of a dicot leaf and herbaceous stem. In a dicot leaf section, the upper epidermis and transparent cuticle admit light, chloroplast-rich palisade mesophyll carries out much photosynthesis, spongy mesophyll air spaces support gas exchange, and guard cells control stomata. Vascular bundles contain transport tissues.

In a leaf vein, xylem is usually nearer the upper surface and phloem nearer the lower surface. In a herbaceous stem vascular bundle, xylem lies towards the centre and phloem towards the outside. Identify tissues from their position and visible structure before inferring function. Xylem moves water and mineral ions, while phloem moves food. A root hair cell has a long extension for large surface area, a thin wall for a short uptake path, and many mitochondria to support active ion uptake.

Chlorophyll absorbs light energy and converts it to chemical energy used to form carbohydrates: $\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}$. Most life depends on photosynthesis for food energy and oxygen.

Plants use the carbohydrates in respiration, convert them into storage substances, and build materials for growth. Consumers depend directly or indirectly on this captured chemical energy.

Carbon dioxide diffuses through stomata to mesophyll cells. Investigate light intensity, carbon dioxide concentration, or temperature by changing one factor, controlling the others, and measuring oxygen production or another justified rate indicator.

Transpiration is water-vapour loss through stomata. Evaporation from leaves and continuing water loss create a transpiration pull that draws water through xylem. Higher temperature gives water molecules more kinetic energy and increases evaporation. Lower humidity makes the water-vapour concentration difference between leaf and air steeper. Greater light intensity commonly opens stomata for photosynthesis, increasing the diffusion pathway for water vapour. These trends apply when other factors are controlled and the plant is not water-stressed.

Investigate one environmental factor with a leafy shoot in a potometer, allow the apparatus to equilibrate, measure bubble movement over a fixed time, and keep the other factors and leaf area constant. A potometer estimates transpiration from water uptake rather than measuring water-vapour loss directly. Wilting occurs when water loss exceeds uptake and cells lose turgor. Translocation is the movement of food, mainly sucrose, through phloem from producing or storage tissues to tissues that use or store it.