Quick questions on Plant cells, tissues and water transport: VCE Biology Unit 1

Plants are multicellular eukaryotes. Their cells specialise into tissues that work together as organs (roots, stems, leaves, flowers). The four main plant tissue types:

Root hair cells. Found in the root epidermis. Each cell has a long thin extension (the root hair) that vastly increases surface area for water and mineral uptake. Root hairs absorb water by osmosis because the soil solution has a lower solute concentration than the root cell cytoplasm.

Water moves from soil to atmosphere as a continuous column.

Phloem transports sucrose and amino acids from sources (photosynthesising leaves, storage organs in spring) to sinks (growing tips, developing fruits, storage organs in autumn). The pressure-flow hypothesis: sucrose is actively loaded into the sieve tube at the source; water follows by osmosis, raising the pressure; the high pressure pushes the sap through the sieve tubes to the sink, where sucrose is unloaded and water leaves. Phloem transport is bidirectional and requires active loading (so it does need ATP, indirectly).

Found in the root epidermis. Each cell has a long thin extension (the root hair) that vastly increases surface area for water and mineral uptake. Root hairs absorb water by osmosis because the soil solution has a lower solute concentration than the root cell cytoplasm.

Hollow tubes made of dead cells with thickened, lignified walls. Vessel elements are wide and have lost their end walls, forming continuous open tubes. Tracheids are narrower with pitted walls.

Living cells with perforated end walls (sieve plates) that link them into continuous tubes. They have lost most internal organelles; they are kept alive by adjacent companion cells with full nuclei and many mitochondria. Phloem transports sugars (mainly sucrose) and other organic solutes.

Loosely packed parenchyma in the leaf with many chloroplasts, where photosynthesis occurs. Water moves from xylem to mesophyll and evaporates from mesophyll cell walls into the air spaces inside the leaf.

Pairs of crescent-shaped cells flanking each stoma (pore) on the leaf surface. They open and close the stoma by changing turgor pressure, regulating gas exchange and water loss.

Distinguish between xylem and phloem with respect to direction of transport, contents, and cellular composition. [3 marks]

A wheat leaf has stomata that close as soil water potential drops. Describe the role of guard cells in this response, naming one hormone involved. [3 marks]

Refer to a river red gum's water transport. (a) Define transpiration. (b) Explain how the cohesion-tension theory accounts for water rising tens of metres against gravity.