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.

Stomata pose a trade-off: they must open to let CO2 in for photosynthesis, but every open stoma also loses water.

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.

Plants do not pump water. The xylem column is pulled by transpiration; the cells are dead. This is passive transport at the whole-plant scale.

Xylem: dead cells, water and minerals, one direction (roots up). Phloem: living cells, sugars and organic solutes, bidirectional (source to sink).

Stomata open and close in response to light, humidity, CO2, and water stress.

Without hydrogen bonding between water molecules (cohesion) and to xylem walls (adhesion), the water column would break under tension.

Transpiration is the cost of opening stomata for photosynthesis, but it also pulls water and minerals up the plant and cools the leaves.