specialisation and organisation of plant cells into tissues for specific functions in vascular plants, including intake, movement and loss of water

What this dot point is asking

VCAA wants you to describe how plant cells specialise into tissues, especially the vascular tissues (xylem and phloem), and explain how water is taken up, moved and lost by a vascular plant.

The answer

Plant cell specialisation

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

• Dermal tissue (epidermis). A single outer layer that protects the plant and limits water loss. The epidermis on leaves and stems is often covered by a waxy cuticle.
• Ground tissue. The bulk of the plant. Includes parenchyma (storage, photosynthesis), collenchyma (flexible support), and sclerenchyma (rigid support).
• Vascular tissue. Xylem and phloem (see below).
• Meristematic tissue. Regions of undifferentiated dividing cells at root and shoot tips and in the cambium; the source of new cells for growth.

Specialised cells for water movement

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.

Xylem vessels and tracheids. 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. Both transport water and dissolved minerals from roots to leaves.

Phloem sieve tube elements. 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.

Mesophyll cells (leaf). 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.

Guard cells. 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.

Water movement: the cohesion-tension theory

Water moves from soil to atmosphere as a continuous column.

1. Uptake at roots. Water enters root hair cells by osmosis (down a water potential gradient from the soil). It moves across the root cortex by three pathways: the apoplast (through cell walls), the symplast (through cell cytoplasm via plasmodesmata), and the vacuolar pathway. At the endodermis, the Casparian strip (a waxy band in the cell wall) forces water into the cytoplasm, allowing the plant to selectively take up minerals.
2. Loading into xylem. Water and dissolved minerals enter the xylem in the root stele.
3. Movement up the xylem. Water is pulled up by transpiration pull:
   • Water evaporates from the surfaces of mesophyll cells inside the leaf into the air spaces.
   • The water vapour diffuses out through stomata into the atmosphere (transpiration).
   • This evaporation creates negative pressure (tension) at the top of the xylem column.
   • The tension is transmitted down the xylem because water molecules stick to each other (cohesion, via hydrogen bonds) and to the xylem walls (adhesion).
   • The continuous water column is pulled up like a rope.
4. Loss at leaves. Transpiration through the stomata is the final step.

This is the cohesion-tension theory. It is passive: the plant does not pump water. The energy comes from the sun, which drives evaporation.

Stomata, guard cells and trade-offs

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

• Open at midday when light drives photosynthesis. Guard cells take up K+ ions actively, water follows by osmosis, they become turgid and bow apart to open the pore.
• Closed at night when no photosynthesis is needed, conserving water. Guard cells lose K+ and water; they become flaccid and close.
• Closed in drought even during the day, triggered by abscisic acid; this saves water but stops photosynthesis.

Plants in dry environments often have stomata sunken into pits, surrounded by hairs, or only open at night (CAM plants), all to reduce transpiration.

Phloem: not water, but worth knowing alongside xylem

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).

Worked example

On a warm sunny day, a tomato plant opens its stomata. Water evaporates from mesophyll cell walls inside the leaf and diffuses out through the stomata. Negative pressure develops at the top of the xylem column in the leaf; this is transmitted down the xylem to the roots through a continuous water column held together by cohesion. Water enters the root hair cells by osmosis (because the soil has lower solute concentration than the root cytoplasm) and is replaced as fast as it leaves the leaves. On a hot dry day, if soil water runs short, the plant releases abscisic acid; guard cells lose K+ and water, the stomata close, transpiration stops, and the plant survives at the cost of pausing photosynthesis.

Common traps

Saying water is pumped up the plant. 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.

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

Saying stomata are "always open". Stomata open and close in response to light, humidity, CO2, and water stress.

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

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

In one sentence

Plant cells specialise into tissues including root hairs (uptake), xylem (dead, lignified tubes that transport water and minerals upwards by the cohesion-tension theory driven by transpiration through stomata), phloem (living sieve tubes that transport sucrose from source to sink under pressure flow), and stomata with guard cells (which open for photosynthesis and close to conserve water), allowing vascular plants to move water from the soil to the atmosphere as a continuous column.