Plant tissues, organs and transport

Plants don't move, but they have an extensive transport system to move water, minerals and sugars around. Two key tissues do almost all the work.

The leaf as an organ

A typical dicot leaf in cross-section, from top to bottom:

• Waxy cuticle — waterproof; reduces evaporation from the upper surface.
• Upper epidermis — transparent, lets light through.
• Palisade mesophyll — tall cells packed with chloroplasts; main site of photosynthesis.
• Spongy mesophyll — irregular cells with air spaces for gas exchange.
• Lower epidermis — contains stomata (pores) flanked by guard cells.
• Veins with xylem (top, water in) and phloem (bottom, sugars out).

Each layer is its own tissue type cooperating to make a leaf an organ for photosynthesis and gas exchange.

Xylem and phloem

Xylem — hollow tubes of dead cells (with no end walls and lignified spiral thickening for support). Carry water and dissolved mineral ions UP the plant from roots to leaves. Movement is in one direction.

Phloem — living cells (sieve tubes with companion cells). Carry dissolved sugars (sucrose) and amino acids from leaves to wherever they're needed (growing tips, storage organs like potato tubers, fruits) — this is translocation. Movement is in BOTH directions.

A handy mnemonic: Xylem like an Xpress lift to the top; phloem moves food.

Transpiration

Transpiration is the loss of water vapour from a plant, mostly through stomata in the leaves. The driving sequence: water evaporates from spongy mesophyll surfaces into the air spaces, diffuses out through stomata, and this loss pulls more water up the xylem from roots — known as the transpiration stream.

Factors that increase transpiration:

• High temperature — water molecules have more KE, evaporate faster
• Low humidity — bigger water-vapour gradient leaf → air
• Wind / air movement — sweeps water vapour away, keeps gradient steep
• High light intensity — stomata open wider for photosynthesis

Plants can be measured for transpiration using a potometer — measures water uptake (good proxy when most is lost as vapour).

Stomata and guard cells

A stoma is a pore; it's controlled by a pair of guard cells. In light, guard cells take in water by osmosis, become turgid, and bend apart (their inner walls are thicker) — opening the pore. This lets in CO₂ for photosynthesis and lets out water vapour. In drought / darkness, guard cells lose water, become flaccid, and the pore closes.

There's a trade-off: stomata must open enough for CO₂ but not so wide that the plant wilts.

Translocation (HT-friendly)

Sugars made in leaves (sources) move in phloem to where they're used (sinks): roots, fruits, growing tips. Movement requires energy from respiration (active loading into sieve tubes) — so phloem cells need companion cells with many mitochondria.

Links

Builds on B1.3 (osmosis = the engine of stomatal opening), and prepares for B4.1 (photosynthesis demand for CO₂).