Plant Transport

xylem vessels

Hollow thick-walled tubes which transport water through flowering plants.

transpiration

The main force that pulls water through the xylem vessels in the stem of a plant is the evaporation of water from leaves through stomata.

stomata

Tiny pores which guard cells control the opening and closing of. If the stomata are open, water vapour molecules diffused out of the air spaces into the surrounding air.

cohesion

Attraction between molecules of the same type - how water molecules form hydrogen bonds between one another and hence tend to stick together.

transpiration pull

How a column of water is pulled up the xylem as a result of transpiration.

cohesion-tension theory

The main factor that is responsible for the movement of water up the xylem, from the roots to the leaves. Transpiration pull puts the xylem under tension (there is negative pressure within the xylem) and because of the cohesive nature of water (due to hydrogen bonds between water molecules) there is a continuous stream of water being pulled across the mesophyll cells and up the xylem.

potometer

A piece of apparatus which enables the rate of water loss in a plant to be measured.

phloem

The tissue which transports biological molecules in flowering plants. It is made up of sieve tube elements, long thin structures arranged end to end. Their end walls are perforated to form sieve plates. Associated with the sieve tube elements are cells called companion cells.

translocation

The process by which organic molecules and some mineral ions are transported from one part of a plant to another.

sieve tube element

These are living, tubular cells that are connected end to end. The end cell walls have perforations in them to make sieve plates. The cytoplasm is present but in small amounts and in a layer next to the cell wall. It lacks a nucleus and most organelles so there is more space for solutes to move. The cell walls are made of cellulose so solutes can move laterally as well as vertically. Next to each sieve tube element is a companion cell.

companion cell

Since the sieve tube element lacks organelles, the companion cell with its nucleus, mitochondria, ribosomes, enzymes etc., controls the movement of solutes and provides ATP for active transport in the sieve tube element. Strands of cytoplasm called plasmodesmata connect the sieve tube element and companion cell.

mass flow theory

The bulk movement of a substance through a given channel or area in a specified time. Sucrose is transferred into sieve elements from photosynthesising tissue and there can be mass flow of sucrose solution down a hydrostatic gradient in sieve tubes (caused by active transport of sucrose into sieve tubes at the source and out of sieve tubes at the sink, and osmosis - movement of water into sieve tubes near source and out of sieve tubes near sink).

ringing

An experiment when a section of outer layers (protective layer and phloem) is removed around the complete circumference of a woody stem while it is still attached to the rest of the plant. This results in the region of the stem immediately above the missing ring of tissue swelling because the sugars of the phloem accumulate above the ring and it leads to tissues dying below the ring because of the interruption of flow of sugars to this region. It shows that the phloem is responsible for translocating sugars.

tracer

Radioactive isotopes can be used to trace the movement of substances in plants. 14CO2 is used so plants incorporate this isotope into the sugars produced during photosynthesis. These radioactive sugars can then be traced as they move within the plant using autoradiography. This shows that sugars are found where phloem tissue is in the stem.