8. Transport in plants

Water and dissolved mineral ions taken up by roots travel in xylem. Sucrose made by photosynthesis and amino acids are transported in phloem to parts of the plant that use or store them.

First identify the organ from its overall tissue pattern. Then locate each vascular bundle and use the inside–outside or upper–lower relationship to distinguish xylem from phloem.

lignified continuous hollow vessel → strong unobstructed pathway → transport of water and mineral ions plus structural support

In a diagram, look for one cell on the outside of a root with a single hair-like extension. The projection is part of the cell, not a separate root.

many long root hairs → larger surface area in contact with soil → more membrane available for uptake at the same time → increased uptake of water and mineral ions

A root system with many root hairs can explore more soil-water contact than a smooth root surface of the same size, increasing the rate and total opportunity for uptake.

The sequence connects uptake at the root surface to delivery inside the leaf. Xylem is the long-distance transport stage between the root cortex and leaf mesophyll.

The stain appears in the xylem of the stem and leaf veins. This shows that water moves through the above-ground parts of the plant in xylem.

Water reaches a leaf as liquid in the xylem but leaves it as water vapour. The loss occurs mainly through stomata in the leaf surface.

Diffusion occurs because the concentration of water vapour is usually higher inside the leaf air spaces than in the surrounding air.

Increasing temperature increases transpiration rate. Increasing wind speed also increases transpiration rate. Plot the measured rate against the changed factor and describe the pattern using the data.

Together, the large mesophyll surface and stomatal exit area allow more water to evaporate and diffuse out at the same time, increasing water-vapour loss.

The pull draws a column of water molecules upwards through the xylem. Forces of attraction between water molecules hold the molecules together, so the pull is transmitted down the continuous column.

water vapour lost from leaves → transpiration pull develops → continuous water column is pulled upward → attraction between water molecules keeps the column together

The highest rate is expected in warm, windy, low-humidity air; the lowest in cool, still, high-humidity air, provided stomata and water supply do not become limiting.

Plant cells lose water, their vacuoles shrink and turgor pressure falls. The cells become flaccid and no longer press firmly against one another, so leaves and non-woody stems droop.

water loss exceeds uptake → cells lose water → turgor pressure falls → cells become flaccid → leaves and stems wilt

Movement may be upwards or downwards in the plant because the location of a source relative to a sink can differ.

Phloem therefore links producing or releasing regions to regions of growth, respiration, reproduction or storage.

For example, a potato tuber can be a source in spring while a new shoot grows, then become a sink later when mature leaves send sucrose back for storage.