xylem and phloem

why do plants need transport systems

• need substances like water, minerals and sugars

• need to get rid of waste substances

• multicellular so have a small surface area to volume ratio

• have a high metabolic rate

• exchanging substances by direct diffusion would take too long

what does the xylem transport

water and mineral ions up the plant

what does the phloem transport

sugars up and down the plant

root cross section

• xylem is in the centre surrounded by phloem to provide support for the root as it pushes through the soil

stem cross section

• the xylem and phloem are near the outside to provide support for the root as it pushes through the soil

leaf cross section

xylem and phloem make up a network of veins which support the thin leaves

longitudinal cross section of a leaf

How are xylem vessels adapted for transporting water and mineral ions

1. xylem vessels are very long, tube like structures formed from cells joined end to end

2. there are no end walls on these cells, making an uninterrupted tube that allows water to pass up through the middle easily

3. their cells are dead so contain no cytoplasm

4. walls are thickened with a woody substance called lignin which helps to support the xylem vessels and stops them from collapsing inwards. Lignin can be deposited in xylem walls in different ways eg. in a spiral or as distinct rings

5. the amount of lignin increases as the cell gets older

6. water and ions move into and out of the vessels through small pits in the wall where there’s no lignin

what does phloem tissue contain

phloem fibres, phloem parenchyma, sieve tube elements and companion cells

are xylems or phloem used for support

just xylem

sieve tube elements

• living cells that form the tube for transporting solutes through the plant

• they are joined end to end to form sieve tubes

• the ‘sieve’ parts are the end walls, which have lots of holes in them to allow solutes to pass through

• although they are living cells, they have no nucleus, a thin layer of cytoplasm and few organelles

• the cytoplasm of adjacent cells is connected through the holes in the sieve plates

cross section of phloem cells

companion cells

• lack of a nucleus are other organelles in a sieve tube means that they can’t survive on their own so each sieve tube has its own companion cell

• they carrying out the living functions for both themself and their sieve cells. For example, they provide the energy for the active transport of solutes

how to dissect plant cells

• use a scalpel to cut a cross-sections of the stem

• cut the sections as thin as possible

• use tweezers to gently place the cut sections in water until you come to use them. This stops them from drying out

• transfer each section to a dish containing a stain eg, toluidine blue, and leave for one minute. TBO stains the lignin in the walls of the xylem vessels blue-green. This will let you see the position of the xylem vessels and examine their structure

• rinse off the sections in water and mount each one onto a slide