Plants - transport systems

Why do plants need a transport system?

1. Metabolic demands

2. Size

3. Surface area : volume ratio

Why do plants need a transport system - metabolic demands?

• cells containing chlorophyll make glucose and oxygen in photosynthesis

• underground parts don’t photosynthesise = they need waste products removed and glucose and oxygen supplied

• hormones need to be transported to areas where they have an effect

• mineral ions absorbed by roots need to be transported to all cells (in order to make proteins)

Why do plants need a transport system - size?

• some are very large e.g. the coastal redwood tree

• need very efficient systems to move substances up and down the whole plant

Why do plants need a transport system - SA : Vol ratio?

• leaves have large SA : Vol ratio

• whole plant (stems, trunks, roots) have relatively small SA : Vol ratios

• cannot rely on diffusion alone to provide cell with everything they need

Structure of the leaf

What is vascular tissue?

1. Xylem

2. Phloem

Found together throughout the plant in vascular bundles.

Arrangement of vascular bundles in the roots

• Vascular bundles in the middle = withstand tugging forces as plant is blown in the wind

• Xylem tissue is the strongest so is in the centre (X structure)

• Phloem in four separate sections

Arrangement of vascular bundles in the stem

• Vascular bundles are arranged in a ring around the outer part of the stem

• Helps withstand bending forces

• Vascular tissue forms complete rings (tree rings) in older woody stems

Vascular tissues in stems

• Xylem is located on the inside of each vascular bundle

• In between xylem and phloem is a layer of meristem cells called the cambium

• Cambium can divide to form new xylem and phloem

Arrangement of vascular bundles in the leaf

Midrib = main vein carrying the vascular tissue and helping to support the leaf

• smaller branching veins spread through the leaf functioning both in transport and support

Vascular bundle in the leaf

• xylem is located on the top of the phloem

• this only applies to dicotyledonous plants, other plants have different structure

Structure of xylem vessels

• consist of xylem vessel elements: dead cells stacked end-on-end to make a continuous tube

• cells contain no cytoplasm - hollowed out to form a lumen

• the walls of the xylem are strengthened by lignin

Development of xylem

1. living cells become elongated and the cytoplasm produces lignin

2. lignin becomes embedded in cell wall

3. this gradually kills the cells and end walls disintegrate

4. this forms hollow tubes

Lignin

Can form different patterns to provide support whilst still allowing flexibility

• rings (anhular)

• spirals

• broken rings (reticulate)