The Transport of Substances in Plants

Based on the Integrated Curriculum for Secondary Schools. KBSM.
Biology Form 5, Chapter 1-Transport,
1.7 The Transport of Substances in Plants

You must concentrate on these points:
☑ State the necessity for transport of substances in plants
☑ Identify the vascular tissue in stem, root and leaf.
☑ State the role of vascular tissue in the transport of substances
☑ Describe the structure of the vascular tissue
☑ Relate the structure of xylem to transport
☑ Relate the structure of phloem to transport
☑ Predict the effect of removing a ring of phloem tissue from a plant.

The necessity for transport of substances in plant? (answered by Pounds, thanks! x )
a) transport water and mineral salts absorbed from roots.
◎ water-solvent for biochemical reaction & reactant for cell metabolism (light reaction: photosynthesis)
◎ mineral salts / ions - synthesis chlorophyll, for healthy plant growth and development
b) be able to carry out photosynthesis. 
c) be able to synthesis organic substances.
d) transport carbohydrates (food) produced by leaves to stem and roots for respiration, growth and storage.

The problem that could be faced by plant in transporting substances and how it is overcome in plants?
Problems: 

◎ The roots absorb water and mineral salts which are transported upwards to the leaves and other parts of the plant several metres up.
◎ Plants are unable to pump necessary substances through great distances
◎ Sometimes requiring the need to defy gravity.
How is it overcome?
◎ Plants use a combination of root pressure, capillary action and transpiration to provide enough force to transport water to the shoot.

Transport in plant is provided by the vascular tissues. 

2 types of vascular tissues: xylem & phloem.

Vascular tissue in stem, root and leaf

is found in:-

◎ ROOTS

◎ STEMS

◎ LEAVES

Xylem

★ Transports water and dissolved mineral salts (absorbed in the roots, up the stem and to the leaves)

★ (in woody plants), provide mechanical support to the plant.

Phloem

★ Transport organic substances downwards (from the leaves to the storage organs), 

★ and also upwards (from the storage organs) such as the roots to the growing regions such as the buds.

THE STEM has an:

★ epidermis layer that helps maintain the shape of the stem.

✐ (in young plants) epidermis cells may secrete a waterproof cuticle.

✐ (in older plants) the epidermis may be absent, replaced by bark.

Just inside the epidermis is:

★ cortex layer 

✐ made up of parenchyma cells .

✐ which provide support and flexibility to the stem.

Inner part of stem (central region) consist of:

★ pith & vascular tissues

★ pith is: 

✐ used for food storage in young plants.

✐ may be absent in older plants.

In dicotyledonous plants, 

✿ the vascular tissues of the stem → grouped together → form vascular bundles.

✿ vascular bundles are arranged in a ring around the pith, the central region.

✿ in each bundle, the xylem is found towards the inside of the stem, phloem is found towards the outside.

✿ cambium ( a tissue) is found between xylem and phloem.

✿ cambium cells can divide resulting in an increase in the radius of the stem.

In monocotyledonous plants,

✿ the vascular bundles are scattered throughout the stem.

THE ROOTS

The cross section of a dicotyledonous root is shown in photograph below.
(Photograph found on: http://sci.waikato.ac.nz/farm/images/dicot%20stem_labelled_web.png)

★ The outermost layer is the epidermis.

✐ does not have waxy cuticles.

★ In one region of the root, specialised epidermal cells grow outwards to form root hairs.

✐ increase the surface area for water absorption.

✐ a single plant may have more than 10 million root hairs.

★ The region next to the epidermis is called the cortex. 

✐ made up of parenchyma cells ( which may store starch grains)

★ Located immediately after the cortex is a single layer of cells called the endodermis.

★ Inside the endodermis is the pericycle. 

✐ consists of sclerenchyma tissue which provides mechanical support for the root.

★ In roots, the vascular tissues are located in vascular cylinder.

✐  consists of the vascular tissues (xylem and phloem) and the pericycle.

✐  the vascular tissues in roots are continuous with the vascular tissues of stems.

✐  xylem radiates from the centre of the vascular cylinder, forming a star shape.

✐  phloem fills the area between the xylem.

The cross section of a monocotyledonous root is shown below

★ vascular cylinder has a central core called the pith.
✐ pith contains parenchyma cells
✐ vascular tissues form a ring around the pith with the xylem tissues alternating with the phloem tissues. 

THE LEAF

★ consists of a board portion called the blade

★ blade is connected to the stem by a leaf stalk called the petiole.

★ inside the petiole are vascular tissues of xylem & phloem that are continuous with those in the stem, root and blade.

★ the leaf blade contains leaf veins. 

★ vascular tissues are found in the leaf veins.

★ xylem forms upper part of a vascular bundle in the leaf

★ phloem forms lower part of the vascular in the leaf

The structure of xylem in relation to transport.

Xylem contains 4 types of cells:-

★  xylem vessels,

★  tracheids,

★  parenchyma (to store food substances) and 

★  fibres (to provide support) ( a type of sclerenchyma)

✐ xylem vessels and tracheids are water-conducting cells. 

✐ they are elongated cells arranged end to end.

Tips: make sure you also know the cross sections an longitudinal sections of xylem.

During development, 

✐ the walls of xylem vessels and tracheids are thickened with lignin deposits, making them woody and impermeable.

✐ mature xylem vessels and tracheids are perforated by a series of holes called pits (allow water and mineral salts to pass sideways between the cells)

Difference between xylem vessels and tracheids. 

✐ Tracheids are longer and have a smaller diameter compared to xylem vessels.

✐ Tracheids are pointed at the ends. 

✐ The end walls break down in pits that allow water to pass from cell to cell.

✐ The end walls of xylem vessels are open (so that the cells join end to form a continuous hollow tube)

✐ this allows water to flow upwards continuously. 

✐ The cell walls are thickened with lignin ( makes the xylem vessels strong, do not collapse under the tension created by the upward pull of water during transpiration) 

The structure of phloem in relation to transport.

Phloem is composed by 4 types of cells:-

★ Sieve Tube, 

✐ organic substances are transported along the sieve tube, 

✐ cylindrical column of long cells arranged end to end, 

✐ is a living cell, no nucleus and its cytoplasm is pushed to the sides of the cell when mature, 

✐ end walls of each cell are perforated by pores to form sieve plates, 

✐ each sieve tube cell is kept alive and supported in their function by one or more companion cells

★ Companion Cell, 

✐ is a normal cell with a nucleus and a large number of mitochondria, 

✐ indicating that it is very active metabolically

Tips: Check out the longitudinal sections of phloem showing sieve tubes and companion cells too.

★ Parenchyme, 

✐ to store food substances

★ fibres. 

✐ to provide provide support

What is the effect of removing a ring of phloem tissue from a plant? ( Bark Ringing) 

Aim: To carry out bark ringing to show the role of phloem in a plant.
Materials: A pot containing two woody plants (such as hibiscus) and petroleum jelly
Apparatus: Knife and ruler
Procedure:

1. A complete ring of bark including the phloem and cambium is removed from one of the hibiscus stems. The ring is about 4 cm wide and about 15 cm long from the ground.
2. The exposed xylem is covered with a layer of petroleum jelly.
3. The plants are watered regularly.
4. After four weeks, the parts above and below the ring are observed. 

Observation: 

Discussion:

1. Bark ringing is the removal of ring of tissue external to the xylem from around the trunk of a woody plant.
2. The ring is covered with petroleum jelly to prevent it from drying up.
3. Complete ringing affects the transport of organic substances from the leaves to the roots due to the roots due to the disconnection of phloem at the ring.
4. After a few weeks, there is swelling above the ring and reduced growth below the ring.
5. Prediction: Gradually the roots will die because they cannot get food from the upper part of the plant. When the roots die, there will be no absorption of water and mineral salts. Eventually, the whole plant dies.

Conclusion: Phloem transports organic substances manufactured in the leaves to the roots.

Questions:

Get yourself a piece of paper and ready to answer these questions.

You can get the answers from the notes above. :)

1. What is meant by the term vascular system in a plant?

2. Draw the cross sections of dicot stem and dicot root.

3. Predict the effect of removing a ring of phloem tissue from a woody plant.

4. How does the arrangement vascular bundles in dicotyledonous stems differ from the monocotyledonous stems?

5. State the main function of xylem. What are the common cells found in xylem and phloem tissues?

6. State one similarity between the phloem sieve tube and the xylem vessel.