Transport in Plants
Xylem and phloem appear in the midrib of the leaf, as well as in the leaf veins.
Figure below shows a stem cut across (transversely) and down its length (longitudinally) to show its internal structure.
Like the leaf epidermis, this is a single layer of cells that help to keep the shape of the stem and cuts down the loss of water vapour. Stomata in the epidermis allow the tissues inside to take up oxygen and get rid of carbon dioxide. In woody stems, the epidermis is replaced by bark, which consists of many layers of dead cells.
These are made up of groups of specialised cells that conduct water, dissolved salts and food up or down the stem. The vascular bundles in the roots, stem, leaf, stalks and leaf veins all connect up to form a transport system throughout the entire plant. The two main tissues in the vascular bundles are called xylem and phloem. Food substances travel in the phloem; water and salts travel mainly in the xylem. The cells in each tissue form elongated tubes called vessels (in the xylem) or sieve tubes (in the phloem) and they are surrounded and supported by other cells.
The cells in the xylem that carry water become vessels. A vessel is made up of a series of long cells joined end to end. Once a region of the plant has ceased growing, the end walls of these cells are digested away to form a continuous, fine tube. At the same time, the cell walls are thickened and impregnated with a substance called lingin, which makes the cell wall very strong and impermeable. Since these lignified cell walls prevent the free passage of water and nutrients, the cytoplasm dies. This does not affect the passage of water in the vessels. Xylem also contains many elongated, lignified supporting cells called fibres.
The conducting cells in the phloem remain alive and form sieve tubes. Like vessels, they are formed by vertical column of cells. Perforations appear in the end walls, allowing substances to pass from cell to cell, but the cell wall are not lignified and the cell contents do not die, although they do lose their nuclei. The perforated end walls are called sieve plates.
Phleoem contains supporting cells as well as sieve tubes.
Xylem and phloem
Substances need to be transported for long distances throughout a plant’s body – sugars, for example, are produced in the photosyntheising cells of the leaves and may need to be transported to storage cells in the roots. The water and ions absorbed by the roots may be required by cells at the growing tip (the meristem) of the shoot. These long-distance transport functions are carried out by two specialised plant tissues – the xylem and the phloem. These are tubes running through the plant, collected together in groups the vascular (transport) bundles.
Moving vital substances from sources to sinks
The transport tissues are arranged in the stem and root, to offer:
- the most efficient transport of materials from sources (where they are taken in or made) to sinks (where they are used or stored)
- the most effective support in air (the stem) and soil (the root)
The transport functions of xylem and phloem have been investigated in a number of ways, as shown in the diagram below.
Functions of xylem and phloem
- Xylem vessels carry water and minerals from the roots to the leaves.
- Phloem tubes carry sugar and other organic nutrients made by plant from the leaves to the rest of the plant.
Structure of phloem tissue
This is a long tube that runs alongside the xylem tissue. They are made of long narrow tubes with perforated sieve plates along the length.
The function of the phloem tissue is to transport food nutrients such as glucose and amino acids from the leaves and to all other cells of the plant, this is called translocation.
Unlike the xylem, the phloem tissue is made of columns of living cells, which contains a cytoplasm but no nucleus, and its activities are controlled by a companion cell next to it which has a nucleus, but companion cells have no function in translocation.
Structure of a xylem tissue
Xylem vessels consist of dead cells. They have a thick, strengthened cellulose cell wall with a hollow lumen. The end walls of the cells have disappeared, so a long, open tube is formed. The wall of the xylem vessel contains holes called pits which water enters through.
The xylem vessel is specialised to transport water and dissolved minerals from the root up to all the other parts of the plant, and also to help supporting the stem and strengthening it.
Distribution of xylem and phloem in roots, stems and leaves
In the roots xylem and phloem are in the centre to withstand stretching forces.
In the stems, they are arranged in bundles near the edge to resist compression and bending.
They are grouped into veins and vascular bundles as they pass through leaves.
The positions of xylem and phloem tissues as seen in transverse sections of unthickened, herbaceous, dicotyledonous roots, stems and leaves:
Transport of the products of photosynthesis
Aphids (greenfly) are serious pests of many crops. They can take food meant for the growing regions of plants by inserting their mouthparts (the stylet) into the plant tissues.
If feeding aphids are anaesthetised with carbon dioxide their bodies can be ‘flicked away’ from the plant surface, leaving the style in place. The contents of the phloem, the sap, will slowly leak out of the stylet and can be analysed. The results show that the phloem transports sucrose (sugar), the main product of photosynthesis.
Application: aphids eat themselves to death!
Many insects kill useful insect species as well as pests. Systemic insecticides are sprayed onto the plant and absorbed into the phloem tissues, so they only kill aphids.
- Vascular tissues of the plant consist of xylem vessels and the phloem
- Xylem vessels are elongated hollow tubes that are made of xylem cells linked end to end. Xylem cells are dead at maturity.
- Functions of xylem tissue:
- Conduct water and mineral salts from the roots to the leaves
- Mechanical support
- Adaptations to these functions include:
- Absence of protoplasm and cross-walls which could impede water flow through the lumen (central space)
- Deposition of lignin on the cell walls which strengthens vessel walls, providing support
- The phloem tissue consists of sieve tube elements and companion cells
- Sieve tube elements are elongated thin-walled living cells. They have degenerated protoplasm, which means they lack organelles such as the nucleus, ribosomes and the large central vacuole
- Sieve tube elements are arranged end to end, with porous walls called sieve plates between them
- There is one companion cell closely associated with each sieve tube element. Companion cells contain nuclei, cytoplasm and numerous mitochondria, and are responsible for performing the metabolic functions of the sieve tube elements
- The function of the phloem is to conduct sugars and amino acids from the leaves to other parts of the plant
- Adaptations to this function include:
- Porous sieve plates that allow uninterrupted flow of food substances through the sieve tubes
- Numerous mitochondria in the companion cells that provide energy for them to help load sieve tube members with sugar
Position of vascular tissue in dicotyledonous stems
- In dicotyledonous stems, the vascular bundles are arranged in a ring around a central pith
- Between the ring of vascular tissue and the epidermis is the cortex. The epidermis is covered by waterproof cuticle that minimises water loss in the stem
- Within the vascular bundles, the phloem tissue is found on the side facing the cortex and the xylem on the side facing the pith. Between the xylem and phloem is a layer called the cambium. Cambium cells can differentiate into new xylem and phloem tissues
- Food is stored in the cortex and pith
All organisms respire, therefore all organisms need to exchange gases with the environment.
Unicellular organisms: exchange gases directly through their cell membrane. They can do this because their surface area is large compared to their volume (large SA:Vol ratio). They do not need a circulatory system.
Multicellular organisms: cannot exchange gases directly through their skin. Their surface area is very small compared to their volume (small SA: Vol ratio); therefore they need to have specialised gas exchange organs (e.g. leaf, lung and gill) and a circulatory system.