Transport in Plants Flashcards
Why do multicellular plants require transport systems for transport of substances?
Multicellular plants have small surface area: volume ratio. Cells requiring substances are at a distance from site production and this distance would be too great for diffusion, making the diffusion rate very slow thus the need for transport systems.
What is the xylem?
A non-living, heavily lignified plant transport vessel responsible for the transport of water and mineral from the roots to the shoots and leaves.
What is the phloem?
A living plant transport vessel responsible for the transport of assimilates to all parts of the plant. The phloem consists of sieve tube elements and companion cells.
Where are xylem and phloem located in transverse section of roots, stems and leaves?
In roots and stems, xylem is located in the inner part with the phloem surrounding it. In leaves, xylem is located on the other part with the phloem on the inside.
State what is meant by the term ‘vascular bundles’ in plants.
It consists of two transport vessels, the xylem and the phloem.
Why are the vascular bundles in the stem situated towards the outside?
To provide support or strength to the plant.
Describe the arrangement of the vascular bundle in dicotyledonous roots.
Xylem vessels arranged in an X shape in centre of vascular bundle.
X shape of xylem vessels is surrounded by endodermis, an outer layer of cells which supply xylem vessels with water.
Presence of inner layer of meristem cells known as the pericycle.
Describe the arrangement of the vascular bundle in dicotyledonous stems.
Xylem located on inside to provide support to stem.
Phloem found on outside of vascular bundle.
Layer of cambium between xylem and phloem.
Cortex found between vascular bundle and epidermis.
Describe the arrangement of the vascular bundle in dicotyledonous leaves.
Vascular bundles form the midrib and veins of a leaf.
What is the function of xylem vessel elements?
Transports water and dissolved minerals from the roots to the rest of the plant and also provides physical support.
Mention three adaptations of xylem vessels for transport of water.
- Walls are thickened with lignin, which provides support to the plant.
- Cells are dead and empty, which reduces resistance to flow of water.
- Contains no end walls which allows it to form elongated hollow tubes which reduces resistance to flow of water.
What is the function of phloem sieve tube elements?
Transports sugars up and down the plant.
Mention three adaptations of phloem sieve tubes for transport of assimilates.
- Contains sieve pores for easy flow of sucrose from sieve tube element to sieve tube element.
- Contains a sieve plate which may prevent sieve tubes from bursting.
- Has little cell contents to reduce resistance to flow of sucrose.
Mention three differences between a xylem vessel and a phloem sieve tube.
- Xylem vessels have no cytoplasm, however phloem sieve tubes have a very small amount of cytoplasm.
- Xylem vessels contain lignin, phloem sieve tubes do not.
- Xylem vessels contain no sieve pores, phloem sieve do.
Describe the transport of water from the root through the xylem.
- Evaporation of water at walls of mesophyll cells.
- Diffusion of water vapour through the stomata out of the leaf.
- Loss of water creates hydrostatic pressure through the stem, causing a column of water to move up by transpiration pull.
- Due to strong cohesive forces of attraction between water molecules (hydrogen bonding) and adhesive force between water molecules and walls of xylem.
