Transport systems in plants 9.1 Flashcards
Why do plants need transport systems?
They have a big metabolic demand, their size and their SA:V ratio
Metabolic Demands
Plants produce their own glucose and oxygen by photosynthesis in the leaves and green parts of the plant. They need to transport this to places that don’t photosynthesize as well as transporting water and mineral ions.
Size
Some plants can be small, but some others are very big and therefore there is too much distance for diffusion to be used as the substances need to travel a long distance which would best be supported by a transport system.
SA : V ratio
Some parts of the plant like the leaves have a very big surface area to volume ratio, however if you look at the whole plant, you will see that the SA:V is still very small.
Dicotyledonous plants
These are plants that produce seeds with two cotyledons in them. These are organs that are used as food stores for the growing embryo.
Herbaceous Dicotyledons
These are plants that have soft tissue and a relatively short life cycle e.g. house plant
Woody (Arborescent) Dicotyledons
These are plants that are made of strong lignified tissues and have a long life cycle e.g. trees
What is the transport system called?
It’s the vascular system which is many vessels running through the leaves, stems and roots. It mainly consists of xylem and phloem vessels.
Cross section of a stem
The vascular bundles are found around the edge with the parenchyma tissue in the middle. This helps to reinforce the structure and keep it strong. The phloem is on the outside of the bundle, the xylem is on the inside. There is also a cortex and epidermis surrounding the bundle
Cross section of a root
The vascular bundles are found right in the middle to keep the root strong for it to withstand tugging from the wind etc.. It has an exodermis, epidermis and cortex on the outside. The vascular bundle is surrounded by an endodermis and has the xylem in the middle like a cross, with phloem surrounding it.
Cross section of a leaf
Leaves have a midrib which is the main vein of the plant which is where the vascular bundle lies. This helps maintain the structure of the leaf and keep it strong. The phloem is on the outside and the xylem is on the inside. It is mainly made of mesophyll palisade cells apart from that.
Xylem Structure
It’s a non living tissue and it’s made up of long hollow tubes with no cytoplasm that is made up of dead cells that are fused together with their inner cell walls destroyed. It has lignified walls and a long spiral of lignin lining it to reinforce it’s structure. It also has bordered pits which are little gaps in the tube where there is no lignin and water can flow out of the xylem. There are parenchyma packs that surround it to give it a food store and also contain tannin deposits which has a bitter taste so stops predators eating it.
Xylem Function
Transports water, minerals and ions from the roots to the shoots and tips. This means it only has one direction of flow. It is also used to keep the plants structure solid and to keep it strong
Phloem Structure
Phloem is a living tissue that is composed of living cells. These cells are called sieve tube elements and are all lined up end-to-end and form a long hollow structure. The cell walls are not lignified like the xylem and the younger sieve tube elements will have a nucleus etc… The cell walls in the middle of the phloem will start perforating, forming gaps in between so they are now called sieve plates. They allow substances to flow through, but as the phloem matures, and the sieve plates become bigger, the nucleus and organelles will leave, making it hollow. They can also contain supporting tissue and cells with really thick cell walls.
Phloem Function
They transport glucose and other substances needed as organic solutes. These will help the plant respire and synthesize more molecules. The substances travel from the leaves where they are made by photosynthesis, to the rest of the cells in the plant, the flow can go both ways.
