Why Do Plants Need A Specialised Transport System Flashcards
Why do multicellular plants need transport systems
-Need water, minerals and sugars to live and also need to get rid of waste substances
-Multicellular plants have a low SA:V ratio, however they are relatively big with a high metabolic rate
-Exchanging substances by direct diffusion would be too slow to meet their metabolic needs.
-So plants need transport systems to move substances to and from individual cells quickly
Function of the xylem
-To transport water and mineral ions in solution to all parts of the plant
-To provide structural support
Function of phloem
To transport sugars in solution up and down the plant
How are xylem vessels adapted to their function (4)
-No end walls on cells, making an uninterrupted tube that allows water to pass up through the middle easily
-Cells are dead so they contain no cytoplasm so water can pass through easily
-Cell walls are thickened with lignin-helps support xylem vessels and stop them from collapsing inwards. Amount of lignin increases as cell gets older.
-water and ions move in and out of the cell through small pits where there’s no lignin.
How are the structure of the sieve tube elements in the phloem adapted to their function? (5)
-Living cells that form tubes for transporting solutes through the plant
-Joined end to end to form sieve tubes
-‘sieve’ parts are perforated end walls which allow solutes to pass through
- have no nucleus, few organelles and a thin layer of cytoplasm, to make space for maximum transport of sucrose.
-Cytoplasm of adjacent cells connected through the holes in the sieve plates.
What is the role of companion cells, how is their structure adapted to their function?
-Lack of nucleus and few organelles in sieve tube elements mean they are not self sufficient. Companion cells carry out the living functions of both themselves and their sieve cells, e.g. provides energy for active transport of solutes.
-have lots of mitochondria, a large nucleus and a dense cytoplasm.
Mass flow (source)
-Solutes are loaded into the phloem via active transport.
This results in a low water potential in the sieve tube elements.
-water from the xylem and the companion cells diffuse into the sieve tubes via osmosis, resulting in a high pressure inside the sieve tubes at the source end of the phloem.
Mass flow (sink)
-Solutes load into the companion cells at the source end of the phloem.
-As water potential is higher in the sieve tubes it diffuses out back into the xylem and the companion cells via osmosis.
-This results in a low pressure at the sink end of the phloem m. There is a pressure gradient. Source end has higher pressure and sink end has low pressure.
-Solutes move down their pressure gradient from the source into the sink.
