section 3: transport in plants - topic 1: xylem & phloem Flashcards
what are the 2 transport systems a plant has?
xylem
phloem
why do plants need transport systems?
- need water, minerals, and sugars to live
- need to get rid of waste substances
- direct diffusion is too slow to meet their metabolic needs (relatively big = high metabolic rate)
need transport systems to move substances to and from individual cells quickly
what are the 2 types go tissue involved in transport in plants?
xylem tissue
phloem tissue
what does xylem tissue transport?
in which direction?
water and mineral ions (in solution)
up the plant - from roots to leaves
what does phloem tissue transport?
in which direction?
mainly sugars (in solution)
both up and down the plant
what is the other function of xylem?
support
where are the xylem and phloem found in roots? function?
they are in the centre to provide support for the root as it pushes through the soil
where are the xylem and phloem found in stems? function?
they are near the outside to provide a sort of ‘scaffolding’ that reduces bending
where are xylem and phloem found in leaves? function?
they make up a network of veins which support the thin leaves
what part of xylem tissue actually transports the water and ions?
xylem vessels
what are xylem vessels?
very long, tube-like structures formed from cells (vessel elements) joined end to end
why can water pass through the middle of xylem vessels easily?
they have no end walls, making an uninterrupted tube
why do the xylem vessels not contain any cytoplasm?
because they are dead
what are the cell walls thickened by?
lignin
what is the function of lignin?
it helps to support the walls and stops them from collapsing inwards
what are the 2 ways in which lignin can be deposited in xylem walls?
spiral
distinct rings
what does lignin being deposited in these patterns allow?
flexibility and prevents the stem from breaking
does the amount of lignin increase or decrease as the cell gets older?
increases
how do water and mineral ions move into and out of the vessels?
through small pits in the walls where there’s no lignin
what is phloem formed from/
cells arranged in tubes
how does phloem differ from xylem?
it is purely a transport tissue - it is not used for support as well
what does phloem tissue contain?
phloem fibres
phloem parenchyma
sieve tube elements
companion cells
what are the most important cell types in phloem for transport?
sieve tube elements and companion cells
what are sieve tube elements?
living cells that form the tube for transporting sugars through the plant
sieve tube elements are joined end to end to form what?
sieve tubes
what are the ‘sieve’ parts of sieve tube elements? function?
they are the end walls
they have lots of holes in them to allow solutes to pass through
what is unusual about sieve tube elements?
they have no nucleus, a very thin cytoplasm, and few organelles
why do sieve tube elements need companion cells?
sieve tube elements can’t survive on their own due to their lack of a nucleus and other organelles
what do companion cells do?
they carry out the living functions for both themselves and their sieve cells
for example, they provide the energy for the active transport of solutes
