Download Xylem and Phloem

Xylem and Phloem
Things we know:
• Plants need water, minerals, sugars, but they also
need to get rid of waste
• Multicellular = small SA:V
• Diffusion is too slow
• TF they need a transport system!
The vascular bundle
• Vascular tissue is distributed throughout the plant
• Vascular tissue is made up of xylem and phloem
tissue
• As well as the transport of substances the vascular
bundle has structural support purposes
Transpiration System
• The movement of water molecules and dissolved
minerals ions
• Xylem vessels
• Passive process
Translocation System
• The movement of sugars (Sucrose) & amino acids
• Phloem vessel – sieve & companion cells
• Active process
Xylem
• The X on the root diagram, always on the inside strongest
• Transports water and mineral ions
Phloem
• On the outside
• Transports dissolved substances e.g. sugar
• Hint: ‘f’ sound – phloem = food
Where?
All parts of the plant!
• In the root they are at the
centre to provide support
as it pushes through the
soil
• In other parts of the plant
(e.g. stem) they are found
around the edges –
scaffolding
• In the leaf we see them as
veins – still supporting!
Xylem
• Continuous tubes made of cells
called vessel elements.
• No end walls on cells – water and
mineral ions can flow (1D!)
• from the root
• DEAD cells = no cytoplasm
• Lignin spirals INSIDE the walls –
stops tube collapsing inwards
• Older cells have more lignin
• Water and ions move in through
pits where there is no lignin (like
pores)
• PASSIVE TRANSPORT
Root pressure – passive transport
• Water in xylem flows from root to tip
• Osmotic pressure causes water to rise up the plant at
night
• Mineral ions build up in the roots, leading to a decrease in
water potential
• This leads to water entering the
root from the soil by osmosis
• The pressure of this water pushing
on the rigid xylem cells causes the
water to rise up the stem
Fun fact: root pressure can raise
water up to 7m!
Phloem
• Also in tubes
• Not used for support – no
lignin
• Solutes flow in both
directions
• Two important parts for
solute transport: sieve
tube elements and
companion cells
• ACTIVE TRANSPORT
Phloem tissues used for transport
Sieve tube elements
• Living cells that make up the tubes
• Have holes on the end walls for solutes to pass through
• No nucleus, very little cytoplasm, and few organelles – cannot
live on their own!
So they have…
Companion cells
• One for every sieve tube
element
• Carry out the functions for
both cells to provide energy
for active transport of
solutes
Pressure Flow (Hypothesis) –
active transport
Sugars travel from areas of high water potential, called sources
(leaves), to regions of low water potential, called sinks (growing
tissues).
1. Sugar -> sieve tube elements - active transport
2. Water -> sieve elements - osmosis
3.
Water creates turgor pressure in the sieve elements, which
forces the sugars and fluids down the phloem tubes toward the
sinks
4. At the sinks, the sugars are actively removed from the phloem
and water follows osmotically
5. High water potential and low turgor pressure are restored,
pressure flow can continue!
Exam Question
• (b) hint: think about the
structure of the xylem cells
Mark Scheme