Transport in plants

How is water transported in plants ?

How is water transported in plants ?

Through the xylem vessels; long continuous columns which also provide structural support to the stem

Explain the cohesion-tension theory

• water lost from leaf by transpiration

• Lowers the water potential of mesophyll cells

• Water is pulled up the xylem creating tension

• Water molecules are cohesive due to hydrogen bonding

• Forming columns of water

• Water sticks to walls of the xylem due to its adhesion

What are the three components of phloem cells :

• sieve tube elements - form a tube to transport sucrose in the dissolved form of sap.

• Companion cells - involved in ATP production for active loading of sucrose into sieve tubes

• Plasmodesmata - gaps between cell walls where the cytoplasm links allowing substances to flow

How does sucrose in the leaf move into the phloem

• Sucrose enters companion cells of the phloem by active transport

• This uses ATP and a diffusion gradient of hydrogen ions which are co-transported

• Sucrose is then actively transported from companion cells into the sieve tube elements through the plasmodesmata

How do phloem cells transport sucrose around the plant

• when sucrose moves into the sieve tube elements the water potential decreases

• Water enters via osmosis from the xylem which increases hydrostatic pressure

• Water moves along the sieve tube towards areas of lower hydrostatic pressure

• Sucrose diffuses to where it is needed

Evidence for mass flow hypothesis of translocation

• sap released when the stem is cut - indicates pressure in the phloem

• Higher sucrose concentration in leaves than in roots - increased sucrose levels in leaves results in increased sucrose levels in the phloem

Evidence against mass flow hypothesis of translocation

• structure of sieve tubes seems to hinder mass flow

• Not all solutes move at the same speed

• Sucrose is delivered at the same rate throughout the plant rather than to areas with lowest sucrose concentration first

How can can ringing experiments be used to investigate transport in pants

• bark and phloem of tree removed leaving the xylem

• The tissues above the missing ring swells due to accumulation of sucrose as tissue below starts to die

• Therefore sucrose must be transported in the phloem

How can tracing experiments be used to investigate transport in plants ?

• plants are grown in radioactive co2 which is incorporated into plants sugars

• Using autoradiography can see that the areas exposed to radiation correspond to where the phloem is

What substances do plants transport and in what vessels

Transport takes place in two vessels: xylem which transports water and mineral salts, phloem actively transports organic molecules such as sucrose from photosynthesis

Cambium

The cambium are cells that sit between the xylem and phloem that can differentiate into either xylem cells or phloem cells

Vascular bundle

Xylem, phloem, cambium cells

How many directions does active transports take place

Active transports happens in two directions and uses energy from ATP

Structure of the xylem

Made up of dead cells that form continuous tubes

How does water move from the soil into the root hair cells

Osmosis from a high water potential to a low water potential