TRANSPORT IN PLANTS Flashcards
Xylem tissue
Transports water and mineral ions in solution from the roots to the leaves
Phloem tissue
Transports sugars up and down the plant
Xylem vessels
Adapted for transporting water and mineral ions
Long tube structures formed from vessel elements joined end to end
No end cell walls
Cells are dead with no cytoplasm
Walls thickened with lignin
Water ions move into and out of vessels through small pits in the walls where no lignin is present
Phloem tissue
Adapted for transporting solutes mainly sucrose around plants
Cells arranged in rows
Contains phloem fibres, phloem parenchyma, siege tube elements and companion cells
SIEVE TUBE ELEMENTS- living cells forming he tube for transporting solutes
Joined end to end to form sieve tubes
Sieve pets are end walls which contain lots of holes to allow solutes through
No nucleus, very thin, cytoplasm containing few organelles
Cytoplasm of adjacent cells connected through parenchyma
COMPANION CELLS
Support sieve tube elements
Carry out living functions for themselves and their sieve cells
Provide energy for active transport of solutes
Water transport in roots- symplast pathway
Living parts of the cell
The cytoplasm
The cytoplasm neighbouring cells connect through plasmodesmata
Osmosis
Water transport in roots- apoplast pathway
Non living parts of the cell
The cell wall
The walls are very absorbing so water can simply diffuse through them and through spaces between them
Water can carry solutes and move from area of high hydrostatic pressure to area of low (mass flow)
Water reaches the capsarian strip and then has to take the symplast pathway
Water must go through the membrane which is partially permeable and can control which substances in the water get through
Once through this barrier, water enters the xylem
Apoplast pathway provides less resistance
Mechanisms to move water through the transpiration stream
COHESION AND TENSION
Water evaporates from the leaves (transpiration)
This creates a tension which pulls the water into the leaf
Water molecules are cohesive and stick together so when some are pulled into the leaf, others follow. This means the whole column of water in the xylem moves upwards
ADHESION
Water molecules are attracted to the walls of the xylem vessels
This helps water rise up through the xylem vessels
Factors affecting transpiration rate
Light
Temperature
Humidity
Wind
Xerophytic plants
Cacti and marram grass Adapted to dry conditions Stomata in sunken pits Has hairs on the epidermis Marram grass plants roll their leaves Thick each layer on their endodermis Cacti have spines Cacti close their stomata at the hottest times
Hydrophilic plants
Live is aquatic habitats
Need to cope with a low oxygen level
Air paces in the tissues help the plants float and can act as a store of oxygen for use in respiration
Floating= increases amount of light they receive
Air spaces allow oxygen to move from the floating leaves down to parts that are underwater
Stomata present on upper surface of floating leaves maximises gaseous exchange
Flexible stem and leaves
Translocations
Movement of assimilates to where they are needed in the plant
Energy requiring process in the phloem
Source -> sink
Enzymes maintain concentration gradient from source to sink by changing the dissolved substances at the sink
Mass flow hypothesis
Active transport used to actively load the solutes into the sieve tubes of the phloem at the source
This lowers the water potential inside the sieve tubes so water enters the tubes by osmosis from the xylem to the companion cells
Creates a high pressure inside the sieve tubes at the source end of the phloem
At the sink the solutes are removed from the phloem to be used up
This increases the water potential inside the sieve tubes so water leaves the tubes by osmosis
This lowers the pressure inside the sieve tubes
The result is a pressure gradient from the source end to the sink end
The gradient pushes solutes along the sieve tubes to where they are needed
Active loading at the source
In the companion cell, active transport is used to actively transport hydrogen ions out the cell into surrounding tissue cells
This sets up a concentration gradient where there are more H+ ions in the surrounding tissue than in the companion cell
A H+ ion binds to a cotransported protein in the companion cell membrane and renters the cell
A sucrose molecule binds to the cotransporter protein at the same time
The movement of H+ ions is used to move the sucrose molecule into the cell against the concentration gradient
Sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process
