Transport in plants Module 3 Flashcards
Why do plants require transport systems?
Multicellular so have a small SA:V, high metabollic rate, and big so difficult for direct diffusion of substances into the cells within the plant
What does xylem tissue to generally?
Transports water and mineral ions in solution, from the roots to the leaves
What does phloem tissue do generally?
Transports sugars in solution up and down the plant
What makes up the vascular system in plants?
Phloem and xylem vessels
Describe the structure of the vascular system, in the root, in the stem and in the leaf?
In a root:
Xylem is in the centre surrounded by phloem to provide support as it pushes through the soil
In the stems:
Xylem and phloem are near the outside to provide scaffolding that reduces bending of the stem
In the leaves:
Xylem and phloem make up a network of veins
How are xylem vessels adapted for transporting water and mineral ions?
Formed from long tube like cells joined end to end, there are no end walls on these cells, so water and mineral ions can pass through easily
The cells are dead so contain no cytoplasm
Their walls are thickened with a woody substance called lignin, which supports them and stops them collapsing inwards
Water and ions move from cell to cell through small pits in the wall where there is no lignin
Phloem tissue is made up of sieve tube elements and companion cells, describe their structure?
Sieve tube elements:
Living cells that form the tube for transporting solutes through the plant, joined end to end to form sieve tubes
The sieve part are the end walls which have lot of holes in them to allow solutes to pass through
Have no nucleus, very few organelles, and cytoplasm is connected through holes in sieve plates of cells
Companion cells:
Sieve tube elements can’t survive on their own (lack of nucleus), so there’s a companion cell for every sieve tube element
They carry out the living functions for themselves and the sieve tube elements eg. provide the energy for active transport of solutes
What stain do you use to stain lignin walls?
Toludine blue
How does water enter the plant through it’s roots?
Via osmosis (water potential high in soil, low in roots), goes from soil to root hairs, then the root cortex including the endordermis to reach the xylem
What are the 2 ways water moves through the roots to the cortex and into the xylem?
Through the symplast pathway, or the apoplast pathway
Describe the symplast pathway?
Water goes through the living parts of cells- the cytoplasm
The cytoplasms of neighbouring cells connect through plasmodesmata (small channels in the cell walls)
So water moves through the sympoast pathway via osmosis
Describe the apoplast pathway?
Water goes through the non living parts of cells - the cell walls
Water can just diffuse between the cell walls
Water moves an area of high hydrostatic pressure, to an area of low hydrostatic pressure this is an example of mass flow
However, when the water in this pathway reaches the endodermis cells, it’s pathway is blocked by a waxy Casparian strip, which forces the water to take the symplast route
Water moves into the xylem
Why is it useful that water in the apoplast pathway will eventualy need to take the symplast route
Forces the water to go through a partially permeable membrane, so will control which solutes enter, so can prevent toxins entering the xylem
Describe using cohesion, tension and adhesion how water moves up a plant against the force of gravity?
Water evaporates from the leaves at the “top” of the xylem
This creates tension which pulls more water into the leaf
Water molecules are cohesive (stick together), so when some are pulled into the leaf other follow
Also the water is adhesive (attracted to the walls of the xylem) which also aids it travelling up the xylem
What 4 factors affect transpiration and describe how?
Light intensity: The lighter it is the faster the transpiration rate, as the stomata open when it’s light so CO2 can diffuse into the leaf for photosynthesis. So water moves out
Temperature: The higher the temperature the higher the rate of transpiration as warmer water molecules have more energy, increasing the rate at which they evaporate, and therefore increasing the water potential gradient
Humidity: The LOWER the humidity the faster the transpiration rate, as if the air is dry around the leaf, the water potential gradient between the plant and the air is decreased
Wind speed: The faster the wind speed, the faster the trasnpiration rate, as lots of air movement blows away water molecules from around the stomata, increasing the water potential gradient, which increases the transpiration rate
Brief description of how a potometer can be used to estimate transpiration rate?
Cut a shoot underwater at a slant to prevent air entering xylem and slanted to increase water uptake
Assemble potometer, and measure the distance the bubble travels due to transpiration pull in a certain amount of time
(can use reservoir of water to return bubble to beginning for repeats)
What are Xerophytes?
Plants that are adapted to live in dry conditions
Describe how Xerophytes are adapted to live in dry climates?
Can have stomata that are sunk into pits so are shielded from the wind, decreasing their transpiration rate
Can have a layer of hairs on the epidermis, that traps moist air around the stomata, which reduces the water potential gradient between the leaf and the air, decreasing the transpiration rate
Can roll their leaves traps moist air, reduces surface area for water to be lost, and shields from the wind
Can have a thick waxy layer on the epidermis, reducing water loss by evaporation
Can close their stomata at hottest points in day, where transpiration rates would be highest
What are hydrophilic plants?
Plants that area adapted to survive in water
Describe the adaptations of hydrophillic plants so they can survive in water?
Air spaces in the tissues help the plant to float, and can act as a store of oxygen for use in respiration
Stomata are only present on the upper surface of floating leaves, maximises gas exchange
They have flexible leaves and stems, as already supported by water, and flexibility helps them not get damaged by strong currents
What is translocation?
The movement of dissolved substance to where they are needed in the plant, it moves them from a source to a sink
What’s a source?
Where the substance is made (high concentration)
eg. leaves for sucrose
What’s a sink?
The area where it’s used up (by enzymes so lower concentration there)
How can the roots be a source and a sink?
Are a sink, as they require sucrose for growth from the leaves, but also can store sucrose and it’s transported to other parts of the plant
Describe the mass flow hypothesis for phloem transport?
Active transport is used to load the solutes into the sieve tubes of the phloem at the source
This lowers the water potential inside the sieve tube elements, so water enters the tubes by osmosis from the xylem and companion cells
This creates a high pressure inside the sieve tube tubes at the source of the phloem
At the sink end solutes are removed from the phloem to be used up, this increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis
Lowering the pressure in the sieve tube elements at the sink end
So a pressure-gradient is formed from the source end to the sink end, and this gradient pushes the solutes to where they are needed
Describe exactly how substances enter the phloem at the source via active loading?
In the companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell and into the surrounding tissue cells
This sets up a conc gradient there are more H+ ions in the surrounding cells than in the companion cells
A H+ ion binds to a co-transporter protein in the companion cell membrane, and re-enters the cell down the concentration gradient
A sucrose molecule binds to the co-transporter protein at the same time. The movement of the H+ ion is used to move the sucrose into the companion cell against it’s concentration gradient
Sucrose molecules are then transported out of the companion cells and into the the sieve tube elements via the same process
