Transport in Plants Flashcards
What do plants need to live
Substances such as water, minerals and sugars
Why are they multicellular
Small SA:V ratio and relatively big with high metabolic rate
Why do plants need transport systems
Exchanging substances by direct diffusion would be too slow to meet metabolic needs and so it is quicker
What are the two tissue types involved in transport in plants
Xylem and Phloem
What does xylem transport and direction
Water and mineral ion sin solution moving up the plant from the roots to the leaves.
What does phloem tissue transport and direction
Mainly sugars both up and down the plant
Xylems function in a root
In the centre to provide support for the root as it pushes through the soil
Xylem and phloems function in the stems
Near the outside to provide a scaffolding reducing bending
Xylem and phloems function in a root
Make up a network of veins that support the thin leaves
Where is the xylem located in the root/leaf/stem
The xylem is in the middle of the cell and the phloem in outside in the root. The xylem is on the top of the phloem in the leaf.
What are xylem vessels
Very long tube-like structures formed from cells joined end to end (vessel elements)
Feature of xylem vessels
- No end walls making an uninterrupted tube allowing water to pass up through the middle easily
- The cells are dead so do not contain cytoplasm
- Their walls are thickened with woody substance called lignin, helping to support the xylem vessels and stops them collapsing inwards.
- Water and ions move into and out of the vessels through small pits in walls when no lignin present
Features of Phloem tissue
- Transport solutes mainly sugars
- Is formed from cells arranged in tubes but is purely a transport tissue not used for support
- Contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
What are sieve tube elements
- Living cells that form the tube for transporting solutes through the plant
- Joined end to end to form sieve tubes
- Have lots of holes in them allowing solutes to pass through
- No nucleus only a thin layer of cytoplasm and few organelles
- The cytoplasm of adjacent cells is connected through holes in sieve plates
What are companion cells
- Carry out living functions for both themselves and sieve cells. Provide energy for active transport of solutes
- The do not have a nucleus and other organelles in sieve tube elements so it supports them
How to dissect plant stems
1) Use a scalpel to cut a cross section of the stem and cut the sections a thinly as possible as better for viewing
2) Use tweezers to gently place the cut sections in water until you use them so they do not dry out
3) Transfer each section to a dish containing a stain and leave for a minute (toluidine blue O)
4) Rinse off the section in water and mount each one onto a slide
How does water enter a plant
Through the soil, root hair cells, root cortex, endodermis and into the xylem
How is water drawn into the roots
Through osmosis as it travels down a water potential gradient
How does water always move (wpg + and in the roots)
From area of higher water potential to lower water potential down a water potential gradient. Soil around the roots have a high water potential and leaves to lower in the root hair cells.
What is the symplast pathway
Goes through the living parts of cells (the cytoplasm). They cytoplasm of neighboring cells connect through plasmodesmata (small channels in cell walls). Water moves through the symplast pathway via osmosis.
What is the apoplast pathway
Goes through the non-living parts of cells (cell walls). The walls are very absorbent and water can simply diffuse through them, as well as pass through the spaces between them. The water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure. This is an example of mass flow.
What happens when water goes through the apoplast pathway to endodermis cells in the root and why is it useful
It’s path becomes blocked by the waxy strip called Casparian strip so the water now has to enter the symplast pathway. It is useful as the water has to go through a cell membrane which is partially permeable and are able to control whether substances can get through to the xylem.
Which pathway provides the least resistance
Apoplast
What happens when water reaches the xylem vessels
It transports water all around the plant. At the leaves, water leaves the xylem and into the cells mostly by the apoplast pathway and then evaporates from the cell walls into the spaces between cells in the leaf. When the stomata open the water diffuses into surrounding air down water pot gradient. This is called transpiration.
What assists the transpiration stream
Cohesion, adhesion and tension
How does water move up against the force of gravity
1) Water evaporates from the leaves at the top of the xylem
2) This creates tension (suction) and pulls more water into the leaf
3) Water molecules are cohesive (stick together) to each other so when some are pulled into the leaf others follow and adhesive to the walls of xylem (clings). So the whole column of water in xylem moves upwards
4) Water enters the stem through the root cortex cells
Why is water loss a consequence of transpiration
1) A plant opens its stomate to let in carbon dioxide so that it can produce glucose by photosynthesis
2) This also lets out water as there’s a higher concentration of water inside the leaf than in the air outside so water moves out the leaf down the water potential gradient when stomata open
Four factors affecting transpiration rate
Light, Temperature, Humidity, Wind
How does light affect transpiration rate
The lighter the faster the transpiration rate as stomata open when it is light so CO2 can diffuse into the leaf for photosynthesis. When it is dark stomata closes so little transpiration rate.
How does temperature affect transpiration rate
Higher temp = faster transpiration rate. Warmer water molecules have more ATP so they evaporate from the cells inside the leaf faster increasing the water potential gradient between the inside and outside of the leaf making water diffuse out the leaf faster.
How does humidity affect transpiration rate
The lower the humidity, the faster the transpiration rate. If the air around the plant is dry the water potential gradient between the leaf and the air is increased, increasing transpiration
How does wind affect transpiration rate
The windier it is the faster the transpiration rate as air movement blows away water molecules from around the stomata increasing the water potential gradient and therefore transpiration rate.
What is a potometer
Used to estimate transpiration rates as measures water uptake but then assumed it is directly related to water loss by the leaves
How to prepare and use a potometer
1) Cut a shoot underwater preventing air from entering the xylem at a slant to increase SA available for water uptake
2) Assemble the potometer in water and insert the shoot underwater so no air can enter
3) Remove the apparatus from the water but keep the end of capillarry tube submerged in a beaker of water
4) Check that the apparatus is watertight and airtight
5) Dry leaves allow time for the shoot to acclimate and then shut the tap
6) Remove the end of the capillary tube form the beaker of water until onw air bubble has formed, then put the end of the tube back into the water
7) Record the starting position of the bubble
8) Start a stopwatch and record the distance moved by the bubble per unit time. Rate of bubble movement = transpiration rate
What are xerophytes
Adapted to live in dry climates
How is marram grass adapted to reduce water loss
- Has stomate that are sunk in pits so are sheltered from the wind slowing down transpiration rate
- Has a layer of ‘hairs’ on the epidermis trapping moist air round the stomata reducing water potential gradient between the leaf and the air slowing transpiration down
- They roll their leaves trapping moist air and slowing down transpiration. Also reduces the SA for losing water and protects stomata from the wind.
How are cacti adapted to reduce water loss
- Have spines instead of leaves reducing the SA for water loss
- Close their stomata at the hottest time of the day when transpiration rates are the highest
What are hydrophilic plants
Live in aquatic habitats and don’t need adaptions to reduce water loss but need help with low oxygen level
Adaptations of hydrophilic plants
- Air spaces in the tissues help the plants to float and can act as a store of oxygen for respiration and allow oxygen to move from the floating leaves down to parts of the plant underwater. Also allows plants to float on the surface increasing light received .
Name a hydrophilic plant
Water lilies
What is translocation
The movement of dissolved substances (assimilates) to where they are needed in the plant. It is energy requiring occuring in the phloem moving substances from ‘sources’ to ‘sinks’.
What is the source of a plant
Where the assimilate is made so has a high concentration
What is the sink of a plant
Where the assimilate is used up so has a lower concentration
What is the stages of the mass flow hypothesis
1) Active transport is used to actively load the solutes into the sieve tubes of the phloem at the source
2) This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells
3) This creates a high pressure inside the sieve tubes at the source end of the phloem
4) At the sink end, solutes are removed from the phloem to be used up
5) Increases water potential inside the sieve tubes so water also leave the tubes by osmosis
6) Lowers the pressure inside sieve tubes and results in a pressure gradient from the source end to the sink
7) Ultimately pushing solutes along sieve tubes to where they are needed.
How do enzymes maintain a concentration gradient from the source to the sink
By changing the dissolved substances at the sink making sure there is a lower concentration at the sink than the source
What is active loading
Moving substances into the companion cells from surrounding tissues and from the companion cells into the sieve tubes against a concentration gradient.
In active loading where is the concentration of sucrose highest
In companion cells than surrounding tissue cells and higher in sieve tube cells than companion cells.
How is sucrose moved by active transport
- ATP is used
- A concentration gradient is set as more H+ ions are in the surrounding tissue than the companion cells
- H+ ion binds to a co-transport protein in the companion cell membrane and re-enters the cell
- A sucrose molecule binds to the co-transport protein at the same time so the movement of H+ ion 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
