transport in plants full topic

the main sources of assimilates are

• green leaves and green stems

• storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period

• food stores in seeds when they germinate

main sinks in a plant

• roots that are growing and actively absorbing mineral ions

• meristems that are actively dividing

• any parts of the plant that are laying down food stores, such as developing seeds, fruits or storage organs

Mass flow hypothesis

• active transport is 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 and companion cells

• this creates a high pressure inside the sieve tubes at the source end 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

• this lowers the pressure inside the sieve tubes

• the result is a pressure gradient from the source end to the sink end

• this gradient pushes solutes along the sieve tubes to where they’re needed

active loading

• in the companion cell, ATP is used to actively transport hydrogen ions out of the cell and into surrounding tissue cells

• this sets up a concentration gradient - there are more H+ ions in the surrounding tissue then in the companion cell

• A H+ ion binds to a co-transport protein in the companion cell membrane and re-enters the cell (down the concentration gradient)

• A sucrose molecule binds the the co-transport protein at the same time. The movement of the H+ ions is used to move the sucrose molecule into the cell, against its concentration gradient

• sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process

evidence for active loading

• advances in microscopy allow us to see the adaptations of the companion cells for active transport

• if the mitochondria of the companion cells are poisoned, translocation stops

• the flow of the sugars in the phloem is about 10,000 times faster than it would be by diffusion alone, suggesting an active process is driving the mass flow

• Aphids can be used to demonstrate the translocation of organic solutes in the phloem. Using evidence from aphid studies, it has been show that there is a positive pressure in the phloem that forces the sap out through the stylet. The pressure and therefore the flow rate in the phloem is lower closer to the sink than the source. The concentration of sucrose in the phloem sap is also higher nearer to the source than the sink

cohesion and tension

• water evaporates from the leaves at the top of the xylem (transpiration)

• this creates a tension which pulls more water into the leaf

• water molecules are cohesive (they stick together) so when some are pulled into the leaf others follow. This means the whole column of water in the xylem, form the leaves down to the roots, moves upwards

• water enters the stem through the root cortex cells

adhesion

• water molecules are also attracted to the walls of the xylem vessel

• this helps water to rise up through the xylem vessels

transpiration as a consequence of gas exchange

• a plant needs to open its stomata to let in carbon dioxide so that it can product glucose (by photosynthesis)

• but this also lets water out - there is a higher concentration of water inside the leaf that in the air outside, so water moves out of the leaf down the water potential gradient when the stomata open

• so transpiration is really a side effect of gas exchange

factors affecting transpiration rate

• light - the lighter it is the faster the transpiration rate. This is because the stomata open when it gets light, so CO2 can diffuse into the leaf for photosynthesis. When its dark the stomata are usually closed so there’s little transpiration

• temperature - the higher the temperature, the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the water potential gradient between the inside and outside of the lead, making water diffuse out of the leaf faster

• humidity - the lower the humidity, the faster the transpiration rate. If the area around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration

• wind - the windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata. This increases the water potential gradient, which increases the rate of transpiration

How do you set up a potometer

1. cut a shoot underwater to prevent air from entering the xylem. Cut it at a slate to increase the surface area 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 the capillary tube submerged in a beaker of water

4. check that the apparatus is watertight and airtight

5. dry the leaves, allow time for the shoot to acclimatise, and then shut the tap

6. remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back in the water

7. record the starting position of the air bubble

8. start a stopwatch and record the distance moved by the bubble per unit time. The rate of air bubble movement is an estimate of the transpiration rate

How are xerophytic plants adapted to reduce water loss

• marram grass has stoma that are sunk in pits, so they’re sheltered from the wind. This helps to slow transpiration down

• it also has a layer of ‘hairs’ on the epidermis - this traps moist air around the stomata, which reduces the water potential gradient between the leaf and the air, slowing transpiration down

• in hot or windy conditions marram grass plants roll their leaves - again this traps moist air, slowing down for transpiration. It also reduces the exposed surface area for losing water and protects the stoma from the wind

• both marram grass and cacti have a thick, waxy layer on the epidermis - this reduces water loss by evaporation because the layer is waterproofed

• cacti have spines instead of leaves which reduces the surface area for water loss

• cacti close their stoma at the hottest times of day when transpiration rates are the highest

how to hydrophilic plants survive in water

• air spaces in the tissues help the plants to float and can act as a store of oxygen for use in respiration. Eg. water lilies have large air spaces in their leaves. This allows the leaves to float on the surface of the water, increasing the amount of light they receive. Air spaces in the roots and stems allow oxygen to move from the floating leaves down to pairs of the plant that are underwater

• stomata are usually only present on the upper surface of floating leaves. This helps maximise gas exchange

• hydrophytes often have flexible leaves and stems - these plants are supported by the water around them, so they don’t need rigid stems for support. Flexibility helps prevent damage from water currents

evidence for cohesion tension theory

• changes in the diameter of trees. When transpiration is at its height during the day, the tension in the xylem vessels it at its highest too. As a result the tree sinks in diameter. At night when transpiration is at its lowest, the tension is the xylem vessels is at its lowest and the diameter of the tree increases. This can be tested by measuring the circumference of a suitably sized tree at different times of day

• when a xylem vessels is broken - for example when you cut flower stems to put then in water - in most circumstances air is drawn in to the xylem rather than water leaking out

• if a xylem vessel is broken and air is bulled in as described in the previous bullet, the plant can no longer move water up the stream and the continuous stream of water molecules held together by cohesive forces has been broken

How does water travel through a plant

• enters through root hair cells and then passes through the root cortex, including the endodermis, to reach the xylem

• water is drawn in by osmosis so it travels down a water potential gradient

• the soil around roots generally has a high water potential and leaves have a low water potential

• this creates a water potential gradient which keeps water moving through the plant in the right directions

the symplast pathway

• goes through the living parts of cells - the cytoplasm

• the cytoplasm of neighbouring cells connect through plasmodesmata (small channels in the cell walls)

• Water moves through the symplast pathways via osmosis

The apoplast pathway

• goes through the non-living parts of the cells - the cell walls

• the walls are very absorbent and water can simply diffuse through them, as well as pass through the spaces between

• the water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure (along a pressure gradient)

• this is an example of mass flow

• the main pathways because it provides the least resistence

the Casparian strip

• when the water in the apoplast pathway gets to the endodermis cells in the root, its path is blocked by a waxy strip in the cell walls, called the casprian strip. Now the water has to take the symplast pathway

• this is useful because it means the water has to go through a cell membrane. Cell membranes are partially permeable and are able to control whether or not substances in the water get through

• once past the barrier, the water moves into the xylem

How does the water get from the xylem to out of the leaves

• xylem vessels transport the water all around the plant

• at the leaves, water leaves the xylem and moves into the cells mainly by the apoplast pathway

• water evaporates from the cell walls into the spaces between cells in the leaf

• when the st

uses of water in plants

• turgor pressure (or hydrostatic pressure) as a result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves.

• turgor also drives cell expansion - it is the force that enables plant roots to force their way through tarmac and concrete

• loss of water by evaporation helps to keep plants cool

• mineral ions and the products of photosynthesis are transported in aqueous solutions

• water is a raw mineral for photosynthesis

how are root hair cells adapted

• their microscopic size means they can penetrate easily between soil particles

• each microscopic hair has a large SA.V ratio and there are thousands on each growing tip

• each hair has a thin surface layer through which diffusion and osmosis can take place quickly

• the concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell

evidence for the role of active transport in root pressure

• some poisons, such as cyanide, affect the mitochondria and prevent the production of ATP. If cyanide is applied to root cells so there is no energy supply, root pressure disappears

• root pressure increases with a rise in temperature and decreases with a fall in temperature, suggesting chemical reactions are involved

• if levels of oxygen or respiratory substrates fall, root pressure falls

• xylem sap may exude from the cut end of stems at certain times. In the natural world, xylem sap is forced out of special pores at the ends of leaves in some conditions - for example overnight when transpiration is low (known at guttation)

why do plants need transport systems

• need substances like water, minerals and sugars

• need to get rid of waste substances

• multicellular so have a small surface area to volume ratio

• have a high metabolic rate

• exchanging substances by direct diffusion would take too long

what does the xylem transport

water and mineral ions up the plant

what does the phloem transport

sugars up and down the plant

root cross section

• xylem is in the centre surrounded by phloem to provide support for the root as it pushes through the soil

stem cross section

• the xylem and phloem are near the outside to provide support for the root as it pushes through the soil

leaf cross section

xylem and phloem make up a network of veins which support the thin leaves

longitudinal cross section of a leaf

How are xylem vessels adapted for transporting water and mineral ions

1. xylem vessels are very long, tube like structures formed from cells joined end to end

2. there are no end walls on these cells, making an uninterrupted tube that allows water to pass up through the middle easily

3. their cells are dead so contain no cytoplasm

4. walls are thickened with a woody substance called lignin which helps to support the xylem vessels and stops them from collapsing inwards. Lignin can be deposited in xylem walls in different ways eg. in a spiral or as distinct rings

5. the amount of lignin increases as the cell gets older

6. water and ions move into and out of the vessels through small pits in the wall where there’s no lignin

what does phloem tissue contain

phloem fibres, phloem parenchyma, sieve tube elements and companion cells

are xylems or phloem used for support

just xylem

sieve tube elements

• living cells that form the tube for transporting solutes through the plant

• they are joined end to end to form sieve tubes

• the ‘sieve’ parts are the end walls, which have lots of holes in them to allow solutes to pass through

• although they are living cells, they have no nucleus, a thin layer of cytoplasm and few organelles

• the cytoplasm of adjacent cells is connected through the holes in the sieve plates

cross section of phloem cells

companion cells

• lack of a nucleus are other organelles in a sieve tube means that they can’t survive on their own so each sieve tube has its own companion cell

• they carrying out the living functions for both themself and their sieve cells. For example, they provide the energy for the active transport of solutes

how to dissect plant cells

• use a scalpel to cut a cross-sections of the stem

• cut the sections as thin as possible

• use tweezers to gently place the cut sections in water until you come to use them. This stops them from drying out

• transfer each section to a dish containing a stain eg, toluidine blue, and leave for one minute. TBO stains the lignin in the walls of the xylem vessels blue-green. This will let you see the position of the xylem vessels and examine their structure

• rinse off the sections in water and mount each one onto a slide