What substances need transporting in plants? Oxygen? Carbon dioxide? Water? Minerals? Nutrients? Sugars,amino acids. Hormones?

Transport in multicellular plants Carbon dioxide Needed for photosynthesis in all green parts of the plant, mainly leaves. It diffuses from the air into leaves down the concentration gradient(high conc in air low conc in leaves) The large surface area/volume ratio of leaves helps this It is NOT moved by the plant transport system CO2

Oxygen All plant cells need it for respiration Photosynthesising cells produce their own supplies For other cells such as roots it must diffuse in from air in the soil Excess oxygen from leaves diffuse out Plants have much lower energy needs than animals ,so need oxygen much less rapidly Oxygen is NOT transported around the plant.

Organic nutrients Photosynthetic cells make their own nutrients These include glucose & amino acids These need to be transported to other parts of the plant This involves the phloem Inorganic ions and water Ions such as magnesium,potassium,nitrates etc are needed by cells These including water are taken up by the roots and transported in the xylem

The plant transport system Only WATER, MINERAL IONS and ORGANIC NUTRIENTS need transporting Because of the LOW ACTIVITY of plants these are not needed rapidly The plant transport system is much simpler than animals Fluids move much more slowly There is no pump to move the fluids The cells transporting substances are known as vascular tissue There are two tissues consisting of systems of tubes called xylem & phloem

WATER TRANSPORT In a plant water moves down a water potential gradient From HIGH water potential (ψ) in the soil to LOW water potential (ψ) in the air around the leaves Because of this there is a constant flow of water through the plant

Soil to Root Hair Water is absorbed by the roots Most of the root is impermeable to water Only the root hair cells have thin permeable cell walls These are found in a region behind the root tip They grow out between the soil particles and provide a large surface area. They are very delicate and easily damaged,often lasting only a few days before being replaced

Mycorhizas Some trees have fungi growing in their roots These associations are called mycorhizas The fungi form a mass of fine threads which help absorb nutrients especially phosphates Some trees growing on poor soils cannot survive without the fungi

Water enters the root hair cell by osmosis The root hair cell contains dissolved nutrients and minerals, this gives it a low water potential These minerals have been pumped into the cells by active transport Soil water has higher water potential Water enters the cell by osmosis from a high WP to a low WP

Water Transport : From root hair to xylem Water enters the root hair cells It then passes through the cortex and into the stele This consists of the endodermis,pericycle and into the xylem Water potential in the xylem is lower than in the root hair cells Water moves down the water potential gradient

Root Section Ranunculus: t.s young root Tissue plan

Apoplast & Symplast There are three routes for the water through the cortex cells 1. Apoplast pathway The water moves through the cell walls These are made of cellulose fibres and can soak up water It moves between cells via the intercellular spaces or via touching cell walls. It does not pass through any membranes, meaning mineral ions can be transported

2. Symplast pathway Here the water enters the cells It passes through the cytoplasm It passes from cell to cell through the cytoplasm in plasmodesmata It is thought that this pathway is most often used 3. Vacuolar pathway This is similar to the symplast but also includes the vacuoles Apoplast may be used when a lot of water is being lost from the plant and flow through the root needs to be rapid

The endodermis When the water reaches these cells it must follow the symplast pathway This is because the cell walls contain a thick waterproof band called a casparian strip. This is made of a waxy substance called suberin This blocks the flow of water by the apoplast route

It is thought that this may allow control of the minerals entering the xylem as they must pass through cell membranes Endodermis cells pump minerals into the xylem by active transport This lowers the ψ and helps move water in to the xylem by osmosis It may also help generate root pressure Casparian strip also blocks water return out of the xylem

Apoplast & Symplast

How does water move up the stem? Root pressure Transpiration stream Capillary action

Water transport: From root to leaf Water passes from the roots into the stem and then into the leaves It does this through the XYLEM TISSUE

Xylem Tissue Xylem tissue is a group of cells that work together to transport water & minerals and support the plant It contains several different types of cells Xylem vessel elements Tracheids (Sclerenchyma) fibres Parenchyma cells

Xylem vessel elements Xylem vessels transport water They are made up of many elongated vessel elements. These form a hollow pipe These began life as a normal plant cell But a substance called lignin has been laid down in the walls

Lignification Lignin is a complex carbohydrate It is hard,strong and impermeable to water It can be stained red in microscope slides This thickening of the cell wall with lignin kills the cell This leaves a hollow space, a lumen, through which water can pass

Types of lignification Thickening starts off as circular or spiral, then reticulate and finally pitted

Types of thickening

Pits Pits are not open They have the original cell wall in them This is fully permeable to water Pits allow water to enter and leave the xylem They can also be used to bypass blockages

Tracheid Also dead , hollow cells with lignified walls Their ends are not completely open and taper Water passes between cells via pits They are the main water conducting tissue in more primitive plants More modern plants(angiosperms) make more use of xylem vessels

Fibres Small dead elongated cells Lignified walls Small lumen Main role is supporting the plant

Stem Vascular Structure

Vascular Bundle

Leaf Section(dicot TS)

Leaf section

Transpiration Stream To understand this we first need to consider: Movement of water: Leaf to air The mesophyll cells of the leaf have wet cell walls Water evaporates into the air spaces saturating them Water will move by osmosis from the xylem vessels (via pits) across the cells of the leaf to replace water lost If the air outside the leaf has a lower water potential than inside, water vapour will diffuse out of the leaf through the stomata This loss of water by evaporation from the aerial parts of plants is called TRANSPIRATION

Transpiration: the price plants pay for photosynthesis Carbon dioxide is needed for photosynthesis Stomata must be open so this can diffuse into the leaf This means water is lost through the open stomata If water is being lost too quickly the stomata may be closed Leaves also wilt giving less surface area to lose water Transpiration can also be important in cooling plant leaves as evaporation use heat

Transpiration Stream : root xylem to leaf xylem As water evaporates from the leaf cells water replaces it from the leaf xylem vessels by osmosis Removing water from the xylem lowers the hydrostatic pressure This is now lower than the pressure in the roots causing water to move up the xylem (similar to being sucked up a straw) If you suck too hard on a straw the pressure causes the walls to collapse To prevent this xylem vessels walls are strong, lignified. The transpiration stream is a passive process, relying on evaporation of water from the leaves.

Mass Flow This movement of water is called mass flow The water molecules move all together as a mass It can do this because the water molecules are held together by H bonds This is called COHESION They are also attracted to the lignin in the walls of the xylem This is ADHESION

If the column of water broke an air bubble would form and this would stop the mass flow The small diameter of the xylem vessels makes this less likely to happen

Capillary action If thin tubes are placed in water the water will move some way up the tube This is capillary action due to adhesion & cohesion

Root Pressure Hydrostatic pressure at the root end of the xylem can be increased Cells surrounding the xylem in the roots pump mineral ions into the xylem by active transport This lowers the water potential and more water enters the xylem from the cells by osmosis Hydrostatic pressure increases This is not essential though and only helps

Measuring the rate of transpiration The rate of transpiration varies This will depend on: Temperature: The higher the temperature the faster the rate of evaporation inside the leaves and the faster the rate of diffusion out of the stomata Water potential gradient between leaf and air The steeper the gradient the faster the water vapour will diffuse out of the leaf

Moving air will remove water vapour from around the leaf Moving air will remove water vapour from around the leaf. Keeping the gradient steep and speeding up diffusion Dry air will mean a steeper gradient Number of stomata The more stomata the quicker water vapour will be lost Position of stomata Stomata on lower surface are not exposed to the heating effect of the sun.So less water lost Light Stomata open in the light. Soil water availability Low levels of soil water may mean wilting,plant stress and stomatal closure Waxy cuticle Its presence reduces water evaporation from leaf surface

Factors affecting the rate of transpiration Factor affecting transpiration rate How it affects water loss

Movement of meniscus each minute(mm) Using the potometer Label the diagram fully The potometer does not measure the accurate uptake of water,it can only compare water uptake.Why? Why should the leafy shot stem be cut at an angle under water and placed in the potometer? What must the potometer be to ensure it works correctly? Explain how you can estimate water uptake using the potometer Explain how you can change the following conditions when using the potometer: humidity, light intensity, air currents, temperature. Explain the role of the reservoir. Plot a graph of your results and explain them. condition Movement of meniscus each minute(mm) 1 2 3 4 5

Using the potometer Fill the apparatus and syringe totally with water by placing it in a bowl – air bubbles will block the water flow Cut a shoot under water – this ensures the xylem does not get blocked with air bubbles Place the shoot in the apparatus under water Ensure it is a tight fit and seal with vaseline – any leaks will let air into the apparatus. Set up on a stand As water evaporates from the leaves water will be taken up from the apparatus to replace it. This will not give real total of water uptake as a small amount of the water taken up will be used for photosynthesis Measure the movement of the water level in a set time. The water level can be reset by adding water from the syringe. Try moving air, lower temperature, higher humidity to see their effect on transpiration rate.

Xerophytes Plants adapted to living in very dry areas. Explain using examples how xerophytic plants reduce water loss by transpiration Include: Leaves reduced to spines Waxy cuticle Position of stomata Sunken stomata Hairy leaves Rolled leaves

Phloem Tissue This tissue transports ASSIMILATES These are substances the plants make themselves and include sugars and amino acids Phloem tissue consists of 2 main types of cells Sieve elements Companion cells There are also parenchyma cells and fibres

Sieve Elements Sieve elements are the single cells which join up to form sieve tubes These are living cells They have a Cellulose cell wall Cell membrane Only a thin lining of cytoplasm containing Endoplasmic reticulum & mitochondria No nucleus,chloroplasts or ribosomes

Sieve plates The ends of the cells are not completely open but have a sieve plate containing open sieve pores so materials are free to move through

Companion cells These are next to the sieve elements They have the “normal plant cell” structure However they have more mitochondria and ribosomes than normal Smaller vacuole There are many plasomdesmata between the cells so materials can easily pass between the cells

Microscopy Both light and electron microscopy show protein fibres in the phloem It was thought these had a role to play It has now been shown they are only a response to damage by the cells when the sections are taken

Translocation The transport of soluble organic molecules within the plant (assimilates) Takes place through the sieve tubes It takes place by mass flow Due to hydrostatic pressure differences at either end of the phloem But this is an active process , not passive as in transpiration Assimilates are actively loaded into the phloem and unloaded out of it

Sources and Sinks The area where assimilates are being made is called the SOURCE It is most often the leaf where many organic molecules are made during photosynthesis However it could be a storage organ such as a potato tuber where sugars are being loaded into the phloem

Assimilates are transported from the source to the SINK This is where the assimilates are unloaded and are being used by the plant They might include: A growing point of the root or stem A storage organ in the root, building up starch A developing fruit or flower A nectary in a flower As sinks could be above or below the sources materials can move in both directions in phloem

How translocation occurs Loading of assimilates The most widespread assimilate which is loaded into the phloem is sucrose This is formed from triose sugars(C3) made in the mesophyll leaf cells in photosynthesis This is a disaccharide that is the transport carbohydrate for plants It moves across the leaves to the phloem via apoplast or symplast This varies between species

When it reaches the companion cells it is moved into them(loaded) by active transport A H+ protein pump moves hydrogen ions out of the companion cell by active transport This is against their concentration gradient and requires energy from ATP This is provided by the many mitochondria in the companion cells This means a high concentration of H+ outside the cell Another protein in the membrane allows the H+ ions back into the cell down their concentration gradient This is linked to bringing sucrose into the cell This protein is a co-transporter protein

Mass Flow Sucrose can then diffuse into the sieve tube via the plasmodesmata This lowers the water potential of the sieve tube Water enters them by osmosis from the xylem raising the hydrostatic pressure The sucrose solution is pushed down the sieve tubes At the sink sucrose diffuses out of the phloem and water follows This lowers the hydrostatic pressure

Unloading at the sink The mechanism of this is unclear sucrose may just diffuse out into the cells that need it The sucrose may be converted to glucose & fructose by the enzyme invertase, and be used for respiration or changed to starch and stored In both cases its concentration will decrease maintaining a concentration gradient In some cases it is possible that sucrose is unloaded by active transport

Evidence for the mechanism of translocation Sources of evidence come from: Light and electron microscopy Composition of phloem sap Speed of flow of sap and hydrostatic pressure Potential difference readings from inside and outside cells

Phloem sap This is not easy to collect. As soon as you cur phloem tissue it rapidly blocks the sieve pores with the protein plug Within hours this is replaced by callose a polysaccharide similar to cellulose This “clotting” mechanism stops loss of sap and entry of microbes. Castor oil sap can be collected Solute Concentration (m/l) Sucrose 250 K+ 80 Amino acids 40 Cl- 15 phosphate 10 Mg2+ 5 Na+ 2 ATP 0.5 nitrate auxins traces

Using Aphids Aphids can place their proboscis into the phloem sieve tube The proboscis is so fine that it does not activate the plants protective mechanism They can then be anaesthetised and heads removed Samples of sap will then exude from the cut end of the stylet

Evidence for Translocation theory Phloem sap has a high pH about 8.0 , suggesting a low concentration of H+ ions This would be the case if they are being removed by active transport. There is a -150mv potential across the membrane, again supporting the removal of H+ ions, making the inside –ve. ATP is present in the sap which would support active transport Flow rate of sap is 10,000x faster than it would be for diffusion and match readings of hydrostatic pressure differences at source & sink Phloem sap is under pressure and will leak out if cut

Evidence against Why do the sieve pores still exist? They obstruct mass flow and you would think evolution would have removed them! Phloem transports to several different sinks at one time, not to the one with the lowest hydrostatic pressure!

The differences between sieve elements and xylem vessels Do exercise SAQ 10.9 p147