Ch. 7: Transport in Plants AICE Biology 2017

Introduction Plants use photosynthesis to convert light energy to chemical energy Simple organic substances, such as CO2, H2O and ions are used in their raw form to produce glucose and other carbohydrates.

Introduction How does the plants obtain H2O and CO2? Does the plants have a circulatory system like us? How these trees lift water against the force of gravity? (i.e. American sequoias -giant redwood; Height of 60 meters)

Objectives Explain the need for transport systems in multicellular plants Describe the distribution of xylem and phloem tissue in roots, stems and leaves Explain the absorption process in roots Describe transport mechanisms (various ways of movement; both water, food and assimilates- compounds that the plant makes) List factors that affects rate transpiration Describe xerophyte properties List the series of events that leads to translocation  

Types of Plant Tissues

Plants have 2 separate transport tissues (VASCULAR TISSUE!) Xylem tissue: Water and ions travel upwards (1 direction) Roots Stems Leaves Flowers Fruits Phloem tissue: Sucrose and other assimilates travel upwards and downwards Movement of water in the xylem and phloem is by mass flow. Everything travels in the same direction within each of column of xylem or phloem Note that neither plant transport system carries O2 or CO2

Xylem and Phloem In both the walls of the tubes are further thickened by the addition of: Cellulose (organic compound –polysaccharide) Lignin (woody material)

Water transport in 3 parts Transpiration (or evapo-transpiration) is the transport of water and minerals from roots to leaves. It involves 3 basic steps: 1- Absorption at the roots. 2 - Capillary action in the xylem vessels. 3 - Evaporation at the leaf.

Xylem (vascular tissue) Function? Types of cells? Vessel elements Tracheids Direction of movement? Transpiration Stomata Guard Cells

Roots Root hair Single-celled extensions of some cells Very thin (200-250 µm) A single root can have thousands Increases the surface area Absorbs water by osmosis

Roots Osmosis: Movement of H2O molecules from an area of high concentration to an area of lower concentration Lower solute concentration in the soil (high water potential) Higher solute concentration in the root (lower water potential)

Two different routes for water to move into root: Apoplast pathway: “AROUND” When H2O soaks through the cell walls and then seeps across the root from cell wall to cell wall and through the spaces between cells Symplast pathway: “CYTOPLASM” When H2O enters the cell walls and moves from cell to cell by osmosis Or through strands of cytoplasm that makes direct connection between adjacent cells- plasmodesmata

When water reaches the stele the apoplast pathway is blocked. Endodermis cells (stele) have suberin (waterproof)

Casparian strip: belt of waxy material, allows only minerals in the symplast to pass into the vascular cylinder through the plasma membrane of endodermal cells. Cells in the vascular cylinder transport water and minerals throughout the plant.

Xylem Long narrow cells Xylem elements Start as living cells (nucleus, cell wall) Then differentiated into specialised structures and died No living material Just empty shells

Stem Root cross section

Leaf Cross section

Leaf cross section

Leaf tissue: Palisade and spongy mesophyll cells have very large internal surface for gas exchange (CO2- As the carbon dioxide concentration in the air is so low (0.04%), the surfaces are large so that enough can be absorbed for photosynthesis.

The air inside leaves is always fully saturated with water vapour. Usually, the air outside is less saturated and so a concentration gradient for water vapour exists between the air spaces and the outside. Therefore, water vapour diffuses down this humidity gradient. (High to Low H2O potential) The pathway with the least resistance is through the stomata.

How does the water goes up? Water properties??? Capillary Action Cohesion-Adhesion tension theory Bonding Animation

Transpiration drives the movement of water in plants The loss of water from leaves by transpiration causes water to travel upwards through the plant by mass flow. The mechanism is called ‘cohesion-tension’ and it works as follows:

Cohesion-tension theory Water loss caused by transpiration Causes a pulling force Negative pressure produced Transpiration pull

Cohesion-tension theory Root Absorption through osmosis Endodermal cells actively secrete mineral salts ( Why?- To keep the water potential in the xylem lower) Causing water to be drawn through the endodermis “pulling” of water caused by cortex cells produce positive hydrostatic pressure inside the xylem , forcing water upwards Root pressure

How does the water goes up? Transpiration pull (negative pressure) Root pressure (positive pressure) 2 important factors of the water: Cohesion: H2O molecules tend to stick together Adhesion: H2O molecules tend to stick to the inside of the xylem

Transpiration Spongy mesophyll cells are not tightly packed (air spaces are in direct contact with the air outside the leaf, through stomata) If air outside the leaf contains less H2O vapour then inside There is a H2O potential gradient from the air spaces inside the leaf to the outside

Potometer Measures the water absorption Estimate the rate of transpiration Air/water tight Water transpired Water entering to xylem

Factors affecting rate of transpiration Light intensity: Affects the opening and closing of the stomata ROT Indirect effect

Factors affecting rate of transpiration Humidity: Humid atmosphere Contains a lot of H2O molecules Reduction of the water potential gradient between the air spaces and atmosphere ROT decreases Low humidity increases ROT

Factors affecting rate of transpiration Temperature: Temperature kinetic energy Rate of diffusion through the stomata pores Air is able to hold more water molecules at higher temperatures ROT

Factors affecting rate of transpiration Wind speed: Still air makes the H2O molecules to accumulate around the stomata pores (leaves) Reduces the H2O potential gradient and slows the ROT Wind disperse H2O molecules gradient in H2O potential ROT

Xerophytes A plant adapted to live in dry conditions They have a range of adaptations to reduce the loss of water vapour by transpiration. A plant adapted for survival in soil with a limited supply of water. Capillary water is absent from the surface horizons of the soil for extended periods of time. Some xerophytes, such as cacti and succulents, store water in their stems to survive extended periods of extreme drought. Cactus spines are modified leaves which provide protection against browsing animals. Because photosynthetic leaves are absent from most mature cactus plants, photosynthesis occurs within chloroplasts in the stems. In addition, cactus plants typically inhabit well-drained alluvial slopes and have shallow, surface roots to absorb the scant rainfall.

xerophytes Leaves Small to reduce the surface area Thick to reduce surface area: volumes ratio

xerophytes Sunken Stomata

xerophytes Stomata Set deep inside the leaf so that they are at the base of a depression full of water vapour Some plants open their stomata at night to store and absorb CO2

xerophyte Thick waxy cuticles reduce water loss through the epidermis

Xerophytes Rolling up of leaves Lower surface faces inside and traps humid air next to the stomata Varies with conditions

Xerophytes Leaf hairs Trap damp air Reduces air movement Cut down transpiration

Transport in the Phloem Most photosynthesis occurs in the leaves. The reactions take place in the chloroplasts. The compounds that the plant makes are called assimilates. Animation Video notes Many of these are exported form the leaves to the rest of the plant in the phloem.

Sources and Sinks The transport of these assimilates is called translocation (means ‘from place to place’) Assimilates are loaded in the phloem in the leaves, they are often called sources. They are transported to other parts of the plant, such as roots, stems, flowers, fruits and seeds. These are called sinks.

Movement in the Phloem in an active transport The transport of these assimilates is called translocation Sucrose and other assimilates travel throughout a plant in phloem sieve tubes. These are made from cells called sieve elements.

Sieve tube Made of sieve elements Living cells No nucleous Ribosomes or tonoplast Diameter 10 15 um End walls: sieve plates Large pores

Alongside sieve tubes are companion cells. Mesophyll cells in the leaf are close to veins containing sieve tubes. Sucrose travels to the phloem companion cells in two ways.

From cell to cell through the plasmodesmata. Along cell walls in the mesophyll. Carrier proteins in the cell surface membranes of companion cells actively pump sucrose into the cytoplasm. From here it passes through plasmodesmata into a sieve element.

The accumulation of sucrose and other solutes, such as amino acids, in sieve elements lowers the water potential so that water diffuses in by osmosis from adjacent cells and form the xylem. This creates pressure in the sieve elements causing the liquid (phloem sap) to flow out of the leaf.

Phloem sieve elements are adapted for transport as it has: End walls that have sieve pores allowing sap to flow freely. Little cytoplasm to impede the flow of sap. Plasmodesmata to allow assimilates to flow in from companion cells.

Phloem cells vs. xylem cells: Sieve elements differ form xylem vessels because they are alive. Sieve cells have some cytoplasm with organelles. Sieve cells are not lignified, as they do not need to withstand the same forces as exist in the xylem.

Sucrose is unloaded at sinks. This is taken up by the cells and is respired or stored as starch. This reduces the concentration of phloem sap and lowers the pressure, so helping to maintain a pressure gradient form source to sink so the sap keeps flowing in the phloem. http://www.youtube.com/watch?v=MxwI63rQubU&feature=related

REVIEW! Could you answer all objectives thoroughly?? Explain the need for transport systems in multicellular plants Describe the distribution of xylem and phloem tissue in roots, stems and leaves Explain the absorption process in roots Describe transport mechanisms (various ways of movement; both water, food and assimilates- compounds that the plant makes) List factors that affects rate transpiration Describe xerophyte properties List the series of events that leads to translocation