1.7 Understandingthe Transport of Substances in Plants

Why is it essential for substances to be transported in plants?Water- acts as a solvent and acts as a reactant in cell metabolismMineral ions- required for chlorophyll synthesis, healthy plant growth and development.Organic food materials- need to be sent to the growing regions, storage organs and other parts.

Small multicellular plants have a large TSA/V ratio and short distance for substance diffusion.They do not require an internal transport system

Large flowering plants have a smaller TSA/V ratio and substances have to move farther.Transport is carried out by an internal system of tubes.

Vascular Tissues in Stems, Roots and Leaves Two types:Xylem Transports water and dissolved mineral saltsProvides mechanical support to woody plants

PhloemTransports organic substances

The stem

The rootCross Section of Dicot Root

The leaf

Structure of Xylem in Relation to TransportFour types :

a)xylem vessels b) tracheids, c) fibres d)parenchyma

How are xylem vessels and tracheids adapted to their functions?Xylem vessels consist of long, hollow, continuous tubes This allows water to flow upwards continuously

Walls are thickened with lignin depositsThis prevents the collapse of the vesselsAlso provides mechanical support

Conifers and ferns do not have xylem vessels.They have tracheids which are less efficient In conducting waterTracheids have pointed ends. The end walls break down in the pits and allow water to pass from cell to cell.

Structure of Phloem in Relation to TransportFour types:

Sieve tubeCompanion cellsParenchymaFibres

Sieve tubeA cylindrical tube consisting of elongated living sieve tube cells.The end walls are perforated by small poresThey look like sieves and are called sieve platesLong strands of cytoplasm pass through the pores to allow substances to pass throughIn a mature sieve tube, the nucleus degeneratesA thin layer of cytoplasm lining the cellulose wall allows flow of liquid through the sieve tube

Companion cellFound only in flowering plantsHas a nucleus, dense cytoplasm and many mitochondriaHelp to transport manufactured food from the leaf cells into the sieve tubesProvide the sieve tube cells with proteins, ATP and other nutrients

1.8 The Transport of Organic Substances and Water in Plants

Translocation

Translocation is the two-directional transport, upwards or downwards, of soluble organic food materials by the phloem from the leaves to other parts of the plant Sugar and amino acids are transported to growing regions for growth and development, to cells for metabolism while the excess is stored in organs such as roots.

TranspirationTranspiration is the loss of water in the form of water vapour from a plant to the atmosphere.Most of the water is lost through the stomata of the leaves.About 90% of the water that a plant absorbs is lost in transpiration; the other 10% of the water is used in the chemical reactions in the plant tissues.Transpiration creates a transpirational pull that draws water and dissolved mineral salts from the roots to the leaves.

Water is used for photosynthesis while mineral salts are used for cell activities.Water that is carried to the cells keep them turgid to give support to leaves and young stems. Evaporation of water from the leaves requires latent heat and keeps the plant cool in hot weather.

Pathway of Water from the Soil to the Leaves

Movement of water through the roots

Mineral ions are pumped from the root cells into the solution in the xylem vessels.The vessels become more concentratedWater enters the xylem from the roots through osmosisThis creates an upward force called root pressureThis helps to push water up to a certain height in plantsOther factors involved are capillary action and transpirational pull

Movement of water through the stemThe upward movement of water through the xylem vessels is because of capillarity.Capillarity is due to combined forces of cohesion and adhesionWater molecules form a continuous water column in the xylem vessels due to cohesionAdhesive forces between water molecules and xylem walls enable water to move up the narrow xylem vessels

Movement of water from the leaves to the atmosphere

Opening and Closing of StomataEach stoma has a pair of kidney-shaped guard cells surrounding a stomatal poreGuard cells have thinner elastic outer walls and thicker, less elastic inner wallsThey have a nucleus, cytoplasm and chloroplasts

During the day, photosynthesis takes placeGlucose is produced, increasing osmotic pressure in cellsPotassium(K+)ions are actively transported from the epidermal cells into the guard cellsAccumulation of potassium ions increases osmotic pressure in guard cellsWater enters by osmosis into the guard cellsThe guard cells become turgid, curve outward and the stomata openH2OK+

At night, photosynthesis does not occurPotassium ions move out of the guard cells into the epidermal cellsOsmotic pressure in the guard cells decreasesWater diffuses out into the epidermal cellsThe guard cells become flaccid and the stomata close.H2OK+

Factors Affecting the Rate of Transpiration

Light Intensity

Stimulates stomatal opening and increases rate of transpirationSunlight provides heat energy for the leavesIncreases evaporation of waterThe reverse is true in the dark

TemperatureHigh temperature increases rate of transpirationIncreasing temperature increases the kinetic energy of water molecules, making them move faster through the stomataReduces the humidity of the surrounding airThe above effects are reversed by low temperatures

Relative humidityHigh humidity

Air is saturated with water vapourWater is unable to evaporate from cellsTranspiration rate decreasesLow humidity

Increases the concentration gradient of water between leaves and surrounding airTranspiration rate increases

Air humidityMoving air carries water vapour away rapidly outside the stomataThis is so that more water molecules can diffuse to the surroundingsThe rate of transpiration increases in windy conditionsWhen there is little air movement, water vapour accumulates around the stomata and the transpiration rate reduces

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