Transcript Slide 1
Transport in plants Plant transport systems Forces acting on transport Forces in the leaves • Transpiration – water lost from leaves • Evapo-transpiration – water lost from leaves as water evaporates • Transpiration stream – water is pulled up the stem as water is lost from the leaves in transpiration Forces in the stems • Adhesion – water molecules stick to the sides of the walls in the xylem • Cohesion – water molecules stick to each other • Capillarity/capillary action – water is drawn upwards through thin tubes Forces in the roots • Active transport – salts are actively absorbed, increasing the osmotic pressure within the roots • Osmosis – water is pulled in due to the concentration gradients Translocation Movement of sugar • Translocation – movement of sugar – sugar is actively transported from leaf to phloem (source) and from phloem to roots (sink), thus setting up a concentration gradient from leaf to roots • Diffusion – sugar will diffuse downwards because of this concentration gradient • Osmosis – water will be pulled out of the xylem near the leaves, and move downwards, then return to the xylem near the roots, due to the concentration gradient. Plant transport systems Roles of • Leaves – carry out photosynthesis and transpiration • Stomata – allow water and gases to enter and leave the leaf. Opening is controlled by guard cells • Xylem – transports water and salts upwards • Phloem – transports sugars, mostly downward • Roots – draw in water and salts • Root hairs – increase surface area Leaves and stomata 1 Leaves and stomata 2 • Leaves are responsible for photosynthesis and exchange of gases and water • Gases and water enter and leave through the stomata • Epidermis provides protection • Palisade cells carry out photosynthesis • The spongy mesophyll layer allows storage of air and water vapour • Vascular bundles contain xylem and phloem for the transport of water, salts and sugars Control of stomal opening Control of stomal opening 2 • Stomatal opening is controlled by turgor pressure in the guard cells • This is controlled by pumping salts into the cells, thus bringing in more water (opening stoma) or pumping salts out of the cells, thus forcing water to leave (closing stoma) • Turgor pressure increases when water availability is high • Turgor pressure decreases when water availability is low Structures in the stem Xylem and phloem Xylem Phloem Dead Living Thick Lignin Impermeable Thin Cellulose Permeable Cross walls None Sieve plates Cytoplasm None Yes Function Carries water & salts Carries sugars Direction of flow Upwards Down and up Special features Fibres Companion cells Cells living/dead Cell walls: Thickness Material Permeability Roots and water transport Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman Root hairs and water transport Water moves in by osmosis • Osmotic pressure in root hair cell is higher than in soil • This can be maintained by active transport of salts into the root hairs • Root hairs increase the surface area available Measuring water loss • This can be done with a device called a potometer • The rate of transpiration is shown by movement of a bubble of air through the tubing • Key features include – air tight seal between plant and tubing, narrow tubing, intact stem (cut under water so it will draw up water), air bubble, scale Water loss in plants Water is lost when the stomata open to allow gas exchange eg for photosynthesis Factors causing an increase in water loss • Temperature • Increased air movement • Low water availability • Increase in light intensity Factors causing a decrease in water loss • Humidity • High water availability If the rate of photosynthesis increases then water loss is likely to increase If the plant tries to reduce water loss, the rate of photosynthesis may also decrease Arid regions Usually hot and dry eg Australian & African deserts & savannah Can be cold & dry eg icecaps & tundra Plant adaptations - arid Problems faced • Water availability low • Humidity low • Temperature high • Light high • Air movement high Solutions include Increased roots – either deep or wide and shallow Water storage (roots, leaves or stems) eg cacti, boabs Reduced leaves Reduced stomata in leaves Protection for leaves – cuticle, thick epidermis, curling, sunken pits, hairs to guard stomata Closing stomata in hot conditions Arid region adaptations Water storage Reduced leaves Reduced leaves Wide shallow root system Water storage succulent leaves Water storage - trunk Reduced leaves – loses leaves in summer Long deep roots Coastal environments Hot, dry, saline, moving sand which can cover plants Plant adaptations - coastal Problems faced • Water availability low • Humidity low • Temperature high • Light high • Air movement high – lots of sand and salt Solutions include Wide shallow root systems Rapid growth Can cope with burial Rolled leaves, sunken stomata, reduced stomata and/or hairs Succulent leaves Salt secretion in leaves Rainforest Low light, high humidity Plant adaptations - wet Problems faced • Water availability high • Humidity high • Temperature varies high (tropical) to medium (temperate) • Light can vary – high in the canopy, low at ground level • Air movement usually low Solutions include Large leaves to trap light Deep veins to carry water away from plant Usually many stoma and thin epidermis Large air spaces within leaves Aquatic environments Plenty of water, problems with water logging/lack of air Marine and estuarine plants must cope with high salinity Plant adaptations - aquatic Problems faced • Water availability high • Humidity usually high • Temperature varies • Light usually high (may vary if plant deeper under water) • Air low Solutions include Stomata on surfaces of leaves (eg water lilies) Large air spaces for buoyancy and gas storage Aerial roots (eg mangroves) Salt secretion in leaves (mangroves) Leaf adaptations 1 Leaf adaptations 2 Leaf adaptations 3 Look at the stomata in these leaves. Which of these is most likely to be adapted to arid conditions? This one- it has fewer stomata Leaf adaptations 4 What adaptations can be seen in these that allow them to survive arid conditions? Rolled leaf Thick cuticle and epidermis Rolled leaf Sunken pits Hairs to protect stoma Reduced stoma Sunken pits Thick cuticle and epidermis Leaf adaptations 5 What adaptations can be seen in these that allow them to survive in aquatic conditions?