Transport in Plants

The Need for Transport Systems in Plants Write the word equation for photosynthesis Unlike animals, plants do not transport water, mineral ions and organic molecules together in a single transport system. Transport of water and mineral ions Photosynthetic cells in the leaf need water and mineral ions which are available only in the soil. The transport of water and ___________ ions from the ____________ to the leaves is essential ; these molecules are transported upwards in hollow tubes formed by dead cells called xylem tissue. Transport of the products of photosynthesis (photosynthates) The glucose produced during photosynthesis is used to make ________________ and ____________ ____________ (photosynthates). These molecules must be moved from the leaves to other organs of the plant (e.g. ___________ and ______________). The photosynthates are transported upwards and downwards in phloem tissue.

Vascular Tissue in Plants The distribution of vascular tissue (xylem and phloem) differs in stems, leaves and roots. Draw labelled diagrams to show the arrangement of vascular tissue in transverse sections through the leaf, stem and root under low power. Leaf Stem Root

Uptake of Water and Mineral Ions by the Root Mineral ions such as nitrates - NO3- are ___________ ____________from the soil into root hair cells. This lowers the water potential inside the root hair cell so water enters by ___________. Water and dissolved mineral ions then travel through the root cells of the cortex to the endodermis down a water potential ____________. Transport of water across the root occurs by the symplast, apoplast and vacuolar routes. Vacuolar Pathway – Apoplast Pathway – water and ions travel through the cell walls, being sucked along by the transpiration stream Symplast Pathway-

Uptake of Water and Mineral Ions by the Root Once the water and mineral ions reach the endodermis all the water and ions are forced to travel along the symplast pathway. This is because the walls of the cells of the endodermis are impregnated with a waxy substance called suberin (i.e. the _______________ strip). Suberin is impermeable to ____________ and mineral ions. Therefore the only way that water and ions can pass through the endodermis to the pericycle and into the xylem is by the _____________ pathway.

Diagram of a root endodermal cell showing the Casparian strip How do mineral ions and water enter the xylem from the endodermis? Mineral ions are actively transported from outside the endodermal cells into the cytoplasm and from there to the pericycle and then into the xylem. Water follows by osmosis, down a water _________________ __________________. Why does the plant need to control entry of mineral ions into the xylem ? Why do plants need to absorb nitrates from the soil? Name some other mineral ions absorbed by plants and state their uses Give two features of root hair cells that are adaptations for uptake of water and minerals.

Transpiration Transpiration is the evaporation of water from inside the leaves through the stomata to the atmosphere. Water is absorbed by the _______ hair cells Water moves through the root tissue, into the xylem and is transported up the xylem in the plant stem to the leaf. Water moves from the xylem in the leaf by ____________ to the cells of the spongy mesophyll, where it evaporates from the surface of the cells into the ______ spaces. Water vapour then diffuses from the air spaces out of the leaf through the ___________ down a water potential gradient.

The Transpiration Stream Water molecules are attracted to each other by hydrogen ________. This is known as a _________________ force. Water molecules stick to the hydrophilic lining of the xylem. This known as an _________________ force. Capillarity - the forces of adhesion and cohesion allow water molecules to rise up narrow tubes, for a short distance which is useful in small plants. Why is capillarity not useful in large trees? Root Pressure – another force that helps to transport water upwards in xylem. This pressure is caused by osmosis, as water moves down its water potential gradient following __________ uptake in the root tissues (see p. 4-5) The Cohesion-Tension Theory (Transpiration Pull) Movement of water up the xylem of large trees can be explained by the cohesion-tension theory. As water molecules _____________ from the leaves, the water column is pulled up the xylem in one unbroken stream. Water evaporates from the leaves faster than it can enter through the ________ and therefore the water column is put under tension.

Factors Affecting Transpiration Any factor that increases the water potential gradient between the water vapour in the leaf and the surrounding atmosphere increases the rate of transpiration. The rate of transpiration will be affected by: These factors do not work independently of each other but interact with each other e.g. on a dry windy day ... Factor Rate of Transpiration Explanation Increased Temperature   Increased Air Speed Increased Humidity Increased Light Intensity

Potometer This apparatus measures the rate of uptake of water by the shoot which indicates the rate of transpiration. Why is the rate of uptake only an estimate of transpiration rate? Setting up the equipment:

Xylem Tissue Xylem tissue is composed of 4 different types of cell Vessels - Tracheids - Fibres - Xylem parenchyma – Vessels are made up of a continuous column of dead cells arranged end to end. The cross-walls between these cells dissolve completely to form a long, hollow tubes that transport large volumes of __________ from roots to leaves. The vessels have secondary cell walls that are thickened with lignin, this makes them impermeable to water and solutes. The lignin also provides the plant with mechanical strength and ______________. Tracheids are very similar to vessels but are more elongated with tapering ends. The cell walls are also thickened and impregnated with lignin. Both vessels and tracheids have perforations in their side walls called pits. In vessels, these pits allow sideways movement of ____________ between adjacent vessels. In tracheids the pits are involved in the sideways movement of water to nearby __________ tissue. Why is it important that vessel walls are impermeable to water and solutes? Lignin is hydrophilic, why is this important for xylem function?

Structure of Xylem Xylem cells Xylem structure of hardwood Xylem vessels

Phloem Tissue Phloem is made up of four different types of cells: Sieve tubes Companion cells Phloem fibres Phloem parenchyma Label - mitochondria, sieve plate, companion cell, sieve tube and plasmodesmata

Phloem tissue contains sieve tubes to transport organic compounds (photosynthates) e.g. _________________ and ____________ ____________. The sieve tubes are formed from cells called sieve elements placed end to end. The thin cellulose wall at the ends of these cells is perforated to form sieve plates that allow the cytoplasm from one cell to run into an adjacent cell. Phloem protein (filaments in cytoplasm) runs through the sieve plates. Smaller strands of cytoplasm run through the side walls of sieve tube cells into adjacent companion cells through the __________________. The companion cells do not transport organic materials but provide ______ and ___________ for the sieve tubes to control their activity. This is an important function since sieve tube cells lose their ____________ and other cell organelles as they mature. Give 3 ways in which sieve tube elements are adapted to their function:

Translocation Translocation is the transport of soluble organic materials produced by photosynthesis e.g. sucrose and amino acids, in the phloem. Movement of these molecules may be downwards e.g. from the leaf to the root or _______________e.g. from the leaves to fruits and shoot tips. The source is the region where the products of photosynthesis (photosynthates) are produced and exported. The sink is the region where the products of photosynthesis (photosynthates) are stored or used for growth. Identify the source and sink regions of this plant Source: Sink:

Evidence for translocation in phloem Ringing experiments Removal of a ring of outer bark tissue from a woody stem removes the phloem. Analysis of the phloem contents just above and below the ring shows that organic compounds (e.g. ______________ and amino acids) cannot be transported past the region where the bark has been removed. Aphid experiments. Aphids are small insects that can be used to collect the contents of individual phloem sieve tube cells. The contents can then be chemically analysed. Aphids, such as ______________ have specialised mouthparts called __________, which they use to penetrate ____________ tubes and sup out the sugary ______ inside. If the aphids are anaesthetised with carbon dioxide and cut off , the _________remains in the ______ so pure phloem ______ can be collected through the _________for analysis. This surprising technique is more accurate than a human with a ___________ as the aphid’s ________ ensure that the ________ doesn’t get blocked.

Radioisotope labelling The image shows the result of an experiment using a plant supplied with radioactive 14CO2. The radioactive 14CO2. is supplied to one leaf (marked with an arrow). After 20 minutes the whole plant is placed on photographic film. Dark areas on the film negative (autoradiograph) show areas containing radioactivity. The result shows that the radioactive carbon (14C) is fixed into the sugar produced at the source during photosynthesis (leaf marked with an arrow) . It is then translocated to sink parts of the plant (dark regions). This technique shows that the sugar is transported bidirectionally (upwards and __________________) since the radioactivity is seen in the aerial parts of the plant as well as the ___________. Which parts of the plant contain the most radioactivity? Explain your answer using the terms source and sink

The Mass Flow Hypothesis The main theory to explain translocation is mass flow. Sugars flow passively from areas of high concentration in the leaf (source) to areas of low concentration e.g. growing tissue (sink). How it works: Photosynthesis produces organic molecules including sucrose, in the source (the leaves) Sucrose is loaded by active transport into the sieve tubes, using ATP. Water enters the sieve tubes along a water potential gradient by osmosis. The pressure in the sieve tubes increases and the sucrose solution moves through the phloem towards the sinks (where the sucrose is being used). Sucrose is unloaded by active transport into the cells at the sinks (e.g. the roots or the growing shoots of a plant. The sucrose is used for respiration to provide ATP for growth and cell division or may be converted to starch for energy storage. Water moves by osmosis out of the phloem as the sucrose is removed and the pressure in the phloem tissue becomes lower at the sink. There is a pressure difference between source and sinks that maintains the movement through the phloem. This is called the mass flow hypothesis  

Mass flow hypothesis model

Arguments for and against the Mass Flow Theory The mass flow theory does not explain the observations that sucrose is transported in sieve tubes at a rate of 25-100 cm per hour. Diffusion alone would give a transport rate of 0.2 mm per day. There are a number of arguments against the theory and some recent theories put forward. Summarise the argument against the theory and the alternative theories: Arguments against Alternative theories

Mesophytes, Xerophytes and Hydrophytes Mesophtyes Most plants in temperate regions and most crop plants (grown for food) are mesophytes. They grow best in well drained soils and moderately dry air. A number of plant behaviours allow mesophytes to survive at unfavourable times of the year, e.g. Hydrophytes Hydrophytes are water plants, that live submerged or partially submerged in water.

Features of hydrophyte Explanation of this feature Little lignin in plant tissues Xylem tissue poorly developed Leaves have thin cuticle Stomata on upper surface of leaf Stems and leaves have large air spaces Transverse section of Water-lily (Nymphaea) leaf showing hydrophytic adaptations. Label the key features

Xerophytes Xerophytes are adapted to conditions of _______water availability. They live in hot, dry desert conditions, or cold regions where soil is ____________ for much of the year, or exposed windy locations. Marram grass is a xerophyte that specialises in living on sand dunes Tranverse section through Marram grass (Amophilia) leaf

Feature of Marram grass Explanation of this feature Rolled leaves Sunken Stomata (in pits) Hairs on leaf surface of leaf Thick cuticle Describe the xerophytic adaptations of cacti Why do pine trees have needle like leaves? Why do many xerophytes open stomata at night and close them during the day?