Presentation on theme: "A. state the functions of xylem and phloem. b. identify the positions of xylem and phloem tissues as seen in transverse section of roots, stems and leaves."— Presentation transcript:
a. state the functions of xylem and phloem. b. identify the positions of xylem and phloem tissues as seen in transverse section of roots, stems and leaves. c. investigate, using a suitable stain, the pathway of water in a cut stem.
How is water transported against gravity from the roots, up the xylem and to the leaves?
Think Like a Scientist Scientists use ‘thought experiments’ to help them solve problems. 3
I wonder where trees get water from? Well, obviously from the ground. What are the processes involved?
How does water move through the transport system of a plant IF it does not have a heart to act as a pump? PAUSE to PONDER How is water lifted against gravity from the ground to the leaves through this transport system? Are the products of photosynthesis also carried in a set of vessels from the leaves to the roots? 5
How do nitrate ions get into plants? Are they directly absorbed from the air? No. Even though the air has 79% of nitrogen, it is highly unreactive. PAUSE to PONDER How are ions transported around in plants?
Transport in Plants Plants need a transport system so that cells deep within the plants tissues can receive the nutrients they need for cell processes The problem in plants is that roots can obtain water, but not sugar, and leaves can produce sugar, but can’t get water from the air
What substances need to be moved? The transport system in plants is called vascular tissue Xylem tissue transports water and soluble minerals Phloem tissue transports sugars
Structure of Xylem Used to transport water and minerals from roots to leaves Consists of tubes for water, fibres for support and living parenchyma cells
Xylem vessels Obvious in dicotyledonous plants Long cells with thick walls containing lignin Lignin waterproofs walls of cells and strengthens them Cells die and ends decay forming a long tube Lignin forms spiral, annular rings or broken rings (reticulate) Some lignification is not complete and pores are left called pits or bordered pits, allowing water to move between vessels or into living parts
Adaptations of Xylem to Function Xylem can carry water and minerals from roots to shoot tips because: Made of dead cells forming continuous column Tubes are narrow so capillary action is effective Pits allow water to move sideways Lignin is strong and allows for stretching Flow of water is not impeded as: there are no end walls, no cell contents, no nucleus, lignin prevents tubes collapsing
Structure of Phloem Function to transport sugars from one part to another Made of sieve tube elements and companion cells
Sieve Tubes Sieve tube elements not true cells as they have little cytoplasm Lined up end to end to form a tube Sucrose is dissolved in water to form a sap Tubes (known as sieve tubes) have a few walls across the lumen of the tube with pores (sieve plates)
Companion cells In between sieve tubes Large nucleus, dense cytoplasm Many mitochondria to load sucrose into sieve tubes Many gaps in cell walls between companion cells and sieve tubes for flow of minerals
The Vascular Tissues Xylem and phloem are found together in vascular bundles, that sometimes contain other tissues that support and strengthen them
Vascular bundles: xylem & phloem Xylem transports water & dissolved minerals from roots to leaves. Phloem transports food (sugar) made in leaves to all other parts of the plant.
Stem The vascular bundles are found near the outer edge of the stem The xylem is found towards the inside of each vascular bundle, the phloem is found towards the outside In between the xylem and phloem is a layer of cambium Cambium is a layer of meristem cells that divide to make new xylem and phloem
Xylem & phloem tissues in stems.
Phloem Xylem Stem
Root The vascular bundle is found in the centre There is a large central core of xylem- often in an x-shape This arrangement provides strength to withstand the pulling forces to which roots are exposed Around the vascular bundle are cells called the endodermis which help to get water into the xylem vessels Just inside the endodermis is the periycle which contains meristem cells that can divide (for growth)
Distribution of vascular bundles in roots xylem phloem xylem phloem
Leaf The vascular bundles (xylem and phloem) form the midrib and veins of the leaf A dicotyledon leaf has a branching network of veins that get smaller as they branch away from the midrib Within each vein, the xylem can be seen on top of the phloem
Distribution of vascular bundles in leaves. xylem phloem
xylem phloem Diagram showing a section through a leaf.
A = Xylem B = Phloem C/D = Upper/Lower epidermis Leaf
Root vs. stem vs. leaf The vascular bundle differs depending on if it is a root or stem
Transport Structures LeafStemRoot Xylem Phloem Xylem Phloem
d. describe the structure and functions of root hair cells in relation to their surface area, and to water and ion uptake. e. define transpiration. f. describe how factors (e.g humidity, temperature, light intensity) affect the rate of transpiration. g. describe how wilting occurs.
Transport of Water
Root Stem Leaves
Water uptake in ROOT Animation
Root Pressure Osmotic pressure that build up within the root cells which forces water up the root xylem Recap on Osmosis How does the root maintain a concentration gradient?
Root Pressure Uptake of water by the root hair by Osmosis Uptake of dissolved minerals through active transport Cell sap within the root hairs becomes more concentrated than the water in the soil
Root Pressure Occurs when the soil moisture level is high either at night or when transpiration is low during the day Can only raise the water in some plants up to 20cm Not the main force
Water transport in STEM You have learnt that water is transported from the root to the stem and the leaf. How does the plant transport water upwards against gravity??
Transpiration Evaporation of water from the plant Loss of water vapor through the stomata on the underside of the leaves 1.The mesophyll cells in the leaves are covered with a thin layer of moisture Essential for efficient gas exchange to occur 2.Some of this moisture evaporates into the intercellular spaces which diffuses through the stomata into the ‘drier’ air Water is pulled upwards through osmosis (Transpiration Pull) 3.The waterway in which the water moves from a higher water potential to a lower water potential Transpiration Stream
Potometer The rate at which plants take up water depends on the rate of transpiration- the faster a plant transpires, the faster it takes up water.
Capillary Action The attractive force between the molecules of a particular liquid is known as Cohesion Water coheres to each other via chemical bonds called hydrogen bonds (holds the droplets of water together) The attractive force between two unlike materials is known as Adhesion Adhesion causes water to stick to the inside of the glass When water passes up the thin xylem vessels, it adheres to the surface of the vessels, while the force of osmosis gently ‘pushes’ the water molecules, which cohere to each other, upwards
Forces that promotes uptake of water 1.Root Pressure Promotes uptake of water in the root 2.Transpiration pull Main force to ‘suck’ up the water 3.Capillary Action Pushes the water upwards due to the adhesion between the walls of the xylem vessels and water molecules as well as the cohesion between the water molecules
Transport of Food
Transport of Food in plant Food substances (sugar & amino acids) are manufactured in the green leaves through the process called photosynthesis Translocation is the process of transporting food substances downwards from the leaves to all other parts of the plant, through the phloem
3 experiments to show phloem transport food substances 1.Using Aphids Aphid penetrates the stem into the phloem using its mouthpart called stylet and sucks the plant sap A feeding aphid can be anaesthetized and the stylet cut off The phloem sap flows out through the stylet and can be analysed. It is found to contain sugars and other organic substances
3 experiments to show phloem transport food substances 2.The Ringing Experiment 1.Swelling above the ring 2.Reduced growth below the ring 3.Leaves are unaffected
3 experiments to show phloem transport food substances 3.Using radio-isotopes
Wilting of plant
Wilt Temporary or permanent loss of turgor pressure due to excessive transpiration of the leaves & shoot Result in flaccid appearances of leaves & shoot Characterized by drooping & shriveling of soft tissue
a.Is wilting necessary a disadvantage to the plants? Why? b. Wilting reduces loss of water in leaves. How does it affect intake of water for photosynthesis? c. How is it the prolong process of wilting cause harm to plants? d. What other conditions may cause plants to wilt?
Advantage of wilting Rate of transpiration reduces as the leaves folds up, reducing surface that is exposed to sunlight. This will cause guard cells to become flaccid and the stomata will close to conserve water in the plant. Disadvantage of wilting Rate of photosynthesis will be reduced due to the following reasons: Water becomes a limiting factor now. The amount of sunlight that can be absorbed becomes a limiting factor as leaves shriveled up. The amount of carbon dioxide becomes a limiting factor as the stomata are closed.
Things to ponder What time of the day should you water your plant? Too much water, is it good for your plant?
People often give potted houseplants more fertilizer than they need. As a result, the plants begin to wilt and eventually die instead of getting larger and healthier. What could be the reason for this result?
“Girdling” or ring-barking is a term referring to the bark of the tree in a complete ring around the trunk or a branch. Predict the effect that girdling will have on a tree. Explain. Beneficial or Destructive to plants?