IB TOPIC 9 | PLANT BIOLOGY
2016 | SYJ0014
Topic 9.1: Plant biology – Transport in the xylem of plants
Structure and function are correlated in the xylem of plants.
• Understanding: Transpiration is the inevitable consequence of gas exchange in the leaf.
Stomata: pores in the epidermis of a leaf that allows for gas exchange
Guard cells: cells found in pairs on each side of a stoma that can control the opening of the stoma
Absisic acids: hormones that control the opening and closing of the stoma by stimulating guard cells
Transpiration: loss of water vapour from the leaves and stems of plants
When plants absorb carbon dioxide and excrete oxygen during photosynthesis, water vapour also can escape
Water evaporates out due to a water potential gradient inside and outside of the plant
Therefore, transpiration is the inevitable consequence of gas exchange
Water is lost from the spongy mesophyll cells through stomata to the atmosphere
• Understanding: Plants transport water from the roots to the leaves to replace losses from transpiration.
Movement of water from roots to leaves:
Stoma: water is lost from stoma due to concentration gradient
Leaf: water leaving stoma is replaced from xylem
Xylem: water in xylem climbs stems due to low concentration gradient in the
leaf (transpiration pull)
Root: water moves into soil into roots by osmosis, and water in root replace the
water in xylem that is released
Pathway for water absorption in root:
Apoplast: water is absorbed through the cell wall (red)
Symplast: water is absorbed through cytoplasm
• Understanding: The cohesive property of water and the structure of the xylem vessels allow transport under tension.
Plants and cohesive property of water:
Cohesion property of water: water has strong cohesive property as they attract each other strongly with hydrogen bonding
Application in plants: plant utilizes the strong cohesion property to pull up water in a continuous stream through various structure
Long continuous tube: adjacent cell wall area is broken down to maximize transport of water
Thickened wall: cellulose and lignin harden the xylem wall so cells can withstand low pressure
• Understanding: The adhesive property of water and evaporation generate tension forces in leaf cell walls.
Plants and adhesive property of water:
Adhesive property of water: water molecules are polar and thus are attracted to other polar molecule
Application in plants: plant utilizes the adhesive property to pull up water in a continuous stream against gravity
Transpiration pull forms: evaporation from the leaf to the atmosphere causes a transpiration pull
Tension force forms: tension force is generated as the low water potential in the leaf pull the water from the root
Water pull: force of adhesion alongside others allows xylem to pull up water against gravity
• Understanding: Active uptake of mineral ions in the roots causes absorption of water by osmosis.
Active uptake of mineral ions in the root:
Solute concentration difference: solute concentration inside the root is greater than that in the soil
Active transport: to maintain concentration of mineral ions, the difference is established via active transport
Use of ATP: ATP is consumed during active transport
Mitochondria abundance: root epidermal cells are rich in mitochondria and requires oxygen
Diffusion: water diffuses into the root by osmosis
Characteristics of root for absorption:
Branched roots: roots are branched to increase surface
Root hairs: extensions of epidermis that increase surface area
Multiple different pathway: different methods of the water can travel through
Mutualistic relationship: plants build relationship with fungus that absorb mineral ions that is supplied to the root
• Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.
Porous pots:
Target of experiment: modelling transpiration pull; cohesion and adhesion property of water
Situation: water evaporates out form the surface of the porous pots
Explanation: water is drawn up from glass tubing and is lost from trough as there is a transpiration pull
Capillary tubing:
Target of experiment: modelling cohesion and adhesion property of water and other liquids
Situation: capillary tube is dipped into water with dye and mercury
Capillary tube in water: dyed water climb up the tube due to adhesion and cohesion of water
Capillary tube in mercury: mercury does not climb up the tube due to lack of both adhesion and cohesion
Explanation: water has stronger adhesion and cohesion property due to its polar nature and intermolecular hydrogen bonding
LAST EDITED 2017-03-17 | 1
2016 | SYJ0014
Topic 9.1: Plant biology – Transport in the xylem of plants
Structure and function are correlated in the xylem of plants.
• Understanding: Transpiration is the inevitable consequence of gas exchange in the leaf.
Stomata: pores in the epidermis of a leaf that allows for gas exchange
Guard cells: cells found in pairs on each side of a stoma that can control the opening of the stoma
Absisic acids: hormones that control the opening and closing of the stoma by stimulating guard cells
Transpiration: loss of water vapour from the leaves and stems of plants
When plants absorb carbon dioxide and excrete oxygen during photosynthesis, water vapour also can escape
Water evaporates out due to a water potential gradient inside and outside of the plant
Therefore, transpiration is the inevitable consequence of gas exchange
Water is lost from the spongy mesophyll cells through stomata to the atmosphere
• Understanding: Plants transport water from the roots to the leaves to replace losses from transpiration.
Movement of water from roots to leaves:
Stoma: water is lost from stoma due to concentration gradient
Leaf: water leaving stoma is replaced from xylem
Xylem: water in xylem climbs stems due to low concentration gradient in the
leaf (transpiration pull)
Root: water moves into soil into roots by osmosis, and water in root replace the
water in xylem that is released
Pathway for water absorption in root:
Apoplast: water is absorbed through the cell wall (red)
Symplast: water is absorbed through cytoplasm
• Understanding: The cohesive property of water and the structure of the xylem vessels allow transport under tension.
Plants and cohesive property of water:
Cohesion property of water: water has strong cohesive property as they attract each other strongly with hydrogen bonding
Application in plants: plant utilizes the strong cohesion property to pull up water in a continuous stream through various structure
Long continuous tube: adjacent cell wall area is broken down to maximize transport of water
Thickened wall: cellulose and lignin harden the xylem wall so cells can withstand low pressure
• Understanding: The adhesive property of water and evaporation generate tension forces in leaf cell walls.
Plants and adhesive property of water:
Adhesive property of water: water molecules are polar and thus are attracted to other polar molecule
Application in plants: plant utilizes the adhesive property to pull up water in a continuous stream against gravity
Transpiration pull forms: evaporation from the leaf to the atmosphere causes a transpiration pull
Tension force forms: tension force is generated as the low water potential in the leaf pull the water from the root
Water pull: force of adhesion alongside others allows xylem to pull up water against gravity
• Understanding: Active uptake of mineral ions in the roots causes absorption of water by osmosis.
Active uptake of mineral ions in the root:
Solute concentration difference: solute concentration inside the root is greater than that in the soil
Active transport: to maintain concentration of mineral ions, the difference is established via active transport
Use of ATP: ATP is consumed during active transport
Mitochondria abundance: root epidermal cells are rich in mitochondria and requires oxygen
Diffusion: water diffuses into the root by osmosis
Characteristics of root for absorption:
Branched roots: roots are branched to increase surface
Root hairs: extensions of epidermis that increase surface area
Multiple different pathway: different methods of the water can travel through
Mutualistic relationship: plants build relationship with fungus that absorb mineral ions that is supplied to the root
• Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.
Porous pots:
Target of experiment: modelling transpiration pull; cohesion and adhesion property of water
Situation: water evaporates out form the surface of the porous pots
Explanation: water is drawn up from glass tubing and is lost from trough as there is a transpiration pull
Capillary tubing:
Target of experiment: modelling cohesion and adhesion property of water and other liquids
Situation: capillary tube is dipped into water with dye and mercury
Capillary tube in water: dyed water climb up the tube due to adhesion and cohesion of water
Capillary tube in mercury: mercury does not climb up the tube due to lack of both adhesion and cohesion
Explanation: water has stronger adhesion and cohesion property due to its polar nature and intermolecular hydrogen bonding
LAST EDITED 2017-03-17 | 1
