Lesson 8: water transport in plants

Lesson 8: water transport in plants

Xylem Vessels and Phloem Vessels

• Plants need a way to transport materials such as Water, Sugars and Dissolved Minerals throughout there various tissues

• In order to accomplish this task they contain specialized vascular tissues called the Xylem and Phloem

Xylem

• Transports Water and dissolved minerals from the soil to the leaves

• Xylem cells are dead at maturity and act like pipes within the plant

• The cells are linked to each other forming continuous tubes called Xylem Vessels

• Xylem Vessels can be divided into two groups:

1. Tracheid’s

2. vessel elements

• Water transport in Xylem Vessels allows for Photosynthesis and Turgor Pressure in plants

Phloem

• Transports Sugars produced by Photosynthesis from the Leaves to the rest of the plant

• Phloem cells are alive at maturity and also act as pipes within the plant

• However, Phloem cells are porous allowing the exchange of materials between the Phloem cells and neighbouring tissue cells

• The Phloem cells are linked to each other forming continuous tubes called Phloem Vessels (Note: Cells are separated from each other by Sieve Plates)

• Phloem Vessels can be divided into two groups:

1. Sieve Tubes

2. Companion Cells

• Sap transport in Phloem Vessels allows for nutrients to reach the various tissues in the plant

Water Uptake in Roots

• Water and minerals enter a plant from the Roots

• At the core of the root are the Xylem and Phloem

• Epidermal tissue covers the root

• At its tip, the epidermal cells are permeable to water and water enters the root here by osmosis

• Root hairs help to increase the surface area of the roots

• Water continues to diffuse into the root tissue until it reaches the xylem

• Although water diffuses easily across the cell membrane, minerals do not

• The plant must use Facilitated Diffusion or Active Transport to move minerals across the membrane

• The water and minerals that enter the Xylem is called the Xylem Sap

• The sap travels up through the Tracheid's and Vessel Elements of the Xylem Vessels towards the leaves As the Xylem sap enters the Leaves the Xylem Vessels branch into veins and deliver the water and minerals to the cells of the leaves

Properties of Water

• How does the Xylem Sap rise up to the top of the Plant?

• Water is a Polar Molecule and the negatively charged Oxygen of one water molecule will attract the positively charged Hydrogens of another water molecule

• This phenomenon is called Cohesion and it helps to drag the water up the Xylem Vessels

Cohesion:

• Cohesion allows water molecules to transport through the Xylem like a chain where each water molecule is a separate link in the chain

• If there is a bubble in the Xylem or the water freezes the Cohesion can be disrupted and only the water molecules above the break will continue upwards

Adhesion:

• Water also has the ability to attract to other molecules

• This attraction is called Adhesion and allows the water molecules to climb up the walls of the Xylem Vessels .

Transpiration Pulls

• Root Pressure, Cohesion and Adhesion will work for small plants but what about huge trees?

• Transpiration helps move the water up the xylem by evaporating water out of the leaves

• Since Water drags the water up from the roots as the water evaporates out of the leaves

Root Pressure Pushes

• Cohesion and Adhesion help to drag the water molecules up the Xylem Vessels of the plant, however, the roots also help to push the water up the plant

• Turgor Pressure within the roots helps to force the Xylem sap into and up the Xylem Vessels

• As Minerals are Actively Transported into the root it makes the root Hypertonic and thus brings in more water by osmosis adding more root pressure helping the Xylem Sap to move upwards

Sugar Transport in Phloem

• After Photosynthesis has occurred in the leaves the sugars must be transported to the rest of the plant to be used as energy

• Phloem transports the sugars (and other materials) throughout the plant

• As the Sugars enter the Phloem Vessels the solution becomes Hypertonic and draws in water by osmosis

• This solution is then called Phloem Sap and the Sap will move throughout the Phloem Vessels down a concentration gradient to the rest of the Plant tissues