Mass Transport in Plants Flashcards
Plant roots, Symplastic and apoplastic pathway, Transpiration, Structure of xylem cells, Factors affecting rate of transpiration, Measuring the rate of transpiration, Translocation and mass flow and Mass flow evidence
Name the structures of a root
. Xylem . Phloem . Endodermis . Cortex . Epidermis . Root hair
How does water enter the plant?
Root hair cells actively transport ions from the soil across their membranes into their cytoplasm, via carrier proteins. This lowers the water potential of the cytoplasm, to below the water potential in the soil. Water enters the root hair cells by osmosis down a water potential gradient, it then passes across the cells of the cortex, across the endodermis, and into the xylem vessels. The xylem vessels transport water up the plant to the leaves
How are the roots adapted for water uptake?
. Root hair cells have extensions that increase their SA to increase the uptake of water
. They also have a thin cell wall to decrease the diffusion pathway
What are the two ways water can pass from the root hair cells, across the cells of the cortex and to the xylem?
. Symplastic pathway - through the cytoplasm of the cells, across the cell membranes and through plasmodesma from one cell to another. Water moves down a concentration gradient via osmosis, this process is quite slow as there is resistance to the movement of water
. Apoplastic pathway - water passes through adjoining cell walls from one cell to another, this movement is relatively fast and is by diffusion. The water can travel this way util it reaches the endodermis which has a waterpoof strip (casparian strip), the water is forced to pass through the cytoplasm of the cells. The cells of the endodermis actively transport ions into the xylem. This lowers the water potential in the xylem, causing the water to move into the xylem down a water potential gradient. This enables the movement of water from the root hairs to the centre of the root, down a water potential gradient
What is the first step in translocation?
Water leaves the leaf through the stomata by transpiration
What is the second step in translocation?
This lowers the water potential at the mesophyll layer which causes water to travel up the xylem creating tension
What is the third step in translocation?
Th water molecules cohere by hydrogen bonds which makes the water flow in a continuous column
What is the fourth step in translocation?
Water is also attracted to the walls of the xylem creating adhesion
How are xylem vessels adapted to help them carry out their function?
. Dead cells, which form hollow tubes - allows the water potential not to be affected so all the water stays within the xylem vessel
. Lignin, strengthens the cell walls - the xylem is waterproof
. End wall breaks down so xylem forms a continuous tube - water can form a continuous column
. Pits in the xylem - allows water to move between xylem vessels
What are the 4 main factors that affect the rate of transpiration?
. Light - stomata open in more light
. Temperature - increases kinetic energy, so diffusion rate increases
. Humidity - the greater the difference in humidity the greater diffusion (more humid = less diffusion)
. Air movement - it moves water away from the stomata so increases diffusion
What is used to measure the rate of transpiration?
Potometer
What are the steps in measuring the rate of transpiration?
- A leafy shoot is cut under water. Care is taken to prevent getting water on the leaves
- The potometer is filled carefully with water making sure there is no air bubbles
- Using a rubber tube, the leafy shoot is fiited to the photometer under water
- The photometer is removed from the water and all joints are sealed with waterproof jelly
- An air bubble is introduced into the capillary tube
- As transpiration happens, water moves through the capillary tube, taking the bubble with it
- The distance moved over a period of time is recorded and the mean is calculated of a number of repeats
- The volume of water lost over a period of time can be calculated
What are the precautions a student should take when carrying out the rate of transpiration experiment?
. Ensure the apparatus is watertight be sealing joints . Cut shoot under water . Cut shoot at a slant . Dry the leaves . Insert the shoot into the apparatus under water . Ensure no air bubbles are present . Shut tap . Not where the bubble is at the start
What is the mass flow hypothesis?
- The palisade cell in the leaf turns glucose into sucrose which establishes a concentration gradient between the leaf and the companion cell. Facilitated diffusion transports the sugar into the companion cell
- The sucrose is then actively transported against it’s concentration gradient into the phloem which has a high concentration of sucrose. This lowers the water potential so water enters from the phloem via osmosis increasing the hydrostatic pressure, forcing the solution away towards the sink
- At the sink the sucrose is loaded into the companion cell (where it will be used for processes like respiration or stored) lowering the water potential in the cell causin g water to leave the phloem via osmosis, decreasing the hydrostatic pressure creating a pressure gradient from source to sink
What evidence supports the mass flow theory?
. Aphids - they pierce the phloem then their body is removed. Sap flows out suggesting the phloem is under pressure
. Ringing - bark containing phloem is removed, swelling above the phloem then occurs indicating sugars can’t move past the area with no phloem and the sugar travels downwards
. Radioisotopes - the plant photosynthesises with radioactive CO2, when traced with x-ray film it could be detected in the phloem
