Module 3- Transport in plants Flashcards
Describe the structure of xylem
-long tubes of dead cells that run up the stems of plants
-vessel elements (xylem cells) are stacked on top of one another
-there are no cell walls at the end of each vessel element, creating a continuous tube for water to flow through
-the walls of xylem are lined with a waterproof polymer called lignin which reinforces the xylem walls and provides structural support
What are the cells of xylem called?
vessel elements
Describe the cohesion tension theory
-most of the water in the leaves evaporates via evapotranspiration
-the loss of water from the leaves creates tension in the xylem where hydrogen bonds are formed between water molecules and the sides of the xylem elements
-water in the xylem is pulled upwards by the tensions towards the leaves
-individual water molecules also form hydrogen bonds with each other (cohesion) making other molecules of water move upwards
-this continuously pulls water upwards to replace water lost by transpiration
-when water is pulled up the stem the water potential at the bottom of the plant decreases so water diffuses into the roots by osmosis down its water potential gradient
How do you investigate transpiration rate?
-the potometer is filled with water and a cutting of a shoot is placed inside (this must be done under water to ensure no water enters the xylem)
-remove the end of the capillary tube from the water beaker and wait for a bubble of air to form in the capillary tube
-mark the starting position of the air bubble and use a stop watch to record the distance moved by the air bubble in a given time period
-calculate the transpiration rate (rate of movement of bubble per hour)
-change variables e.g light or temperature
What is a potometer?
piece of equipment used to estimate the volume of water taken up by a plant in a given time
What habitats are xerophyotic plants specialised to?
dry habitats where water availability is low
How are xerophyotic plants adapted to their environment?
-have fewer stomata to avoid water lost when they are open during the day
-have sunken stomata which are sunk into the pits in the epidermis as the pits trap moist air which decreases the water potential gradient between inside leaves and outside leaves slowing the rate of diffusion of water out of the stomata
-have fine hairs covering the epidermis which trap moist air and decrease water loss
-curled leaves so stomata are sheltered from the wind
-have a waxy cuticle which is waterproof so reduced the rate of evaporation of water
What is the structure of phloem?
-sieve tube elements are the cells that make up the phloem vessel which run up the stem on top of each other
-living cells contain a cytoplasm but no nucleus
-the walls of sieve tube elements are made of cellulose
What are companion cells connected to sieve tube elements by?
via plasmodesmata
What does plasmodesmata do?
allows cytoplasm to be shared between companion cells and sieve tube elements
Do companion cells have a nucleus? yes/no
yes
Structure , function and location of sieve plates?
-found at either end of the sieve tube elements
-have large pores that allow sap to move through the sieve tube elements
-allow sugars to be transported through the phloem
What is translocation?
process where sugars produced in photosynthesis are transported from the leaves to other parts of the plant
Describe the process of active transport of sucrose in translocation?(1st stage)
-sucrose is produced during photosynthesis in the leaves so the leaves are the source
-sucrose is actively transported into the companion cells in the phloem
-energy is provided for active transport from the mitochondria in the companion cells
Describe the process of diffusion of sucrose in translocation? (2nd stage)
-the loading of sucrose into the phloem causes concentration of sucrose to increase
-sucrose diffuses from the companion cells into the sieve tube elements
