4.4 Flashcards
transport in plants
describe the xylem
carries water and dissolved minerals from the roots to the photosynthetic parts of a plant
always moves upwards
made up of dead cells
describe the phloem
transports dissolved solutes around the plant
can move up and down
made of living cells
what is the cambium
layer of unspecialized plant cells that divide to form both the xylem and phloem
what is the protoxylem
the first xylem formed that can stretch and grow because the walls are not fully lignified
summarise the formation of the xylem
it begins as living cells as the protoxylem
the cellulose microfibrils in the walls lay vertically and provide strength
as the stem ages cells stop growing which increases the amount of lignified tissue (metaxylem) in the walls making the cells impermeable to water and other substances
the contents of the cell dies and end walls between cells breaks down developing transverse plates to eventually form hollow tubes
what is metaxylem
consists of mature xylem vessels made of lignified tissue
how does water move in and out of the xylem
water and minerals are taken in by the roots and travel up the xylem and transport the water and minerals to cells at undignified areas or specialized pits (holes) in the xylem walls
what are pits
specialized holes in the xylem wall to allow water and minerals to move out into surrounding cells
what are parenchyma cells
relatively unspecialised plant cells that act as packing in stem and roots to give support through turgor pressure
what are sclerenchyma cells
plant cells that have thick lignified cell walls and an empty lumen with no living contents
what are collenchyma cells
plant cells with areas of cellulose thickening that give mechanical strength and support to the tissues
how can you experiment the movement of water in the xylem
- cut the end of a shoot and place it in a solution of eosin dye that can then be seen to be carried in the xylem
- remove and ring of bark from a tree and kill cells then place dye in water and it will be taken up by the xylem
- autoradiography
summarize autoradiography
radioactive label is placed on substance taken up by the xylem (water containing deuterium can be used)
the radioactive substance is taken up and can be traced by placing plant against photographic film to produce a autoradiograph or traced using a scintillation counter
how is the phloem formed
phloem sieve tubes join up and their walls become perforated forming sieve plates
organelles in the sieve tube such as nucleus and tonoplasts break down and the tube is filled with phloem sap
they are then supported by companion cells by many plasmodesmata which move sucrose into and out of the phloem
how are companion cells well adapted
cell membrane contains many infoldings to increase surface area for transportation of sucrose
contain many mitochondria to supply ATP for active transport of sucrose
what are root hairs and why are they needed
microscopic hairs that are extensions of the membrane of the outer cells of the root to increase surface area for the absorption of water and minerals down the concentration gradient by osmosis and allows close contact with soil particles
how does water travel from the soil to they xylem generally
it is taken up by the root hair down the concentration gradient by osmosis and then passes through neighboring cells by osmosis to the xylem
what is the symplast pathway
water moves by diffusion down the concentration gradient from the root hair cells to the xylem through interconnected cytoplasm in the plasmodesmata, gaps in the cellulose cell wall that allow strands of cytoplasm to pass through them
what is the apoplast pathway
water is pulled by the hydrogen bonds between water molecules across adjacent cell walls from the root hair cell to the xylem through the open network structure of the cell wall
what is the casparian strip
the waterproof layer of waxy tissue in the walls of the endodermis which minerals must move through via active transport against the concentration gradient to enter the cytoplasm
why is the casparian strip important
it allows cells to control the amount of water and minerals moving from the soil into the xylem
what is translocation
the movement of substances around the plant
define transpiration
the loss of water vapour from the surfaces of the plant that has evaporated from the surface of the spongy mesophyll cells within the leaves
how does transpiration occur
water reaches the leaves and moves by osmosis from the xylem in the veins to the spongy mesophyll where it then evaporates from the cellulose walls into air spaces and moves out through open stomata by diffusion along the concentration gradient
what is the cohesion tension theory of transpiration
water is lost by transpiration and therefor more water leaves the xylem and enters the leaves by osmosis down the concentration gradient creating tension in the column of water due to the cohesion of the water as a result of its polar nature forming hydrogen bonds the water molecules ‘stick together’ which gives the column high tensile strength
therefor when a water molecule is lost through transpiration it creates tension along the column and pulls more water up the column to replace it
explain the adhesion of water in the xylem and why it is important
water molecules adhere strongly to the walls of the narrow xylem due to the attraction between unlike molecules such as tiny channels and pores within the cellulose cell walls which helps to support the column of water and pull the whole column of water upwards
what is the transpiration stream
the movement of water up from the soil through the root hair cells and across to the xylem, then up the xylem across the leaf until it is lost by transpiration from the spongy mesophylls and out of the stomata
summarize the experiment in measuring the movement of water in plants
get a fresh leafy shoot and cut under water then transfer to potometer ensuring roots are covered
create an air bubble at the start of the capillary tube of known diameter in the potometer
measure the start point of the air bubble
how can you check if water is being lost from the plant in the experiment
seal the plant in a bell jar to condense on the glass of the bell jar and cobalt chloride or copper sulfate paper can be used to show the presence of water as there will likely be very small amounts
why must the stem be cut under water and kept underwater for the experiment
to avoid air bubbles
what is the reservoir of water in a potometer used for
resetting the air bubble
how can you ensure the seal around the plant in the potometer is air tight
use petroleum jelly
what are the main factors that effect transpiration
light
temperature
air movement/wind
air humidity
describe and explain the graph for the effect of light on transpiration rate
as stomata open in the light and close in the dark for photosynthetic gas exchange as light intensity increases rate of transpiration also increases until all of the stomata are open then transpiration rate levels off
describe and explain the graph for the effect of temperature on transpiration rate
as temperature increases rate of transpiration increases due to several factors:
increased evaporation
increased amount of water vapour the air can take before becoming saturated which increases the concentration gradient
increased kenetic energy so molecules move faster increasing rate of diffusion
until another limiting factor such as light intensity causes rate of transpiration to level off
explain the effect of air movement/wind on rate of transpiration
reduces the shell of still air around the stomata which increases the concentration gradient so rate of transpiration increases
describe and explain the graph for the effect of air humidity on transpiration rate
as humidity increases rate of transpiration decreases because there is a higher concentration of water vapour in the air which reduces the concentration gradient so lower rate of transpiration until the water concentration is greater outside than inside the leaf therefor rate of transpiration levels off
what is guttation
a process in which drops of water may be forced out of the leaves as a result of root pressure
define root pressure
the pressure that results when salts are actively secreted from the root cells into the xylem sap, increasing the concentration across the root and moving water in by osmosis
why is root pressure thought to be an active process
the pressure disappears if cells are killed by steam (heat) or poison therefor enzymes for active transport have been denatured
why is glucose converted to sucrose for translocation
sucrose has less of an osmotic effect than glucose
what are assimilates
substances that are transported by the phloem (mainly sucrose)
where does translocation occur
phloem
what are the sources of sucrose in plants
leaves and stem (green parts)
what is a sucrose sink
a region of a plant that is low in sucrose and removes sucrose from the phloem
give examples of a sucrose sink
actively dividing cells (meristem), storage organs
what is the symplast pathway of phloem loading
sucrose moves by diffusion down the concentration gradient from the source cells through to the companion cells then into the phloem sieve tubes causing water to also enter the phloem sieve tubes resulting in a positive hydrostatic pressure which moves the phloem sap towards the sinks
what is the apoplast pathway
sucrose moves by diffusion down the concentration gradient through the cell walls and cell spaces into the companion cell wall it is then actively transported into the cytoplasm of the companion cell making a high sucrose concentration. sucrose then passes into the sieve tubes through the plasmodesmata and water moves into the companion cells by osmosis producing a positive hydrostatic pressure which moves assimilates into the phloem sieve tubes and then towards sinks where the hydrostatic pressure is lower and sucrose concentration is lower. This concentration gradient is maintained by the removal of sucrose by mass flow of cell sap to the sucrose sinks
summarise the mechanism that moves assimilates through the phloem
sucrose accumulates in the phloem which increases turgor pressure and forces sap to regions of lower pressure in sinks
explain phloem unloading
sucrose diffuses out of the sieve tubes down the concentration gradient then quickly onto other cells or is converted into other compounds in order to maintain the concentration gradient, as sucrose leave water potential of the sap increases and so moves out down the concentration gradient to other cells and often into the xylem
what evidence is there for translocation
radioactive carbon-14 can be made available in plant leaves and therefor creates radioactive glucose and then sucrose is labelled with this radioactive marker that can be traced using autoradiography
use a jet of steam to kill a complete outer ring of bark then below the dead tissues will not have movement of solutes
what is the transport medium in translocation
phloem sap
what provides the force for movement in translocation
turgor (hydrostatic) pressure
