Transport In Plants

Question 1

define dicotyledonous plants

Answer 1

plants with two seed leaves and a branching pattern of veins in the leaf

Question 2

define meristem

Answer 2

a layer of dividing cells, here it is called the pericycle

Question 3

define phloem

Answer 3

transports dissolved assimilates

Question 4

define vascular tissue

Answer 4

consists of cells specialised for transporting fluids by mass flow

Question 5

define xylem

Answer 5

transports water and minerals

Question 6

why do plants need a transport system

Answer 6

simple- they need a transport system to move water and minerals from the roots up to the leaves
-to move sugars from the leaves to the rest of the plants

all living things need to take substances from, and return wastes to their environment .as with animals , larger plants have a smaller surface area to volume ratio . Therefore , they need to have specialised exchange surfaces and a transport system

every cell of a multicellular plant needs a regular supply of oxygen , water, nutrients and minerals. plants are not very active , and their respiration rate is low - therefor demand for oxygen is low. this demand can me met by diffusion . however the demand for water and sugars is still high. plants can absorb water and minerals at the roots , but they cannot absorb sugars from the soil . the leaves can perform gaseous exchange and manufacture sugars by photosynthesis , but they cannot absorb water from the air, therefore plants need a transport system

Question 7

the transport system consists of

Answer 7

specialised vascular tissues , the phloem and xylem

Question 8

describe the xylem and phloem in the young root

Answer 8

the vascular bundle is found at the centre of a young root . there is a central core of xylem, often in the shape of an X . the phloem is founf in between the arms of the x shaped xylem tissue . this arrangement provides strength to withstand the pulling forces to which roots are absorbed

around the vascular bundle is a special sheath of cells called the endodermis. the endodermis has a key role In getting water into the xylem vessel, just inside the endodermis is a layer of meristem cells called the pericycle

Question 9

describe the xylem and phloem in the stem

Answer 9

the vascular bundles are found near the outer edge of the stem. in non woody plants the bundles are separate and discrete. in woody plants the bundles are separate in young stems , but become a continuous ring in order in older stems. this means that there is a complete ring of vascular tissue just under the bark of a tree. this arrangement provides strength and flexibility to withstand the bending forces to which stems and branches are exposed

the xylem is found towards the inside of each vascular bundle and the phloem towards the outside . in between the xylem and the phloem is a layer of cambium . the cambium is a layer of meristem cells that divide to produce new xylem and phloem

Question 10

describe the xylem and phloem in the leaf

Answer 10

the vascular bundles form the midrib and veins of the leaf. a dicotyledonous leaf has a branching network of veins that get smaller as the spread away from the midrib . within each vein the xylem is located on top of the phloem

Question 11

define companion cells

Answer 11

the cells that help to load sucrose into the sieve tubes

Question 12

define sieve tube elements

Answer 12

make up the tubes in phloem tissues that carry sap up and down the plants . the sieve tubes elements are separated by sieve plates

Question 13

define xylem vessels

Answer 13

the tubes which carry water up the plant

Question 14

describe the structure of the xylem

Answer 14

is a tissue used to transport water and mineral ions from the root up to the leaves and other parts of the mineral ions from the roots up to the leaves and other parts of the plant

• vessels to carry the water and dissolved mineral ions
• fibres to help support the plant
• living parenchyma cells which act as packing tissue to separate and support the vessels

Xylem vessels consist of dead cells. They have a thick, strengthened cellulose cell wall with a hollow lumen.

Question 15

describe xylem vessels

Answer 15

as xylem vessels develop, lignin impregnates the wall of the cells, making the walls waterproof. this kills the cells. the end walls and content of the cells decay , leaving a long column of dead cells with no contents - a tube called the xylem vessel. the lignin strengthens the vessel walls and prevents the vessel from collapsing , this keeps vessels open even at times when water may be in short supply

the lignin thickening forms patterns in the cell walls. these may be spiral, annular (rings) or reticulate (a network of broken rings ) this prevents the vessel from being too rigid and allows some flexibility of the stem or branch

in some places lignification is not complete , leaving gaps in the cell wall, these gaps form pits or bordered pits . the bordered pits in two adjacent vessels are aligned to allow water to leave one vessel and pass into the next vessel . they also allow water to leave the xylem and pass into the living parts of the plants

Question 16

describe the adaptations of xylem to its function

Answer 16

• they are made from dead cells aligned end to end to form a continuous column
• the tubes are narrow , so that the water column does not break easily and capillary action can be effected
• bordered pits in the lignified walls allow water to move sideways from one vessel to another
• lignin deposited in the walls in spiral , annular or reticulate patterns allow xylem to stretch as the plants grow, and enables the stem to branch or bend

the flow of water is not impeded because

• there is no cross walls
• there are no cell contents, nucleus or cytoplasm
• lignin thickening prevents the walls from collapsing

Question 17

structure and function of phloem

Answer 17

phloem is a tissue used to transport assimilates around the plant. the sucrose is dissolved in water from sap, phloem tissue consists of sieve tubes , made up of sieve tube elements and companion cells
consists of columns of living cells.

Question 18

describe sieve tube elements

Answer 18

elongated sieve tube elements are lined up end to end to form sieve tubes . they contain no nucleus and very little cytoplasm , leaving space for mass flow of sap to occur. at the ends of the sieve tube elements are perforated cross walls called sieve plates . the perforations in the sieve plate allows movement of the sap from one elements to the next. the sieve tubes have very thin walls and when seen in transvers section are usually five or six sided

Question 19

describe the companion cells

Answer 19

in between the sieve tubes are small cells, each with a large nucleus and dense cytoplasm. these are the companion cells , they have numerous mitochondria to produce the ATP needed for active processes. the companion cells carry out the metabolic processes needed to load assimilates actively into the sieve tubes

Question 20

define plasmodesmata

Answer 20

gaps in the cell wall containing cytoplasm that connects two cells

Question 21

what are the three pathways taken by water

Answer 21

apoplast
symplast
vacular

Question 22

describe the apoplast pathway

Answer 22

water passes through the spaces and between the cell walls and between the cells . it does not pass through any plasma membranes into the cells. this means that the water moves by mass flow rather than by osmosis . also dissolved mineral ions and salts can be carried with the water.

Question 23

describe the symplast pathway

Answer 23

water enters the cell cytoplasm through the plasma membrane . it can then pass through the plasmodesmata from one cell to the next

Question 24

describe the vacuolar pathway

Answer 24

this is similar to the symplast pathway but the water is not confined to the cytoplasm of the cells. it is able to enter and pass through the vacuoles as well

Question 25

describe water potential

Answer 25

is a measure of the tendency of water molecules to move from one place to another.

Question 26

water always moves from a region of ? water potential to a region of ? water potential

Answer 26

higher

lower

Question 27

the water potential of pure water is

Answer 27

zero

Question 28

in a plant cell, the cytoplasm contains mineral ions and solutes this will ? the water potential , this is because

Answer 28

reduce

there are fewer free water molecules available than in pure water , as result the water potential in plant cells is always negative

Question 29

describe water uptake

Answer 29

if you place a plant cell in pure water , it will take up water molecules by osmosis . this is because the water potential in the cells is more negative (lower) than the water potential of the water . water molecules will move down the water potential gradient into the cells . but the cell will not continue to absorb water until it bursts . this is because the cell has a strong cellulose cell wall. once the cell is full of water it is described as being turgid . the water inside the cell starts to exert pressure on the cell wall, called the pressure potential . as the pressure potential builds up, it reduces the influx of water

Question 30

describe water loss

Answer 30

if a plant is placed in a salt solution with a very negative (low) water potential , then it will lose water by osmosis. this is because the water potential of the cell is less negative (higher) than the water potential of the solution , so water moves down the water potential gradient out of the cell . as water loss continues , the cytoplasm and vacuole shrink. eventually the cytoplasm no longer pushes against the cell wall, and the cell is no longer turgid. if water continues to leave the cell, then the plasma membrane will lose contact with the wall- a condition known as plasmolysis. the tissue is now flaccid

Question 31

describe the movement of water between cells

Answer 31

when plant cells are touching each other, water molecules can pass from one cell to another. the water molecules will move from the cell with the less negative (higher) water potential to the cell with the more negative (lower) water potential . this is osmosis

water moves from the cell with the higher water potential to the cell with the lower (more negative ) water potential

Question 32

define a potometer

Answer 32

a device that can measure the rate of water uptake as a leafy stem transpires

Question 33

define transpiration

Answer 33

is a loss of water vapour from the upper parts of the plant - particularly the leaves . some water may evaporate through the upper leaf surface , but this loss is limited by the waxy cuticle . most water vapour leaves though the stomata , which open to allow gaseous exchange for photosynthesis . since photosynthesis occurs only when there is sufficient light, the majority of water vapour is lost during the day

Question 34

typical pathway taken by water in a leaf

Answer 34

• water enters the leaf through the xylem , and moves by osmosis into the cells of the spongy mesophyll . it may also pass along the cell walls via the apoplast pathway
• water evaporates from the cell walls of the spongy mesophyll
• water vapour moves diffusion out of the leaf through the open stomata. this relies on a difference in the concentration of water vapour molecules in the leaf compared with outside the leaf . this is known as the water potential gradient . there must be a less negative (higher) water vapour potential inside the leaf than outside

Question 35

what is the importance of transpiration

Answer 35

• transports useful mineral ions up the plant
• maintains cell turgidity
• supplies water for growth , cell elongation and photosynthesis
• supplies water that, as it evaporates , can keep the plant cool on a hot day

Question 36

name the environmental factors which effect the rate of transpiration

Answer 36

• light intensity
• temperature
• humidity
• air movement
• water available

Question 37

describe how light intensity effects transpiration

Answer 37

in light , the stomata open to allow gaseous exchange for photosynthesis . higher light intensity increases the transpiration rate

Question 38

describe how temperature effects transpiration

Answer 38

a higher temperature will increase the rate of transpiration in three ways

• increase the rate of evaporation from the cell surfaces so that the water vapour potential in the leaf rises
• increase the rate of diffusion through the stomata because the water molecules have more kinetic energy
• decrease the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf

Question 39

describe how relative humidity effects transpiration

Answer 39

higher relative humidity in the air will decrease the rate of water loss. this is because there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside

Question 40

describe how air movement (wind) effects transpiration

Answer 40

air moving outside the leaf will carry away water vapour that has just diffused out the leaf. this will maintain a high water vapour potential gradient

Question 41

describe how water availability effects transpiration

Answer 41

if there is little water in the soil, then the plant cannot replace the water that is lost, if there is insufficient water in the soil , then the stomata close and the leave wilts

Question 42

define adhesion

Answer 42

the attraction between molecules and the walls of the xylem vessel

Question 43

define cohesion

Answer 43

the attraction between water molecules caused by hydrogen bonds

Question 44

the transpiration stream is the movement of water from the ….. to …..

Answer 44

soil through the plant , to the air surrounding the leaves

Question 45

describe water take up and the movement across the root

Answer 45

the outermost layer of cells of a root contains root hair cells. these are cells with a long extension (root hair) that increases the surface area of the root . these cells actively absorb mineral ions and water from the soil making the water potential of the cytoplasm more negative . the water then moves across the root cortex down a water potential gradient to the endodermis of the vascular bundle. water may also travel through the apoplast pathway as far as the endodermis , but must then enter the symplast pathway, as the apoplast pathway is blocked by the casparian strip

Question 46

describe the role of the endodermis

Answer 46

• movement of water across the root is driven by an active process that occurs at the endodermis . the endodermis is a layer of cells surrounding the medulla and xylem. this layer of cells is also known as the starch sheath , as it contains granules of starch , as sign that energy is being used
• the casparian strip blocks the apoplast pathway between the cortex and the medulla
• this ensures that water and dissolved mineral ions have to pass into the cell cytoplasm through the plasma membrane
• the plasma membrane contains transporter proteins, which actively pumps mineral ions from the cytoplasm of the cortex cells into the medulla and xylem
• this makes the water potential of the medulla and xylem more negative , so that water moves from the cortex cells into the medulla and xylem by osmosis
• once the water has entered the medulla , it cannot pass back into the cortex , as the apoplast pathway of the endodermal cells is blocked by the casparian strip

Question 47

describe the movement of water up the stem of a plant

Answer 47

movement of water up through the xylem is by mass flow - a flow of water and mineral ions in the same direction. there are three processes that help to move water up the stream

Question 48

what are the three processes that help move water up the stream

Answer 48

root pressure
transpiration pull
capillary action

Question 49

describe root pressure

Answer 49

the action of endodermis moving minerals to the medulla and xylem by active transport draws water into the medulla by osmosis, pressure in the root medulla builds up and forces water into the xylem , pushing the water up the xylem, root pressure can push water a few metres up a stem, but cannot account for water getting to the top of all trees

Question 50

describe transpiration pull

Answer 50

the loss of water by evaporation from the leaves must be replaced by water coming up from the xylem . water molecules are attracted to each other by forces of cohesion . these cohesion forces are strong enough to hold the molecules together in a long chain or column. as molecules are lost at the top of the column , the whole column is pulled up as one chain . the pull from above creates tension in the column of water. this is why the xylem vessels must be strengthened by lignin. the lignin prevents the vessels from collapsing under tension

because this mechanism involves cohesion between the water molecules and tension in the column of water. it is called the cohesion tension theory. it relies on the plant maintaining an unbroken column of water all the way up the xylem. if the water column is broken in one xylem vessel , then the water column can still be maintained through another vessel via the bordered pits

Question 51

describe capillary action

Answer 51

the same forces that hold water molecules together also attract the water molecules to the sides of the xylem vessels. this is called adhesion. because the xylem vessels are very narrow , these forces of attraction can pull the water up the sides of the vessel

Question 52

describe the transpiration stream

Answer 52

• minerals actively transported into xylem . this lowers the water potential in the xylem and water follows by osmosis
• cohesion of water molecules enables water to move by mass flow, pulled upwards by tension from above
• movement of water out of xylem creates low hydrostatic pressure and thus tension
• osmosis moves water across the leaf
• evaporation of water from the cell surface
• diffusion of water vapour out of the leaf

Question 53

describe how water leaves the leaf

Answer 53

most of the water that leaves the leaf exists as vapour through the stomata. only a tiny amount leaves through the waxy cuticle. water evaporates from the cells linings the cavity immediately above the guard cells. this lowers the water potential in these cells ,causing water to enter them by osmosis from the neighbouring cells. in turn, water is drawn from the xylem in the leaf by osmosis, water may also reach theses cells by the apoplast pathway by the xylem

Question 54

define hydrophyte

Answer 54

a plant adapted to living in water or where the ground is very wet

Question 55

define xerophyte

Answer 55

a plant adapted to living in dry conditions

Question 56

living plants on land must be adapted to

Answer 56

reducing water loss

-replacing the water which is lost

Question 57

most terrestrial plants can reduce their water losses by structural and behavioural adaptions, for example

Answer 57

• a waxy cuticle on the leaf will reduce water loss due to evaporation through the epidermis
• the stomata are often found on the under surface of leaves not on the top surface , this reduces the evaporation due to direct heating from the sun
• many stomata are closed at night , when there is no light for photosynthesis
• deciduous plants lose their leaves in winter , when the ground may be frozen(making water less available )and when temperatures may be too low for photosynthesis

Question 58

adaptations of marram grass

Answer 58

• the leaf is rolled longitudinally as that air is trapped inside , the air becomes humid , which reduces water loss from the leaf. the leaf can roll more tightly in very dry conditions
• there is a thick waxy cuticle on the outer side of the rolled leaf (upper epidermis) to reduce evaporation
• the stomata are on the inner side of the rolled leaf so that they are protected by the enclosed air spaces
• the stomata are in pits in the lower epidermis , which is also folded and covered by hairs, these adaptations help to reduce air movement and therefore loss of water vapour
• the spongy mesophyll is very dense with few air spaces , so there is less surface area for evaporation of water

Question 59

describe the adaptations of a cacti

Answer 59

• they’re succulents, so they store water in their stems which become fleshy and swollen . the stem is often ribbed or fluted so that it can expand when water is available
• the leaves are reduced to spines . this reduces the surface area of the leaves . when the total leaf surface area is reduced , less water is lost by transpiration
• the stem is green for photosynthesis
  
  • the roots are very wide spread, in order to take advantage of any rain that does fall

Question 60

describe other xerophytic features/ adaptations

Answer 60

• closing the stomata when water availability is low will reduce water loss and so reduce the need to take up water
• some pants have a low water potential inside their leaf cells . this is achieved by maintaining a high salt concentration in the cells. the lower water potential reduces the evaporation of water from the cell surfaces as the water potential gradient between the cells and the leaf air spaces is reduced
• a very long tap root that can reach water deep underground

Question 61

adaptions of a water lily

Answer 61

• many large air spaces in the leaf . this keeps the leaves afloat so that they are in the air and can absorb sunlight
• the stomata are on the upper epidermis , so that they are exposed to the air to allow gaseous exchange
• the leaf stem has many large air spaces . this helps with buoyancy , but also allows oxygen to diffuse quickly to the roots for aerobic respiration

Question 62

how do plants transpire

Answer 62

transpiration is the loss of water vapour from the surfaces of the leaves , but the water will not evaporate into water or into air that has a very high humidity. if water cannot leave the plant, then the transpiration stream stops and the plant cannot transport mineral ions up to the leaves. many plants contain specialised structures at the tips or margins of their leaves called hydathodes . these release structures can release water droplets which may then evaporate from the leaf structures

Question 63

define assimilates

Answer 63

substances that have become a part of the plant

Question 64

define sink

Answer 64

a part of the plant where those materials are removed from the transport system

Question 65

sink example

Answer 65

the roots receive sugars and store them as starch . at another time of the year , the starch may be converted back to sugars and transported to a growing stem, so the roots can also be a source

Question 66

define source

Answer 66

a part of the plant that loads materials into the transport system

Question 67

source example

Answer 67

the leaves photosynthesise and the sugars made are moved to other parts of the plants

Question 68

define translocation

Answer 68

the transport of assimilates through a plant

Question 69

translocation occurs in the

Answer 69

phloem

Question 70

describe active loading

Answer 70

sucrose is loaded into the sieve tube by an active process .this involves the use of energy from ATP in the companion cells . the energy is used to actively transport hydrogen ions (H+) out of the companion cells . this increases their concentration outside the cells and decreases their concentration inside the companion cells .as a result , a concentration gradient is created. the hydrogen ions diffuse back to the companion cells through special cotransporter proteins . these proteins only allow the movement only allow the movement of the hydrogen ions into the cell if they are accompanied by sucrose molecules . this is known as cotransport. it is also called secondary active transport , as it results from the active transport of the hydrogen ions out of the cell and moves sucrose against its concentration gradient . as the concentration of sucrose in the companion cells increase , it can diffuse through the plasmodesmata into the sieve tubes

Question 71

describe the movement of sucrose

Answer 71

movement of sucrose along the phloem is by mass flow. a solution of sucrose , amino acids and other assimilates flows along the tube. the solution is called sap and it can be made to flow either up or down the plant as required

the flow is caused by a difference in hydrostatic pressure between the two ends of the tube , which produces a pressure gradient . water enters the tube at the source , increasing the pressure and it leaves the tube at the sink, reducing the pressure . therefore the sap flows from the source to the sink

Question 72

describe the movement of sap in the phloem

Answer 72

• sucrose is actively loaded into the sieve tube elements and reduces the water potential
• water follows by osmosis and increases the hydrostatic pressure in the sieve tube elements
• sap moves down sieve tube from high hydrostatic pressure at source to lower hydrostatic pressure at sink
• sucrose is removed from the sieve tube by the surrounding cells and increased the water potential in the sieve tubes
• water moves out the sieve tube and reduces the hydrostatic pressure

Question 73

describe the movement of in the source

Answer 73

sucrose entering the sieve tube elements ,makes the water potential inside the sieve tubes more negative . as a result, water molecules move into the sieve tubes elements by osmosis from the surrounding tissues . this increases the hydrostatic pressure in the sieve tube at the source

a source is any part of the plant that loads sucrose into the sieve tube. in early spring this could be the roots , where energy stored as starch is converted to sucrose and moved to other parts of the plant in order in order to enable growth in the spring. the most obvious is the leaf , sugars made during photosynthesis converted to sucrose and loaded into the phloem sieve tubes, this occurs during late spring , summer and early autumn . whilst the leaves are green. the sucrose is transported to other areas of the plant that may be growing , or to areas such as the roots for storage

Question 74

describe the movement of sucrose in the sink

Answer 74

a sink is anywhere that removes sucrose from the phloem sieve tubes. the sucrose could be used for respiration and growth in a meristem, or it could be converted to starch for storage in a root. where the sucrose is being used in the cell, it can diffuse out of the sieve tube via the plasmodesmata . it may also be removed by active transport. the removal of sucrose from the sap makes the water potential less negative , so that water moves out of the sieve tubes into the surrounding cells . this reduces the hydrostatic pressure in the phloem at the sink

Question 75

describe the movement of sucrose along the phloem

Answer 75

water entering the sieve tube at the source increases the hydrostatic pressure. water leaving the sieve tube at the sink reduces the hydrostatic pressure . therefore a pressure gradient is set up along the sieve tubes , and the sap flows from higher pressure to lower pressure. this could be in either direction depending upon where sucrose is being produced and where it is needed. it is even possible that sap could be flowing in opposite directions in different sieve tubes at the same time .since the sap in one tube is moving in the same direction , this is mass flow