TOPIC 9 : TRANSPORT IN PLANTS

Question 1

3.3.1 XYLEM AND PHLOEM
Why do plants need transport systems?

Answer 1

plants need substances like water, minerals and sugars to live
also need to get rid of waste
animals and plants = multicellular -» means they have a small surface area to volume ratio&raquo_space; but have a high metabolic rate
exchanging substances by direct diffusion ( form outer surface to the cells) would be too slow to meet their metabolic needs
plants need transport systems to move substances to and form individual cells quickly

Question 2

3.3.1 XYLEM AND PHLOEM
What is the structure and function of the vascular in the roots?

Answer 2

in the roots the xylem and phloem are in the centre to provide support for the root as it pushes through the soil

Question 3

3.3.1 XYLEM AND PHLOEM
What is the structure and function of the vascular in the stem?

Answer 3

in the stems, the xylem and phloem are near the outside to provide a sort of ‘ scaffolding’ that reduces bending

Question 4

3.3.1 XYLEM AND PHLOEM
What is the structure and function of the vascular in the leaves?

Answer 4

in a leaf, xylem and phloem make up a network of veins which support the thin leaves

Question 5

3.3.1 XYLEM AND PHLOEM
What is the role of the xylem tissue?

Answer 5

transports water and mineral ions in solution
these substances move up the plant from the roots to the leaves

Question 6

3.3.1 XYLEM AND PHLOEM
What is the role of the phloem tissue?

Answer 6

mainly transports sugars both up and down the plant

Question 7

3.3.1 XYLEM AND PHLOEM

Answer 7

the xylem and phloem make up a plants vascular system
they are found throughout a plant and they transport materials to all parts
where they’re found in each part is connected to the xylem’s other function, which is support
the position of the xylem and phloem in the root, stem and leaf are shown in the transverse cross-sections
transverse means the sections are cut through each structure at a right angle to its length
you can also get longitudinal cross-sections
these are taken alon the length of a structure

Question 8

3.3.1 XYLEM AND PHLOEM
What is the structure of the xylem vessels?

Answer 8

made from several differnt cell types xylem vessels are very long, tube like structure formed from cells joined end to end
there are no end walls on these cells, making an uniterrupted tube that allows water to pass through the middle easily
the cells are dead, so they contain no cytoplasm

they are made up of
tracheids
- tapered cells with sloping end walls
Xylem vessels
- shorter and fatter than tracheids
each are perforated by pits to allow sideways movement of water and minerals
fibres and xylem parenchyma

Question 9

3.3.1 XYLEM AND PHLOEM
What is the adaptation of the xylem vessels?

Answer 9

the cell walls are thickened with a woody substance called lignin, which helps to support the walls and stops them collapsing inwards
lignin can be deposited in xylem wallls in different ways
being deposited in these patterns allows flexibility and prevents the stem from breaking
the amoun of lignin increases as the cell gets older
water and mineral ions move into ans out of the vessels through small pits in the walls where there’s no lignin
this is how other types of cells are supplied with water

Question 10

3.3.1 XYLEM AND PHLOEM
What is the funtion of the phloem tissue?

Answer 10

transports solutes mainly sugars like sucrose, round plants
like xylem, phloem is formed from cells arranged in tubes
it isn’t used for support as well
phloem tissue contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
sieve tube elements and companion cells are very important cell types in phloem for transport

Question 11

3.3.1 XYLEM AND PHLOEM
What are the adaptations of the phloem tissue? ( sieve tube elements )

Answer 11

SIEVE TUBE ELEMENTS
these are living cells that form the tube for transporting sugars through the plant
they are joined end to end to form sieve tubes
the sieve parts are the end walls, which have lots of holes in them to allow solutes to pass through
unsually for living cells, sieve tube elements have no nucleus, a very thin layer of cytoplasm and few organelles
the cytoplasm of adjacent cells is connected through the holes in the sieve plates

Question 12

3.3.1 XYLEM AND PHLOEM
What are the adaptations of the phloem tissue? ( companion cells )

Answer 12

they lack of a nucleus ans other organelles in sieve tube elements means that they can’t survive on their own
so there’s a companion cell for every sieve tube elements
companion cells carry out the living functions for both themselves and their sieve cells
e.g they provide the energy for the active transport of solutes

narrow thin wall with abundance of cytoplasm and a nucleus
function - keep the sieve tube alive = provides nutrients
contains many mitrochondria = provides energy for translocation

Question 13

3.3.2 WATER TRANSPORT
Why do plants need transport systems?

Answer 13

water, minerals and sugars to live
get rid of waste
multicellular ( so they have a small SA/V )
relatively big - high metabolic rate
diffusion too slow using outer surfaces
transports allows quick movement to and from cells

Question 14

3.3.2 WATER TRANSPORT
How does water enter a plant?

Answer 14

Water is taken into the root hair by osmosis : it moves into the root hair cells due to the higher water potential in the soil
it then moves across the root tissue from the root hair ( higher water potential ) to the xylem ( lower water potential )

water moves across the root in one of two ways:
- symplast pathway water travels down a water potential gradient by osmosis through the cytoplasm of the cell and travels along cytoplasmic strands called plasmodesmata between cells
- apoplast pathways water travels through the non-living part of the cell, the cell transports this way relies on adhesion and a pressure gradient and is an exmaple of mass flow. Most water moves through a plant via this pathway. At the endodermis, a waxy layer in the cell wall ( casparian strip ) forces water to take the symplastic pathway allowing better control of uptake due to the role of cell membranes ( partially permeable )

water then moves into the xylem down a water potential gradient via pits in the xylem vessels

Question 15

3.3.1 XYLEM AND PHLOEM
What is the structure of a phloem tissue?

Answer 15

transports sucrose and amino acids
main conducting cells are sieve tube members
comapnion cells assist in the loading of sugars

Question 16

3.3.2 WATER TRANSPORT
How does water transport in xylem?

Answer 16

water molecules cling together by hydrogen bonding between the molecules
known as cohesive forces and help water to be pulled through the plant
along with the adhesive forces of water molecules to the xylem walls, create a strong tention force within the xylem vessels
xylem vessles has a cell wall strengthened with lignin which thickens them making them stronger
cohesive property provides an unbroken column of water in the xylem throughout a plant
failure to do this would stop all flow of water through the xylem vessels

Question 17

3.3.2 WATER TRANSPORT
How does water transport through the leaves?

Answer 17

water leaves the xylem tissue and across the spongy mesophyll cells by osmosis
as spongy mesophyll cells the water leaves by osmosis and evaporates into water vapour
water vapour diffuses out of the leave through the stomata from a high water vapour concentration to a low water vapour concentration

• the evaporation of water from a plants surface is called transpiration
• the movement of water from roots to the leaves is called a transpiration stream

Question 18

3.3.2 WATER TRANSPORT
How does the transpiration stream work?

Answer 18

1) Mineral ions ( e.g. nitrates ) enter by active transport which decreases water potential and drives osmosis: water moves into root hair cells down the water potential gradient. This forces water upwards - root presure
2) Cohesion - water molecules stick together due to hydrogen bonds. It moves as a column/ chain. This is mass flow
3) tension - water is pulled into leaves form xylem down a pressure gradient, then via sympalst & apoplast pathways, to enter cells in the mesophyll layer. Xylem doesn’t collapse under low pressure due to lignin
4) Evaporation - water vapour is lost form air pockets in spongy mesophyll layer via the open stomata. Water diffuses out, down a water potential gradient

Question 19

3.3.3 TRANSPIRATION
Why does transpiration happen?

Answer 19

its the result of gas exchange
a plant needs to open its stomata to let in carbon dioxide so that it can produce glucose ( by photosynthesis )
but this also lets water out - there’s a higher concentration of water inside the lead than in the air outside, stomata open
so transpiration’s really a side effect of the gas exchange needed for photsynthesis

Question 20

3.3.3 TRANSPIRATION
How does light intensity affect the transpiration rate?

Answer 20

the lighter it is the fater the transpiration rate
this is because the stomata open when it gets ligh ( the lighter it gets, the wider they open )
when its dark the stomata are usually closed, so there’s little transpiration

Question 21

3.3.3 TRANSPIRATION
How does temperature affect the transpiration rate?

Answer 21

the higher the temperature the faster the transpiration rate
wamer water molecules have more energy so they evaporate from the cells inside the leaf faster
this increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf

Question 22

3.3.3 TRANSPIRATION
How does humidity affect the transpiration rate?

Answer 22

the lower the humidity, the faster the transpirsation rate
if the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration rate

Question 23

3.3.3 TRANSPIRATION
How does wind affect the transpiration rate?

Answer 23

the winder it is, the faster the transpiration rate
lots of air movement blows away molecules form around the stomata
this increases the water potential gradient, which increases the water potential gradient, which increases the rate of transpiration

Question 24

3.3.3 TRANSPIRATION
What are xerophytes?

Answer 24

these are plants like cati and marram grass ( which grow on sand dunes )
they’re adapted to live in dry climates
their adaptations prevent them losing too much water by transpiration

Question 25

3.3.3 TRANSPIRATION What are the adaptations of a cati?

Answer 25

have thick, waxy layer on the epidermis - this reduces water loss by evaporation because the layer is waterproof ( water can't move through it) they have spines instead of leaves - this reduces the surface area for water loss cati also close their stomata at the hottest time of the day when transpiration rates are the highest

Question 26

3.3.3 TRANSPIRATION What are xerophytes?

Answer 26

these are plants which can tolerate low water availability due to low rain fall or high winds such as a desert or an ice/ snow covered region their adaptions are to conserve as much water a possible e.g cati and marram grass

Question 27

3.3.3 TRANSPIRATION What are the adaptations of maraam grass?

Answer 27

> marram grass has stomata that are sunk in pits, so they're sheltered from the wind. this traps moist air in the pits and helps to slow transpiration down by lowering the water potential gradient. > it also has layers of ' hairs' on the epidermis - this also traps moist air round the stomata, which reduces the water potential gradient between the leaf and the air, slowing transpiration down > in hot or windy conditions marram grass plants roll their leaves - again this traps moist air, slowing down transpiration. it also reduces the exposed surface area for losing water and protects the stomata from wind > like cati, marram grass has a thick waxy layer on the epidermis to reduce water loss by transpiration

Question 28

3.3.3 TRANSPIRATION What are the adaptations of xerophytes?

Answer 28

deep roots to reach water far underground which are also shallow spreading to collect occasional rainfall leaves reduced ro spines with minimum surface area for transpiration reduced number of stomata to reduce transpiration rate rolled leaves, leaf hairs and stomata sunk in pits to trap moist air, increasing humidity and slowing diffusionof water vapour from the stomata thick waxy leaf cuticle which is impermeable to water ( preventing evaporation ) stomata opening at night and closed at midday when evaporation rate would be highest ( reversed stomatal rhythm ) storage of water in succulent tissues

Question 29

3.3.3 TRANSPIRATION adaptations help collect and store as much water as possible - why?

Answer 29

for photsynthesis dilute mineral ions chemical reaction within the cell

Question 30

3.3.3 TRANSPIRATION What are hydrophytes?

Answer 30

these plants can tolerate very high levels of water & are either be particially or fully submerged in water their adaptations are to absorb as much oxygen and light as possible e.g water lilly and watercress

Question 31

3.3.3 TRANSPIRATION What are the adaptations of hydrophytes?

Answer 31

leaf shape: the submerged leaves have a very large surface area for maximum light absorption = maximum rate of photosynthesis lack of protective layer: the epidermal layer has very little, if any cuticle, as water loss is not a problem xylem tubes: very few or absent as all surface cells absorb water, nutrients&dissolved gases by diffusion from surrounding water location of stomata: the green pigement-containg chloroplasts & stomata are also found only on the upper surface of the leaf no strengthening tissue: stems and leaf petioles are normally supported by water. Mechanical support would be disadvantageas it would limit flexibility in the event of changes in water level or water movements roots: often also reduced and their main function is anchorage. The root hairs are often absent and the roots themselves may be entirely missing air pockets: many to ensure leaves float on the water's surface to rapidly exchange gases and capture light energy

Question 32

3.3.3 TRANSPIRATION All adaptations help keep leaves on the surface - why is this important?

Answer 32

photosynthesis -> light also trying to do gas exchange

Question 33

3.3.3 TRANSPIRATION

Question 34

3.3.3 TRANSPIRATION Whar are halophytes?

Answer 34

these plants can tolerate very high levels of salt, loose soil and low levels of oxygen. The roots are fully submerged in water most or some of the day, which is often anoxic their adaptations are to reduce internal salt levels, stay upright and absorb as much oxygen as possible

Question 35

3.3.3 TRANSPIRATION What are the adaptations of hydrophytes?

Answer 35

leaf shape: the submerged leaves have a very large surface area for maximum light absorption = maximum rate of photsynthesis lack of protection layer: the epidermal layer has very little, if any cuticle, as water is not a problem xylem tubes: very few or absent as all surface cells absorb water, nutrients & dissolved gases by diffusion from surrounding water location of stomata : the green pigment - containing chloroplasts & stomata are also found only on the upper surface of the leaf no strengthening tissue: stems and leaf petioles are normally supported by water. Mechanical support would be disadvantage as it would limit flexibility in the event of changes in water level or water movements roots: often also reduced and their main function is anchorage. the root hairs are often asent and the roots themselves may be entirely missing stomata: usually only present on the upper surface of floating leaves. this helps to maximise gas exchange air pockets: many to ensure leaves float on the water's surface to rapidly exchange gases and capture light energy

Question 36

3.3.3 TRANSPIRATION

Question 37

3.3.3 TRANSPIRATION What is halophytes?

Answer 37

these plants can tolerate very high levels of salt, loose soil and low levels of oxygen. the roots are fully submerged in water most or some of the day, which is often anoxic their adaptations are to reduce internal salt levels, stay upright and absorb as much oxygen as possible e.g mangroves

Question 38

3.3.3 TRANSPIRATION Describe the xerophytic features of a leaf and explain how each feature reduces loss of water vapour

Answer 38

reducing the number of stomata - can reduce transpiration rate, as ot can minimise the overall surface area through which water vapour can escape thus reducing the rate of water loss sunken stomata - which creates small pits to trap moist air around the stomata, this reduces water potential gradient between the inside of the leaf and the outside, further slowing down the rate of water vapour iffusion out of the stomata thick cuticles - on their leaves and stems which acts as a barrier to water loss. this cuticle reduces the asmount of water that can evaporate directly from the leaf surface storage of water - because of their high water holding capacity, by retaing water they can reduce surface area exposed to the air, which slows down evaporation and minimises water loss through transpiration

Question 39

3.3.3 TRANSPIRATION Compare xerophytes and hydrophytes

Answer 39

XEROPHYTES: Root System: Often have well-developed root systems to absorb water from the soil. Stomata: May have sunken stomata or stomata covered by hairs to reduce water loss. Cuticle: Usually have a thick, waxy cuticle to minimize water loss. Leaf Adaptations: Some xerophytes have reduced leaf size, spines, or modified stems for photosynthesis. Water Storage: Some xerophytes store water in their stems, leaves, or roots. HYDROPHYTES: Root System: May have poorly developed root systems as they absorb nutrients and water through their leaves and stems. Stomata: Often have numerous stomata on their leaves, allowing for gas exchange. Cuticle: Usually have a thin or absent cuticle as they don't need to conserve water. Leaf Adaptations: Some hydrophytes have thin, flat leaves with large air spaces for buoyancy. Water Absorption: They absorb nutrients and water directly from the water environment.

Question 40

3.3.4 TRANSLOCATION What is translocation?

Answer 40

movement of dissolved substances to where they are needed in a plant what substances? - sucrose, amino acids, nitrate energy requiring process happenes in phloem translocation moves substances from source to sink source - where a substance is made : green leaves and green stem , storage organs sink - where it is used : roots, meristems, being stored enzymes maintain conc gradient from source to sink by changing dissolved substances at the sink e.g. breaking them down into something else. this means there is always a lower conc at the sink than the source

Question 41

3.3.4 TRANSLOCATION translocation - the movement of assimilates in a plant

Answer 41

sugars and amino acids ( assimilates ) move through a plant by translocation ( change of location ) sugars are transported as sucrose through a plant they are added to the phloem by sources ( photsynthetic leaves or tubers ) and are removed from the phloem by sinks ( non- photosynthetic, respiring tissues, meristems or tubers ) the assimilates move by mass flow

Question 42

3.3.4 TRANSLOCATION Mass flow hypothesis - first stage SOURCE

Answer 42

active transport is used to actively lad solutes ( e.g sucrose from photosynthesis into seive tubes of the phloem at the source ( e.g the source ) this lowers the water potential inside the sieve tubes, so water enters by osmosis from xylem and companion cells. This creates high pressure inside the sieve tube at source end of phloem

Question 43

3.3.4 TRANSLOCATION Mass flow hypothesis - second stage SINK

Answer 43

at the sink end, solutes are removed from the phloem to be used up usually by diffusion ( passive ) because solutes are at a higher conc in phloem than surround tissues at the sink the removal of solutes increases water potential inside sieve tubes so water leaves by osmosis this lowers the pressure inside the sieve tubes

Question 44

3.3.4 TRANSLOCATION Mass flow hypothesis - third stage FLOW

Answer 44

the result is a pressure gradient from the source end to the sink end this gradient pushes solutes along the sieve tubes towards the sink when they reach the sink the solutes will be used ( e.g. respiration ) or stored ( e.g. starch ) starch = insoluable as it doesn't affect water potential

Question 45

3.3.4 TRANSLOCATION What is active loading at sources?

Answer 45

used as the source to move substances to the companioon cells form surronding cells and from here to the sieve tubes

Question 46

3.3.4 TRANSLOCATION What is a cotransporter?

Answer 46

carries protein and binds two molecules at time found in membranes

Question 47

3.3.4 TRANSLOCATION What is the process of active loading at sources?

Answer 47

hydrogen ions are actively pumped out of companion cells into surrounding tissue ( using ATP ) sets up a diffuision gradient so H+ ions diffuse back in via cotransporter proteins in cell surface membrane as they diffuse back in, they bring sucrose with them so sucrose concentration increases inside the companion cells this sets up a concentration gradient between them and the sieve tube elements = sucrose to diffuse into them via plasmodesmata

Question 48

3.3.4 TRANSLOCATION How does movement along the phloem occur?

Answer 48

as the sucrose concentration increases in the sieve tube elements, the water potential decreases therfore, water moves into the phloem from the surrounding tissues ( often the xylem ) by osmosis this increases the hydrostatic pressure in this area, setting up a hydrostatic pressure gradient inside the phloem this cause the mass flow of assimilates in either direction

Question 49

3.3.4 TRANSLOCATION What occurs once at the sink?

Answer 49

sucrose either diffuses into sink cells or moves by active transport the sucrose is either used by the sink cells in respiration or stored as starch this reduces the concentration of sucrose in the cell so it maintains a concentration gradient = more sucrose arrives as sucrose leaves the phloem, the water potential increases = water moves out of phloem into surrounding tissues by osmosis and hydrostatic pressure reduces