Topic 3: Exchange + Transport: Transport in Plants

Question 1

Why do multicellular plants need transport systems?

Answer 1

Multicellular plants need transport systems because:
-Metabolic demands = underground parts of plants don’t photosynthesise but still require exchange of substances and transportation of hormones and mineral ions absorbed by roots
-Size = plants grow large -> needs effective transport systems to move substances up and down between tips and roots
-Surface area = small sa:vol ratio -> cannot rely on diffusion alone to supply cells

Question 2

What are dicotyledonous plants (dicots)?

Answer 2

Dicotyledonous plants make seeds that contain two cotyledons (organs that act as food stores for the developing embryo plant and form the first leaves when the seed germinates)

Question 3

What are cotyledons?

Answer 3

Cotyledons are organs that act as food stores for the developing embryo plant and form the first leaves when the seed germinates

Question 4

Types of dicots (dicotyledonous plants)

Answer 4

Types of dicots:
-Herbaceous dicots
-Woody (arborescent) dicots

Question 5

Herbaceous dicots

Answer 5

Herbaceous dicots - type of dicot - soft tissues and relatively short life cycle (leaves and stems that die down at the end of the growing season to the soil level)

Question 6

Woody (arborescent) dicots

Answer 6

Woody (arborescent) dicots - type of dicot - hard, lignified tissues and a long life cycle (sometimes hundreds of years)

Question 7

Vascular systems in dicotyledonous plants

Answer 7

-Vascular system: dicots have a series of transport vessels running through the stem, roots and leaves which is known as the vascular system

Question 8

Vascular bundles in dicotyledonous plants

Answer 8

-Vascular bundles: transport tissues (eg phloem + xylem) are arranged together in vascular bundles in leaves, stems and roots of harbaceous dicots

Question 9

Why are vascular bundles around the edge in stems?

Answer 9

Vascular bundles are arranged around the edges in stems to give strength and support

Question 10

Why are vascular bundles in the middle of roots?

Answer 10

Vascular bundles are in the middle of roots to help the plant withstand tugging strains that result as the stems and leaves are blown in the wind

Question 11

The midrib of a dicot leaf

Answer 11

The midrib of a dicot leaf is the main vein carrying the vascular tissue through the organ. It also helps to support to the structure of the leaf. Many small, branching veins spread through the leaf functioning both in transport and support.

Question 12

Function of the xylem

Answer 12

Function of the xylem: large non-living tissue that has two main functions - the transport of water and mineral ions, and support
-The flow of materials in the xylem is up from roots to shoots and leaves
-Made up of dead cells
-Long, hollow structures made by columns of cells fusing together end to end

Question 13

Two other tissues associated with the xylem in herbaceous dicots

Answer 13

Two other tissues associated with the xylem in herbaceous dicots:
-Thick-walled xylem parenchyma packs around the xylem vessels, storing food and containing tannin deposits
-Xylem fibres with lignified secondary walls that provide extra mechanical strength but doesn’t transport water

Question 14

Tannin in dicots

Answer 14

Tannin is a bitter, astringent-tasting chemical that protects plant tissues from attack by herbivores

Question 15

Xylem parenchyma in dicots

Answer 15

Thick-walled xylem parenchyma packs around the xylem vessels, storing food and containing tannin deposits

Question 16

Xylem fibres in dicots

Answer 16

Xylem fibres in dicots:
Long cells with lignified secondary walls that provide extra mechanical strength but do not transport water

Question 17

Ways of which lignin can be laid down in the walls of xylem vessels

Answer 17

Ways of which lignin can be laid down in the walls of xylem vessels:
Rings, spirals or relatively solid tubes with lots of small unlignified areas called bordered pits

Question 18

Bordered pits in plants

Answer 18

Bordered pits is where water leaves the xylem and moves into other cells of the plant

Question 19

Functions of the ploem

Answer 19

-Phloem is living tissue that transport food in the form of organic solutes around the plant from leaves where they are made by photosynthesis
-Supplies cells with sugars and amino acids needed for cellular respiration and for synthesis of all other useful molecules
-Flow of materials in phloem can go both up and down the plant

Question 20

Sieve tube elements in phloems

Answer 20

Sieve tube elements - the main transporting vessels of the phloem - made up of cells joined end to end forming a hollow structure
-Not lignified

Question 21

Sieve plates in the phloem

Answer 21

Sieve plates - areas between cells where walls have become perforated (hole-y) - tonoplants (vacuole membrane), nucleus and some organelles break down - phloem becomes tube filled with sap and mature phloem cells have no nucleus

Question 22

Companion cells in the phloem

Answer 22

Companion cells form along with sieve tube elements - cells are linked to the sieve tube elements by many plasmodesmata - maintains nucleus and organelles
-Active cells and function as ‘life support system’ for sieve tube cells, which have lost normal cell functions

Question 23

What is plasmodesmata?

Answer 23

Plasmodesmata are microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells

Question 24

Tissues that support the phloem tissue

Answer 24

Fibres and scelerids (cells with with extremely thick cell walls) are supporting tissues found within the phloem

Question 25

What are sclereids?

Answer 25

Scelereids are cells with extremely thick cell walls, and act as supporting tissue (eg they are found in the phloem)

Question 26

Vascular cambium in stems

Answer 26

Vascular cambium in stems - located inbetween the xylem and phloem cells - contains meristem cells

Question 27

Arrangement of xylem and phloem in stems

Answer 27

Stem arrangement: circular, xylem and phloem vessels located around it, with xylem being inside and phloem being on the outside, vascular cambium located inbetween the xylem and phloem vessels and contain the meristem cells

Question 28

Arrangement of xylem and phloem in roots

Answer 28

Xylem is located at the center of roots in an 'X' drill-like structure, with phloem in four seperate sections around the xylem

Question 29

Why is water important in both the structure and in the metabolism of plants?

Answer 29

Water is important in both the structure and metabolism of plants because: -Turgor pressure (or hydrostatic pressure) as result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves (around 1.5MPa in plants) -Turgor drives cell expansion - force enables plant roots to force their way through tarmac and concrete -Loss of water by evaporation helps to keep plants cool -Mineral ions and the products of photosynthesis are transported in aqueous solutions -Water is a raw material for photosynthesis

Question 30

Root hair cells function

Answer 30

Root hair cells are the exchange surface in plants where water is taken into the body from the plant from the soil -Root hair is long, thin extension from root hair cell

Question 31

How are root hairs well adapted as exchange surfaces?

Answer 31

Root hair adaptations: -Microscopic size = penetrate easily between soil particles -Each microscopic hair = large sa:v ratio -Each hair has thin surface layer which means diffusion and osmosis can take place quickly -Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell

Question 32

Why does soil water have a high water potential?

Answer 32

Soil water has a high water potential due to having a low concentration of dissolved minerals

Question 33

Why does the cytoplasm and vacuolar sap of the root hair cell have low water potential?

Answer 33

The cytoplasm and vacuolar sap of the root hair cell has a low water potential due to containing many different solvents including sugars, mineral ions and amino acids - as a result water moves into the root hair cells by osmosis

Question 34

Three different pathways that water can move across the root to the xylem once it has entered the root hair cell?

Answer 34

Two pathways water can go leading up to the xylem vessel: -The symplast pathway -The apoplast pathway -Vacuolar pathway

Question 35

What is the symplast of plants?

Answer 35

The symplast is the continuous column of living plant cells that is connected through the plasmodesmata

Question 36

What is the apoplast in plants?

Answer 36

Apoplast in plants: the cell walls and the intercellular spaces of plants

Question 37

Movement of water into the xylem

Answer 37

Movement of water into the xylem: Water moves across root (apoplast or symplast pathway) until epidermis -> water in the apoplast pathway forced into cytoplasm and joins water in symplast pathway, passing through a selectively permeable membrane -> once inside vascular bundle, water returns to apoplast ptathway to enter xylem

Question 38

Why does water in the apoplast pathway pass through selectively permeable membranes before entering the symplast pathway on the route to xylem vessels?

Answer 38

passing through selectively permeable cell surface membranes excludes potentially-toxic solutes in soil water from reaching living tissues

Question 39

What causes root pressure?

Answer 39

The active pumping of minerals into the xylem to produce movement of water by osmosis results in root pressure - independant of any effects of transpiration

Question 40

Root pressure

Answer 40

Root pressure gives water a push up the xylem, but under most circumstances it is not the major factor in movement of water up from roots to leaves

Question 41

Evidence for the role of active transport in root pressure

Answer 41

Evidence for the role of active transport in root pressure: -Some poisons, eg cyanide, affect the mitochondria and prevent ATP production - if cyanide is applied to root hair cells there is no energy supply and root pressure disapears -Root pressure increases with a rise in temp and falls with a decrease in temp, suggesting enzyme controlled reactions are involved -Levels of oxygen or respiratory substrates fall - root pressure falls -Guttation: sap and water moves out cut stems, suggesting they are actively pumped out rather than being drawn out by transpiration

Question 42

Symplast pathway

Answer 42

Symplast pathway = water moving between cells through the plasmodesmata between the cell cytoplasms, water potential of cells decrease up the plant, and so water is continuously drawn up

Question 43

What causes the continuous pull of water up the plant?

Answer 43

Water is continously pulled up the plant by the decreasing water potential, as water moves down the water potential gradient by osmosis

Question 44

The vacuolar pathway

Answer 44

The vacuolar pathway is the same as symplast pathway, but water travels through vacuoles as well as the cytoplasms, and is therefore the slowest route of water up the plant to the xylem

Question 45

The apoplast pathway

Answer 45

The apoplast pathway is the quickest route of water up the plant to the xylem, and is where water moves through the cell walls and intracellular spaces - this is caused by the cohesion and tension forces acting on the cell walls that cause water to move up the plant

Question 46

The casparian strip in plants

Answer 46

The casparian strip is an impermeable layer of suberin (a waxy material), as a result all the water in the apoplast pathway is forced into the symplast pathway - the purpose of this is to remove the toxins and pathogens to prevent them entering the xylem as it goes across the selectively permeable plasma membrane

Question 47

What is the purpose of the casparian strip in plants?

Answer 47

The casparian strip prevents toxins and pathogens entering the xylem as water is forced across a selectively permeable plasma membrane, and as a result water from the apoplast pathway enters the symplast pathway

Question 48

Cohesion-tension theory of water in the plant

Answer 48

Cohesion-tension theory: force between the water and wall is greater than the weight of the water pulling it down - resulting in water being pulled up and evaporated from the leaf rather than falling down

Question 49

Where is endodermis in plants found?

Answer 49

Endodermis is found at the casparian strip within plants

Question 50

Endodermis in plants

Answer 50

Endodermis in plants acts as an apoplastic barrier, as the prescense of the casparian strip provides a selectively impermeable layer to remove toxins and pathogens from entering the xylem vessel

Question 51

The process by which water enters the cells of roots from the soil

Answer 51

Process by which water enters cells of roots from soil: -Low water potential inside -Moves down water potential gradient -Through partially permeable membrane

Question 52

Why measure a large number of stomata to calculate a mean in an experiment?

Answer 52

Measuring a large number of stomata to calculate a mean in an experiment accounts for variation of stomata, and removes chances of anomalies

Question 53

Two ways in which photographing a leaf surface makes it easier to measure stomata

Answer 53

Two ways in which photographing a leaf surface makes it easier to measure stomata: -Provides a permanent record -Allows to be enlarged to view detail -Still image - as stomata opens and closes in response to conditions

Question 54

Adaptations of xylem vessels for the transport of water

Answer 54

Adaptations of xylem vessels for the transport of water: -Long hollow tubes made up of dead cells - continuous flow of water -Contains pits - allows water to move sideways between the vessels -Lignin - thickens vessels - found in spirals - allows them to remain flexible

Question 55

How does the xylem work in plants?

Answer 55

How the xylem works in plants: -Works by using the transpiration pull and root pressure to move water and nutrients from the roots to the rest of the plant

Question 56

What is the transpiration pull in the xylem?

Answer 56

The transpiration pull is the process by which water is drawn up xylem vessels due to loss of water through the stomata on leaves - as water evaporates from leaves, it creates a negative pressure that pulls water up from the roots, therefore keeping the plant hydrated

Question 57

What is root pressure in the xylem?

Answer 57

Root pressure in the xylem is the pressure that is exerted by the roots to move water and nutrients up xylem vessels, it is created by active transport, which then exerts pressure onto the surrounding water

Question 58

Difference of the xylem and the phloem in plants

Answer 58

Xylem: responsible for transporting water and nutrients, made up of dead cells Phloem: responsible for transporting sugars and other organic compounds from leaves to rest of the plant

Question 59

Sugar transported by the phloem

Answer 59

The phloem transports sugar as sucrose, because: - it is more energy efficient as it is a disaccharide that contains more energy than monosaccharides, -non reducing sugar = no intermediate reactions with cells - lasts longer than glucose would've

Question 60

Movement of water into roots of plants

Answer 60

Water moves into the roots of plants by osmosis (high -> low concentration)

Question 61

Adaptations of the leaves

Answer 61

Leaf adaptations: -Increased surface area -Surface covered with waxy cuticle which makes them water proof - important as it prevents the leaf cells from losing water rapidly

Question 62

Stomata function

Answer 62

Stomatas - microscopic pores in the leaf - opened and closed by guard cells which surround the stomatal opening - open to allow for gaseous exchange

Question 63

What is transpiration?

Answer 63

Transpiration is the loss of water vapour from the leaves and stems of plants caused when the stomata is open as water vapour moves out by diffusion

Question 64

Why is stomata open during the day?

Answer 64

Stomata is open during the day to allow for carbon dioxide to be taken in for photosynthesis

Question 65

Why is the stomata open at night?

Answer 65

Stomata opens at night to take in oxygen for cellular respiration when no oxygen in being produced in photosynthesis

Question 66

What is the transpiration stream?

Answer 66

The transpiration stream in plants is when water vapour moves into the external air through the stomata along a diffusion gradient

Question 67

Capillary action in plants

Answer 67

Capillary action in plants refer to the process by which water can rise up by a narrow tube against the force of gravity

Question 68

Evidence for the cohesion-tension theory

Answer 68

Evidence for cohesion-tension theory: -Changes in diameter of trees -Broken xylem vessels

Question 69

How does change in diameter of trees provide evidence for the cohesion-tension theory?

Answer 69

When transpiration is high -> tension in xylem vessels are high -> tree shrinks in diameter When transpiration is low -> tension in xylem vessels low = tree increases in diameter (can be tested by measuring circumference of the tree at diffrent times of the day)

Question 70

How would broken xylem vessels provide evidence for the cohesion-tension theory?

Answer 70

Broken xylem vessels

Question 71

Steps involved in the process of the cohesion-tension theory

Answer 71

Cohesion-tension theory: 1). Transpiration - water evaporated off plant 2). Water loss creates tension (formation of H bonds between water molecules and sides of the xylem vessel elements), water pulled upwards by tension 3). Cohesion - water forms H bonds with eachother - continuous pull upwards of water to replace water lost in transpiration 4). Water being pulled up decreases water potential at roots and so water diffuses in roots via osmosis

Question 72

Equipment used to measure transpiration

Answer 72

Potometer is used to measure transpiration by estimating the volume of water taken up by a plant in a given time

Question 73

Calculating transpiration rate

Answer 73

Calculating transpiration rate: rate of bubble movement per hr

Question 74

Problems with transpiration

Answer 74

Problems with transpiration: -Limited water availability -Stomata opens for gaseous exchange in photosynthesis which increases water loss especially in conditions of high sunlight intenstity

Question 75

How turgor affects stomata

Answer 75

How turgor affects stomata: -Low turgor -> configuration of the guard cell walls closes the stomata -When environmental conditions are favourable, guard cells pump in solutes by active transport to increase turgor

Question 76

Cellulose hoops

Answer 76

Cellulose hoops prevents cells from swelling in width, so they extend lengthways -> inner wall of guard cells is less flexible than outer wall, cells become bean-shaped and stomata opens

Question 77

Plants adaptive response to lack of water

Answer 77

Scarce water -> hormonal signals from roots trigger turgor loss from guard cells to close the stomatal pore and conserve water

Question 78

Factors that affect transpiration

Answer 78

Factors that affect transpiration: -Light intenstity = increases with transpiration -Relative humidity = increase of humidity decreases transpiration as there is reduced water vapour gradient -Temperature = increases with transpiration -Air movement = increased diffusion gradient will increase transpiration -Soil-water availbility

Question 79

How does light intensity affect transpiration?

Answer 79

Light intensity -> stomata opens in the light for gaseous exchange -> increased light intensity increases number of open stomata -> increases rate of water vapour diffusing out -> increases evaporation from leaf surface

Question 80

How does relative humidity (the measure of the amount of water vapour in the air in comparison to the total concentration of water the air can hold) affect transpiration

Answer 80

Relative humidity -> increased relative humidity decreases transpiration -> as there is a reduced water vapour potential gradient between inside leaf and outside air -> dry air has opposite effect

Question 81

How does temperature affect transpiration?

Answer 81

-Increased temp -> increased kinetic energy of water molecules -> increased rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf

Question 82

How does air movement affect transpiration?

Answer 82

Air movement -> each leaf has layer of still air around it trapped by the shape and features of the leaf eg hairs -> decreases air movement close to the leaf -> so water vapour that diffuses out the leaf accumulates in this layer and so increases water vapour potential around stomata -> increased diffusion gradient -> increases transpiration

Question 83

How does soil-water availability affect transpiration?

Answer 83

Soil-water availability -> amount of water available in soil -> dry = plant under stress and reduces transpiration, and vice versa

Question 84

Why is it important to produce very thin slices of plant tissue in celery dissection?

Answer 84

It is important that the plant tissue is really thin, so that all of the cellular structures can be seen under the microscope as the electrons can then pass through

Question 85

Why is it important that in celery dissection, the tissue is cut in transverse/longitudinal ways rather than at an angle?

Answer 85

It's important that the celery tissue is cut in transverse/longitudinal ways as it makes the internal structures - such as the xylem and phloem - more clearly seen, as if it they were cut at an angle, parts of the tissue would be too thick for structures to be seen

Question 86

Why are stains useful in microscopy?

Answer 86

Stains are useful in microscopy as it binds to specific cells/tissue, allowing them to be seen clearly under the microscope as clear distinct features

Question 87

Why is toludine blue useful in celery dissection?

Answer 87

Toludine blue is useful in celery dissection as it provides a colour difference for lignified and non-lignified cell walls, allowing for the xylem and phloem to be identified separately, and so we can observe the adaptations

Question 88

Movement of water in the transpiration stream

Answer 88

Movement of water in the transpiration stream: -Water evaporates from the leaves which decreases the water potential of the air space inside the mesophyll -Water moves into air spaces from adjacent cells -Water moves out of the xylem into the cells in the leaves -Water hydrogen bonds to itself (cohesion) and bond to walls of the xylem vessel walls (tension) resulting in capillary action

Question 89

Result of water being a polar molecule in the xylem

Answer 89

Water is polar molecule - water molecules arrange so that positive and negatively charged poles lie next to eachother in the xylem - causing them to cohere and stick to eachother

Question 90

Turgid stomata

Answer 90

Turgid - water moves into vacuoles by osmosis - outer wall more flexible than inner wall - cell bends to open stomata

Question 91

Flaccid stomata

Answer 91

Flaccid - water moves out of vacuoles by osmosis - outer wall more flexible than inner wall - cell bends back to close stomata

Question 92

Potometer

Answer 92

Potometer - actually measures water uptake by plant - but this is assumed to be transpiration

Question 93

How is water lost by leaves replaced in the potometer practical?

Answer 93

Water lost by leaves is replaced from the water in the capillary tube

Question 94

Precautions taken in the potometer practical to measure transpiration to ensure results are valid:

Answer 94

Precautions taken in the potometer practical to measure transpiration to ensure results are valid: -Set up under water to ensure no air bubbles enter the apparatus -Cut stem under water to prevent air entering the xylem -Cut stem at an angle to provide large surface area in contact with the water -Dry the leaves -Keep conditions constant -Insert shoot in apparatus under water -Shut screw clip

Question 95

Long hairs from epidermis cell effect on transpiration

Answer 95

Long hairs -> traps more air around plant -> increases humidity -> decreases transpiration

Question 96

Why are measurements taken from a potometer not an actual measurement of transpiration rate?

Answer 96

-Potometers actually measure the rate of water uptake by the plant -But this is an estimated rate of transpiration as it is assumed that uptake refers to water loss by transpiration from the leaves -Some of the water is taken up by photosynthesis

Question 97

Plasmolysis

Answer 97

Plasmolysis = the process of contraction or shrinkage of the protoplasm (all material inside the cell) away from the cell wall - happens when plants lose water after being placed in a solution that has a higher concentration of solutes than the cell does (hypertonic solution) - result of osmosis

Question 98

Affect of salt solution on permeability of cell walls

Answer 98

Salt solutions increases the permeability of cell walls - as a concentration gradient is set up as salt solutions have a lower concentration of water and so water moves out by osmosis across the semi-permeable membrane

Question 99

How would you investigate the effect of wind speed on transpiration rate?

Answer 99

Investigate wind speed by placing apparatus in front of a fan

Question 100

How to investigate the effect of light intensity on transpiration rate?

Answer 100

Investigate light intensity by placing apparatus in front of a lamp

Question 101

How to investigate the effect of temperature on transpiration rate?

Answer 101

Investigate temperature by using a heater

Question 102

Xerophytes

Answer 102

Xerophytes: plants in dry habitats that have evolved a wide range of adaptations that enable them to live and reproduce in places where water availability is very low

Question 103

Xerophyte examples

Answer 103

Xerophyte examples: conifers, marram grass, plants in cold icy conditions, cacti

Question 104

Ways for xerophytes to conserve water

Answer 104

Ways for xerophytes to conserve water: -A thick waxy cuticle -Sunken stomata at pits -Reduced number of stomata -Reduced leaves -Hairy leaves -Curled leaves -Succulents -Leaf loss -Root adaptation -Avoiding the problems