Transport in Plants

Question 1

what do xylem tissues consist of?

Answer 1

xylem vessels/ tracheids
xylem parenchyma
xylem fibres (sclerenchyma)

Question 2

describe xylem vessels

Answer 2

Xylem vessels are long, hollow tubes with walls made of cellulose and lignin

Xylem vessels have a large empty lumen

Xylem vessels have pits in the walls (non-lignified areas)

There are no horizontal cross-walls

Forces of adhesion can occur between the water molecules and the walls of the vessels enabling upward movement of the columns of water.

Question 3

why is there no cytoplasm in xylem vessels?

Answer 3

so a large volume of water can fit inside of the lumen.

Question 4

function of pits in xylem vessels

Answer 4

they allow horizontal movement of water and minerals through the cellulose walls to neighbouring cells

Question 5

why are there no horizontal cross-walls in xylem vessels?

Answer 5

so there is no obstruction to the free upward movement of columns of water and minerals inside of xylem vessels

Question 6

what is the purpose of long tubes for xylem vessels?

Answer 6

The long tubes provide a large surface area over which water molecules can be attracted to the walls as they move upwards over long distances.

Question 7

what are tracheids?

Answer 7

These are non-living cells with lignified walls but they have horizontal cross-walls present

Question 8

what is lignin?

Answer 8

strong, hard substance that is impermeable to water

Question 9

how is lignin deposited in walls of xylem vessels?

Answer 9

Annular (rings)
Pitted
Helical
Reticulate
Scalariform

Question 10

function of lignin in xylem vessels

Answer 10

Lignin strengthens the walls and prevents them from collapsing when columns of water inside of them are under tension.

Question 11

what is the function of pits in the tracheids?

Answer 11

to allow the movement of water and minerals between tracheids and into other cells

Question 12

what is xylem parenchyma?

Answer 12

These are living cells with cytoplasm which are found filling spaces between other types of xylem cells.
They have thin walls

Question 13

what is xylem sclerenchyma?

Answer 13

These are dead, empty cells with thin walls, smaller than xylem vessels.
They provide strength and support.

Question 14

what are the three pathways of water from one cell to another?

Answer 14

Apoplast
Symplast
Vacuolar pathway

Question 15

how does the apoplast pathway work?

Answer 15

water moves along by mass flow along the cell walls without entering the cytoplasm of the cell.
This is the pathway of least resistance

Question 16

how is water transported from one cell to another by the vacuolar pathway?

Answer 16

In this case the water molecules move from one cell to another by passing through the cell wall, cell surface membrane → cytoplasm → tonoplast →into the vacuole of one cell → then moving across the tonoplast, cytoplasm, cell surface membrane and cell wall out of the plant cell → into another vacuole in the same way.

In this case, water moves by osmosis from one vacuole to another due to differences in water potential between the 2 cells.

Question 17

How does the symplast pathway work?

Answer 17

In this pathway, water moves by mass flow from 1 cell to another by entering into the cytoplasm after crossing the cell wall and cell surface membrane → then flowing within the cytoplasm via the plasmodesmata, into an adjacent cell.
The water does not enter the vacuole of the cell but remains in the cytoplasm as it flows along from one cell to another.

Question 18

what is the endodermis?

Answer 18

This is the innermost layer of cells in the cortex of the root. It separates the cortex from the stele or vascular tissue of the root.

Question 19

what does the stele consist of?

Answer 19

xylem
pericycle
parenchyma
phloem

Question 20

features of the endodermis

Answer 20

made up of a single layer of cells and surrounds one or two layers of cells which make up the pericycle of the root

have a hydrophobic material called suberin deposited in bands in their walls

Question 21

what is the purpose of suberin deposited in bands in the endodermis walls?

Answer 21

It forms the Casparian Strip, which blocks the free movement of water and minerals along the apoplast pathway

When water and minerals travelling the apoplast pathway reach the Casparian Strip, they must face the partially permeable cell surface membrane of the endodermal cells. only allow certain minerals and other substances

Question 22

what are the roles of the endodermis?

Answer 22

1. To regulate the movement of water and minerals between the cortex and the xylem of the root.
2. To ensure a one-way flow of minerals from the soil to the xylem and the rest of the plant
3. To protect the plant from the entry of pathogens and toxic ions that can be present in the soil water.
4. It plays a role in the development of root pressure in the roots of plants, which is needed to assist in the upward movement of minerals and water in the plant.

Question 23

how does water move into plant roots

Answer 23

water and mineral ions are present in soil solution. WP of soil > WP of vacuole in root hair epidermal cell

water enters by osmosis across cell wall, cell surface membrane, cytoplasm, tonoplast and enters vacuole.

vacuole becomes dilute and WP of cell increases

water moves along water potential gradient in vacuolar pathway by osmosis until it reaches xylem vessels

in xylem vessels, WP is low compared to WP of soil solution

Question 24

how is water potential of xylem vessels made lower?

Answer 24

by the pumping of mineral ions from the endodermis into the xylem

Question 25

what happens to water potential as you go up the plant?

Answer 25

WP is highest in soil and root and decreases going up due to increasing amounts of dissolved substances then it is lowest in the air

Question 26

how so minerals enter the root?

Answer 26

either passively or actively Minerals that enter passively move by mass flow along the apoplast pathway and travel along the cell walls until they reach the endodermis (Casparian Strip that blocks the apoplast pathway) The ions would have to be able to cross the partially permeable membranes of the endodermal cells in order to diffuse further. Some ions can also cross the partially permeable membranes of the epidermal cells via facilitated diffusion.

Question 27

what affects passive movements?

Answer 27

no ATP required occurs along diffusion gradient higher temperatures speeds up kinetic energy of ions and increases the rate of diffusion

Question 28

how are cations taken up into root hair cells?

Answer 28

1 The root hair cells secrete H+ out of the root cells into the soil solution using H+ pumps in the cell surface membrane. This creates a slight negative charge in the root epidermal cells . 2. Cations are then moved actively from the soil solution into the root cells via carrier proteins (located in the cell surface membranes) with the expenditure of ATP. This movement is assisted by the negative charge in the root epidermal cells since the cations will be attracted into the cells.

Question 29

what mechanism is used to transport anions across membranes of root hair cells?

Answer 29

cotransport mechanism

Question 30

what occurs in the cotranport mechanism?

Answer 30

H+ pumps in the cell surface membranes of root epidermal cells actively move H+ from the root epidermal cells into the soil solution using ATP. A high H+ concentration is established outside of the root in the soil solution. Both the H+ and the Cl- ions then move passively from the soil solution via a cotransport protein that functions only if both types of ions are present. This mechanism is called indirect active transport. The active part of the process is affected by oxygen concentration etc.

Question 31

what do ions do once they enter endodermal cells?

Answer 31

they can move from those cells via the symplast pathway or be pumped Into the pericycle and then move into the xylem. a high concentration of ions in the xylem lowers the water potential and attracts water from the cells around the stele, the cortex, and the soil solution.

Question 32

what is root pressure?

Answer 32

a high pressure that develops in the xylem as water enters it. It pushes water and minerals a short way upwards into the xylem of the stem

Question 33

what factors is root pressure affected by?

Answer 33

temperature availability of ions in soil absence of oxygen

Question 34

how does temperature affect root pressure?

Answer 34

higher temperatures increase root pressure

Question 35

how does the availability of ions in the soil affect the root pressure?

Answer 35

there is less pressure when fewer ions are available

Question 36

how does a lack of oxygen affect root pressure?

Answer 36

it lowers the root pressure

Question 37

what is the cohesion-tension theory about?

Answer 37

explaining how water can move upwards in a plant against gravity from soil into the air

Question 38

what is water potential

Answer 38

the tendency of water to move out of a system by osmosis via a partially permeable membrane

Question 39

define mass flow

Answer 39

bulk movement of molecules together from one point to another due to differences in hydrostatic pressures

Question 40

what is hydrostatic pressure?

Answer 40

pressure exerted by a column of liquid against its containing vessel

Question 41

explain cohesion

Answer 41

force of attraction due to hydrogen bonding between water molecules for energy

Question 42

define adhesion

Answer 42

force of attraction between water molecules and the walls of the xylem vessels to aid in the upward movement of water

Question 43

what is capillarity?

Answer 43

the upward movement of water molecules in the xylem vessels due to the force of cohesion and adhesion

Question 44

explain transpiration pull

Answer 44

a suction effect in the leaves when water diffuses out and also evaporates from cells into the atmosphere. followed by a pulling effect on water molecules in the xylem due to cohesion in order for it to occur, there must be a loss of water from the leaves

Question 45

what is the tensile strength of a water column?

Answer 45

refers to the ability of a column of water to remain unbroken despite the transpiration pull and force of gravity

Question 46

where is the stomata found?

Answer 46

in the epidermis of leaves, stems and flowers

Question 47

how many guard cells are in a stoma?

Answer 47

2 with a pore between them

Question 48

characteristics of guard cells

Answer 48

firmly joined at both ends but can separate in the mid region of their length have unevenly thickened walls (wall near he pore is thick and wall far away is thin) have bundles of cellulose microfibrils which are arranged as hoops around the width of the cell

Question 49

why are there bundles of microfibrils in guard cells

Answer 49

So that when the guard cells become turgid, they cannot expand in width, but can expand in length. However, because the ends are joined together, the guard cells will curve and therefore, the pore will open

Question 50

what are the factors that affect transpiration?

Answer 50

temperature relative humidity in the air inside or outside of the leaf air movements watter supply or availability of water in the soil for the plant light number of stomata present surface area of the leaf thickness of the cuticle on surface of the leaf whether the stomata are at the surface of the leaf or sunken

Question 51

how does light affect transpiration?

Answer 51

In the presence of light, photosynthesis occurs in the guard cells. Water enters them from the subsidiary cells, causing them to become turgid, swell, curve and open the stomata. Rate of transpiration is high. the opposite occurs when there is no light

Question 52

how does water availability affect transpiration?

Answer 52

if there is no water in the soil, stomata closes due to no water in the soils, stomata closes due to no water available to replace water that is lost from the leaves. Guard cells become flaccid and the stomata closes. Transpiration stops.

Question 53

what is the role of air movements on transpiration rate?

Answer 53

in windy conditions, the leaders of the moist air are removed quickly from the surfaces of the leaves. This facilitates the diffusion of water vapour from the leaf spaces through the stomata. Excessively high wind speeds cause the stomata to close because the plant is under stress. This prevents the loss of too much water from the plant.

Question 54

how does relative humidity affect transpiration rate?

Answer 54

if the humidity of the surrounding air is high and the humidity in the air spaces in the leaf is also high, there will not exist a steep diffusion gradient, so the rate of transpiration will be very low. On the other hand, if the humidity of the air around is low, a steep diffusion gradient will exist between the air spaces in the leaf and the air around it is low, a steep diffusion gradient will exist so rate is high

Question 55

how does temperature affect transpiration?

Answer 55

An increase in temperature increases the water holding capacity of the air. As a result, warmer water can hold more vapour and there will be an increase in the rate of transpiration. When temperatures are lowered, transpiration rates decrease because cooler air can hold less vapour.

Question 56

what are the roles of transpiration?

Answer 56

1. Cooling of the plant due to evaporation of water from surface of the leaf 2. Supplying nutrients from the soil ie. minerals to all parts of the plant 3. Ensuring that all cells reduce the water 4. Allowing the exchange of gases between the plant and surrounding air.

Question 57

what does the phloem tissue consist of?

Answer 57

phloem sieve tubes companion cells phloem parenchyma phloem sclerenchyma

Question 58

what are phloem sieve tubes?

Answer 58

A sieve tube consists of a number of elongated sieve elements or sieve tube elements that are joined end to end vertically to form a long, continuous tube whose cross walls are perforated by pores.

Question 59

Describe the elements of the sieve tube

Answer 59

Each sieve element is living with a plasma membrane and a small amount of cytoplasm, which contains a few small mitochondria, as well as a small amount of endoplasmic reticulum. The cytoplasm forms a thin lining inside the cell just under the cell surface membrane. There is no nucleus, ribosomes, vacuole, or golgi. There are proteins present in the cytoplasm of he sieve elements There are pits between the companion cells and sieve elements

Question 60

how is a sieve plate formed?

Answer 60

Where the end walls of the 2 sieve elements meet, a sieve plate forms. This is perforated by large sieve pores. Normally, the pores are not blocked out strands of callose (polysaccharide) can be found blocking the pores after the elements are damaged.

Question 61

what is the function of the pit between the companion cell and sieve elements?

Answer 61

they are allow communication between companion cells via the plasmodesmata

Question 62

what organelles are found in phloem companion cells?

Answer 62

Each one contains a nucleus, ribosomes, vacuole, cytoplasm , many large mitochondria, golgi body, and rough endoplasmic reticulum

Question 63

what is the function of companion cells?

Answer 63

they are involved in movement of solutes into and out of sieve tubes

Question 64

features of companion cells

Answer 64

they are located adjacent to each sieve tube element many plasmodesmata pass through cell walls of companion cell and adjacent sieve tube element

Question 65

explain phloem parenchyma

Answer 65

These are living cells which fill spaces in the phloem tissue. They also play a role in collecting and transferring solute into the companion cells.

Question 66

what is the phloem sclerenchyma?

Answer 66

This consists of non-living cells which are elongated and have thickened walls. They have deposits of lignin in the walls for strengthening.

Question 67

define translocation

Answer 67

Translocation refers to the movement of dissolved solute by mass flow due to differences in hydrostatic pressure between the source and sink.

Question 68

what is a source?

Answer 68

A source is that part of the plant from which dissolved food materials are moved by translocation via the phloem.

Question 69

explain the sink

Answer 69

A sink is that part of the plant to which dissolved food materials are being translocated via the phloem.

Question 70

what are the similarities between sieve tubes and companion cells

Answer 70

both are part of the phloem both are living cells at maturity both play a role in translocation pits between them allow for communication

Question 71

differences between sieve tubes and companion cells

Answer 71

companion cells are smaller, narrower and densely packed with cytoplasm and organelles Sieve tubes are elongated cells that form a continuous column Sieve tube is dependent on companion cell for metabolic activities while the companion cells are responsible for movement of solutes into and out of sieve tubes

Question 72

change that was made to the mass flow hypothesis

Answer 72

Original: both solutes and water move at the same rate and in the same direction Revised: movement was due to hydrostatic pressure gradients in the phloem, and that these were brought about by active processes which utilized ATP

Question 73

what does translocation require for it to occur?

Answer 73

the establishment of a hydrostatic pressure gradient in the phloem sieve tubes and energy

Question 74

where does active loading of sucrose occur?

Answer 74

from the photosynthetic tissues of the leaf into the phloem at the source

Question 75

what happens at the sink?

Answer 75

Sucrose diffuses out of the phloem by facilitated diffusion or sometimes it leaves the phloem sieve tube by active transport and moves into the sink. Water follows and this lowers the hydrostatic pressure in the phloem sieve tube.

Question 76

where does the hydrostatic pressure gradient start?

Answer 76

in the sieve tubes and not in the photosynthetic tissue or storage tissues where unloading happens

Question 77

how are transfer cells adapted for active loading of solutes in phloem to occur?

Answer 77

internal projections on the cell walls many mitochondria cell surface membrane water attracted unloading at sink

Question 78

how does the mechanism of Indirect Active Transport of Solutes from Phloem Parenchyma into Companion Cells happen?

Answer 78

a protein pump actively transports protons (H+) from the companion Cells into the phloem Parenchyma against the concentration gradient (H+ concentration in phloem parenchyma established) Another transport protein in the companion cells functions as a cotransport protein and allows the H+ and sucrose to move passively across the membrane into the companion cell from a high to low concentration water will enter the cell by osmosis and the hydrostatic pressure of the companion cells will increase

Question 79

evidence to support pressure flow hypothesis

Answer 79

Rate of transport of solutes in phloem= 10 000 times faster than it would be if substances moved by diffusion. Phloem sap always has a relatively high pH (about 8) as expected if H+ were actively pumped out of companion cells. The concentration of sugars in phloem varies from 15-30% compared with 0.5% in leaf mesophyll (parenchyma) cells. Large amounts of ATP in phloem companion cells. Higher H+ concentration of sucrose in leaves and lower H+ concentration in roots.

Question 80

criticisms of mass flow hypothesis

Answer 80

Mass flow is passive in the sieve tubes but sucrose transfer into the phloem is not; hypothesis does not take into consideration The need for living sieve tubes in order for mass flow to occur. Pressure flow/ mass flow does not explain the bi-directonal movement of solutes in the same sieve tube at the same time. The use of KCN stops respiration in the phloem and translocation stops, suggesting that pressure flow alone cannot account for the process, but an active process is also involved. The pressure gradients that actually exist are insufficient to account for the movement along sieve pores which actually impose resistance to the mass flow of solutes in the phloem.

Question 81

what drives the movement of water along a vascular plant?

Answer 81

The movement of water occurs along a water potential gradient, starting from the soil, through the plant, and into the surrounding air.

Question 82

what is the role of transpiration in water movement?

Answer 82

Transpiration is a passive process where water vapour diffuses out of the leaf through open stomata, lowering water vapour concentration in the air spaces of the leaf.

Question 83

How is water replaced in the mesophyll cells after evaporation?

Answer 83

Water evaporating from the mesophyll cells is replaced by water pulled from the xylem through cohesion.

Question 84

What happens to the hydrostatic pressure in the xylem when water is lost at the top of the plant?

Answer 84

The loss of water reduces the hydrostatic pressure at the top of the xylem, creating a suction effect that pulls water upwards

Question 85

how do cohesion and adhesion help in water movement?

Answer 85

Cohesion: Water molecules stick together, forming an unbroken column. Adhesion: Water molecules are attracted to the xylem walls, aiding the upward movement.

Question 86

why doesn't the xylem vessel collapse under tension?

Answer 86

Xylem vessels are strengthened by bands of lignin, which keep them open despite the tension and narrowness.

Question 87

what creates root pressure in the xylem?

Answer 87

Root pressure develops when ions are actively pumped into the xylem from the endodermis, followed by water movement, increasing hydrostatic pressure.

Question 88

how does water move from the soil solution into the root?

Answer 88

Water moves into the root cortex along a water potential gradient, replacing water that enters the xylem.

Question 89

does transpiration require ATP?

Answer 89

No, transpiration is a passive process and does not require ATP.

Question 90

what establishes the suction effect in the leaves?

Answer 90

The suction effect is established by water evaporation and diffusion from the mesophyll cells, reducing water potential in the leaf.

Question 91

how does water move up the xylem?

Answer 91

Water moves up the xylem due to a hydrostatic pressure gradient and is pulled upwards by cohesion and adhesion forces.